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Contents
Preface viii2.7.4 Driven piles 5
Examples ix2.7.5 Driving precast piles 5
2.7.6 Test loading 5
Bibliography 6
1 Site investigations 1
1.1 Walk-over survey 1 3 Foundations in cohesive soils 6
1.2 Desk study 2 3.1 Introduction 6
1.3 Site investigation: field work 4 3.2 Settlements in cohesive soils 61.3.1 Trial pit logs 4 3.3 Consolidation settlement 6
1.3.2 Borehole record 43.3.1 Bearing capacity of cohesive soils 61.4 Site investigation procedure 4 3.3.2 Vertical stress distribution 7
1.4.1 Borehole logs 4 3.3.3 Construction problems on clay sites 71.4.2 Trial pit logs 4 3.3.4 Foundation" designs on clay soils 71.4.3 Groundwater 4 3.3.5 Settlements in clay soils 71.4.4 Standard penetration tests 4 3.4 Moisture movements 7
1.5 Interpretation of laboratory testing 9 3.4.1 Liquid limit test 761.5.1 Chemical tests 9 3.4.2 Plastic limit test 77
1.6 Solution features 11 Bibliography 79
1.6.1 Limestones 11
1.6.2 Chalk 11
1.6.3 Salt 13 4 Foundations in sands and gravels 81
.6.4 Gypsum 14 4.1 Classification of sands and gravels 81Case study 1.1 Investigation of former mining site, 4.1.1 Composite sands and gravels 81
Sheffield 14 4.1.2 Dilatant sands 82Bibliography 21 4.1.3 Calcareous sands 82
4.2 Relative densities of granular soils 82
4.2.1 Field density assessment 822 Foundation design 23 4.2.2 Visual observations 82
2.1 Introduction 23 4.2.3 Groundwater levels 83
2.1.1 Width of footing 23 4.2.4 The standard penetration test 83
2.1.2 Soft spots 23 4.2.5 Interpretation of SPT results 84
2.1.3 Stratum variation in excavation 23 4.2.6 Ultimate bearing capacities 84
2.1.4 Firm clays overlying soft strata 25 4.3 Construction problems in granular soils 86
2.1.5 Depth of footings 26 4.4 Foundation design in granular soils 87
2.2 Widened reinforced strip footings 29 4.5 Plate bearing tests 89
2.3 Reinforced strip footings on replacement 4.6 Piling into sands and gravel strata 89
granular fill 324.6.1 Bored piles 90
2.4 Trench fill foundations 33 4.6.2 Continuous flight auger piles 90
2.5 Raft foundations 34 4.6.3 Design of bored piles 90
2.6 Pad and pier foundation 44 4.6.4 Set calculations 92
2.6.1 Disused wells 44 4.6.5 Dynamic pile formula 92
2.7 Piled foundations 47 4.6.6 Re-drive tests 93
2.7.1 Bored piles 47 4.6.7 Base-driven steel tube piles 93
2.7.2 Design of a bored pile 48 4.6.8 Top-driven steel piles 93
2.7.3 Design of bored and driven piles 48 Bibliography 94
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Contents
95 7.2.2 Additional weight of regrade fillBuilding in mining localities
7.2.3 Changes in the groundwater level or5.1 Coal mining, past and present 95
surface runoff5.2 Coal shafts 96
7.2.4 Excavations for deep drainage5.3 Shallow mineworkings 97
7.2.5 Removal of trees and vegetation5.4 Drilling investigations 99
7.2.6 Split-level housing5.5 Stabilizing old workings 99
7.3 Retaining systems5.5.1 Collapsed workings 99
7.3.1 Gravity type retaining systems5.5.2 Special conditions 100
7.3.2 Cantilever walls: reinforced concrete5.6 Foundations in areas with shallow workings 100 or brickwork5.7 Active mining 100
7.3.3 Gabions, crib walling, reinforced earth5.8 Future mining 100
7.3.4 Steel sheet piling5.9 Mitigating the effects of mining subsidence 101
7.4 Designing retaining walls5.9.1 Longwall mining (advancing system) 101
7.4.1 Active pressure on walls5.9.2 Designing buildings for future mining
7.4.2 Surcharge loadingsubsidence 101
7.4.3 Passive resistance (granular soils)5.9.3 CLASP system of construction 102
7.5 Cantilevered retaining walls5.9.4 Mining rafts 103
7.5.1 Mass brick or block walls5.9.5 Irregular-shaped units 104
7.5.2 Reinforced cavity walls5.9.6 Designing strip footings in active
7.5.3 Pocket-type wallsmining areas 107
7.6 Damp-proofing to retaining walls5.9.7 Movement joints 109
7.6.1 Type A structures: tanked protection
Bibliography 112 7.6.2 Type C structures: drained cavityconstruction
Bibliography6 Sites with trees 113
6.1 Foundation design 113
Building on filled ground.1.1 Climatic variation 119 8
6.1.2 Distances between trees and8.1 Opencast coal workings
foundations 1198.2 Foundations
6.1.3 Foundation depths related to8.2.1 Stiff raft foundations
proposed tree and shrub planting 1218.2.2 Piled foundations
6.1.4 Measurement of foundation depths 1218.3 Suspended ground-floor construction
6.2 Building on wooded sites 1228.4 Compaction of fills to an engineered
"
6.2.1 Piled foundations 122specification
6.2.2 Deep trench-fill concrete foundations 123 8.4.1 Procedure6.2.3 Deep strip footings with loose stone
