FU HUA CHEN (Eds.) Foundations on Expansive Soils 1975

Further titles in this series:

1. G. S ANGLER AT THE PENETROMETER AND SOIL EXPLORATION

2. Q. ZARUBA AND V. MENCL LANDSLIDES AND THEIR CONTROL

3. E.E. WAHLSTROM TUNNELING IN ROCK

4A.R. SILVESTER COASTAL ENGINEERING, I Generation, Propagation and Influence of Waves

4B. R. SILVESTER COASTAL ENGINEERING, II Sedimentation, Estuaries, Tides, Effluents and Modelling

5. R.N. YOUNG AND B.P. WARKENTIN SOIL PROPERTIES AND BEHAVIOUR

6. E.E. WAHLSTROM DAMS, DAM FOUNDATIONS, AND RESERVOIR SITES

7. W.F. CHEN LIMIT ANALYSIS AND SOIL PLASTICITY

8. L.N. PERSEN ROCK DYNAMICS AND GEOPHYSICAL EXPLORATION Introduction to Stress Waves in Rocks

9. M.D. GIDIGASU LATERITE SOIL ENGINEERING

10. Q. ZARUBA AND V. MENCL ENGINEERING GEOLOGY

11. H.K. GUPTA AND B.K. RASTOGI DAMS AND EARTHQUAKES

Developments in Geotechnical Engineering 12

FOUNDATIONS ON EXPANSIVE SOILS

by

FU HUA CHEN

President, Chen and Associates, Inc., Consulting Soil Engineers, Denver, Colo., U.S.A.

ELSEVIER SCIENTIFIC PUBLISHING COMPANY Amsterdam — Oxford — New York 1975

E L S E V I E R S C I E N T I F I C P U B L I S H I N G C O M P A N Y

3 3 5 J a n v a n G a l e n s t r a a t

P . O . B o x 2 1 1 , A m s t e r d a m , T h e N e t h e r l a n d s

A M E R I C A N E L S E V I E R P U B L I S H I N G C O M P A N Y , I N C .

5 2 V a n d e r b i l t A v e n u e

N e w Y o r k , N e w Y o r k 1 0 0 1 7

I S B N 0 - 4 4 4 - 4 1 3 9 3 - 6

C o p y r i g h t © 1 9 7 5 b y E l sev i e r S c i e n t i f i c P u b l i s h i n g C o m p a n y , A m s t e r d a m

All r i g h t s r e s e r v e d . N o p a r t o f t h i s p u b l i c a t i o n m a y b e r e p r o d u c e d , s t o r e d

in a r e t r i e v a l s y s t e m , o r t r a n s m i t t e d in a n y f o r m o r b y a n y m e a n s , e l e c t r o n i c ,

m e c h a n i c a l p h o t o c o p y i n g , r e c o r d i n g , o r o t h e r w i s e , w i t h o u t t h e p r i o r w r i t t e n

p e r m i s s i o n o f t h e p u b l i s h e r .

E l s ev i e r S c i e n t i f i c P u b l i s h i n g C o m p a n y , J a n v a n G a l e n s t r a a t 3 3 5 , A m s t e r d a m

P r i n t e d in T h e N e t h e r l a n d s

To my wife Edna with love and appreciation.

vi

P R E F A C E

The problems associated wi th expansive soils are no t widely appreciated outs ide areas of

their occurrence . The a m o u n t of damage caused by expansive soils is alarming. It has been

es t imated tha t the damage t o buildings, roads , and o the r s t ruc tures founded on expansive soils

exceeds t w o billion dollars annual ly.

In the past 20 years, considerable progress has been m a d e in unders tanding the na ture of

expansive soils. This new knowledge can be separated in to two categories. The first emphasizes

the theoret ical approach and is the result of studies mos t ly by academic ins t i tu t ions . Ins t i tu t ional

research involves soil mineralogy, s t ruc ture , and modif icat ion. Academicians have also advanced

new theories such as effective stress, soil suct ion, and osmot ic pressure which reveal proper t ies of

swelling soils previously little k n o w n to engineers. The second category is concerned wi th the

field performance of expansive soils wi th emphasis on design criteria and cons t ruc t ion

precaut ions for s t ructures founded on expansive soil. Practical approaches of combat ing the

swelling soils problem are mos t ly under t aken by soils engineers; therefore , they mus t offer

practical and economical solut ions to their clients, so tha t the s t ruc ture will be free from

damaging foundat ion movemen t .

Unfor tuna te ly , present day knowledge of expansive soils has n o t reached a stage where

rat ional solut ions can be assigned to the p rob lem. It is difficult for the public to unders tand why

the soils engineer is n o t capable of offering easy solut ions. When the first crack appears in a

s t ruc ture , a lawsuit is th rea tened .

This book provides the practicing engineer wi th a summary of the state-of-the-art of

expansive soils and practical solut ions based u p o n the au thor ' s exper ience. Part I discusses theory

and pract ice, and summarizes some of the theoret ical physical proper t ies of expansive soils. It

also discusses various techniques employed to found s t ructures on expansive soils such as drilled

pier foundat ion , mat foundat ion , mois ture cont ro l , soil replacement , and chemical stabil ization.

Part II presents typical case studies. The au tho r has found tha t few records are available on the

cause of s tructural distress, their remedial measures , and more impor t an t , the degree of success

after those measures have been comple ted .

In . the last 15 years , the au tho r has investigated m a n y thousands of building sites*in

expansive soil areas in the R o c k y Mounta in region. He has also investigated over 1,000 cracked

buildings and has suggested remedial measures . It is the au thor ' s hope tha t by sharing his

knowledge and the knowledge of o the r practicing engineers, a be t t e r unders tanding of expansive

soil p rob lems can be achieved.

The au tho r wishes to thank the ent i re staff of Chen and Associates for sharing the work load

while the au thor was devoting his t ime to writ ing this book and also the assistance given by them

in the prepara t ion of the manuscr ip t . Many thanks to the various consult ing firms, especially

Woodward-Clyde and Associates, Jorgensen and Hendr ickson, Ketchum-Konkel-Barret t -Nickel-

Aust in, and Ε. H. Tippets Company for allowing the publ icat ion of their valuable findings. Mr.

Byron Eskesen has conduc ted mos t of the field investigation and labora tory test ing presented in

this book .

Denver, Colorado

August , 1975

Chapter 1

NATURE OF EXPANSIVE SOILS

INTRODUCTION

The prob lem of expansive soils was no t recognized by soil engineers unti l the lat ter par t of

1930. Prior to 1920, most of the lightly loaded buildings in the United States consisted of frame

dwellings. Such s t ructures could wi ths tand considerable m o v e m e n t wi thou t exhibi t ing not iceable

cracks. By 1930, brick veneer residences became widely used. It was then tha t the owner found

cracks developing in the brick course. The damages were a t t r ibu ted to shoddy cons t ruc t ion and

se t t lement of the foundat ion at one corner , w i thou t recognit ion of the role of expansive soils.

The U.S. Bureau of Reclamat ion [1] * first recognized the swelling soil problem in 1938 in

connect ion wi th a founda t ion for a steel s iphon at their Owyhee Project in Oregon. Since tha t

t ime, engineers realized the cause of damage was somet imes o the r than se t t lement . The

increasingly extensive use of concrete slab-on-ground cons t ruc t ion , after 1940, has further

increased the damage to s t ructures caused by expansive soils.

Today , there is a world-wide interest in expansive clays and shales. Engineers from Canada,

Australia, Sou th Africa, Israel, and the United States have cont r ibu ted immensely to the

knowledge and the proper design for s t ructures on expansive soils. The first significant na t ional

conference on expansive clay probably was one held at the Colorado School of Mines in Golden,

Colorado in 1959. The Internat ional Research and Engineering Conference on Expansive Soils

held their first and second conferences at Texas A & M University in 1965 and 1969, and their

third conference in Haifa, Israel, in 1973 .

ORIGIN O F EXPANSIVE SOILS

G. W. Donaldson [2] classified the parent materials tha t can be associated wi th expansive

soil in to two groups.

The first group comprises the basic igneous rocks , such as the basalts of the Deccan Plateau

in India, the doleri te sills and dykes in the central region of South Africa and the gabbros and

norit ies west of Pretoria Nor th , Transvaal. In these soils, the feldspar and pyroxene minerals of

the parent rocks have decomposed to form montmor i l lon i t e and o ther secondary minerals .

The second group comprises the sedimentary rocks that conta in montmor i l lon i t e as a

cons t i tuent which breaks d o w n physically to form expansive soils. In Nor th America, bedrock

shale found in the Pierre Fo rma t ion and the more recent Laramie and Denver Format ions are

examples of this type of rock. In Israel, there are the marls and l imestones and in South Africa,

the shale of the Ecca Series.

*Numbers in brackets refer to items in the references at the end of each chapter.

2 FOUNDATIONS ON EXPANSIVE SOILS

Γ / V / Λ )

i i — Ά — ι — r . .

I. Highland source of. sediments S volcan*:

; materials I

V

m Marine mud j and clay in I ocean basin

or

Figure 1. Geographic setting of deposition of Pierre and Bearpaw Shales and related rocks in Late Cretaceous time in Rocky Mountain and Great Plains region. (After Tourtelot, 1973)

Tour te lo t [3 ] recons t ruc ted the paleogeographic condi t ion in the Rocky Mounta in and

Great Plains regions as shown on figure 1.

In the Late Cretaceous t ime, to the west of the Rocky Mounta ins were high-to-moderate

uplands, and to the east the Great Plains regions were once ocean basins where the Pierre and

Bearpaw shales and their equivalent were deposi ted. The source of the sediments consists of

volcanic rock in the no r the rn par t (Montana) and a range of rock types in the southern part .

Separating the coastal plain from the ocean basin is a belt of sandy deposi t . The shale is sandier

and siltier adjacent to the coast and possesses a lower swelling potent ia l . Inland, the shale consists

almost entirely of clay-size material with high swell po ten t ia l .

NATURE OF EXPANSIVE SOILS 3

The montmor i l lon i t e was probably formed from t w o separate origins. The p roduc t s of

weathering and erosion of the rocks in the highlands were carried by s treams to the coastal plains.

The fine grained soils eventually became shale accumulat ing in the ocean basin. Meanwhile,

volcanic e rupt ions , sending up clouds of ash, fell on the plains and the seas. These ashes were

altered to montmor i l lon i t e .

Figure 2 i l lustrates, in a general way, the abundance of montmor i l lon i t e in bedrock geologic

formations in the United States. Montmori l loni te is regionally a b u n d a n t in con t inuous geologic

formations t h roughou t the Rocky Mounta ins , mos t of the Great Plains, large parts of the Gulf

Coastal Plains, and the Mississippi Embay m e n t as well as California and the Pacific Nor thwes t .

DISTRIBUTION O F EXPANSIVE SOILS

G. W. Donaldson [21 summarized the dis t r ibut ion of repor ted instances of expansive soils

around the world (fig. 3) . The countr ies in which expansive soils have been repor ted are as

follows:

Argent ina Iran

Australia Mexico

Burma Morocco

Canada Rhodesia

Cuba South Africa

Eth iopia Spain

Ghana Turkey

India U.S.A.

Israel Venezuela

Figure 3 indicates tha t the potent ia l ly expansive soils are confined t o the semi-arid regions

of the tropical and t empera te climate zones . Expansive soils are in abundance where the annual

évapotranspira t ion exceeds the precipi ta t ion. This follows the theory tha t in semi-arid zones, the

lack of leaching has aided the format ion of montmor i l lon i t e .

Potent ial ly expansive soils can be found almost anywhere in the world . In the

underdeveloped na t ions , m u c h of the expansive soil p rob lems may n o t have been recognized. It is

to be expected that more expansive soil regions will be discovered each year as the a m o u n t of

cons t ruc t ion increases r

World problem of expansive soils

The s ta tus of the art of dealing with world problems on expansive clay soils was summarized

in the Internat ional Panel Review during the first conference on expansive clay soils a t Texas A &

M, 1965. The following are the typical findings in each coun t ry :

Australia — The major city tha t experienced expansive soil p roblems is Adelaide in Sou th

Australia. Even though the damage caused by expansive soils is modera t e , in a city of

4F

OU

ND

AT

ION

S ON

EX

PA

NS

IVE

SO

ILS

Figure 2. General abundance of montmorillonite in near-outcrop bedrock formations. (Modified from Tourtelot, 1973)


Figure 3. Distribution of reported instances of heaving. (After G. W. Donaldson)

some 600 ,000 inhabi tan ts , the aggregate damage associated with foundat ion cracks is a

substantial a m o u n t .

Canada - The wide range of climate and geology in Canada produces a great variety of

foundat ion problems. In Western Canada, including Saskatchewan and Alberta ,

expansive clay prob lems are strongly evident . The soils in this region are generally

desiccated. Also, in this area of Canada, shallow basements placed on shallow footings

are commonly used. There have been very m a n y cases where pressures of the expansive

clays have caused lateral deflections of basement walls. Basement floors have been

k n o w n to heave as m u c h as 6 inches in 18 m o n t h s .

India - The so-called black co t t on soils cover a large area of approx imate ly 2 0 0 , 0 0 0 square

miles, in the hear t of India. This soil is characterized by its ex t reme hardness when dry

and with high swelling potent ia l during the process of wet t ing.

Israel — Expansive soil p roblems exist t h roughou t Israel. Israel has a rainy winter season and

a ho t , dry summer . The soils are primarily alluvium or reworked t ranspor ted alluvium

which originates from the weather ing of ei ther basalt or l imestone . In the clay soil area,

montmor i l lon i te may be present in quant i t ies ranging from 40 to 80 percent of the

soil.

Mexico — Mexico City has a wor ld- renowned repu ta t ion for se t t l ement p rob lems . The

prob lem of expansive clays in Mexico is no t considered to be very serious t o da te . So


far, they have been encounte red in only about five towns of ra ther med ium size, bu t

the problem is potent ia l ly more serious because new towns are being cons t ruc ted and

small towns are being expanded .

South Africa — In South Africa, the problem of expansive soils was b rough t to the a t t en t ion

of the engineers as early as 1950. The South African Ins t i tu t ion of Civil Engineers

published the first sympos ium on expansive clays in 1957. Severe founda t ion

movemen t problems were recorded at Leeuhof, Vereeniging, and Pretoria in Transvaal,

where the fluvio-lacustrine deposits are the source of swelling soils. The Ecca shale,

covering a large par t of South Africa, is responsible for the foundat ion movemen t

p rob lems at Odendaalsrus in the Orange Free States Goldfields.

Spain — In Spain, many clay format ions of sedimentary origin with high plast ici ty can be

found. In mos t par ts of the coun t ry , the climate is arid and the évapotranspira t ion is

several t imes greater than the precipi ta t ion resulting in swelling phenomena . Among

the various regions where such p h e n o m e n a have been observed, there are t w o provinces

which m a y be regarded as typical ; Andalucia and Madrid. In the province of Madrid,

the soils for the most par t consist of montmor iHoni t ic clays. These soils reach a liquid

limit of 250 , though generally they do n o t go over 80. In a great par t of the

met ropol i tan areas, the highly plasticity clays are covered wi th a sufficient dep th of

sandy clay sediments , therefore , present n o swelling problem.

Venezuela — The first repor t of swelling clays in Venezuela came from the vicinity of the

City of Coro where many buildings are badly cracked. In one instance near the city,

shales with expansive propert ies are found. Some of these soils have swelling pressures

of 13 tons per sq. ft. and occasionally up to 28 tons per sq. ft.

Distribution of expansive soils in the United States

In the United States, from the Gulf of Mexico to the Canadian Border and from Nebraska

to the Pacific Coast, the abundance of Montmori l loni te is c o m m o n in b o t h clays and claystone

shales.

The repor ted problem areas are most ly located in the regionally abundan t montmor i l lon i t e

areas indicated in figure 2. Research has been carried ou t on expansive soils in m a n y states

th roughou t the United States. Figure 4 indicates the states where the State Highway Depar tmen t s

have sponsored research concerning expansive soils [ 4 ] . It is interest ing to no te the similarity

be tween figures 2 and 4 . The states tha t experience various degrees of expansive soil p roblems are

listed as follows:

Severe: Colorado

Texas

Wyoming

Modera te : California

Utah

Nebraska

South Dakota

Figure 4 . State Highway Departments that are sponsoring or have recently sponsored research concerning expansive clay soils. (After Sallbert and Smith)

NA

TU

RE

O

F

EX

PA

NS

IVE

S

OIL

S

7


Mild: Oregon

Montana

Arizona

Oklahoma

Kansas

Alabama

Mississippi

DAMAGE CAUSED BY EXPANSIVE SOILS

Jones and Holtz repor ted in ASCE in 1973 [5] the est imated damage a t t r ibu ted to

expansive soil movement as follows:

Est imated average annual loss, Cons t ruc t ion category millions of dollars

Single-family homes $ 3 0 0

Commercia l buildings 360

Multi-story buildings 80

Walks, drives, parking areas 110

Highway and streets 1,140

Underground utilities and service 100

Airports 4 0

Urban landslides 25

Others 100

Total $ 2 ,255

According to the above es t imate , expansive soil damages n o w exceed the combined average

annual damages from floods, hurr icanes, ea r thquakes , and tornados .

A great deal of s t ructural movemen t has been unduly b lamed on expansive soils. Many floor

slabs const ructed in an expansive soil area crack and somet imes heave due to improper ly designed

concrete . It is a well k n o w n fact tha t improper curing of concre te , in addi t ion to the lack of

expansion jo in ts , will cause cracking. Curling of concrete slabs has a s t rong resemblance to

heaving floors caused by swelling soils. This is especially t rue for large warehouse floors where

proper curing and design is essential.

In expansive soil areas, the soils are generally stiff, and the chance of lightly loaded

structures cracking due to se t t lement is ra ther r emote . At the same t ime, there are a large n u m b e r

of instances where heavy cracks have appeared in the basement walls tha t were no t caused by

foundat ion heaving bu t by ear th pressure exerted on the wall, generally compounded by seepage

pressure. In mos t cases where vertical or hor izonta l cracks developed in the basement wall, ea r th

pressure problems are suspect . Diagonal cracks tha t develop below windows and above doors are

a s trong indicat ion of swelling movement .


Somet imes , basement wall cracks are caused by careless cons t ruc t ion crews. Backhoe or

o ther ear th moving equ ipmen t bumping against the wall can cause vertical or hor izon ta l cracks.

Expansive soils are of tent imes blamed for arching of a wall when actually improper

re inforcement and restraint is the real p rob lem. Backfill should n o t be placed against the wall

unti l the wall has been proper ly restrained at t o p and b o t t o m . Failure to do so may result in an

arched condi t ion . Such p h e n o m e n o n somet imes is er roneously in terpre ted as hor izon ta l swelling

pressure being exer ted against the wall.

While it is possible tha t a large a m o u n t of swelling pressure can be exer ted hor izonta l ly

against a wall, generally backfill is so loosely compac ted tha t distress caused b y lateral expansion

of backfill is very u n c o m m o n .

Structural defects are somet imes mis taken for distress caused by swelling soils. Split level

houses are generally cons t ruc ted with grade beams placed at different levels. Such grade beams, if

no t proper ly tied together wi th re inforcement , can result in cracks and movemen t .

While it is t rue tha t swelling soils are p robably responsible for mos t of the cracking and

movement of lightly loaded s t ructures , o ther aspects of founda t ion movemen t canno t and should

no t be ignored.

CLAY M I N E R A L S

Most soil classification systems arbitrarily define clay particles as having an effective

d iameter of t w o microns (0 .002 m m ) or less. Particle size alone does no t de te rmine clay mineral .

Probably the most i m p o r t a n t grain p roper ty of fine-grained soils is the minealogical compos i t ion

[ 6 1 . Fo r small size part icles, the electrical forces acting on the surface of the particle are m u c h

greater than the gravitational force. These particles are said to be in the colloidal s ta te . The

colloidal part icle consists primarily of clay minerals tha t were derived from paren t rock by

weathering.

The three mos t i m p o r t a n t groups of clay minerals are montmor i l lon i t e , illite, and kaolini te ,

which are crystalline hydrous aluminosil icates. Montmor i l lon i te is the clay mineral t h a t presents

mos t of the expansive soil p roblems.

The n a m e ' ' m o n t m o r i l l o n i t e " is used current ly b o t h as a g roup name for all clay minerals

wi th an expanding lat t ice, except vermiculi te , and also as a specific mineral name [ 7 ] .

Absorp t ion of water by clays leads to expansion. F r o m the mineralogical s t andpo in t , the

magni tude of expansion depends u p o n the kind and a m o u n t of clay minerals present , their

exchangeable ions, e lectrolyte con ten t of aqueous phase, and the internal s t ruc ture .

Formation of clay minerals

The clay minerals are formed th rough a compl ica ted process from an assor tment of paren t

materials. The paren t materials include feldspars, micas, and l imestone. The a l tera t ion process

that takes place on land is referred to as weather ing and tha t on the sea floor or lake b o t t o m as

halmyrolysis . The al terat ion process includes disintegrat ion, ox ida t ion , hydra t ion , and leaching.


Tour te lo t [31 po in ted ou t tha t the sett ing for the format ion of montmor i l lon i t e is ex t reme

disintegration, s t rong hydra t ion , and restricted leaching. The s i tuat ions in which montmor i l lon i t e

can form require tha t leaching be restr icted, so tha t magnesium, calcium, sod ium, and iron

cations may accumula te in the system. Thus , the format ion of montmor i l lon i t i c minerals is aided

by an alkaline envi ronment , presence of magnesium ions, and a lack of leaching. Such condi t ions

are favorable in semi-arid regions wi th relatively low rainfall or highly seasonal mode ra t e rainfall,

part icularly where evaporat ion exceeds precipi ta t ion. Under these condi t ions , enough water is

available for the al terat ion process, b u t the accumula ted cations will n o t be removed by flush

rain.

The parent minerals for the format ion of montmor i l lon i te often consist of ferromagnesium

minerals, calcic feldspars, volcanic glass, and m a n y volcanic rocks . Bentoni te is a clay composed

primarily of montmor i l lon i te which has been formed by the chemical weathering of volcanic ash.

Swelling clays are commonly referred to as ben ton i t i c soils by laymen. Since commercia l

ben ton i te is whi te , the whi te calcium streaks present in stiff clays are often mis taken for

ben ton i te . Actual ly, clays wi th an abundance of calcium seldom exhibi t swelling characterist ics.

Cation exchange

Clay minerals have the p rope r ty of sorbing certain anions and cations and retaining t h e m in

an exchangeable s ta te . The exchangeable ions are held a round the outs ide of the silica-alumina

clay-mineral s t ructural uni t , and the exchange react ion does no t affect the s t ructure of the

silica-alumina pocke t . In clay minerals , the mos t c o m m o n exchangeable cat ions are Ca^, Mg**", FT,

K+, N H 4 +, Na+, frequently in abou t tha t order of general relative abundance .

The existence of such charges is indicated by the ability of clay to absorb ions from the

solut ion. Cat ions (positive ions) are more readily absorbed than anions (negative ions) ; hence ,

negative charges mus t be pe rdominan t on the clay surface. A cat ion, such as Na+, is readily

a t t racted from a salt solut ion and a t tached to a clay surface. However, the absorbed Na+ ion is

no t pe rmanen t ly a t t ached ; it can be replaced by K+ ions if the clay is placed in a solut ion of

potassium chloride KCL. The process of replacement by excess cat ions is called cation exchange

[ 8 ] .

The cat ion exchange capaci ty is the charge or electrical a t t rac t ion for cat ion per uni t mass

as measured in millequivalent per 100 grams of soil.

The cat ion exchange capacity of different types of clay minerals m a y be measured by

washing a sample of each wi th a solut ion of a salt such as a m m o n i u m chloride N H 4 C L and

the a m o u n t of adsorbed N H ^ by measuring the difference be tween the original and the final

concent ra t ion of the washing solut ion.

Typical ranges of cat ion exchange capacities of various clay minerals are shown in table 1.

F r o m table 1, it is seen tha t montmor i l lon i tes are 10 t imes as active in absorbing cat ions as

kaolinites. This is caused by the large net negative charge carried by the montmor i l lon i t e particle

and its greater specific surface as compared wi th kaolini te and illite.

Certain relat ionships exist be tween soil proper t ies such as At terberg l imits, the t y p e of clay

mineral , and the na ture of the adsorbed ion. Table 2 indicates the liquid limit and the plasticity

index of each group of clay minerals. F r o m tables 1 and 2, it is seen tha t the cat ion exchange


Table 1 - . Ranges of cation exchange capacities of various clay minerals

Kaolinite Illite Montmorillonite

Particle 0 . 5 - 2 0.003 - 0 . 1 Less than thickness microns microns 9.5 A

Particle 0 . 5 - 4 0.5 - 10 0.05 - 10 diameter microns microns microns

Specific surface 1 0 - 2 0 65 - 180 50 - 840 (sq. meter/gram)

Cation exchange 3 - 1 5 1 0 - 4 0 7 0 - 8 0 capacity (milliequivalents per 100g)

(After Woodward-Clyde & Associates, 1967)

capacity of a clay has definite relation with the At terberg l imits. The greater the cation exchange

capacity of clay, the greater the effect of changing the adsorbed cat ion.

Cat ion exchange p h e n o m e n o n takes place in everyday life. A simple and well known

example of the ion exchange react ion is the softening of water by the use of pe rmut i t es or carbon

exchangers. The basic principle involved in the chemical stabilization of expansive soil is the

increase in the ionic concen t ra t ion in the free water and base exchange p h e n o m e n o n .

Clay structure

Philip Low [9] po in ted ou t the two fundamenta l molecular s t ructures as the basic uni t s of

the latt ice s t ruc ture . These are the silica t e t rahedron and the a lumina oc tahedron .

The silica t e t r ahedron consists of a silicon a tom sur rounded te t rahedral ly by four oxygen

ions as shown on figure 5a. The alumina oc tahedron consists of an a luminum a t o m sur rounded

octahedral ly by six oxygen ions as shown on figure 5b . When each oxygen a tom is shared by t w o

te t rahedra , a plate-shaped layer is formed. Similarly, when each a luminum a tom is shared by two

oc tahedron , a sheet is formed.

The silica sheets and the alumina sheets combine t o form the basic s t ructural uni t s of the

clay particle. Various clay minerals differ in the stacking configurat ion.

The results of studies using the e lectron microscope and X-ray diffraction techniques show

that the clay minerals have a latt ice s t ruc ture in which the a toms are arranged in several sheets ,

similar to the pages of a book . The ar rangement and the chemical compos i t ion of these sheets

de termine the type of clay mineral . The basic building blocks of the clay minerals are the silica

t e t rahedron and the alumina oc tahedron . The blocks combine in to te t rahedral and octahedral

sheets to p roduce the various types of clays.

Kaolinite is a typical two-layer mineral having a single te t rahedra l sheet jo ined by a single

octahedral sheet to form what is called a 2 to 1 latt ice s t ruc ture .


Table 2 - . Atterberg-limit values of clay minerals with various adsorbed cations

Cation

Na+ Ca" Mg"

Cation

Liquid

limit,

percent

Plasticity

index,

percent

Liquid

limit,

percent

Plasticity

index,

percent

Liquid

limit,

percent

Plasticity

index,

percent

Liquid

limit,

percent

Plasticity

index,

percent

Clay mineral

Kaolinte

Illite

Montmorillonite

29

61

344

1

27

251

35

81

161

7

38

104

34

90

166

8

50

101

39

83

158

11

44

99

(After W.A. White, 1958)

Figure 5. Polyhedra composing the structure of montmorillonite: (a) the silica tetrahedron, (b) the alumina octahedron. (After Philip Low, 1973)


Montmori l loni te is a three-layer mineral having a single oc tahedra l sheet sandwiched

be tween two te t rahedral sheets to give a 2 to 1 lat t ice s t ruc ture as shown on figure 6.

Illite has similar s t ruc ture with tha t of montmor i l lon i t e , bu t some of the silican a toms are

replaced by a luminum, and, in addi t ion , potass ium ions are present be tween the te t rahedra l sheet

and adjacent crystals.

In the clay-water-air system, the water within the clay is called adsorbed water , the water

and ions with the clay latt ice cons t i tu te the diffuse double layer. T w o forces exist in t h e system,

the at tractive and the repulsive forces.

The closer the dipolar water molecules and cat ions are t o the flat plate surface, the more

strongly they are a t t rac ted . At small interlayer distances, two at tract ive forces p redomina te .

1. Electrostat ic force - depends on the composi t ion of the mineral .

2. Van der Waals' force — depends on the distance be tween the layers.

The high concent ra t ion of cat ions near the surface of the clay particle creates a repulsive

force be tween the diffuse double-layer system. The inter layer solut ion has a higher concen t ra t ion

of dissolved e lectrolyte than the external solut ion and the subsequent en t ry of water by osmosis.

The resulting repulsive pressure is, therefore, the osmot ic pressure.

The double-layer theory assumes tha t the clay particle is a flat, charged condenser plate and

the ions are assumed to be non-interact ing poin t charges. Hence, it is possible to use Poisson's

equat ion from the theory of electrostat ics . By combining Poisson's equat ion wi th Bol tzmann ' s

equat ion of osmot ic pressure, the resulting equat ion is the Poisson-Boltzmann equa t ion and is the

Figure 6. Model of a layer of montmorillonite. (After Philip Low, 1973)


basic differential equa t ion of the double-layer theory . The typical result of the in tegra t ion of the

Poisson-Boltzmann equa t ion is given in figure 7, in which the surface charge densi ty is

de te rmined by dividing the cat ion ion exchange capacity by the surface area. It is seen from

figure 7 tha t the calculated repulsive pressures increase rapidly as the half-distance be tween the

particles decreases.

Warkent in and Bolt [101 observed tha t exper imen al curves of swelling pressure versus

inter layer half-distance for Na-montmor i l loni te has the same shape as figure 7.

The basic relat ion be tween dry densi ty and swelling pressure developed by the au tho r in

figure 28 also assumes the same pa t te rn .

Osmotic pressure

Osmosis is the passage of solvent th rough a semi-permeable m e m b r a n e from a solut ion of

lesser concen t ra t ion t o one of higher concen t ra t ion , and osmot ic pressure is the pressure which

mus t be applied to the solut ion to prevent the flow of solvent which tries to dilute the solut ion.

Osmot ic pressure can be evaluated by Van ' t Hoff equa t ion as follows:

P 0 = R T ( C i - C 2 )

In which: P Q = Osmot ic pressure

R = Gas cons tan t (Bol tzmann cons tan t )

Τ = Absolute t empera tu re

C j = Concent ra t ion of any ionic species

(in ions per c m 3 )

C 2 = Ionic concent ra t ion of the ionic species in the external

solut ion (in ions per c m 3 )

It is well recognized tha t osmot ic pressure can be expec ted to take place in the soil-water

sys tem. Assuming tha t the double layer system exists in the soil la t t ice, the concen t ra t ion of ions

being held by the at t ract ive force prevents the ions from moving away from the double layer.

However, water is able to move in and dilute the concent ra t ion , and, consequent ly , a

semi-permeable m e m b r a n e effect is achieved.

Research made in the last decade strongly suggests tha t osmot ic pressure indeed develops in

the soil-water system and is responsible for the swelling mechanism. G. H. Bolt , as early as 1956,

[11] concluded tha t the swelling of b o t h illitic clays and montmor i l lon i t e clays is caused by the

excess osmot ic pressure in the adsorbed layer of ions. Bolt claimed that the osmot ic pressure of

the sys tem might reach a value of 50 to 100 tons per square foot. It is therefore, no t surprising

tha t the swelling pressure of expansive clays somet imes reaches more than 25 tons per square

foot .

Based on the theory tha t osmot ic pressure is the only internal pressure acting be tween

particles, if the soil is subjected to external pressure, the distance be tween particles will decrease

and water will be squeezed out . As a result , the ion concent ra t ion be tween the particles will

increase and the osmot ic pressure in turn increases. An equil ibr ium is finally reached when the

osmot ic pressure equals the external pressure. The reverse process involves the decrease of


Figure 7. Calculated repulsive pressures at different half-distances between adjacent montmorillonite particles (layers) for two values of the surface charge density (cr). (After Philip Low)


external pressure and the suct ion of liquid by osmot ic pressure be tween the particles t o dilute the

concent ra t ion of ions. The distance be tween the particles would increase, resulting in volume

increase and a reduct ion of osmot ic pressure. This process cont inues unti l a new equi l ibr ium is

established. The imbibat ion of water is the mos t impor t an t cause of swelling.

RECOGNITION O F EXPANSIVE SOILS

There are three different m e t h o d s of classifying potent ia l ly expansive soils. The first,

mineralogical identif icat ion, can be useful in the evaluation of the material bu t is no t sufficient in

itself when dealing wi th natural soils. The various m e t h o d s of mineralogical identif icat ion are

impor tan t in a research labora tory in exploring the basic proper t ies of clays, bu t are impract ical

and uneconomical for practicing engineers.

Ano the r group includes the indirect m e t h o d s , such as the index p rope r ty , PVC m e t h o d , and

activity m e t h o d which are valuable tools in evaluating the swelling p roper ty . Soil suct ion may

prove to be very useful wi th more general appl icat ion and improved testing techniques . None of

the indirect m e t h o d s should be used independent ly . Er roneous conclusions can be drawn wi thou t

the benefit of direct tests.

The third m e t h o d , direct measurement , offers the most useful data for a practicing engineer.

The tests are simple to perform and d o n o t require any costly and exot ic labora tory equ ipmen t .

A word of caut ion should be in t roduced here . Testing should be performed on a n u m b e r of

samples ra ther than of a few to avoid er roneous conclusions.

Mineralogical identificatioη

The mineralogical composi t ion of expansive soils has an impor t an t bearing on the swelling

potent ia l as explained under "Clay S t ruc tu re . " The negative electric charges on the surface of the

clay minerals, the s t rength of the interlayer bonding, and the cat ion exchange capacity all

cont r ibu te to the swelling potent ia l of the clay. Hence, it is claimed by the clay mineralogist tha t

the swelling poten t ia l of any clay can be evaluated by identif ication of the cons t i tuen t mineral of

this clay. The five techniques which may be used are as follows:

X-ray diffraction,

Differential thermal analysis,

Dye adsorpt ion ,

Chemical analysis, and

Elect ron microscope resolut ion.

The various me thods listed above should generally be used in combina t ion . Using

combinat ions of the m e t h o d s , the different types of clay minerals present in a given soil can be

evaluated quant i ta t ively . Unfor tuna te ly , t hough a great deal of research has been done in the

various fields of mineralogical s tudy , the test results require exper t in terpre ta t ion and the


specialized appara tus required are costly and no t economical ly available in mos t soil test ing

laboratories . A brief descript ion of the various techniques is as follows:

X-Ray Diffraction Method . The X-ray diffraction m e t h o d used in de termining the p ropo r t i on of

the various minerals present in a colloidal clay consists essentially of comparing the rat ios of the

intensities of diffraction lines from the different minerals wi th the intensit ies of lines from the

s tandard substance. G. W. Brindley [12] claimed tha t the use of self-recording coun te r

spect rometers in lieu of pho tograph ic techniques increases considerably b o t h the accuracy and

the convenience of the X-ray m e t h o d . Brindley also believes tha t the X-ray m e t h o d for

quant i ta t ive de te rmina t ions should be applied wi th considerable c i rcumspect ion, and tha t in

favorable cases the possibility of1 identifying species by X-ray analysis can be regarded wi th

restrained opt imism.

Differential Thermal Analysis. Differential thermal analysis when used in conjunct ion wi th X-ray

diffraction and chemical analysis enables the identif icat ion of otherwise difficult materials . It is

well established as a technique for the control of materials which undergo characterist ic changes

on heating. The use of differential thermal analysis technique in identifying expansive soil is n o t

always accurate [ 1 3 ] .

Dye Adsorp t ion . Dyestuffs and o the r reagents which exhib i t characterist ic colors when adsorbed

by clay have been used to identify clay. When a clay sample has been pre t rea ted with acid, the

color assumed by the adsorbed dye depends on the base exchange capacity of the various clay

minerals present . The presence of montmor i l lon i t e can be de tec ted if its a m o u n t is greater than

abou t 5 to 10 percent . The relatively simple testing procedure and speed of dye staining tests

compared wi th X-ray diffraction and differential thermal analysis justify wider appl icat ion of the

color me thod .

Chemical Analysis. Chemical analysis can be a valuable supplement to o the r m e t h o d s such as

X-ray analysis in identifying clays. In the montmor i l lon i t e g roup of clay minerals , chemical

analysis can be used to de termine the na ture of i somorphism and to show the origin and locat ion

of the charge on the lat t ice. According t o Kelley [ 1 4 ] , the i somorphous character of the

montmor i l lon i te group can probably be shown in n o o ther way. The i somorphism involves three

basic variat ions in the subs t i tu t ion : the subs t i tu t ion for Al for Si in te t rahedral posi t ions in the

la t t ice; the subs t i tu t ion of Fe for Al in the octahedral coord ina t ion ; and the subs t i tu t ion of Mg

for Al in the oct rahedral posi t ions .

Electron Microscope Resolut ion. Microscopic examinat ion of clay minerals offers a direct

observation of the material . T w o clays may give the same X-ray pa t t e rn and the same differential

thermal curve b u t will show up dist inct morphological characterist ics unde r e lectron microscope

resolut ion. The main purpose of the microscopic examina t ion is to de te rmine minéralogie

composi t ion , t ex tu re , and internal s t ruc ture .


Ravina [15] made extensive s tudy of the mineralogical composi t ion of expansive clays by

the use of the scanning electron microscope. It showed that the nonswelling clays appear as flat,

relatively thick plates while montmor i l lon i tes have a crinkly, ridged, honeycomb-l ike t ex ture . It

might be possible to evaluate some proper t ies of the expansive soil by observing the degree of

crinkling and interpart icle bonding from scanning an electron microscope.

Single index method

Simple soil p roper ty tests can be used for the evaluation of the swelling poten t ia l of

expansive soils. Such tests are easy to perform and should be included as rout ine tests in the

investigation of building sites in those areas having expansive soil. Such tests may include:

At te rberg limits tests,

Linear shrinkage tests,

Free swell tests, and

Colloid con ten t tests.

At terberg Limits. Holtz and Gibbs [16] demons t ra ted in 1956 tha t plasticity index and liquid

limit are useful indices for determining the swelling characteristics of most clays. Seed,

Woodward, and Lundgren [17] have demons t ra ted that the plasticity index alone can be used as

a prel iminary indicat ion of swelling characteristics of most clays.

The swell potent ia l is defined as the percentage swell of a laterally confined sample which

has soaked under a surcharge of 1 p o u n d per square inch after being compac ted to m a x i m u m

densi ty at o p t i m u m mois ture con ten t according to the AASHO compac t ion test. F rom this, Seed,

Woodward, and Lundgren established the following simplified relat ionship:

in which:

and

S = 60K(PI) 2M

S = Swell potent ia l

Κ = 3.6 X 1 0 " 5 and is a cons tant .

The above equat ion applies only to soils wi th clay con ten t s be tween 8 and 65 percent and

the compu ted value is probably accurate t o within abou t 33 percent of the labora tory

determined swell potent ia l .

Since liquid limit and swelling of clays bo th depend on the a m o u n t of water a clay tries to

imbibe, it is n o t surprising that they are related.

Relat ion be tween swelling poten t ia l of clays and plasticity index can be established as

follows:

Swelling poten t ia l Plasticity index

Low 0 - 15

Medium 10 - 35

High 2 0 - 5 5

Very high 35 and Above


While it may be t rue that high swelling soil will manifest high index p rope r ty , the converse is n o t

t rue.

Linear Shrinkage. The swell potent ia l is presumed to be related to the opposi te p rope r ty of linear

shrinkage measured in a very simple test. In theory it appears tha t the shrinkage characterist ics of

the clay should be a consistent and reliable index to the swelling potent ia l .

It was suggested b y Al tmeyer in 1955 [18] as a guide to the de te rmina t ion of potent ia l

expansiveness for various values of shrinkage limits and linear shrinkage as follows:

Shrinkage limit Linear shrinkage Degree of as a percentage as a percentage expansion

Less than 10 Greater than 8 Critical

1 0 - 1 2 5 - 8 Marginal

Greater than 12 0 - 5 Non-critical

Recent research, however , failed t o show conclusive evidence of the correlat ion be tween

swelling potent ia l and shrinkage limit.

Free Swell. Free swell tests consist of placing a k n o w n volume of dry soil in water and not ing the

swelled volume after the material sett les, w i thou t any surcharge, to the b o t t o m of a graduated

cylinder. The difference be tween the final and initial vo lume, expressed as a percentage of initial

volume, is the free swell value. The swell test is very crude and was used in the early days when

refined testing me thods were n o t available.

Exper iments conduc ted by Holtz [16] indicated tha t a good grade of high swelling

commercial ben ton i t e will have a free swell value of from 1200 t o 2 0 0 0 percent . Hol tz suggested

that soils having free swell value as low as 100 percent can cause considerable damage to lightly

loaded s t ructures , and soils having free swell value below 50 percent seldom exhibi t appreciable

volume change even under very light loadings.

Colloid Con ten t . The grain size characterist ics of a clay appear t o have a bearing on its swelling

potent ia l , part icularly the colloid con ten t . Seed, Woodward , and Lundgren [17] believed tha t

there is no correlation* be tween swelling potent ia l and percentage of clay sizes. However , for a

given clay type , the a m o u n t of swell will increase wi th the a m o u n t of clay present in the soil as

shown on figure 8.

For any given clay t ype , the relat ionship be tween the swelling potent ia l and percentage of

clay size can be expressed by the equa t ion :

S = K C X

where : S = Swelling potent ia l , expressed as percentage of swell unde r 1-psi

surcharge for a sample compac ted at o p t i m u m mois ture c o n t e n t t o

m a x i m u m density in s tandard AASHO compac t ion test ,


70

50

% 4 0

Clay co mponent: C >mmercial Ε îentonite

NOTE ι l 1

: Percent swell measured under 1 psi surcharge for sample compacted at optimum water content to maximum density IP «tnnHnrri Δ Δ ^ ΗΠ t**t

J

\ y h i Comme -cial I l l i te / Bentonite

I , ,6\ Com rcercial Kao unite/Senti >nite

/ ^3-1 Com nercial III) e/Bentonit 1

JL-C< •mmercial I 1 lite

A ^ < _ | : | Commercial

1 Commercial

Illite/Kaol

Kaolinite

nite

20 30 4 0 50 60 70 PERCENT CLAY SIZES (finer than 0 . 0 0 2 mm)

100

Figure 8. Relationship between percentage of swell and percentage of clay sizes for experimental soils. (After Seed, Woodward & Lundgren)

C = Percentage of clay sizes finer than 0 .002 m m ,

X = An exponen t depending on the type of clay, and

Κ = Coefficient depending on the type of clay.

Where the quan t i ty of the clay size particles is de termined by a h y d r o m e t e r test , the

quali ty or kind of colloid, which is reflected by X and Κ in the above equa t ion , controls the

a m o u n t of swell. Colloid con ten t as well as At terberg limits should be included in the rou t ine

labora tory investigation on expansive soils.

Classification method

By utilizing rou t ine labora tory tests such as At terberg l imits, colloid conten ts , shrinkage

limits, and others , the swelling potent ia l can be evaluated w i thou t resort ing to direct

measurement . Some of these m e t h o d s are as follows:

USBR Method — Developed by Hol tz and Gibbs [16] is based on the s imultaneous considerat ion

of several soil propert ies . The typical relat ionships of these proper t ies with swelling potent ia l are

shown on figure 9.

Based on the curves presented in figure 9, Hol tz [19] proposed the identif icat ion criteria of

expansive clay as follows:


Table 3-Data for making estimates of probable volume changes for expansive soils

Data from index tests* Probable Colloid content, expansion, Degree percent minus Plasticity Shrinkage percent total of

0.001 mm index limit vol. change expansion

> 2 8 >35 <11 > 3 0 Very high 20-13 25-41 7-12 20-30 High 13-23 15-28 10-16 10-30 Medium

>15 < 1 8 >15 < 1 0 Low

•Based on vertical loading of 1.0 psi. (After Holtz & Gibb)

It should be pointed out tha t figure 9 is based on actual expansion tests for only 45

undis turbed and remolded samples and, therefore , the da ta accumulated is no t sufficient t o form

accurate empirical relat ionship be tween measured expansion and three indicator tests . Especially,

considerat ion should be given in the differentiat ion of soil behavior be tween undis turbed and

remolded samples.

The au thor has over the past 15 years performed m a n y thousands of tests on poten t ia l swell

and index proper t ies . F rom the test results of 321 undis turbed samples, a regression curve can be

fitted as shown on figure 10. The relat ionship be tween swell potent ia l and plasticity index can be

expressed as follows:

S = B e A( p i)

in which A = 0 .0838 , and

Β = 0 .2558

F rom figure 10 it is seen tha t with increase of plastici ty index, the increase of swelling

potent ia l is m u c h less than predicted by Hol tz and Gibbs or from Seed, Woodward and Lundgren .

All tests refer to a surcharge pressure of 1 psi wi th mois ture con ten t be tween 15 and 20 percent

and dry densi ty be tween 100 and 110 pcf.

Activity Method . - The activity m e t h o d proposed by Seed, Woodward , and Lundgren (16) was

based on remolded , artificially prepared soils composed of 23 mixtures of ben ton i t e , illite,

kaol ini te , and fine sand. The expansion was measured as percent swell on soaking from 100

percent m a x i m u m densi ty and o p t i m u m mois ture con ten t in s tandard AASHO compac t ion test

under a surcharge of 1 psi. The activity for the artificially prepared sample was defined as:

Activity = J 2 _

In the above, C denotes the percentage clay size finer than 0 .002 m m . The proposed classification

chart is shown on figure 11.

The activity m e t h o d appears t o be an improvement over the USBR m e t h o d in tha t the

shrinkage limit did no t en ter in the evaluation of swell potent ia l . Also, an a t t e m p t has been m a d e

t o differentiate be tween undis turbed and remolded samples.


/ · 1 /

/ ./

. / — /

1 • 1.

\

1

• •

!•/ h r V \ V

. 1 ·

\ 1 ';•/ • A \ λ* A τ

f 7 • / / '

- - 0 r - / i -/ •l %

/ / • • 40

COLLOID CONTENT (% less than 0.001 mm)

20 40 24

PLASTICITY INDEX SHRINKAGE LIMIT (%)

Figure 9. Relation of volume change to colloid content, plasticity index, and shrinkage limit (air-dry to saturated condition under a load of 1 lb. per sq. in.) (After Holtz and Gibbs)

Indirect measurement

Indirect measurement of swelling potent ia l of expansive soils has been approached by m a n y

investigations. The Ladd and Lambe m e t h o d aided by a PVC me te r is p robab ly the simplest and

quickest m e t h o d , while the soil suction m e t h o d is considered to be a new approach toward the

measurement of swelling potent ia l and swelling pressure.

PVC Meter. - The de te rmina t ion of the potent ia l volume change (PVC) of soil was developed by

T. W. Lambe under the auspices of the Federal Housing Adminis t ra t ion [ 2 0 ] . Remolded samples

were specified. The sample was first compac ted in a fixed ring consol idometer with compac t ion

effort of 55 ,000 ft.-lbs. per cu. ft. Then an initial pressure of 200 psi was applied, and water

added to the sample which is partially restrained from vertical expansion by a proving ring. The

proving ring reading is taken at the end of 2 hours . The reading is converted to pressure and is

designated as Swell Index. F rom figure 12, the swell index can be converted to potent ia l volume

change. Lambe established the following categories of PVC rat ing:

PVC Rating Category

Less than 2 Non-critical

2 — 4 Marginal

4 - 6 Critical

Greater than 6 Very critical


Figure 10. Relationship of volume change to plasticity index as predicted by Holtz, Seed, and Chen.


*>

Swelling Potential = 25%

Swelling Potential = 5% Swelling Potential = 1.5%

I l _ I

Ο 10 20 30 40 50 60 70 80 90

Percent Clay Sizes (finer than 0.002mm)

Figure 11. Classification chart for swelling potential (After Seed, Woodward & Lundgren)

100

The PVC me te r m e t h o d has been widely utilized by the Federal Housing Adminis t ra t ion as

well as the Colorado State Highway Depa r tmen t . It should be po in ted ou t tha t the PVC me te r

test in itself does n o t measure the swell po ten t ia l . The t rue swell potent ia l of clay measured can

be much greater than the indicated value. The PVC mete r test should be used only as a

comparison be tween various swelling soils.

Ladd and Lambe [21] proposed a classification system in 1961 whereby soils are classified

with respect to potent ia l vo lume change due to b o t h swelling and shrinkage. The m e t h o d has no t

received wide a t ten t ion .

Soil Suct ion. — In theoret ical analysis, the to ta l suct ion can be considered t o consist of the

osmotic (or solute) potent ia l , gravitational potent ia l , and matr ix or capillary potent ia l . In engi-

neering pract ice, however , it is considered satisfactory to conduc t labora tory analysis by simulat-

ing the actual capillary potent ia l in the soil. The capillary potent ia l can be considered as being

equivalent t o the negative pore pressure at low level of mat r ix suct ion. The capillary potent ia l of

an unsa tura ted soil is often identified in te rms of its soil suct ion.


2 3 4 5 6 7 8 9 ΙΟ II 12

Non Critical Marginal Cri t ical Very Cri t ical _

POTENTIAL VOLUME CHANGE IPVC)

Figure 12. Swell index versus potential volume change. (From "FHA Soil PVC Meter Publication," Federal Housing Administration Publication No. 701)


Soil suct ion is expressed in a term designated as p F which is the log of the equivalent

capillary rise in cent imeters of water . Thus , a p F of 2 represents 100 cent imeters of hydros ta t ic

heads (205 psf), p F of 4 represents 10,000 cent imeters (20 ,500 psf), and so forth [ 2 2 ] .

The a m o u n t of soil suct ion of a sample at equil ibrium wi th free water is zero. U p o n drying,

the a m o u n t of soil suct ion rises rapidly. At oven dry condi t ion , the value m a y be several

thousand a tmospheres .

Obermeier [23] claimed tha t for a saturated clay mass, the stress release during excavation

can result in significantly m o r e negative pore pressure in the under ly ing soils. Thus , water can

flow in to the soil benea th the excavated area and cause swelling. Obermeier further believed that

b o t h shear and tensile stresses m a y have been an impor t an t con t r ibu t ion t o the heaving of clay

shales. The long-term heave potent ia l tha t results from stress release during the excavat ion of

clay-shales m a y someday be predicted by the use of a suct ion test.

The u l t imate goal of the measurement of soil suct ion is the predic t ion of mois ture

movemen t and mois ture equilibria ra ther than the direct measurement of t he swell potent ia l .

Osmot ic cell-consolidometer apparatus has recently been developed to measure the swelling

propert ies of soils unde r variable suction condi t ions . Obermeier developed an osmot ic

consol idometer which is similar to that used by Kassiff and Ben-Shalom [24] bu t is inexpensive,

simple in cons t ruc t ion , and easily adapted to existing labora tory equ ipment . It consists of t w o

units separated by a semi-permeable membrane . A solut ion of polye thylene glycol is placed in

unit I and the soil sample in uni t II. Since the m e m b r a n e is pervious to ions of dissolved salts in

the soil-water, the system controls matr ix suct ion. The disadvantage of such tests is tha t there is a

long t ime required t o reach equi l ibr ium. Bet ter labora tory techniques for measuring heaving

potent ial of swelling soils subjected to stress release are needed.

Direct Measurem en t

The mos t satisfactory and convenient m e t h o d of de termining the swelling potent ia l and

swelling pressure of an expansive clay is by direct measurement . Direct measurement of expansive

soils can be achieved by the use of the convent ional one-dimensional consol idometer . The

consol idometer can b e platform type , scale t ype , or o the r arrangement . The load can be applied

with air as in the case of Conbel consol idometer or by direct weight as in the case of cantilever

consol idometer . The soil sample is enclosed be tween two porous plates and confined in a meta l

ring. The d iameter of the ring ranges from 2 to 4 inches depending u p o n the type of sampling

device. T h e thickness of the sample ranges from one-half t o 1 inch. Tne soil sample can be

flooded b o t h from the b o t t o m and from the t op . Vertical expansion measurement is repor ted as

percentage of the initial height of the sample and is frequently referred to as the percent of swell.

Such a device enables an easy and accurate measurement of the swelling potent ia l of a clay

unde r various condi t ions . After the soil has reached its m a x i m u m volume increase, the sample can

be reloaded and swelling pressure de termined (chapter 2) . Thus , swelling pressure can be

evaluated easily w i thou t resorting to devices to hold the soil volume constant .

A great deal of da ta has been accumulated in files of soil engineers, academic or

governmental organizat ions on expansive tests using a consol idometer . Unfor tuna te ly , dissimilar

test procedures have been used. Thus , it is difficult to evaluate and compare the test da ta [ 2 5 ] . A


s tandardizat ion of test p rocedure of a one-dimensional swell test does no t appear difficult and

will salvage m u c h of the valuable da ta accumulated in the hands of the private consul tants . In the

performance of a typical swell test , the more impor t an t variables involved are as follows:

1. State of sample. For an undis turbed sample, this would include the condi t ion of the

sample, sampling m e t h o d , and stress h is tory of the sample . F o r remolded samples, this

would include the m e t h o d of compac t ion , curing t ime before and after compac t ion , and

compac t ion densi ty .

2. Moisture con ten t . The lower the initial mois tu re con ten t the higher the swell. The initial

mois ture con ten t is affected b y :

(a) The t ime allowed for the sample to remain in the ring before wet t ing,

(a) The ex ten t of evaporat ion allowed while the sample is in the ring, and

(c) The t empera tu re and humid i ty of the labora tory .

3 . Surcharge load. Increasing the applied load will reduce the magni tude of swell. Surcharge

load for mos t l abora tory practice ranges from 1 to 10 psi. Somet imes , a t t emp t s were

m a d e to dupl icate the surcharge load with the actual footing dead load.

4. Time allowed. The t ime required to fully comple te the swell process m a y vary

considerably and depends on the permeabi l i ty of the clay, the molding water con ten t , the

dry densi ty , and thickness of the sample. Fo r an undis turbed sample having a thickness of

1 inch, it m a y require as m u c h as several days to comple te the total available swell.

Undoub ted ly , the direct measurement m e t h o d is the mos t impor t an t and reliable test on

expansive soils. By standardizing the above variables, a reliable and reproducible test can be

obta ined. Also, if the concept of swelling pressure as discussed in chapter 2 is fully unders tood ,

m a n y of the variables ment ioned above can be simplified.

PHYSICAL P R O P E R T I E S O F EXPANSIVE SOILS

It is well k n o w n to soil engineers tha t montmor i l lon i t e clays swell when the mois ture

con ten t is increased, while swelling is absent or l imited in illite and kaol ini te . The types of soils,

and the condi t ions under which the mos t critical s i tuat ion exists, can be outl ined as follows:

Moisture content

Irrespective of high swelling potent ia l , if the mois ture con ten t of the clay remains

unchanged, there will be n o volume change; and s t ructures founded on clays wi th constant

mois ture con ten t will n o t be subject to m o v e m e n t caused by heaving. When the mois ture con ten t

of the clay is changed, volume expansion, b o t h in the vertical and hor izonta l d i rect ion, will take

place. Comple te sa turat ion is n o t necessary to accomplish swelling. Slight changes of mois ture

conten t , in the magni tude of only 1 to 2 percent , are sufficient to cause de t r imenta l swelling. In

the labora tory , clay samples swell in the consol idometer wi th slight increase of humid i ty . It is

k n o w n tha t floor slabs founded on expansive soils cracked mos t severely when the mois ture

con ten t increased slightly due to local wet t ing. If the floor slab is f looded, as in the case of a

rising water table , the floor will heave bu t the ex ten t of cracking will no t be severe.


The initial mois ture con ten t of the expansive soils controls the a m o u n t of swelling. This is

t rue b o t h for soils in undis turbed and in remolded states. As previously discussed, the

relat ionship be tween the initial mois ture con ten t and the capabil i ty of swelling has been s tudied

by Holtz [ 1 6 ] , Seed [ 171 , and m a n y others .

Very dry clays wi th natural mois ture con ten t below 15 percent usually indicate danger.