8.4.2 Site testing before backfillingbackfill 124
8.4.3 Foundations6.2.4 Stiff raft foundations on a thick
8.4.4 Roads and drainagecushion of granular fill 124
8.4.5 Groundwater6.2.5 Deep pad and stem foundations 125
8.5 Ground improvement techniques6.3 Precautions to take when there is evidence of
8.5.1 Dynamic consolidationclay desiccation 125
8.5.2 Surcharge loading6.3.1 Suspended floors 126
8.6 Compaction of structural fills6.3.2 Drainage and services 126
8.6.1 Materials specification6.3.3 Protection to drainage 126
8.6.2 Definitions6.3.4 Precautions against clay heave 127
8.6.3 Suitable fillmaterials6.4 Foundations in granular strata overlying
8.6.4 Unsuitable materials
shrinkage clays 134 8.6.5 CompactionBibliography 136
8.6.6 Testing on site
Bibliography
7 Developing on sloping sites 137
7.1 Stability of slopes 1379 Ground improvement
Case study 7.1 Sloping site with clay fills over
9.1 Vibro-compaction techniquesoulder clay 141
9.1.1 Types of treatment.2 Developing on sloping sites 146
9.1.2 Ground conditions.2.1 Additional weight of dwellings 146
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Content
147 9.1.3 Engineering supervision 190 11.4.1 Asbestos 23
9.1.4 Design of vibro-compaction stone 11.4.2 Scrap yards 23
columns 193 11.4.3 Sewage treatment works 23
9.1.5. Foundations on vibro-cornpaction sites 197 11.4.4 Timber manufacturing and timber
9.2 Dynamic consolidation 198 treatment works 23
9.2.1 Testing 201 11.4.5 Railway land 23
9.3 Preloading using surcharge materials 201 11.4.6 Petrol stations and garage sites 23
9.4 Improving soils by chemical or grout injection 202 11.4.7 Gasworks sites 23
Bibliography 203 11.4.8 Metal smelting works 23
11.4.9 Old mineral workings 2311.4.10 Toxicological effects of
10 Building up to existing buildings 205 contaminants 23
10.1 Site investigation 20511.5 Landfill sites 23
10.2 Foundation types 20511.5.1 Gas migration 24
10.3 Underpinning 21211.5.2 Gas monitoring 24
11.5.3 Carbon dioxide 2410.3.1 Beam and pad solution 216
11.5.4 External measures 2410.3.2 Pile and needle beam solution 216
11.6 Desk study 24Case study 10.1 Investigation and underpinning
of detached house on made11.6.1 Local geological study 24
ground, York 21811.6.2 Industrial history of the site 24
Case study 10.2 Differential settlement, Leyburn 21911.6.3 Mining investigation 24
11.6.4 Site reconnaissance 24Case study 10.3 House on made ground, Beverley 222
11.7 Site investigation 24Case study 10.4 House founded on sloping
rock formation, Scarborough 22311.7.1 Trial pits 24
10.4 Shoring 22611.7.2 Boreholes 24
Bibliography 22711.7.3 Testing for toxic gases 24
11.7.4 Chemical analysis 24
11.7.5 Safety 24
11 Contaminated land 22911.7.6 Conclusion 24
Case study 11.1 246
ILl Contaminated sites 229 Bibliography 248
11.2 UK policy on contaminated land 229
11.3 Risk assessment 230
11.4 Industrial processes 233 Index 251. .
147
147
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148149
150
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Site investigations
SITE SURVEYS LTD BOREHOLE RECORD BOREHOLE
Site Address: V<lle Avenue. LocationNUMBER
Wa.keFie1d . , W Yorks. Type of Dri ll ing: Qol-ru-~ i P€("cvs$./ve 5BORING SAMPLING RECORD OF STRATA Sheet No
1
DATEASING WA De~th Type NYC Nt Depth Level
Key Descr ip tion of stratam) TER (m % ROD. (m) (m)
0·00 10Q-o ADD
l 2 S 2 2 f ' i1o.de. 6roIJIld - Bla.c.k Ash
~"- FIrM. MediUM browl\. SQ.lldj~
2.·0 LI ~. " . s.~ CLAY w d - h ~- par 8s--.--
4-'cm~
...=-
.....I...I:Jk.oI- fo ~dill»1 e r ~ - brc11.Jl/l-
l- .......... F i 7Le t- o ~d l . u l l ' l (lro. . i .ne d. . ,......
f1a.53j . sANO~ONE:. .. ......Oo:.a. '$10 nal: ~hin b<Mc: is o r...... ...
.....
C l ~ o f ~sl-roJ-a.
r-.... .
f'''4s).....
B~ -.... ..... ..... ..... ..... .
WoJ-w ......... .
rE
'''jress I-q·oo...............
-... ... '"........ ..... ...... ..... .
\0'00. -" ..... .
f==MediUM fo DarK Er~-bro"'A
F==U"o/\ Sh~..t\ed !;~ S~i.~tJl).sTONE wiHI. iA!:erb Cicled
F= siJ1-s I-oi\e ba.t1dU!~F=
r-'?"50 r==
_~F==
MediUM to cia ..rk: ere3 -brow!\..!==i=== lool\. sl-cWt.ed ~~ s V i a . A ~
I- !== l"v\uOSTONE u . i c H t "'-f-eree.Clde..d -1=== 1 ? 1 c u : ~ c=J!:j .sl1c:d.e b:v1ds.t===i== -F==
I- !== .,
t===17·00 i==
i== Moo. i l /Vl to dark: (rE?j evld1==
r-" i=== bla .c .k r fodl-.j sVJ~ r l4 .ud.sJoAe _
F== uJi . I -~ bq_" d S o o F b a .ck
i=== co~ sho.le.s .
r=s=r
'2.1·0 f= t = I '1 Q ~ Borehol-e
Remarks and groundwater observations
Grool\d wal-er i¥l~re.ss 0} ~·OM. °3 .