Such clays will easily absorb mois ture to as high as 35 percent wi th resul tant damaging expansion

t o s t ructures . Conversely, clays wi th mois tu re con ten t s above 3 0 percent indicate t h a t mos t of

the expansion has already taken place and further expansion will be small. However , mois t clays

may desiccate due to lowering of wate r table or o ther changes in physical condi t ions and u p o n

subsequent wet t ing will again exhibi t swelling potent ia l .

Dry density '

Directly related to initial mois ture con ten t , the dry densi ty of the clay is ano the r index of

expansion. Soils wi th dry densities in excess of 110 pcf generally exhibi t high swelling potent ia l .

Remarks m a d e by excavators complaining that the soils are as hard as a rock is an indicat ion that

soils inevitably will present expansion problems.

The dry densi ty of the clays is also reflected by the s tandard pene t ra t ion resistance test

results. Clays wi th pene t ra t ion resistance in excess of 15 usually possess some swelling potent ia l .

In the highly expansive clay areas of Denver, pene t ra t ion resistances as high as 30 are no t

u n c o m m o n .

Index properties

The au tho r has accumulated years of test da ta on expansive soils in the Rocky Mounta in

area and found tha t it is more convenient to correlate the expansive propert ies wi th the

percentage of silt and clay ( - 2 0 0 ) , liquid l imit, and field pene t ra t ion resistance. Since mos t

lightly loaded s t ructures will exer t a m a x i m u m dead load pressure of abou t 1,000 psf on the

footings, it is realistic to use a vertical load of 1,000 psf to gauge the swelling potent ia l . Table 4 is

a guide for est imating the probable volume changes of expansive soils.

The simplified classification of the expansive proper t ies can be convenient ly used by

engineers as a guide for the choice of t ype of foundat ion on expansive soils. Fo r example , for

Table 4 - . Data for making estimates of probable volume changes for expansive soils

Laboratory and field data

Standard Probable Percentage Liquid penetration expansion, Swelling Degree passing No. limit, resistance, percent total pressure, of

200 sieve percent blows/ft volume change ksf expansion

>35 > 6 0 > 3 0 > 1 0 > 2 0 Very high 60-95 40-60 20-30 3-10 5-20 High 30-60 3040 10-20 1-5 3-5 Medium

<3 0 <30 <30 < 1 1 low


soils wi th a low degree of expansion, spread footing type foundat ions can usually be used, if

sufficient re inforcement is provided in the founda t ion walls to compensa te for slight movements .

Fo r soils of m e d i u m degree of expansion, individual footings or pads can be used where the dead

load of the s t ruc ture can be concent ra ted to an in tensi ty of 3 ,000 to 5,000 psf. F o r soils of

high-to-very-high degree of expansion, special considerat ion should be given as to the foundat ion

type . Piers wi th sufficient dead load pressure and enough anchorage as described in chapter 4

should be used.

Fatigue of swelling

A clay sample is subjected to full swelling in the consol idometer , al lowed t o desiccate to its

initial mois ture con ten t , then is saturated again. This is repeated for a n u m b e r of cycles. It was

observed that the soil showed signs of fatigue after each cycle of drying and wet t ing [ 2 6 ] . This

p h e n o m e n o n has no t been unde r full investigation. It has been no ted tha t pavements founded on

expansive clays which have undergone seasonal m o v e m e n t due to wet t ing and drying have a

tendency to reach a po in t of stabil ization after a n u m b e r of years. The fatigue of swelling

probably can furnish the answer. Figure 13 shows a typical l abora to ry fatigue curve of swelling.

Fat igue of swelling was also observed by Chu [27] in his research on control led suct ion test.

Chu believed tha t if drying and wet t ing cycles are repeated, the swelling dur ing the first cycle

would be appreciably higher than that in subsequent cycles.

5 ι 1 1 1 1 1 1 1

ζ ο </) ρ ζ I < £ Claystone Soil Sample. UJ Sample saturated to allow full expansion, then

dessicated to initial moisture content ( 1 1 . 5 % ) , then allow full expansion again.

Ο 1 2 3 4 5 6 7

NUMBER OF CYCLES OF WETTING 8 DRYING

Figure 13. Fatigue of swelling (After Chen, 1965)


R E F E R E N C E S

[ I ] Holtz, W. G. and Gibbs, H. J., "Engineering Properties of Expansive Clays," Proceedings, ASCE, Vol. 80,

1954.

[2] Donaldson, G. W., "The Occurrence of Problems of Heave and the Factors Affecting its Nature." Second

International Research and Engineering Conference on Expansive Clay Soils, Texas A & M Press, 1969.

[3] Tourtelot, Η. Α., "Geologic Origin and Distribution of Swelling Clays," Proceedings of Workshop on

Expansive Clay and Shale in Highway Design and Construction, Vol. I, 1973.

[4] Salbert, J. R. and Smith, P.C., "Pavement Design over Expansive Clay: Current Practices and Research in

the United States," Engineering Effects of Moisture Change in Soils, International Research and

Engineering Conference on Expansive Clay Soils, Texas A & M Press, 1965.

[5] Jones, D. E. Jr., and Holtz, W.G., "Expansive Soils - The Hidden Disaster," Civil Engineering, Aug. 1973,

Vol. 43, Nov. 8.

[6] Peck, R., Hanson, W. and Thornburn, T., "Foundation Engineering," John Wiley & Sons, 1974.

[71 Mielenz, R. C. and King, M. E., "Physical-Chemical Properties and Engineering Performance of Clays," Clay

and Clay Technology, Bulletin 169, 1955, State of California, Dept. of Natural Resources.

[81 Grim, R. E., "Clay Mineralogy," McGraw-Hill Book Co., 1968.

[9] Low, P.F., "Fundamental Mechanisms Involved in Expansion of Clays as Particularly Related to Clay

Mineralogy," Proceedings of Workshop on Expansive Clays and Shales in Highway Design and

Construction, Vol. I, 1973.

[101 Warkentine, B.P., Bolt, G. H. and Miller, R. D., "Swelling Pressure of Montmorillonite," Soil Science

Society of America, Proceeding 21, 1957.

[ I I ] Bolt, G. H., "Physical-Chemical Analysis of the Compressibility of Pure Clays," Geotechnique, Vol. 6, No.

2, 1956.

[12] Brindley, G. W., "Identification of Clay Minerals by X-Ray Diffraction Analysis," Clays and Clay

Technology, Div. of Mines Bulletin 169, 1955.

[13] Dodd, C. G., "Dye Adsorption as Method of Identification of Clays," Clays and Clay Technology, Div. of

Mines, Bulletin 169, 1955.

[14] Kelley, W. P., "Interpretation of Chemical Analysis of Clays," Clays and Clay Technology, Div. of Mines,

Bulletin 169, 1955.

[15] Ravina, I., "Swelling of Clays, Mineralogical Composition and Microstructure," Proceedings of the Third

International Conference on Expansive Soils, Haifa, Israel, 1973.

[16] Holtz, W. G. and Gibbs, J. J., "Engineering Properties of Expansive Clays," ASCE Transactions Paper No.

2814, Vol. 121, 1956.

[17] Seed, H. B., Woodward, R. J. and Lundgren, R., "Prediction of Swelling Potential for Compacted Clays,"

Journal ASCE, Soil Mechanics and Foundations Div., Vol. 88, 1962.

[18] Altmeyer, W. T., "Discussion of Engineering Properties of Expansive Clays," Proceedings ASCE, Vol. 81,

Separate No. 658, March, 1955.

[19] Holtz, W. G., "Expansive Clay - Properties and Problems," Colorado School of Mines Quarterly, Vol. 54,

No. 4, 1959.

[20] "The Character and Identification of Expansive Soils." A Report Completed for the Technical Studies

Program of the Federal Housing Administration, May, 1960.

[21] Ladd, C. C. and Lambe, T. W., "The Identification and Behavior of Expansive Clays," Proceedings, 5th

International Conference on Soil Mechanics and Foundation Engineering, Paris, Vol. I, 1961.

[22] "A Review Paper on Expansive Clay Soils," by Woodward-Clyde & Assoc., Vol. I, 1967.

[23] Obermeier, S. I., "Evaluation of Laboratory Techniques for Measurement of Swell Potential of Clays,"

Proceedings of Workshop on Expansive Clay and Shale in Highway Design and Construction, Vol. I, 1973.

[24] Kassiff, G. and Ben-Shalom, Α., "Apparatus for Measuring Swell Potential Under Controlled Moisture

Intake," ASTM Journal of Materials, March, 1971.


[25] Kraynski, L. M., "The Need for Uniformity in Testing of Expansive Soils," Proceedings of Workshop on

Expansive Clays and Shales in Highway Design and Construction, Vol. I, 1973.

[26] Chen, F. H., "The Use of Piers to Prevent the Uplifting of Lightly Loaded Structures Founded on

Expansive Soils," Engineering Effects of Moisture Changes in Soils, Concluding Proceedings International

Research and Engineering Conference on Expansive Clay Soils, Texas A & M Press, 1965.

[27] Chu, T. and Mou, C. H., "Volume Change Characteristics of Expansive Soils Determined by Controlled

Suction Tests," Proceedings of the Third International Conference on Expansive Soils, Haifa, Israel, 1973.

Chapter 2

MECHANICS OF SWELLING

INTRODUCTION

In chapte r 1, the origin, mineralogical composi t ion , and the basic s t ruc ture of expansive soil

were out l ined. Obviously, if the envi ronment of the expansive soil has n o t been changed, swelling

will no t take place. Envi ronmenta l change can consist of pressure release due t o excavat ion,

desiccation caused by t empera tu re increase, and volume increase because of the in t roduc t ion of

mois ture . By far the mos t impor t an t e lement and of mos t concern to the practicing engineer is

the effect of water on expansive soils.

Kraynski [22 ] s ta ted in his review paper on expansive soils, "There mus t be a potent ia l

gradient which can cause wate r migrat ion and a con t inuous passage th rough which wate r transfer

can take p lace . " With the in t roduc t ion of water , volumetr ic expansion takes place. If pressure is

applied to prevent expansion, the pressure required to main ta in the initial volume is the swelling

pressure.

This chapte r presents a discussion on the migrat ion of water , swelling potent ia l , and swelling

pressure.

M O I S T U R E M I G R A T I O N

The pa t te rn of mois ture migrat ion depends on the geological format ions , climatic

condi t ions , topographic features, soil types , and ground-water level. I m p o r t a n t differences in the

mois ture migrat ion pa t t e rn be tween covered and natura l areas have been studied extensively by

the C o m m o n w e a l t h Scientific and Industrial Research Organizat ion in Australia. Much research

has been conduc ted in recent years by highway organizat ions in Australia, Sou th Africa, and the

United States in an a t t e m p t to stabilize pavements cons t ruc ted in expansive soil areas.

Moisture transfer

The mos t c o m m o n m e t h o d of mois ture transfer is by gravity. The seepage of surface water ,

precipi ta t ion, and snow mel t ing in to the soil are c o m m o n examples . T h e mois ture migrat ion can

occur in all di rect ions. Under artesian condi t ions , the flow can be upward . In stiff clays and in

shale bedrock , the flow generally occurs in the bedding planes or follows con t inuous fractures

and fissures. Shrinkage cracks which develop due t o surface desiccation provide easy access of

water in to the deep soils.

In fine grained soils, capillary force is a significant means of water transfer. The height of

water rise i n t o the capillary fringe varies inversely wi th the radius of the capillary tube . In clean,

coarse gravel, the capillary rise is insignificant. In clean sands, the rise is a few inches; in fine


sands, the rise is 1 or 2 feet; in silt, u p to 10 to 12 feet; and in clay, a rise of m o r e than 1000 feet

is theoret ical ly possible.

It is well recognized by observant soil engineers tha t the heaving of expansive soils m a y take

place w i thou t the presence of free water . Vapor transfer plays an impor t an t role in providing the

means for the vo lumne increase of expansive soils. Water vapor at a t empera tu re higher than its

surroundings will migrate toward the cooler area to equalize the thermal engery of the two areas.

When wate r reaches the cooler area, generally the covered area benea th a s t ruc ture , condensa t ion

can take place and provide sufficient mois ture to init iate swelling.

Thermal gradients can also cause mois ture migrat ion th rough the liquid phase of the soils.

Exper iments conducted at Pr inceton University show tha t a t empera tu re differential of Γ C was

at least equivalent to a hydros ta t i c head of 3 feet in its ability to cause mois ture migrat ion.

Thermal gradient reaches m a x i m u m efficiency when the mois ture con ten t in the soil is near

plastic limit. Covering the ground surface around the building wi th plastic membranes creates a

thermal gradient which m a y encourage mois ture from lawn water ing t o transfer t o the

foundat ion soils.

Vapor and liquid mois ture transfer unde r thermal gradient can be an impor t an t cause of the

swelling of moisture-deficient soils.

Moisture equilibria

In natural ground, the mois ture con ten t of the partially saturated soil is in general

equil ibrium wi th the applied stress, the forces due to evaporat ion and t ranspirat ion at ground

surface, and the capillary forces. When the area is covered by a building or pavement , the

evaporat ion and t ranspira t ion forces are el iminated and a new set of equil ibr ium mus t be

established. The new equil ibrium requires the flow of mois ture compat ib le with the new

condi t ion . The force causing the mois ture change or flow is te rmed soil suct ion.

The review panel of engineering concepts of Moisture Equilibria and Moisture Changes in

Soil Beneath Covered Areas [28] states, " T h e major change induced in soil by a surface cover is

caused by an al terat ion in the rates and quant i t ies of water able to enter and leave the soil at the

surface." Soil suct ion as briefly described in chapter 1 is considered a basic parameter to

soil-water equilibria and movemen t of wa te r in soil.

The pore pressure of an undis turbed sample obta ined at the ground surface (above the water

table wi th n o external ly applied stress) is less than a tmospher ic . The difference be tween the pore

pressure and a tmospher ic pressure is the value of soil suct ion [ 2 9 ] .

Below the ground surface or unde r s t ructural load, the value of soil suct ion is reduced by the

overburden pressure and /o r the external stress. The magni tude of pore pressure in the soil can be

obtained from the soil suct ion as follows:

u = a P - s

where : a = fraction of the normal pressure effective in changing the

value of soil suct ion,

Ρ = the to ta l normal stress on the soil e lement ,

s = the measured soil suct ion, and

u = the pore wate r pressure.

MECHANICS OF SWELLING 35

In sa turated clay a is u n i t y - i n nonconsol ida table soil a is zero. In unsa tura ted clay, the

value of a varies be tween zero and un i ty .

The soil suct ion concep t provides a means of de termining , w i t h o u t direct measurement , the

soil mois ture con ten t . The calculated mois ture will correspond to measured field mois ture

con ten t if field pore pressure can be accurately de te rmined [ 3 0 ] .

Aichison and Richards [31] claim tha t in unsa tura ted soils, the pore pressures are negative

and tha t the permeabi l i ty Κ is no longer assumed t o be a cons tan t . Consequen t ly , the classic

Darcy 's Law should be modified to conform with flow of wa te r in unsa tura ted soils.

Aichison presented empirical m e t h o d s predict ing equil ibr ium soil suct ion unde r s t ruc tures

b y utilizing the results of a broad-scale field investigation benea th sealed pavement t h roughou t

Australia. S tudy showed tha t the soils under a covered area are isolated from rapid changes and

tend toward a stable equi l ibr ium mois ture d is t r ibut ion. The to ta l suct ion benea th the covered

area tends t o change wi th cl imatic condi t ions .

Depth of moisture fluctuation

Kraynski [22] explained the mois ture con ten t variat ion wi th d e p t h in a homogeneous soil

b y figure 14. In a covered area, the mois ture profile is shown by curve 1. There is no gain or loss

of mois ture t o the a tmosphere . The mois ture con ten t of the soil decreases wi th dep th . Curve 2

indicates the mois ture con ten t variation wi th d e p t h in the same area in uncovered natura l

condi t ions . Evapora t ion causes loss of mois ture in the soil near the ground surface. However , the

influence of evaporat ion decreases wi th d e p t h and at some dep th , H^ , the mois ture con ten t

equil ibrium remains the same as the covered condi t ion . Kraynski referred to this d e p t h as D e p t h

of Desiccation. The value of H^ depends c n the cl imate condi t ion , the type of soil, and the

locat ion of the wate r table. This d e p t h represents the to ta l thickness of mater ia l which has a

potent ia l t o expand because of wate r deficiency. It is impossible t o de te rmine the value of H(j.

The ho t t e r and drier the cl imate, the greater the d e p t h of desiccat ion. The m a x i m u m d e p t h of

H(j is equal t o the d e p t h t o the water table , and the m i n i m u m d e p t h is equal to the d e p t h of the

seasonal mois ture con ten t f luctuat ion described below.

During wet m o n t h s wi th heavier precipi ta t ion and higher humid i ty , t he mois ture c o n t e n t of

near-surface soil increases and the mois ture profile represented by curve 2 alters its shape to curve

3 . The upper por t ion of curve 3 can extend beyond curve 1 in very wet seasons and behind curve

1 in dry seasons. The* d e p t h of seasonal mois ture con ten t f luctuat ion, H s, indicated in figure 14

depends on the variat ion of surface mois tu re , permeabi l i ty of the soils, and climatic condi t ions .

In areas where prec ip i ta t ion and evaporat ion are fairly cons tan t , the H s d e p t h m a y be only a few

feet. When a long drought is followed by an intense rainfall, the H s d e p t h can reach 10 feet or

more .

It should be not iced tha t in the above evaluation of the d e p t h H s, n o considerat ion has been

given to the man-made envi ronment . The water ing of lawns, plant ing of trees and shrubs,

discharge of roof drains, format ion of drainage channels and swales, and the possibility of ut i l i ty

line leakage will all increase the value of H s. It is n o t u n c o m m o n tha t H s d e p t h can reach as m u c h

as 25 feet (see chapte r 7).


INCREASING MOISTURE CONTENT OF SOIL

2 - Dessicated Moisture Content

3 - Wet Season Moisture Content

H$- Depth of Seasonal Moisture Content Fluctuation

Hd-Depth of Dessication

Figure 14. Moisture content variation with depth below ground surface. (After Kraynski, 1967)

2 - Dessicated Moisture Content


When areas are covered by s t ructures such as buildings, pavements , sidewalks or aprons ,

evaporat ion is blocked or partially re tarded. The mois ture con ten t benea th the covered area

increases due t o gravitational migrat ion, capillary act ion, vapor and liquid thermal transfer and, in

the course of several years, the d e p t h of seasonal mois ture con ten t f luctuat ion H s can approach

t o d e p t h of desiccat ion H^ .

The shifting of the mois ture profile of a swelling soil from the natural condi t ions

represented by curves 1 and 2 t o curve 3 in figure 14 for covered condi t ions is the cause of

significant damage. Since mois ture transfer is a slow process, it is no t surprising tha t distress of a

building often takes place several years after occupancy . In the course of investigating a cracked

building, it is n o t unusual to find that the mois ture c o n t e n t of the soils benea th the covered area

or in the vicinity of the covered area had substantial ly increased.

Shrinkage

In reviewing l i terature on expansive soil, m u c h a t t en t ion has been focused by various

investigators on the mechanics of shrinkage. It has been claimed tha t shrinkage is the mi r ror

reflection of expansion. It is expected by measuring soil suct ion which evaluates the to ta l

negative pressures in t he soil-water system, the p h e n o m e n o n of swelling and shrinkage can be

explained at the same t ime.

It is believed tha t seasonal mois ture con ten t f luctuat ion can result in heaving and sett l ing of

the s t ruc ture . The cyclic u p and d o w n m o v e m e n t is believed to occur in phases, b u t lags behind

rainfall [ 3 2 ] . Theoret ical ly , shrinkage can result in se t t lement , bu t there is very lit t le evidence

that there is appreciable downward movemen t unde r covered areas in a building. Seasonal

f luctuat ion of mois ture c o n t e n t along the edges of highway pavement or parking areas can be

expec ted , b u t at the central por t ion of a covered area, shrinkage seldom takes place even under a

prolonged arid cl imate.

Movement measurements of buildings in good condi t ion have seldom been per formed.

Monitor ing cracked building movemen t canno t reflect the effect of seasonal mois ture variat ion.

An unusual o p p o r t u n i t y afforded the mak ing of a precise measuremen t of a school building

founded on piers. The movemen t of the piers was measured each m o n t h for a period of 11

mon ths . The movemen t of b o t h the inter ior and exter ior piers was recorded and p lo t t ed along

wi th the average m o n t h l y precipi ta t ion, as shown in figure 15.

The dry and wet per iods coincide fairly well wi th the upward and downward m o v e m e n t of

the exter ior piers. However , for the inter ior piers, the m o v e m e n t graph lags behind the

precipi ta t ion graph. This is expected since it takes addi t ional t ime for mois ture or vapor t o

migrate from the exter ior of the building to the inter ior piers.

In the school building previously discussed, it should be noted tha t over a period of 11

m o n t h s , the difference be tween the m a x i m u m and m i n i m u m pier m o v e m e n t seldom exceeded

one-half inch. Such m o v e m e n t was insufficient to manifest not iceable damage to the building. In

fact, such movemen t would n o t have been no ted had no t a precise leveling been conduc ted .

The end result of shrinkage around or benea th a covered area seldom causes any s t ructural

damage, hence, is n o t an impor t an t i tem to be considered by soil engineers. It is the con t inuous

increase in mois ture con ten t benea th the covered area tha t in t roduces the damage in an expansive

soil area.


„ MOVEMENT OF PIER

MONTHS (SHIFTED)

ι 1 1 1 1 1 1 1 1 1 1 1 1 1 1

N O J F M A M J J A S O N DJ MONTHS

Figure 15. Movement of exterior and interior piers in a school building with respect to precipitation.

SWELLING P O T E N T I A L

Al though the swelling p h e n o m e n o n has been fully recognized for m a n y years, a definite

m e t h o d of measuring the swelling potent ia l of clay has no t been established.

In 1962, Seed [17 ] defined swelling potent ia l as the percentage of swell of a laterally

confined sample on soaking unde r a 1-psi surcharge, after being compac ted to m a x i m u m densi ty

at o p t i m u m water con ten t in the s tandard AASHO compac t ion test .

Lambe [33] used the swell index to measure the expansion characterist ics of clay. The swell

index is defined as the slope of the e-log ρ curve. The pressure increment from 1.0 to 0.1 kg per

c m 2 was used.

It is clear tha t b o t h defini t ions have their l imitat ions. Seed's defini t ion is confined to

remolded soil while the slope of the e-log ρ curve depends on init ial-moisture con ten t as well as

surcharge pressure. T h e difficulty in providing a suitable yardst ick for measuring swelling

characteristics is the presence of the numerous variables involved. Consequent ly , to this da te ,

when high swelling soil in San An ton io , Texas is discussed, it is no t the same standard as the

swelling of the clay shale in the Rocky Mounta in area.

There is an urgent need for uni formi ty in testing expansive soils. A s tandard m e t h o d for

general use was proposed by W. G. Hol tz [ 3 4 ] . The proposed m e t h o d utilizes the convent ional

fixed-ring consol idometer for conduct ing the tests. Procedures are given for testing b o t h

undis turbed and remolded specimens. T h e s tandard procedure involves the testing of two similar

samples, one for loaded-and-expanded test and the o the r for expanded-and-loaded test . The

m e t h o d also involves shrinkage tests and permeabi l i ty tests . The m e t h o d has been submit ted for

considerat ion as a s tandard to ASTM Commi t t ee D-18 and is given in Append ix A.


In the preceding chap te r u n d e r the "Physical Proper t ies of Expansive Soi l , " the impor tance

of initial mois ture con ten t and initial dry dens i ty has been discussed. These and o the r

environmental condi t ions are ex t remely i m p o r t a n t in de termining the a m o u n t of swell. The

following factors influence the results obta ined in loaded swell tests on soils of any mineralogical

compos i t ion :

1. Initial mois ture con ten t . In test ing undis turbed samples, care should be taken in selecting

the sample wi th the m o s t critical mois tu re con ten t . Usually, tests should be performed on

the driest sample. F requency of testing is impor t an t so as to cover all possible condi t ions .

F o r remolded samples, it is obvious tha t the initial mois tu re con ten t will con t ro l the

vo lume change. Field condi t ion and cons t ruc t ion specifications d ic ta te t he mois ture

requi rement . In addi t ion , a great deal of a t t en t ion should be directed to the t ime

e l e m e n t - t h e t ime elapsed be tween sampling and testing and the t ime elapsed when the

sample is placed in the consol idometer , and when the wet t ing takes place or when the

load is applied.

2 . Initial d ry densi ty . The single mos t i m p o r t a n t factor affecting swelling characterist ics of

swelling soils is densi ty . Detailed discussions are given later u n d e r "Swelling Pressure ." In

remolded tests, the initial compac t ion cond i t ion is critical. Swell tests m a y decide the

degree of compac t ion required in the p lacement of fill. Since mois ture c o n t e n t and d ry

densi ty are closely related, they should be examined concur ren t ly .

3 . Surcharge pressure. A small surcharge load in the range of 0.35 t o 1 psi has been

suggested for a seating load in the swell test . Since swell is very sensitive to changes in

pressure in the lower ranges of pressure (less than abou t 1 psi), the use of low surcharge

pressure may lead to errat ic and e r roneous test results. Since mos t footing foundat ions

can exer t a pressure of abou t 1,000 psf on the soil, it is r ecommended tha t this value be

used for a surcharge load.

4 . T ime allowed for swell. The t ime required for the soil to reach its m a x i m u m swell

po ten t ia l may vary considerably depending essentially on the initial densi ty , permeabi l i ty ,

and the thickness of the sample. F o r remolded samples, generally 24 hours is sufficient t o

obta in 95 percent of the tota l available swell. At the same t ime, for undis turbed high

densi ty clay shale, it m a y require several days or even a week before comple te sa tura t ion

can be achieved. F o r remolded samples, the initial added water mus t be evenly

d is t r ibuted . This requires a m i n i m u m curing t ime of 6 hours for reproducible results.

5 . Size and thickness. Sample thickness affects the t ime required for to ta l sa tura t ion . T o

expedi te testing t ime , sample thickness of less than 1 inch should be used. Greater

thickness m a y in t roduce excessive side friction. At the same t ime, in a cut t ing from an

undis turbed sample, small thickness may in t roduce surface d is turbance and exclude the

possible effect of granular part icles, fissures, and seams in the soil.

The d iameter of the sample is also significant, a l though only vertical rise is measured in the

consol idometer . The smaller the d iameter , the larger the effect of side friction. Sample d iamete r

is control led by a sampling device. In general , t he d iamete r should n o t be less than 2 inches.

S tandardiza t ion on b o t h size and thickness of sample is apparen t ly necessary for comparable

results.

Factors affecting volume change


F r o m the preceding analysis, it is obvious tha t for a certain clay wi th k n o w n proper t ies , the

swelling characteristics will vary greatly wi th the variation of one or m o r e of the above

environmenta l or p lacement condi t ions . It would be er roneous to compare the swelling

characteristics of different soils w i thou t first clearly defining the p lacement condi t ion .

A reliable and reproducible test which is to be considered as a basis for the classification of

potent ia l expansive soil mus t be s tandardized at least for the following envi ronmenta l condi t ions :

For undis turbed sample:

Surcharge pressure

Size and thickness

T ime required for the test

Initial mois ture con ten t and densi ty mus t be qualified

Fo r remolded sample:

Initial mois tu re con ten t

Initial d ry dens i ty

Method of compac t ion

Surcharge pressure

Size and thickness

Time required for the test

Curing t ime allowed

Clearly, one set of s tandards can hardly cover the very complicated variables involved. With

the in t roduc t ion of the concep t of swelling pressure, the above environmental condi t ion can be

simplified and test results can be compared as later explained under "Swelling Pressure ."

Total heave

The de te rmina t ion of swelling potent ia l in the labora tory , as out l ined above, leads to the

predict ion of to ta l heave or the m a x i m u m potent ia l magni tude of heaving of a s t ructure . The in

situ variables involved are always more complicated than those used in labora tory tests, especially

where only a single sample is used to represent an entire foundat ion . Some of the variables are

described as follows:

1. Climate. Climate condi t ions involving precipi ta t ion, evaporat ion and t ranspirat ion affect

the mois ture in the soil as previously discussed under "Mois ture Transfer ." The d e p t h and

degree of desiccation affects the a m o u n t of swell in a given soil hor izon. Climate

condi t ion partially affects the desiccation.

2. Soil profile. The thickness of expansive soil u n d o u b t e d l y affects the magni tude of to ta l

heave. If the thickness of high swell potent ia l soils is thin, the potent ia l to ta l heave will be

small. At the same t ime, for a thick s t ra tum of low swelling soil, the to ta l heave can be

considerable over a long period of t ime. As long as the thickness of the swelling soil is less

than 24 inches, l i t t le damage to the foundat ion can take place.

3 . Ground water . Evidently, swelling soil located below ground water will no t pose a

p rob lem to the s t ruc ture . The thickness of the swelling soil s t ra tum is, therefore , limited

by the d e p t h to ground water . However, ground wate r f luctuates, and receding ground

water somet imes cont r ibu tes to addi t ional swelling. Damaging swelling soil p roblems are

M E C H A N I C S O F S W E L L I N G 41

seldom encounte red for a soil profile wi th ground water located shor t distances benea th

the footings. Capillary m o v e m e n t and vapor transfer will be of such magni tude tha t it

takes only a shor t period of t ime before the thin layer of swelling soil below the s t ructure

becomes comple te ly sa tura ted.

4 . Drainage. As expec ted , good surface drainage a round the s t ruc ture reduces the swelling

p rob lem as discussed in detail in chapter 8.

Various m e t h o d s have been proposed to predict the a m o u n t of tota l heave u n d e r a given

s tructural load. These are the doub le oedome te r m e t h o d [ 3 5 1 , the McDowell m e t h o d [ 3 6 ] , the

Lambe and Whi tman m e t h o d [ 3 3 ] , and the F H A m e t h o d [ 3 7 ] . All suggested m e t h o d s have

l imitat ions. The double o e d o m e t e r m e t h o d developed by Jennings and Knight is based on the

concept of effective stress and has received wide a t t en t ion . The general test p rocedure is given as

follows:

T w o consol idometer rings are filled wi th undis turbed samples from adjacent locat ions . The

first sample is kep t at its na tura l mois ture con ten t and a confined compression test is per formed.

The second sample is f looded wi th wate r at low pressure of 20 psf. After the sample is fully

we t t ed , a consol idat ion test is perfor«med in the convent ional manner . The t w o compress ion

curves are p lo t t ed on the same diagram and one of the curves is selected for vertical adjus tment

so as to coincide wi th the virgin sections of the curves as shown on figure 16.

A soil sample taken at d e p t h ζ has an overburden pressure P 0 = Yz where Y is the densi ty of

the soil. The void-ratio at the overburden pressure is e G. Se t t l ement due to load increment Δ Ρ can

be calculated from the change of void-ratio e 0 - t l . If no load is applied, the soil unde r a covered

area will gain mois ture and swelling will t ake place. T h e condi t ion will alter P 0, resulting in e 0

having a new effective pressure P 0 + U L represented in the uppe r saturated curve by P 0 + U L and

e 2. If D is the d e p t h to the water table and Y w is the uni t weight of water , U L = Y w (D - z) . The

effect of the load increment is then again taken in to considerat ion and the final values are ( P 0 +

U L + Δ Ρ and e 3 ). The final condi t ions of m o v e m e n t may then be predicted by adding the void-

rat ion changes over the whole profile.

The doub le o e d o m e t e r m e t h o d is based on the assumpt ion tha t there is a po in t dur ing

compression at which the initially unsa tura ted soils pass from an applied pressure t o an effective

p h e n o m e n o n and the compression curve joins wi th the virgin consol idat ion curve. Typical heave

calculations are shown on table 5.

The search by various investigators for a reliable m e t h o d for predict ing the to ta l heave is

probably affected by the concep t of u l t imate se t t l ement in the theo ry of consol idat ion. F o r

m a n y years engineers have been familiar wi th the calculat ion of u l t imate se t t l ement and

differential se t t l ement of a s t ruc ture founded on clay, and it is assumed tha t the tota l heave can

also be predic ted . There are some fundamenta l differences be tween the behavior of settl ing and

heaving soil. Some of them are as follows:

1. Se t t lement of clay unde r load will take place w i t h o u t the aid of wet t ing, while expansion

of clay will n o t be realized w i thou t mois ture increase.

2. The to ta l a m o u n t of heave depends on the envi ronmenta l condi t ion , such as the ex ten t of

wet t ing , the dura t ion of wet t ing , and the pa t t e rn of mois ture migrat ion. Such variables

canno t be ascertained, and consequent ly , any to ta l heave predic t ion can be ent i rely

er roneous .


Figure 16. Log P curves showing adjustment to bring straight line portions coincident. (After Jennings & Knight, 1958)

3. Differential settlement is usually described as a percent of the ultimate settlement.

However, in the case of expansive soils, one corner of the building may be subjected to

maximum heave due to excessive wetting while another corner may have no movement.

Therefore, in the case of swelling soils, differential heaving can equal the total heave. No

correlation between differential and total heave can be established.

Effective stress

Terzaghi developed the principle of effective stress in the early 1920's. It is believed that the

principle can be applied to all soil behaviors. In gernal terms, the principle of the effective stress

states that the behavior of an element of soil mass depends not on the total stress applied to the

element but rather on the difference between this total stress and the stress present in the pore

fluid. For saturated soil, the effective stress is defined as:

Ö = G-yL

where: Ú - effective stress

î - total normal stress

M = stress in pore fluid or pore water pressure


Table 5-. Results of heave calculations

1 2 3 4 5 6 VOID RATIO

11 12 13

7 8 9 10 ο Β

Lay

er N

o.

Dep

ths

at t

op a

nd b

ott

om

of

laye

r, f

eet

Dep

th o

f dou

ble

Oed

om.

test

, fe

et

P 0 a

t m

ean

dep

th,

ton

/sq

. ft

.

ΔΡ

at m

ean

dep

th,

ton

/sq

. ft

.

UL

at m

ean

dep

th,

ton

/sq

. ft

. eo ei e 2 e 3

<u ο

<5 CO

•o .s ο ^

Ο Ο)

> ι ο

DU I - H

S) Β ra

Χ )

ο g C ^

c Ό Β &

ë ι ο

s κ * 00 I —π

Β Ό

=• r 1 »°

ζ 1 +

1 4.00- 5.25 0.30 0.10 0.97 0.419 0.418 0.492 0.481 1.585 0.021 1.342

1 6.56 6.56- 7.87 0.45 0.07 0.89 0.428 0.427 0.460 0.455 0.623 0.019 0.525

ζ 8.87

•j 8.87- 9.87 0.59 0.06 0.84 0.418 0.417 0.449 0.447 0.506 0.016 0.474 «5 10.80

A 10.80- 11.74 0.73 0.04 0.78 0.544 0.543 0.563 0.562 0.362 0.020 0.343 f 13.35

5 13.35-18.81 18.81-

14.85 0.97 0.04 0.70 0.626 0.625 0.640 0.639 0.565 0.040 0.524

6

13.35-18.81 18.81- 18.87 1.21 0.04 0.59 0.523 0.522 0.524 0.523 0.017 0.017 0

(After Jennings & Knight, 1958) TOTALS: 3.658 0.133 3.208

The preceding equat ion is based on saturated soils. For par t ly saturated soils, considerat ion

should be given t o t he fact tha t pore pressure acts only on par t of the area of any plane th rough

the soil. In addi t ion , an electro-chemical force of a t t rac t ion and repulsion force m a y act th rough

the spaces n o t filled wi th water . Consequent ly , the effective equa t ion should be modified by

adding the vector sum of at tract ive forces and subtract ing the vector sum of repulsive forces.

Aichison and Richards [31] s ta ted, " I t is n o t only possible b u t essential for the effective

stress principle to be used in the quant i f icat ion of expansive soil behavior . "

Lambe [33] s tated in 1959 after making in-depth research on the applicat ion of the concept

of effective stress to explain the swelling soil behavior , " O n e wonders whe ther , for example , we

are hindering our unders tanding of the na tu re of the ex t remely plastic swelling soils b y forcing

them to fit t he effective stress c o n c e p t . "

The au thor shares the th inking of Lambe and believes tha t the mechanics of expansive soil

are total ly different from the theory of consol idat ion and the shearing s t rength of soils. It should

be considered as a new phase of soil mechanics .

SWELLING P R E S S U R E

In the course of the last 15 years , thousands of swell tests were conduc ted by the au tho r on

various k inds of expansive soils found in the R o c k y Mounta in area. T h e s tandard p rocedure used

to conduc t these tests is to place the undis turbed sample in a conso l idomete r unde r a surcharge


load of 1,000 psf (about 7 psi) for 24 hours , sa tura te the sample, measure and record the a m o u n t

of vo lume change.

The a m o u n t of volume change exhibited by various soils unde r various p lacement condi t ions

varies greatly. Fu r the rmore , it was found tha t soil obta ined from benea th a s t ruc ture tha t has

undergone severe foundat ion m o v e m e n t may no t possess high swell potent ia l . It was suspected

tha t there mus t be a single soil p rope r ty tha t governs the swelling characterist ics. After the

sample had swelled to its m a x i m u m ex ten t , the specimen was loaded unti l it re turned to its initial

volume and the pressure required t o do this was designated as swelling pressure.

It is suspected tha t the swelling pressure is the built-in p rope r ty of expansive soil and will

no t be affected b y p lacement condi t ion or environmenta l condi t ion .

Test procedure

The clay selected for making this s tudy is a c laystone shale typical of those found in

southeast Denver. Such soil has caused a great deal of damage to lightly loaded s t ructures such as

residential houses. Most of the houses in this area are founded wi th piers b o t t o m e d in a zone

where a change of mois ture con ten t is unlikely t o take place; however , slabs placed on such soil

have experienced generally severe movemen t . Uplift m o v e m e n t in excess of 6 inches is no t

u n c o m m o n .

The physical proper t ies of such clay are as follows:

Liquid limit 44 .4%

Plasticity index 24 .4%

Shrinkage limit 14.5%

Sand 0%

Silt 63 .0%

Clay (percent smaller than 0 .005 m m ) 37 .0%

O p t i m u m mois ture con ten t 19 .5%

Maximum d r y dens i ty (s tandard

Proc to r tes t ) 108.4 pcf

Free swell (USBR m e t h o d ) 75 .0%

Specific gravity 2.67

In addi t ion to the above, the mineral con ten t , in percent , of such clay is as follows:

Montmor i l lon i te 25 .0

Calcite 5.0

Quar tz 25 .0

Feldspar 10 t o 25 .0

Kaolini te 5.0

Because of the errat ic format ion of natural soil s trata , for research purposes , it will be

necessary to use only remolded samples so tha t the p lacement condi t ion can be dupl icated.

The air dry sample is prepared by passing it th rough a No. 4 0 sieve. Moisture con ten t is

added to the air dry clay and then allowed to age in a sealed conta iner for a period of 48 hours .


The sample is compacted in a 2-inch-diameter, 1-inch-thick consolidometer ring in three layers with predetermined compaction effort. The required density is obtained as near as possible by an experienced technician. It should be noted that in the test results presented, some deviation of density has taken place which results in some erratic test results.

Surcharge pressure

It is a well recognized fact that if sufficient load is applied on an expansive clay, the detrimental volume increase can be controlled. The surcharge pressure applied to the soil sample in the consolidometer simulates the dead load pressure exerted on the footings or pier foundation. Figure 17 and table 6 indicate that with a surcharge pressure of 1,000 psf, upon wetting the clay swelled 5.9 percent with a swelling pressure of 12,000 psf. By increasing the surcharge pressure to 5,000 psf, the amount of volume increase was limited to 1.6 percent, but the swelling pressure remains unchanged.

Figure 18 indicates the relationship between volume change with surcharge pressure. These curves have a hyperbolic shape and the intersection of the curves with the abscissa indicates the pressure required for zero volume change. This pressure by definition is the swelling pressure.

Figure 17. Relationship between surcharge pressure and volume increase for constant density and moisture content samples.


Table 6—. Effect of varying pressure on volume change and swelling pressure for constant density and moisture content samples.

Applied pressure

(psf)

Moisture content, percent

Initial density,

pcf

Volume increase, percent

Swelling pressure,

psf

Applied pressure

(psf) Initial Final

Initial density,

pcf


Swelling pressure,

psf

1,000 11.90 24.58 105.58 5.90 12,000

2,000 11.90 25.08 106.08 3.90 13,000

3,000 11.90 24.94 105.96 2.80 12,000

5,000 11.90 25.02 105.90 1.60 12,500

7,000 11.90 25.49 105.96 1.00 12,500

Average 11.90 25.02 105.95 12,400

SURCHARGE PRESSURE ( p s f )

Figure 18. Effect of varying pressure on volume change for constant density and moisture content sample.

Surcharge load is essential t o control foundat ion movemen t . With swelling pressure

de te rmined , a reasonable foundat ion design can be approached . If the swelling pressure is n o t

excessive, say, on the order of 5 ,000 psf, a spread footing foundat ion can be used. The

requi rement will be to assign a min imum dead load pressure of 5,000 psf so tha t the volume

change of the soil will n o t be allowed even in excessive wet t ing condi t ion . In the design of the


footing founda t ion , it m a y be possible t o allow certain a m o u n t s of uplift movemen t so as t o

minimize the required dead load pressure. Uplift m o v e m e n t can be tolerated in certain s t ructures

in the same m a n n e r as some se t t l ement can be to lera ted in mos t s t ruc tures . A differential uplift

of three-fourths of an inch generally is considered to be tolerable . With 3/4-inch allowable

differential uplift , the required dead load pressure can be drastically reduced. F o r a rat ional

design, it is advisable t o actually de te rmine the swelling pressure. F r o m the pressure versus

volume change curve and the tolerable uplift , a working swelling pressure can be established.

Fo r highly swelling soil wi th swelling pressure in excess of 5,000 psf, a pier foundat ion will

be required to concen t ra te the dead load pressure on a small area. It is no t difficult to assign a

dead load pressure in excess of 20 ,000 psf in a small d iameter pier. However , a t t en t ion m u s t be

given to the addi t ional swelling effect on the shaft of the pier embedded in swelling soil. The

swelling pressure exer ted on the shaft of the pier can be m a n y t imes greater than the pressure

exer ted at the b o t t o m of the pier. Dead load pressure alone generally is no t sufficient to prevent

the uplifting of the pier. Anchorage of the pier in a zone no t affected by mois ture change should

be used t o assist the dead load pressure requi rement .

Since the volume change for surcharge pressure of 1 psi and the volume change under a

surcharge pressure of 10 psi will vary considerably, the mer i t of using swelling pressure as a direct

measurement of the swelling characterist ics can be seen at once .

Degree of saturation

This series of tests was performed t o s tudy the effect of dura t ion of wet t ing on swelling

characteristics. It is a well established p h e n o m e n o n tha t prolonged wet t ing will result in m o r e

damage t o a s t ructure than shor t dura t ion wet t ing.

Since it is difficult in a short sample height t o cont ro l the dura t ion of wet t ing, t o achieve

the same effect in the labora tory the degree of sa tura t ion on the sample was varied. The samples

were compac ted in the consol idometer wi th uniform densi ty and mois ture con ten t , and a

measured a m o u n t of wate r was then in t roduced in to the sample. Sufficient t ime was allowed for

all added water t o soak in to the sample. The a m o u n t of volume change and the swelling pressure

recorded are shown on table 7. F r o m figure 19 it is seen tha t the a m o u n t of vo lume change

Table 7 - . Effect of varying degree of saturation on volume change and swelling

pressure for constant density and moisture content samples.

Moisture content, percent Initial

density, pcf


Swelling pressure,

psf

Degree of

saturation, percent Initial Final

Initial density,

pcf


Swelling pressure,

psf

Degree of

saturation, percent

9.66 13.07 106.6 1.83 16,000 61.0

9.66 14.53 106.0 3.35 15,500 67.0

9.66 17.58 105.6 4.35 12,000 82.0

9.66 18.50 106.7 5.53 17,000 86.3

9.66 19.93 105.9 6.25 15,000 93.0

rage 9.66 106.2 15,100

Figure 19. Effect of varying degree of saturation on volume change for constant density and moisture content sample.



increases in direct p ropor t ion t o the degree of sa tura t ion at the end of test. Figure 20 indicates

tha t the swelling pressure is cons tan t , or the pressure required to mainta in cons tan t volume is

independent of the dura t ion of wet t ing or the degree of sa tura t ion .

Comple te sa tura t ion is n o t required to result in a large volume change. This reflects directly

in the misconcept ion tha t by removing free wate r the swell can be control led. It has been a

c o m m o n pract ice t o install drain tiles a round the building in an a t t e m p t to remove free water and

to s top foundat ion movement . A subdrain will no t arrest the migrat ion of mois ture .

Consequent ly , swelling can be substantial .

Wi thout super imposed load, the swelling of the soil canno t be control led even wi th a

min imum a m o u n t of mois ture change. Since the swelling pressure remains cons tant , a short

dura t ion wet t ing can cause equally heavy damage to lightly loaded s t ructures as long durat ion

wett ing. This is the reason why it is so difficult to control slab movement for slab-on-ground

const ruct ion .

Initial moisture content

Expansive soils will no t be subject to volume change unless there is an increase in mois ture

con ten t . A drier soil will swell more than a wet soil. In this series of tests, an a t t e m p t is made to

Figure 20. Relationship between degree of saturation and volume increase for constant density and moisture content samples.


de te rmine the effect of increasing the initial mois ture con ten t on the volume change as well as

swelling pressure. Figure 21 shows the results of a series of tests indicating the a m o u n t of volume

change of. soil samples compac ted at cons tan t densi ty b u t varying mois ture con ten t s . As

expec ted , the soils wi th low initial mois ture con ten t swell mos t . Thus , the slope of the e-log ρ

curve decreases as the initial mois ture con ten t increases. However, the swelling pressure required

for zero volume change remained practically cons tant .

Kassiff & Baker in 1971 [ 3 8 ] s tated tha t if clay is given enough t ime for aging, that for the

same dry densi ty , the swell pressure is no t affected by mois ture con ten t . The result shown in

table 8 confirms this s t a tement . Table 8 indicates tha t the swelling pressure for the various

mois ture con ten t s ranges from 7,000 to 12,500 psf. Due to variation in labora tory control led

condi t ions , the initial dens i ty is n o t entirely cons tant , and some variation in swelling pressure

occurs. F o r all practical purposes , however , the swelling pressure is a cons tan t value. Figure 22

indicates the variation of mois ture con ten t versus volume change. These tests indicate tha t wi th

mois ture con ten t slightly higher than o p t i m u m mois ture con ten t , the volume change should be

negligible.

The results of the above labora tory tests indicate that the increase of mois ture con ten t of an

expansive soil is no t a positive m e t h o d of control l ing the expansion of the soil. Even with high

mois ture con ten t , footings founded on swelling soil will experience the same swelling pressure

Figure 21. Relationship between initial moisture content and volume increase for constant density samples.

M E C H A N I C S O F S W E L L I N G

Ο » 1 1 I I \ 0 5 10 15 20 25

Moisture Content (%)

Figure 22. Effect of varying moisture content on volume changes from constant density samples.

51


Table 8 - . Effect of varying moisture content on volume change and swelling pressure for constant density samples.

Initial density,

pcf



Swelling pressure,

psf

Initial density,

pcf Initial Final


Swelling pressure,

psf

106.97 5.84 20.34 7.71 9,500

105.93 9.95 20.77 5.55 9,500

106.27 10.77 18.75 5.03 12,500

105.60 12.48 22.09 4.30 9,500

106.47 12.92 20.54 3.48 9,000

106.37 14.84 19.59 3.30 10,500

105.46 17.97 18.50 2.15 7,000

105.73 18.59 19.41 1.38 7,500

106.35 19.37 20.18 0.75 9,000

îrage 106.13 20.02 9,333

and the same a m o u n t of dead load pressure will be required to insure zero volume change. High

moisture con ten t soils will experience less uplift, b u t the pressure required to maintain constant

volume will no t be al tered. This also indicates tha t the commonly accepted procedure of

prewet t ing the foundat ion excavat ion to el iminate the swelling characteristics is no t a reliable

procedure . Wetting of the foundat ion soil, if it can be accomplished, can only serve to decrease

the a m o u n t of swelling. A foundat ion placed on such soil will still require the same a m o u n t of

dead load pressure.

Soil engineers are often deceived by the low volume change of a high mois ture con ten t soil.

If such soils possess a high swelling pressure, they will cause severe damage to s t ructures if

allowed to dry and are subsequent ly wet ted .

Stratum thickness

Labora tory research has been further ex tended to explore the effect of s t ra tum thickness on

the a m o u n t of volume change and swelling pressure. In this series of tests, the sample thickness

ranged from 1/2 to 1-1/2 inches. Again, the samples were compac ted to uniform moisture

conten t and densi ty and sufficient t ime was allowed for comple te saturat ion of the thickest

sample.

As could be predic ted , from the results shown in figure 24 , the magni tude of the volume

change is p ropor t iona l t o the sample thickness and the percentage of volume increase remains

constant . The shape of the e-log ρ curve remains almost identical for various sample thicknesses

(fig. 23) and the swelling pressure is cons tan t ( table 9) .

This series of tests indicates that if the weight of a s t ructure is capable of exert ing pressure

to various dep ths benea th the footing with equal in tens i ty , then the volume increase can be

arrested. Unfor tuna te ly , dead load pressure exer ted on the footing can only control volume

change of the near surface soils. At lower dep ths , pressure exerted on the footing is dis t r ibuted


over a larger area and is no t effective in prevent ing volume change. Deep seated swelling is

control led only by the weight of the overburden soil and no t by dead load pressure exer ted on

the foundat ion sys tem.

Table 9—. Effect of varying sample thickness on volume change and swelling pressure for constant density and moisture content sample.

Initial density,

pcf


Sample thickness,

in.


Volume increase,

in.

Swelling pressure,

psf

Initial density,

pcf Initial Final

Sample thickness,

in.


Volume increase,

in.

Swelling pressure,

psf

105.20 10.10 22.30 0.504 5.66 0.0285 11,000

106.33 10.10 20.92 0.748 5.75 0.0430 11,500

105.31 10.10 21.14 1.007 5.15 0.0520 11,000

106.05 10.10 20.49 1.250 5.60 0.0700 15,000

106.05 10.10 20.58 1.500 5.60 0.0840 12,500

Avg. 105.78 10.10 21.08 5.54 12,200

Figure 23. Relationship between sample thickness and volume increase for constant density and moisture content samples.


Ο 0.50 0.75 1.00 1.25 1.50

Sample Thickness ( Inches )

Figure 24. Effect of varying sample thickness on volume change for constant density and moisture content samples.


Initial density

Initial densi ty , whe the r und is tu rbed or r emolded , is the only e lement tha t affects t h e

swelling pressure. As seen in figure 25 and table 10 for cons tan t mois ture samples, the volume

change increases wi th dry densi ty , as does the swelling pressure. The family of curves have t h e

same shape and are approx imate ly parallel t o each o ther . A similar relat ionship was found b y

Kassiff and Shalom [ 3 9 ] . Figure 26 establishes a straight line relat ionship be tween dry dens i ty

and volume change. The relat ionship be tween the dry densi ty and swelling pressure can b e

p lo t ted ei ther in semi-log scale or in rectangular scale. F o r the semi-log scale, the curve is a

straight line as shown in figure 2 7 . The curve can be expressed as:

log y = ax- b

where : y = swelling pressure,

χ = dry densi ty , and

a and b = cons tan ts depending on soil p rope r ty

and " a " is the slope of the curve.

Dry densi ty and swelling pressure relat ionship when p lo t t ed in rectangular scale are shown

on figure 28 . The curve can be expressed by the following exponen ta l form:

y = k c x

where : k = 10" b

c = 1 0 a

It is seen tha t when dry densi ty decreases, swelling pressure rapidly approaches zero. When

dry densi ty increases, swelling pressure rapidly increases and approaches infinity. The soil

engineer is in teres ted, only wi th in a narrow range of dry densi ty , ranging from 100 to 130 pcf.

Table 1 0 - . Effect of varying density on volume change and swelling pressure for constant moisture content samples.

Initial density,

pcf


Initial degree

of saturation,

percent


Swelling pressure,

psf

Initial density,

pcf Initial Final

Initial degree

of saturation,

percent


Swelling pressure,

psf

94.3 12.93 21.27 45.0 2.7 2,600

99.4 12.20 24.92 48.1 3.8 4,600

100.2 12.93 19.93 52.1 4.2 5,000

103.3 12.93 20.51 56.3 5.1 7,000

109.1 12.93 20.56 65.4 6.7 13,000

110.8 12.20 19.03 64.7 7.3 14,000

114.5 12.20 19.17 71.6 8.2 21,000

118.9 12.20 17.08 81.2 8.6 35,000

Average 12.55 21.08


Figure 25. Relationship between density and volume increase for constant initial moisture content samples.

Since the foregoing established that the swelling pressure of a given soil is a constant and

varies only with the dry density, swelling pressure can be conveniently used as a yardstick to

measure the swelling characteristics of the soils. For undisturbed soil, dry density is the in situ

characteristic. Therefore, the swelling pressure at the in situ dry density can be used directly to

describe the swelling characteristics. For remolded soil, the swelling pressure varies with degree of

compaction. It is useful to introduce maximum Proctor density as a guide. In other words, the

swelling pressure of remolded clay can be defined as the pressure required to keep the volume of

a soil at its Proctor density constant.

In this particular case, the standard Proctor density is 108.4 pcf. At this density the swelling

pressure is 12,000 psf. It is intresting to note that at the site where the sample is taken in its

undisturbed state, the dry density of the soil is 110 pcf corresponding to a swelling pressure of

14,000 psf.

Conclusions

1. The swelling pressure of a clay is independent of the surcharge pressure, initial moisture

content, degree of saturation, and the thickness of the stratum.

2. The swelling pressure increases with the increase of initial dry density.

90 95 100 105

Dry Density (pcf)

110 115 120

Figure 26. Effect of varying density on volume change for constant moisture content samples.

M E C H A N I C S O F S W E L L I N G 57


4 0 0 0 0

2000

100 105

Dry Density (pcf)

Figure 27. Effect of varying density on swelling pressure for constant moisture content samples.


70,000

60 ,000

0 20 40 60 80 100 120 140 DRY DENSITY (pcf)

Figure 28. Effect of varying density on swelling pressure for constant moisture content samples.


3. For undis turbed soil, the swelling pressure can be defined as the pressure required to

keep the volume of a soil at its na tura l dry densi ty cons tant .

4. Fo r remolded soil the swelling pressure can be defined as the pressure required to keep

the volume of a soil at its m a x i m u m Proc tor densi ty cons tant .

5. Swelling pressure can be used as a yardstick for measuring swelling soil. Swelling pressure

reflects only the swelling characterist ics of the soil and will no t b e changed by p lacement

condi t ions or environmental condi t ions .

R E F E R E N C E S

[28] "Moisture Equilibria and Moisture Changes in Soils Beneath Covered Areas," Butterworth, Australia, 1965.

[29] Croney, D. and Coleman, J. D., "Soil Moisture Suction Properties and their Bearing on the Moisture

Distribution in Soils," Proceedings, Third International Conference on Soil Mechanics and Foundation

Engineering, Zurich, Vol. I, 1953.

[30] "A Review of Literature on Swelling Soils," Department of Highways, State of Colorado, University of

Colorado and Bureau of Public Roads.

[31] Aichison, G. D. and Richards, B. G., "The Fundamental Mechanics Involved in Heave and Soil Moisture

Movement and the Engineering Properties of Soils which are Important in Such Movement," Second

International Research-and Engineering Conference on Expansive Clay Soils, Texas A & M Press, 1969.

[32] Jennings, J. E., "The Theory and Practice of Construction on Partially Saturated Soils as Applied to South

Africa Conditions." "Engineering Effects of Moisture Changes in Soils," International Research and

Engineering Conference on Expansive Clay Soils, Texas A & M Press, 1965.

[33] Lambe, T. W. and Whitman, R. V., "The Role of Effective Stress in the Behavior of Expansive Soils,"

Quarterly of the Colorado School of Mines, Vol. 54, No. 4, October, 1959.

[34] "Special Procedures for Testing Soil and Rock for Engineering Purposes," 5th Edition, 1970, ASTM, STP

479,1970.

[35] Jennings, J. E. and Knight, K., "The Prediction of Total Heave from the Double Oedometer Test," 1957-58

Symposium on Expansive Clays, South African Institution of Civil Engineers, Johannesburg.

[36] McDowell, C , "Interrelationship of Load, Volume Change and Layer Thickness of Soils to the Behavior of

Engineering Structures," Proceeding, Highway Research Board, Vol. 35, 1956.

[37] "Criteria for Selection and Design of Residential Slab-on-Ground," Building Research Advisory Board,

Report No. 33 to the Federal Housing Administration, 1968.

[38] Kassiff, G. and Baker, R., "Aging Effects on Swell Potential of Compacted Clay," Journal of the Soil

Mechanics and Foundation Division, ASCE, Vol. 97, SM 3. Proc. March, 1971.

[39] Kassiff, G. and Shalom, A. B., "Experimental Relationship Between Swell Pressure and Suction,"

Geotechnique, Vol. XXI, No. 3, September, 1971.

Chapter 3

FIELD AND LABORATORY INVESTIGATIONS

I N T R O D U C T I O N

The stabili ty of a s t ruc ture founded on expansive soil depends u p o n the subsoil condi t ions ,

ground surface features, type of cons t ruc t ion , and possibly even the meteorological variat ions.

Subsoil condi t ions can be explored by drilling and sampling, seismic surveying, excavating test

pits, and by s tudying existing data . Ground surface features are control led b y surface geology and

physiography. A s tudy of existing s t ructures in the immedia te vicinity of the site can be of pr ime

impor tance in de termining the type of cons t ruc t ion in an expansive soil area. This is necessary in

the investigation of soil for a building addi t ion.

Elaborate site investigation of tent imes cannot be conduc ted due to limited assigned

cons t ruct ion costs. Fo r very favorable sites, e laborate site investigation m a y no t be warran ted .

However, if the area is suspected of having swelling soil p rob lems , extensive soil investigation will

be necessary even for very minor s t ructures . General ly, it is the small building wi th inadequa te

funding, insufficient planning, and low-bidding con t rac to r w h o unwisely economizes in

construct ing the building tha t presents the mos t problems. The soil engineer should no t accept

jobs in an expensive soil area which will n o t allow a thorough subsoil investigation.

SITE INVESTIGATION

Before init iating the site investigation, the soil engineer should obtain informat ion regarding

site topography , surficial geology, and existing s t ructures . This can be accomplished by reviewing

available data , s tudying topographic and geologic maps , and making a reconnaissance survey.

Topography

The topographic condi t ion is an essential par t of the site investigation. Generally, for larger

projects a topographic survey is available. However, care mus t be taken to insure tha t the survey

is correct . Many t imes, site grading has comple te ly altered the survey. The elevation of each test

hole should be recorded.

One impor t an t aspect is the selection of a bench mark . Every effort should be made to tie in

the elevations wi th the archi tect ' s reference poin t . Since the floor level governs the selection of

foundat ion type , this aspect canno t be overemphasized. Whenever possible, the bench mark

should be referenced to establish da tum such as an existing building, manho le invert , cross on a

sidewalk, t op of fire hyd ran t and so forth.


The locat ion of na tura l and m a n m a d e drainage features is also of considerable impor tance .

Erecting a s t ructure across a na tura l gully always poses a future drainage prob lem. The wate r level

in any nearby streams and rivers should be measured and recorded. Irrigation di tches carry large

a m o u n t s of wate r dur ing irrigation season. Water leaking from the d i tch can supply mois ture to

the foundat ion soil and cause swelling of footings and slabs. Pier uplift due to the infi l tration of

wa te r from an irrigation d i tch is no t u n c o m m o n . Water leaking from the di tches can also cause

o the r p roblems such as basement damage. The locat ion and elevation of the d i tch should be

included as a par t of the field records.

Streams and nearby rivers natural ly are of impor tance in the site investigation. Of par t icular

interest , in avoiding flooding, is the ex ten t of flood plains. Such prel iminary informat ion can

usually be obta ined from the U. S. Geological Survey and the U. S. Depa r tmen t of Agriculture

soil survey repor ts .

The steepness of valley slopes is of special concern for sites to be located in m o u n t a i n o u s

areas. Some envi ronmenta l agents classify valley slopes in excess of 30° as po ten t ia l hazard areas.

Slope stability depends u p o n the slope angle of the rock and soil format ion, evidence of past

slope movemen t , and drainage features. The field engineer should be aware of the possible slope

problems associated wi th landslides, local slope failure, m u d flow or o ther problems. The

vegetative cover on the slope, shape of the trees, and the behavior of any neighboring s t ructures

should also be observed.

Surficial geology

General surficial geology of the area includes the s tudy of slopes, t r ibutary valleys,

landslides, springs and seeps, sinkholes, exposed rock sections, and the na ture of the

unconsol ida ted overburden.

An inspect ion of upland and valley slopes m a y provide clues to the thickness and sequence

of format ions and rock s t ruc ture . The shape and character of channels and the na ture of the soil

(residual, colluvial, alluvial, e tc . ) provide evidence of the past geologic activity.

An engineering geologist should identify and describe all geologic format ions visible at the

surface and no te their topographic posi t ions. The local dip and strike of the formations should be

de te rmined and no te made of any stratigraphie relat ionships or s t ructural features tha t may cause

problems of seepage, excessive wa te r loss, or sliding of the e m b a n k m e n t .

T o insure adequa te planned deve lopment of a subdivision, Senate Bill 35 of Colorado

requires the subdivider to submi t i tems such as:

1. Repor t s concerning s treams, lakes, topography , geology, soils, and vegetat ion.

2 . Repor t s concerning geologic characterist ics of the area tha t would significantly affect the

land use and the de te rmina t ion of the impac t of such characteristics on the proposed

subdivision.

3 . Maps and tables concerning suitabili ty of types of soil in the proposed subdivision.

Similar bills exist in m a n y o the r states in response to environmental zeal of the na t ion . In

moun ta in areas, the mapping of surficial geologic features is highly desirable. Johnson [40]

suggested tha t the features to be shown on the m a p should include:

1. Tex tu re of surficial deposi ts ,

FIELD AND LABORATORY INVESTIGATIONS 63

2. S t ruc ture of bedrock , including dip and strike, faults or features, stratif ication, poros i ty

and permeabi l i ty , schistosity, and weathered zones,

3 . Ground-wate r features, including seeps, springs, observable water table , and drainage,

4 . Area of m o d e r n deposi t (result of accelerated erosion) ,

5. Unstable slopes, slips, and landslides, and

6. Faul ts and fault zones .

Of special impor tance is tha t the geologist should be able to recognize a potent ia l swelling

soil p roblem. Fo r ins tance, the red si l tstone format ion in Laramie, Wyoming, will n o t pose a

swelling problem while a few miles to the east where the claystone of the Pierre Fo rma t ion is

encounte red , swelling will be critical.

Existing structures

T h e behavior of existing s t ruc tures has an impor t an t bearing on the selection of the

proposed s t ruc ture . All possible informat ion should be obta ined concerning s t ructures in the

immedia te p rox imi ty . Inquiry should be made as to the condi t ion of the s t ruc ture , age, and type

of the foundat ion . If adjacent existing s t ructures have experienced water problems, the

possibility of a high-water table condi t ion in the area exists. If the existing s t ruc ture exhibi ts

cracks, this usually indicates that the existing foundat ion system is no t adequa te . An experienced

field engineer can readily identify the type of distress which has occurred in the existing buildings

and then de te rmine if it is associated wi th swelling soils. The age of the building should also be

considered. If t h e cracks are old and new cracks have n o t appeared in the pa tched areas, the

foundat ion movemen t m a y have been stabilized. However, even if the existing s t ruc ture is in

excellent condi t ion , this does no t mean tha t the existing foundat ion system should be dupl icated

in new cons t ruc t ion . T h e existing s t ruc ture could have been originally overdesigned. This is

especially t rue for old s t ruc tures where massive founda t ion systems were usually used.

D R I L L I N G AND SAMPLING

Geophysical techniques are generally used for prel iminary subsoil investigation. Both the

seismic and the resistivity geophysical m e t h o d s have been successfully used. Geophysical

equ ipment has been c o m m o n l y used in de termining the surface of bedrock , major changes in

subsoil condi t ions , and ground-water dep th . However, such a survey should always be

supplemented by drilling.

Probably , the mos t accurate subsoil investigation m e t h o d is by the opening of test pi ts . In a

test pit , the field engineer can examine in detail the subsoil s trata , stratif ication, layers and lenses,

as well as taking samples at the desired locat ion. However, the dep th of the test pi t is l imited to

the reach of a backhoe , generally 12 feet. Also when the water table is high, test pi t investigation

becomes useless. In locat ions where the subsoils consist essentially of large boulders and cobbles ,

the use of test pit investigation is mos t favorable. Auger drilling th rough boulders and cobbles is

difficult, if n o t impossible, and in addi t ion the cost of ro ta ry drilling m a y n o t be warranted for

small projects .


Ano the r possible investigation m e t h o d is to drill a large-diameter caisson hole to the

required dep th . By physically entering the caisson hole , the subsoil s t rata can be clearly

examined, and undis turbed samples can be obta ined at the desired dep th .

The drawback of b o t h the test pit m e t h o d and the caisson hole m e t h o d is tha t s tandard

pene t ra t ion tests canno t be performed. Probably 95 percent of all site investigations are

conduc ted by drilling test holes , e i ther by auger drilling, ro tary drilling, percussion drilling, or

o ther m e t h o d s .

Test holes

The n u m b e r of test holes required for a project depends u p o n the type of foundat ion

system, uni formi ty of subsoil condi t ions , and to some ex ten t the impor tance of the s t ruc ture .

If prel iminary investigation indicates tha t shallow spread footings will be the mos t likely

type of foundat ion , then it will be desirable to drill shallow test holes close together to be t te r

evaluate the subsoil condi t ions within the loaded d e p t h of the footings. If, however , the shallow

foundat ion system is no t feasible and a deep foundat ion system is likely to be required, then the

n u m b e r of test holes can be decreased and the spacing increased.

As a rule of t h u m b , test holes should be spaced at a dis tance of 50 to 100 feet. In n o case

should the test holes be spaced more than 100 feet apar t when in an expansive soil area.

The client somet imes has the misconcept ion tha t drilling of test holes is the major cost of

the subsoil investigation, and consequent ly , there is a t endency to drill as few holes as possible,

preferably only one. The risk involved in such an under tak ing is eno rmous . Errat ic subsoil

condi t ions can exist be tween widely spaced test holes ; therefore , logs of these test holes cannot

be considered as representat ive of the overall site subsoil condi t ion .

Closely spaced test holes are especially impor t an t where the presence of expansive soils is

suspected. F o r instance, if sands tone and sil tstone bedrock are encountered at a shallow dep th , a

logical r ecommenda t ion is t o found the s t ruc ture directly on the bedrock with spread footings

designed for high pressure. However , if m o r e test holes had been drilled, it is qui te possible that

claystone bedrock containing high swelling potent ia l might have been revealed. The presence of

swelling soil, even in only one test hole , can change the foundat ion recommenda t ions completely.

The d e p t h of test holes required is generally governed by the type of foundat ion. Before

drilling, the t y p e of foundat ion canno t be comple te ly envisioned. Therefore, the first hole the

field engineer drills should be a deep hole , deep enough to provide informat ion per t inent to b o t h

shallow and deep foundat ion systems. Samples in the hole should be taken at frequent vertical

intervals, preferably n o t more than 5 feet apart . After the comple t ion of the deep test hole , the

field engineer should have a fairly good idea of the possible foundat ion system. Consequent ly , for

the subsequent holes, m o r e a t t en t ion should be directed to the upper soil if a shallow foundat ion

is likely. If, however , a deep foundat ion system is con templa ted , drilling to bedrock would be

necessary for all holes.

Oftent imes, the d e p t h to bedrock is a cri terion as to the dep th of test holes. Where bedrock

is wi th in economical reach, say, within 4 0 feet, it is advisable t o drill a few holes in to t he

bedrock. In Colorado , the d e p t h t o the t op of shale bedrock is ext remely erratic, and the dep th


to bedrock can increase as m u c h as 30 feet wi thin a short dis tance of 100 feet. This will have to

be t aken in to considerat ion w h e n de termining the locat ion and d e p t h of test holes.

In some cases, deep holes are required, no t from the subsoil requ i rement b u t for the

de te rmina t ion of the wa te r table elevation. Fo r deep basement cons t ruc t ion , the d e p t h of test

holes should be at least 20 feet to preclude the possibility of ground water becoming a problem

in the lower floor.

Penetration test

In performing the s tandard pene t ra t ion test , a soil sampler k n o w n as a split spoon is used. It

is an open-ended steel cyl inder which is split longitudinally in to halves. These two halves are held

together by a cut t ing shoe at the lower end and a coupling which connec ts the sampler to the

drill rod. The split spoon is driven 18 inches in to the ground by means of a 140-pound h a m m e r

falling a free height of 30 inches. The n u m b e r of h a m m e r blows required to drive 12 inches is

called the s tandard pene t ra t ion resistance Ν which represents the n u m b e r of blows per foot, or

the blow count .

The s tandard pene t ra t ion test has been and will con t inue t o be an impor t an t and practical

field test . The following should be considered in using the pene t ra t ion test :

1. The value of Ν in cohesionless soils is influenced by the d e p t h at which the test is made .

At a great d e p t h , the same soil wi th the same relative densi ty would give a high

pene t ra t ion resistance. T h e influence of overburden pressure can be approx imated by the

following equa t ion :

Ν = N ' \ P + 1 0 /

where : Ν = Adjusted value of s tandard pene t ra t ion resistance,

N ' = Standard pene t ra t ion resistance as actually recorded, and

ρ = Effective overburden pressure, psi

The above equat ion was derived by the U. S. Bureau of Reclamat ion th rough actual

exper iment and has been extensively used all over the world .

The au thor found tha t while the increase in pene t ra t ion resistance at greater d e p t h canno t

be ignored, the calculated values are ra ther high. Using the Bureau of Reclamat ion

equat ion and a value of N ' = 12, at 20 feet below the ground surface wi th Ύ = 110 pcf,

the adjusted pene t ra t ion resistance is almost doubled . This results in the designing

engineer using a pene t ra t ion resistance value larger than required for a safe design.

2. Pene t ra t ion resistance is reliable only if the driving condi t ion is no t abused. The h a m m e r

should be entirely free falling w i t h o u t being subject t o u n d u e friction. T h e s tandard

pene t ra t ion barrel should no t be packed by overdriving since this forces the soil against

the sides of the barrel and causes incorrect readings. An increase in b low coun t b y as

m u c h as 50 percent can somet imes be caused by a packed barrel. Driving a s tandard barrel

in to gravelly soils presents a p rob lem. The barrel will b o u n c e when driving on cobbles,


and hence, n o useful value can be obta ined. Somet imes , a small piece of gravel will jam in

the barrel thereby prevent ing the ent rance of soil in to the barrel , thus substantial ly

increasing the blow count .

3 . Considerable e c o n o m y can be achieved by combining pene t ra t ion resistance wi th

sampling. A slight modif icat ion in the design of the barrel will allow the insert ion of thin

wall lining in the barrel and provide a blow coun t as well as an undis turbed sample. The

modified barrel is c o m m o n l y referred t o as a California sampler. Field tests have been

conduc ted compar ing the results of the pene t ra t ion resistance of the California sampler

wi th tha t of the s tandard pene t ra t ion tests. The tests indicate tha t the results are

commensurable wi th the except ion of very soft soil (N < 4 ) and very stiff or dense soil

(N > 3 0 ) . By combining pene t ra t ion resistance test wi th the sampling device, m o r e tests

can be m a d e and undis turbed samples can be obta ined w i thou t resorting to the use of

Shelby tubes.

Sampling

Some cont rac t s call for a pene t ra t ion test for every 5 feet and sampling for the same interval

o r every change of soil s t r a tum. This m a y prove unnecessary. The field engineer should use his

j u d g m e n t t o guide the frequency of sampling and avoid unnecessary sampling so tha t the cost of

investigation can be held to a min imum. Samples in the upper 10 or 15 feet are impor tan t , as

this is generally the site of shallow foundat ions ; also, soil characterist ics at this level govern the

slab-on-ground cons t ruc t ion and earth-retaining s t ructures . Sampling and pene t ra t ion tests at

lower dep th , say, in bedrock , become critical when a deep foundat ion system is required.

Sampling above the proposed floor level should be limited since this material will be

excavated. Somet imes , for deep basement cons t ruc t ion , no sampling or pene t ra t ion tests will be

necessary for the first 15 feet below ground level.

Sampling and testing at an in termedia te d e p t h generally is no t t oo critical, except when

friction piers are unde r considerat ion. Instead of assigning the field engineer definite inst ruct ions

as to the frequency of sampling, it would be be t te r to leave the m a t t e r up to his discret ion.

A reasonably good sample can be obta ined when driving in to shale bedrock . Auger drilling

in m o s t cases can be successfully conduc ted in shale bedrock . Fo r o the r types of bedrock, such as

l imestone and granite, ro tary drilling will be necessary and rock cores obta ined. Core samples are

b r o u g h t up by the drill and can be visually examined. The general characteristics, part icularly the

percentage of recovery, are of impor tance to foundat ion design and cons t ruc t ion .

L A B O R A T O R Y TESTING

Soil testing is essential in establishing the design criteria. Dist inct ion mus t be made be tween

t h e needs of the practicing engineer and those of the research engineer. Fo r a practicing engineer,

t h e purpose of labora tory testing is mainly to confirm his preconceived concep t derived from

field drilling, pene t ra t ion tests , visual examina t ion of the samples, and personal experience in the


area. Exot ic l abora tory equ ipmen t and refined analysis are in the realm of the research engineer.

Nei ther t ime n o r budget will allow the practicing engineer to follow the researcher 's p rocedure .

Swell test

The most impor t an t l abora tory test on expansive soils is the swell test . The s tandard

one-dimensional consol idat ion test appar tus is similar t o tha t used in mos t soil laborator ies for

consol idat ion studies. This appara tus , k n o w n as a consol idometer , is shown on figure 29 and is of

the fixed-ring type [ 4 1 ] . It can a c c o m m o d a t e a remolded or undis turbed sample from 2 t o 4 .25

inches in d iameter and from 0.75 to 1.25 inches in thickness. Porous s tones are provided at each

end of the specimen for drainage or sa tura t ion . The sample uni t is placed on a platform scale

table and the load is applied by a y o k e ac tua ted by a screw jack. The load imposed on the

specimen is measured by the scale beam and a dial gage is provided to measure the vertical

movement .

The advantage of such appara tus is tha t it is possible to hold the u p p e r loading bar at a

cons tant volume and allow the measurement of the m a x i m u m uplift pressure of the soil w i t h o u t

Figure 29. Platform scale type consolidometer.


volume change. This requires a cons tan t load adjustment by an opera tor . A m o r e advanced

scheme is to use the consol idometer wi th a triaxial frame along with the a t t achmen t s and

electrical circuits [ 4 2 ] . Such a device will allow the au tomat i c load increment and measure

swelling pressure w i thou t allowing volume change to take place.

The consol idometer can also be used to measure the a m o u n t of expansion unde r various

loading condi t ions . Since swelling pressure can be evaluated by loading the swelled sample to its

original volume as explained in chapter 2 , it is simple t o convert the pla t form scale

consol idometer in to a lever-type consol idometer as shown on figure 30 . A simple consol idometer

can be made locally wi th low cost. The average soil labora tory should have a train of

consol idometers to speed u p test ing procedures .

Interpretation of test results

Labora tory testing of dis turbed and undis turbed soils can be performed in mos t soil

laboratories by an efficient labora tory technician. Labora tory test results are reliable only to the

extent of the condi t ion of the sample. Results of testing on badly dis turbed samples or samples

not representative of the s trata no t only are useless bu t add confusion to the comple te analysis.

Equally impor t an t to testing of representative samples is the frequency of testing. Testing of

a few samples on a single project and basing the final analysis on such testing is n o t only

undesirable b u t somet imes dangerous , especially when evaluating swelling soils. The potent ia l of

swelling generally canno t be de te rmined visually, and labora tory swell tests mus t be conduc ted .

By testing only a few samples, the high swelling sample m a y have been missed and er roneous

conclusions drawn. T o o little testing is worse than n o testing at all.

Figure 30. Simplified lever-type consolidometer.


In terpre ta t ion of test results should be conduc ted by an experienced engineer, having the

ability to screen the test results and exclude the dubious ones, to de te rmine when the min imum

or m a x i m u m values are t o be used, and when average values should be used.

R E F E R E N C E S

[40] Johnson, A. I. "Suggested Method for Geologic Reconnaissance of Construction Site" Special procedures

for testing soil & rock for engineering purposes. ASTM stp 479, 1970.

[41 ] Lambe, T. W., "Soil Testing for Engineers," John Wiley & Son, Inc.

[42] Agarwal, K.P. and Sharma, S. C , "A Method for Measuring Swelling Pressure of an Expansive Soil,"

Proceedings of the Third International Conference on Expansive Soils, Haifa, Israel.

Chapter 4

DRILLED PIER FOUNDATIONS


Drilled pier, or drilled caisson, or caisson foundat ion is widely used in the R o c k y Mounta in

area. Drilled piers, when made wi th an enlarged base, are c o m m o n l y referred to as belled piers

and when m a d e w i t h o u t an enlarged base are referred t o as straight-shaft piers.

The drilled pier founda t ion is used to transfer the s t ructural load from the uppe r uns tab le

soil t o the lower stable soil. The use of drilled pier founda t ion covers a wide range of possibili t ies;

some of which follow:

1. Piers drilled in to hard bedrock for suppor t ing high co lumn load.

2. Fr ic t ion piers b o t t o m e d on stiff clays for suppor t ing light s t ructures ,

3 . Belled piers b o t t o m e d on sand and gravel for suppor t ing m e d i u m co lumn load, and

4 . Long, small-diameter piers drilled in to a zone unaffected by mois ture change in swelling

soil areas.

The drilled pier founda t ion is a rat ional solut ion to c o m b a t the prob lem of expansive soils;

however , the design and cons t ruc t ion mus t be closely control led.

As s ta ted by Richard Woodward [ 4 3 1 , " I f investigation and design were always perfect ;

supervision always c o m p e t e n t and adequate , inspect ion always con t inuous , exper ienced and aler t ;

and the con t rac to r ' s personnel always exper t and conscientious—if all these condi t ions prevailed,

all the t ime on a project , then there would be n o defective p ie rs . "

PIER CAPACITY

Piers bearing on bedrock or shale have been designed using mos t ly empirical considerat ions

derived from exper ience , l imited load test da ta , and the behavior of the existing s t ructures . F o r

instance, because of its errat ic characterist ics, the u l t imate load carrying capaci ty of the Denver

Blue Shale has never been de te rmined . Nonetheless , piers suppor t ing a co lumn load in excess of

1,000 kips are c o m m o n l y designed and cons t ruc ted in the Rocky Mounta in area wi th satisfactory

performance .

Woodward , Gardener and Green [43] stated on empirical design of piers, "Many piers,

part icularly where rock bearing is used, have been designed using strictly empirical considera t ions

which are derived from regional exper ience . " They further stated tha t , "Where subsurface

condi t ions are well established and are relatively uni form, and the per formance of past

cons t ruct ions well d o c u m e n t e d the design by exper ience approach is usually found to be

sat isfactory."


The bearing capaci ty of drilled piers b o t t o m e d on bedrock is a combina t ion of the

end-bearing capaci ty and the skin friction developed be tween the pier wall and bedrock .

Bearing capacity

The bearing capacity of bedrock for piles or piers can be found in mos t building codes. The

values given are generally conservative and the type of bedrock usually no t clearly defined. The

me thods for de termining the bearing capacity are as follows:

1. Penet ra t ion resistance of bedrock ,

2. Unconfined compressive s t rength or triaxial shear s t rength of undis turbed bedrock core

sample,

3 . Consol idat ion tests on bedrock core sample unde r high load,

4 . Actual record of se t t lement of existing building founded on similar bedrock ,

5. Load test on bedrock , and

6. Menard pressuremeter test on bedrock in the test holes.

Unfor tuna te ly , all of the above approaches have their l imitat ions. Penet ra t ion resistance on

hard bedrock involves blow counts in excess of 100. When a soft seam or a very hard lens is

encountered , the actual pene t ra t ion resistance of the s t ra tum cannot be accurately de termined.

Therefore, to obta in and accurate de te rmina t ion of pene t ra t ion resistance, m a n y tests, possibly

more than 50 , should be conduc ted . After the representat ive pene t ra t ion resistance value has

been de te rmined , the soil engineer should convert the blow coun t to allowable bearing capaci ty.

One widely used m e t h o d is:

Ν

% = 2

where : q a = allowable bearing capacity in ksf, and

Ν = blow count

For instance, if the representat ive blow coun t is 4 0 , the allowable bearing capacity selected will

be 20 ,000 psf. This approach is qui te conservative. Extensive labora tory testing indicates that the

more realistic value for q a should be in the range of Ν to 0 .75N. That is, wi th Ν = 40 , the

allowable bearing capaci ty should be more nearly 30 ,000 to 4 0 , 0 0 0 psf.

Allowable bearing capacity derived by penet ra t ion resistance data should be checked by b o t h

the unconfined compressive s t rength test and the consol idat ion test . Unconfined compressive

strength test at best can only represent the lower limit of the actual bearing capacity of bedrock.

Unconfined compressive s t rength performed on drive samples can only be used for comparat ive

purposes. Samples obta ined from large d iameter cores obta ined from core drilling are far m o r e

reliable, b u t the presence of slickensides and o the r effects of d is turbance limit the validity of

such tests.

High capaci ty consol idat ion test on a reasonably good sample can somet imes be used to

de termine the a m o u n t of pier se t t lement . The actual se t t lement value taken as a percentage of

laboratory consol idat ion value should be left to the j udgmen t of the soil engineer.

Probably , the mos t reliable m e t h o d of est imating bedrock bearing capacity is the

observation of the behavior of existing s t ructures . Unfor tuna te ly , such records are scarce. In the

DRILLED PIER FOUNDATIONS 73

Denver area, piers bearing of Denver Blue Shale F o r m a t i o n designed for a bearing capaci ty of

60 ,000 psf have per formed satisfactorily wi th long-term se t t l ement of less t han 1 inch.

Load tests on piers are costly and t ime consuming, especially for large-diameter piers.

Limited da ta on the testing of small-diameter piers indicates tha t the convent ional approaches are

ext remely conservative.

The m o s t direct and r e c o m m e n d e d m e t h o d in de termining bearing capacity is by the

insert ion of a Menard pressuremeter in to the b o t t o m of the test hole and evaluating the actual

bearing capaci ty. An exper ienced opera to r will be required to conduc t the tests.

Skin friction of shale

The load-carrying capaci ty of a pier depends no t only u p o n its end bearing value bu t to a

great ex t en t on the skin friction or side shear be tween concre te and its sur rounding soils. The

skin friction value is unfor tuna te ly difficult to de te rmine as discussed later unde r "Fr ic t ion

Piers ."

Soil engineers in the Rocky Mounta in area chose the es tabl ishment of skin friction value by

the following two assumpt ions :

1. Assuming tha t skin friction value in bedrock is one- ten th of the end bearing value of

bedrock , t hen for an end bearing value of 60 ,000 psf, the skin friction value is 6 ,000

psf. This value obviously has exceeded the cohesion of c laystone and the to ta l

hor izonta l pressure against the shaft surface. It is generally though t tha t the skin

friction value may no t develop fully along the surface of the shaft bu t tha t a lubricated

surface may exist be tween the soil and shaft. Consequent ly , a t t empt s have been made

to provide a shear ring in the por t ion of the pier in bedrock . Shear rings are made by

the use of grooving tools which cut slots a round the circumference of the hole . The

circular slots are abou t 1-inch deep and 3/4-inch thick at intervals of 8 inches. It is

believed tha t by grooving the hole , adequa te development of skin friction can be

assured. Exper ience shows tha t such an installat ion is seldom justified. The walls of a

pier hole are seldom smoo th , wi th except ion of those drilled in to oily shale. If it is

found tha t a pier hole is t o o smoo th , a t o o t h p ro t rud ing about one-half inch can be

inserted o n t o an auger and wi th several tu rns , the surface of the hole can thus be

roughened artificially.

Load tests on small d iamete r piers (12 inches) drilled in to bedrock wi th n o end

bearing (pier poured on elastic material) indicate tha t the skin friction value exceeds

the design value by several t imes. It is believed tha t for small d iameter piers, skin

friction has actually t aken all t he load exer ted on the pier w i t h o u t transferring the load

t o the pier b o t t o m .

2. In designing the pier load, the skin friction developed in the por t ion of pier embedded

in the overburden soil is usually ignored. Such values are considered t o be added

factors of safety in the pier system. Actual ly, for long piers, the skin friction value

developed in the overburden soils can be considerable. Assuming the drilling of a

42-inch-diameter pier t h r o u g h 2 0 feet of dense sand and gravel i n to the bedrock , t he

uni t skin friction value be tween the concrete pier and the granular soil is at least 4 0 0


psf. Total skin friction value can exceed 90 kips. This added factor of safety to the

drilled pier system is r ecommended . F o r major s t ructures , it is i m p o r t a n t tha t an

adequate factor of safety be incorporated to allow for unforeseen cons t ruct ion

contingencies.

Design capacity

After end bearing and skin friction values have been established, it is no t difficult to

calculate the tota l load-bearing capaci ty of a pier. Experience wi th shale bedrock in the Rocky

Mounta in area generally indicates t ha t t he hardness of bedrock increases wi th dep th . This has

been verified by pene t ra t ion resistance and by pressuremeter tests. Therefore, for piers

penet ra t ing deep in to bedrock , a higher load-carrying capacity can be assigned. Experience

indicates tha t b o t h the end-bearing capaci ty and the skin friction value increases wi th dep th at

t he rate of 3 percent per foot. Using this assumpt ion, the load-carrying capacity of a

straight-shaft pier can be expressed as follows:

Q = A(E + 0.03 EL) + 0.1 (E + 0.03 EL) C L

in which: Q = to ta l pier carrying capacity (kips)

A = end area of pier (sq. ft.)

Ε = end bearing capacity of pier (psf)

L = d e p t h of pene t ra t ion in to bedrock (ft.)

C = per imeter of pier (ft.)

Figure 31 was prepared using the above equa t ion , and it was assumed tha t the end bearing

value of t op of bedrock is 50 ,000 psf. It should be poin ted out tha t the above equat ion has its

l imitat ions as no ted :

1. The bearing capacity of the t op of bedrock should be carefully evaluated. Oftent imes,

the t op of shale is highly weathered and only a low value can be assigned. Sometimes,

deep excavat ion will expose the t o p of shale and the material will be subject to severe

disintegrat ion. Therefore , it is usually safe t o assign a bearing capacity value for tha t

por t ion of pier 2 to 4 feet below the surface of bedrock , and then increases the bearing

value for addi t ional pene t ra t ion .

2. The increase of bearing capacity wi th d e p t h of pene t ra t ion has its l imitat ions.

According to the formula, the bearing capacity of the pier will be doubled when the

dep th of pene t ra t ion exceeds 30 feet. Obviously, a load-carrying capacity of such

magni tude canno t be assigned to the pier. Extensive experience in the Rocky Mountain

area indicates t ha t the absolute m a x i m u m of end bearing value for piers in shale is

100,000 psf or abou t 700 psi.


2 , 2 0 0

< 1 ,800

1,000

8 10 12 14

PENETRATION INTO BEDROCK · FEET

Figure 31. Design load versus depth into bedrock.

Using the above approach in designing drilled piers, the load-carrying capacity of a single

60-inch-diameter pier can easily reach 3 ,000 kips wi th reasonable pene t ra t ion . Such load is of

sufficient magni tude t o accommoda te mos t co lumn loads of a high-rise s t ruc ture .

MECHANICS O F PIER UPLIFT

As stated earlier in the chapter , the principle of the use of drilled piers is t o provide a

relatively inexpensive way of transferring the s t ructural loads down to stable material or to a

stable zone where mois ture changes are improbable . There should be no direct contac t be tween

the soil and the s t ructure wi th the except ion of the soils suppor t ing the piers. Figure 32 is a

sketch of the grade beam and pier sys tem.

It is seen from figure 32 tha t the uplifting forces which tend to pull the pier ou t of the

ground are a direct funct ion of the swelling pressure. The wi thhold ing force consists of the dead

load pressure exer ted on the pier and the skin friction along the unwe t t ed por t ion of the pier.

For safe pier design, the wi thholding force mus t balance the uplifting force. These forces are

analyzed as follows.


Dead load pressure

2r

Reinforcement for - tension

Grade Beam

Air space beneath Grade Beam

Uplifting Pressure U = uf

Swelling Pressure

Skin friction

Assumed circular plane of failure

STRAIGHT SHAFT-PIER FOUNDATION BELL - PIER FOUNDATION

Figure 32. Grade beam and pier system.

Uplifting force

The to ta l uplifting force of the soils surrounding the pier can be wr i t ten as follows:

U = 2 7 r r f u ( D - d )

where : r = radius of the pier (ft.)

d = d e p t h of the zone of soils unaffected by wett ing, (ft.)

D = tota l length of the pier, (ft.)

u = swelling pressure, (psf)

f = coefficient of uplift be tween concrete and soil,

U = to ta l uplifting force, (lbs.)

The soils surrounding the pier expand b o t h vertically and hor izontal ly u p o n wett ing.

Parcher and Liu [441 have shown tha t compacted clay soils exhibi ted greater uni t swelling in the

hor izonta l direct ion than in the vertical direct ion. Since the magni tude of the difference of

swelling is small, we can assume tha t the vertical swelling pressure can be used in est imating the

uplifting force on the piers.

The coefficient of uplift be tween the soil and surface of the concre te pier is no t k n o w n .

With the mode l pier test described in the subsequent sect ion, it was a t t e m p t e d t o establish a

rat ional value for the coefficient of uplift for design purposes . It should be poin ted ou t here tha t

the uplifting pressure of the pier may no t be control led by the shear s t rength of the clay. Our


exper iment discussed later unde r "Model Test for Pier Upl i f t" indicates tha t the pier actually

slips ou t of the soil in very m u c h the same manne r as ext rac t ing a pile. The con tac t surface

be tween the concre te and the soil remained clean.

Withholding force

The wi thholding force tha t keeps a pier from pulling ou t of the ground can be wri t ten as:

W = πι2 ρ + 2πΓ sd

where : ρ = uni t dead-load pressure, (psf)

s = skin friction surrounding the pier, (psf)

W = total wi thholding force, (lbs.)

d = d e p t h of zone of soils unaffected by wet t ing , (ft.)

r = radius of the pier, (ft.)

The skin friction of the soils surrounding the pier is again a factor which canno t be fully

evaluated. According t o Mohan and Chandra [45] " T h e frictional resistance of bored concrete

piles in m e d i u m to hard clays is abou t half the average undis turbed shear s t rength of the clay

along the pile shaft and has the same value in loading and pul l ing." Experience wi th piers

embedded in shale indicates tha t the skin friction along the shaft of the pier is equal to at least

1/10 of the end bearing value of the pier. The value is conservative and depends u p o n the

roughness of the pier shaft and the compressive s t rength of the concre te as previously discussed

under "Skin Fr ic t ion of Shale" .

Zone of wetting

In pier design it is impor t an t to de te rmine the d e p t h of the zone of wet t ing. Field tests

indicate tha t expansive soils are so impermeable tha t surface water can pene t ra te only abou t 2

feet in to the soil. However , in actual cases, it was found tha t soils surrounding piers were wet to a

dep th of as m u c h as 15 feet below the ground surface. This is possibly due to the following

reasons:

1. The source of wet t ing may no t be derived from surface water . A b roken wate r main or

sewer pipe can cause a m u c h more severe wet t ing condi t ion than surface water .

2. Surface water seeps th rough seams and fractures in the stiff fissured clays and clay shales.

3 . Water tends to seep along the walls of the pier shaft in to the surrounding soils.

For the above reasons, it is no t possible to precisely define the d e p t h of the zone of wet t ing.

Fo r design purposes , 5 feet can be arbitrarily assigned as the probable d e p t h of wet t ing .

Model test for pier uplift forces

The soil sample used for conduct ing the test was typical of Southeast Denver 's highly

expansive clay. The physical proper t ies of the soil were as follows:

Liquid limit, percent

Plastic l imit, percent

= 50 .0

= 24 .3


Plasticity index, percent = 25.7

Passing no . 200 sieve, percent = 82.4

O p t i m u m mois ture con ten t , percent = 21 .0

Maximum dry densi ty , pcf = 96.5

The soil was packed in to a steel conta iner to a d e p t h of 9 inches in thin layers and at a

mois ture con ten t of 14.0 percent . T w o sets of holes were drilled in the soil.

The first set of t w o piers was prepared for s tudying the behavior of friction piers. The piers

were 2 inches in d iameter and were b o t t o m e d on the steel container . The length of the pier was

embedded in the full d e p t h of soil which is 9 inches; therefore , any uplift pressure exer ted on the

pier would be only along the pier shaft and none on the base.

The second set of two piers was prepared for s tudying the behavior of end-bearing piers.

Holes 2-1/2 inches in d iameter and 6 inches deep were drilled in the soil sample. The piers, being

only 2 inches in d iameter , did n o t have any direct con tac t wi th the soil along their c ircumference,

b u t were bearing direct ly on the soil at the b o t t o m of the hole . Consequent ly , the only pressure

act ing on these piers was at the base.

The ar rangement of the tests is shown on figures 33 and 34 . Piers A and Β are friction piers.

The uplifting movemen t of Pier A was recorded by placing a dial gauge directly on the t op of the

pier. The uplifting pressure of Pier Β was measured with a proving ring. The proving ring was ,

centered on t o p of the pier wi th a ball bearing be tween the pier and proving ring. An adjustable

screw was placed benea th the ball bearing. Dial A, which was placed on top of the pier, was

carefully observed for m o v e m e n t ; as soon as uplifting m o v e m e n t was observed on Dial A, the

adjustable screw was turned to keep the pier in its original posi t ion th roughou t the test. The

stress registered by the proving ring represents the t rue uplifting pressure of the pier [ 4 6 ] . Piers C

and D were end-bearing piers. The set up for these two piers was identical to Piers A and B.

Figure 33. Apparatus for the determination of the coefficient of uplift

DRILLED PIER FOUNDATIONS

r

Friction Pier A End Bearing Pier C Strain Gage Strain Gage

ο ο ο ο ο ο

° φ ° ° (φ) ° ο ο r> ο ί

ο \ ο / Drain Pipes /

Dial Support7

Friction Pier Β End Bearing Pier D Stress Gage Stress Gage

ο ο ο ο ο — ^ ο

ο (| ι ι JJ ο ο (ι_4 L J ] Ο

V \ / Μ— Proving Ring \ > - y J

Dial A - ^ 0 /0 Dial A -^ZT^ο σ ο

PIER UPLIFTING TEST APPARATUS PLAN VIEW

Load Frame

SECTION A A

Figure 34 . Pier uplift ing test apparatus—plan and section.

79


Perforated plastic pipes were inserted a round the per iphery of the piers as shown on figure

34 . Water was added to the soil th rough the perforated pipes to obta in saturated condi t ions . Both

stress and strain readings were t aken at frequent intervals. The test was carried out for a period of

2 weeks. Results of the time-strain and time-stress relat ionships are shown on figures 35 and 36

respectively.

The results of the tests can be summarized as follows:

T y p e Maximum uplifting Maximum upward

Pier of pressure, movemen t ,

No . pier p o u n d s inches

A Fric t ion _ 0.20

Β Fr ic t ion 195.0 -

C End Bearing - 0 .16

D End Bearing 77 .0 -

Fo r comple te wet t ing condi t ions and wi thou t pressure being exerted on the b o t t o m of the

piers, the tota l uplifting force is:

U = 27rrfuD

The to ta l uplifting force exer ted on an end-bearing pier wi th the pier free from the

surrounding soils can be wr i t ten as:

U = 7rr2u

Δ

0 .20

FRICTK >N P I E R - ^ 0

ο—

t - •

-END BEAR! IG PIER

ί 1. 4/17 4/18 4/19 4 /20 4/21 4/22 4 / 2 3 4 /25 4/26 4/27

DATE

Figure 35. Time-strain relationship of friction and end-bearing piers in clay.


3 0 0

2 5 0

ο

FRI CTION PIER-

ο ο ο Ô

" Ο _ 0

/ Ε ND BEARING PIER

" Λ Λ /

• ·

4/17 4/18 4/19 4 / 2 0 4/21 4 / 2 2 4 / 2 3 4 / 2 4 4 / 2 5

DATE

Figure 36. Time-stress relationship of friction and end-bearing piers in clay

Subst i tu t ing the actual uplifting pressure measured in the mode l tes t ;

Fo r end-bearing piers: Fo r friction piers:

77 = π ( I ) 2 u 195 = 2 π ( l ) f u ( 9 )

u = 24.5 psi fu = 3.45 psi.

Assuming tha t the vertical swelling pressure is equal to the hor izonta l swelling pressure, then

fu = 3.45 psi, from which the value f is de termined to equal 0 .14 .

F r o m the results of the exper iment , and from experience wi th pier systems in this area, it

appears tha t the uplifting pressure along the surface of the concre te exer ted by the soil in

soil-concrete pier sys tems is abou t 15 percent of the vertical swelling pressure. This value is of

significant magni tude and can be the governing factor in the design of grade beam and pier

systems. The following example will i l lustrate the impor tance of uplifting pressure along the

surface of the pier.

Assume a 12-inch-diameter pier embedded 10 feet in expansive soils having a swelling

pressure of 10,000 psf. Assuming tha t the uppe r 5 feet of the soil becomes we t t ed , then the tota l

uplifting force exerted on the pier will be :

U = 2 7 r r f u ( D - d )

= 2π (0 .5) (0 .15) (10 ,000) (10-5)

= 23 ,600 lbs.

Fo r piers spaced on abou t 10-to-l 5-foot centers , the dead load pressure normal ly assigned t o the

piers is about 20 ,000 psf. The tota l dead load pressure exer ted on a 12-inch-diameter pier is

15,700 lbs. The unbalanced uplifting pressure is 23 ,600-15 ,700 = 7 ,900 lbs. Compensa t ion must


b e made for this unbalanced uplifting pressure. This is accomplished by the pier in the lower

u n w e t t e d por t ion of the soil where friction along the surface of the pier will provide a restraint to

upl i f t .

In ex t reme cases, it can be considered tha t the entire length of soil surrounding the pier is

w e t t e d . The tota l uplifting pressure then will be 4 7 , 2 0 0 lbs. , and the unbalanced uplifting

pressure will be 31 ,500 lbs. Unde r this condi t ion , uplifting of the pier is inevitable and structural

cracking results.

It was concluded from the above described model pier tests tha t the uplifting pressure along

t h e surface of concre te was abou t 15 percen t of the vertical swelling pressure as previously

discussed. Theoret ical analysis of the magni tude of the uplifting pressure is no t presently feasible

since the mechanics of swelling have no t ye t been translated in to a suitable mathemat ica l model .

It is interest ing to k n o w the shape of the failure plane when the pier is lifted from the

g ro u n d . One theory is tha t when the pier is being lifted, a cone of soil is carried by the pier as

s h o w n on figure 37a. By convent ion , the cone is assumed to be 6 0 ° . Ano the r theory is that the

p ie r is pulled ou t from the ground relative to the surrounding soils as shown on figure 37b . In our

m o d e l test, we found tha t after the comple t ion of the test , Pier A had actually pulled from the

sur rounding soils leaving a gap be tween the b o t t o m of the pier and the surface of the container , a

d i s tance approximate ly equal t o the vertical movement of the pier, as indicated on the strain

gauge. This indicated tha t the failure t o o k place along the interface of soil and concre te as

ind ica ted on figure 37b .

Rational pier formula

A rational pier formula can be derived by equat ing the tota l uplifting force and the tota l

w i thho ld ing force acting on a pier.

2 7 r r f u ( D - d ) = 7rr2p + 27rrsd

then : p = ~ [ f u ( D - d ) - sd]

where ρ = uni t dead load pressure

T o solve the above equat ion , it is necessary to assign values to u, f, s, and D-d. The expansive soils

usual ly encounte red in this area belong to the categories of med ium or high degree of expansion

( t ab le 4) . Fo r usual design purposes , it is possible to assign values of the soil propert ies in the

above equat ion and obtain rat ional solut ions. The following assumptions are made to simplify the

c o m p u t a t i o n :

1. The soil is uniform for the full length of the pier. If shale bedrock is encountered at the

b o t t o m of the pier, the error will be on the safe side.

2. Surface wet t ing will affect only the upper 5 feet of the pier.

3 . Skin friction, s, of the soils surrounding the pier in the unwet t ed zone is about 500 psf.

4 . The swelling pressure acting on the pier for soils wi th high degree of expansion is abou t

10,000 psf and for soils wi th med ium degree of expansion it is abou t 5 ,000 psf.

5. The coefficient of uplift, f, be tween the pier and the soil is 0 .15 .


Based on the above simplified assumpt ions , figure 38 was prepared. F rom the figure, it is

possible for the s t ructural engineer to select the size of the pier, the dead load pressure, and t h e

required to ta l length of the pier.

BELLED PIERS

Piers drilled in to mater ials o ther than bedrock are often enlarged at the b o t t o m of the h o l e

for the purpose of increasing the bearing area, thus increasing the to ta l load-carrying capac i ty .

Such piers are c o m m o n l y referred to as belled piers, or under reamed piers. The ideal bell is in t h e

shape of a frustum wi th a vertical side at the b o t t o m . The vertical side may be 6 to 12 inches

high. The sloping side of the bell should be at an angle at least 60 degrees wi th the hor i zon ta l .

Most drillers are capable of providing bells wi th diameters equal to three t imes the d iameter o f

the shaft.

The advantages and disadvantages of the belled system are presented in the following t w o

paragraphs.

Advantage of belled piers

It is seen in figure 32 tha t the uplift forces exer ted on the belled pier system are as fol lows:

U = P + F W + F S

where : U = to ta l uplifting force due to the swelling of

the soils surrounding the pier shaft in the

unwe t t ed zone , (lbs.),

Ρ = total vertical pressure exerted on the pier, (lbs.),

F w = to ta l weight of the soil above the bell, as shown

on figure 32 , (lbs.),

F s = to ta l shearing resistance along the assumed circular

line of failure in the unwe t t ed zone , d iameter

of the t o p of frustum 2R, (lbs.)

Probably , the greatest advantage of the belled pier is tha t the resistance against uplift will

no t be affected by loss of friction in the zone unaffected b y wet t ing. As previously discussed

under "Pier Upl i f t ," the wi thhold ing force depends u p o n the skin friction of the pier in the z o n e

unaffected by wet t ing. If, for some reason, such as the rise of ground water , the skin friction is

lost, then pier uplift is unavoidable . With belled piers, the tota l weight of the soil above the bel l ,

F w, will no t be affected b y mois tu re change. Hence , there is always an added factor of safety o f

this system against uplift.

In areas where the uppe r soils are highly expansive, bedrock is shallow, and there is a s t rong

possibility of deve lopment of a perched water table condi t ion , the use of a belled pier sys tem

should be favorably considered.


Uplift Uplift

Original position ̂ ^"'^ a of pier bottom b

Figure 37. Soil failure plane resulting from pier uplift.

Since a belled pier system relies entirely u p o n the anchorage of the lower por t ion of the pier

in the zone unaffected by wet t ing against uplift, this type of pier can be used for co lumns wi th

very light load. The magni tude of dead load pressure exerted on the pier is no t a factor.

Disadvantage of belled piers

The shaft of a belled pier mus t be sufficiently large to allow cleanout and inspect ion. The

min imum shaft size is 24 inches, a l though 3 0 inches is desirable. F o r straight-shaft piers with a

large shaft d iameter , the uplift pressure exerted on the pier is several t imes higher than if the

smaller 12-ineh-diameter shaft is used. More re inforcement will be required in a belled pier than

in a small-diameter straight-shaft pier. If the re inforcement is inadequate , tension cracks can

develop in the vicinity of the pier shaft and the bell.

The mos t p rominen t disadvantage of belled piers is the cost and the difficulty of inspect ion.

Bells can be formed mechanical ly wi th a special belling device. In hard bedrock , a mechanical

device may be unsuccessful. When it is necessary to send a man d o w n the hole wi th a j ackhammer

to comple te the bell, the cost can be prohibi t ive. In the Denver area where the cost of pier drilling

is exceedingly compet i t ive , it is es t imated tha t in a favorable drilling condi t ion , the cost of belling

can easily exceed the cost of drilling an extra 10 feet of pier shaft. The advantage of anchorage in

a bell pier system can easily be offset by drilling an addit ional 10 feet in to bedrock with a

straight-shaft pier.

Cleaning a belled pier is m u c h more difficult than cleaning a straight-shaft pier. Proper

cleaning of a belled pier can only be accomplished by sending a man d o w n the hole with a shovel.

With present safety regulations, this means tha t all pier shafts will have to be cased. The

condi t ion of the b o t t o m of the straight-shaft pier can be inspected from the ground level with a

torch or a mirror , b u t the condi t ion of the bell canno t be inspected from ground level.

The difficulty of belled pier cons t ruc t ion increases markedly where the materials

immediately overlying bedrock are subject to caving. It is nearly impossible to bell in granular

soil. The problem is further complicated if ground wa te r is encounte red .


4 6 8 10 12 14 16 18 2 0

REQUIRED TOTAL PIER LENGTH - FEET

Figure 38. Rational pier design chart.

With due considerat ion of the cost , the advantages of the belled pier can be achieved by the

use of straight-shaft piers wi th a larger pene t ra t ion in to a zone unaffected by wet t ing.

Isolation of pier uplift

Since the mos t damaging act ion in a drilled pier system is the uplifting pressure exer ted on

the per imeter of the pier, the simple solut ion appears to be isolating the pier shaft from the

uplifting forces.

The obvious m e t h o d is t o drill an oversized pier hole and provide soft, compressible mater ia l

a round the pier so that the surrounding soils canno t exer t uplifting pressure on the wall of the

pier. However, by so doing, the tota l load carrying capaci ty of the pier will be greatly reduced.

Since mos t piers rely heavily on the skin friction to carry the co lumn load, e l iminat ion of the

skin friction necessi tates the use of large bells for suppor t t o justify the use of such a system. T h e

shor tcomings of this system are as follows:

1. The load carrying capaci ty of the pier is reduced to tha t of a pad footing foundat ion ,

2. No lateral resistance is available for the system,

3 . The annular space m u s t be filled wi th compressible mater ia l such as sawdust , ash, o r

straw. The use of loose sand or pea gravel will be satisfactory initially, b u t repeated

seasonal cycles of expansion and shrinkage will pack the filling and allow the surrounding

soils to grip the pier shaft, and


4 . The annular space filled wi th compressible material affords a free pa th for wate r to reach

the b o t t o m of the pier and heave the ent ire pier from the b o t t o m . Consequent ly , the

purpose of placing the pier in a zone unaffected by wet t ing is defeated. The end result of

using such isolated-shaft belled piers is n o different from using individual pads . This

shor tcoming can be partially corrected by sealing the upper 3 feet of the annular space

wi th compac ted clay. The process is costly and in mos t cases no t effective in preventing

the seepage of water .

One possible applicat ion of such a system is where the upper expansive soils are underlain

b y nonexpansive gravelly soils. An oversized pier hole can be drilled to the top of the gravel

s t ra tum, a small d iameter Sono tube can then be inserted in to the hole and the annular spaces

filled wi th sawdust , and lastly, the Sono tube filled wi th concre te . With such subsoil condi t ion,

the infiltration of water to the b o t t o m of the pier hole will no t cause uplift , as long as the pier

shaft is free from contac t wi th the expansive soils. Such systems have been used wi th great

success in a notor ious ly t roublesome area where the thickness of the upper swelling soil is about

20 feet.

Ano the r approach has been devised to break the bond be tween the pier and the clay. The

concept was init iated in San A n t o n i o , Texas [43] where highly expansive clays are found. A pipe

is in t roduced in to the pier. The concrete-filled pipe is designed to carry the column load at the

top . The outside of the pipe from the t op d o w n to the b o t t o m of the potent ia l ly wet ted zone is

coated wi th a b i tuminous mast ic mater ial . When the pier is lifted by swelling soils, the annulus of

concrete outs ide the coated section breaks in tension, the mast ic coating breaks the bond , and

the upward force is l imited by the viscosity of the mast ic . Figure 39 illustrates this system.

F R I C T I O N PIERS

Since the purpose of a drilled pier foundat ion is to transfer the building load to a zone

where the mois ture con ten t will n o t be affected by surface wet t ing, it is, therefore, unnecessary

t o drill all piers in to bedrock or very hard format ions . Where bedrock is deep and the upper

overburden clays are expansive, a logical solut ion is the use of friction piers. In the design of a

friction pier, a rat ional value should be assigned to the skin friction.

Skin friction

It is generally recognized tha t the skin friction be tween cohesive soils and the pier shaft

cannot exceed the cohesion of the soil. Cohesion m a y be assumed to be equal to one-half of the

unconfined compressive s t rength of the soil. In stiff or hard clays, however , the bond between

the concre te and the soil may be less than cohesion. According to Teng [ 4 7 ] , the m a x i m u m

design value for skin friction of all cohesive soils should be limited t o abou t 1,000 (or u p t o

1,500) psf.

Ul t imate pier shaft resistance can be calculated as the sum of the u l t imate shearing

resistance imposed by the various s trata which are in contac t wi th the pier. The u l t imate shaft

Figure 39 . Design of belled pier for relief of uplift due to expansion of upper clay layer. Note that the outer annulus of concrete is expected to break in tension near the bottom of the expansive clay layer. (Raba and Associates, Consulting Engineers, San Antonio, Tex.)



resistance of a pier drilled in uniform clay strata can be expressed in terms of the undra ined shear

s trength S u and a reduct ion factor α as follows:

S = n r2aSu

The undra ined shear s t rength of clays can be de te rmined in the labora tory from the

unconsol idated-undrained triaxial test . F o r stiff clays, an unconfined compression test will

generally serve the purpose . O the r more direct m e t h o d s in evaluating shear s t rength such as the

use of a pressuremeter inserted in to the drill hole at the desired d e p t h can be used. A vane shear

test is no t applicable for use in stiff clays b u t can be valuable in soft clays. Shear value can also be

interpreted indirectly from field pene t ra t ion resistance tests or cone pene t ra t ion resistance data.

The shear s t rength reduct ion factors a as derived from analysis of p r o t o t y p e field load tests

in clays, shales, and tills, are listed in table 11 .

The shear s t rength reduct ion factor, according to Woodward , Gardener and Greer [43] is

also influenced by cons t ruc t ion effects and the mois ture sensitivity of the suppor t ing materials.

Water migrating from the concre te to the pier wall also reduces the shear s t rength. O the r

e lements such as the dura t ion of exposure of open shafts can also be impor t an t factors.

Design of friction piers

The principle for the design of friction piers is given in a previous section under "Rat iona l

Pier F o r m u l a . " The following considerat ions should be given:

1. Fr ic t ion piers generally bear on stiff clays. With the use of small d iameter piers, the end

bearing capacity of such piers can generally be neglected.

2. Sufficient field pene t ra t ion resistance tests should be performed no t only to establish the

proper friction value b u t also to insure tha t soft soils are no t encounte red within the

length of the pier.

3 . Since the upper 5 feet of soil around the pier is subject to surface wet t ing and uplifting,

this length should be excluded in calculating the pier load capacity.

Table 11—. Shear strength reduction factors (a) for drilled piers.

Material Properties

Moisture content Plasticity Index Shear strength, Material Wn, percent I , percent tons per sq. ft. a Reference

Stiff clay 23 35-55 1.2 0.44 Whitaker and Cooke Stiff clay 25 20-60 1.2 0.62 Reese and O'Neill Massive shale 15 7-16 5.0 0.64* Matich and Koziki Glacial till 12 2-16 2.5 0.64*+ Matich and Kozicki Stiff clay . .. . . .. 1.1 0.52 Woodward et al.