Fig. 1.9 Typical borehole log.
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SOILS SEDIMENTARY ROCKS
Site investigation procedure
Made ground Chalk
Limestone
Conglomerate
Breccia
Sandstone
Siltstone
Mudstone
Shale
Coal
Pyroclastic(volcanic ash)
Gypsum.
Rocksalt etc.
IGNEOUS ROCKS
Topsoil
Boulders and cobbles
I : . : . : 0~ · I_ T
Gravels
Sands
Silts
1 = - - = - = = 1 Clay
Peat
Note: Composite soil typeswil l be identif ied by
combined symbolse.g.
Dilty sand
METAMORPHIC ROCKS
~ Coarse-grained
~ Medium-grained
§Fine-grained
Fig. 1.10 Key to soil symbols in trial pit logs.
1 · · · · · · ·' 1..... - _ -o e ••••••
I : : : ; 1
I I
I ~ t t + t l . Coarse-grained
I'_·+~L·+ ...I
· . Medium-grained
IV"V.JV V.vV I
. Fine-grained
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Site investigations
LABORATORY RESULTS: SYMBOLS
B Bulk sample: disturbed
Cohesion (KN/m2 )
Effective cohesion intercept (kN/m2 )
California Bearing Ratio
Jar sample: disturbed
Laboratory vane test
Coefficient of volume decrease (m2JkN)
Standard penetration test value (blows
/300mm)
Acidity / alkalini ty index
Plast icity index (%)
c
c'
CBR
D
LVT
mv
N
pH
P I
Soil strengths. Soil tests used on cohesive strata fall into
two main categories. The undrained triaxial test is carried
out on a soil which is stressed under conditions such that no
changes occur in the moisture content. This reproduces the
Table 1.1. SPT values for cohesive soils
Consistency Undrained shear
strength
(kN/m2)
Very stiff > 150
Stiff 75-150
Firm 40-75
Soft 20-40
Very soft <20
Nvalue
> 20
10-20
4-10
2--4
<2
Table 1.2. SPT values for sands and gravels
Couistency Nvalue
Very dense
Dense
Medium dense or compact
Loose
Very loose
> 50
30-50
10-30
4-10
<4
Table 1.3. Field assessment of soil s trengths
Consistency Method of testing Approximate undrained
of soil shear strength
(kN/m2)
Very soft Exudes between fingers <20
when squeezed in one's
handSoft Moulded by light finger 20--40
pressure
Firm Moulded by strong finger 40-80
pressure
Stiff Indented by thumb pressure 80-150
Very stiff Indented by thumbnail 150-300
Hard Difficult to indent with >300
thumbnail
8
Soluble sulphate content
Undisturbed 100 mm dia, samples
Undrained triaxial compression test
Water sample
Natural moisture content (%)
Liquid limit (%)
Plastic limit (%)
Bulk density (kg/m))
Dry density (kg/mJ)
Angle of shearing resistance (degrees)
Effective angle of shearing resistance (degrees)
Table 1.4. Typical ground bearing capaci ties
Maximum safe beari
(kN/m2) capacity
Types of rock and soil
10 700
Rocks
Igneous and gneissic rocks in sound
condition
Massively bedded limestones and hard
sandstones
Schists and slates
Hard shales, mudstones and soft
sandstones
Clay shales
Hard solid chalk
Thinly bedded limestones and sandstones
Heavily shattered rocks
Non-cohesive soils"
Compact , well-graded sands and
gravel-sand mixtures
Loose, well-graded sands and
gravel-sand mixtures
Compact uni form sands
Loose uniform sands
Cohesive soils"
Very st iff boulder clays and hard clays
with a shaly structure
St iff clays and sandy clays
Firm clays and sandy clays
Soft clays and si lts
Very soft clays and siltsd
Peats and made ground=
4300
3200
2200
1100
650
Dry Submerge
430-650 220-320
220-430 110-220
220--430 110-220
110-220 55-110
430-650
220--430
110-220
55-110
55-nil
a To be assessed a fter inspect ion
bWith granular soils the width of foundation is 10 be nOI le ss tha n 900 m
'Dry' means that the goundwater level is at a depth not less than 900 mmbe low the foundation ba se
C Cohes ive soi ls a re suscept ib le to long- leon conso lida tion
d To be determined a fter inves tiga tion
Source: BSI (1986) BS 8004.
conditions most likely to occur under the actu
foundations. The undrained test gives the apparent cohesio
Cu and the angle of shearing resistance I / I u . With saturate
non-fissured clays I / I u tends to zero and the apparen
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free
ratio
values,30
non-
n using
hen the
classes.
d in the
9 relates
eness to
if
cement
ee
82 .1
SOIL SURVEYS LTDTRIAXIAL COMPRESSIONTEST RESULTS(QUICK UNDRAINED)
Client:
Location: Leeds Ave.
r s . c . J.;::.Q"Sf:.NCb~ Lb::L .