0.9 0.49 Stiff clay 19 36-46 1.4 0.30 Mohan and Jain

*Failure was not reached. +Sandy gravel with cobbles and approximately 50 percent silty clay, Ν > 45 blows per ft. (After Woodward, Gardner, and Greer)


4. Fr ic t ion piers should n o t be used at a site where ground wate r is high or where there is

the possibility of future high ground-water condi t ion .

An example of typical friction pier design is as follows:

Data

Pier capacity

Uplifting force

Stiff clay t o d e p t h 4 0 feet, no ground water

encounte red . Average pene t ra t ion resistance :

Average unconfined compressive

s t rength

Pier length

Pier d iamete r

Shear s t rength reduct ion factor :

Ul t imate skin friction

25 blows per foot

4 ,000 psf

20 feet

12 inches

0.5

4 ,000 x 0 . 5

Total load carrying capacity

= ( 2 0 - 5 ) x 1,000 x 3 .14

Using a factor of safety of 3

Design load

When designing for a dead load plus full live load, a

factor of safety of 2 can be used.

Then design load

1,000 psf

47 .1 kips

15.7 kips

Swelling pressure

Coefficient of uplift

Tota l uplift = 3.14 χ 10,000 χ 0.15 χ 5

Withholding force Ul t imate skin friction

Ul t imate wi thhold ing force

= ( 2 0 - 5 ) x 1,000 x 3 . 1 4

= 23.6 kips.

= 10,000 psf

= 0.15

= 23.5 kips

= 1,000 psf

= 47 .1 kips

The above calculat ions indicate tha t the design is safe as the wi thhold ing force is larger than

the uplifting force.

F A I L U R E O F T H E PIER SYSTEM

A proper ly designed drilled pier system involves the coord ina t ion of the floor slab, grade

beam, void space, re inforcement , expansion jo in t , and floor system. A typical detai l is shown on

figure 40 .

The grade beam and pier system offers the mos t logical solut ion for lightly loaded s t ructures

founded on expansive soils. However , if incorrect ly designed, or incorrect ly cons t ruc ted , a

building wi th a pier foundat ion is jus t as vulnerable, if no t more vulnerable, to m o v e m e n t than a

building founded on spread footings.


Figure 40. Typical detail of grade beam and pier system.

Considerable experience is required to de te rmine the cause of cracking of a building

founded on piers. Of tent imes , the cracks are caused by slab movemen t as discussed in chapter 6.

Typical pier uplift m o v e m e n t generally takes place a short dis tance from the pier and has a 45

degree pa t te rn . Cracks often are wider at the t o p and nar rower at the b o t t o m . Generally, the type

of cracking depends u p o n the s t ructural configurat ion of the building.

Masonry walls and cinderblock walls are mos t sensitive to movement . Consequent ly , the

first sign of pier movemen t will be reflected as cracks tha t develop in the brick wall as shown on

figures 41 and 4 2 . Basement walls are s tructural ly more resilient to differential movemen t than

masonry walls. When severe diagonal cracks appear in the basement as shown on figure 4 3 , pier

uplifting can be considered a cer ta inty.

DRILLE D PIE R FOUNDATION S 91

Figure 4 1 . Typical cracking caused by pier upl i f t .

Figure 42. Heaving of piers of a three-story structure. Insufficient pier length is the cause.


Figure 43. Typical cracks developing in the basement wall immediately beneath the window well. Note that crack is wide at the top and narrow at the bottom.

The mos t c o m m o n errors in design are insufficient pier length, excessive pier diameter , or

the absence of pier re inforcement . The mos t c o m m o n errors in cons t ruc t ion are excess concrete

on t op of pier resulting in m u s h r o o m s at the t op of the piers and the absence of, or defective air

space benea th the grade beams. These are discussed in detail as follows:

Excessive pier size

Many laymen have the impression tha t the larger the d iameter of the pier, the safer the

building. Actual ly, to exer t enough dead load pressure on the pier, it is necessary to use small

d iameter piers in combina t ion wi th long spans. The mos t economical spacing of the piers is

limited by the a m o u n t of re inforcement in the grade beams or the economical size of the floor

beams. Normal ly , piers should have a m i n i m u m spacing of 12 feet.

Most small drill rigs in the Rocky Mounta in area are equipped to drill 12-inch-diameter pier

holes. Auger sizes of 8 and 10 inches are also available. However, wi th pier holes less than 12

inches in d iameter , considerable difficulty can be encountered in cleaning the holes. A pier hole

with as little as 2 inches of loose soil at the b o t t o m will experience excessive se t t lement at a later


da te . The use of 12-inch-diameter piers for residential and light commercia l cons t ruc t ion is

r ecommended .

insufficient pier length

As explained in the previous sect ion, the stabil i ty of the pier against uplift depends u p o n the

a m o u n t of dead load pressure exer ted on the pier and the anchorage provided in the lower

por t ion of the pier. If the length of the pier is short , the possibility of the soil at the b o t t o m of

the pier becoming wet ted is great, thus the skin friction along the pier providing the anchorage

would be lost and the pier would have to depend u p o n dead load pressure alone t o resist

uplifting.

The dead load pressure for a lightly loaded building is n o t of sufficient magni tude to resist

the uplifting. This is especially t rue in the case of the interior , lightly loaded piers. Therefore , the

function of a shor t pier actually is n o different or more desirable than individual pad footings.

This is i l lustrated on figure 44 .

Fo r short piers, the uplifting pressure is the sum of the swelling pressure acting on the

b o t t o m of the pier, plus the uplifting pressure acting u p o n the per imeter of the pier.

Failure of the pier system due to insufficient pier length (fig. 45 ) is commonplace . The

practice of using piers of insufficient length is usually observed in areas where claystone shale is

near ground surface and the engineer specifies only the d e p t h of pene t ra t ion in to bedrock . F o r

example , it is c o m m o n in this area to specify a m i n i m u m bedrock pene t ra t ion of 4 feet. In m a n y

instances, this results in a pier wi th a tota l length of only 4 feet. There is a good possibili ty of

wet t ing of the ent i re length of such a pier and subsequent heaving of the pier.

Dead load pressure Dead load pressure

! \

t t

\ H U H Swelling pressure on sur face of pier

Swell ing pressure

Swel l ing pressure

SHORT P I E R FOUNDATION PAD F O U N D A T I O N

Figure 44. Swelling pressure acting on short pier foundation and pad foundation.


Figure 45. Typical example of short pier foundation (insufficient pier length). Pier drilled into highly weathered claystone. Note: Adjacent Sonotube placed for underpinning.

It is impor t an t tha t the engineer also specify the min imum total pier length in the

foundat ion system to insure tha t the piers are anchored sufficiently deep in the unwet t ed zone of

the bedrock .

Uniform pier diameter

After the pier hole is drilled, and dur ing the placing of concre te , excess concre te is usually

no t removed from the t o p of the pier, resulting in a m u s h r o o m occurring at the t op of the pier as

indicated on figures 46 and 4 7 .

At t imes, the mush room has been k n o w n to have a d iameter three t imes the d iameter of the

pier. Soil benea th the grade beams will exer t direct uplifting pressure on the unders ide of the

mush room. F o r a 12-inch-diameter pier wi th a 36-inch-diameter m u s h r o o m , the tota l area


GRADE BEAM Reinforcement

Void beneath grade beam Uplifting pressure exerted on mushroom of pier.

Figure 46. Effect of mushroom on the uplifting of the pier.

subjected t o uplifting is 6.25 square feet. For a modera te ly expansive soil having a swelling

pressure of 10,000 psf, the to ta l uplifting pressure exer ted on the mush room is about 6.2 kips.

This pressure alone will be sufficient to lift a pier provided it is no t adequate ly anchored .

It is r ecommended tha t Sonotubes , or a similar p roduc t , be used to form the upper por t ion

of the pier to assure uniform pier d iameter .

Pier reinforcement

Since the lower por t ion of the pier is anchored in to bedrock by skin friction, and the

uplifting pressure is acting u p o n the upper we t ted por t ion of the pier, tensile stress develops

wi thin the pier. The maximum, a m o u n t of tensile force developed can roughly be calculated as

follows:

T = 27rrsd -P

where : Τ = tota l tensile force in lbs, and

Ρ = to ta l dead load pressure exer ted on the pier in lbs.


Figure 47. Typical mushroom 26 inches in diameter on top of a 12-inch-diameter pier.

Fo r a 12-inch-diameter pier wi th a 15-kip dead load pressure having 4 feet of unwet ted

length, and assuming tha t the skin friction of c laystone is 2 ,000 psf, the to ta l possible uplifting

force is 10.1 kips or 89 psi. This stress can be taken by lightly reinforced piers; however,

w i t h o u t re inforcement the pier will fail in tension. The locat ion of the tension cracks is usually at

the b o u n d a r y of the we t ted and unwe t t ed po r t ion of the pier. This zone generally is located at

least 3 feet below grade beam; therefore , the normal dowel bars used in the piers will no t provide

the required resistance to tension. General ly, 0.6 t o 1.0 percent re inforcement is sufficient, bu t

there are cases when it is necessary to use as m u c h as 7 percent . Reinforcement of the full length

of the pier is essential to avoid tensile failure. A typical tension crack in a pier is shown on figure

4 8 .

Air space

T o prevent the lower soils from exert ing uplifting pressure on the grade beams, it is essential

t ha t there be n o con tac t be tween soil and grade beams. The required void space can be formed by

the use of sand, cardboard , or o the r similar mater ia l which can be removed after the grade beam


Figure 48. Tension crack developed at approximately 3 feet below grade beams.

is poured . The mos t convenient m e t h o d is by the use of a void-forming cardboard form k n o w n as

"Ver t i ce l " . The cardboard material is wrapped in plastic and has adequa te s trength t o suppor t

the concrete b u t will de ter iora te after the plastic is punc tu red as shown on figure 4 9 . It is n o t

necessary t o remove the cardboard material after the comple t ion of the grade beams. The

cardboard form material also p ro tec t s the backfill soils from plugging u p the void space. Figure


Figure 49. Deterioration of Vertical (void-forming cardboard) beneath the grade beam.

50 indicates the locat ion of the cardboard mater ia l . The thickness of air space provided by the

cardboard ranges from 3 to 4 inches. It is assumed that the a m o u n t of expansion of the soils

benea th the grade beam will n o t exceed 3 inches; however , there have been instal lat ions where a

4-inch air space was comple te ly closed by highly expansive soils. In ex t reme cases, it may be

necessary to provide a 6-inch air space.

Pier settlement

If piers are drilled deep in to bedrock to provide the necessary anchorage in a swelling soil

area, pier se t t lement should no t pose a problem. Under normal pier load, the magni tude of pier

se t t lement in compe ten t bedrock should be be tween 1/4 and 1/2 inches.

However, there are cases where excessive se t t lement of the pier has taken place resulting in

severe cracking of the building. These cases c o m m o n l y occur in small projects where there is no

cons t ruc t ion cont ro l and the pier driller de termines the length of the pier and the a m o u n t of

pene t ra t ion . If the pier is no t b o t t o m e d on bedrock but instead on the upper stiff clays, excessive

se t t lement can occur. Stiff clays somet imes have a strong resemblance to c laystone bedrock , and

a mis taken identif icat ion of the bearing soil occasionally occurs.

A small a m o u n t of dry or plastic cut t ings on the b o t t o m of a pier hole will no t affect the

bearing capacity of the pier. If, however, there are 1 or 2 inches of soft soils at the b o t t o m of the

pier hole and concre te is poured on the soft m u d , then excessive se t t lement can occur. Such

p h e n o m e n o n usually takes place in small-diameter piers (10 to 12 inches in d iameter ) where the

driller is unable to remove the mud accumula ted at the b o t t o m of the pier hole by spinning the

auger. Se t t lement of a single pier can usually be bridged by the adjacent piers and is usually

unnot iced . Bearing capaci ty of the pier is usually m u c h larger than the design bearing value. An


Figure 50. The use of Verticel (void-forming cardboard) beneath the grade beam.

except ion is the ch imney of a house . Chimney foundat ion pads are generally suppor ted by t w o to

three piers. Se t t lement of one pier can cause visible lifting and separat ion of the ch imney as

shown on figure 5 1 .

Void in pier shaft

Voids in pier shafts have been of great concern t o foundat ion engineers since the p rob lem

developed in a major s t ruc ture in Chicago, 111. Voids or discont inui t ies in the pier shaft often

result when a concre te which is t oo stiff is used. In addi t ion , voids are caused by collapse of the


Figure 51. Separation between chimney and house caused by pier settlement.

casing, by squeezing of the soft format ion , or by hang-up of concre te in the casing while being

pulled.

Fo r tuna te ly , piers drilled in expansive soil areas often encoun te r stiff clays and the problem

of squeezing of the soft format ion seldom takes place. However, cases are k n o w n where

reinforcing cage was inserted in to cased small-diameter piers wi th stiff concre te resulting in voids.

Uplifting of foundation walls

Theoret ical ly, wi th a carefully designed grade beam and pier system, there should be n o

movement of a building even under severe wet t ing condi t ions . However, an impor tan t factor

cont r ibut ing t o the movemen t of a building which is generally ignored is the possibility of

uplifting pressure exer ted on the exter ior surface of the basement walls and grade beams.


Assuming tha t the soil has a swelling pressure of 10,000 psf and backfill is in con tac t wi th 8

feet of height of the exter ior wall, then the con tac t area be tween the soil and concre te is 8 square

feet per foot of wall. In ex t r eme cases wi th comple te wet t ing , the uplfit ing pressure along the

face of the concre te basement wall will be 10,000 χ 0.15 χ 8 = 12,000 lbs per running foot of the

wall. With piers at 15-foot intervals, the uplifting pressure exer ted on each pier would be abou t

180 kips.

The above calculation is based on an ex t reme case. In actual condi t ions , such an uplifting

pressure seldom occurs because the backfill a round the basement walls is loosely compac ted and

complete wet t ing of the backfill rarely takes place. However, swelling pressure acting along the

face of the basement walls canno t be ignored. All backfill material a round the basement walls, in

expansive soil areas, should consist of nonexpansive soils t o minimize the risk of wall uplift .

Lateral pressure on foundation walls

Backfill against the basement wall no t only exerts uplifting pressure on the wall, b u t also

exerts full hor izonta l expans ion pressure against the wall. This pressure equals at least the full

swelling pressure of the soil. In an 8-foot basement backfilled wi th expansive soils, and wi th an

expansion pressure of 10 ,000 psf, the a m o u n t of lateral pressure exer ted on the wall can reach as

high as 80 kips per running foot of the wall. It should be no ted tha t the lateral pressure due t o

swelling differs from active ear th pressure in tha t it is of uniform in tens i ty , depending only u p o n

the dep th of wet t ing.

Lateral m o v e m e n t of t h e basement walls is prevented at t he t o p by floor jois ts which are

anchored t o the wall by means of wooden sills and anchor bol ts and at the b o t t o m of floor slabs.

The F H A Specifications call for the use of 1 /2-inch bolts embedded no t less than 6 inches wi th a

m a x i m u m spacing n o t less than 4 feet. The size and spacing of bol ts is insufficient t o prevent

lateral wall movemen t . Several houses were examined where the long side of the basement wall

actually assumed a b o w shape wi th m a x i m u m deflect ion in excess of 3 inches and all of the

anchor bol ts were b e n t and pushed in.

To reduce such lateral movemen t , selected backfill material is desirable. In all cases,

backfill material should be nonexpansive and impervious. Nonexpansive material will minimize

the expansive pressure exer ted on the wall, and at the same t ime , well compac ted , impervious

backfill will prevent surface water from seeping th rough the backfill in to the foundat ion soils.

Rise of ground water

The theory behind the drilled pier system is tha t there must be sufficient dead load pressure

and the pier must be long enough so tha t the lower par t of the pier is embedded in a zone

unaffected by mois ture change. Assuming tha t surface water will n o t pene t ra te more than abou t

15 feet, a 20-foot-long, heavily loaded pier should theoret ical ly be free from any possible

movement .

The except ion being the rise of ground water where the once dry por t ion of soil sur rounding

the pier becomes comple te ly sa tura ted . The skin friction used for pier wi thhold ing is n o w

complete ly lost, and the pier will lift.


Rising of ground water can somet imes cause expansion of soil in an otherwise relatively

stable soil area. In the Cherry Creek Dam area of Denver, houses generally were founded wi th

spread footings and had n o expansion prob lems for m a n y years. In 1965, during the area flood,

the water level at Cherry Creek Dam reached an all t ime high and the ground-water level in the

surrounding area rose. Subsequent ly , m a n y cases of cracked houses were repor ted . Such

foundat ion movemen t was directly a t t r ibuted to the rise of ground water .

R E F E R E N C E S

[43] Woodward, R., Gardner, W. S. and Greer, D. M., "Drilled Pier Foundation," McGraw-Hill Book Company,

1972.

[44] Parcher, J. V. and Liu, P. C , "Some Swelling Characteristics of Compacted Clays," Journal of the Soil

Mechanics & Foundation Division, ASCE, Vol. 91 , pp. 1-17.

[45] Mohan, D. and Chandra, S., "Frictional Resistance of Bored Piles in Expansive Clays," Geotechnique, Vol.

XI, No. 4, pp. 294-301.

[46] Seed, H. B., Mitchell, J. K. and Chan, C. K., "Studies of Swell and Swelling Pressure Characteristics of

Compacted Clays," Highway Research Board Bulletin 313.

[47] Teng, W. C , "Foundation Design," Prentice-Hall, Inc. 1962.

Chapter 5

FOOTING FOUNDATIONS

INTRODUCTION

Foot ing foundat ions can be successfully placed on expansive soil provided one or more of

the following criteria are me t :

1. Sufficient dead load pressure is exer ted on the foundat ion ,

2. The s t ructure is rigid enough so that differential heaving will no t cause cracking, or

3 . The swelling poten t ia l of the foundat ion soils can be el iminated or reduced.

CONTINUOUS F O O T I N G S

The mos t c o m m o n type of foundat ion for lightly loaded s t ructures is the con t inuous

footings. Local building codes somet imes specify the min imum allowable wid th of footing as 20

inches which is n o t applicable for footings which are to be placed on expansive soils. To

concent ra te sufficient dead load pressure on expansive soils, the wid th of the footing should be as

narrow as possible.

It should be no ted tha t con t inuous spread footings canno t be expected to function well in

highly expansive soil areas. The use of this system should be l imited to soils wi th a low degree of

expansion; those having a swelling potent ia l of less than 1 percent and a swelling pressure of less

than 3 ,000 psf.

Generally, the dead load pressure exer ted on a con t inuous foundat ion is low and in the

following range:

To insure tha t a dead load pressure of at least 1,000 psf is exerted on the soil, it will be

necessary to use very narrow footings, in mos t cases less than 12 inches wide.

Wall footings

Engineers often specify the erect ion of basement walls direct ly on the soil w i thou t the use

of footings. This reduces the bearing wid th to abou t 9 inches and increases considerably the uni t

dead load pressure exer ted on the soils. Such a concept is sound from the expansive soil

Single s tory schools

Basement house

Butler type building

2 ,000 to 4 ,000 lb. per ft.

1,000 to 1,500 lb. per ft.

< 500 lb. per ft.


s tandpoin t . However , care should be exercised to insure the rigidity of the system by checking

the following condi t ions before cons t ruc t ion begins:

1. De te rmine if there are any soft pocke t s in the excavat ion tha t may in t roduce se t t lement ,

2 . Insure tha t there is sufficient con t inuous re inforcement in the foundat ion wall to provide

rigidity, and

3 . Make sure tha t the walls are proper ly restrained against ear th pressure.

An ext reme case recent ly occurred which involved a wall bearing directly u p o n expansive

soil. The upper wall heaved and impar ted hor izonta l pressure to the basement wall resulting in

heavy bowing of the wall even before the house was comple ted (figs. 52 and 53) .

Box construction

The use of heavy re inforcement in the foundat ion wall can p ro tec t the s t ruc ture from

cracking due to differential heaving. The average height of a concre te basement wall is 6 feet.

Such walls can span an unsuppor t ed length of at least 10 feet, and can therefore tolerate

considerable differential movemen t w i thou t exhibi t ing cracks. Weak points do appear at po in ts of

d iscont inui ty , such as doors , deep windows, and change of elevation.

Box cons t ruc t ion is based u p o n the principle tha t there is n o discont inui ty of s t ruc ture ;

therefore, there are n o weak sections. Box const ruct ion is economical for s t ructures having

simple configurations. F o r split-level residential houses or basements with walk-out doors , such

cons t ruc t ion is more difficult. Considerat ion should then be given t o the use of a cons t ruc t ion

jo in t t o separate the s t ruc ture i n to t w o o r more uni ts . Each uni t will then act independen t ly and

differential movemen t can be confined to the jo in ts .

S. Shraga and D. Amir [48] repor ted the use of box cons t ruc t ion in Kibutz Gat , Israel,

where the s t ructure consists of t w o reinforced concrete boxes each about 22 by 35 feet in

dimension. The s t ruc ture did n o t exhibi t damage after 17 years despi te the considerable

differential movements of u p t o 5 inches be tween the corners of individual boxes . Shraga and

Amir concluded tha t b o x cons t ruct ion can structural ly wi ths tand movemen t and tension wi thou t

cracking.

Masonry bricks and cinder blocks canno t wi ths tand movement and should no t be used for

foundat ion walls. The small saving derived from using masonry cons t ruc t ion instead of concrete

foundat ion walls may later result in heavy loss of p rope r ty in the event of foundat ion movement .

Reinforced br ickwork has been widely used in South Africa. D. L. Webb [49] repor ted the

use of re inforcement in the external wall panels be tween jo in ts to resist bending stresses and

shear stresses resulting from foundat ion movement . The arrangement is shown on figure 54.

PAD F O U N D A T I O N S

The pad foundat ion system consists essentially of a series of individual footing pads placed

on the upper soils and spanned by grade beams. The principle of a pad foundat ion system is

similar t o tha t of a drilled pier foundat ion in tha t the load of the s t ructure is concent ra ted at

several points , the difference being tha t the pads bear on the upper soils and skin friction is no t

involved.

FOOTING FOUNDATIONS 105

Swelling Pressure

SECTION

t 1

Side W a l l ^

Basement

I

Basement

I

1 PLAN VIEW

Figure 52. Plan and section of foundation wall bearing directly on expansive soil. Note pressure distribution.


Figure 53. Foundation wall bearing directly on expansive soil without footings. Heaving has pushed the side wall toward the basement wall resulting in heavy bowing of the basement wall.

Under the following condi t ions , the use of a pad foundat ion system can be advantageous:

1. Where bedrock or bearing s t ra tum is deep and cannot be economical ly reached by drilled

piers,

2 . Where the water table or a soft layer exists preventing the use of a friction pier,

3 . Where the upper soils possess modera te swell potent ia l , and

4. Where the bearing capacity of the upper soils is relatively high.

Design

By loading an expansive soil so tha t the pressure exer ted on the soil is greater than the swell-

ing pressure of the soil, heaving movement can be prevented.

By using an individual pad foundat ion system, it is theoret ical ly possible t o exer t any desirable

dead load pressure. Actually, the capacity of the pad is l imited by the allowable bearing capacity

of the foundat ion soils. If a pad is founded directly on bedrock , m a x i m u m allowable soil pressure

will n o t pose a problem. However, if the pads are placed on stiff swelling clays, the max imum

bearing capacity of the pad is l imited by the unconfined compressive s t rength of clay. Generally,

the m a x i m u m bearing capacity should be abou t 5 ,000 psf. Consequent ly , the practical dead load

pressure tha t can be applied t o the pad is abou t 3 ,000 psf (considering the rat io of dead and live

load t o be abou t 2 to 3) . Occasionally, pads founded on clay are designed t o wi ths tand a dead

F O O T I N G F O U N D A T I O N S 107

Figure 54. Section through externally reinforced brick wall. (After D. L. Webb)

load pressure as high as 5 ,000 psf. With this l imitat ion, an individual pad foundat ion system can

only be used in those areas where the soils possess only a medium degree of expansion with

volume change—on the order of 1 to 5 percent and a swelling pressure in the range of 3 ,000 to

5,000 psf.

T o allow for the concen t ra t ion of dead load pressure on the individual pads , a void space is

required benea th the grade beam and should be cons t ruc ted in the same manne r as grade beams

and pier system (fig. 55) . Figure 56 shows a grade beam and pad foundat ion system tha t failed

because the dead load pressure was no t sufficient to prevent the heaving of the foundat ion soils.

Peck, Hanson & T h o r n b u r n stated in the second edi t ion of F o u n d a t i o n Engineering tha t ,

"Swelling can be prevented only in a localized zone benea th the footings or piers where the

stressed inducted by the foundat ion are concen t r a t ed . " This is shown on figure 57 .


Figure 55. Grade beams and pads constructed with void space between pads.

At a comparat ively shallow dep th benea th the foundat ion , the intensi ty of added stress is

small and swelling may occur below this level, even if it is entirely prevented above. In the area

be tween the footings, swelling is undiminished.

Deep pads

In areas where the layer of swelling soils is relatively th in , deep individual pads placed on

nonswelling soil can be economical ly used. A typical example is where 2 to 3 feet of swelling

clays are underla in by sand and gravel or by nonswell ing bedrock such as granite or sandstone.

Pads placed as deep as 5 feet below the ground surface can be economical ly used in areas

where drill rigs are unavailable. Care should be exercised to insure that uplifting pressure will no t

be exerted on the sides of t he pad. The excavation should be larger than the footing pad and the

space be tween the concrete and the soil filled wi th loose backfill.

The use of a deep pad system usually applies to cons t ruct ion areas where the problem soil

ranges in thickness from 0 to 5 feet. In such cases, it is desirable t o place all footing pads on

uniform nonswelling soils.

In those parts of the world where hand labor is inexpensive and drilling equ ipment no t

readily available, the use of a deep pad system can be an advantage from a cost considerat ion.


Figure 56. Typical crack which developed in the basement of house founded with grade beams and individual pads. Dead load pressure was not sufficient to prevent pad uplift.

Interrupted footing

In ter rupted footings are used in conjunct ion wi th a wall footing system. With foundat ion

walls bearing direct ly on swelling soil, the m a x i m u m unit dead load pressure exer ted on the soil is

about 2 ,000 lbs. per ft. By placing a void space at intervals, the bearing area will be decreased,

thus increasing the dead load pressure. In this manner , the dead load pressure exerted on the soil

can be easily doub led .

This principle of in te r rup ted footings has been successfully applied to the correct ion of

cracked buildings having a con t inuous footing foundat ion . By in t roducing some void space

benea th the footings, the dead load pressure can be substantial ly increased, thus prevent ing

further foundat ion movements . (See Case Study IV for i l lustrations)

F O O T I N G S ON SELECTED F I L L

The removal of na tura l expansive soils and their replacement wi th nonexpansive soil is the

mos t obvious m e t h o d of preventing s t ructural damage due to soil heaving. In a few cases, it m a y

be possible to comple te ly remove the expansive strata, thus el iminating the heaving prob lem. In

mos t cases, the expansive mater ial ex tends to t o o great a d e p t h to allow comple te removal and


Figure 57. Diagram illustrating influence on swelling of high contact pressure beneath footing. If net pressure at base of footing is 8,000 Ib/sq ft and swelling pressure at zero volume change is 2,000 Ib/sq ft, swelling will be prevented within shaded areas only. (After Peck, Hanson & Thornburn)

backfill. The problem is t hen t o de te rmine the a m o u n t of excavat ion and the type of backfill

required to prevent heaving. Detail discussion is given in chap te r 8 unde r "Soil Rep lacemen t . "

Probably the most impor t an t single factor affecting the success of footings placed on

selected fill is the drainage control used during cons t ruc t ion . If the excavation is wet ted

excessively before the p lacement of the selected fill, the imbibed mois ture in the soil will cause

the soil t o swell and heave and exer t pressure against the selected fill resulting in severe damage

to the s t ructure . Many school buildings have been successfully placed on selected fill, b o t h for

the ent ire system and for slabs alone. Coincidental ly , there have also been failures when such

cons t ruct ion has been used, mainly because the site was flooded during cons t ruc t ion .

For success in placing footings and slabs on selected fill, the following precaut ions should be

observed:

1. There should be at least 3 feet of selected fill benea th the b o t t o m of footings and slabs.

2. The fill should ex tend beyond the building line for a dis tance of at least 10 feet in every

direct ion.

3 . The fill should consist of nonexpansive soil, preferably impervious and granular.

4 . The fill should be compacted t o at least 90 percent s tandard Proc to r densi ty for

support ing slabs and 100 percent s tandard Proc tor densi ty for suppor t ing footings.

5. Before the p lacement of fill, care should be taken to avoid the excessive wet t ing of

natural soils.


MAT F O U N D A T I O N

Mat foundat ions , somet imes referred to as s t ructural slab-on-ground or reinforced and

stiffened slabs, are considered to be b o t h a load suppor t ing as well as a separating e lement . The

slab receives and t ransmits all the s t ructural load to the underslab soils. The slab should be

designed to resist b o t h the positive and the negative m o m e n t . Positive m o m e n t includes that

induced by b o t h dead and live load pressure exer ted on the slab. Negative m o m e n t consists

mainly of those pressures caused by the swelling of the underslab soils. Since the swelling

pressure in an expansive soil area can reach m a n y thousand pounds per square foot, negative

m o m e n t considerat ion generally controls the design of the mat foundat ion. If all s t ructural

e lements are t o be placed on a stiffened slab, t hen slab movemen t will no t affect the stability of

the s t ructure . However, there could be tilting of the mat , bu t the performance of the building

would no t be s tructural ly affected. Such concept ion has been studied by the "Building Research

Advisory Board" [ 5 0 ] . A s tudy on the work in the Rocky Mounta in areas indicates tha t there

are l imitat ions on the use of such a system as follows:

1. The success of such system so far is limited to modera te swelling soil areas.

2. Configurat ion of building mus t be relatively simple.

3 . The load exerted on the foundat ion mus t be light. Past performance has been limited t o

residential cons t ruc t ion .

4. Single level cons t ruc t ion is required. It would be difficult t o apply such cons t ruc t ion to

basement houses wi th an a t tached garage or split level houses.

Design

The design of a ma t foundat ion is generally based on the following parameters :

1. Slab dimensions ,

2. The suppor t index, and

3 . The dead and live load acting on the slab.

From the above parameters , the designer must develop a s t ructure capable of satisfying the

shear, bending m o m e n t , and deflection condi t ions .

The first s tep in the design is to de te rmine the suppor t index. The suppor t index is based

upon the climatic rat ing, plasticity index, and length-to-width rat io of the foundat ion .

It is assumed that the mois ture con ten t in the soil is affected by climatic condi t ions , and the

volume change of the soil is affected by mois ture con ten t . Consequent ly , b o t h the swelling and

the se t t lement of the soil will be affected by cl imate. A s tudy of weather da ta indicates tha t the

yearly annual precipi ta t ion, d is t r ibut ion of precipi ta t ion, f requency of precipi ta t ion, dura t ion of

precipi ta t ion, and a m o u n t of each precipi ta t ion all affect the consistency of c l imate. Based on

data obta ined from 122 weather s ta t ions , the U. S. Nat ional Weather Service has developed

informat ion which has been t ransformed in to frequency isolines on a map of the Cont inenta l

United States as shown on figure 58 . F rom figure 58 , the climate rating C w is selected. Fo r

instance, in Colorado , the cl imate rating is be tween 20 and 25 .


Figure 58. Climatic ratings C w for Continental United States, (after Federal Housing Administration)

The second major factor necessary for design is the suppor t index. The suppor t index is

direct ly related to the cl imate factor and the soil proper t ies . Figure 59 shows the relat ionship of

the various proper t ies . The soil proper t ies are related t o the At terberg limits, percent swell in the

PVC meter , and swell index. Of these, the swell index is the mos t reliable factor of the three for

predict ing potent ia l volume change of the foundat ion soils.

Soils wi th identical plastici ty index exhibi t greatly varying swell potent ia l . Also, the PVC

meter is based on test ing soils in a remolded state which can material ly differ from that in the

undis turbed s ta te .

T o obtain the swell index, the percentage swell for a specific soil s t ra tum should be obta ined

th rough swell tests using convent ional consol idometer test equ ipment on undis turbed soil

samples and pressure corresponding to the in situ overburden pressure plus the average of the

to ta l dead and live loads on the slab. The undis turbed samples should be obta ined under soil

mois ture condi t ions representat ive of condi t ions prevailing at the t ime of cons t ruc t ion .

With the swell index or the percent of swell under specific loading condi t ion and the climate

rating de te rmined , the suppor t index can then be de te rmined from figure 59 . The design of the

stiffened slab section will be based on the value of the suppor t index.

R. L. L y t t o n and J. A. Woodburn [51] of Texas A & M University have performed

considerable research on the design procedure for stiffened mats on expansive clay. Ly t ton and


ΙΟ I

20 1

30 1

4 0 1

50 1

60 70 "Ρ 9p PI

I.I I

3.0 1

4.8 1

6 4 1

7.7 1

8.9 1

10.0 1

n o I

12.0 PVC

1.0 2.0 1

3.0 1

4.0 1

5.0 1

6.0 7.0 1

8 0 1

9.0 1

Swell Index (%)

Figure 59. Support index C based upon criterion for soil sensitivity and climatic rating C w. (after Federal Housing Administration)

Woodburn de te rmined the suppor t index algebraically from the average foundat ion pressure,

subgrade modu lus , m a x i m u m expected differential heave of the soil and the m o u n d e d area.

Ly t t on and Woodburn prepared a nomograph for de termining the suppor t index as shown on

figure 60 .

With the suppor t index de te rmined , the design of the m a t foundat ion is within the realm of

a s t ructural engineer. Typical m a t foundat ion design is shown on figure 6 1 .

Behavior

Stiffened slab cons t ruc t ion has been widely used in southern Texas where mode ra t e swelling

soils are encounte red . The so-called waffle slabs have been in use in San An ton io for m o r e than

25 years and are also required for F.H.A.-sponsored cons t ruc t ion in Montgomery , Alabama.

In Denver, such foundat ion system was first considered in 1970. Subsequent ly , 52 houses

were buil t using waffled slabs in West Field Park, Jefferson C o u n t y . The soils are generally stiff

clays wi th a plasticity index ranging from a low of 3.6 to a high of 3 2 . 1 . The swell potent ia l

ranged from a low of 1 to a high of 5.5 percent u n d e r l o a d s ranging from 500 to 1,000 psf. The

soils are considered to have modera t e swell potent ia l .

In the same year, 12 houses were buil t using the stiffened slab system in Lake Arbor

Subdivision in Nor th Denver. The soils in the Lake Arbor area possess m u c h higher swell

potent ia l than tha t of West Field Park, exhibi t ing swelling pressure as high as 10,000 psf. Typical

design and cons t ruc t ion details of these houses are given in figures 62 th rough 68 .

A survey of these houses was made in 1974 and their condi t ion was excellent . None of these

houses exhibi ted not iceable cracks. Differential elevation be tween the opposi te corners of the

house in some cases reached 1 inch, b u t distress, e i ther in the interior or the exter ior , has n o t


Figure 60. Support index nomograph (After R. L. Lytton and J. A. Woodburn).

Figure 61. Typical mat foundation design. (Sheet 1 of 2)

taken place. In the same Lake Arbor area, some o the r houses have required replacement of their

basement floor slab three t imes within 4 years. This indicates tha t the stiffened slab system used

for the 12 houses appears t o be highly successful.

In the Denver area dur ing 1970 to 1 9 7 1 , the increased cost of using stiffened slabs ra ther

than convent ional pier and grade beam system was abou t 50 cents per square foot . This amoun t s

t o an increase in cost of about $750 per house which is negligible when compared to the

problems and rehabi l i ta t ion costs encountered where the stiffened slab system had n o t been used.

As stated in the opening remarks of the advisory board [ 5 0 1 , " I t is recognized tha t

experience and the state of engineering knowledge are such tha t precise answers to m a n y of the

problems posed mus t , of necessi ty, be considered b e y o n d a t t a inmen t in the immedia te foreseeable


SECTION A SECTION Β

- O N E 3/β β STRAND

SECTION C SECTION D

TOTAL 3 TENDONS

IN FIREPLACE

TWO **5 BARS

6"xl8" CURTAIN WALL

SECTION Ε

Figure 61. Typical mat foundation design. (Sheet 2 of 2)

FOOTIN G FOUNDATION S 117

Figure 62. Trenching the cross-beams.

Figure 63. Placing reinforcement.


Figure 64. Post-tensioning.

Figure 65. Placing concrete.


Figure 66. Completed mat.

Figure 67. Interior partitions.


Figure 68. Completed residence founded on mat foundation

foreseeable future. Nevertheless, the approach recommended herein is considered to be

sufficiently valid to warrant applicat ion n o w . "

Based on our experience and performance relating to the Denver project , the stiffened slab

system of cons t ruc t ion can be successfully applied to low to modera t e swelling soil areas. Much

research will be required t o de te rmine and unders tand the m a n y variables, especially the

relationship, of swelling characteristics wi th suppor t index. As discussed previously in "Design",

the suppor t index should be related to swelling pressure. Thus , the loading condi t ion can be

eliminated from the design as well as the climatic rating.

R E F E R E N C E S

[48] Shraga, S., Amir, D., and Kassiff, G., "Review of Foundation Practice for Kibbutz Dwelling in Expansive

Clay." Proceedings of the Third International Conference on Expansive Soils, 1973.

[49] Webb, D. L., "Foundations and Structural Treatment of Buildings on Expansive Clay in South Africa,"

Second International Research and Engineering Conference on Expansive Clay Soils, Texas A & M Press,

1966.

[50] "Criteria for Selection and Design of Residential Slabs-on-Ground," Building Research Advisory Board.

[51] Lytton, R. L. and Woodburn, J. Α., "Design and Performance of Mat Foundations on Expansive Clay,"

Proceedings of the Third International Conference on Expansive Soils, 1973.

Chapter 6

SLABS ON EXPANSIVE SOILS

INTRODUCTION

Slab-on-ground cons t ruc t ion , when on expansive soils, is a very difficult aspect to cont ro l .

In the category of slabs are inter ior floor slabs, exter ior sidewalks or aprons , and pa t io slabs.

General ly, floor slabs d o no t suppor t any appreciable live load, and the dead load actually

exerted on the slab is small. Consequent ly , movemen t of the slab is t o be expected when the

underslab mois ture con ten t increases, and it should be designed accordingly. T h e m o v e m e n t of

slabs no t only presents unsightly cracks bu t , in mos t cases, also direct ly affects the stabili ty of

the s t ructure .

SLAB-ON-GROUND

Concrete slabs, placed direct ly on the ground, are m u c h less expensive than s t ructural floor

slabs or "crawl space" type cons t ruc t ion . This is especially t rue where basement cons t ruc t ion is

involved. Since 1940, mos t of the residential houses , school buildings, industrial , and warehouse

s t ructures call for the use of slab-on-ground cons t ruc t ion . It was no t unt i l the discovery of the

expansive soil problem that engineers began to ques t ion the wisdom of using slab-on-ground

cons t ruc t ion .

Types of slab-on-ground

Slab-on-ground, somet imes referred t o as slab-on-grade, are concre te slabs placed direct ly on

the ground wi th little considerat ion given t o their s t ructural requi rements . These slabs are

const ructed b o t h wi th or w i t h o u t re inforcement .

T h e unreinforced slabs are generally const ructed in residential houses or where light floor

load is expected . The limits of the length of the unreinforced slab are based u p o n the a m o u n t of

shrinkage cracking cont ro l desired. Normal ly , shrinkage cracks are control led by designed

weakened plane jo in ts .

A lightly reinforced slab is normal ly reinforced wi th t empera tu re cont ro l as a pr ime design

factor. The Port land Cement Association [52] r ecommended the use of a 4-inch-thick slab

reinforced wi th 6x6 - 10/10 mesh or No . 3 bar at 24 inches on center each way for slabs placed in

modera te ly swelling soil areas. Fo r high swelling soil areas, the Association r ecommended the use

of 6x6 - 6/6 mesh or N o . 3 bar at 18 inches on center each way .

The choice be tween an unreinforced slab and a lightly reinforced slab depends upon the

subsoil condi t ions as well as the loading condi t ions . Reinforcement in the slab will reduce the


opening of t empera tu re cracks bu t will no t prevent cracking of the slab caused by heaving of the

underslab soils.

In 1968, a repor t concerning residential slab-on-ground cons t ruc t ion was prepared by the

Building Research Advisory Board [50] for use by the Federal Housing Adminis t ra t ion which

provided criteria for the selection and design of residential floor slabs. The repor t r ecommended

the use of unreinforced concre te slabs for firm, nonexpansive soils. Nomina l re inforcement was

recommended where the subgrade m a y undergo slight movemen t . Bo th reinforced and

unreinforced slabs are considered to have the limiting function of separating the ground from

living space.

Slab-on-ground cons t ruc t ion on expansive soil will always pose a cracking and heaving

problem unless the subgrade soils are t reated or replaced. In commercial buildings such as

warehouses and storage areas, where floor loads as high as 3 ,000 psf are expected , special design

will be required, no t only from the s tandpoin t of expansive soils, b u t also to mainta in the

s t ructural integrity of the building. Minor floor cracking of slab-on-ground cons t ruc t ion is

difficult if no t impossible to prevent .

Slab movement

In expansive soil areas, floor movemen t is invariably associated wi th the increase of mois ture

con ten t of the underslab soils. The source of water tha t enters in to the underslab soils can

generally be associated wi th the following:

1. Rise of ground water , usually perched water , can cause excessive swelling. Heaving of

slabs, in excess of 6 inches, is no t u n c o m m o n as shown on figure 69 . Water marks and

severe floor cracks indicate the ex ten t of damage.

2. Broken uti l i ty lines often cont r ibu te wate r to the underslab soils. Water and sewer lines

buried in expansive soils are subject to stress. Differential heaving can break pipes and

cause leakage. Such leakage can cont inue for a long period of t ime wi thou t being

de tec ted . In one case, the con t rac to r neglected to connec t inter ior sewer line to the street

sewer, and this fault wen t unde tec ted unti l extensive damage had taken place. Figure 70

shows floor m o v e m e n t in a boiler r o o m . The floor drain to the boiler room became

plugged resulting in severe slab heaving from uplift.

3 . A most c o m m o n source of mois ture entering the underslab soils is derived from irrigation,

lawn watering, and roof downsprou t s . Surface water enters the loose backfill and causes a

wet t ing condi t ion.

The above sources of water that enter the underslab soils are the obvious ones. Moisture

migrat ion due t o thermal differential as ment ioned in chapter 2 can also cause damage to

slab-on-ground wi thou t the observance of free water .

F loor cracking caused by swelling soils mus t be differentiated from that caused by shrinkage

of concrete . In an expansive soil area, soil heaving is unjustly blamed for all cracks tha t develop in

a floor. F loor cracks due to heaving generally take place along the bearing wall as shown in figure

69 . In the absence of jo in ts , shrinkage cracks can take place at approx imate equally spaced

intervals. For concre te floors covering a large area, the Port land Cement Association recommends

the installation of cont ro l jo in ts at intervals of approximat ley 20 feet. Isolation jo ints separating


•

Figure 69. Differential slab heaving of up to 12 inches in a newly completed basement.

123


Figure 70. Heaving of a floor slab in a boiler room. Source of water derived from inadequate floor drain system. Uplift is 2 inches.

concrete slabs from columns, footings, or walls to permi t b o t h hor izonta l movemen t due to

vo lume changes and vertical m o v e m e n t caused by differential se t t lement or heaving are also

r ecommended .

Curling of concre te slabs in large floor areas due to improper curing is no t u n c o m m o n .

Concre te curling has a s t rong resemblance t o uplift of slabs due to heaving of unders lab soils.

Underslab gravel

Convent ional slab cons t ruc t ion uses 4 inches of gravel benea th all concre te floors. A widely

accepted theory pertaining to slab-on-ground cons t ruc t ion in expansive soil areas is that water

from a single source, such as from a b roken pipe or from an improper ly located downspou t , will

travel w i thou t resistance th roughou t the gravel bed and saturate the entire area undernea th the

slab. Therefore , more extensive damage to the floor will take place when the gravel is used. T o

da te , this theory has n o t been proven.

The use of gravel benea th the slab allows the uniform dis t r ibut ion of floor load and the

uniform curing of concre te , thus reducing the shrinkage cracks and somet imes the curling of

concrete . The main advantage of using gravel benea th the floor slab, however , is to p ro tec t the

building from the rise of ground water . If a perched water condi t ion develops benea th a basement

which has nei ther a subdrainage system nor a gravel bed benea th the slab, there is no easy m e t h o d

of removing the water . The installat ion of a subdrainage system inside or outs ide of a comple ted

building is a major under taking. If, however, free draining gravel has been previously installed

SLABS ON EXPANSIVE SOILS 125

benea th the slab, it m a y only be necessary to install a sump p u m p in the basement as the water

will flow through the gravel toward the sump.

In any event, the advantages of providing gravel benea th the slab far exceeds any possible

disadvantages.

S T I F F E N E D SLABS

Slab-on-ground cons t ruc t ion cannot be safely used in an area where the subsoil possesses

high swell potent ia l . Fo r m a n y years , b o t h s t ructural and soil engineers a t t empted to devise an

economical floor system which would comba t the problem of swelling soil. Unfor tuna te ly , such a

system has no t been devised to da te . The systems now include the s t ructura l floor slab, the raised

floor system, and the h o n e y c o m b system.

Structural floor slabs

The best m e t h o d to prevent floor movemen t is t o cons t ruc t a s t ructural slab suppor ted on

each side by grade beams and provide a void benea th the slab t o prevent contac t be tween the soil

and the slab. T h e shor tcomings of this system lie no t only in the cost of cons t ruc t ion , which is

m u c h m o r e expensive than the convent ional slab-on-ground me thod , bu t also u p o n the

cons t ruc t ion technique .

The mos t convenient cons t ruc t ion m e t h o d is to provide a crawl space benea th the slab. This

can be readily provided in major s t ructures such as schools and office buildings. The crawl space

provides access for inspect ion, can be venti lated, and can also serve as a convenient area for

uti l i ty pipes and condui ts . Ei ther t imber or concre te floors can be used in this type of

cons t ruc t ion .

Oftent imes, it is no t possible t o const ruct a crawl space, and the s t ructura l floor mus t then

be cons t ruc ted wi th only a few inches of air space be tween the slab and ground. This is typical

where a s t ructural slab is to be cons t ruc ted in a basement area. The problem wi th this t ype of

cons t ruc t ion is tha t of providing a forming material to allow the placing of concre te . The use of

Verticel, J-void, or o ther forming material similar t o tha t used benea th the grade beams in the

pier foundat ion is satisfactory. Forming materials are costly and there is no assurance tha t the

mater ial will comple te ly de ter iora te benea th the slab, thereby allowing the bui ld-up of uplifting

pressure. One possible alternative is the use of bal loons as a forming material which could be

deflated after the concre te has reached its initial set. Commercia l prestressed, hollow core, flat

slabs are available in sufficient length to span 20 feet, thereby eliminating the need for void

forming material . The use of prestressed slabs in large quant i t ies can prove to be economical .

Raised floor system

The Port land Cement Association [52] has approached the problem of the cons t ruc t ion of a

s t ructural floor slab on expansive soils by utilizing a concre te floor raised above grade by

intersecting concre te ribs formed in a waffle pa t t e rn .


The raised floor system is cons t ruc ted by placing Verticel or J-void (waxed cardboard

boxes) u p o n a level subgrade. The spaces be tween the boxes conta in reinforcing and

form-support ing concrete ribs. The actual floor slab, containing wire fabric, is placed over the

suppor t ing ribs and cardboard boxes in a monol i th ic concrete p lacement . A typical plan and

cross-section is shown on figure 7 1 .

The spacing of the ribs and the thickness of the slab depends upon the swelling potent ia l of

the surface soils and the dead load imposed on the waffle s t ructure . The advantage of such a

system is tha t it offers a clear, rat ional approach for the s tructural engineers, and the formed

voids provide a means of relieving upward swelling pressure. The system can also incorporate

uti l i ty routes , such as heat ing and cooling, th rough the floor. The disadvantage of such a floor

system is the inability t o exer t sufficient dead load pressure upon the ribs t o counterac t the

swelling pressure. This t ype of const ruct ion is also expensive. In addi t ion, the floor area must be

very finely graded to provide a level base for the void forming material so that uniform

thicknesses result. This grading is an addit ional cost.

— — i 1 — ' ' ,.' 1 1

TYPICAL 32 SQ. BOXES

EVERY 3 6 " ON CENTER

— ' ' ,.' 1 1

TYPICAL 32 SQ. BOXES

EVERY 3 6 " ON CENTER

—

EA CH WAY , 2 1/2 si _AB

—

— I

— —

1 1

SECTION A

FLOOR PLAN Values and dimensions are for i l lus t ra t ive purposes only.

Figure 71. Raised concrete floor system (after Portland Cement Association).

Honeycomb system

The deve lopment of the h o n e y c o m b system was based upon the assumption that

comparat ively slight movements of some clays reduce or relieve swelling pressures [ 5 3 ] . The

foundat ion consists of longitudinally split Sonotubes that are placed with the openings toward

the soil as shown on figure 72 , the b o t t o m 2 inches of the space be tween Sonotubes being filled

wi th sand. The Sono tube forms stand up well during placing of the concrete bu t disintegrate after

being wet ted . After the tube dist integrates, the sand runs out from under the joists.

It was theorized that as the clay swells, it could expand in to these openings and reduce the

swelling pressure. The system has been tried in a few limited cases in the Denver area with

doubtful success.


8" 8"

SECTION A - A

Figure 72. Typical honeycomb form system.

F L O A T I N G SLABS

A floating slab refers to a slab-on-ground cons t ruc t ion in which the slabs are total ly

separated from the grade beam and building s t ruc ture . Theoret ical ly , the slab is capable of

moving independent ly wi thou t being in contac t wi th the surrounding s t ruc ture .

Slip joints

Inter ior floor slabs should be total ly separated from the grade beams and inter ior co lumns

to allow for free slab movement . If the slab is no t separated from the grade beam, heaving of the

slab can t ransmit pressure to the grade beam and, in turn , lift the piers.

In pract ice, the slab is separated from the grade beams by the use of asphalt felt expansion

jo in ts . When the backfill exerts lateral pressure on the grade beam, the expansion jo in t is unde r

compression and part of the uplift pressure benea th the slab is then t ransmit ted to the grade

beam. In nearly all basement buildings which have been subjected to uplifting, the central por t ion

of the slab raised while the area along the per imeter of the grade beam remained essentially in

place. In many cases, cracks appeared abou t 2 feet from and parallel to the grade beam, as shown

on figure 7 3 .


Figure 73. Floor cracks parallel to the foundation wall resulting from lack of slip joints.

Theoret ical ly, if the swelling pressure of the underslab soil is 5 ,000 psf, as much as 10,000

pounds per linear foot of uplifting pressure can be t ransmi t ted to the grade beam. Natural ly, as

soon as the slab cracked, the uplifting pressure was relieved. However; the initial uplift force is

somet imes sufficient to cause heavy damage.

An improved cons t ruc t ion m e t h o d is the installation of a lubricated slip jo in t be tween the

grade beam and the slab as shown on figure 74 . This installation involves the use of two 1/8-inch

masoni te strips wi th silicone lubr icant be tween them. This type of jo in t system is no t affected by

lateral pressure thereby allowing free slab movement .

Some architects prefer to extend the floor slab in to the exter ior foundat ion wall as shown

on figure 7 5 . The result is obvious; heaving of the floor slab no t only produces floor cracks, bu t

also tilts the exter ior wall, causing great s t ructural damage. Figure 76 indicates the results of

faulty design.


2 - l / 8 " x 9 " continuous tempered masonite. Coat smooth sides with silicone lubricant. Tape smooth sides together. Provide temporary support by taping to wall. Do not nail. N .

Figure 74. Typical slip joint detail between slab-on-ground and foundation wall.

Exterior slabs

Exterior patio slabs can also transmit swelling pressure to the structure. Conventional

practice calls for the exterior slab to be tied in with the grade beam by the use of dowel bars. In

this manner, full swelling pressure is transmitted into the foundation walls. Such type design is

not recommended. See also "Aprons" for a subsequent discussion of similar problems.

Figure 77 shows a typical case where the patio slab has transmitted pressure through dowel

bars to the foundation wall causing considerable damage.

In another case, the exterior sidewalk slab was extended about half an inch into the brick

course for aesthetic reasons (fig. 78). This resulted in the flaking and damaging of the brick wall,

as shown on figure 79.

Oftentimes, the patio slabs are tied into the top of the foundation wall with dowel bar as

shown on figure 80. Heaving of the patio slab can cause severe cracking of the upper structure

without any sign of movement being apparent in the basement portion of the structure.

Partition wall

The single largest factor that causes damage to structures founded on expansive soils is

partition walls that bear directly on a slab. When the floor slab heaves, everything resting on the


τ

Figure 75. Tilting of exterior wall caused by slab heaving and improper slip joints.


Figure 76. Exterior brick course buckled due to heaving of interior floor slab.

floor will rise. The a m o u n t of floor heaving depends upon the swelling por tent ia l of the underslab

soils as well as to the degree of wet t ing. Slab heaving ranging from a fraction of an inch t o as

m u c h as 12 inches has been observed. The i tems affected by floor heaving are:

Stud walls,

Sheet rock,

Wall paneling,

Cinder block par t i t ions ,

Staircase walls,

Door frames,

Water lines,

Furnace duc t s , and

Shelves and bookcases


Floor Joist

Brick Course

Dowel bar . Patio Slab

t u t 4 -Swelling Pressure

Basement Wall

Figure 77. Patio slab dowelled into wall. Crack appeared parallel to wall. Magnitude of swelling pressure transmitted is estimated to be about 10,000 pounds per running foot.

Any one, or several, of the above i tems can impar t pressure to the upper floor joist or

beams; initially, doors will bind followed by the occurrence of severe cracking. Almost every

building investigated suffered some degree of damage due to the uplifting of the slab-bearing

par t i t ion walls.

For stud wall cons t ruc t ion , it is relatively easy to provide slip jo in ts in the system so the

wall is free to move wi thou t exer t ing pressure on the upper s t ructures . A typical detail of such

const ruct ion is shown on figure 8 1 . The slip jo ints can ei ther be installed at the t op (floor

suppor ted) or at the b o t t o m (hung par t i t ion wall) . The disadvantage of using a f loor-supported

wall is tha t when the wall lifts, vertical cracking will occur be tween the par t i t ion wall and the

exter ior walls as shown on figure 82 . Figure 83 indicates the archi tectural detail of a school

building in which the load-bearing walls are suppor ted by piers and the inter ior masonry walls are

placed on the slabs. Heaving of the slab-on-ground has resulted in severe cracking of the

slab-bearing par t i t ion wall while the walls suppor ted by the grade beams and piers remain stable

as shown on figure 84 .


Figure 78. Flaking of brick course caused by slab heaving.


Figure 79. Sidewalk heaving caused flaking of the brick course as a direct result of the slab extending into the brick course.

F requen t ly , the s tud walls have been proper ly provided with slip jo in ts ; bu t sheet rock,

applied on b o t h sides of the s tuds , resting on the floor as shown on figure 85 negates the slip

joint instal lat ion. Sheet rock is capable of t ransmit t ing sufficient pressure to the floor jo in t or

ceiling resulting in great damage.

A similar si tuation occurs when basement walls are paneled. The studs may be free from the

floor, b u t the paneling bears directly on the floor. If uplifting occurs , it may result in the popping

of the paneling as shown on figure 86 .

Staircase walls are the mos t frequently neglected when it comes to providing proper uplift

precaut ions. When instruct ions are to provide slip jo ints to all slab bearing walls, the staircase is

usually neglected. One single 2 x 4 can exert great uplift pressure and thus damage the upper

s t ructure , and the force may even extend to the second level in the case of split level buildings as

shown on figure 87 .

Figure 88 indicates a proper ly formed slab-bearing part i t ion wall with slip jo in ts at the

b o t t o m .


Figure 80. Patio slab attached to basement wall by dowel bar. Swelling pressure results in damage to brick course.

Door frames and utilities

Door frames should be hung from the t op and n o t suppor ted on slabs. Slab heaving can

t ransmit high intensi ty pressure th rough the door frame to the upper s t ructures .

A very c o m m o n distress in residential houses is the separat ion of garage d o o r frames from

masonry walls. This is essentially caused by heaving of the garage slab which results from failure

to provide a grade beam, across the ent rance for the garage d o o r opening.

Figure 89 indicates a c o m m o n sight of garage floor slab heaving in a swelling soil area. The

central por t ion of the slab heaved; however, the edges remained in place because they were

restrained by the d o o r frame.

Figure 90 indicates the crushing and dis tor t ion of furnace duc ts resulting from heaving of a

basement slab. Figure 91 shows the severe bending of water line also caused by floor heaving.

Such distress generally brings immedia te alarm to the h o m e owner . If the uti l i ty lines above the

slab are being damaged, those below the slab can also be seriously damaged.


FLOOR SUPPORTED PARTITION WALL HUNG PARTITION WALL

Figure 81. Detail of slip joints used in a partition wall. (After Jorgensen and Hendrickson, Inc.)


Figure 82. The cracking of slab bearing partition wall at the junction of the exterior wall.

Aprons

Concre te sidewalk slabs around a building will prevent surface wate r from enter ing th rough

the backfill and in to the foundat ion soils. However , the concre te apron should no t be doweled

in to the foundat ion wall for the reasons previously discussed.

Concre te sidewalks or aprons will heave and crack. Heaving of concre te walks can somet imes

result in the drainage being directed toward the building, allowing surface water to en ter th rough


P R O J E C T 2l'-IO"

\Z2

0 C L A S S R M . 30-/0

CRACKED SLAB BEARING WALL

Figure 83. Architectural detail of school building.

138

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139

Figure 84. Heaving of interior slab-bearing part i t ion. Note the

uncracked wall, on the right, is supported by structural grade Figure 85. Buckling of stud wall due to floor heaving, beams.

140

FO

UN

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ON

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Λ / % η 1 Ι. χ „ ι. ι ri ι Figure 87. Staircase 2x4 s rest directly on f loor. Note bow of Figure 86. Buckling of wall paneling due to f loor heaving. 0 ~ , ,

3 2x4 s due to f loor heaving.

Figure 89. Typical garage f loor heaving at the central port ion.


Figure 88. Properly formed slab-bearing partition wall wi th slip joint at bot tom.

! 42

FO

UN

DA

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NS

ON

EX

PA

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IVE

SO

ILS

Figure 90. Crushing of furnace duct caused by heaving of base- R g u re gi D j s t o r t J on of w a t er p i pe d ue to f loor heaving,

ment floor slab.

SL

AB

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143

Figure 93. Sidewalk slab heaving transmits pressure through slab Figure 92. Typical heaving of sidewalk. bearing timber frame to the building causing heavy structure

cracking.


the jo in t be tween the apron and the wall. F requen t expansion jo in t s will be necessary to prevent

excessive cracking of the slab. Figure 92 shows the heaving of a concrete slab.

Ano the r serious mis take is t o place the posts of a pat io on a concrete walk as shown on

figure 9 3 . As the concre te walk heaves, it t ransmits pressure to the s t ruc ture through the

connect ing girders causing building damage.

R E F E R E N C E S

[521 "Recommended Practice for Construction of Residential Concrete Floors on Expansive Soils," (Vol. II),

Portland Cement Association, Los Angeles, California.

[531 Means, R. E., "Buildings on Expansive Clay," Quarterly of the Colorado School of Mines, Vol. 54, No. 4,

1959.

Chapter 7

MOISTURE CONTROL

INTRODUCTION

Terzaghi stated tha t , "Wi thou t any water there would be no use for soil mechan ics . "

Terzaghi had only limited knowledge of swelling soils; however, his s t a tement can be accurately

applied to the behavior of expansion soils. Ever since the acknowledgment of expansive soil

problems, engineers have been a t t empt ing to isolate wate r from the foundat ion s tructure. .

It is a relatively simple under tak ing to remove free water which m a y seep in to a building

foundat ion by providing adequa te surface drainage and proper ly installed subdrainage systems.

However, it is difficult to isolate the migrat ion of mois ture from an exter ior locat ion to a covered

area. Vapor barriers, b o t h hor izonta l and vertical, have been used wi th only a l imited degree of

success in impeding mois ture migrat ion. Fu r the r research is necessary in b o t h the field and

labora tory to establish a practical and economical m e t h o d of control l ing mois ture migrat ion.

H O R I Z O N T A L M O I S T U R E B A R R I E R S

Horizonta l mois ture barriers can be installed around a building in the form of membranes ,

rigid paving, or flexible paving. The purpose of the hor izonta l barriers is to prevent excessive

in take of surface mois ture . The use and effectiveness of these mois ture barriers are discussed

below.

Membranes

A widely used hor izonta l mois ture barrier is a combina t ion of a po lye thy lene m e m b r a n e

extending beyond the limits of backfill and loose gravel placed on t o p of the membrane .

Somet imes a plank is installed along the edge of the m e m b r a n e . The purpose of such installation

is t o prevent surface water from seeping th rough the backfill in to the building and to prevent the

growth of weeds. Figure 94 indicates the typical design.

It should be realized tha t the dry soils benea th an impervious m e m b r a n e will, in t ime,

become wet regardless of the presence of such m e m b r a n e because evaporat ion can n o longer take

place.

The thickness of the polye thylene m e m b r a n e ranges from about 4 t o 20 mils. The

m e m b r a n e tears easily and eventually develops holes. Surface water ponding in a depressed area

will in t ime leak th rough the holes and edges of the m e m b r a n e and en ter the soil benea th , while

evaporat ion and drying of the soil benea th the m e m b r a n e is impossible. Even in the case of a

perfect impervious m e m b r a n e , mois ture migrat ion due to thermal transfer as explained in chapter

2 will in t roduce addit ional mois ture to the foundat ion soils.


Foundation wall

Figure 94. Impervious membrane along exterior walls.

Thus , it appears tha t the ques t ionable advantage of using a m e m b r a n e around the building is

t o increase the t ime required for mois ture pene t ra t ion and make the mois ture d is t r ibut ion more

uni form. In the course of several years, the backfill soil benea th a m e m b r a n e will be total ly

sa tura ted . By lifting the m e m b r a n e , it is easy to find that the soil has a mois ture con ten t greater

than the plastic limit.

Concrete aprons

The installation of concrete aprons or sidewalks has been found effective in controll ing

mois ture f luctuat ion. The advantage of using concre te aprons ra ther than plastic membranes is

tha t the former offers a positive barrier to water . Obviously, within reason, the wider the

concre te apron the more p ro tec t ion it offers to the building. Paving the entire non-building area

is impractical and unsightly. Nonetheless , it has been observed tha t foundat ion movement due to

expansive soils seldom takes place in gasoline service stat ions where the ground surface is

extensively covered.

Mohan and Rao [ 5 4 ] installed a 4-foot-wide concre te apron around distressed buildings

founded on black co t ton soils, which proved effective in controll ing movement . They claim that

t he function of the apron is t o move the marginal mois ture variat ion away from the building.

While the use of concrete aprons around the exter ior of the building may prove beneficial,

care should be exercised in obtaining an effective seal be tween the aprons and the foundat ion

walls. Swelling soils can heave an apron so tha t surface drainage is toward the building rather than

away. With poor ly const ructed jo in ts , water will en ter the jo in t and seep into the foundat ion soil,

MOISTURE CONTROL 147

thus an apron can cause m o r e damage than good. In those areas where concre te aprons are used,

constant care and main tenance is required.

Asphalt membranes

As early as 1933 , the Texas Highway Depa r tmen t used asphalt membranes to prevent

surface water from enter ing the expansive clay subgrade [ 5 5 ] . T h e concre te pavement was placed

u p o n an asphalt ma t 48 feet wide. This m e m b r a n e instal lat ion m a y retard swelling bu t will n o t

prevent it.

Research conduc ted by the Asphalt Ins t i tu te [56] advocates t ha t asphalt membranes

const ructed from catalytically b lown asphalt can be effective in preventing mois ture from

intruding in to subgrade soils. Specifications for catalytically b lown asphalt cement are given in

table 12.

Van L o n d o n [57 ] used the 50-60 pene t ra t ion asphalt m e m b r a n e to comple te ly envelop the

highway e m b a n k m e n t . The purpose of the m e m b r a n e was to mainta in a cons tan t mois ture

con ten t in the e m b a n k m e n t soil, thus preventing volumetr ic change of the fill mater ial .

Subsequent mois ture de te rmina t ions of the enclosed e m b a n k m e n t indicated very little change in

mois ture con ten t over the 10-year period subsequent to cons t ruc t ion , and the pavement

remained in a stable condi t ion .

A n o t h e r t ype of asphalt m e m b r a n e consisted of prefabricated asphalt sheets, less than

one-half inch thick, 3 t o 4 feet wide, and up t o 20 feet long. Such mater ial can be convenient ly

handled and easily placed.

Table 12.—Specification for catalytically-blown asphalt cement.

Test Designation Test Method 50-60 Test Designation

ASTM Penetration Grade

Flash point D 92 425° F. Min. Softening point Penetration, 77° F.

D 36 175°F.-225°F. Softening point Penetration, 77° F.

100 gms., 5 sec. D 5 50-60 Penetration, 32° F.,

200 gms., 60 sec. D 5 30 Min. Penetration, 115° F.,

50 gms., 5 sec. D 5 120 Max. Ductility, 77° F.

(5 cm per min) cm D 113 3.5 Min. Loss on heating

325° F. in 5 hrs. D 6 1.0 Max. Penetration of residue,

77° F. (100 gms., 5 sec. compared to original) percent 60.0 Min.

Solubility in C C I 4 , percent D 165 97.0 Min.

Asphalt used as a membrane shall be 50-60 penetration grade. This material shall be prepared by the catalytic blowing of petroleum asphalt. The use of iron chlorides or compounds thereof will not be permitted. The asphalt shall be homogeneous, free of water and shall not foam when heated to 347° F. It shall meet the above tabulated requirements.


Asphalt membranes can be used t o cover the surface of expansive soils so tha t nonexpansive

fill can be placed on t o p of the membranes . This will minimize the infiltration of surface water

into the underslab soils. Where slab-on-ground cons t ruc t ion is required, such t rea tment can be

very advantageous. The a m o u n t of asphalt cement required to const ruct a membrane , according

t o the Asphalt Ins t i tu te , is abou t 1.3 gallons per square yard.

The use of asphalt membranes in connec t ion wi th the cons t ruct ion of a swimming pool in

an expansive soil area is part icularly desirable. Fu r the r research and field observation will be

required.

V E R T I C A L M O I S T U R E B A R R I E R S

Vertical mois ture barriers have been used around the per imeter of the building to cut off the

source of water tha t m a y en te r the underslab soils. Theoret ical ly, vertical barriers should be more

effective than hor izonta l barriers in minimizing seasonal drying and shrinking of the per imeter

foundat ion soils, as well as maintaining long term uniform mois ture condi t ions benea th the

covered area.

Installation

Buried vertical barriers m a y consist of po lye thy lene membrane , concrete , or o ther durable ,

impervious materials . The pa th of mois ture migrat ion when using a vertical barrier is shown on

figure 9 5 .

As seen from figure 9 5 , the installation of a vertical barrier prevents edge wet t ing due to

lateral mois ture migrat ion wi th in the d e p t h t o which the m e m b r a n e ex tends . However, over a

period of t ime, rainfall, melt ing snow, and lawn irrigation water will accumulate near the b o t t o m

Rainfall and yard watering

Most migration "blocked" by membrane.

With time, some moisture migrates below membrane depth.

Depth of seasonal moisture change

Moisture content relatively constant, but can still absorb moisture with time.

Figure 95. Path of moisture migration blocked by vertical barrier. (After Woodward-Clyde-Sherard and Associates).


of the m e m b r a n e and the mois ture will be sucked in to the mositure-deficient soils benea th the

building. Thus , the same degree of wet t ing of the foundat ion soil could result wi th or w i thou t the

use of a barrier, bu t would occur over a longer per iod of t ime . By installing a mosi ture barrier,

the potent ia l for damage would be less because of the slower rate of mosi ture migrat ion and the

more uniform mois ture con ten t of the soil at any part icular t ime.

Vertical mois ture barriers should be installed to a d e p t h equal to or greater t han the d e p t h

of seasonal mois ture change.

A s tudy in Sou th Africa [58 ] has shown tha t wet t ing of soils benea th a house occurred to a

d e p t h of at least 24 feet in a fissured-clay profile. Most of this wet t ing was a t t r ibu ted to lateral

migrat ion of mois ture from seasonal rains, ra ther than from capillary rise.

The following should be considered in the instal lat ion of a vertical mois ture barr ier :

1. Vertical mois tu re barriers cannot be effectively installed a round basement s t ructures .

2. Vert ical mos i ture barriers should be installed 2 t o 3 feet from the per imeter foundat ion

to permit mach ine excavat ion of the t rench for the m e m b r a n e .

3 . T h e vertical barr ier is somet imes a t tached t o a hor izonta l barrier t o prevent wet t ing

be tween the vertical barr ier and the building.

4. Ei ther concrete or po lye thy lene m e m b r a n e can be used. The m e m b r a n e should be of

sufficient thickness and durabi l i ty t o resist punc tures dur ing backfilling of the t rench.

5. It is also possible t o use semi-hardening, impervious slurries installed in a narrow t rench.

Theoret ical ly , vertical mois tu re barriers have a dis t inct advantage over hor izonta l mois ture

barriers. However, in view of the high cost involved in the installation of a vertical mois ture

barrier, especially where great d e p t h is required, it is doubt fu l tha t such an installat ion is of

sufficient meri t to war ran t the expense .

Backfill

An impor t an t e lement involved in bui lding cons t ruc t ion which is usually slighted is the

backfill a round a building. When proper ly cons t ruc ted , backfill serves the same purpose as d o

vertical mois ture barriers. This is especially t rue in basement building where proper ly compac ted

backfill can prevent surface wa te r from enter ing the founda t ion soils.

Unfor tuna t ley , backfill is seldom compacted proper ly . Many builders choose to push the

loose soil i n to the excavat ion wi th n o further compac t ion effort. Others a t t e m p t t o consol idate

the backfill by puddl ing. It is obvious t ha t p roper compac t ion of t h e backfill canno t be achieved

by such processes. When improper ly compac ted , almost all of the backfill along the foundat ion

walls is in a loose s ta te . Surface water can then en ter the backfill and seep freely in to the

foundat ion soils. T h e result of se t t l ement of loose backfill is shown on figure 9 6 .

Compac t ion of backfill in restricted areas, such as in ut i l i ty t renches , cannot be performed

by large compact ing equ ipment . These areas should be compac ted , by hand-opera ted vibrating

plates, in th in lifts of n o t more than 4 inches. Because of p o o r compact ive effort, se t t l ement of

backfill a round a building as well as se t t lement of ut i l i ty t renches is the rule, ra ther than the

except ion.


Figure 96. Typical case of loose backfill around the building. Loose backfill allows surface water to enter the foundation soils. Note depressions which trap water.

One recent ly advertised hydraul ic-operated compac to r claimed to have the ability to

compact backfill in a t rench, in one effort, from the ground surface. Such a claim appears to be

false while, in fact, the compac to r has probably a lift compac t ion capacity of no more than 12

inches.

The so-called puddl ing process has been widely used by cont rac tors in small jobs with the

assumption tha t the soils will consolidate w i thou t compact ion . No t only does such practice make

it impossible to obtain the required densi ty bu t it is somet imes dangerous. In cohesive backfills,

puddling inevitably leads t o weakening and softening of the soil and to future loss of stability and

subsidence. In uniformly granular soils, puddl ing will cause the collapse of the ext remely loose,

unstable zone associated wi th bulking. Excess puddl ing, which frequently results when a hose is

left discharging in to the backfill overnight, can easily in t roduce water in to the foundat ion soils

and benea th the slab, resulting in great damage. In expansive soil areas, such damage will be

reflected in the s t ructure for m a n y years.

By using impervious clay compacted t o 85 percent or more of s tandard Proc tor densi ty at

op t imum mois ture con ten t , the backfill acts as a very effective vertical mois ture barrier. Such

barriers are more effective than membranes .


SUBSURFACE D R A I N A G E

The purposes of a subsurface drainage system are as follows:

1. In tercept t he gravity flow of free water ,

2. Lower the ground water or perched water , and

3 . Arrest the capillary mois ture movemen t and movemen t of mois ture in the vapor s ta te .

In tercep ting d rains

Intercept ing drains are effective in minimizing the wet t ing of the foundat ion soils where the

wet t ing is due t o the gravity flow of free wa te r in a subsurface pervious layer such as a layer of

gravel or fissured clay. This is shown on figure 9 7 . To insure the in tercept ion of free water , the

drain mus t be comple te ly filled wi th gravel and the t rench should be deep enough t o reach the

water-bearing layer.

Intercept ing drains are mos t effective when located along the toe of a slope where ground

water leaves the deep strata and where it may emerge to the surface. When a s t ructure is located

near an irrigation di tch or canal wi th a leakage p rob lem, the installat ion of an in tercept ing drain

will p ro tec t against the infi l tration of seepage water . Intercept ing drains are also widely used for

improving slope stability and preventing landslides.

INTERCEPT GRAVITATIONAL FREE WATER.

Figure 97. Typical function of an intercepting drain. (After Woodward-Clyde-Sherard & Associates).

Perched water

As ment ioned previously unde r "Slabs on Expansive Soils ," a perched wate r table condi t ion

can develop in areas where bedrock is shallow. Surface water accumulated from yard irrigation

will no t pe rmea te the bedrock and can create a local perched water condi t ion . A perched wate r

table can also be created by a relatively impermeable s t ra tum of small areal ex ten t and by a zone

of aerat ion above the main b o d y of ground water [59] as shown on figure 9 8 .

Where bedrock is s i tuated at a slight d e p t h benea th the ground surface, a perched water

table condi t ion m a y develop d u e t o the following:

1. The upper soils are relatively pervious and surface wate r is capable of seeping through the

uppe r soils encounter ing relatively low resistance.


Ground surface

Perched water tables

Water table

Unconfined aquifer

Figure 98. Perched aquifers. (After Todd).

2. The lower bedrock is impervious and will no t allow the infi l tration of water . However,

there are seams and fissures in the bedrock which provide passage for water . A large

volume of water is capable of flowing in the fissures of the bedrock.

3 . With surface irrigation and precipi ta t ion, the surface wate r tends to seep th rough the

upper soil and accumulate on top of bedrock . Part of the accumulated wate r will flow on

t o p of the bedrock and par t of it will run through the fissures of the bedrock . When a

deep basement is cons t ruc ted , the basement excavation can cut th rough the fissures of

bedrock and the water can accumula te in the low basement area, creating a perched water

table condi t ion.

Wells tapping perched aquifers yield only t empora ry or small amoun t s of water . However,

such water can enter basements and cause considerable damage. The installation of a subsurface

drainage system around the per imeter of the lower level of a s t ructure can pro tec t against

infil tration from perched water . T o be effective, an intercept ing drain should be installed at least

2 feet below the floor and should lead to a suitable out le t where water can be removed by gravity

or by a sump p u m p .

Peripheral drains

Figure 99 indicates the suggested locat ion of peripheral drains. These drains can be installed

around ei ther the inter ior or exter ior of the building. The subdrainage system is effective in

minimizing general wet t ing of the foundat ion soils, which occur no t only because of gravitational

flow of free water , bu t also because of mois ture migrat ion [ 6 0 ] . As explained in chapter 2,

mois ture migrat ion includes capillary mois ture m o v e m e n t in the liquid state and movemen t of

mois ture in the vapor state due to t empera tu re differential. Where the water table is deep ,


•5 ï

Foundation Wall

Grade down 0 .5% to sump

Drain Trench

Sump 3 deep can be located at any convenient location in the basement.

Grade beam or Foundation wall

J / 2 " Expansion joint material

4 Void Material

Concrete Pier

SECTION A A

Figure 99. Typical sub-drain detail.


capillary act ion and vapor transfer are probably the major causes of wet t ing of the

moisture-deficient soils in a covered area. The possible p lacement of a subdrain to in ter rupt

mois ture movemen t is shown on figure 100.

It should be noted tha t , for a subdrainage system to be effective in preventing mois ture

m o v e m e n t discussed above, it m u s t be designed as a capillary break; and addit ional ly, the vapor

pressure in the drain should be at a lower value than the vapor pressure in the foundat ion soils.

The gravel used t o fill the subdrain t rench should have a gradat ion be tween 3 /4 and 2 inches in

size wi th percent of fines less than 5.

Positive out le ts should be provided for the subdrainage system. If a gravity out le t is no t

possible, the drain should be discharged to a sump where water can be removed by pumping. In

some instances, it m a y b e permissible to connec t the subdrain to the gravel bed benea th the street

MOISTURE MOVEMENT WITHOUT SUBDRAIN Legend

Gravitational flow of free water (in shrinkage cracks and fissures)

Saturated soil at surface

Capillary or vapor movement in soil mass.

SUBDRAIN INTERRUPTS MOISTURE MOVEMENT

Figure 100. Interruption of moisture movement by subdrain. (After Woodward-Clyde-Sherard & Associates)


sewer line. However, because of the small gradient differential be tween sewer line in take and

subdrain out le t , such in ar rangement is no t usually satisfactory.

Experience indicates tha t t o be fully effective, the peripheral drain should be placed at least

12 inches below the floor level, preferably 24 inches.

S U R F A C E D R A I N A G E

The ground surface a round a building should be graded so tha t surface water will drain away

from the s t ructure in all direct ions. This usually is n o t accomplished due to negligence, cost ,

limited p rope r ty size and o the r reasons. In m a n y cases, the area around the building had been

proper ly graded after cons t ruc t ion , bu t the grade was later changed to improve the appearance of

the landscape. As a result , it is n o t u n c o m m o n to find buildings wi th surface drainage directed

toward the foundat ion walls. Moisture change at the per imeter of the building apears to be the

mos t significant con t r ibu to r to damage. Therefore , by improving the drainage, a beneficial effect

is inevitable.

Sprinkling system

Lawn sprinkling systems often create foundat ion soil p roblems. Lawn sprinkling systems

should be installed at least 10 feet from the building. Nozzles of the sprinkling system should

never be directed toward a building. An au toma t i c t iming device should be provided for all

sprinkling systems so tha t excessive water ing is avoided. Before use, all out le ts in the system

should be pressure checked to de tec t the presence of any possible open out le t underground

where water could flow unchecked for a long period of t ime w i thou t arousing suspicion.

Vegetation

From an archi tectural s tandpoin t , it is pleasing to have shrubs and flower beds p lanted

adjacent to buildings. However, since it is necessary to irrigate flower beds and shrubs, the excess

wate r will pene t ra te th rough the loose backfill in to the foundat ion soils. Exper ience indicates

that in practically every investigation of a cracked building, shrubs and flower beds were located

adjacent to the building. Figure 101 indicates a typical case where drainage away from the

building is obs t ruc ted by the paved walk and improper compac t ion results in a depression along

the building creating a ponding condi t ion . Shrubs planted along a wall is typical of m a n y school

buildings and residential houses.

Many studies have shown that large bushes and trees can cause differential drying [611 of

the foundat ion soils and result in damage to the building from shrinkage. Most of the damage

caused by shrinkage takes place in non-swelling or low-swelling soil areas. It is doubt fu l whe the r

large tress will pose a p rob lem in high-swelling soil areas. Nevertheless, it is good practice t o plant

trees and shrubs at least 10 feet from a s t ruc ture .


Figure 101. Plantation strip around a school building allowing the ponding of water between the sidewalk and the foundation wall.

Roof drain

Roof downspou t s mus t be directed away from a s t ructure so tha t water will no t seep in to

the foundat ion soils. The downspou t s should extend well beyond the per imeter of the

foundat ion and should discharge to an area where the surface drainage is adequate to carry off

the water rapidly and prevent any possible ponding of water . If necessary, the water from

downspou t s should be carried in a closed pipe or lined di tch to the street . Many modern buildings

are constructed w i thou t downspou t s , and wate r from the roof drains freely through the loose

backfill in to the foundat ion soils.

Somet imes an open cour tyard is cons t ruc ted in the central por t ion of a building. The

courtyard is usually covered wi th lawn, flower beds, and trees. Such a cour tyard const i tu tes a

major drainage problem because surface water will be unable to drain unless an adequate

subsurface drainage system is provided.


Interior plumbing

Inter ior p lumbing, including sewer and wate r lines, should be carefully checked for leakage.

Sewer lines laid benea th the basement are subjected to stress when the surrounding soils expand ,

and in ex t r eme cases, shearing stress has caused pipe breakage resulting in flooding.

In one instance, while investigating a cracked house , it was found that the plug on the drain

t rap benea th the shower stall was missing. As a result, all the water from the shower drained in to

the crawl space area for a period of at least 3 years.

Leakage from wate r lines is less frequent . Somet imes leakage is found near the wate r me te r

which generally is si tuated near the sidewalk in front of the house . Leaking wate r follows the

loose backfill around the water pipe in to the founda t ion soils causing damage.

R E F E R E N C E S

[54] Mohan, D. and Rao, B. G., "Moisture Variation and Performance of Foundations in Black Cotton Soils in

India," Moisture Equilibria and Moisture Change in Soils Beneath Covered Areas, Australia, Butterworth.

[55] "Report of Committee on Warping of Concrete Pavements," Highway Research Board Proceedings, Vol.

25, 1945.

[56] "Asphalt Membranes and Expansive Soils," Information Service No. 145 (IS-145) May 1968.

[57] Van London, W. J., "Waterproofing Value of Asphalt Membranes in Earth Fills for Gulf Freeway,"

Proceedings, Association of Asphalt Paving Technologists, Vol. 22, 1953.

[58] Collins, L. E., "Some Observations on the Movement of Buildings on Soils in Vereening and Odendaalsrus,"

Symposium on Expansive Clays, South African Institution of Civil Engineers.

[59] Todd, D. K., "Ground Water Hydrology," John Wiley & Sons, Inc., 1959.

[60] "Remedial Methods Applied to Houses Damaged by High Volume Change Soils," Woodward-Clyde-Sherard

& Associates, Oakland, California, 1968.

[61] Hammer, M. J. and Thompson, Ο. B., "Foundation Clay Shrinkage Caused by Large Trees," Journal ASCE,

Soil Mechanics and Foundation Division, Vol. 92, No. SM 6, Nov. 1966.

Chapter 8

SOIL STABILIZATION


In theory , the swelling potent ia l of an expansive clay can be minimized or comple te ly

el iminated by one of the following m e t h o d s :

1. F lood the in-place soil t o acheive swelling pr ior to cons t ruc t ion ,

2 . Decrease the densi ty of the soil by compac t ion cont ro l ,

3 . Replace the swelling soils wi th nonswelling soils,

4 . Change the proper t ies of expansive soils by chemical injection, or

5. Isolate the soil so there will be n o mois ture change.

Isolation of the soil has been extensively discussed in chapter 7.

PREWETTING

An old established concept among engineers and cont rac tors as well as laymen in dealing

wi th swelling soils is prewet t ing . As explained in chapter 2 , mois ture can migrate from a

modera te -dep th water table t o an upper moisture-deficient soil by means of capillary rise.

Moisture migrat ion can also take place from a h igh- tempera ture area to a low- tempera ture area by

means of thermo-osmosis or o the r mechanisms. Normal ly this mois ture evaporates at the surface

and mois ture equil ibr ium is mainta ined in the soil. The presence of covered areas, such as floor

slabs, pavements , or similar s t ructures which inhibi t this evaporat ion increases the mois ture

con ten t of the founda t ion soil wi th resul tant swell.

The prewet t ing theory is based on the assumpt ion tha t if soil is allowed to swell by wet t ing

prior t o cons t ruc t ion and if the high soil mois ture con ten t is mainta ined, the soil volume will

remain essentially cons tant , achieving a no-heave state and therefore s t ructural damage will no t

occur.

Ponding

The present p rewet t ing pract ice usually involves direct flooding or ponding of the building

area. The foundat ion and floor area is f looded by const ruct ing a small ear th berm around the

outs ide of the foundat ion t renches to impound the water . A n o t h e r pract ice includes first

prewet t ing the foundat ion t renches , then placing the foundat ion which is used as a dike to flood

the floor area. In some cases, where the mois ture con ten t at footing d e p t h is stable, it is possible

to place concre te footings and utilize them as dikes so tha t only the floor area is pre wet ted .


The effect of ponding or flooding on the moisture content at various depths has been

investigated by the Texas Highway Department [62] . A section of Interstate Route 35 north of

Waco, Texas was chosen for the experiment. The subgrade was ponded and the moisture content

at various depths was taken. The moisture variation at specific depths beneath the ponding area is

shown on figure 102. The following observations were made:

1. The moisture content achieved a significant penetration of only 4 feet below the pond

during a period of 24 days.

2. To obtain desirable moisture distribution at greater depths, ponding should extend

approximately 30 days.

Experience in Southern California [63] indicates that pre wetting moderately expansive soils

to a condition of 85 percent saturation at a depth of 2-1/2 feet is often satisfactory. In the case

of highly expansive soils, prewetting to as much as 3 feet may not be sufficient.

For slab-on-ground construction, after completing of the prewetting treatment, the ground

surface must be kept moist until the slab is placed. A gravel or sand bed 4 to 6 inches thick

should be placed over the subgrade prior to the prewetting period. The gravel layer prevents the

clay from drying and shrinking.

The prewetting operation must not be at the discretion of a contractor or owner. The

treatment should be based upon an engineering investigation and evaluation of the site, subsoil

condition, swelling potential, climatic condition, foundation system, and prior local experience.

The moisture content profile should be checked frequently by tests in the field to assure that the

desired results are achieved.

Figure 102. Subgrade moisture movement on IH35. (After McDowell) McLennan Co., Texas.

SOIL STABILIZATION 161

Practice

Ponding or sprinkling, to increase the soil mois ture t o a degree tha t will prevent harmful

heaving u p o n subsequent wet t ing, has been used in the cons t ruc t ion of the San Luis Drain on the

San Luis Unit of the Bureau of Reclamat ion ' s Central Valley project in California.

Bara [64 ] claimed tha t if the dense clays wi th a part icular liquid limit could be expanded to

densities at or above critical na tura l density-liquid limit reference line, a stable or near u l t imate

mois ture condi t ion would have been approached and future volume changes would be small. The

liquid limit versus dry densi ty relat ionship is shown on figure 103. A soil wi th liquid limit of 70

intercepts the reference line at 90 pcf densi ty . Similarly, the water content- l iquid limit

relat ionship was developed for soil liquid limit ranging be tween 4 0 and 100 as shown on figure

104. Moisture con ten t s above the reference line in figure 104 would assure tha t the densities were

on the non-crit ical side of the reference line in figure 103.

Figure 104 indicates t ha t clays at liquid limit 4 0 require only abou t 23 percent mois ture ,

while those near liquid limit 100 require at least 37 percent mois ture before they are considered

to be relatively nonexpansive in situ.

Large scale exper iments of flooding of foundat ion soil for building sites have been

conduc ted in Vereeniging, Sou th Africa [ 6 5 ] . Here, the effect of wet t ing was accelerated by a

grid of vertical 4-inch-diameter wells each 20 feet deep . At the end of 96 days , over 90 percent of

Figure 103. Clays encountered along San Luis Drain, (after Bara).


LIQUID LIMIT (%)

4 0 5 0 6 0 7 0 8 0 9 0 100 4 0 J 1 1 1 1 1 1

cc

10

0 I I I I I I

Figure 104. Minimum water content required for soil liquid limit, (after Bara).

the m a x i m u m surface heave had taken place. It is concluded by the au thors tha t the acceleration

of heave by flooding is a feasible pre-construct ion procedure for light s t ructures .

E. J. Felt [66 ] discusses a prewet t ing project in which the soil mois ture con ten t did no t

increase appreciably after the first m o n t h of prewet t ing . Fo r 5 m o n t h s thereafter , soil swelling

cont inued. It was suggested tha t the first infil tration of water was probably taken by seams and

fissures present in the clay and, therefore , full soil expansion did n o t occur. As t ime passed, the

water moved from the fissures in to the b locky soil mass, and swelling took place th roughou t the

mass of the soil and no t merely along a seepage pa th .

At a housing project near Aust in , Texas, the expansive soil benea th the foundat ion was

prewet ted by filling the foundat ion t rench wi th water . After 6 weeks of soaking, the water was

pumped ou t , the foundat ion placed on the wet soil, the t renches again filled with water , and the

soil kep t wet ted thereafter. This house heaved b o t h during and after cons t ruc t ion .

It was concluded by Dawson [67] tha t it is ex t remely difficult t o sa tura te high plasticity

clays within a reasonable period of t ime. Expansion of partially saturated clays will cont inue

after comple t ion of the s t ruc ture .

Evaluation

Most highway engineers strongly endorse the use of prewet t ing t o minimize subgrade

heaving. In view of the past experience and actual case studies, it is doubtful if prewet t ing can be

successfully used with lightly loaded s t ructures . The effective migrat ion of mois ture , dep th of

penet ra t ion , t ime required for sa tura t ion, and swelling of partially saturated soils is no t fully

unders tood . Prewett ing practice is m u c h more complicated than assumed by most laymen. A


great a m o u n t of research will be required before comple te evaluation of the prewet t ing practice

can be made . Some of the disadvantages of the prewet t ing m e t h o d are as follows:

1. Allowing the ponding water to migrate in to the lower moisture-deficient soils. Exper ience

indicates tha t in a covered area, the mois ture con ten t of the unders lab soil seldom

decreases. Wet soil will induce swelling. After the swelling has reached its m a x i m u m

potent ia l , mois ture migrates to the lower moisture-deficient soil and induces fur ther

swelling. This procedure can cont inue for as long as 10 years.

2. F r o m a cons t ruc t ion s tandpoin t , the t ime required for prewet t ing can be critical. A

mois ture condi t ion of less than sa turat ion is often adequa te to inhibit object ionable

uplift . The length of ponding t ime required is usually abou t 1 t o 2 m o n t h s . Even this

length of t ime may be object ionable as being too great.

3 . It is highly ques t ionable if a uniform mois ture con ten t can be obta ined in pre wet ted

areas. Water can only seep in to the stiff clay th rough fissures, and consequent ly , uni form

dis t r ibut ion of mois ture con ten t is n o t likely to take place. As a result , differential

heaving can be critical even after a prolonged period of prewet t ing .

4 . Exper iments indicate tha t ponding water can effectively pene t ra te the soil t o a d e p t h of 4

feet within a reasonable t ime. Such d e p t h is insufficient to provide a balanced mois ture

zone for the cons t ruc t ion of impor t an t s t ructures .

5. While prewet t ing m a y prove t o be a possible m e t h o d of stabilizing the soil benea th the

floor slab, pavement , or canal lining, it is doubtful tha t footing foundat ions can be placed

on p rewet ted soil. In saturated condi t ions , the bearing capacity of a stiff clay can be

reduced t o a very low value, less than 1,000 psf, which prohib i t s the use of convent ional

footing foundat ions .

While prewet t ing may play an impor t an t role in the cons t ruc t ion of slabs, it is doubtful tha t

this m e t h o d will be an impor t an t cons t ruc t ion t echn ique for building foundat ions on expansive

soils.

COMPACTION C O N T R O L

The a m o u n t of swelling tha t occurs when a s t ructural fill is exposed t o addi t ional mois ture

depends upon the following:

1. The compac ted dry densi ty ,

2. The mois ture con ten t ,

3 . The m e t h o d of compac t ion , and

4 . The surcharge load.

The last t w o requi rements are no t critical in actual cons t ruc t ion . The m e t h o d of compac t ion

is generally l imited by available equ ipment . Fo r lightly loaded slabs, the surcharge load is usually

very small.


Placement condition

As early as 1959, Dawson [67] suggested that highly expansive soils be compacted to some minimum density rather than to a maximum density.

Holtz and Gibbs [68] show the influence of density and moisture on the expansion of a compacted expansive clay, as shown on figure 105.

IVS 1÷/ bV &.*/ %/VS *J%* TV

MOISTURE CONTENT • PERCENT OF DRY WEIGHT

Figure 105. Percentage of expansion for various placement conditions when under unit psi load. (After Holtz and Gibbs).


It can be seen tha t expansive clays expand very little when compac ted at low densities and

high mois ture b u t expand greatly when compac ted at high densit ies and low mois tures .

Gizienski and Lee [69 ] show tha t when their test soil was compac ted at abou t 4-1/2 percent

above o p t i m u m , which is 10-1/2 percent , the swell was negligible for any degree of compac t ion .

The main reason mois ture con ten t is i m p o r t a n t is tha t mois ture con ten t can generally result

in low densi ty fill; n o t tha t high mois ture con ten t will reduce swelling. The control l ing e lement is

densi ty. Compact ing stiff clay at 4 to 5 percent above o p t i m u m is very difficult. The process of

recompact ing swelling clays at mois ture con ten t s slightly above their na tura l mois tu re c o n t e n t

and at a low densi ty should be an excellent approach .

Referring to chapter 2 , it was established tha t the swelling pressure of clay is i ndependen t of

the surcharge pressure, initial mois ture con t en t , degree of sa tura t ion , thickness of s t r a tum, and

increases only with the increase of initial dry densi ty . For ins tance, wi th reference to figure 28

and table 10, by decreasing the dry densi ty of a typical expansive clay from 109 t o 100 pcf, the

swelling pressure decreases from 13,000 t o 5,000 psf and the swelling poten t ia l decreases from

6.7 t o 4.2 percent . All of this can be accomplished wi thou t changing the mois ture con ten t .

The main advantage of using this approach is tha t the swelling poten t ia l can be reduced

wi thou t the adverse effects caused by in t roducing excessive mois ture in to the soil. Figure 22

indicates tha t to decrease the swelling potent ia l from 6.7 to 4.2 percent , an increase of mois ture

con ten t of abou t 5 percent will be required.

The shor tcomings of prewet t ing m e t h o d s men t ioned in the preceding section can be

el iminated by compac t ion cont ro l . Excess water will no t be present in the soil; therefore , there

will no t be migrat ion of mois ture t o the under lying moisture-deficient soils and long wa i t ing

periods, pr ior t o cons t ruc t ion , will be unnecessary. A reasonably good bearing capaci ty can be

assigned to the low densi ty soil.

With m o d e r n cons t ruc t ion techniques , it is possible to scarify, pulverize, and r ecompac t the

natura l soil effectively wi thou t substantial ly increasing the cons t ruc t ion costs.

Design

Leonard Kraynski of Woodward , Clyde & Associates suggests the following design p rocedure

on compac t ion con t ro l :

1. Adequa te mix should be prepared for three Proc to r cylinders at each mois ture con ten t .

The cylinders are to be compac ted using three different efforts, such as 12 ,400 ft.-lb. per

cu. ft. (S tandard AASHO) , 23 ,000 ft.-lb. per cu. ft., and 56 ,200 ft.-lb. pe r cu. ft.

(Modified AASHO) . Thus , a to ta l of 12 t o 15 samples will be adequa te t o define

mois ture-densi ty curves as shown on figure 106.

2 . F rom each compac ted sample, a 2-inch-diameter core may be ext rac ted and tes ted in the

consol idometer for swell. The samples are subjected t o 144-psf surcharge pressure, t hen

submerged in wate r and allowed t o swell. F r o m the measured percent expansion, curves

of equal swell were p lo t t ed as shown on figure 107.

3 . F r o m a s tudy of these results , a mois ture con ten t of 19 t o 23 percent and a dry densi ty

ranging from 9 6 to 102 pcf were selected as design specifications. Using the p lacement

condi t ions , the average swell under a surcharge load of 144 psf is predic ted t o be 5


Figure 106. Preparation of specimens for earthwork specifications. (after Woodward-Clyde and Associates)

percent with maximum swell potential of less than 8 percent. Such average and maximum

swell is considered to be acceptable for the proposed type of construction.

4. The required depth of compaction depends upon the degree of expansion and the

magnitude of the imposed loads. Generally, 1 to 5 feet of compacted material will be

adequate with the range of 2 to 3 feet being the most commonly used.

SOIL REPLACEMEN T

A simple and easy solution for slabs and footings founded on expansive soils is to replace

the foundation soil with nonswelling soils. Experience indicates that if the subsoil consists of

more than about 5 feet of granular soils (SC-SP), underlain by highly expansive soils, there is no

danger of foundation movement when the structure is placed on the granular soils. The

mechanics and the path of surface water seeping through the upper granular soils and into the

expansive soils is not clear. It is concluded 1}hat either seepage water has never reached the

expansive soils, or the heaving of the lower expansive soils is so uniform that structural

movement is not noticeable.

This is not true in the case of man-made fill. For economic reasons, the extent of the

selected fill must be limited to a maximum of 10 feet beyond the building line. Therefore, the

possibility of edge wetting exists. A guideline has not been established as to the thickness


0 5 10 15 20 25 30

Moisture Content (%)

Figure 107. Determination of fill placement moisture and density, (after Woodward-Clyde and Associates)

requ i rement for the selected fill. A min imum of 3 feet should always be insisted upon , a l though 5

feet is preferred. This th ickness refers t o thickness of selected fill benea th the b o t t o m of the

footings or b o t t o m of floor slabs.

The per t inen t requi rements concerning soil replacement are the type of rep lacement

material , the dep th of replacement , and the ex ten t of replacement .

Type of material

Obviously, the first requi rement for the replacement soil is tha t it be nonexpansive . All

granular soils ranging from GW t o SC in the Unified Soil Classification System may! fulfill the

nonexpansive soil requi rement . However, for clean, granular soils such as GW and SP, surface

water can travel freely th rough the soil and cause wet t ing of the lower swelling soils. In the o the r

ex t reme , SC material wi th a high percentage of plastic clay somet imes will exhib i t swelling

potent ia l . The following criteria have been used with a certain degree of success:


Liquid limit,

percent No. 200 sieve

Percent minus

Greater than 50

30 - 50

Less than 30

15 - 30

10 - 40

5 - 5 0

It is becoming increasingly difficult t o locate materials , fulfilling the above requi rements , in

expansive soil areas such as Metropol i tan Denver. If necessary, the requi rement for impervious-

ness can be forfeited. Any selected fill will be satisfactory provided the material is nonexpansive .

Also, swell tests are the only positive m e t h o d of de termining the expansiveness of the material .

When in doub t , such tests should be conduc ted ra ther than relying on plasticity tests .

A great deal of emphasis has been given t o the possibility of blending granular soil wi th the

on-site swelling soils, thus reducing the a m o u n t of impor ted fill required. Theoret ical ly , such a

m e t h o d is reasonable; bu t in practice it is difficult t o incorpora te granular soil wi th stiff, dry

expansive clays. Disc harrows and plows will be required to break the clay in to reasonably sized

clods. Such an under tak ing will p robably be as expensive as using the lime stabilization m e t h o d .

Depth of replacement

The d e p t h of influence is a most complicated quest ion tha t must be answered w h e n dealing

wi th soil t r ea tmen t benea th the slabs or footings. T o what d e p t h should the natura l soil be

recompac ted? How m a n y feet of overexcavation will be required? How many cubic yards of

nonexpansive soil will have t o be impor ted? These quest ions cannot be intelligently answered

unt i l t he a m o u n t of movemen t tha t will occur benea th the slabs or footings can be assessed.

Theoret ical ly , the a m o u n t of uplift can be evaluated from the da ta derived from swell tests

and pressure dis t r ibut ion me thods . Gizienski and Lee [701 evaluated the theoret ical ly com-

puted uplift derived from labora tory test da ta and the actual measurement taken from a small

scale field test . They found tha t the actual heave in the field was only one-third of tha t est imated

from the results of labora tory tests .

The Colorado Highway Depa r tmen t established curves which show the relat ionship be tween

to ta l swell and the d e p t h below the surface of the subgrade [ 7 1 ] . Studies have shown that the

swelling can take place down to a d e p t h of as much as 50 feet. Also, 60 percent of the swell in

many of the Colorado subgrade clays can occur down to a 20-foot dep th .

While b o t h the theoret ical approach and actual measurement concerning d e p t h of influence

are urgently needed, the following should be poin ted ou t :

1. The potent ia l vertical rise of a soil mass, say 10-by 10-by 3-feet, (such as tha t used in

Gizienski 's exper iment ) unde r uniform saturat ion condi t ions , can be less than tha t of the

same mass subject to local wet t ing only. Uniform wett ing tends t o equalize heaving.

2. There is a definite gain in placing the s t ructure on a nonexpansive soil cushion. Even if

the deep seated soils swell, the movemen t will be more uni form, and consequent ly , more

tolerable.


3 . The d e p t h of selected fill should never be less than 36 inches and preferably 4 8 inches.

The swelling poten t ia l of the soil benea th the fill is very i m p o r t a n t as dens i ty and

mois ture condi t ions change at various locat ions . It should be no ted tha t wi th 4 feet of fill

plus the weight of concre te , a uniform pressure of abou t 600 psf is applied to the surface

of expansive soils. Fo r modera te ly swelling soil, such surcharge load can be i m p o r t a n t in

preventing poten t ia l heave.

4 . The failure of the soil replacement m e t h o d generally occurs during cons t ruc t ion . If the

subgrade or open excavat ion becomes wet ted excessively before the p lacement of the fill,

the t rapped wate r will cause heaving. In such case, de t r imenta l heaving will occur

regardless of thickness of the selected fill. The soils engineer should have the oppo r tun i t y

of supervising the p lacement of fill, or such a scheme should no t be adop ted .

5. The thickness of the impor ted fill can be reduced if a combina t ion of the soil

r ecompac t ion and soil replacement m e t h o d s is used. The natura l soil is scarified and

recompac ted as described under " C o m p a c t i o n C o n t r o l " for a thickness of abou t 2 feet,

then ano the r 2 feet of selected compac ted fill placed. The combined thickness of 4 feet

should be adequa te to con t ro l heaving.

6. The degree of compac t ion of the selected fill depends upon the type of suppor t ing

s t ruc ture . Fo r suppor t ing slabs, 90 percent of s tandard Proc tor densi ty should be

adequa te . Fo r suppor t ing footings, a degree of compac t ion of 95 to 100 percent should

be achieved.

Extent of replacement

The main reason tha t an artificially selected fill cushion is less effective than a natura l

granular soil b lanket is tha t in natural condi t ions , the b lanket extends over a large area, m u c h

larger than in the artificial condi t ion . In an artificial fill s i tuat ion, it is always possible for surface

water to seep in to the deep-seated expansive soil at the per imeter of the fill. Therefore , the larger

the area of replacement , the more effective the fill.

Figure 108 shows the suggested ex ten t of replacement for b o t h basement and nonbasemen t

condi t ions . With this ar rangement , the possibility of surface water enter ing the foundat ion soil is

greatly reduced. The t y p e of material used for backfill should be the same as used for the

underslab selected fill.

Evaluation

With present technology on expansive soils, soil replacement is the best m e t h o d to use in

obtaining a stabilized foundat ion soil. The following are the evaluations of soil rep lacement

m e t h o d :

1. It is possible to compac t the replaced nonexpansive soil to a high degree of compac t ion ,

thus enabling the mater ial to suppor t e i ther heavily loaded slabs or footings. Such

capabili ty cannot be obta ined by the prewet t ing m e t h o d . Also, wi th the compac t ion

cont ro l m e t h o d , a high degree of compac t ion on expansive soils is no t desirable, and,

consequent ly , the load carrying capacity is l imited.


-Ground surface

Overexcavated and replaced with nonexpansive f i l l .

7 Min.

-Drilled Pier

Building Lines

NON BASEMENT CONDITION

f— Drilled Pier

Building Lines

DEEP BASEMENT CONDITION

Figure 108. Suggested extent of fill replacement.

2 . The cost of soil replacement is relatively inexpensive when compared to chemically

t reat ing the soil. N o special cons t ruct ion equ ipmen t , such as disc har row, spreader, o r

mixer will be required. The cons t ruct ion can be carried ou t w i thou t delay as is

encoun te red in the prewet t ing m e t h o d .

3 . The granular soil cushion also serves as an effective barrier against the rise of ground

water or perched water .

4 . With the except ion of a s t ructura l floor slab (suspended floor), soil replacement provides

the safest approach to slab-on-ground cons t ruc t ion .

5. To guard against unexpec ted condi t ions which might cause heaving, it is strongly

suggested tha t floating slab cons t ruc t ion be used. Slip jo in ts mus t be provided for all


slab-bearing par t i t ion walls so there is n o chance of slab m o v e m e n t disturbing the

s t ruc ture .

6. Surface drainage a round the building mus t be proper ly mainta ined so there is n o

oppo r tun i t y for water t o en te r the expansive soils benea th the selected fill.

LIME STABILIZATION

The use of lime to stabilize subgrade soil has been k n o w n to engineers all over the world for

a long t ime. F o r centuries , the Chinese have used lime as a stabilizing agent in foundat ion soils.

Modern engineering rejected the use of lime—in preference to cement—because the cementa t ion

react ion of lime requires m a n y m o n t h s and the gain in s t rength is m u c h smaller t han with

cement . Since s t rength is no t a requi rement , lime is a favorable agent t o reduce the swelling

potent ia l of foundat ion soils. Most of the lime stabil ization projects were carried ou t by the

highway depa r tmen t s of various states. Fo r ins tance, the Texas State Highway Depa r tmen t used

nearly 1/2 million tons of lime for stabil ization in 1969. Al though the success of l ime-treated

subgrade is ques t ionable in many instances, the use of the l ime stabil ization m e t h o d has been

steadily increasing.

Reaction

It is generally recognized tha t the addi t ion of lime t o expansive clays will reduce the

plasticity of the soil and, hence , its swelling potent ia l . The chemical react ion occurr ing be tween

lime and soil is qu i te complex . The stabil ization apparent ly occurs as the result of t w o processes.

In one process, a base exchange occurs wi th the s t rong calcium ions of lime replacing

the weaker ions such as sodium on the surface of the clay particle [12]. Also, addit ional

non-exchanged calcium ions may be adsorbed so tha t the to ta l ion densi ty increases. The net

result is a low base-exchange capaci ty for the particle wi th a result ing lower volume change

potent ia l .

In the o the r process, a change of soil t ex tu re th rough flocculation of the clay particles

takes place when lime is mixed wi th clays. As the concen t ra t ion of lime is increased, there is a

reduct ion in clay c o n t e n t and a corresponding increase in t he percentage of coarse part icles. The

react ion results in reduct ion of shrinkage and swell and improved workabi l i ty .

W. G. Hol tz [73] found tha t lime drastically reduces the plasticity index and

drastically raises the shrinkage limit of montmor i l lon i t i c clays, as shown on figure 109.

Application

The a m o u n t of lime required t o stabilize the expansive soils ranges from 2 t o 8 percent by

weight. The recently comple ted Dallas-Fort Wor th Regional Airpor t [74 ] claims to have

under taken the world ' s largest lime stabil ization project , consuming abou t 3 0 0 , 0 0 0 tons of l ime.

The subsoil consists of 8 t o 16 feet of expansive clay wi th a po ten t ia l vertical expansion

equivalent t o 10 percent of t he layer thickness . The clays are underlain by shale of the Eagle

Ford Forma t ion .


PORTERVILLE CLAY

( U S B R . Data) FT. THOMPSON, S D . WEATHERED

(U .S .BR. Oota)

S 4 0

PERCENT LIME ADMIXTURE BY WEIGHT PERCENT LIME ADMIXTURE BY WEIGHT

HOUSTON BLACK CLAY

(From C. McDowell) S D . PIERRE 'SHALE'

(From Roods 8 Streets)

2 3 4 5 6

PERCENT LIME ADMIXTURE • BY WEIGHT

MONTGOMERY COUNTY CLAY, ILLINOIS

(From M R . Thompson)

PERCENT LIME ADMIXTURE BY WEIGHT

Figure 109. Effect of lime on plastic characteristics of montmorillonitic clays. (After Holtz).

The thickness of the t r ea tmen t ranged from 9 inches for taxiways and runways to 18 inches

for aprons. For stabil ization, 6 t o 7 percent of lime was required.

The stiff clay subgrade was broken down with a disc har row to m a x i m u m sized clods of 4 to

6 inches. Lime was applied in slurry consisting of one part lime to two par ts water by weight. The

slurry was applied t o the subgrade at 4 0 to 60 pounds pressure using water t rucks . The

application rate was sufficient to p roduce , within the stabilized layer, a dry lime con ten t of 6


percent . Exper ience wi th this project indicated tha t the lime t r ea tmen t no t only t ransformed the

soil to a nonswelling, friable mix tu re , bu t also improved the s t ructura l capaci ty of the t rea ted

layer.

In in ters ta te highway cons t ruc t ion in Florida, Ok lahoma , and o ther states lime stabil ization

was used to a large ex ten t . In Ok lahoma [ 7 5 1 , a deep plowing technique was used. The subgrade

was overexcavated 2 feet, then deep plowed by r ipper- type equ ipmen t for an addi t ional 2 feet.

Lime was then added, and the deep plowing opera t ion was con t inued unti l a good mix was

obta ined. After compac t ion , the 2 feet of soil which had been removed was replaced in

6-inch-thick layers, mixed wi th l ime, and compac ted . The a m o u n t of lime used was abou t 3

percent by weight.

The successful use of mixing lime in expansive soils for highway and airport cons t ruc t ion is

encouraging, a l though the d e p t h of t r ea tmen t required and the results of the t r ea tmen t on a long

term basis has no t been evaluated.

Mixing lime in foundat ion soils t o reduce swelling has no t been serious considered in the

past . It appears tha t , wi th the knowledge gained from airport and highway cons t ruc t ion using

lime, t r ea tmen t of underslab soils wi th lime deserves more a t t en t ion . This is especially t rue in the

case of large warehouses or school buildings where the floor covers a large area and a s t ructural

floor slab is no t feasible d u e t o the high cost. By overexcavating the site b o t h in d e p t h (3 t o 4

feet) and area and replacing the soil in compacted layers having adequa te lime t r ea tmen t , a stable

slab can be expec ted . With the present day limited knowledge of lime stabil ization, footing

foundat ions should no t be placed on t reated expansive soils.

Pressure injection

The pressure injection m e t h o d of lime stabil ization has been used in Jackson, Mississippi, in

Calexico, California, and in Tucson , Arizona [ 7 7 1 . The m e t h o d consists of pressure injecting

l ime-water slurry in to the soil th rough closely spaced drill holes as shown on figure 110.

The drilled holes were 5 feet deep , located adjacent to the building, and on 3-foot centers .

In Jackson, Mississippi, where the soils benea th 200 houses were t rea ted , it was repor ted tha t an

est imated 10 percent of the t reated soils had to be re t rea ted , and 1 percent received three

t r ea tments .

Quest ions arise concerning the lime pressure-injection m e t h o d and the ex ten t of lime

migrat ion in to the swelling soils. Expansive clays are generally stiff and practically impervious.

Lime slurry will disperse from the injection po in t th rough roo t holes , fissures in clay, and

desiccation cracks. The ex ten t of such migrat ion is probably l imited. Addi t ional informat ion on

the swelling potent ia l or swelling pressure of the soils unde r t r ea tmen t in Jackson, Calexico, and

Tucson is no t known , b u t it is very possible tha t the a m o u n t of swell in these areas is mild.

L. K. Davidson [76] stated in 1965 tha t the results of labora tory studies show tha t lime

does diffuse in to a soil-water sys tem. Fo r the exper imenta l condi t ions , the rate of diffusion was

very slow and given by the equa t ion :

L = 0 .081 t*

Where: L = lime pene t ra t ion dis tance, in.

t = t ime, days


DRILLED HOLES OR • t z - PRESSURE INJECTION

POINTS

DRILL OR TRENCH THROUGH CONCRETE .

TYPICAL PLAN

3/4" DIAMETER INJECTION HOLES AT 3 FOOT CENTERS (TYPICAL)

INJECT SLURRY USING TWO PIPE SYSTEM -OUTER PIPE 3 /4 INCH DIAMETER, POINTED AT BOTTOM AND PERFORATED IN LOWER FOOT WITH 1/8" HOLES-, INNER PIPE IS 1/4" IN. DIAMETER. THE PIPES ARE JETTED IN THE GROUND.

CONTINUE TO INJECT SLURRY IN EACH HOLE UNTIL SLURRY COMES OUT OF GROUND AROUND ΓΗΕ PIPE. REPORTED INJECTION PRESSURES AT NOZZLE ARE IN THE RANGE OF 2 0 0 TO 4 0 0 PSI

TYPICAL SLURRY PROPORTIONS •• 50 SACKS OF HYDRATED LIME ( 5 0 LBS /SACK) TO 9 0 0 GALLONS OF WATER, Co ( 0 H ) 2 CONTENT IN LIME AVERAGES 9 5 % . LIME AND WATER SLURRY ARE MIXED IN A BLENDING TANK PRIOR TO INJECTION.