Date: 2.7'/' q27'+0OB N°·
.....(~.~()!.e. N .o .5 D ~.H 1... .Q .:§2 .M . OescripliM: S C M c l ( J C /~
A p.~ .. . .ohe; (9 . t. \ . 4 -5 kNj",~ A~I :e . . . . . f . . S .h~~ f < tJ s1 ~ .r q , . . . .':t.~...
Fig. 1.11 Mohr circle stress diagram.
1.6 SOLUTION FEATURES
Solution feature
Damage to structures can often occur because of severe
subsidence caused when certain types of strata below ground
are affected by water and become soluble. Materials such as
salt, gypsum, chalk and certain limestones can all be affected
in this way.
sink-holes. These sink-holes generally develop where joints
in the limestone intersect, and they can result in large open
galleries. The size of these galleries and sink-holes depends
on the geological structure and the existence of layers o
impervious strata .
1.6.2 Chalk
Limestones are generally very jointed and the action of
acidic ground waters can produce solution features known as
Chalk is a pure soft limestone, which usually contains
approximately 95% calcium carbonate. Solution features do
not generallyform underground in the form of caverns as the
chalk being softer than limestone collapses as it goes into
1.6.1 Limestones
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Site investigations
1.6.4 Gypsum
Gypsum is more readily soluble than soft limestone, and
sink-holes and large caverns can therefore develop in thick
beds, especially when water is a dominant factor. The most
significant areas are in Ripon in Yorkshire and the surround-
ing areas north of Ripon where subsidence hollows have
been recorded along the outcrop of the gypsum beds within
the marl strata. Extensive geological mapping of these
solution features has been carried out by the British
Geological Survey. There is evidence that shows that a lot of
Investigate to provethe adequacy ofthe points of
structural support
Scope of hazard cannotbe fully defined
Design and constructspecial foundations
(address any residualhazard for externalareas and services)
subsidence took place many thousands of years ago,
there are areas where gypsum is still present and could b
problem in the future.
From the statistical evidence available it appears that
building in a hundred may be at risk in the Ripon area an
is considered prudent to provide stiff raft foundations in
floundering area designated by the British Geological Surv
Old subsidence hollows have often been filled with orga
silts and peats and in these situations the only solution i
use a piled raft with the piles taken below the gypsum.
assessment(geophysics,trial pitting,probing,
boring or drilling)
I Further work to I
L~I~e~e~~~...-----..___--,Design andconstructstandard
foundations(subject to
normalperformancecriteria)
Fig. 1.14 Decision flowchart for siteson chalk (Edmonds and Kirkwood, 1989).
Case study 1.1 Investigation of former mining site, Sheffield
INTRODUCTION
A large tract of land is to be offered for sale by treaty to a
major housebuilder and developer for future residential
development. The site is 11 km north of Sheffield city
centre, South Yorkshire, and the site location north of
Wortley Road isshown in Fig. LI5.
Preliminary enquiries with British Coal mining surveyors
have revealed that there may be unrecorded mine shafts and
possible shallow mine workings over part of the site. Other
14
shafts and spoil resulting from ironstone workings could a
be present on the site.A desk study report on the geological and past mini
situation has been commissioned by the developers. T
report is to outline the geological, geotechnical and p
mining problems that may be encountered in
development of the site. In addition the report shou
include a detailed soils investigation using trial pits a
recommendations as to the need for a borehole investigati
to establish the nature and extent of any shallow c
workings beneath the site.
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ago, but
be a
that one
and it
ns in the
Survey.
organic
tion is to
ld also
t mining
ers. This
and past
in the
should
pits and
coal
Case study 1
Case study 1.1 (contd.)
RESEARCH SOURCES 2 . The British Coal Opencast Division at Burton upo
Trent;
3. Ordnance Survey County Series plans 1850 edition an
1905 edition;
4. The County Series geological maps and memoi
including consultations with the British GeologicSurvey at Keyworth. Nottingham;
In compiling this report we have examined information and
records and made enquiries from the following sources:
1. British Coal Mining Surveyors at Technical Head-
quarters. Burton upon Trent to examine the shaft registerheld in the archives;
Fig. 1.15 Case study 1.1: site location and layout.
.1
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Site investigations
Case study 1.1 (cantd.)
SITE GEOLOGY. Sheffield City Technical Services Dept;
6, Mining Records Office at Rawmarsh, Rotherham, South
Yorkshire;
7. Plans of abandoned mines and quarries contained in
the archives of the Health and Safety Executive,London;
8. Mineral Valuer District Offices; Sheffield.
The 1:10560 scale County Series geological map NZ282
published by the British Geological Survey shows most
the site to be overlain by deposits of boulder clay of glacorigin. The thickness of these drift deposits is not record
on the geological maps but from information gleaned fro
:::::::8:,:;;:;;..................
Coal
PF
Silkstone rock
Coal
Sandstone
Silkstone coal
Sandstone
Sandstone
Sandstone
Coal
Sandstone
Sandstone
Whinmoor coal
Coal
16
Fig. 1.16 Case study 1.1: generalized vertical section of strata below the superficial deposits. Unlabelled areas are shales or mudstones.
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Case study 1.1 (contd.)
Case study 1
previoussite investigations in the site locality, the boulder
clay has been proven up to depths of 8 m beneath the
existingsurfacelevels.
Research into the history of the site, described later in
this report, also established the existence of previous
colliery spoil heaps within the site boundaries. There is a
possibility therefore that some areas of the site could be
overlain by deposits of colliery spoil. In addition there may
be backfilledpits on site as a result of ironstone extraction;
2 82 SE
st of
acial
from
these were indicated on the 1905Ordnance Survey ma
The old Ordnance plans showthe Thorncliffe Ironworks
be close to the northern boundaryof the site. The bounda
is in fact marked by the old mineral line which served t
ironworksand isnowdisused.A local shaft isnoted to ha
been worked close to the ironworks from Thornclif
Colliery and a generalized vertical section of the site
shown in Figs 1.16 and 1.17 taken from the Thornclif
Colliery shaft records.
------
._-------
.....................
~ ~~ H ~ ~ ~ ~ E E E ~ ~ ~ [ E:: :::::::::::;:::::::........................................... ._- .:: :::::::::::::::::::....................... .:: ::::::::::::::::::::: ::;:::;::::::::::::................... _ ... .
Description of
strata
Bind
Descnption of strata fromBritish Geological Surveyof Mining Terms
~Brassband
Silkstone coal
Dirt
Silkstone coal
Spavin
Strongstone
Imperfectly laminatedMUDSTONESor any f inegrained rock
Iron pyrite
Soft shaley materialinterbedded with coal
Seatearth - coarse clayor mudstone withrootlets
Sandstone
Fig. 1.17 Case study 1.1:vertical section of strata at Thomcliffe colliery.
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Site investigations
Case study 1.1 (contd.)
SOLID GEOLOGY
The published geologicalmaps show that the drift measures
are underlain by Middle and LowerCoal Measure strata of
the Carboniferous Age. These consist of sandsto
siltstones and mudstones with interspaced coal and fire
horizons. Several collieries existed close to the site, not
the Thorncliffe Colliery about 0.50 miles east of the
2·7M
(rial P i t : : No. 51
E
(3 ) Bcvlder c1~
Q ) SOIL
® CLAY
Sand4j topSOil
tO M
*5'O~E w
@ C~ / a.sh / bride fJ J
o " ' ILL ..: L-aose er't) ~ CI/ ld ask fill
® F I. .L: Loose lY"j ~ / o . . sk Ibicks ek
G) CLAY: 80vklu cia.LL darll brOL<ltt, sl1(f 5CU1~
Mai5t-, s " fP iM.e. c i. due to fill. OJ:ove
@ cLAY : Boolw ~ / daN< brow/!. I V€(3 sf-iff;
$ 1 i t J ~ m<Mt
'5~: BuLl: P a t K ® B . ®u q . . f i " o M . @
18
Fig. 1.18 Case study 1.1: trial pitlogs SI and S2.
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Case study 1
Case study 1.1 (contd.)
fireclay
notably
site.
There was also an ironworks close to the site referred to as
the Thomcliffe Iron Works.
The Silkstone coal seam, average thickness 1.80 m, is
conjectured to outcrop south of Wortley Road with the
Whinmoor coal seam in excess of 40m below the site. Oth
workable coal seams are the Hard Bed coal seam (Ganister
the Coking coal seam and the Pot Clay coal seam. Howeve
it is considered that if these seams have been worked in t
Tri-o l Pi b N o . S3
)
® C.I~
C D FILL
@ eLAY: Baolrkr c~ r dark browll. I shff
sa.Yl~ wiHt . ahundcutt- 8 ( a . J . I e 1
TriaL P J .:: No. S'f-
1. T~
@ Top~aiL: SCU1~ EPpsoil
@ FILL louse 95: w(Jh ash. mist-ore:
@ cLAY 6aulckr c .la j - \/~ .s r- vf f / rna i l5r
Fig. 1.19 Case study 1.1:tr ial pit logs 53 and 54.
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Site investigations
Case study 1.1 (cori td . )
past then any surface subsidence associated with the workings
will have long since ceased. British Coal have stated in their
mining report that no future mining isexpected to take place.
The coal seams are known to dip to the north at a
gradient varying between 1 in 5 and 1 in 15 and i
therefore considered that the Silkstone seam will be at s
a depth below the site that there will be sufficient rock co
over any possible workings.
1,101 Pib No. S5
W
1\-~_~)5M
~ = = = = = = = = = = = = = = = = = = = = ~ £1TopsoiL
2·30~
Bulk. froNt0 and: U4-
@ SOIL
C d ) FILL
@ CLAY
- r :o p 5 O " i.L / SCU1~
Le rose 8~ d~ ( l J ' " f c : ; { o. . .sk
BcrokW- c . .~ f ~IC brawl\.,
ver~ sl-iff wiJ-h. abUllda(rt
8rcweLs Q . ¥ I c L some coal
frOof:J~ts
Fig. 1.20 Case study 1.1:trial pit logs S5 and S6.
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Case study 1.1 (contd.)
Bibliography
nd it is
at such
ckcover
Geological faults TRIAL PITS
The geological maps also show that the site locality is
traversedbynumerous geologicalfaults which have resulted
in localizedvariations in the stratification of the coal seams
in regard to their depth beneath the site and degree of dip.
The Silkstone Coal is present at depths which should
provide an adequate cover of competent rock over any
possibleold workings.
SITE HISTORY
The various editions of the Ordnance SurveyCounty Series
wereexamined. The 1850and 1905publications record the
positionof the old Thorncliffe Colliery northeast of the site,
and the Thorncliffe Iron Works.
The 1905Ordnance plans showseveral ironstone pits on
the site and an air shaft in the northwest comer.
The later Ordnance Survey plan also shows ponds
together with colliery spoil heaps in the northern
area adjacent to the mineral line. An inspection of the
site reveals that the spoil heaps have been removed or
regraded.