Figure 110. Lime stabilization - pressure injection method. (Calexico, Calif. & Jackson, Miss.).

Using this formula results in a pene t ra t ion distance of 1.5 inches in 1 year.

It is the conclusion from b o t h labora tory and field experience tha t lime migrat ion in to

expansive soils is ext remely slow. The rate of migrat ion can probably be increased by int roducing

large quant i t ies of wate r t o carry the lime slurry. There is the potent ia l danger of triggering an

excessive a m o u n t of swelling in the deep seated soils.

Woodward-Clyde-Sherard & Associates [ 7 7 1 , in their investigation, concluded tha t the

success of lime t r ea tmen t is p robably because of mois ture barrier effects ra ther than because of

any widespread changing of soil proper t ies .


CHEMICAL STABILIZATION

Besides the use of l ime, o the r chemicals, b o t h organic and inorganic, can be used to

stabilize expansive soils. Cemen t and fly ash have b o t h been used in the labora tory wi th

successful results. Of course, the cost of cement is considerably more than tha t of l ime. Fly ash is

somet imes added to the soil-lime mix tu re to increase pozzolanic react ion.

O the r inorganic chemicals such as sodium silicate, calcium hydrox ide , sodium chloride,

calcium chloride, and phosphor ic acid have been used t o stabilize expansive soil. Most of these

chemicals are effective u n d e r labora tory condi t ions , b u t their applicat ion in the field is very

difficult. There is no suppor t ing evidence tha t any of the chemicals has economical ly wor thwhi le

benefi ts [ 7 8 ] .

Cement stabilization

The hydra t ion p roduc t s of por t land cement include calcium silicate hydra tes , calcium

aluminate hydra tes , and hydra t ed l ime. During hydra t ion , por t land cement releases a large

a m o u n t of l ime. It is believed tha t the base exchange and cement ing act ion of por t land cement

wi th clay is similar t o tha t of l ime. In addi t ion t o the above act ions, the incorpora t ion of

por t land cement in clay increases the s t rength of the mix tu re . The resulting p roduc t c o m m o n l y

k n o w n as soil-cement is familiar t o mos t soil engineers.

The act ion of cemen t on clay minerals is t o reduce the liquid limit, plasticity index, and

potent ia l volume change, and t o increase the shrinkage limit and shear s t rength [ 7 9 ] .

Spangler and Patel [80] repor ted on the labora tory t r ea tmen t of an expansive Iowa

gumbot i l wi th por t l and cement . The addi t ion of 2 percen t and 4 percent of por t land cement

considerably reduced the potent ia l volume change of the soil.

Jones [72] added 2 t o 6 percent of por t land cement t o the expansive Porterville clay of

California which resulted in the p ronounced reduct ion of volume change characterist ics.

The effect of cement and of lime was abou t the same in reducing soil expans ion , b u t the

cement reduced the shrinkage of air-dried specimens about 25 t o 50 percent more than did the

l ime.

The mixing and dispersing m e t h o d s for cement are nearly identical t o those for l ime. The

difficulties of uniformly in t roducing por t land cement in to very fine-grained soils are generally

greater than with lime because it is less soluble.

Bo th cement and lime have been used in highway cons t ruc t ion for modifying the swelling

p roper ty of the subgrade soil. The use of cement and lime to stabilize underslab soil in buildings

is seldom repor ted . There appears t o be a great potent ia l for using cement to modify the

underslab soils. With 2 t o 6 percent cement incorpora ted in the clay, the resulting soil-cement

mix tu re acts as a semi-rigid slab. If the deep-seated soil expands , the swelling effect tends t o

dis t r ibute uniformly, thus reducing damage caused by differential heaving. Such cons t ruc t ion is

part icularly favorable for the t r ea tmen t of a large warehouse floor where a crack-free, level floor

is essential and the use of a s t ructural floor slab is economical ly prohibi t ive. Due to lack of

s t rength, the use of lime canno t provide a semi-rigid e lement benea th the slab.


A great deal of research and field s tudy will be required before cement stabil ization can be

economical ly applied. An effective applicat ion m e t h o d , ei ther by mixing or by slurry injection,

mus t be perfected before the scheme can be considered in pract ice .

Organic compound

Organic c o m p o u n d s stabilize expansive soils by waterproofing, by retarding water

adsorpt ion, or by hardening the soil w i th resins. Organic c o m p o u n d s such as Arguard 2HT or

4-Terf-Butylpyrocatechol have been used wi th a limited degree of success.

Davidson and Glab [ 8 1 ] , in labora tory investigation of highly plastic Iowa subgrade soils,

have shown tha t certain organic c o m p o u n d s which furnish large organic cat ions when dissolved in

water have considerable promise as admixtures t o increase the stability of such soils. They found

tha t water solut ions of chemical admixtures of this type decreased plast ici ty, shrinkage, and

swelling of plastic soil samples.

A propr ie tary liquid k n o w n as Fluid 7 0 5 , 7 0 6 , and 707 was in t roduced by Soil Technology

Corpora t ion in Denver, Colorado. The fluid was mixed wi th swelling clays and tested in the

labora tory for physical characterist ics, swelling potent ia l , and permeabi l i ty .

Expansion tests were performed on three remolded specimens of Denver clay shale. One

specimen was t reated wi th distilled water , the second wi th propr ie ta ry fluid 7 0 5 , and the third

with propr ie tary fluid 706 . A surcharge load of 100 psf was applied t o each specimen. The

specimens were saturated wi th distilled water and the a m o u n t of expansion de te rmined . The

specimen t reated wi th fluid 705 did n o t expand. The specimen t reated wi th fluid 706 was

modera te ly expansive. T h e specimen t reated wi th distilled water was highly expansive.

Figures 1 1 1 , 112, and 113 give the test results and the change of At terberg limits from high

plasticity t o nonplast ic .

The permeabi l i ty tests were performed on specimens comprised of a mix tu re of 15 percent

clay and 85 percent silica sand by weight. The sand used was Silica Sand Natural Grain, supplied

by the O t t awa Silica Company , Ot tawa , Illinois. The clay used was Aquagel supplied by the

Baroid Division of the Nat ional Lead C o m p a n y , Hous ton , Texas.

Cons tant head permeabi l i ty tests were performed on t w o remolded specimens of clay and

silica sand using distilled wa te r in one test and the propr ie tary Fluid 707 in the other . The

material was found t o be impervious t o distilled water during the 34-day testing period. The.

coefficient of permeabi l i ty of the propr ie tary Fluid 707 t reated soil was de termined to be

approximate ly 6 feet per year.

Permeabil i ty test is impor t an t , as in actual applicat ion the fluid mus t be able to migrate in to

the soil. The ability of t he fluid t o permea te in the impervious soil is encouraging.

The first large-scale exper iment on the use of t he propr ie tary fluid took place in December

1974 in Denver, Colorado . Specially designed equ ipment as shown on figures 114 and 115 was

used. The machine could hydraulical ly bore 1-1/2-inch-diameter holes, three at a t ime, to a d e p t h

of m o r e than 10 feet in stiff clay and claystone shale. The auger was advanced by a pressure of

3 0 0 t o 500 psi. Thus , the holes could be advanced a to ta l of 10 feet in less than a half minu te .

The holes were spaced 36 inches apart and in highly impervious soil the spacing was reduced t o

18 inches. Propr ie tary fluid was in t roduced in to the holes unde r a pressure of about 10 psi.


PLACEMENT CONDITIONS' Dry Density = 76.3 pcf Moisture Content = 1 4 . 0 % Atterberg Limits- Liquid Limit = 6 8 %

Plasticity Index = 17 %

^ 3 0 . 8 % Expansion at 100 psf when wetted

ο ο ιοο ιοοο

APPLIED PRESSURE (psf)

Figure 111. water only.

Swell-consolidation test results on remolded sample of Denver clay shale treated with distilled

The t r ea tmen t was in tended to ex tend for a d e p t h of at least 6 feet. It was in tended to

reduce the plasticity index of the expansive clays from abou t 4 0 to 10 percent and the swelling

potent ia l from mode ra t e swelling to nonswelling. Undis turbed soil samples were taken before and

after t r ea tmen t to de te rmine the effectiveness of the appl icat ion. The results were no t as

expected. Bo th the plasticity index and the swell potent ia l did n o t significantly reduce. Valuable

experience was gained from the exper iment , however , some of which follows:

1. T h e holes should have a m a x i m u m spacing of 12 inches.

2. The fluid mus t be applied unde r a pressure of n o t less than 250 psi.

3 . Pressure gages should be provided t o indicate a pressure d rop when the fluid flows in to

the seams and fissures in the clay. The auger should then be advanced t o avoid the

fissures.

It is believed tha t wi th further s tudy on field appl icat ion and mechanical improvemen t , the

above m e t h o d will eventually find an impor t an t place in the realm of chemical stabil ization.


PLACEMENT CONDITIONS' Dry Density =81.4 pcf Moisture Content = 1 1 . 5 % Atterberg Limits = Non-plastic

8

No Expansion when wetted

100 1000


Figure 112. Swell-consolidation test results on remolded sample of Denver clay shale treated with Fluid 705.


PLACEMENT CONDITIONS' Dry Density =78 .6 pcf Moisture Content = 1 2 . 2 % Atterberg Limits = Non-plastic

2 Ο CO

2 X

2 Ο < Ο ο CO

ο ο

3.7% Expansion at 1 0 0 p s f when wetted

100 1000


Figure 113. Swell-consolidation test results on remolded sample of Denver clay shale treated with Fluid 706.

180

FO

UN

DA

TIO

NS

ON

EX

PA

NS

IVE

SO

ILS

Figure 114. Equipment used for chemical injection. Figure 115. Injection heads bored hydraulically.


R E F E R E N C E S

McDowell, C , "Remedial Procedures Used in the Reduction of Detrimental Effect of Swelling Soils,"

Texas Highway Department.

"Recommended Practices for construction of Residential Concrete Floors on Expansive Soil" Portland

Cement Association Vol. II, Los Angeles, California, 1970

Bara, J. P., "Controlling the Expansion of Desiccated Clays During Construction," Second International

Research Conference on Expansive Clay Soils, August, 1969.

Blight, G. E., and Wet, J. Α., "Acceleration of Heave of Structures on Expansive Clay," Moisture Equilibria

and Moisture Changes in the Soils Beneath Covered Areas.

Felt, E. J., "Influence of Vegetation on Soil Moisture Content and Resulting Soil Volume Changes,"

Proceedings, Third International Conference on Soil Mechanics and Foundation Engineering, Zurich, Vol.

I, 1953.

Dawson, R. F., "Modern Practices Used in the Design of Foundations for Structures on Expansive Soils,"

Quarterly, Colorado School of Mines, Vol. 54, No. 4, 1959.

Holtz, W. G., and Gibbs, H. J. "Expansive Clay-Properties and Problems," Quarterly of Colorado School

of Mines, Vol. 54, No. 4, October, 1959.

Gizienski, S. F. and Lee, L. J., "Comparison of Laboratory Swell Tests to Small Field Tests," Concluding

Proceedings, International Research and Engineering Conference on Expansive Clay Soils, Texas A and M

Press.

Gizienski, S. F. and Lee, L. J., "Comparison of Laboratory Swell Tests to Small Scale Field Tests,"

International Research and Engineering Conference on Expansive Soils, 1965.

"Lime Shaft and Lime Tilled Stabilization of Subgrades in Colorado Highways," Interim Report 1967,

Planning and Research Division, Dept. of Highways, State of Colorado.

Jones, C. W., "Stabilization of Expansive Clay with Hydrated Lime and with Portland Cement," Bulletin,

Highway Research Board, No. 193, 1958.

Holtz, W. G., "Volume Change in Expansive Clay Soils and Control by Lime Treatment," Proceedings of

the Second International Research and Engineering Conference on Expansive Soils, Texas A & M Press,

1969.

Kelly, J. E., "Lime Stabilization of Expansive Clays at the Dallas-Fort Worth Airport," Proceedings of

Workshop on Expansive Clays and Shale in Highway Design and Construction.

Thompson, M. R., "Lime Stabilization: Deep Flow Style," Road and Streets, March 1969.

Davidson, L. K., Demirel, T., and Hardy, R. L., "Soil Pulverization and Lime Migration in Soil-Lime

Stabilization," Highway Research Board, 1965.

"Remedial Methods Applied to Houses Damaged by High Volume-Change Soils," Woodward-Clyde-Sherard

& Associates, FHA Contract H-799.

Gromko, G. J., "Review of Expansive Soils," Journal of the Geotechnical Engineering Division, June 1974.

Croft, J. B., "The Influence of Soil Mineralogical Composition on Cement Stabilization," Geotechnique,

London, England, Vol. 17, June, 1967.

Spangler, M. G. and Patel, Ο. H., "Modification of a Gumbotil Soil by Lime and Portland Cement

Admixtures," Proceedings, Highway Research Board, Vol. 29, 1949.

Davidson, D. T. and Glab, J. E., "An Organic Compound as a Stabilization Agent for Two Soil Aggregate

Mixtures," Proceedings Highway Research Board, Vol. 29, 1949.

Chapter 9

INVESTIGATION OF FOUNDATION MOVEMENT


Investigating the cause of foundat ion m o v e m e n t of an existing building and prescribing

remedial measures requires careful field investigation, exhaust ive labora tory test ing, and m a n y

years of experience. In some respects, this is similar to the t r ea tmen t of a pat ient . Inquiry of the

pa t ien t ' s medical record, a physical examinat ion , and a labora tory diagnosis will be necessary to

diagnose the cause of the sickness. Prescript ion and t r ea tmen t will be relatively simple once the

cause of illness has been de te rmined . As in t he case of a doc to r , n o examina t ion and test ing can

replace exper ience, and exper ience can only be obta ined by trial and error.

In the past 20 years, t he au tho r has had the oppo r tun i t y to s tudy more than 1,200 cases of

cracked buildings in the States of Colorado and Wyoming. These cases include residences, school

buildings, offices, warehouses , swimming pools , apa r tmen t buildings, religious s t ructures , and

pavements . Most of the cracked buildings were the result of founda t ion m o v e m e n t caused by

swelling soils.

H ISTORY STUDY

The first s tep in the investigation of a building is t o obta in comple te in format ion pertaining

t o the building. Unfor tuna te ly , such informat ion is of tent imes absent and it is necessary t o

uncover m u c h of the required informat ion by soil explora t ion .

Foundation information

Effort should be m a d e to obta in the existing informat ion foundat ion relative to the soil. For

buildings erected before 1960, such informat ion is generally ske tchy . Soil tests on individual sites

have b e o m e a requ i rement after 1960. F rom the soil test da ta , it will be possible to de te rmine the

following:

1. T y p e of foundat ion ,

2. Design criteria,

3 . Water table condi t ion ,

4 . Type of foundat ion soils,

5. Moisture con ten t of founda t ion soils, and

6. Swelling potent ia l of foundat ion soils.


Somet imes, the subsoil investigation is n o t conduc ted for a specific building b u t for a

general area. In such case, the subsoil informat ion has only limited use. Care should be exercised

to locate the building u n d e r investigation to the nearest test hole , so tha t it is possible to

de termine as closely as possible the subsoil condi t ion benea th the building.

The above soil test da ta can be invaluable toward finding the cause of s t ruc ture movemen t .

The second step is to check the foundat ion plan. Again, such informat ion may no t be available,

either because the drawing is lost, or the concerned par ty does no t want to p roduce it. The

foundat ion plan will reveal if the r ecommenda t ions given in the soil repor t have been followed.

These are:

1. The dead load pressure exer ted on the footings or piers,

2. The size of footings or piers,

3 . T h e length of t he piers,

4 . Pier re inforcement , expansion jo in t , dowel bars , underslab gravel, and o the r details, and

5 . Subdrainage system.

If the above informat ion is available, the investigation will be greatly simplified. This is

similar to the case where the comple te medical record of a pa t ien t is at the disposal of the

examining doc tor . It is also necessary t o examine the qualifications of the designer, whe ther the

design is made by a registered professional engineer or by the cont rac tor .

If t he above informat ion is n o t available, it will be necessary t o expose t he foundat ion

system by excavat ion. In t he case of a nonbasement building, excavat ion can be easily made

outside of the building adjacent to the grade beam. In the case of basement cons t ruc t ion , it will

be necessary t o break the concrete slab t o reach the foundat ion . It would be a difficult j ob to

expose the ent ire length of the pier, b u t m a n y t imes it is advisable to examine the pier to

ascertain a problem such as uplift.

Logs kep t by t he driller are somet imes available. In such cases, a comple te informat ion of

the pier system will be apparent . This will also provide informat ion on the d e p t h of pene t ra t ion

into bedrock , for b o t h in ter ior and exter ior piers, as well as the water table condi t ion.

Movement data

Effort should be m a d e to obta in chronological da ta on the building movemen t , i tems such

as when the building was comple ted , when the first occupant moved in, and when the first crack

appeared. All in format ion obta ined from the owner should be carefully scrutinized for its

validity. If the owner in tends to sue the bui lder to recover his damages, he tends to exaggerate his

findings. However, wi th careful in terrogat ion and keen observation, the actual s tory can be

revealed.

When examining the exter ior of the building, it is helpful to de te rmine the lawn watering

practice, the set t ing of the au toma t i c sprinkling system, and the condi t ion of the backfill. Most

owners deny excessive irrigation of the lawn and flower beds .

In the inter ior of the building, pr imary informat ion can be obta ined in the basement area.

Water marks or effloresce on the wall usually tell t he s tory of seepage water . A comple te record

on seepage water should be ob ta ined ; the first appearance of water in the basement , the locat ion

INVESTIGATION OF FOUNDATION MOVEMENT 185

of seepage, the a m o u n t of observed water , and whe the r seepage has taken place after heavy

precipi ta t ion.

Also impor t an t is the performance of ut i l i ty lines. Has there been p lumbing difficulty

experienced in the past years? Has the floor drain been plugged? In one instance, investigation

revealed tha t the inter ior house sewer was never connec ted to the street sewer, b u t empt ied in to

the underslab soils. The defect was no t discovered unti l an odo r was de tec ted in the basement .

In ano the r case, the basement shower drain was no t connected to the sewer line. F o r years, the

error remained unde tec ted unt i l the crawl space was entered and an excessive wet t ing condi t ion

discovered.

It is no t always possible to de te rmine the site condi t ions during cons t ruc t ion , b u t if such

informat ion is secured by an observant owner , it can un lock m a n y m o v e m e n t puzzles. There are

instances where the soils were flooded during cons t ruc t ion , and heaving m o v e m e n t t ook place

even before the building was comple ted . Investigation of partially comple ted house revealed tha t

the basement was covered wi th more than 2 feet of snow which the con t rac to r had failed t o

remove before enclosing the s t ruc ture . Fo r drilled pier foundat ions , it is dangerous t o allow the

surrounding soils to become wet ted before the appl icat ion of dead load pressure. Pier uplift can

begin before the placing of the foundat ion concre te .

In the informat ion gathering process, all hearsay should be screened. Stories such as an

underground river running under the s t ruc ture , the building is sliding downhi l l , the ben ton i t e in

the soil has pulled the bui lding apart and o thers should be dismissed as hearsay b y an experienced

engineer, and only substant ive evidence considered.

DISTRESS STUDY

The first sign of founda t ion movemen t , for s t ructures founded on expansive soils, is the

cracking of the floor slab. This is generally followed b y doors binding, windows sticking, and

cracks appearing in the exter ior and inter ior walls and even in the ceiling.

Crack pattern

F o u n d a t i o n m o v e m e n t s are reflected as cracks. Cracks caused by swelling soils have the

same general pa t t e rn as se t t lement cracks, a l though swelling cracks are generally wide at the t o p

and nar row at the b o t t o m . The same crack pa t te rns can develop from se t t l ement . However , in t he

mos t severe se t t l ement cases, diagonal cracks are usually associated wi th a series of hor izonta l

cracks as shown on figure 116. In the Rocky Mounta in area, the problem caused b y expansive

soil is well k n o w n , n o t only t o the soil engineer, b u t of tent imes even to the layman. A t t he first

sign of cracking, t he immedia te react ion is tha t the problem is caused by swelling soils. Figure

117 indicates a severe crack tha t developed in a twin-tee s t ructura l slab. T h e building is founded

wi th drilled piers, and the exter ior walls are in excel lent condi t ion . These cracks had no th ing t o

d o wi th foundat ion movemen t .

It is no t always t rue tha t founda t ion m o v e m e n t of a specific po r t ion of a s t ruc ture is

responsible for certain cracks appearing in the immedia te vicinity of tha t movemen t . T h e


Figure 117. Floor cracks due to shrinkage of twin-tee topping.

186

Figure 116. Typical settlement cracks; crack pattern varies f rom horizontal to diagonal which is quite different f rom heaving cracks.


Figure 118. Vertical cracks beneath the beam pocket caused by lifting of I-beam.

s tructural a r rangement of a building, especially tha t of a house , is complex . Movement of one

por t ion of the building can cause cracks to appear at the opposi te end of the building. It is always

p ruden t to explain the cause of m o v e m e n t in a general sense and t reat and s tudy the movement

as a uni t . The following crack analysis can serve as a guide:

1. Diagonal cracks below exter ior windows or above exter ior doors generally indicate

footing or drilled pier foundat ion movemen t .

2. If such cracks appear only in the exter ior br ick course bu t no t on the inter ior dry wall,

the cracks can be caused by exter ior pa t io slab heaving.

3 . Hairline cracks appearing above inter ior doors and closets could be caused b y plaster

shrinkage or t imber shrinkage are no t necessarily foundat ion movemen t .

4 . Vertical cracks below the I-beam in the basement concre te wall can be caused by the

lifting of the I-beam, resulting in tension cracks as shown in figure 118.

5. Separat ion of the window frame from the brick course as shown on figure 119 generally

indicates differential heaving. Such movemen t has a s trong resemblance t o lateral

movemen t . Actual ly , a lmost all lateral separat ion is caused by differential heaving.


Figure 119. Separation of window frame from brick course.

Stress build-up

Movement of inter ior s t ructural member s can result in stress build-up in the s t ruc ture . The

mos t c o m m o n instance is the uplift of the I-beam caused by the uplift of steel pipe columns.

When the I-beam lifts, the joist system in the upper level is d is turbed, doors stick and closets

canno t be opened. The owner generally planes the d o o r only to find tha t it fails to open and

close again after a period of t ime. The I-beam in the basement is commonly suppor ted by two to

three steel pipe columns. When the one pipe co lumn foundat ion heaves, the o the r pipe column is

usually rendered idle and can be shaken loose by hand.

Pipe columns are provided wi th a screw jack at the t op . The si tuat ion can be corrected, at

least temporar i ly , by lowering the screw jack and revealing the I-beam. The doors are then able t o

be opened and closed freely again.


Stress bui ld-up caused by slab bearing par t i t ion walls has been discussed under "Slabs on

Expansive Soils ."

Owners somet imes repor t tha t the cracks in their buildings are subject to opening and

closing and a t t e m p t t o correlate the m o v e m e n t to seasonal cl imate change. This then leads to the

theory tha t the subsoil has undergone cycles of drying and wet t ing . In mos t cases, the opening

and closing of t h e cracks are caused by the shifting of the locat ion of stress concen t ra t ion . When

a new crack appears, the stress d is t r ibut ion is al tered, and this will temporar i ly close an old crack.

Careful observat ion will indicate tha t the to ta l n u m b e r of cracks appearing in a building is

constant ly increasing and seldom decreasing.

INVESTIGATION

Subsoils

To definitely define the cause of foundat ion m o v e m e n t and to r ecommend remedial

measures , it is necessary to de te rmine the subsoil condi t ions and wate r table. Test holes should be

drilled adjacent to the building and sufficient samples should be taken for the de te rmina t ion of

the swelling characterist ics and mois ture con ten t of t he soil. At least one test hole should be

drilled r emo te from the s t ruc ture and in an area unaffected b y building cons t ruc t ion . The

physical characteristics of t he soils obta ined from the adjacent and r emote test holes can be

compared .

It should be noted tha t for a building having cracking, the soils immedia te ly below the

foundat ion level generally have been wet ted excessively. Labora to ry testing will invariably show a

low swell potent ia l . However , careful test ing can reveal tha t the mater ial possesses a high swelling

pressure. Somet imes , the actual swelling characterist ics of the soil can only be revealed by air

drying the soil sample, and then subjecting it to wet t ing. In any event , samples obta ined from

areas unaffected by building cons t ruc t ion should give informat ion relative to the soil behavior at

the t ime of cons t ruc t ion .

The mos i tu re c o n t e n t as well as the dry densi ty of all soil samples should be de te rmined . If

possible, the mois ture c o n t e n t should be carefully compared wi th the mois ture con ten t of the

soil pr ior t o building cons t ruc t ion . Usually, in the course of nearly 1,200 cases investigated, the

mois ture con ten t benea th the building area had increased. The magni tude of increase ranged from

2 to 8 percent . T h e c o m m o n l y assumed theo ry tha t the soils benea th a s t ruc ture are subject to

wet t ing and drying, resulting in expansion and shrinkage, is n o t necessarily t rue . In fact, drying

and shrinkage of soils seldom or never cause cracking in a building. In one building, the soil

immediate ly benea th the furnace in the basement was examined. The underslab mois ture con ten t

was high. N o shrinkage is likely to take place benea th the central por t ion of a covered area.

Survey

T o mos t s t ructural engineers, t he first order of investigation of a cracked bui lding is a survey

t o de te rmine which par t of the building has moved and the magni tude of movemen t . Admi t t ed ly ,


a survey will assist in the de te rmina t ion of the general t rend of building m o v e m e n t ; however , it

should never be used as a clue to foundat ion movemen t .

A m o v e m e n t survey is of little value if n o t compared wi th a previous survey record. Also,

the results will be of doubt fu l value if a reliable bench mark is no t uti l ized. Bench marks such as

the t o p Of a fire hyd ran t , t e lephone pole or manhole cover are subject to m o v e m e n t in an

expansive soil area, and readings which rely on these bench marks can be entirely misleading.

A reliable bench mark should consist of a concre te pier drilled deep in to bedrock in a zone

where mois ture change will n o t take place. The pier should be well reinforced.

By referring t o a reliable bench mark and conduct ing a survey at intervals of once a m o n t h ,

the m o v e m e n t of the building can thus be mon i to red . The survey mus t be conduc ted wi th

control po in t s carefully selected. Such under tak ing is costly and t ime consuming. Only in the

course of an impor t an t s t ruc ture is such a survey war ran ted .

T h e c o m m o n practice of referring the movemen t of the building to a brick course or to the

t o p of the grade beam, assuming tha t these are level at the t ime of cons t ruc t ion , can be total ly

misleading. A m o v e m e n t survey of a cracked building is of l i t t le value unless conduc ted

professionally.

Test pits

The only positive m e t h o d of determining the subsoil condi t ion and cons t ruc t ion details in

the foundat ion system is by opening test pits . If the building has a crawl space, the investigation

procedure can be greatly simplified. Pits should be opened adjacent to the grade beam and next

to the interior suppor ts . By investigating the test pi t , the following can be revealed:

1. The foundat ion system,

2. Condi t ion of the air space benea th the grade beam (drilled pier founda t ion) ,

3 . Condi t ion of the t op of pier, the presence of mush rooms ,

4 . The mois ture con ten t of the soil (soil samples should be taken every 12 inches to a d e p t h

of at least 5 feet) ,

5. The presence of underslab gravel, gradat ion of gravel and thickness of the gravel layer,

and

6. Condi t ion of the concre te slab, t y p e of re inforcement .

Wherever possible, at least one pier should be uncovered for its ent ire d e p t h and examined

for possible tension cracks or voids at the b o t t o m of the pier.

CAUSE O F M O V E M E N T

When all the investigation and s tudy out l ined above have been comple ted , the cause of

movemen t of the building can then be de te rmined . Obviously, no movemen t will t ake place in an

expansive soil area unless the foundat ion soil becomes wet ted excessively. Therefore , the source

of mois ture mus t be de te rmined .


Foundation design

Founda t i on designs m a d e before 1960 are based u p o n soil repor t s wr i t ten wi th a l imited

knowledge of swelling soil problems and their solut ions. Consequent ly , it is no t surprising t o find

tha t the criteria established in these early soil repor ts are no t sufficient t o cope wi th the

complex i ty of the swelling potent ia l of the soil. The usual shor tcomings of the given criteria are

insufficient dead load pressure exer ted on a pier or pad founda t ion system, insufficient pier

length, and lack of re inforcement .

A c o m m o n design defect is the use of m o r e suppor ts than necessary benea th the I-beam.

Dead load pressure exer ted on the inter ior piers is, therefore , very low. These piers are subject to

uplift. A remedial measure would be t o use a heavier beam section wi th fewer suppor t s .

Tradi t ional design is to dowel the exter ior pat io slabs in to the grade beams with dowel bars.

This p rocedure has caused a great deal of heaving problems as explained previously in chapter 6

unde r "Slabs on Expansive Soils ."

Despite the possible deficiency of a drilled pier or footing system, a proper ly engineered

foundat ion system suffers only minor distress even under the mos t adverse condi t ions . Most

buildings tha t suffer severe damage are designed and cons t ruc ted by cont rac tors wi thou t the

benefit of a soil or s t ructural engineer.

Some cont rac tors take the m a t t e r entirely in to their own hands and prewet the foundat ion ,

puddle the backfill, reinforce the footings instead of the founda t ion walls, drill oversized piers

and expect a stable building. Unfor tuna te ly , or for tunate ly , these buildings may remain in good

condi t ion for years and give an excuse to the builder to cont inue this undesirable pract ice.

Construction

Legally, if the con t rac to r follows every detail specified by his consultants—the s tructural

and soil engineers—then his liability in the event of future building damage will be greatly

reduced. Obviously, the con t rac to r is the first target of the owner in a lawsuit filed for

negligence. Damage caused by swelling soils is no t recognized by the cour t as an act of God .

In the cour t , every effort will be m a d e t o prove the con t rac tor ' s negligence. Fo r ins tance,

the absence of slab re inforcement should n o t be impor t an t wi th respect to slab cracking;

however, in the cour t , this appears as a glaring mis take on the par t of the con t rac to r for no t

following the design.

Most cont rac tors a t t e m p t to d o a good j o b in their building. Details such as the separat ion

of slabs from bearing walls, the use of a dowel bar to connect pa t io slabs wi th grade beams, the

absence of void spaces in the slab bearing par t i t ion walls and others are the results of incomple te

specifications and the ignorance of the con t rac to r on the t echn ique of building in expansive soil

areas ra ther than purposely omi t t ing the details. Unfor tuna te ly , in the cour t of just ice , n o

differentiat ion can be made be tween ignorance and in tent ional errors.

There are except ions where the con t rac to r abuses every rule of good cons t ruc t ion pract ice

in the foundat ion cons t ruc t ion and expects to go unde tec ted because the covered foundat ion

excavation will no t be quest ioned after the building is comple ted . In one large apa r tmen t

complex , the piers were drilled off center from the grade beams, some having missed the grade


beams complete ly . Piers were b o t t o m e d on the upper soils instead of the bedrock , resulting in

ch imney tilting. Some of the cons t ruc t ion defects are shown on figure 120.

Drainage

Water entering foundat ion soils can be from one or more of the following sources:

1. Rise of ground water or deve lopment of perched water ,

2. Poor surface drainage causing surface water to enter th rough backfill in to the foundat ion

soils,

3 . Breakage of ut i l i ty lines.

A perched water table condi t ion cannot be foreseen at the t ime of cons t ruc t ion and is

usually no t men t ioned in the soil repor t ; therefore , a subdrain system is generally no t specified in

the design. The responsibili ty of damage caused by perched water is difficult to define. When

Figure 120. Timber supports used to correct the missed piers.


there is a possibility of perched water , a subdrain system should be provided. To provide an

effective subdrain system against perched water condi t ion involves considerable cost. Damage

caused by perched wate r should be in the category of " A n act of G o d " .

Almost 9 0 percent of the water tha t enters foundat ion soils is derived from surface water .

As discussed in chapter 7, inadequate slope around the building, loose backfill, improper locat ion

of the sprinkling system, and shrubs and flower beds planted adjacent to the building are all

potent ia l causes of wet t ing of foundat ion soils.

If civil act ion is instigated resulting from foundat ion problems, the strongest defense the

con t rac tor possesses against the owner ' s suit is tha t the owner has no t provided proper drainage

around the building. An alert developer issues each h o m e o w n e r a manua l on the care of drainage

around the house . In the event of a complain t , the bui lder can then have recourse back to the

owner .

While it is t rue tha t poor drainage in t roduces water in to the foundat ion soils and causes

heaving, the role of drainage in the cracked building has been exaggerated. Moisture con ten t in

foundat ion soils can increase substantial ly to allow heaving even if the drainage a round the house

is in excellent condi t ion .

When investigating a cracked building, the m o s t obvious defect is tha t of improper drainage

around the building. Consequent ly , many investigators con tend tha t by correct ing the surface

drainage, the problem is solved. Usually, this is far from the real cause of foundat ion movemen t .

When a soil engineer provides the foundat ion design criteria, he should design for sa turated

soil condi t ions . In the field, he selects the soil wi th the highest swell potent ia l for testing. In the

labora tory , he saturates the soil sample to de te rmine the m a x i m u m swell potent ia l . His design

criteria are actually based on the worst possible condi t ions . Precaut ions given on drainage are

only an added factor of safety.

Unfor tuna te ly , the art of coping wi th present-day expansive soil p roblems is far from

comple te and the soil engineer can only h o p e that his r ecommenda t ions are carried out in full,

thus minimizing any possible future damage.

R E M E D I A L M E A S U R E S

It is relatively easy t o r ecommend the necessary remedial cons t ruc t ion for a cracked

building when the cause of foundat ion movemen t has been de te rmined . However, it is seldom

possible to re turn the building to its original condi t ion . After the remedial const ruct ion has been

comple ted , it takes a long t ime for the s t ruc ture to adjust to the improved s ta te . In the mean t ime

minor cracks will con t inue to open . It may take as long as a year before the s t ruc ture is finally

stabilized and cosmetic work can be s tar ted.

Remedial cons t ruc t ion differs in each case, just as in the case of the prescr ipt ion submi t ted

t o the pat ient . It is unusual when t w o pat ients receive the same prescript ion. In the last 15 years,

more than one thousand distressed s t ructures have been investigated. Of this n u m b e r m o r e then

50 percent involved residential houses where the investigation merely consisted of visual

inspection. Generally, for the private dwelling the r ecommended remedial cons t ruc t ion is only

partially completed and the problem cont inues .


Othe r cases involve school buildings, warehouses , religious buildings, ins t i tu t ions , swimming

pools , and o the r lightly loaded s t ructures . The remedial measures r ecommended for such

s t ructures are generally followed. In abou t 75 percent of these cases, the owners are happy wi th

the results obta ined from remedial cons t ruc t ion and the case is closed. The remainder of the cases

repor t cont inued movemen t , though varying greatly in degree. Again, referring to the case of

pat ients and doc tors , some pat ients are in terminal condi t ion and t r ea tmen t can only prolong the

life span. O the r pat ients visit t he d o c t o r at an early da te , have the disease accurately diagnosed,

and the problem is solved.

In a broad sense, the c o m m o n l y used remedial measures are as follows:

Fo r drilled pier founda t ion :

1. Loosen soils around the pier to reduce uplift pressure.

2. Recons t ruc t void space benea th the grade beams.

3 . El iminate the m u s h r o o m at the t op of piers.

4 . Cut the t op of pier and adjust the elevation of the piers by shims.

For con t inuous footing foundat ion :

1. Provide voids benea th the footings at calculated intervals to increase the dead load

pressure.

2. Post- tension the foundat ion t o provide s t ructural stabili ty.

3 . Reinforce existing foundat ion walls wi th new reinforced grade beam to tie the

s t ructure as in box cons t ruc t ion .

4 . Underpin the s t ruc ture wi th piers drilled in to bedrock .

For individual pad foundat ion :

1. Decrease pad size to increase dead load pressure.

2. Underpin the pad wi th piers drilled in to bedrock .

For basement :

1. Saw cut the slab along the foundat ion wall to allow free slab movement .

2. Adjust screw jack on t o p of pipe co lumn to relevel interior I-beam.

3 . Provide slip jo in ts to all in ter ior slab-bearing par t i t ion walls, including door frames and

staircase walls.

F o r exter ior :

1. Remove and recompact backfill.

2. Provide positive drainage around the building.

3 . Provide adequa te out le t for all downspou t s .

4 . Provide concre te apron around the house .

5. Relocate all lawn sprinkling heads to a distance of at least 10 feet from the building.

6. Remove all shrubs and flower beds which are planted adjacent to the house .

Fo r subdrain:

1. Provide a subdrain around the building in the inter ior or exter ior at least 24 inches

below the lower floor slab.

2. Provide a positive out le t for the existing subdrain system.

3 . Provide au tomat i c sump pumps in the basement .

Case I

DISTRESS CAUSED BY PIER UPLIFT

G E N E R A L

The case s tudy is tha t of a school building (fig. 121) and is typical of pier uplift. The pier

load is heavy, cons t ruc t ion is in general up to s tandard, design is adequa te , and crawl space

cons t ruc t ion allows the s t ruc ture t o be free of possible damaging effects of slab heaving. Yet

damage t o the building caused by uplift, before positive remedial measures were taken , was so

severe tha t evacuation of the building was considered for reasons of safety.

HISTORY

The school was comple ted in 1962. It is founded wi th piers drilled in to bedrock. The

bedrock consists of essentially c laystone and sandstone shale located at dep ths 4 to 23 feet below

the ground surface. The piers were designed for an end bearing pressure of 20 ,000 psf and a skin

friction value of 2 ,000 psf. The piers were also designed for a min imum dead load pressure of

15,000 psf. The piers were to pene t ra te the shale bedrock by at least 4 feet and only the skin

friction in the bedrock was t o be assumed. The pier design system was considered to be sound

in view of the limited knowledge of pier design in 1960.

Short ly after comple t ion , distress of the building was not iced . In June , 1964, the con t rac to r

was advised to repair the existing damage. In November , 1964, the piers were found to be in good

condi t ion b u t surface drainage had no t been proper ly provided for and water had pene t ra ted

benea th the s t ruc ture causing soil swelling.

In July 1966 various columns were jacked u p to level the building and steel co lumns were

inserted for suppor t . In February 1970, as a safety precaut ion a precast panel over a doorway had

to be removed. In July and December 1970 repairs were made on several piers. In June 1971

further repairs were m a d e to a n u m b e r of inter ior columns. An inspect ion in July 1971 indicated

tha t movemen t was still cont inuing.

In March 1972 the au thor was engaged t o make a comple te independen t investigation in to

the cause of cracking and to de te rmine the necessary remedial measures .

The school building consists of th ree levels. T h e lower level is designated as Wing C. This

level has a lower floor and one s tory above the lower floor. The middle level, designated as Wing

Β Nor th , and the upper level, designated as Wing Β South , are b o t h one s tory high wi th n o lower

floor. The gymnas ium, cafeteria, and music hall are all one s tory with a high ceiling and

designated as Wing D. The no r the rn por t ion of Wing D has a basement locker room. This is the

only por t ion of the entire building where slab-on-ground cons t ruc t ion is used. The remainder of

the building is crawl space type cons t ruc t ion . Wing A is located at the west side of the building


WING Β

WING C

Figure 121. Exterior view of school building under study.

DISTRESS CAUSED BY PIER UPLIFT 197

and is occupied by a l ibrary and adminis t ra t ion building. The remedial cons t ruc t ion on Wing C

began in 1973 and was comple ted in 1974. The various wings are shown on figure 122.

Most of the remedial measures unde r t aken in the past 10 years were centered at Wing C. In

t he crawl space unde r Wing C, m a n y concre te pedestals were removed and replaced wi th steel H

columns. Figure 123 shows tha t the concre te pedestals were crushed by pier uplifting force in the

same m a n n e r as concre te cylinders are crushed in the compress ion testing machine . Along the

no r th wall in the crawl space, water seeped freely in to the crawl space th rough the backfill. Water

has been entering below the grade beam for a lengthy period of t ime. At the nor thwes t corner ,

evidence was found tha t water has flowed in freely and has washed the soil in the crawl space

forming channels. In Wing B, b o t h n o r t h and sou th , t he ground surface was relatively dry , b u t

there was evidence tha t the soil has been wet ted in the past due to infi l trat ion of surface water .

The lower level cons t ruc t ion is confined in Wing C. The east wall of the w o r k s h o p revealed

severe movemen t . The arts and crafts r ooms also showed m a n y areas of extensive damage.

Figure 122. Plan of school building under study.


Figure 123. Compression failure of pedestal placed above pier and beneath grade beam. Uplifting pressure 30,000 psf.

198


Surprisingly, the slab-on-ground por t ion of the locker and boiler rooms showed practically n o

foundat ion movemen t .

In the uppe r level, n u m e r o u s cracks were found in the in ter ior walls of Wings Β and C.

Almost every inter ior par t i t ion in these areas was cracked. The ceiling had pulled away from the

s t ructural walls by as m u c h as 3 inches at the no r th end of the nor th -sou th corr idor below Wing

D and the rest of t he building as shown on figure 124.

Exter ior cracks were found in the grade beams and brick courses, part icularly on the n o r t h

wall of Wing C. Hairline cracks were found in the reinforced concre te beams in the crawl spaces,

part icularly in Wing C.

INVESTIGATION

Swelling potential

Six test holes and t w o test pits were excavated in 1972 at the locat ions shown on figure 122

and undis turbed samples taken from the test holes. Swell tests conduc ted on samples t aken from

test holes located at the sou th side and west side of the building indicate that the expansion is

abou t 1 to 2 percent and the swelling pressure is abou t 3 ,000 to 4 ,000 psf. Since the test holes

were drilled adjacent to the building, it is likely tha t due t o excessive wet t ing condi t ion , the soil

Figure 124. Cracks and separation of brick from ceiling.


has already swelled to its maximum limit. Consequently, the swell tests cannot reveal the initial

soil condition.

Swell tests performed on undisturbed samples taken from test holes drilled outside of the

building area present a different condition. The upper clays swell about 6 percent with a swelling

pressure as high as 25,000 psf.

The typical dry clays found near the crawl space, after remolding and upon subsequent

wetting, exhibit high swell potential as shown on figure 125. The swell characteristic represents

the actual condition of the subsoil at the time the building was constructed.

Assuming the swelling pressure of the upper clays is 15,000 psf, the following calculation

will indicate the stress condition around the piers:

Data: Pier diameter = 30 in.

Pier circumference = 7.8 ft.

Pier end area = 4.9 sq. ft.

Portion of pier in

upper clay = 8.0 ft.

Portion of pier in

bedrock = 4.0 ft.

Swelling pressure of

upper clays = 15,000 psf

Figure 125. Typical swell test performed on remolded samples.


Por t ion of swelling

pressure responsible

to uplift

Skin friction

then

Total uplifting force

•• 15,000 x 0.15

= 2 ,250 psf

= 2 ,000 psf

: (Total area of pier exposed

to wet t ing) χ (Unit uplift)

= 7.8 χ 8 χ 2 ,250 : 140.4 kips

Total withholding force

Net uplift force

= (Total area of pier in

b e d r o c k ) χ (Unit skin friction)

= 7 . 8 x 4 x 2 , 0 0 0

= 62 .4 kips

= 1 4 0 . 4 - 6 2 . 4 = 78 .0 kips

Moisture analysis

Three piers, S-40, T-20, and W-38 were excavated t o their full d e p t h and samples t aken to

show the variation in soil proper t ies wi th respect to b o t h d e p t h and radial distance from the

piers. The mois ture con ten t de te rmined from samples t aken in the test pits and test holes was

compared wi th the values obta ined on samples taken in 1 9 6 1 . Samples t aken in 1972 b o t h

adjacent to the building and well away from the building were compared to de te rmine the na tu re

of mois ture movement s . The da ta are summarized as follows:

Nor th W a l l - U p p e r Clays

Year Locat ion Avg. mois ture con ten t , percent

1961 In test holes 17.1


1972 In test pits 28.1

1972 Along pier T-40 25.8

1972 Along pier S-40 28.9

1972 R e m o t e from the bui lding, 15.2

Test hole 6

The da ta are n o t definitive, bu t suggest tha t a significant increase in mois ture con ten t has

occurred in the soil nex t to the building and around the piers.


N o r t h Wa l l -Bed rock

Year Locat ion Avg. mois ture con ten t , percent


1972 In test holes 20 .3

1972 Along pier T-40 23.7

1972 Along pier S-40 23.4

1972 R e m o t e from the building, 19.3

Test hole 6

The above da ta shows tha t the mois ture con ten t of the lower bedrock has remained fairly

uniform in the past 10 years, indicating tha t it has no t been substantial ly wet ted . The bedrock

immedia te ly adjacent to the pier appears to have slightly increased in mois ture con ten t .

East and West W a l l s - U p p e r Clay

Wall Year Source

Avg. mois ture

con ten t , percent

West 1961 F rom test hole 17.7

1972 F r o m test hole 21.2

East 1961 F rom test hole 18.3

1972 F rom test hole 18.9

A significant increase in mois ture con ten t has occurred for the west wall, bu t the change

along the east wall is negligible. The bedrock actually appeared to be drier in 1972 than in 1961 .

To de te rmine if the mois ture was penet ra t ing along the walls of the piers or soaking d o w n

uniformly from the surface, mois ture con ten t samples were taken adjacent t o the walls of piers

S-40 and T-40 and also 3 feet away. The average mois ture con ten t was as follows:

Pier Avg. mois ture con ten t , percent

Pier Upper clay Bedrock

At wall Three ft. At wall Three ft.

of pier away of pier away

S-40 28.9 20 .3 23 .4 18.8

T-40 25 .8 21 .0 23.7 21.3

The da ta strongly suggest tha t the main mois ture movemen t is immediate ly along the

surface of the piers.

Pier uplift

The increased mois ture con ten t a round piers S-40 and T-40 suggest tha t b o t h have been

subjected to uplift. For pier T-40, it is possible tha t surface water has entered along the face of


this pier and m a y have even reached near the b o t t o m of the pier. Consequent ly , the entire pier

has lifted. F o r pier S-40, a 3/8-inch-wide hor izonta l crack was found just above bedrock . Since

excavation of the pit a round the pier relieved all the uplift forces on the side of the pier in the

clay, the pier should have gradually sett led as the pi t was excavated and it was theorized tha t the

crack mus t have been open by more than 3 /8 inch prior t o excavat ion. Tension cracks developed

in the pier clearly indicate tha t the upper soils have exer ted uplifting pressure on the upper

por t ion of the pier, and the po r t ion of the pier in bedrock is wi thholding the pier.

The uplifting pressure exer ted on the pier depends on the swelling pressure of the

surrounding soils. The uplifting force exerted on each pier m a y reach as high as 200 kips. This

force is sufficient to crush t he concre te pedestal formed on top of the pier. Also, when all the

piers in Wing C were exposed during the remedial cons t ruc t ion , it was found that at least five

piers had a dist inct shear failure pa t te rn as shown on figures 126 and 127.

Figure 126. Failure of pier by shear resulting from uplift.

2 0 4 FOUNDATIONS ON EXPANSIVE SOILS

Figure 127. Failure of pier by shear resulting from uplift.


In general, t he cause of m o v e m e n t of the building is due to the uplifting of the piers. The

movement is more severe at the no r th side unde r Wing C. T o the west of Wing D, the entire

school building is connected wi th grade beams. Wing D is separated from the remainder of the

school building wi th expansion jo in ts . Consequent ly , at the eastern por t ion of the school building

foundat ion m o v e m e n t is dis t r ibuted th roughou t the system and is no t conspicuous, while at the

nor th-south corr idor the ent ire system is separated. This explains why severe movemen t along the

nor th-south corr idor is not iced.

In addi t ion to the uplifting of the piers, several o the r cons t ruct ion defects were found. Two

piers in Wing C are b o t t o m e d on the uppe r clay instead of drilled in to bedrock as shown on figure


128. The piers were 36 inches in d iamete r and 3 to 4 feet in length, ra ther than 12 inches in

d iameter and drilled in to bedrock as had been designed. Since the upper clays have a m a x i m u m

soil bearing value of abou t 3 ,000 psf, it is possible tha t se t t lement of these piers has taken place.

The ent i re length of air space benea th the n o r t h wall in Wing C was carefully inspected.

There was a min imum of air space. Remnan t s of cardboard used for forming the air space were

found, bu t it appears tha t the air space was no t proper ly cons t ruc ted , as shown on figure 129.

Ei ther the air space was no t formed to the specified thickness or the uplifting of the soil has

closed the air space. In any event , along the no r th wall in Wing C, the soil has exer ted uplifting

pressure on the grade beam tha t can reach as high as 25 ,000 psf.

Not all the distress manifest in the building was caused by foundat ion movemen t . All the

par t i t ion walls in the classrooms show cracks. The pa t t e rn of the cracks indicates tha t the beams

support ing the slabs were deflected. The cracks in the par t i t ion wall are typical distress due t o

the deflection and plastic flow of the long-span concre te floor beams.

Figure 128. Improperly placed pier. Pier length should be 20 feet and bearing on bedrock. Actual length only 4 feet and bearing on clay.


Figure 129. Four-inch void, which has completely closed, beneath the grade beam.

It is impor t an t to isolate s t ructural defects from foundat ion m o v e m e n t when investigating a

cracked building so tha t the cause may be de te rmined . In m a n y cases, s t ructural defects and

foundat ion defects take place in the same s t ructure .

R E M E D I A L CONSTRUCTION

Since the cause of foundat ion movemen t and the source of mois ture that entered in to the

foundat ion soils have been defined, the remedial measures should consist essentially of relieving

the uplifting pressure exer ted on the piers and preventing addit ional water from entering the

foundat ion soils. The remedial measures consist of the following:

1. Remove all backfill a round the building and replace compac ted t o at least 90 percent

s tandard Proc tor densi ty at o p t i m u m mois ture con ten t . Backfill along the no r th wall of

Wing C should consist of nonexpansive soils instead of the original soil. The adequate

compact ion of the backfill soil is very impor t an t to insure tha t any surface water will n o t

pene t ra te th rough the backfill and in to the foundat ion soils.

2. All void space benea th the grade beam should be re-formed to insure that there will be at

least 4 inches of space be tween the soil and the grade beam. At t he same t ime , care

should be taken to insure tha t there will be n o large mush rooms present on t op of the


piers. T h e air space should be formed adjacent t o the sides of the piers. With the air space

proper ly formed, the load of the building will then be exer ted on the piers.

3 . In Wing C, it will be necessary t o loosen or remove the soils above bedrock from around

all piers. Such under tak ing will have t o be per formed by hand inside the crawl space. The

d e p t h of loosening or removing of soil should be at least 8 feet.

4 . Drainage around the building mus t be improved, and should consist of the following:

a. Improve the drainage in the cour tyard area. Remove the asphalt paving in the

cour tyard and replace with concre te .

b . Recons t ruc t the concre te side Walk a round the building to provide an adequa te slope.

Also provide an adequa te expansion jo in t be tween the sidewalk and the grade beam.

c. Slope the ground surface around the building away from the s t ruc ture to allow p roper

drainage.

The above remedial measures will prevent further damage to the school s t ruc ture due to

expansive soils. After the above remedial measures are made , the dead load pressure will be fully

exerted on the piers and the uppe r soils will no t exert uplifting pressure on the piers. By

preventing water from entering the crawl space area, m o v e m e n t of the piers should be arrested.

The releveling procedure can be s tar ted, as follows:

1. Carefully establish the elevation of all piers benea th the school building b y referring t o

the established b e n c h mark at the n o r t h side of the building.

2. A s t ructura l engineer should be consul ted to de te rmine the appropr ia te new elevation of

t he school bui lding commensura te wi th the initial cons t ruc t ion .

3 . The piers around the exter ior of the school building have lifted; however , at the central

por t ion of the building the piers have mainta ined their original posi t ion. It is reasonable

to lower the exter ior piers and allow the inter ior piers to mainta in their original posi t ion.

In 1972, remedial measures as r ecommended above were s tar ted. T o facilitate cons t ruc t ion ,

the ent ire crawl space area benea th Wing C was lowered. This no t only allowed w o r k m e n to move

freely in the work area bu t also remove at least 4 feet of soil a round the piers (fig. 130) . The

ent ire crawl space was lighted and conveyor bel ts installed for ear th removal. Each pier was

carefully examined for defects after the surrounding soil was removed. Steel rings were installed

around t h e t o p of those piers t ha t suffered shear failure. All grade beams were examined for

s t ructural s t rength. Heavy steel girders were in t roduced t o s t rengthen the defected beams (fig.

131). O t h e r remedial measures such as providing adequa te air space benea th the grade beams,

removing and recompact ing backfill, installing sump pumps t o el iminate perched water , and

relocating those piers having insufficient length were performed u n d e r close supervision.

Then the opera t ion of releveling s tar ted. The grade beams were raised wi th high-capacity

jacks (fig. 132), and the t o p of pier cut-and-shimmed wi th steel plates. Three or four piers were

releveled in one opera t ion . A to ta l of 56 piers were releveled in Wing C over a period of 4

m o n t h s . During the leveling opera t ion , careful surveys were conduc ted to de te rmine the vertical

movemen t . Typical records are shown on figure 133 .


Figure 130. Loosening of soil around the pier to eliminate uplifting pressure.

F o u r sets of major readings were taken as follows:

1. Pier elevation before remedial cons t ruc t ion ,

2 . Pier elevation after air space benea th the grade beams was cleared and load of building

concentra ted on the piers,

3 . Pier elevation after the removal of soils surrounding the piers, thus partially eliminating

the uplifting pressure exer ted on the face of the piers, and

4 . Pier elevation after the releveling of the piers.

F rom figure 133 , the effect of the various stages of remedial cons t ruc t ion can be reflected

by the se t t lement of the piers.

The case s tudy of this school is a typical example of failure due to pier uplift. In addi t ion t o

the const ruct ion defects , present knowledge of a drilled pier system in expansive soils calls for


Figure 131. Steel ring placed around the defective pier and steel girder installed to strengthen the grade beam.

Figure 132. Jacking the grade beam in the releveling operation.

210

F

OU

ND

AT

ION

S O

N E

XP

AN

SIV

E S

OIL

S

PIER ELEVATION AFTER LOOSENING SOILS AROUND THE PIERS PIER ELEVATION AFTER RELEVELING OPERATION

Figure 133 . Pier settlement after various stages of remedial construction.


b o t h adequa te re inforcement of the pier t o resist tension and deep pene t ra t ion in to bedrock t o

provide for anchorage. Such precaut ions could have resisted the uplift pressures.

Remedial cons t ruc t ion for this school building has been confined t o Wing C. After a period

of 6 m o n t h s , the building is still undergoing s t ructural adjustment . Minor cracks appeared in the

block wall as the result of releveling adjustment . It is expected tha t a stabilized condi t ion can be

achieved in the building wi th in a year.

Case II

DISTRESS CAUSED BY IMPROPER

PIER DESIGN AND CONSTRUCTION

G E N E R A L

This is a typical case of improper design and cons t ruc t ion of a drilled pier founda t ion

system. T h e building is a residential house located in west Denver, Colorado .

EXISTING CONDITION

Design

The residence is a split-level s t ructure facing east, wi th the finished basement at the sou th

end, crawl space unde r the living por t ion and garage at the n o r t h end. It is a brick veneer and

wood frame s t ruc ture wi th a trussed roof system and suppor ted on piers (fig. 134).

A subsoil investigation was m a d e before cons t ruc t ion . The subsoils consist of abou t 4 feet of

stiff clays overlying claystone bedrock . T h e wate r table was found at a d e p t h of 7 feet be low the

original ground surface.

A pier foundat ion was r ecommended . The piers were designed for a m a x i m u m end pressure

of 15,000 psf, a skin friction of 1,500 psf and a min imum dead load pressure of 15,000 psf. It

was also r ecommended tha t the piers should be drilled at least 4 feet in to c laystone. (As

claystone bedrock was practically exposed in the excavat ion, the length of the piers does n o t

exceed 4 feet).