PAST MINING ACTIVITY
The available records showthat coalmining has taken place
beneath the site in several main seams. The depth of the
Silk~ne seam dips from approximately 16m south of
Wortley Road away to the north. It is likely that the
Silkstone coal seam has been mined by pillar and stall
methods in the past but it isconsidered that theypresent no
riskto stabilityon this site.
Researches ofmine records and topographical plans have
confirmed the positions of one air shaft within the site
area. The recorded position of this shaft has been
investigated and identified using a JCB backacting
excavator to carry out slit trenching under our supervision.
Detailed records of the shaft do not exist; it will require
capping off at rockhead.
Coalmining has ceased in the locality, the main collieries
having been closed. The possibility of future underground
mining for coal or any other mineral can reasonably bediscounted.
The trial pit investigation proved up to 1.50 m of collier
spoil and ironstone debris in various parts of the site
underlain by natural brown boulder clays. The maximum
thickness ofboulder clayrecorded in the trial pits was3.0m
There maybeother areas of the sitewhich are overlain b
colliery spoil and which have not been revealed in thi
investigation.
The trial pit records are included in the appendix to thi
report (see FigsU8-l.20).
CONCLUSIONS AND RECOMMENDATIONS
The site is underlain by superficial deposits which ar
variable in their thickness and lateral extent. These deposi
are generally natural boulder clays but parts of the site ar
shown to be overlain byapproximately 1.50m thickness o
colliery spoil and ironstone minerals. These fillscould be
result ofbell pit workingsfor ironstone.
In those areas of the 'site underlain directly by natura
boulder clays it is recommended that standard-width stri
footings be used for dwellings of 2-3 storeys. These
foundations should be reinforced with a nominal layer o
mesh type B283top and bottom.
Where colliery spoil is evident, fo~ndations will need t
be taken down below the s n into the natural boulder clayfor a minimum distance of300 mm.
Though the soluble sulphate content of the colliery spoi
waswithin the Class 1 range it isrecommended that a Clas
3 concrete mix be adopted owing to the lowpH values.Almortar belowground shouldusesulphate-resistingcement.
Ground floor construction should be a voided precas
beam and block system. .
We are of the opinion that the Silkstone coal seam is a
such a depth beneath the site that any abandoned mine
workingswithin these seams would not present a source o
potential surface instability.
It is therefore recommended that excavations during the
site strip are examined to ensure there are no other mine
shafts present on the site. Should any be encountered the
local British Coal mining surveyorshouldbe informed.
It is recommended that the old air shaft be capped off a
rockhead level. Should the depth to rockhead be excessiv
the shaft infill should be grouted down to rockhead prior to
capping at the surface.
BIBLIOGRAPHY
BRE(1991) BREDigest363: Concrete in sulphate-bearing soils
and groundwater, BuildingResearchEstablishment.
BSI (1992) BS 882: Part 2. Specification for aggregates from
natural sources for concrete, BritishStandardsInstitution.
BSI(1999) BS5930: Code of practice for site investigations, Britis
StandardsInstitution.
BSI (1986) BS 8004: British Standard code of practice for
foundations, BritishStandardsInstitution.
21
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Site investigations
BSI (1990) BS 1377: Methods of testing for civil engineering
purposes, British Standards Institution.
Edmonds, C.N. and Kirkwood, J.P. (1989) Suggested approach to
ground investigation and the determination of suitable substructure
solutions for sites underlain by chalk. Proc. International Chalk
Symposium, paper 12, Thomas Telford, London.
Joyce, M.D. (1980) Site Investigation Practice, E. & F.N. Spon,
London.
22
NHBC (1977) NHBC Foundation Manual: Preventing Found
Failures in New Buildings, National House-Building Co
London (now rewritten as NHBC Standards Chapter 4.1)
Tomlinson, M.J. (1980) Foundation Design and Construction
edn, Pitman.
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Foundation design
225mmminimumthickness
.,f'\' / ' 1 '
Widened reinforcedstrip footing.1.0 mwideC.25P Concrete
Well-compactedstone fill
II)
II >.~l'tl>
-5c.
~
6 . • ~. A.
Main wirestransverse
High-yield .I(
fabric mesh
./40 mm cover
J . , :. . •. . .Width varies and is dependent on
the ground strength
Fig. 2.1 Widened reinforced strip footing.
20 mm chipboardPolythene vapour barrier (500 g)
H Tfabric
Fig. 2.2 Pseudo-raft foundation.
Where rock is encountered it is essential that the house is
placed wholly on the rock stratum. If the rock stratum cannot
be excavated at a reasonable depth, i.e. less than 3 m, then a
different foundation solution should be considered. Figure
2.3 indicates the use of a raft slab on a thick cushion of
crushed stone fill. Alternatively, piles and ring beams o
and pad with ring beams are the usual options worth
sidering when these conditions are encountered. The u
concrete manhole rings to form mass concrete piers is a
tical, economic and safe construction method and, w
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.
.
s or pier
rth con-
e use of
a prac-
where
Substantiatthickness of consolidated granularf il l placed in 200 mm layers
Introduction
~::::=.._!..----;----_ -__-_--. 1 - '_ - = - - - _ - - - _ . _ ~ -
___!--------;---- --\.-'.c--------------'-----;r 1 Rock stratum
Maximum depth of clay stratumover rock to be no morethan twice the thicknessof granular fill cushionto limit differential settlement
Fig. 2.3 Sloping rock formation.