Distress

The house was buil t in 1961 . Cracks appeared in the house 6 m o n t h s after occupancy . The

ex ten t of m o v e m e n t began increasing steadily. A subdrainage system leading to a sump p u m p was

later installed in the crawl space area of the house . The mos t severe movemen t t o o k place

be tween the crawl space area and the living room area. The separat ion of t he crawl space from

the two-story por t ion of the house is shown on figure 135. The wid th of the separat ion measures

as m u c h as 1-1/2 inches. T h e pic ture window above the crawl space area is also separated from

the wall by as m u c h as 1 inch.

Cracks also appeared at the rear of the house be tween the one and two-story por t ions .

Exter ior doors were j a m m e d and the pa t io slab was approx imate ly 1 inch lower than its original

posi t ion.


Figure 134. Location of exterior cracks.

In the inter ior of the house , severe cracks were found near the staircase leading to the

basement (fig. 136). Cracks were also found above mos t doors and windows, indicating severe

movement . Most of the doors in the house were j a m m e d . The I-beam which suppor ts the upper

floor appears to have moved . One of the posts in the crawl space area was loose, indicating the

uplifting of the suppor t unde r the I-beam. Slabs were raised.

In general, the ex ten t of cracking in this house is considered to be very severe, and from the

pat tern of the cracks and the na ture of the swelling of the foundat ion soils, severe uplifting

movement of the soil benea th the foundat ion has taken place.


The cause of foundat ion movemen t for this house can be summarized as follows:

1. Undis turbed hand drive samples were taken in the crawl space area benea th the grade

beams. Tests indicated tha t the weathered claystone possessed high swell potent ia l .

Typical test results are shown on figure 137. Figure 137 indicates tha t the swelling

pressure is abou t 16,000 psf.

2. N o air space was found benea th the grade beam near the main ent rance in the crawl

space. The to ta l length of the por t ion of grade beam wi thou t void-forming cardboard is

approximate ly 8 feet. The lower weathered claystone exerted direct uplifting pressure on

the grade beam in this por t ion of the house . With 8-foot-long grade beams, 9 inches wide ,

DISTRESS CAUSED BY IMPROPER PIER DESIGN 215

Figure 135. Separation of living room from the two-story portion. (See previous figure)

w i t h o u t air space, the tota l uplifting pressure exerted on the grade beam can reach as high

as 9 6 , 0 0 0 lbs. This pressure is sufficient to cause the severe m o v e m e n t be tween the

one-story po r t i on and the two-story po r t i on of the house .

3 . Since the piers are only 4 feet in length, the soils exer ted no t only uplift pressure a round

the per imeter of t he pier b u t also acted direct ly on the b o t t o m of t he piers.

T h e m a x i m u m possible uplift in this case is as follows:

Data: Pier d iamete r = 1 2 in.

Pier circumference = 3 . 1 4 ft.

Pier end area = 0 .785 sq. ft.


Figure 136. Separation of house due to differential expansion ·

Por t ion of pier in

bedrock = 3.0 ft.

Swelling pressure in

bedrock = 16,000 psf

Por t ion of swelling

pressure responsible

t o uplift = 16,000 X 0.15

= 2 ,400 psf

then

Total uplift force

From pier end = 16,000 X 0 .785

= 12.5 kips


Ο

2

Noturol Dry Unit Weight = 107.5 pcf

Natural Moisturt Contint = 22.9 p«rctnt

Ε xp( ns ioi Jll (1er con$1 ant ι >res sur e

\ ( c ue t< Mi t ig.

\

\ \

>

\ \ \ \

Λ s

\ \ \

Swel Of i

ling irnil

Pi es inr

re : 16,000

0.1 1.0 10 100 A P P L I E D P R E S S U R E - k i t

Figure 137. Typical sample of weathered claystone obtained from beneath grade beam.

F r o m pier wall = 2 ,400 X 3 X 3 .14

= 22 .6 kips

Total = 3 5 . 1 kips

and

Total withholding force

(Dead load pressure) = 15,000 X 0 .785

= 11.8 kips

It is obvious tha t the dead load pressure exerted on the pier is no t sufficient t o prevent

uplift. The inter ior piers have even less dead load than the exter ior piers. Consequent ly ,


the piers benea th the I-beam have lifted, causing the I-beam to move , thus dis turbing the

entire upper s t ruc ture .

4 . The par t i t ion walls in the two-story po r t ion of the house are slab-bearing, and when the

slabs heave, the walls impar t direct uplifting pressure to the I-beam which dis turbs the

upper s t ruc ture .

5. The cause of wet t ing of the soils benea th the foundat ion is from a high wate r table and

poo r drainage around the house . Initial foundat ion investigation indicates tha t the water

table is near the basement floor level. Initially, considerat ion should have been given t o

the effect of wet t ing on the s t ructural stabili ty.

The cause of m o v e m e n t of this house is due to the swelling of the soils beneach the grade

beam and the uplifting of the piers. The design and cons t ruc t ion of the house cannot

accommoda te the severe uplifting of the soils.


The following remedial measures were r ecommended :

1. Excavate around the basement por t ion of the house to expose the grade beam. Remove

all m u s h r o o m s above the piers and recons t ruc t t he air space in the same manne r as in the

crawl space por t ion of the house .

2. Free the piers from the grade beam.

3 . Precise leveling should be m a d e using the central grade beam as a reference poin t t o

relevel the ent i re house . It is possible t o definitely establish the a m o u n t of adjus tment

which is required for each individual pier to re turn the house to a level posi t ion. It is

expected tha t after this has been done , the existing cracks will be partially closed.

4 . The piers, after adjustment , should be shimmed wi th steel plates.

5. The backfill in the basement por t ion of the house should be provided wi th deep wells

approximate ly 3 feet in d iameter a round the five exposed piers so tha t in the future,

adjus tment of the piers will be possible w i thou t again removing all backfill.

6. The t o p of the wells should be covered wi th suitable material so tha t surface water will

no t seep in to the wells.

7. Readjust the I-beam to level the upper s t ruc ture .

8. Remove all slab-bearing s t ructures , such as the stairway, inter ior cupboards , bookcases ,

furnace, and so for th, and provide slip jo ints so tha t further slab movement will no t

affect the upper s t ruc ture .

9. Check the grade beam benea th the walk-out doo r at the rear por t ion of the house t o

insure tha t the grade beam is tied in as a uni t . If necessary, new grade beams should be

const ructed to span above the walk-out door .

10. Remove the rear pa t io slab for the ent ire length so tha t the slab will be free from the

grade beam.

11. Free the basement floor slab around the per imeter of the grade beams.

12. Ex tend the exter ior subdrainage system from the rear side of the house to the south of

the garage to in tercept all possible sources of free wate r from entering the house .


13. Recompac t the backfill in thin, mois tened layers wi th a mechanical tamper .

14. Regrade the backfill around the house so tha t surface wate r will drain away from the

house .

15. Remove all shrubs and flower beds from around the house and extend the downspou t s .

This specific case wen t to the court and the con t rac tor was ordered to pay the $ 11,000 cost

of remedial cons t ruc t ion which amoun ted to about 50 percent of the cost of the house . The

remedial measures were complete ly carried o u t .

Short ly after the house was releveled, some of the m o r e severe cracks started t o close as

shown on figure 138. It t ook more than 6 m o n t h s before the s t ruc ture was stablized. Some 6

Figure 138. Movement of the house before and after remedial correction.

FOUNDATIONS ON EXPANSIVE SOILS 220

years after the remedial cons t ruc t ion , n o serious foundat ion movemen t had taken place in this

house (fig. 139) ; therefore , readjus tment of the piers was n o t necessary.

Figure 139. Condition of front of house in 1974.

Case III

DISTRESS CAUSED BY HEAVING OF

FOOTING PAD AND FLOOR SLAB

G E N E R A L

This case s tudy is typical of that of underes t imat ing the swelling potent ia l of the soil.

Individual founda t ion pads have heaved and severe floor heave has taken place. Because of cost ,

remedial measures were only partially carried out . More than 2 years have elapsed since the

remedial work was completed and the buildings remain in perfect condi t ion .

HISTORY

The buildings under investigation are in a State-operated ward for housing the severely

retarded and consist of 6 cot tages, 3 to the east and 3 t o the west. The 3 cot tages located at the

western po r t ion of the site are identified as Cherub , Aspen, and Birch. The eastern cot tages are

the Starlight, Crescent , and But te rcup . Each group is connected to the service buildings as

indicated on figures 140 th rough 143 .

The cottages were comple ted in 1962. The original soil repor t r ecommended tha t the

buildings be founded wi th spread footings on a combina t ion of compacted fill and the in-place

natural sandy clays, designed for a m a x i m u m soil pressure of 3 ,000 psf and a m i n i m u m dead load

pressure of 1,500 psf. The s t ructural design of the buildings indicates that they were founded on

individual pads designed for the pressures r ecommended . The footings and slabs are founded

part ly on compac ted fill and par t ly on natural soils. The fill was compac ted to 100 percent

s tandard Proc tor densi ty unde r the footings and to 95 percent s tandard Proc to r densi ty u n d e r the

slabs.

Cracks first appeared in the building in 1963 and have cont inued steadily since. An

investigation in to the cause of cracking was m a d e by a soils engineer in Oc tobe r 1966. At tha t

t ime, it was r ecommended tha t drainage around the exter ior of the buildings be improved.

DISTRESS

In general, the ex t en t of cracking is more severe at the eastern group of cottages than at the

western g roup . Typical k inds of cracking which took place at the various buildings are as follows:

1. Tension cracks near the top of the concre te co lumns ,


Figure 140. View of cottages

2. Corr idors which connect the service buildings to the cot tages were separated bo th

hor izontal ly and vertically. Diagonal cracks are general in the brick at the junc t ion of the

corridors wi th the service buildings,

3 . Slab bearing par t i t ion walls were cracked and slightly buckled ,

4. The entrance to the service buildings had moved and diagonal cracks were found in the

brick veneer, and

5. A por t ion of the floor slabs had moved with respect to the grade beams.

The ex tent of cracking in the Aspen and Cherub cottages was less severe than in the o ther

four cottages. Typical distresses are shown on figures 144 through 146.

The following slab m o v e m e n t da ta was obta ined:

But te rcup 2.3 in. Bu t te rcup t o Service 2.1 in.

Crescent 4.4 in. Crescent to Service 0.3 in.

Starlight 4.1 in. Starlight to Service 0.7 in.

Cherub 2.3 in. Cherub to Service 1.2 in.

Birch 3.1 in. Birch t o Service 0.9 in.

Aspen 2.4 in. Aspen to Service 2.0 in.

DISTRESS CAUSED BY HEAVING OF SLABS 223

Ο

Figure 141. Location of exploratory holes for the cottages.

The above da ta indicates tha t the differential slab m o v e m e n t is the greatest at Crescent,

while the But te rcup building shows only a 2.3 inch differential slab m o v e m e n t . Considering the

a m o u n t of wet t ing of the slab in this building (But te rcup) , it is likely t ha t the slab raised m o r e

uniformly than in the o the r buildings. The corridors connect ing the service buildings to the

various cot tages indicate definite movemen t . This m o v e m e n t can be associated wi th t he different

loading condi t ions in the cot tages wi th respect to the corr idors .


Figure 142. Plan of test hole and test pit location for west cottages.

INVESTIGATION

In August 1968 investigation in to the cause of cracking of the buildings, the source of

mois ture tha t entered the subsoils, and the possibility tha t a neighboring water tank had an

undec tec ted leak tha t provided the mois ture was ini t iated, as well as the possible remedial

measures needed.

In the first phase of the investigation, 24 explora tory holes were drilled at the site, 15 of

which were drilled adjacent to the cracked buildings. The remainder of the test holes were drilled

away from the cracked buildings at locat ions shown on figure 1 4 1 .

A comple te repor t , including recommended remedial measures , was submi t ted in Sep tember

1968. For more than 1-1/2 years, n o corrective act ion was taken. In the mean t ime , the condi t ion

of all buildings cont inued t o de ter iora te . Aspen and Cherub , which were in relatively good

condi t ion in 1968, now showed severe movemen t . The movemen t of the various buildings

became so severe tha t it was necessary to evacuate the pat ients from all six buildings.

In April 1970 a decision was m a d e to init iate a second phase of investigation. With the two

phases of investigation, all possible factors tha t could influence the effectiveness of the remedial

cons t ruc t ion would be covered.


Figure 143. Plan of test hole and test pit location for east cottages.

In the second phase of the investigation, the concrete slab was core drilled in 12 locations in

each building and hand augered in the core hole to a depth of 6 feet. Undisturbed samples were

obtained in each auger hole. A total of 82 holes was drilled inside the building. The location of all

test holes and test pits is shown on figures 141 through 143.

The investigation was directed mainly toward the following items:

1. Determination of the variation of moisture content in the soil beneath the floor slab in

each building to a depth of approximately 6 feet.

2. Determination of the swelling potential and the swelling pressure of the soils beneath the

floor slabs at various depths.

3. Determination of the swelling potential and the swelling pressure of the soils directly

beneath the exterior footings at each building.

4. Determination of the possible sources of moisture which entered the buildings.

5. Determination of the water table elevation in the area.

6. Prediction of the future behavior of the soils and of the effectiveness of the proposed

remedial measures.

The behavior of the soils involves many variables some of which cannot be determined with

certainty. This investigation is based solely on the statistical average behavior of the soils rather


Figure 144. Upper rounds - West complex - Cherub ward - Wall braced to prevent falling in.

than the result of a single observat ion or test . The evaluation of the behavior of the soils is m u c h

more complicated and difficult than for o the r elastic engineering materials . In this investigation,

more than 150 tests on swelling characterist ics, over 300 tests on mois ture con ten t , and m a n y

o ther tests were m a d e so tha t p roper conclusions could be drawn along wi th r ecommenda t ions

for remedial measures .

Subsoil conditions

Subsoil condi t ions at the site consist essentially of 0 t o 8 feet of fill overlying soft to stiff

clays. Bedrock was found at depths ranging from 8 to 29 feet. The characterist ics of the various

subsoil s t rata are described as follows:


Figure 145. Corner of Cherub ward. Interior wall pulling away from exterior wall.

F i l l - T h e fill consists of the on-site soils and it was often difficult to distinguish be tween the fill

and the natural soil. The fill was placed under control led condi t ion and the o p t i m u m mois ture

con ten t ranged from 16.3 to 18.6 percent . The actual mois ture con ten t of the in-place fill ranged

from 10.7 to 16.1 percent .

C l a y - T h e clays at the site had fairly uniform characterist ics. The soil could be classified as on the

borderl ine be tween CL and CH with the liquid limit ranging from 42 .0 t o 53.6 percent and the

plasticity index ranging from 26.8 to 32.5 percent . The stiffness of the clay varied with the

mois ture con ten t . In general, the upper soils were soft and their stiffness increased wi th dep th .

An X-ray diffraction analysis indicated tha t the total clay mineral (not including clay-size

quar tz or calcite, etc.) was probably no t more than 5 percent by volume of the to ta l sample.

Major minerals were quar tz and calcite, especially in the decanted fractions. Minor minerals


Figure 146. Interior Aspen ward — Floor crack.

were montmor i l lon i te (unusually broad 14-angstrom lines) wi th possibly a trace of kaolinite and

mica.

Hydrome te r analysis indicated tha t the clay fraction (percent minus 0 .002 m m ) of the

typical sample was less than 35 percent , and colloid con ten t (percent minus 0.001 m m ) less than

22 percent . The shrinkage limit of typical clays ranged from 9.7 t o 14.0 percent .

The above physical analysis of the clay soils indicated tha t in accordance with the

established m e t h o d s of classifying expansive soils, the foundat ion soil under the various buildings

falls in to the category of "highly expansive soils ."

Bedrock—Bedrock consisted of c laystone and sands tone . The u p p e r por t ion of claystone was

highly weathered. The claystone bedrock had a resemblance to stiff clay and it was difficult t o

distinguish be tween the claystone and the upper clay. The claystone bedrock had essentially the

same physical characteristics as the upper clay.


Stabilized free water in the area was found at dep ths of 5 to 19 feet below the t o p of the

floor level. However, at the site of the buildings, the water table was at least 10 feet be low the

floor level.

Method of approach

After all t he test da t a had been accumula ted , the m e t h o d of approach used in solving the

p rob lem was as follows:

Swelling Po ten t i a l -Swe l l ing potent ia l is an index tha t indicates the degree of volume change of

the soil after sa tura t ion . F rom the swell po ten t ia l , it is possible to es t imate the magni tude of

floor and foundat ion heaving. The swelling poten t ia l of soils u n d e r each building was obta ined

and a curve which graphically summarized the results was proposed b y p lo t t ing the mois ture

con ten t versus swelling potent ia l . A curve was prepared for each building (fig. 147).

Swelling Pressure-Swel l ing pressure can be defined as the pressure required to keep the volume

of the sample cons tan t . F o r cons tan t dens i ty , swelling pressure should have a cons tan t value.

With variable mois ture con ten t and densi ty , the swelling pressure varies as shown on figure 148.

Figure 148 is a typical graph of swelling pressure versus mois tu re conten t for each bui lding.

Average M o i s t u r e - T h e mois ture con ten t of all soil samples benea th b o t h the slab and the

footings was ob ta ined . An average mois ture con ten t was de te rmined , which provided informat ion

on the following:

1. Average mois ture con ten t for the entire building at various dep ths ,

2. Average mois ture consent in the per imeter of the building at various dep ths , and

3 . Average mois ture con ten t at the central por t ion of each building at various dep ths .

The r ecommended remedial measures for each building are essentially based on the swelling

pressure, swelling poten t ia l , and mois ture d is t r ibut ion . The conclusions are based on the

statistical average of soil behavior.

Source of moisture

No volume change will take place in the expansive soils unless there is a change in the

a m o u n t of mois ture in the soil. The increase of mois ture con ten t can b e caused by various factors

as follows:

1. Surface runoff including rain, mel t ing snow, and lawn sprinkler water ,

2. Leaks in the unde r slab heat ing system,

3 . Leaks in the sewer system,

4 . Leaks in the domes t ic wate r sys tem, and

5. Possible rising water table condi t ion due to increase in subsurface water vo lume.

The mos t difficult aspect in investigating the source of water was in de termining whe the r

the underslab soils were wet ted by the in t roduc t ion of surface water due to p o o r exter ior

drainage or by leakage of the underslab ut i l i ty system. Since each building is su r rounded b y grade


Figure 147. Moisture and swelling potential relationship at Aspen.

beams 3 feet deep, surface water can enter the subsoils only at a depth of at least 3 feet below

the top of the floor slab. However, it is most likely that exterior surface water has entered the

underslab soils through the void space beneath the grade beams; therefore, the following

conclusions can be established:

1. If the moisture content directly beneath the concrete slabs (within 24 inches below the

top of the floor slab) is high, then a leak in the underslab utility lines is suggested.

2. If the moisture content around the perimeter of the building at a depth of more than 3

feet below the floor slab is high, then the migration of exterior surface runoff into the

underslab soils is suggested.


20,000

10,000

£ 5,000

1,000

ASPEN

•

•

•

12 14 20 16 18

MOISTURE CONTENT (%)

Figure 148. Moisture and swelling pressure relationship at Aspen.

22 24

3 . If the mois ture con ten t a round the per imete r of the building and the mois ture con ten t of

the building's in ter ior are b o t h low, then n o in t roduc t ion of surface runoff or leakage in

the ut i l i ty lines is suggested.

4 . If the mois ture con ten t a round the per imete r of the building and the mois ture con ten t of

the building's in ter ior are b o t h high, then b o t h the in t roduc t ion of surface runoff and

leakage in the ut i l i ty lines are suggested.

The average mois ture con ten t at various dep ths for each building will give a clear indicat ion

as t o the source of mois tu re tha t has entered in to the buildings.

T R E A T M E N T

Based on the above reasoning, the p rob lem tha t existed in each building can be established

and remedial measures prescribed.


Treatment at Birch

Birch is the east building of the west complex . Founda t i on soils at this building site consist

essentially of control led compac ted fill. A s tudy of the original soil repor t indicates tha t at the

no r th side of this building there is approximate ly 1/2 foot of cut and at the sou th side there is

approximate ly 7 feet of fill.

Three test pits were opened at the exter ior of the building, adjacent to the grade beam. In

Test Pits 101 and 102, water was flowing from unde rnea th the slab. Fu r the r testing indicated

that the underslab heat ing system had leaked and the sewer line had broken . This resulted in the

flooding of the underslab soils.

Most of the compac ted fill soil benea th the footings possessed only low swell potent ia l . The

possibility of foundat ion movemen t is relatively slim. Moisture con ten ts for the entire building at

various dep ths are relatively uniform wi th the lowest mois ture con ten t 15.8 percent and the

highest mois ture con ten t 22.6 percent . Fu r the r slab movemen t should no t exceed 1/2 inch.

The following remedial measures are r ecommended for this building:

1. The sewer line which runs unde r the building and branches in to the various ba th rooms

should be exposed and carefully checked for leakage.

2. Underpinning of the exter ior o r inter ior footings will no t be necessary. Remedial

measures to the footings are no t r ecommended .

3 . The per imeter of the floor slab should be saw cut to insure that the slab is separated from

the grade beams and tha t there will be free movemen t of the slab wi th respect to the

grade beams.

4. The slab-bearing par t i t ion walls in this building should be reconst ructed in such a manner

tha t slab m o v e m e n t will n o t affect the stability of the s t ruc ture . A vertical slip jo in t

should also be provided where the par t i t ion walls connec t with the exter ior walls or

columns.

Treatment at Aspen

Aspen is the n o r t h building of the west complex . The foundat ion soils at this building

consist entirely of the natural soils. The a m o u n t of cut in the site grading ranged from 5 t o

20 feet.

The swelling potent ia l of the soils benea th the exter ior footings is high wi th a percent of

swell of 5.5 percent and swelling pressure of 10,000 psf. The average mois ture con ten t benea th

the interior footings was 14.7 percent . This corresponds to an average swelling potent ia l of 3.2

percent , (fig. 147) and an average swelling pressure of 6 ,200 psf (fig. 148). At a dep th of 6 feet

below the t o p of the slab, the average mois ture con ten t decreased t o 11.6 percent . This corres-

ponds to an average swelling poten t ia l of 6.5 percent and average swelling pressure of 14,000

psf.

The mois ture dis t r ibut ion indicates tha t the lower soils are in a very dry s ta te , and if the

soils become excessively wet ted , swelling will take place.

In 1968, a mechanical engineer found tha t there was only slight leakage in the underslab h o t

water heat ing system by conduct ing pressure tests. Ano the r pressure test was conduc ted in April


1970, and the pressure d ropped from 130 to 27 psi in 20 minu tes . It was obvious tha t in the

preceeding m o n t h s more leakage had developed in the unders lab heat ing system which accounted

for the severe movemen t in this building.

The soils direct ly benea th the floor slab had an average mois ture con ten t of 13.8 percent .

This is low compared wi th unders lab mois ture con ten t of the o ther buildings. The m o v e m e n t of

the floor slab in this building had only begun and future severe floor movemen t will take place

even though the unders lab heat ing system is entirely d isconnected . The existing pocke t s of high

mois ture con ten t soils caused by leakage of the heat ing system will migrate to the drier phase of

soils and cause damage. This canno t be prevented unless all problem soils benea th the slab are

removed.

The following remedial measures were r ecommended for this building:

1. Underpin the exter ior footings wi th piers drilled in to bedrock . The piers should be

designed for a m a x i m u m end pressure of 30 ,000 psf and a skin friction of 3 ,000 psf for

that por t ion of the pier in bedrock . The piers should also be designed for a m i n i m u m

dead load pressure of 20 ,000 psf. The piers can be drilled in a slanted posi t ion, or two

piers can be drilled under each co lumn wi th a grade beam spanning over the two piers.

2. The inter ior footings should also be underp inned ; however , it is almost impossible to

underp in the inter ior footings wi thou t demolishing the entire building. Therefore , it was

r ecommended tha t the inter ior footings be decreased in area by cut t ing off the concrete

pad and thus increasing the uni t dead load pressure. It was es t imated tha t the dead load

pressure on each pad could be increased to over 6 ,000 psf by reducing the area of the

concrete pad.

3 . The floor slabs in this building should be entirely removed and the soils benea th the slab

removed for a d e p t h of 3 feet. These soils should be discarded and replaced wi th

nonexpansive , impervious, granular soils compac ted to at least 90 percent s tandard

Proc tor densi ty at o p t i m u m mois ture con ten t .

If for some reason the unders lab soils canno t be removed, there is every possibility tha t the

floor slab will raise as m u c h as 3 inches above the present level. Slip jo in ts in the par t i t ion walls

will prevent the dis turbance of the upper s t ruc ture , bu t unsightly cracks in the par t i t ion walls and

the floor slab will take place.

Treatment at Cherub

Cherub is the west building of the west complex . The foundat ion soils benea th this building

are most ly fill. At the n o r t h side, there is 1 foot of cut and at the sou th side there is abou t 7 feet

of fill. Bedrock is shallow at the west side of the building.

The average mois ture con ten t of the soils benea th the exter ior footings was abou t 22.8

percent which corresponds to a swelling potent ia l of less than 1 percent . The possibility of

foundat ion movemen t was ra ther r emote . Most of the inter ior footings are placed on natural

soils. The average mois ture con ten t of the soils benea th the inter ior footings was a b o u t 14.5

percent . This corresponds to a swelling poten t ia l of 5.0 percent . Since the mois ture con ten t was


low, the chance of increase in mois ture con ten t benea th the inter ior footings is high and there is a

s t rong possibility tha t the foot ing founda t ion will have future movement .

In 1968, a mechanical engineer again conduc ted pressure tests on the underslab ho t water

heat ing system. The tests indicated tha t there was n o leakage in the system. On January 28 ,

1970, similar tests were m a d e in this building which indicated tha t the pressure d ropped from 90

t o 76 psi in 15 minu tes and from 100 t o 52 psi in 75 minutes . This indicates tha t there is leakage

in the underslab heat ing system.

The mois ture con ten t d is t r ibut ion analysis indicates tha t the mois ture con ten t near footing

level was higher than for the soils direct ly benea th the floor; also, the exter ior mois ture con ten t

was generally high, abou t 4 percent higher than the inter ior mois ture con ten t . This definitely

indicated tha t mos t of the wet t ing of this building had been caused by the migrat ion of surface

water in to the foundat ion soils. The leakage of the underslab heat ing system had taken place only

recent ly and the effect of the leakage had no t been reflected in the mois ture con ten t of the soils.

The soils direct ly benea th the floor slabs had an average mois ture con ten t of 12.1 percent

which corresponds to a swelling potent ia l of more than 8 percent . The movemen t of the floor

slab of this building has only begun and severe floor movemen t will occur even though the

underslab heat ing system is entirely d isconnected. The existing local high mois ture con ten t in the

soil will migrate t o t he drier soil and cause floor damage.

The following remedial measures are r ecommended for this building:

1. It is no t necessary t o underp in the exter ior footings. The mois ture con ten t around the

per imeter of the building is high and further swelling of the soils benea th the footings is

unlikely.

2 . There is a s trong possibility tha t the in ter ior footings will have movement . The inter ior

footings should be decreased in area by cut t ing off the concrete pad and thus increasing

the uni t dead load pressure, and

3 . The floor slabs in this building should be entirely removed and the soils benea th the slabs

removed to a d e p t h of 3 feet. These removed soils should be replaced wi th nonexpansive,

granular soils compac ted to at least 90 percent s tandard Proc tor densi ty at op t imum

mois ture con ten t . If this is d o n e , then the possibility of further floor movement will be

remote .

Treatment at Buttercup

But te rcup is the east building of the east complex . The foundat ion soils at this building site

consist of 2 t o 5 feet of control led compac ted fill. It was suspected tha t all footings, b o t h inter ior

and exter ior , were founded on s t ructural fill.

The average mois ture con ten t of the soils direct ly benea th the footings was 25 .6 percent .

This corresponds to a swelling pressure of 2 ,000 psf and a swelling poten t ia l of less than 1

percent . At a lower dep th , the natural soils were generally dry wi th the mois ture con ten t ranging

from 18 t o 20 percent and the swelling pressure reaching as high as 9 ,000 psf.

Judging from the mois ture condi t ion of the soils benea th the exter ior footings, it was no t

necessary t o underp in the exter ior footings. The possibility of footing foundat ion movement was

relatively r emote .


The average mois ture con ten t benea th the inter ior footings was 22.1 percent . This

corresponds t o an average swelling potent ia l of less than 1 percent . The soils benea th the exter ior

footings had relatively uniform mois ture con ten t wi th a m i n i m u m mois ture con ten t of 20.1

percent and m a x i m u m mois ture con ten t of 26.9 percent . The possibility of foundat ion

movemen t for the inter ior footings was also r emote . It was, therefore , no t necessary t o underpin

or make remedial cons t ruc t ion on the in ter ior footings.

In 1968, a pressure test was made on the underslab heat ing system, the results of which

showed tha t the pressure d ropped from 32 to 0 psi in 60 seconds. This definitely indicates that

there is a large leak in the unders lab heat ing system. Free water was found no t only in the

exter ior test pits b u t also in three test holes inside the building. The a m o u n t of water t rapped in

the underslab soils mus t be near sa turat ion which accounts for the s teady flow of water from the

underslab soils in to Test Pit 110.

Also, sewer tests t ha t were conduc ted indicate t ha t exter ior test pits had filled during the

tests. It was concluded tha t there are very definitely leaks in the sewer lines of the east complex.

The mois ture con ten t of the soils at the per imeter of the building was only slightly higher

than the mois ture con ten t at the central por t ion of the building. This indicates tha t the amount

of wate r tha t seeped in to the soils from the exter ior of the building was relatively low. Most of

the water present in the underslab soils is derived from the leakage of the unders lab heating

system and possibly from leaking sewer lines.

The possible behavior of the floor slabs at this building can be evaluated by s tudying the

mois ture dis t r ibut ion at various dep ths . The following facts were no t iced :

1. The soils direct ly benea th the slabs had an average mois ture con ten t of 21 .0 percent .

This corresponds t o an average swelling poten t ia l of 1.2 percent which is considered to be

low.

2. There was no large difference be tween the mois ture con ten t in the center por t ion and the

mois ture con ten t a round the per imeter of the building (19.9 versus 21.2 percent ) ;

therefore , the migrat ion of mois ture from inter ior t o exter ior is unlikely t o take place.

3 . Due t o the excessive leakage of the underslab heat ing system, a large por t ion of the soils

had reached a state of sa tura t ion.

It is no t necessary to replace the underslab soils and the chance and magni tude of future

slab movemen t is low.

All backfill a round this building should be removed to expose the foundat ion system. After

the backfill has been removed, the excavat ion should remain open for a period of at least 2 weeks

to insure that all water t rapped under the floor slabs can be effectively drained.

Treatment at Starlight

Starlight is the east building of the east complex . The foundat ion soils at this site consist of

b o t h cut and fill. At the n o r t h side, there is 2 feet of cu t and at the sou th side there is 4 feet of

fill. It is likely tha t all footings, b o t h exter ior and inter ior , are founded on the natural soils.


The swelling potent ia l benea th the footings was errat ic , ranging from 0.3 to 3.4 percent ,

wi th swelling pressure ranging from 2 ,500 to 12,000 psf. Since the footings are only lightly

loaded, fur ther movemen t of the footings is possible. A s tudy of the average mois ture con ten t at

the per imeter of the slab indicated tha t the average mois ture con ten t was 21.1 percent , which

corresponds to a swelling potent ia l of 1.5 percent . The poten t ia l movemen t of the exter ior

footings is at least 50 percent .

The average mois ture con ten t benea th the inter ior footings was 15.0 percent . This

corresponds t o an average swelling poten t ia l of 4.5 percent and an average swelling pressure of

6 ,000 psf. At the lower dep th , approximate ly 6 feet below the t op of the slab, the average

mois ture con ten t decreased to 14.1 percent . This corresponds to an average swelling potent ia l of

6.0 percent and an average swelling pressure of 15,000 psf. It was obvious tha t the potent ia l

movement of the inter ior footings is high and in fact, at a d e p t h of 6 feet be low the top of the

floor slab, some of the soils had a swelling potent ia l as high as 9 percent and a swelling pressure as

high as 30 ,000 psf.

Again in 1968, the underslab heat ing system was checked and only slight leakage was found.

During this test, the pressure only d ropped from 34 to 33 .5 psi.

None of the test pits no r test holes, e i ther inside or outside of the building, showed signs of

the presence of free water . This indicated tha t leakage of the ut i l i ty lines was no t the main

problem at this building. The per imeter mois ture con ten t at a d e p t h of 3 feet was 21.1 percent

and the mois ture con ten t at the central por t ion of the building at the same dep th was 15.0

percent . This definitely indicated tha t the source of mois ture tha t entered this building was from

surface runoff. Poor drainage around the building was direct ly responsible for the wet t ing of the

foundat ion soils.

The soils direct ly benea th the floor slab had an average mois ture con ten t of 16.9 percent ,

corresponding t o an average swelling potent ia l of 3.5 percent . Moisture dis t r ibut ion in the

underslab soils was ex t remely erratic, wi th the highest mois ture con ten t being 22.7 percent and

the lowest 10.6 percent . The potent ia l for fur ther floor movemen t at this building is great.

Moisture d is t r ibut ion was errat ic , and even though the source of mois ture t ha t entered the build-

ing had been cut off by improving the drainage, there is still the chance of mois ture migrat ion

from the wet area t o the dry area, and this can cause damage.

The remedial measures given under " T r e a t m e n t at A s p e n " can be applied in their ent i re ty here .

Since the main source of wa te r which entered the building is from the surface runoff, special

a t t en t ion should be directed t o improving the surface drainage condi t ion .

Treatment at Crescent

Crescent is the n o r t h building of the east complex . T h e foundat ion soils at this site consist

of par t fill and par t cut . At the west side there is 3 feet of cut and at the east side there is 1-1/2

feet of fill. It is likely tha t all footings at this building are placed on natura l soils.

The condi t ion of soils benea th the exter ior footings is represented by Test Pits 112 and 113.

Free wate r was found in Test Pit 112. The water seeped in to the pit from the underslab gravel

and drained away in several days . Soils at footing level in Test Pit 112 possessed only low swelling

potent ia l .


In Test Pit 113 , no free wate r was found. T h e swelling potent ia l at footing level was low b u t

at lower dep ths , the swelling potent ia l reached as high as 2.4 percent wi th a swelling pressure of

6 ,000 psf.

The average per imeter mois ture con ten t was on the order of 21.2 percent . This

corresponds t o an average swelling poten t ia l of 1.8 percent and a swelling pressure of 3 ,000 psf.

F rom the s tudy of the soil behavior benea th the exter ior footings, t he possibility of foundat ion

m o v e m e n t is r emote . It is no t necessary t o underp in t he footings.

Most of the inter ior footings are placed on natural soils. T h e average mois ture con ten t of the

soils benea th the inter ior footings was 19.2 percent . This corresponds t o an average swelling

potent ia l of 2.3 percent and an average swelling pressure of 3 ,500 psf. The mois ture con ten t

decreased wi th dep th . A t a d e p t h of 6 feet below the t o p of the floor slab, some of the natura l

soils possessed high swelling potent ia l , as m u c h as 8.3 percent . There is some chance of

movemen t of t he inter ior footings. Again in 1968, definite leakage was found in the underslab

heat ing system. Tests indicated tha t the pressure d ropped from 35 to 22 psi in 5 minutes .

A s tudy of the mois tu re con ten t of the underslab soils indicated tha t the mois ture con ten t

direct ly benea th t he floor slab was high and the per imeter mois ture con ten t was even higher than

the central mois ture con ten t . This indicated tha t the underslab soils were wet ted by leakage of

the uti l i ty lines as well as by surface runoff.

The soils direct ly benea th the floor slab had an average mois ture con ten t of 20.5 percent .

This corresponds to an average swelling potent ia l of 1.9 percent and an average swelling pressure

of 3 ,000 psf. The mois tu re con ten t was relatively uniform wi th in 4 feet below the surface of the

slab. With p rope r drainage, t he chance of further floor movemen t is no t great.

The remedial measures for this building are essentially t he same as those r ecommended for

Birch. Fo r the floor slabs, the r ecommended procedure is a choice be tween removal of the soils

benea th the slab as r ecommended for Aspen or saw-cutt ing the floor slabs and improving the

par t i t ion walls as r ecommended for Birch.

Drainage improvement

In addi t ion t o the remedial measures given for each building, the following general

t r ea tmen t for improving the exter ior drainage was r ecommended :

1. Remove all backfill to expose the founda t ion system. After removal , a careful check

should b e m a d e of t h e void space benea th the grade beams t o assure t h a t it is p roper ly

formed. The excavat ion should remain open for a period of at least 1 week so tha t all

water which is t rapped under the floor slab can be drained out .

2. A concre te walk should be provided around the exter ior of each cot tage. The walk should

b e at least 8 feet wide wi th sufficient slope t o allow free drainage of wate r away from the

cottages. The walks should no t be tied in to the grade beams. An expansion jo in t should

be provided be tween the walk and the grade beam.

3 . The ground surface surrounding the cot tages should be sloped t o drain away from the

cottages. A slope of 10 inches for the first 10 feet is r ecommended . If this slope in

una t ta inable , it m a y be necessary t o install swales at various locat ions outs ide of the

cot tages to lower the ground surface and allow free drainage.


4. The lawn sprinkler heads should be located at least 10 feet from the foundat ion walls of

the buildings. Spray from the sprinkler heads should no t be directed towards the

buildings.

5. The roof d o w n s p o u t s are no t efficient. It is entirely possible tha t during heavy s torms,

wate r could collect, overflow, and no t drain away th rough the downspou t s . Improvement

is necessary in this area.

6. Drainage is no tab ly p o o r in front of the service buildings. These areas should be paved

wi th concrete if positive surface drainage is no t possible.

R E M E D I A L CONSTRUCTION

Remedial cons t ruc t ion started in Sep tember 1 9 7 1 . All r ecommenda t ions were carried ou t

except t he underpinning of the exter ior footings. Budget l imitat ions did no t allow the full

remedial cons t ruc t ion . The remedial cons t ruc t ion , at a cost of a half mill ion dollars, was

comple ted in July, 1972. Figures 149 th rough 154 show certain cons t ruc t ion procedures .

Considering the ex ten t of original damage , the remedial cons t ruc t ion has proven successful.

It is un fo r tuna te tha t the severe swelling potent ia l of the subsoil had no t been recognized in the

design stage. Had the buildings been founded on piers ra ther than on individual pads and had the

underslab heat ing system been el iminated, mos t of the damage could have been avoided.

Figure 149. Installation of subdrains around the perimeter of the building.

DIS

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239

Figure 150. Checking the void forming material beneath the grade beams.


Figure 151. Repair of leaking plumbing.


Figure 152. Drainage around exterior of the building has been improved. Note catch basins in the lawn area.

242

FO

UN

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NS

ON

EX

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IVE

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Figure 153. Severely cracked brick wall , patched and repaired. Figure 154. Concrete apron placed around the building wi th Note: Further cracking has not occurred. properly constructed mastic joints.

Case IV

DISTRESS CAUSED BY HEAVING OF

CONTINUOUS FOOTINGS

G E N E R A L

This case s tudy is typical of w h a t happens when con t inuous footings are placed on

expansive soil w i t h o u t considering uplift forces. In this instance the s tudy involves a house tha t

was const ructed w i t h o u t benefi t of adequa te design. St ructura l s t rength in the founda t ion walls

was lacking; therefore , it was no t possible to underp in the building. Use of post- tensioned steel or

pouring of a new foundat ion wall appears t o be the only possible remedial cons t ruc t ion . By so

doing, the house will be tied toge ther as a b o x and will be able to wi ths tand further differential

movemen t .

HISTORY

The house is located in Broomfield, Colorado , a small c o m m u n i t y n o r t h of Denver, Colo-

rado. This area is well k n o w n for its swelling soil p rob lem. The house was cons t ruc ted in 1960.

Nei ther a soil investigation record no r s t ructural design drawings were available. The house has a

full basement and a t tached garage.

Test pits were excavated t o examine t h e founda t ion system of t he house . This revealed tha t

the house is founded wi th con t inuous spread footings on the na tura l soils. T h e footings are 20

inches wide and 8 inches in d e p t h . T o conform wi th the na tura l ground con tour , the basement

foundat ion wall is s tepped d o w n from full basement height at the sou th end t o only 24 inches at

the nor th end. T h e space be tween the concre te wall and brick course is filled wi th cinder b lock.

N o re inforcement was found in the concre te foundat ion wall o r in t he footings. Such

foundat ion design creates a s t ructural weakness in t he middle por t ion of the basement wall.

Wi thout re inforcement in the concre te , slight foundat ion movemen t will result in severe cracking.

Also, at the no r th end of* the basement , there is a s t ructura l d iscont inui ty at the walk-out

ent rance , where the grade beam is d isrupted.

None of the slab-bearing par t i t ion walls have slip jo in ts e i ther at the t o p or b o t t o m . A n y

slab m o v e m e n t will cause the par t i t ion wall to push against the uppe r floor joists and cause

movemen t in the uppe r levels.

Exterior

At the exter ior of the house , severe cracks were found, b o t h on the west and east sides in

the brick walls above the basement . Most of the cracks appeared direct ly below and above


windows in the middle por t ion of the basement section of the house . Cracks opened (fig. 155), as

m u c h as three-quarters of an inch, and ex tended t o the concre te benea th the cinder b lock. At the

no r th side of the building, severe cracks were also found below the windows. No severe cracks

were found on the sou th side of the building.

Basement interior

The basement floor slab had been removed, exposing a por t ion of the footings. Prior t o

removal, t he basement floor slab was badly cracked. Cracks were found in the footings as well as

in t he foundat ion walls. A t rench was opened around the basement por t ion of the house in an

a t t emp t to install drain tile a round the basement wall, leading to a sump . A 4-inch gravel layer

was placed benea th t he basement slab.

Upper floor

The a m o u n t of cracking in the upper floor is also severe as shown on figure 156. Cracks

were evident above windows, at the east side of the house , and a few doors were j ammed in the

upper level.

Exterior drainage

Exter ior drainage condi t ions of the house are poor , part icularly at the no r th side of the house

where the walk-out basement d o o r is located. Natura l drainage is from west to east, and there is a

s t rong t endency for surface wate r to en ter the foundat ion soils from the west side.

1 T

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zzrr

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Figure 155. Cracks below window.

DISTRESS CAUSED BY HEAVING OF CONTINUOUS FOOTINGS 245

Figure 156. Severe cracking above closet.

SUBSOIL CONDITION

The subsoil cond i t ions at t he site consist essentially of slightly po rous sandy clays at t he

sou th end, and highly wea thered , mois t c laystone at the n o r t h end. Undis turbed hand drive

samples were t aken from the test pi ts . Swell-consolidation tests , performed on the undis turbed

hand drive samples, indicated tha t the uppe r sandy clays at the sou th side of the building

possessed only low swelling po ten t ia l ; while the weathered c laystone at the no r th side of the

building possessed m o d e r a t e t o high swelling potent ia l . T h e soils, at present , are in a very mois t

condi t ion . It is apparen t tha t the soils were in a m u c h drier condi t ion when the house was

cons t ruc ted , and the swelling po ten t ia l of the claystone should be m u c h higher than the tests

indicate .

N o free water was found in the test pits which had a d e p t h of 5 feet benea th the b o t t o m of

the footings.


T h e cause of m o v e m e n t of t he house is d u e t o a combina t ion of uplifting m o v e m e n t of the

foundat ion soils and p o o r s t ructural design as summarized be low:

1. There is a difference in d e p t h of the concre te founda t ion walls, and also the founda t ion

walls are n o t reinforced. T h e s t ruc ture of the house canno t wi ths tand even slight


differential movemen t . This is substant ia ted by the fact tha t the foundat ion wall, as well

as the foundat ion , have b o t h cracked severely.

2. The a m o u n t of swelling of t he claystone at the n o r t h end of t he building is abou t 5 t imes

as m u c h as the a m o u n t of swell of t he sandy clays at t he sou th end of the building under

the same mois ture condi t ions . Consequent ly , the foundat ion at the n o r t h end of the

building has lifted. Measurement confirms tha t there is a difference in elevation be tween

the no r th and sou th ends of the building by as m u c h as 5 inches.

3 . At one t ime, wa te r seeped in to the basement at the west side th rough the seams be tween

the concre te and cinder block wall. Such wet t ing condi t ions have caused the foundat ion

soils t o swell. No free wa te r was found benea th t he footings at the t ime of inspect ion.

Judging from the high mois ture con ten t of the foundat ion soils, it is evident tha t the

entire area has been u n d e r severe wet t ing condi t ions . T h e swelling of the weathered

claystone requires only slight mois ture increase. The presence of free water is no t

necessarily the cause of these swelling condi t ions .

REMEDIAL M E A S U R E S

Since the cause of m o v e m e n t is due t o b o t h t he s t ructural weakness of the building and the

swelling of the claystone benea th the footings, the following remedial measures are

recommended :

1. A new foundat ion wall should be poured around the inter ior of the basement . The new

grade beams should be reinforced wi th t w o 5/8-inch bars , t o p and b o t t o m , and should be

tied in wi th the existing foundat ion wall in the manne r shown in figure 157. The new

foundat ion wall should have a d e p t h of approximate ly the full height of the basement .

This wall will tie the basement toge ther s t ructural ly and eliminate the existing s t ructural

weakness.

2. The soils should be removed from benea th the footings at approximate ly 10-foot

intervals, as shown in figure 158. In so doing, the weight of the building will be

concent ra ted at isolated locat ions and dead load pressure increased. It is est imated tha t

the a m o u n t of dead load pressure exerted on the exter ior footings is abou t 600 psf. Such

dead load pressure is insufficient t o prevent the uplifting of the c laystone benea th the

footings. With voids formed benea th t he footings, t he dead load pressure will be increased

t o abou t 3 ,000 psf, eliminating the uplifting problem. Air space benea th the footings

should be formed by the use of void forming material .

3 . An alternative remedial cons t ruc t ion m e t h o d is installation of post- tensioned steel cables

in b o t h the outs ide and inside of the foundat ion walls in the manne r indicated in figure

159. The purpose of the post- tensioned cables is t o tie the entire s t ructure together t o

prevent unequa l movemen t .

4 . The drainage around the building should be improved so tha t water will drain away from

the building. All shrubs and flower beds adjacent t o the building should be removed and

all roof d o w n s p o u t s should extend well beyond the limit of all backfill.


SECTION A A SECTION Β Β

Figure 157. Sketch of new grade beam for basement.

5. T h e instal lat ion of a subsurface drainage system will n o t improve the present s i tuat ion.

However , the use of a subsurface drainage system will keep the water table be low

basement level. T h e use of a gravel layer benea th t he slab will n o t prevent cracking of the

slab, b u t will b reak any capillary wate r rise.

If the remedial measures presented above are per formed, it is believed tha t m o v e m e n t of t he

house will s top , b u t it will t ake a period of at least 6 m o n t h s before equil ibrium can be

established. In ter ior decora t ing and repairs should n o t begin unt i l equi l ibr ium has been

established. Elevation pins should b e established around the house and records kep t as to the

a m o u n t of movemen t .

Remedia l cons t ruc t ion was star ted short ly after the house was investigated. T h e post-

tensioned cable system was used t o s t rengthen the founda t ion (fig. 160) . T h e residence is in good

condi t ion after t he remedial cons t ruc t ion .


• Pit I

Figure 158. Removal of soils beneath the footings.


PLAN

Existing foundation wall (concrete in good condition)

Post Tension Cables installed maximum Γ-θ" below window. Cable cased in grouted tubes. Applied tension on the order of 100 p.s.i.

7 ^ Floor Slab

SECTION A · A

Figure 159. Remedial measures using post-tensioned steel cables.


Figure 160. Post-tensioning the foundation wall.

Case V

DISTRESS CAUSED BY RISE

OF WATER TABLE

G E N E R A L

This case s tudy involves 39 two-story townhouses founded on a drilled pier system, that had

foundat ion movemen t . The m o v e m e n t of the drilled pier system is essentially caused by a rise of

ground water . The following knowledge was gained from this s tudy :

1. In areas where there is a s t rong possibility of rise of ground water , the use of a drilled pier

system should be carefully considered.

2. Heaving of the floor slab can t ransmit high swelling pressure t o the grade beams.

3 . Chemical t r ea tmen t of the unders lab soil, in this case, is ineffective.

H I S T O R Y

Thir ty-nine townhouses are unde r investigation for foundat ion movemen t . The townhouses

are located in southeas t Denver, Colorado. There are four t o seven uni ts in each building. A total

of 251 uni ts and a c lubhouse were studied (fig. 161).

Initial subsoil investigation for t he site was m a d e in 1965 . The repor t resulting from that

investigation r ecommended tha t the buildings be founded wi th piers drilled in to bedrock

designed for a m a x i m u m end pressure of 20 ,000 psf and a m i n i m u m dead load pressure of 10,000

psf.

Cons t ruc t ion of the building complex was started in September , 1965. Shor t ly after the

comple t ion of the various uni ts , m o v e m e n t of the buildings began, and in some buildings severe

movemen t of b o t h founda t ion walls and floor slabs was not iced . In March, 1970, a consult ing soil

engineer was engaged t o m a k e a prel iminary investigation in to the cause of cracking and

movemen t in t he various uni ts . A repor t out l ining remedial cons t ruc t ion was prepared bu t no

corrective act ion was taken .

The following typical distress was observed in mos t buildings:

Foundation walls

Cracks were found in the brick course in a diagonal pa t t e rn from the t o p of the window or

d o o r on the ground level t o t he b o t t o m of t he window in t he uppe r level. Most of t he cracks had

been patched and some had reopened. The wid th of the cracks ranged from hairl ine thickness t o

as m u c h as 1 inch as shown on figures 162 and 163 . These cracks definitely indicated pier


95

70 GROUNDWATER CûNTO'Jfc

Figure 161. Building location plan and ground-water contour.

movemen t . In addi t ion , b o t h vertical and diagonal cracks were found in the basement foundat ion

walls.

Interior floor slabs

Most of the basement areas were finished. Concre te floor slabs were covered with tile o r

carpeting. The slabs had heaved in m a n y buildings and the cracks generally followed a pa t te rn

parallel t o the foundat ion walls. This is a typical slab crack where separat ion be tween the slab

and foundat ion wall was no t proper ly cons t ruc ted . The slabs bind on the foundat ion walls and

are no t free t o accept vertical movemen t . Consequent ly , cracks appeared parallel to the

foundat ion walls. Such m o v e m e n t no t only caused the slab cracking, bu t also exerted uplift

pressure on the founda t ion walls.

Partition walls

All buildings have uni t s wi th par t i t ions and in mos t cases slip jo in ts were no t provided in the

slab bearing par t i t ion walls. Consequent ly , when the slabs moved upward , they exerted uplifting

pressure on the upper s t ruc ture . As a result , cracks developed in the uppe r stories, j amming the

doors and dis tor t ing the floor system.

Figure 162. Typical cracking of the townhouse exterior walls.

DISTRESS CAUSED BY RISE OF WATER TABLE 253


Fig

ure

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lift

.


In those townhouses which had basement areas tha t were n o t finished, slab m o v e m e n t does

no t con t r ibu te t o the distress of the upper stories; however , t he staircase walls in all cases rest

direct ly on the basement floor. Therefore , slab m o v e m e n t can t ransmit movemen t t o the upper

stories th rough the staircase par t i t ion walls.

Slab treatment

Between February and April of 1966, in several buildings mainly at the western por t ion of

the site, the floor slab was removed and the soil benea th the slab injected wi th stabilizing

chemicals in an effort t o el iminate the swelling poten t ia l of the lower soils. After t he new slab

had been placed, m o v e m e n t of the floor slab was n o t checked and in mos t cases, slabs again

cracked. T h e results of t he swell tests indicated tha t the swelling pressure of t h e unders lab soils

ranged from 500 to 2 ,500 psf and the mois ture con ten t s a t various dep th s were fairly uniform.

Tests indicated tha t the appl icat ion of chemicals on the unders lab soil did no t have a p ronounced

effect in reducing the swelling potent ia l .

Aprons

Concre te aprons const ructed at the rear of the buildings were in mos t cases cracked. Gaps

were found be tween the slabs and the buildings, indicating separat ion. This can be caused by

ei ther t he uplifting of the building relative t o the slab or the se t t lement of the slab due to

inadequate backfill relative to the building. T h e aprons were m u d jacked in several buildings in an

a t t e m p t t o correct the condi t ion .

In general, t he cracks t ha t developed in this apa r tmen t complex typify the cracks found in

buildings founded on expansive soils. There is no d o u b t tha t the m o v e m e n t of the buildings can

be a t t r ibuted t o a swelling soil p rob lem.

SUBSOIL C O N D I T I O N S

Subsoil condi t ions at t h e site consist of 7 t o 22 feet of stiff t o m e d i u m stiff clays overlying

claystone bedrock . T h e clays have an average mois ture con ten t of 20 .3 percent wi th the highest

mois ture con ten t of 23 .8 percent and the lowest of only 11.6 percent . This mois ture con ten t is

considerably higher than the mois ture con ten t repor ted by the test ing laborator ies in August of

1965. At t h a t t ime, t h e average mois ture con ten t of the u p p e r soil was only 14.6 percent . The

swelling po ten t ia l of the u p p e r clays ranges from 0.5 t o 3 percent wi th the swelling pressure

ranging from 0 to 5 ,000 psf.

The lower bedrock consists basically of c laystone and weathered claystone. Some sandstone

lenses were found in the claystone. A s tudy of mois tu re con ten t of the c laystone bedrock also

indicates an increase in the last few years . Since the mois tu re con ten t of the c laystone bedrock is

affected by the pa t t e rn of seams and fissures of bedrock , t he increase of mois ture con ten t is no t

as obvious as t he increase of mois tu re con ten t of the upper clays. The swelling pressure of the

lower claystone ranges from 5,000 t o 25 ,000 psf. The average swelling pressure is abou t 10,000

psf.


It is concluded from the labora tory testing tha t the u p p e r clays possess only low swelling

poten t ia l and the lower c laystone possess high swelling potent ia l . Most of the foundat ion wall

movemen t is the di rect result of the swelling of the lower c lays tone; however , the swelling of the

uppe r clays is sufficient t o cause the floor slabs t o heave and crack.

WATER T A B L E

When the soil and founda t ion investigation was made in August , 1965, no free wate r was

found in any of the 21 exp lora to ry holes. Some of the test holes were m o r e than 20 feet deep . In

July, 1970 when this investigation was made , free water was found in a lmost every hole . Figure

161 indicates an approx ima te c o n t o u r of equal elevation t o water table .

T h e following condi t ions were observed:

1. The water- table elevation is high at the wes tern po r t ion of the site and low at the eastern

por t ion . A difference in t he water- table elevation of 30 feet was observed be tween the

nor thwes t corner and the southeas t corner of the p rope r ty .

2. The water- table c b n t o u r follows fairly well wi th the bedrock con tour . Water was found at

the t o p or immedia te ly below the surface of bedrock .

3 . The wate r table follows fairly well wi th the ground surface con tour . The ground surface

was high in the west and low in the east.

After carefully s tudying the c o n t o u r of equal elevation t o the wate r table and the general

water- table condi t ions in the area, it is concluded tha t in the last 5 years, after area development ,

there was a definite change of water- table condi t ions . The general rise of the water- table is

essentially caused b y a perched wa te r table in the developed residential area. Bedrock in this area

is shallow and composed essentially of c laystone which is relatively impervious. Free water flows

mos t ly on t o p of bedrock and also flows in the fissures and seams of the bedrock .

Surface wate r does no t necessarily pene t ra te direct ly from the ground surface in to the

under lying bedrock b u t will flow from the high poin t to t he low point along the surface of

bedrock .

The rise of wa te r table definitely has a bearing on the foundat ion movemen t of this

apa r tmen t complex . As seen on figure 1 6 1 , the buildings suffering the mos t severe damage are

located in high water- table elevation areas. Where t he water- table elevation is low, relatively

minor damage t o t he buildings was exper ienced.


In reviewing the founda t ion design, the grade beams and pier system and the results of this

investigation, t he following were derived:


Foundation design

The founda t ion design criteria calls for an end-bearing pressure for piers of 20 ,000 psf and a

min imum dead load pressure of 10,000 psf. The piers should pene t ra te the shale a m i n i m u m

d e p t h of 5 feet. T h e side of the pier excavated in to the shale should be roughened t o provide

resistance t o uplift . The drill logs provided by the driller indicated tha t the required pier

pene t ra t ion had been fulfilled in m o s t cases. T h e load carrying capacity of the piers was reviewed,

including t h e dead load pressure requ i rement , and were found t o be in accordance wi th t he

r ecommenda t ions , excep t for t h e piers suppor t ing in ter ior co lumns and beams.

In mos t cases, the swelling pressure of the lower bedrock is abou t 10 ,000 psf wi th a few

cases reaching as high as 25 ,000 psf. T h e r ecommended dead load pressure of 10,000 psf is low,

b u t considering the subsoils and the wate r table condi t ions at t he t ime the subsoil investigation

was made , the design r ecommenda t ions are in a range of sound engineering pract ice .

Foundation construction

F r o m the informat ion obta ined b y excavating from the inside and from the outs ide of the

building exposing the grade beam system, the cons t ruc t ion was in accordance wi th the s t ructura l

foundat ion design. In several places, m u s h r o o m s were found on t o p of the piers and the void

forming mater ia l benea th the grade beams immedia te ly adjacent t o t he piers was absent . Such

defects decrease the dead load pressure exer ted on the piers; however , t he effect is n o t sufficient

to cause the present distress of the various buildings.

Slab construction

The major problem is in the area of slab cons t ruc t ion . In principle, free floating slabs should

be provided. There should be positive expansion jo in t s be tween the slabs and the grade beams t o

allow free slab movemen t . At present , the jo in t be tween t h e slabs and t h e grade beams is n o t

effective in m o s t cases. Slab movemen t due t o expansive soils has t ransmi t ted uplifting pressure

from the slabs t o t h e founda t ion walls. T h e swelling pressure of the unders lab soil is abou t 2 ,000

psf.

In ex t reme cases, the pressure t ransmi t ted from the slab t o the foundat ion wall can reach

2 ,000 p o u n d s per linear foot . With t he piers spaced on 11-foot centers , each pier can be subject

t o abou t 22 kips of uplifting pressure.

Pier uplifting

T h e m o s t i m p o r t a n t reason for t he m o v e m e n t of t he founda t ion walls is t he uplifting of the

piers. In the original founda t ion design, it was assumed tha t the lower bedrock would n o t become

wet ted and the skin friction be tween the bed rock and t h e piers would provide an addi t ional

factor of safety against pier uplifting. With the rise of t he wa te r table , the ent i re lower bedrock

became we t t ed ; consequent ly , the skin friction value dissipated and ra ther t han holding the piers,


the swelling of the bedrock actually lifted the piers. As an example , pier No . 8 in Building 8 was

checked by calculation as follows:

For inter ior piers where t he actual dead load pressure exerted on the pier ranges from 2.6 to

20.9 kips, the unbalanced pressure is in the range of 10 to 30 kips.

It is concluded tha t the wet t ing of the bedrock plus the pressure t ransmit ted from slab

heaving t o the grade beam had lifted the piers.

In one test pit , t he ent ire length of the pier was exposed. A gap of approximate ly 3 inches

be tween the soil and the b o t t o m of the pier was found. This verified tha t the pier actually pulled

ou t of t he ground and would have cracked had it n o t been reinforced.

SOURCE O F M O I S T U R E

The rise of ground wa te r in this deve lopment is mainly derived from surface water . The

source of surface water is from the following:

Precipitation

Precipi tat ion, ei ther from rain or from melt ing snow, can con t r ibu te to the rise of water

table. Before the deve lopment was cons t ruc ted , mos t of the precipi ta t ion drained from the site as

surface runoff. Only a por t ion of the precipi ta t ion penet ra ted the ground.

With building cons t ruc t ion , in isolated areas, precipi ta t ion will pene t ra te the soils th rough

the loose backfill a round each building and due t o poo r drainage condi t ions , tends to accumulate

instead of running off the site. This cons t i tu tes an impor t an t factor toward the rise of ground

water .

Lawn irrigation

After a deve lopment is comple ted , lawn irrigation in the area will generally create perched

water-table condi t ions . It is conceivable tha t a large a m o u n t of lawn irrigation water will travel

through the uppe r soils and become t rapped at the surface of the bedrock . This practice is no t

necessarily limited t o the p roper ty investigated bu t per tains to the entire general area. High

water-table condi t ions prevailed in several areas in t he general vicinity after t he deve lopment was

completed .

Actual dead load pressure on the pier

Pier d iamete r

Average swelling pressure

Pier pene t ra t ion in to bedrock

Uplifting pressure d u e t o sa turat ion

20.6 kips

10 inches

10,000 psf

7 feet

of bedrock - 7X2.62X 10,000X0.15

Unbalanced pressure

27.5 - 2 0 . 6 7.1 kips

27.5 kips


Pipe leakage

Leakage of the water lines, resulting from corroded service saddles, was found in front of

Buildings 80 , 8 3 , and 84 . It is no t k n o w n how long the leakage had taken place, b u t m o n t h l y

water consumpt ion from January t o Oc tobe r of 1969 was 24 .2 mill ion gallons. During this same

period in 1970, the consumpt ion was 28.3 mill ion gallons, which shows an increase of 4 million

gallons over a period of 10 m o n t h s . This increase in consumpt ion can be partially explained by

pipe leakage. These 4.1 mill ion gallons of water will eventually flow on the surface of the bedrock

and cause a general rise of the water table .

E V A L U A T I O N O F BUILDING CONDITIONS

F o r discussion purposes , the t ownhouse complex was divided in to th ree areas (fig. 161) , and

the evaluations are as follows:

Area I

In this area, severe founda t ion m o v e m e n t has t aken place. T h e founda t ion walls as well as

the floor slabs have cracked and heaved. Most of the floor slabs in this area have been replaced

and the soils benea th the floor slabs have been t rea ted wi th special chemicals.

The water- table elevation is high in this area and the bedrock has been wet ted excessively.

F o u n d a t i o n m o v e m e n t in this area is caused b y the uplifting of the piers.

Area II

In this area, relatively mino r foundat ion m o v e m e n t has t aken place. Some of the floor slabs

have been replaced and t r e a tmen t of the underslab soils was made in several buildings.

T h e buildings in this area have no t suffered severe founda t ion m o v e m e n t which is p robably

due to the following:

1. The water- table elevation is relatively deep and the b o t t o m s of t he piers are above the

present wate r table. The piers still main ta in an anchorage effect and expansive soils have

n o t acted u p o n the surface of the piers.

2. The lower bedrock consists of a combina t ion of sands tone and claystone which does no t

possess a high swell po ten t ia l . Therefore , the piers are relatively stable.

It is possible t ha t these buildings will no t suffer severe founda t ion movemen t in t he fu ture ;

however, changing wa te r table and o the r local condi t ions may affect the stabil i ty of the

s t ructure . F o r instance, in Buildings 8 8 , 90 and 9 2 , it is ant ic ipated tha t severe m o v e m e n t will

t ake place in the near future. This is because of the high swell potent ia l of the lower soils and the

close p rox imi ty of the wa te r table.


Area III

This area is located at the southwest po r t ion of the site where ground surface is high and the

water table elevation is deep . At the nor theas t por t ion of the site, the ground water is well below

the b o t t o m of t he piers. T h e condi t ion of these buildings is relatively good. The floor slabs have

no t been replaced, no r have there been any remedial measures taken.


Remedia l measures depend on the ex ten t of the present damage and can be best described

unde r the three areas men t ioned before:

Area I

In this area, t he damage is of such ex ten t t ha t drast ic measures should be taken as

r ecommended :

1. All piers should be cut free from the foundat ion system so tha t t he entire building will

no t be associated wi th the lower bedrock . This is necessary because the source of the

problem, for the buildings in this area, is caused by the expansion of the lower bedrock .

Since the water- table condi t ions canno t be changed, it is necessary t o prevent direct

con tac t of t h e founda t ion system wi th t h e bedrock .

2. Individual pads should be provided benea th the foundat ion walls t o suppor t the building.

The pads should be designed for a m a x i m u m soil pressure of 2 ,500 psf and as m u c h dead

load pressure as possible. Since the swelling pressure of the uppe r soils in this area is

abou t 2 ,000 psf, there should be lit t le danger of foundat ion movemen t due to the

swelling of the uppe r soils.

3 . Shims should be provided on t o p of each pad so t h a t the elevation of t h e building can b e

adjusted. T h e shims should b e adjusted wi th an engineering level immedia te ly after

comple t ion of the pads and should be readjusted after a period of 6 m o n t h s . The above

described underp inning opera t ion can be executed ei ther from inside t he basement area

or from outs ide. Since it is necessary in m o s t cases t o remove the floor slabs, it will

p robably be m o r e economical to perform the underp inning opera t ion inside the

basement .

4 . All floor slabs should be separated from the bearing walls wi th a positive expansion jo in t .

If effective expansion jo in t s cannot be ob ta ined , a gap of approximate ly one half inch

should b e left all a round the slab t o insure t ha t the slabs will no t bind against the bearing

walls. When the basement is finished, this gap can be tiled t o prevent dir t entering the

gap.

5. The use of inter ior slab-bearing par t i t ion walls should be discouraged. If such is necessary,

then slip jo in ts should be provided t o insure free par t i t ion wall movemen t . The slip jo in t

should apply to all d o o r frames and staircase walls. It should be emphasized t h a t sheet-

rock on b o t h sides of the par t i t ion wall should also be provided wi th slip jo in ts .


6. In some cases, it m a y be necessary t o provide a subsurface drainage system around the

per imete r of the basement area.

7. It will be necessary t o carefully check all sewage and wate r pipes t o insure tha t no leakage

has taken place.

The above remedial measures for Area I are expensive and difficult to carry ou t , b u t t o

insure tha t no further founda t ion movemen t will t ake place and t o el iminate some of the existing

damage, such remedial measures are necessary and unavoidable .

Area II

Remedia l measures for t he buildings in Area II depend greatly u p o n individual building

condi t ions . In general, t he following are r e c o m m e n d e d :

1. Increase the dead load pressure exerted on the piers by el iminating a n u m b e r of piers and

increasing the span be tween the piers. This should be carefully designed and planned by a

s t ructura l engineer. T h e dead load pressure exer ted on the piers should be no t less than

20 ,000 psf. The end bearing pressure of t he piers should no t greatly exceed 4 0 , 0 0 0 psf,

wi th a skin friction of 4 ,000 psf for the por t ion of pier in bedrock .

2 . Where existing damage is relatively severe, the ent ire building should be releveled using

shims on t o p of each pier.

3 . Air space benea th t he grade beams should be carefully checked for effectiveness. All

m u s h r o o m s above the piers should be removed.

4 . Careful inspect ion of the condi t ion of the floor slabs should be m a d e . If the slabs are

binding against the grade beams, t hen the slabs should b e removed and replaced wi th an

effective jo in t system. Every effort should be m a d e to avoid the transmission of pressure

from the slabs to the founda t ion walls.

5. Inter ior slab bearing par t i t ion walls and drainage systems should be t rea ted as described

under Area I

Area III

No remedial measures are necessary in this area. The buildings are in relatively good

condi t ion and unnecessary a l terat ions in this area should be avoided. However , close observat ion

of foundat ion m o v e m e n t should be main ta ined . If it is found at a later da te t ha t there are signs of

foundat ion movemen t , b o t h the s t ructural engineer and the soils engineer should be informed so

that they m a y de te rmine if remedial measures are necessary.

Since the investigation covers as m a n y as 251 uni t s , it is difficult t o specify the

r ecommended remedial p rocedure for each building. The soils engineer should be at the site

during the execut ion of t he remedial measures r ecommended by the s t ructura l engineer and the

soils engineer.

APPENDIX A

SUGGESTED METHOD OF TEST FOR ONE-DIMENSIONAL EXPANSION AND UPLIFT PRESSURE OF CLAY SOILS1

S U B M I T T E D B Y W. G. H O L T Z 2

1. Scope

1.1 This method explains how to make expansion tests on undisturbed or com-pacted clay soil samples that have no particle sizes greater than τ$ in. (passing the No. 4 standard ASTM sieve3). The test is made to determine (1) magnitude of volume change under load or no-load conditions, (2) rate of volume change, (3) influence of wetting on volume change, and (4) axial permeability of laterally confined soil under axial load or no-load during expansion. Saturation (no drainage) takes place axially. Per-meant water is applied axially for deter-mining the effect of saturation and permeability. The specimens prepared for this test may also be used to deter-mine the vertical or volume shrinkage as the water content decreases. Total volume change for expansive soils is determined from expansion plus shrink-age values for different ranges of water content.

1.2 Expansion test data may be used to estimate the extent and rate of uplift in subgrades beneath structures or in structures formed from soils, and shrink-age tests may be used to estimate the volume changes which will occur in soils

1 T h i s s u g g e s t e d m e t h o d h a s n o official s t a t u s in t h e S o c i e t y b u t is p u b l i s h e d a s i n f o r m a t i o n o n l y . T h e m e t h o d is b a s e d o n t h e e x p e r i e n c e of t h e s u b m i t t e r . C o m m e n t s are so l i c i t ed .

2 C o n s u l t i n g C i v i l E n g i n e e r , D e n v e r , C o l o . 3 S e e A S T M Spec i f i ca t ion E l l , for W i r e -

C l o t h S i e v e s for T e s t i n g P u r p o s e s , Annual Book of ASTM Standards, P a r t 3 0 .

upon drying, provided that natural conditions and operating conditions are duplicated.

2. Significance

2.1 The expansion characteristics of a soil mass are influenced by a number of factors. Some of these are size and shape of the soil particles, water content, density, applied loadings, load history and mineralogical and chemical prop-erties. Because of the difficulty in evalu-ating these individual factors, the volume-change properties cannot be predicted to any degree of accuracy unless laboratory tests are performed. When uplift problems are critical, it is important to test samples from the sites being considered.

2.2 The laboratory tests described herein are primarily intended for the study of soils having no particles larger than the No. 4 standard sieve size (^ξ in.). If the test is made on the minus No. 4 fraction of soils containing gravel material (plus No. 4), some adjustment is required in any analysis. Gravel reduces volume change because it re-places the more active soil fraction.

3. Apparatus

3.1 Consolidometer—Conventional la-boratory consolidometers are used for the expansion test. Consolidometers most used in the United States are of the fixed-ring and floating-ring types. Figure 1 illustrates the fixed-ring type. Either


of these is suitable. Both types are available commercially. In the fixed-ring container, all specimen movement rela-tive to the container is upward during expansion. In the floating-ring container, movement of the soil sample is from the top and bottom away from the center during expansion. The specimen con-tainers for the fixed-ring consolidometer and the floating-ring consolidometer consist of brass or plastic rings, and other

sion tests the larger diameter consolida-tion rings are preferred as they restrain fhe soil action to a lesser degree. In a test using the floating-ring apparatus, the friction between the soil specimen and container is smaller than with the fixed-ring apparatus. On the other hand,, the fixed-ring apparatus is more suitable for saturation purposes and when perme-ability data are required. Porous stones are required at the top and bottom of the

/Dial gage holder

Clamping ring and water container ^-Gasket | J-Connecting rod

'Base p la te - - ' '

F I G . 1—Fixed-Ring Consolidometer.

component parts. Sizes of container rings most commonly used vary between 4j-in. diameter by 1} in. deep and 2j-in. diameter by J in. deep, although other sizes are used. However, the diameter should be not less than 2 in. and the depth not greater than three tenths of the diameter, except that the depth must not be less than f in. for specimens of small diamter. Lesser depths introduce errors caused by the magnitude of surface disturbance, while large depths cause excessive side friction. For expan-

specimen to allow application of water. The apparatus must allow vertical movement of the top porous stone for fixed-ring consolidometers, or vertical movement for top and bottom porous stones for floating-ring consolidometers, as expansion takes place. A ring gage machined to the height of the ring con-tainer to an accuracy of 0.001 in. is required; thus, the ring gage for lj-in.-high specimens will have a height of 1.250 in. Measure the diameter of the specimen container ring to 0.001 in.

APPENDIX A 265

3.2 Loading Device—A suitable device for applying vertical load to the specimen is required. The loading device may be platform scales of 1000 to 3000-lb capac-ity mounted on a stand and equipped with a screw jack attached underneath the frame. The jack operates a yoke which extends up through the scale platform and over the specimen con-tainer resting on the platform. The yoke is forced up or down by operating the jack, thus applying or releasing load to the soil specimen. The desired applied pressure, which is measured on the scale beam, becomes fully effective when the beam is balanced.

3.2.1 Another satisfactory loading de-vice utilizes weights and a system of levers for handling several tests simul-taneously. Hydraulic-piston or bellows-type loading apparatus are also very satisfactory if they have adequate capacity, accuracy, and sensitivity for the work being performed. Apparatus such as described in ASTM Method D 2435, Test for One-Dimensional Con-solidation Properties of Soils,4 is satis-factory and may be used.

3.3 Device for Cutting Undisturbed Specimens—This apparatus consists of a cutting bit of the same diameter as the ring container of the consolidometer, a cutting stand with bit guide, and knives for trimming the soil. Wire saws or trimming lathes may be used if a uniform tight fit of the specimen to the container is obtained.

3.4 Device for Preparation of Remolded Specimens—Compacted soil specimens are prepared in the consolidometer ring container. In addition to the container, the apparatus consists of an extension collar about 4 in. in depth and of the same diameter as the container. A com-paction hammer of the same type re-quired in Method A of ASTM Method D 698, Test for Moisture-Density Rela-

4 Annual Book of ASTM Standards, P a r t 11 .

tions of Soils, Using 5.5-lb Rammer and 12-in. Drop.4

4. Procedure-Expansion Test

4.1 Preparation of Undisturbed Speci-mens—Perform the tests on hand-cut cube samples or core samples o£ a size that will allow the cutting of approxi-mately I in. of material from the sides of the consolidometer specimen. (Alterna-tively, obtain a core of a diameter ex-actly the same as the diameter of the consolidometer specimen container, and extrude the core directly into the con-tainer. This procedure is satisfactory provided that the sampling has been done without any sidewall disturbance and provided that the core specimen exactly fits the container. Place the undisturbed soil block or core on the cutting platform, fasten the cutting bit to the ring container, and place the assembly on the srmple in alignment with the guide arms. With the cutting stand guiding the bit, trim the excess material with a knife close to the cutting edge of the bit, leaving very little ma-terial for the bit to shave off as it is pressed gently downward. (Other suitable procedures to accommodate guides for wire saws, trimming lathes, or extrusion devices may be used in conformance with the use of alternative apparatus and samples.) In trimming the sample, be careful to minimize disturbance of the soil specimen and to assure an exact fit of the specimen to the consolidometer container. When sufficient specimen has been prepared so that it protrudes through the container ring, trim it flush with the surface of the container ring with a straightedge cutting tool. Place a glass plate on the smooth, flat cut surface of the specimen, and turn the container over. Remove the cutting bit, trim the specimen flush with the surface of the container ring, and cover it with a second


glass plate to control evaporation until it is placed in the loading device.

4.2 Preparation of Remolded Speci-mens—Use about 2 lb of representative soil that has been properly moistened to the degree desired and processed free from lumps and from which particles or aggregations of particles retained by a ï^-in. (No. 4) sieve have been excluded. Compact the specimen to the required wet bulk density after adding the re-quired amount of water as follows : Place the extension collar on top of the con-tainer ring and fasten the bottom of the container ring to a baseplate. Weigh the exact quantity of the processed sample to give the desired wet density when compacted to a thickness \ in. greater than the thickness of the container ring. Compact the specimen to the desired thickness by the compaction hammer. Remove the extension collar and trim the excess material flush with the con-tainer ring surface with a straightedge cutting tool. Remove the ring and speci-men from the baseplate and cover the specimen surfaces with glass plates until the specimen is placed in the loading device. If, after weighing and measuring the specimen and computing the wet density, as described below, the wet density is not within 1.0 lb/ft3 of that required, repeat the preparation of the remolded specimen until the required accuracy is obtained.

4.3 Calibration of Dial Gage for Height Measurements—Prior to filling the con-tainer ring with the soil specimen, place a ring gage in the specimen container with the same arrangement of porous plates and load plates to be used when testing the soil specimen. Place the assembly in the loading machine in the same position it will occupy during the test. After the apparatus has been as-sembled with the ring gage in place, apply a load equivalent to a pressure of 0.35 psi (or 0.025 kgf/cm2) on the soil

specimen. The dial reading at this time will be that for the exact height of the ring gage. Mark the parts of the appara-tus so that they can be matched in the same position for the test.

4.4 Initial Height and Weight of Soil Specimen—Clean and weigh the specimen container ring and glass plates and weigh them to ±0.01 g before the ring is filled. After filling and trimming is completed, weigh the soil specimen, ring, and glass plates to ±0.01 g. Determine the weight of the soil specimen. Assemble the specimen container and place it in the loading device. If the specimen is not to be saturated at the beginning of the test, place a rubber sleeve around the pro-truding porous plates and load plates to prevent evaporation. Apply the small seating load of 0.35 psi (or 0.025 kgf/cm2) to the specimen. By comparing the dial reading at this time with the dial reading obtained with the ring gage in place, determine the exact height of the speci-men. Use this information to compute the initial volume of the specimen, the initial density, void ratio, water content, and degree of saturation. The true water content of the specimen will be deter-mined when the total dry weight of the specimen is obtained at the end of the test.

4.5 Saturation and Permeability Data —To saturate the specimen attach the percolation tube standpipe, fill it with water, and wet the specimen. Take care to remove any air that may be entrapped in the system by slowly wetting the lower porous stone and draining the stone through the lower drain cock. After the specimen is wetted, fill the pan in which the consolidometer stands with water. After saturation has been completed, permeability readings can be taken at any time during the test by filling the percolation tube standpipe to an initial reading and allow the water to percolate through the specimen. Measure the

APPENDIX A 267

amount of water flowing through the sample in a given time by the drop in head.

4.6 Expansion Test: 4.6.1 General Comments—The expan-

sion characteristics of an expansive-type soil vary with the loading history, so that it is necessary to perform a separate test or several specimens for each condi-tion of loading at which exact expansion data are required. However, one procedure is to test only two specimens: (1) loaded-and-expanded, and (2) expanded-and-

permeameter tube head should be suffi-ciently low so that the specimen is not lifted.) As the specimen begins to expand, increase the load as required to hold the specimen at its original height. Then reduce the load to \ , and \ of the maximum load and finally to the seating load of 0.35 psi (or 0.025 kgf/cm2) and measure the height with each load. Use a greater number of loadings if greater detail in the test curve is required. Main-tain all loads for 24 h, or longer if needed, to obtain constant values of height#

I

I" to

rSpecimt *n wetted

\

\ \

\ \

^A^et ted

·> ^ 0 — - —

Specimen wetted,

10 20 Load-psi.

F i g . 2—Load-Expansion Curves.

loaded. From these data, an estimate of expansion can be made for any load condition as shown by Curve C, Fig. 2, in which Specimen No. 1 was loaded and expanded by saturation with water, (Curve B) and Specimen No. 2 was expanded by saturation with water and then loaded (Curve A).

4.6.2 Loaded and Expanded Test—To measure expansion characteristics where the soil specimen is saturated under full load and then allowed to expand, apply the seating load of 0.35 psi (or 0.025 kgf/cm2) to Specimen No. 1, and secure initial dial readings. Then saturate the soil specimen as described in 4.5. (The

Remove the specimen from the ring container and weigh it immediately and again after drying to 105 C. From the water content, dry bulk density, and specific gravity of the specimen, calcu-late the volume of air and, assuming it to be the same as the volume of air following the determination of permeabil-ity, calculate the water content and degree of saturation.

4.6.3 Expanded and Loaded Test—To measure expansion characteristics where the soil is allowed to expand before loading, apply the seating load of 0.35 psi (or 0.025 kgf/cm2) to Specimen No. 2, and secure initial dial gage readings.


Then saturate the specimen as described in 4.5. Allow the specimen to expand under the seating load for 4$ h or until expansion is complete. Load the speci-men successively to | , J, J and—t times the maximum load found in 4.6.2, to determine the reconsolidation charac-teristics of the soil. Use a greater number of loadings, if greater detail in the test curve is required. Follow the procedures specified in 4.6.2 for making loadings and all measurements and determinations.

4.6.4 Individual Load-Expansion Test —When it is desired to perform separate expansion tests for other conditions of loading apply the seating load of 0.35 psi (or 0.025 kgf/cm2) to the specimen and measure the initial height. Then load the specimen to the desired loading, saturate the specimen as described in 4.5, and allow the specimen to expand under the applied load for 48 h, or until expansion is complete. Measure the height of the expanded specimen. Reduce the load to that of the seating load. Allow the height to become constant and measure; then remove the specimen from the ring and make the determination specified in 4.6.2.

5. Procedure—Shrinkage Test

5.1 Specimen Preparation—When measurements of shrinkage on drying are needed, prepare an additional speci-men as described in 4.1 or 4.2. Cut this specimen from the same undisturbed soil sample as the expansion specimens, or remolded to the same bulk density and water content conditions as the expansion specimens. Place the specimen in the container ring, and measure the initial volume and height as described in 4.4. Determine the water content of the soil specimen by weighing unused portions of the original sample of which the specimen is a part, drying the material in an oven to 105 C, and reweighing it.

5.2 Volume and Height Shrinkage Determinations—To measure volume

shrinkage, allow the specimen in the ring to dry in air completely or at least to the water content corresponding to the shrinkage limit (ASTM Method D 427, Test for Shrinkage Factors of Soils).4 After the specimen has been air-dried, remove it from the ring con-tainer, and obtain its volume by the mercury-displacement method.

5.2.1 To perform the mercury dis-placement measurement, place a glass cup with a smoothly ground top in an evaporating dish. Fill the cup to over-flowing with mercury, and then remove the excess mercury by sliding a special glass plate with three prongs for holding the specimen in the mercury over the rim. Pour the excess mercury into the original container and replace the glass cup in the evaporating dish. Then immerse the air-dried soil specimen in the glass cup filled with mercury using the special glass plate over the glass cup to duplicate the initial mercury volume determination condition. (See Method D 427 for general scheme of test and equipment.) Transfer the displaced mercury into a graduated cylinder, and measure the volume. If the shrinkage specimen is cracked into separate parts, measure the volume of each part, and add the individual vol-umes to obtain the total. (A paper strip wrapped around the specimen side and held by a rubber band is effective in holding the specimen intact during handling.)

5.2.2 If the height of the air-dried specimen is desired, place the specimen and ring container in the loading ma-chine. Apply the seating load of 0.35 psi (or 0.025 kgf/cm2), and then read the dial gage.

6. Calculations

6.1 Expansion Test Data—Calculate the void ratio as follows:

volume of voids h — hQ

volume of solids h0

APPENDIX A 269

where: e = void ratio, h = height of the specimen, and

ho = height of the solid material at zero void content

Calculate the expansion, as a percentage of the original height, as follows :

h2 — hi A, percent = X 100

hi

where : Δ = expansion in percentage of initial

volume, hi = initial height of the specimen, and hi = height of the specimen under a

specific load condition.

6.2 Permeability Test Data—Calculate the permeability rate by means of the following basic formula for the variable head permeameter:

Ap X U 1 . 5 i k = X - In —

As X 12 t H{

where: k — permeability rate, ft/year,

A ρ = area of standpipe furnishing the percolation head, in.2,

Aa = area of the specimen, in.2, Ls = length of the specimen, in.,

Hi = initial head, difference in head between headwater and tailwater, in.,

Hf = final head, difference in head be-tween headwater and tailwater, in., and

/ = elasped time, years. 6.3 Shrinkage Test Data—Calculate

the volume shrinkage as a percentage of the initial volume as follows:

Vi

where : Δ8 = volume shrinkage in percentage of

initial volume, i\ — initial volume of specimen (height

of specimen times area of ring container), and

vd = volume of air-dried specimen from mercury displacement method.

Calculate the shrinkage in height as follows:

Δ*. = X 100

where: AhB — height of shrinkage in percentage

of initial height, hi — initial height of specimen, and

ha — height of air-dried specimen. 6.3.1 To calculate the total percentage

change in volume from "air-dry to saturated conditions,,, add the per-centage shrinkage in volume on air drying Δ8 to the percentage expansion in volume on saturation Δ 6, as described in 6.1. This value is used as an indicator of total expansion but is based on initial conditions of density and water content. Since expansion volume data are deter-mined for several conditions of loading, the total volume change can also be determined for several conditions of loading.

6.3.2 To calculate the total percentage change in height from saturated to air-dry conditions, add the percentage shrinkage in height Ahs to the percentage expansion Δ wThen the specimen is saturated under specific load conditions.

7. Plotting Test Data

7.1 Expansion Test—The test data may be plotted as shown on Fig. 2.

8. Reports

8.1 Expansion Test—Include the fol-lowing information on the soil specimens tested in the report :

8.1.1 Identification of the sample (hole number, depth, location).

8.1.2 Description of the soil tested and size fraction of the total sample tested.

8.1.3 Type of sample tested (remolded or undisturbed; if undisturbed, describe the size and type, as extruded core, hand-cut, or other).


8.1.4 Initial moisture and density conditions and degree of saturation (if remolded, give the comparison to maxi-mum density and optimum water con-tent (see Methods D 698)).

8.1.5 Type of consolidometer (fixed or floating ring, specimen size), and type of loading equipment.

8.1.6 A plot load versus volume change curves as in Fig. 1. A plot of void ratio versus log of pressure curve may be plotted if desired.

8.1.7 A plot log of time versus de-formation if desired.

8.1.8 Load and time versus volume-

change data in other forms if specifically requested.

8.1.9 Final water content, bulk dry density, and saturation degree data.

8.1.10 Permeability data and any other data specifically requested.

8.2 Shrinkage Test—For the report on shrinkage, include data on the decrease in volume from the initial to air-dried condition and, if desired, other informa-tion such as the total change in volume and total change in height. Report the load conditions under which the volume change measurements were obtained. Include also Items 8.1.1 through 8.1.5 and 8.1.9.

APPENDIX Β

The following are some ques t ions c o m m o n l y raised by owners , bui lders , and archi tects

concerning the solut ions and precaut ions for s t ruc ture founded on expansive soils. The answers

given are based most ly on exper ience ra ther than on the theoret ical approach .

T R U E A N D F A L S E

1. It is t rue tha t a subsoil investigation should be conduc ted for each s t ruc ture t o be bui l t in an

expansive soil area.

Using subsoil in format ion tha t was obta ined in the vicinity ra ther than at the specific

s t ructure locat ion can be b o t h misleading and dangerous .

2. It is t rue tha t founda t ion design based u p o n subsoil investigation should be obta ined before

cons t ruc t ion .

Proper foundat ion systems, such as size of footing, length of pier, thickness of slab, e tc . ,

mus t be carefully designed.

3 . I t is false t ha t whi te s treaks in clay are ben ton i t e .

White streaks in the clay are calcium deposi ts and n o t ben ton i t e . When we t t ed , the heavy

calcium c o n t e n t in the soil can cause excessive se t t lement b u t no t swelling.

4. It is false tha t heaving act ion will stabilize after a n u m b e r of years .

Once the foundat ion soils have become wet ted , there is n o way to remove the mois ture .

Movement of the s t ruc ture will con t inue , bu t the magni tude m a y vary.

5. It is false tha t expansive soil benea th the foundat ions will sett le back u p o n removal of

source of wa te r or on prolonged drying.

Moisture con ten t benea th any s t ructure seldom decreases, wi th possible except ion of areas

immedia te ly benea th hea t duc ts or furnaces. Upon wet t ing , the soil heaves. After the soil

has reached its m a x i m u m swell po ten t ia l , mois ture migrates t o the drier po r t ion of the soil

and heaving act ion cont inues .

6. I t is false t ha t re inforcement should be placed in con t inuous footings.

The d e p t h of con t inuous footings is usually only 8 to 12 inches. Such d e p t h is no t sufficient

to render an effective re inforcement . Reinforcement should be placed in the foundat ion

walls.

7. It is t rue tha t heavy re inforcement in the foundat ion wall will minimize cracking.

Reinforcement will span across unequal heaving and reduce cracking.


8. It is false tha t chemical stabilization can provide an answer to all expansive soil p roblems.

Present day knowledge of chemical stabil ization is in its infantile stage. Extensive research

will be required, especially in the area of field applicat ion, before chemical stabilization can

be widely adop ted .

9. It is false t o assume tha t if a building is s i tuated on high ground, there will be no swelling

p rob lem.

Ground water does n o t follow ground con tour , and a perched water table usually follows

the bedrock con tour . Therefore, it is possible tha t a high water condi t ion can trigger

swelling even on high ground.

1.0. It is false tha t free water is necessary to cause swelling.

A mois ture increase of as little as 1 percent by weight is sufficient t o cause heaving act ion.

Installation of a subdrainage system around a building may intercept free water b u t will no t

prevent the increase of mois ture con ten t in the foundat ion soils.

11 . It is false tha t b y ponding the soil before cons t ruc t ion , the heaving problem can be

el iminated.

Ponding will affect only soils below ground surface to a shallow dep th , while there is great

danger of triggering the expansion of deep seated expansive soils after the building is

comple ted .

12. It is t rue tha t good surface drainage will reduce the risk of foundat ion movemen t in an

expansive soil area.

Good surface drainage is a necessary requi rement b u t by itself is no t sufficient t o prevent

heaving and cracking. Other factors such as adequate s t ructural design and proper

cons t ruc t ion techniques are equally impor t an t .

13. It is t rue tha t a subdrainage system is always a desirable e lement in the foundat ion system.

Subdrains will prevent free water from enter ing the foundat ion soils. However, to be

effective, the drains should be placed at the p roper dep th wi th proper out le ts .

14. It is t rue tha t if all the ground surface surrounding a building is paved, swelling problems can

be control led.

Extensive paving a round a building can effectively control the migrat ion of surface water

in to the foundat ion soils. This accounts for the surprisingly few cases repor ted on the cracks

of gasoline service s ta t ion s t ructures in expansive soils areas.

15. It is t rue tha t downspou t s should be long enough to drain water away from a building.

Downspou t s are preferred t o the hidden roof drain system. Defective downspou t s can be

immedia te ly detected while the built-in roof drain system may develop leakage tha t goes

unde tec ted for many years.

APPENDIX Β 273

16. It is false tha t by providing plastic membranes a round the house which are t opped with

gravel bedding, water will no t be able to seep in to the foundat ion soils.

Plastic membranes will allow surface water t o leak th rough the seams and holes b u t will n o t

allow evaporat ion, consequent ly , mois ture con ten t in the soil benea th the plastic will

increase steadily. After m a n y years , the accumula t ion of mois ture will eventually cause

problems.

17. It is t rue tha t lawn sprinkling systems should no t be installed adjacent to the building.

Lawn sprinkling systems should be installed at least 10 feet away from the building, wi th

nozzles directed away from the s t ruc ture . Thus the chance of saturat ing the backfill can be

reduced.

18. It is t rue tha t a s t ructural floor system or a suspended floor system is the only solut ion to

slabs-on-ground cons t ruc t ion in expansive soil areas.

Whenever possible a suspended floor system should be adop ted , bu t of tent imes this is no t

economical ly feasible.

19. It is false tha t sand and gravel placed benea th the floor slab will reduce the uplift pressure of

expanding clay by providing void space in to which the clay can flow as it expands .

Swelling clay can exer t uplifting pressure on the inter locking gravel particles causing the

floor t o heave. It is doubt fu l that any clay will flow in to the voids.

20. It is t rue tha t bur ied ut i l i ty lines should be avoided, whenever possible, in an expansive soil

area.

Swelling soil can shear water and sewer lines and cause leakage. Initial small leakage will

trigger more expansion causing greater leakage, resulting in severe damage.

2 1 . It is false tha t puddl ing of backfill can achieve the desired compac t ion .

Simulated labora tory tests can easily demons t ra te tha t puddl ing will n o t increase the soil

densi ty . Excessive puddl ing can in t roduce a large a m o u n t of water in to the foundat ion soils.

22 . It is t rue tha t the use of expansive soil as backfill can exer t swelling pressure on the wall and

cause cracking.

Horizontal swelling pressure is approximate ly equal in magni tude t o the vertical swelling

pressure. However , since most backfill is no t t ightly compac ted , such hor izonta l pressure

seldom fully develops.

2 3 . It is false tha t lateral expansion is the cause of separat ion of windows and doors from

frames.

Most lateral movemen t is caused by differential heaving which creates an impression of

pushing away or pulling apar t .

24 . It is false tha t a drilled pier foundat ion system provides the best answer t o s t ructures

founded on expansive soils.


A drilled pier sys tem, if intell igently designed and cons t ruc ted , can solve m u c h of the

swelling soil p rob lem. However, in areas where there is a possibility of rising ground water , a

drilled pier foundat ion may no t be effective. Statistically, there are p robably more cracked

buildings founded on piers than wi th spread footings.

2 5 . It is t rue tha t by placing the building on a single pier, there would never be a swelling

prob lem.

Who can afford tha t?

INDEX A

Absorption, 9 Activity Method, 16,21 Adsorbed cation, 11 Adsorbed water, 13,176 Airport, 8, 173 Air space, 90, 96, 190, 205, 207, 246 Apron,37,121, 136,146,194, 255 Artesian, 33 Asphalt mat, 147 Asphalt membranes, 147, 148 Atmospheric pressure, 34 Atterberg limits, 10 ,11, 18,20, 112, 176 Attractive force, 13,14,43 Australia, 1 ,3 ,33,35

Β

Backfill, 100,101, 110, 122, 127, 146,149,170, 193,194, 206,219,235

Base exchange, 11, 171 Base exchange capacity, 17, 171 Basement, 5 ,9 ,62 , 121, 124, 125, 149, 184, 246

deep, 65, 66, 170

slab, 136, 244 wall, 90,99, 100,104,106, 134

Bearing capacity, 72, 73, 106, 165 Bearpaw shale, 2 Belled pier

advantage of, 83 cleaning of, 84 construction of, 84 disadvantage of, 83 isolated-shaft, 86 shaft of, 84 system, 83, 84

Bench mark, 61 , 190, 207 Bentonite, 19,21,271 Black cotton, 5, 146 Blow count, 65, 72 Bond,86, 87 Box construction, 104,194, 243 Brick wall, 90,187,251 Building Research Advisory Board, 111, 122

C

Caisson, 64, 71 Canada, 1, 3 Canal, 151,163 Capillary

force, 33, 34

Capillary (cont.) fringe, 33 moisture movement, 41 , 151,152,154 potential, 24 rise, 149, 159,247 tube, 33

Catalytically-blown asphalt, 147 Cation, 10, 13, 176 Cation exchange, 10, 11, 16 Cation exchange capacity, 16 Cement, 175 Cement stabilization, 175,176,177, 272 Chemical analysis, 16, 17 Chemical injection, 159 Chemical stabilization, 11,175, 255 Cinder block wall, 90, 104, 131, 246 Clay

fissured, 77, 149, 151 impervious, 150 mineral, 9, 10, 11, 16, 175 partially saturated, 162 shale, 26, 39, 77 size, 19, 24 stiff, 33 ,113,165, 172,255 structure, 10,11 type of, 20

Claystone shale, 6, 44, 63, 93, 98, 195, 228, 255

Climate condition, 35 ,40 ,111 , 160 Climate rating, 111,112,113,120 Cohesion, 73, 86 Colorado, 6, 62, 64,111,183 Colorado School of Mines, 1 Colorado State Highway Department, 24,

168 Colloid content, 18,19, 20, 21, 22, 228 Colloidal clay, 17 Compaction, 149, 206

control, 159, 163, 165,169,232 degree of, 39,56,169 effort, 149 method of, 27,149,163

Compactor, 150 Cone penetration, 88 Consolidation test, 67, 72 Consolidometer, 26, 27, 29, 43 ,45 , 67, 68

cantilever, 26,68 conbel, 26 fixed-ring, 38 one-dimensional, 26 platform, 26 ,67 ,68 ring, 45

Control joint, 122

276 INDEX

Court yard, 156,207 Covered area, 33, 34, 35, 37,145, 148, 154,

159 Crack pattern, 185, 186 Cracked building, 91, 109,190 Crawl space, 121, 125, 185, 190, 199,207,213,214

214 Curing, 8, 124 Curing time, 27, 39,40 Curling, 8, 124

D

Darcy's law, 35 Dead load, 27, 28, 29, 45,47, 52, 81, 83, 92, 93,

95,103,106,109, 184,194, 207, 233 Deep plow, 173 Degree of expansion, 19, 21, 28 Degree of saturation, 47, 48,49, 56, 165 Denver, 28,44, 73,84, 113, 126, 168,176 Denver Blue Shale, 71, 73 Denver formation, 1 Depth of desiccation, 35, 36,40 Depth of penetration, 74 Desiccate, 28, 29, 33, 173 Design criteria, 66 Differential

drying, 155 heaving, 103, 104 movement, 104 settlement, 41 , 42, 124 thermal analysis, 16,17

Direct measurement, 16, 20, 26, 27, 35 Discontinuity of structure, 104 Distress study, 185 Doorframe, 131,135 Double layer structure, 13, 14 Double oedometer method, 41 Dowel bar, 95,129,191 Drainage, 41 , 192,193,207,229 Drain tile, 49 Drilled pier, 71, 74, 75, 86, 273 Drilling, 63

auger, 63, 66 percussion, 64

rotary, 64, 66

Dry density, 13, 21, 28, 160, 162, 189 initial, 39, 40, 55, 165 maximum, 47, 78

Drying and shrinkage, 160, 189 Drying and wetting, 29 Dye adsorption, 16, 17

Ε

Earth pressure, 8, 101, 104

Ecca shale, 1 Effective stress, 42 Electron microscope resolution, 16, 17, 18 Electrostatic force, 13 End bearing capacity, 72, 74, 78, 195 Environmental condition, 33, 35,40, 41 , 44, 60 Evaporation, 34 Evapotranspiration, 3 Excavation, 33, 169 Existing structure, 63, 72 Expansion joint, 8, 89, 127, 207, 237, 257, 260 Expansive soils, 1, 3, 8, 11, 17, 27, 71

damage caused by, 8 distribution of, 3 nature of, 1 origin of, 1 physical properties of, 27 recognition of, 16 structure of, 33

F

Factor of safety, 73, 74, 83, 193, 25η Fatigue of swelling, 29 Federal Housing Administration, 22, 24, 101,

113,122 Fill, 110, 165, 167, 169,227 Fissures, 151, 152, 162, 255, 256 Floating slab, 127, 170 Flood plain, 62

Flooding, 110,159, 160, 161, 185 Floor level, 61, 66 Floor load, 124 Floor slab, 27, 89,124, 129, 159, 222, 252 Flower bed, 155, 182, 193, 194, 219, 246 Fly ash, 175 Footings, 27,45, 50, 52, 158, 183, 221

continuous, 103, 243 foundation, 103, 173 individual, 29,221 interrupted, 109, 248 pad, 29, 85, 86, 104, 221 wall, 103, 109

Foundation deep, 64 information, 183 mat, 111 movement, 46, 104, 166, 183, 187,190, 259 plan, 184 shallow, 64, 66 system, 63, 64, 93, 190, 271 type, 29, 61, 183 wall, 29, 100, 105, 245, 246, 252

Foundation soil, 135, 145, 150, 157, 173, 271 Free draining gravel, 124 Free swell, 18,19,44 Free water, 34, 151, 246, 256, 272 Furnace duct, 135, 142,271

INDEX 277

G

Garage slab, 135,141 Grade beam and pier system, 75, 76, 89, 218, 256 Gaade

Grade beams, 89, 94, 96, 99,107, 125,127, 129, 136, 190, 192,251

Granite, 108 Granular soils, 110, 165, 167, 170 Gravitational migration, 37 Gravitational potential, 24 Gravity, 33 Gravity flow, 151, 152,154 Ground water, 40, 63, 84, 89, 101

rise of, 101, 122,124, 151, 170, 192, 258

H

Heaving differential, 163, 175, 187 movement, 106 potential, 26, 169

Highway, 8, 37, 171, 175 Honeycomb floor system, 125, 126, 127 Horizontal swelling pressure, 9 Humidity, 27, 35 Hydrometer analysis, 228

I

I-beam, 187, 188,218 Illite, 9, 10, 13,21,27 Impervious, 110, 150, 152,256 Index property, 16,19, 28 India, 1,3,5 Indirect measurement, 22 Initial moisture content, 27, 28, 29, 38, 39,40,

49, 50, 56 Intercepting ditch, 151, 152 Intercepting drains, 151 Ion, 9, 13,26, 171 Irrigation ditch, 62, 151 Isolation of pier uplift, 85 Isomorphism, 17 Israel, 1 ,3 ,5, 104

J

J-void, 125, 126

Κ

Kaolinite, 9, 10, 11 ,21 ,27 ,44

L

Landslide, 62, 63, 151 Laramie formation, 1 Lateral pressure, 101, 127, 128 Lateral resistance, 85 Lateral wall movement, 101 Lawn watering, 35, 122, 148, 184, 194, 258, 272 Lightly loaded structure, 28,44, 89, 93, 192 Lime, 170, 171, 174 Lime stabilization, 171, 173, 174 Lime-treated subgrade, 171 Lime slurry, 173 Linear shrinkage, 18, 19 Liquid limit, 10, 28, 44, 77, 161, 168,175, 227 Load bearing walls, 135 Load test, 72, 73

M

Masonry construction, 90,104 Masonry wall, 135, 136 Mat foundation, 111

behavior, 113 design of, 111, 113, 115, 116, 117

Matrix suction, 24, 26 Membrane, 145

asphalt, 147, 148 plastic, 146, 273 polyethylene, 145, 148, 149

Menard pressurementer, 72, 73, 74, 88 Meteorological, 61 Method of compaction, 40 Mexico, 3, 5 Mexico City, 5 Mineralogical composition, 9, 17, 18, 33, 39 Mineralogical identification, 16 Model pier test, 76, 77, 82 Moisture

content, 21, 27, 34, 35, 111, 202 deficient, 34, 149, 154, 158, 163 distribution, 35, 146, 160 equilibria, 34 fluctuation, 35, 37, 146 migration, 33 ,34 ,41 ,49 , 122, 145, 148, 152,

162,236 movement, profile, 35, 160 transfer, 33, 37, 40

Moisture barrier, 174 horizontal, 145, 149 vertical, 145, 148, 149, 150

Montana, 2, 8 Montmorillonite, 1 ,3 ,6 ,9 , 13, 17, 18,27,44,

171,172,227 Mud jack, 255 Mushroom, 92, 95, 96, 190, 194, 206, 218, 257

Laboratory testing, 66, 189

278 INDEX

Ν

Negative moment, 111 Negative pore pressure, 25, 26 Normal stress, 42

Ο

Optimum moisture content, 19, 38,44, 50, 77,150, 164 ,225

Organic compound, 176 Osmosis pressure, 13, 14, 16 Osmotic consolidometer, 26 Osmotic potential, 24 Overburden pressure, 34,41,112 Overburden soil, 53, 62, 73

Ρ

Pad, 29, 85, 93, 98,104, 106, 260 deep, 108 foundation system, 104,107,189

Partition wall, 129,132, 136,127, 252 Patio, 129,135, 187, 190,218 Pavement, 29 ,34 ,35 , 37,147,162 Penetration resistance, 28,65, 72, 74, 88 Penetration test, 64, 65, 66 Perched water, 83, 122, 124,151,152, 192,

193,256,258,272 Peripheral drain, 152, 155 Permeability, 35, 38, 63, 176 Physiography, 61 Pierre formation, 1, 2, 63 Piers, 29, 37 ,44 ,47 ,195

anchorage, 29,47, 84, 93,210, 259 bearing capacity of, 71,74, 85, 85, 89, 98 belled, 71, 83 design capacity of, 74 diameter of, 94 drilled, 71, 89,104,184 failure of, 89 foundation, 45, 125 friction, 66, 71, 73, 77, 86, 88 length of, 92, 93,184, 191,205 reinforcement, 92, 95, 184, 210 settlement, 72 ,92 ,98 , 205, 208 shaft, 47, 73, 77, 85,99 size, 92,182 spacing, 92 straight-shaft, 71, 74, 84, 85 system, 73,101, 189,208 uplift, 62, 75, 83 ,93 , 202, 254, 257

Placement condition, 39 ,44,164 Plastic limit, 34, 77,146 Plastic membranes, 34 Plasticity Index, 10 ,18 ,21 ,22 ,23 ,44 , 78, 111,

112,159,171,175,227

Plumbing, 157 Poisson's equation, 13 Poisson-Boltzmann equation, 13 Ponding, 155,156,159, 160,161,163, 272,

273 Pore pressure, 34, 35,42 Portland cement, 175 Portland Cement Association, 121,122, 125 Post-tension, 118,194, 246, 247, 249, 250 Potential volume change, 22,25 Precipitation, 3, 10, 33,34, 35, 37, 111, 152,

185,258 Pressure injection, 173 Pressure release, 33 Prewetting, 52 ,159,162,163, 165,190 Proctor density, 44, 56, 60,110,150,169, 206,

221 Proprietary fluid, 176 Puddling, 149,150,192 PVC meter method, 16,21,24,112 PVC rating, 21

Q

Quartz, 44

R

Raised floor system, 125,126

Rational design, 47, 85 Rational pier formula, 82, 88 Reinforced brickwork, 104 Remedial measures, 183, 193 Remolded sample, 21, 28, 38, 39, 44, 55, 56 Repulsive force, 13,43 Residential houses, 44 ,111 , 121,136,193 Resistivity, 63 Rocky Mountain Area, 28, 38, 43, 71, 73, 74, 92,

111 Roof downspout, 122, 156, 194, 238, 246, 272 Roof drain, 35, 156

S

Sampler California, 66 Shelby tube, 66 split spoon, 65

Samples core, 66, 72 disturbe,d, 68 representative, 68 undisturbed (see Undisturbed)

Sample thickness, 40 Sampling, 63, 66 Sampling method, 27 Sandstone, 64, 108,255

INDEX 279

Saturation, 27 complete, 47, 52 partial, 34,43

Seasonal cycle, 86,189 Seasonal moisture change, 148, 149 Seepage, 33 ,62 ,151 ,184 ,185 Seismic survey, 61,63 Selected fill, 109,110,169 Settlement, 37 ,41 , 72 Sewer line, 155, 157, 185,232 Sewer pipe, 77, 122, 155 Shale bedrock, 33, 64, 74, 82 Shear failure, 203, 204, 207 Shear ring, 73 Shear strength, 76, 88, 175 Shear strength reduction, 88 Shear stress, 26 Shearing resistance, 83, 86 Sheet rock 131, 134 Shrinkage, 37,155,189

crack, 33, 121, 124 limit, 1 9 ,2 0 ,2 1 ,22 ,44 ,171 , 175,228 mechanics of, 37 test, 38

Side shear, 73 Sidewalk, 37 ,121, 129, 131,134, 146,143,207 Silt, 28, 34 Siltstone, 63, 64, 131,146 Site investigation, 61 Skin friction, 73, 74, 77, 83, 86,195, 257 Slabs

exterior, 129 floating, 127 interior floor, 121, 128 movement, 49, 90, 111, 121,122, 128,223 patio, 129,132, 134 prestressed, 125 reinforced, 121 structural, 121, 125,170,273

Slab-bearing partition wall, 135,136, 171, 189,194, 222,232,260

Slab-on-ground, 1,44, 49,66, 121,122, 127, 129, 160,170, 199

Slip joint, 127, 129,130,132, 134,136,170, 194, 233,252,260

Slope stability, 151 Soil engineers, 26, 34, 37, 52, 55, 61 , 72, 73,169,

175,191,193 Soil lattice, 11, 17 Soil profile, 40 Soil replacement, 110, 166, 169

depth of, 167,168 extent of, 169,170

Soil stabilization, 159 Soil suction, 16, 21, 24, 26, 34, 35 Soil test, 184 Soil-water equilibria, 39

Soil-water system, 14, 37,173 Sonotube, 86 ,94 ,126,127 South Africa, 1, 3, 6, 33, 104, 149,161 Spain, 3, 6 Specific gravity, 44 Specific surface, 11 Split level, 104, 111,213 Sprinkling system, 155, 193 Staircase wall, 131, 134, 140, 255 Stiffened slab, 111, 112, 113,115,120, 125 Stratum thickness, 5, 165 Stress history, 27 Stress release, 26 Structure engineer, 189 Structural fill, 234 Stud wall, 132, 135 Subdrain, 49, 145, 153,154,192, 194, 272 Subdrainage system, 124,152, 156,184,194, 213,

247,260, 272 Subsoil condition, 64 Suction test, 26, 29 Sump pump, 125, 152,194, 213, 244 Support index, 111, 112, 113, 114,120 Surcharge load, 27, 46,163 Surcharge pressure, 21 , 38 ,40 ,45 ,46 ,165 Surface drainage, 41 , 145,146, 151, 155,192,247,

272 Surface geology, 61 Surface water, 33, 77, 82, 101, 122,151,152,197,

218,229

Surficial geology, 62

Swell index, 22 ,25 ,38 ,112 percent of, 26, 112 test, 39,67, 112,168,200 total, 168

Swelling characteristic, 18, 38, 29,43 definition of, 38 mechanics of, 18,43, 82 potential, 2, 16, 18, 19, 20, 24, 26, 27, 28, 38,

64 ,171,199,225,229 pressure, 6, 14, 26, 27, 39, 4 0 , 4 3 , 4 5 , 4 6 , 4 7 ,

50, 52, 55, 56, 60, 75, 82,94, 106,110, 126,165,199,203,225,229,255

soil, 9, 64,145

Τ

Tensile stress, 94, 95 Tension crack, 84 ,95 ,97 ,190 , 203, 221 Temperature crack, 122 Test hole, 64, 72,73 Test pit, 61 , 63 ,64,190, 245 Texas, 6, 86, 160, 162 Texas A & M, 1,3, 112 Texas Highway Department, 147, 160,171

280 INDEX

Texture, 17 Thermal gradients, 34,122 Thermal-osmosis, 159 Thermal transfer, 37, 145 Time element, 39 Topographie feature, 33, 61, 62 Topographie survey, 61 Total heave, 40, 41 Transpiration, 34 Triaxial shear strength, 72, 88

U

Ultimate settlement, 41,42 Unconfined compressive strength, 72, 86, 88, 106 Underpin, 194, 232,233 Under-reamed pier, 83 Underslab gravel, 124, 190 Underslab soil, 128, 131, 148, 173, 175 Undisturbed sample, 21, 28, 34, 38, 39, 40, 55,

112 Undrained shear test, 88 United States, 3, 6, 33,111 Unsaturated soils, 24, 35 Unwetted zone, 83,93 Uplift, 44, 88, 168,257

coefficient of, 76, 82 differential, 47 movement, 47, 214, 245 tolerable, 47

Uplifting force, 76, 80, 81, 83, 89, 215, 216 Uplifting pressure, 67, 75, 81, 94, 96, 99,108, 128,

129, 194, 198,203,215,252 U.S. Bureau of Reclamation, 65, 161 U.S.B.R. method, 20, 21 Utility lines, 35, 122, 136, 185,231,273 Utility trench, 149, 185, 192

V

Van der Waals' force, 13

Vane shear test, 88 Van't Hoff equation, 14 Vapor barrier, 145 Vapor pressure, 154 Vapor transfer, 34, 37, 41,154 Vegetation, 155 Vermiculite, 9 Verticel ,97,98,99,125, 126 Void-ratio, 41 , 42 Void-space, 89, 96, 107, 109, 189, 194, 227 Volume change, 21, 28, 38, 39 ,43 ,47 , 48, 50,

67 ,111 ,161 , 175

W

Waffle slab, 113,125 Walkout door, 104, 218, 243 Wall paneling, 131, 135, 140 Water level, 33,62 Water line, 131,157,259,273 Water main, 77

Water table, 27, 28, 35, 63, 65, 152, 183, 184, 218,228,256,259

Water transfer, 33 Water vapor, 34 Weathered claystone, 214 Withholding force, 75, 83, 89, 217 Wyoming, 6 ,63 , 183

X

X-ray diffraction, 11,16, 17, 227

Ζ

Zone of aeration, 151 Zone of wetting, 77 Zone unaffected by wetting, 83, 84, 85, 86

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FU HUA CHEN (Eds.) Foundations on Expansive Soils 1975

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