Note:All foundations to. be placed on similar stratum
Quarry backfill /
\\
\ \Fig. 2.4 Pad and stem foundation.
only a few dwellings are affected, avoids the expense of
piling and avoids delays in the construction programme(Fig. 2.4).
2.1.4 Firm clays overlying soft strata
Avoid deep footings where a firm clay stratum overlies a soft
stratum which reduces in strength with depth. Where footings
have to be deepened to cater for existing trees or existing
deep drainage then it is preferable to adopt a pseudo-raft
foundation on a thick cushion of granular fill (Fig. 2 . 5 ) . If a
strip footing is deepened it is important to check that the
footing load does not overstress the softer strata at depth if
excessive settlements are to be avoided. In such ground con-
ditions the width of the footing should be kept as narrow as
possible so keeping the pressure bulb within the firm stratum.
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Foundation design
Foundation width B
Excavation = 2 x B minimum
Well-consolidatedgranular fill placedin200 mm thicklayers and given4-6 passes perlayer with avibratory roller
Fig. 2.8 Double reinforced footing on consolidated granular fill.
.:=> •. • A·,.- . Made ground or .
,weak strata _-
/ > .
(a)
Firm natural
L ..........,r-7T--77.T:m:::!.V stratum for at~ ~, ~ least 1.50 times
Concrete to be a the foundationdense mix if fills widthcontain sulphates orground is acidic
oIDIZ
~
--oH=:foo_1-50-75 mm .Clayrnasterlow-density
_polystyrene _
(11kg/~
Desiccated-clays -
~
E P, . .0 E'6 0
0It)
(b )
Fig. 2.9 Trench-fill foundations: (a) filled ground; (b) catering for existing trees and vegetation.
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;' ,/
/ /to/fi 'W
.~ ~~... < & • ••
'~A·
I: vReinforced concrring beam
~. . .'. '" . J
• •
A, I..-
- .A_ .--- v
I'"[;-Concrete stubcolumn at4-5 m centres
1-,-
1 < ' 4, E E, . .
E ::J
E.6 0'2I I • • 'T--r • C') 'E
a .. ..'-~
Widened reinforced strip footings
ete
MassC20Pconcrete inmanhole
Dowel barlinking beam
, to pier
- .
. ' .1I . " .
Fig. 2.10 Pad and stem foundations: (a) piers formed from formwork; (b) piers formed from concrete-filled manhole rings. Size of pad
foundation to suit bearing capacity of ground.
• Pad and stem foundations. Designed using ground
beams spanning between piers formed from concrete-filled
manhole rings or formwork (Fig, 2.10).
• Bored or driven piling with reinforced ring beams at
ground level.
2.2 WIDENED REINFORCED STRIP FOOTINGS
Designed in accordance with BS 8110.
Unfactored line load including self-weight of footing =40 kN/m
Allowable bearing capacity of ground taken as 40 kN/m2
Therefore footing width = 40/40 = 1,0 m.
Net uplift pressure = 40 - (0.225 x 24) = 34.6 kN/m2
Load factor = 1.50
I . 50 3 60 0,3752
U tirnate transverse moment = 1. x 4. x -- = 3.65 kN m.2
Using the Design Chart No.1 (Fig. 2.11):
M 3.65 X 106- =0.005
bd2 feu - 103 X 1702 X 25
Therefore lever arm, la =0.95d
From BS 8110: Part 3:
feu = 25 N/mm d = 170 mm
3.65x106
A . = 0,87x460xO.95xI70
. . 0.13x103 x225Minimum percentage 292 mm 2
100
Use A393 fabric mesh (which has the required minimum
percentage Of steel in both directions) in bottom. The fin
ished foundation is shown in Fig. 2.13.
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Foundation design
1.0
~
1 \
\- ,1 \\~
\1\
1\
\
i\~
\~
\r\,.\ .. .,
1\
\
\
.~
f\\
l/0.156Iv
0.95
N
E~ttl~Q)
>~
II 0.90
1J> <. . l i e :
~o+-'
~
E~ttl
f f i 0.85
~
0.80
0.05 0.10 0.15M
bcf2 fcu
When Mlbcf2fcu exceeds 0.156, compression steel is required. .A s = M/O.87fyktd. fy = 460 up to 16 mm dia; fy =425 over 16 mm dia;fy =485 wire
Fig. 2.11 Design Chart No.1: lever ann curve for limit state design.
30
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o
C\ I
t;~WW/N) m J
o io 0 io.q- C"') C"') C\ I
oLt)
.\ \\\ '\
---_.---_ . _ -_._-- --------_.-. - _ - " _
---.--. ----- -_ ..'--' _ - - _ ....• ._ _ ... -_ .. - _-
\\\1 \ \ ' f \ \ -- ---- ----- f---- 1-----1----
'\\ \~ ,\
-r-r
'\~\
~ \\
~\\,~~~,
~c----
"--"
~
_ \
-,
------- --_-
----1------- - --
\\
-,
, , , , \
o
Lt)
o
CD io o
oCD.q-
oC\ I
Lt) .,;ST""" . .,
,r:J
"0. ,u. . . .< 2t::
'u. . . .:>."@ jt::';;;
N0
e n 1) Zq ;:.0 'g
0 . < : :
0 U0 C
T""" bI)
T"""- ; ; ;.,Q
r'I. . . .N~fi:
LO
o
oo
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Foundation design
2.3 REINFORCED STRIP FOOTINGS ON be used placed on a thick mat of replacement stone
REPLACEMENT GRANULAR FILL (Fig. 2.8). The bulb of pressure remains within the
depth and the underlying soft clays are not stressed.
Where a weak stratum exists with allowable bearing capa- method can often be utilized instead of a raft foundation
cities between 25 and 50 kN/m2 then a reinforced footing can important to ensure that the thickness of stone fill belo
Table 2.1. Design Chart No 3. Reinforcement: areas of groups of bars
Diameter Area (mm-) for numbers of bars
(mm)2 3 4 5 6 7 8 9 10 II
6 28 57 85 113 142 170 198 226 255 283 311
8 50 101 151 201 252 302 352 402 453 503 553
10 79 157 236 314 392 471 550 628 707 785 864
12 II3 226 339 452 566 679 792 905 1020 1130 1240 1
16 201 402 603 804 1010 1210 1410 1610 1810 2010 2210 2
20 314 628 943 1260 1570 1890 2200 2510 2830 3140 3460 3
25 491 982 1470 1960 2450 2950 3440 3930 4420 4910 5400 5
32 804 1610 2410 3220 4020 4830 5630 6430 7240 8040 8850 9
40 1260 2510 3770 5030 6280 7540 8800 10100 11300 12600 13800 15
50 1960 3930 5890 7850 9820 ll800 13700 15700 17700 19600 21600 23
Diameter Area (mm-) for spacings in mm
(mm)50 75 100 125 150 175 200 250 3
6 566 377 283 226 189 162 142 II3
8 1010 671 503 402 335 287 252 210
10 1570 1050 785 628 523 449 393 314
12 2260 1510 1130 905 745 646 566 452
16 4020 2680 2010 1610 1340 1150 1010 804
20 6280 4190 3140 2510 2090 1800 1570 1260 1
25 9820 6550 4910 3930 3270 2810 2450 1960 1
32 16100 10700 8040 6430 5360 4600 4020 3220 26
40 25100 16800 12600 10100 8380 7180 6280 5030 41
50 39200 26200 19600 15700 13100 II 200 9800 7850 65
Diameter
""'cmm) 6 8 10 12 16 20 25 32 40 5
Mass
(kg/m) 0.222 0.395 0.616 0.888 1.579 2.466 3.854 6.313 9.864 15.4
Fig. 2.13 Widened reinforced footing.
32
footings remains fairly constant. Shear vane result
40kN/m2 to depths of 3.0 m.
At a depth of 450 mm the line load of 40 kN/m
imposes a ground pressure of 40/0.60 = 66 kN/m2.
increased line load of 60 kN/m run, it will be necessacheck the ground pressures at critical levels.
Checking at underside offooting
qm =60/0.60 = 100 kN/m2
This exceeds the allowable and will require the weak c
be replaced by consolidated granular fill. 100 kN/m2
ceptable for a well-compacted granular fill placed in 20
layers.
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filling
e stone
d. This
It is
e low the
12
340
604
942
1360
2410
3770
5890
9650
15100
23600
300
94
168
262
377
670
1050
1640
2680
4190
6550
50
15.413
sults are
run2• For an
ssary to
k clays to
is ac-
200mm
,Trench fill foundations
Table 2.2. Design Chart No.4
Mesh sizes Cross-sectional
(nominal pitch Wire sizes area per Nominal
BS of wires) metre width mass per
reference square metre
Main Cross Main Cross Main Cross
(mrn) (mm) (mm) (mm) (mm) (mm) (kg)
Square mesh fabric
A393 200 200 10 10 393 393 6.16
A 252 200 200 8 8 252 252 3.95
A 193 200 200 7 7 193 193 3.02
A 142 200 200 6 6 142 142 2.22
A98 200 200 5 5 98.0 98.0 1.54
Structural mesh fabric
B 1131 100 200 12 8 1131 252 10.9
B 785 100 200 10 8 785 252 8.14
B503 100 200 8 8 503 252 5.93
B 385 100 200 7 7 385 193 4.53
B 283 100 200 6 7 283 193 3.73
B 196 100 200 5 7 196 193 3.05
Long mesh fabric
C785 100 400 10 6 785 70.8 6.72C 636 100 400 9 6 636 70.8 5.55
C503 100 400 8 5 503 49.0 4.34
C 385 100 400 7 5 385 49.0 3.41
C 283 100 400 6 5 283 49.0 2.61
Wrapping fabric
D49 100 100 2.5 2.5 49.1 49.1 0.770
Checking 1.35m below ground level
Vertical pressure factor = 0.386 (pressure bulb, Fig. 2.14)
5.70sllQall=3.Q
whe~s =undrained shear strength,
Il=plasticity index correction factor
All . 5.7x4OxO.8 4 60 Nt 2owable beanng pressure = 5. k m3
Actual pressure at 1.35m depth = 0.386 x 100 = 38.60 kNtm 2
<45.60
Use a 600 mm wide by 225 mm thick double reinforced
concrete footing reinforced with fabric mesh B283 top and
bottom and specify a concrete mix of C25P.
2.4 TRENCH FILL FOUNDATIONS
These are only suitable if a good bearing stratum is known tobe present at an economic and practical depth. The stratum
below the base of the trench fill must remain competent for
at least 1.5 times the foundation width.
This foundation is useful for sites where deep fills, thick
soft strata or peat beds overlie firmnatural strata. Due regard
must be taken of the possibility of soluble sulphates in the fill
materials,or in acidic materials such as peats. In these situ-
ations the concrete should have a minimum cement content
based on BRE Digest 363 (1991 edition).
This type of foundation is also useful for meeting th
requirements of NHBC Chapter 4.2 where houses are bui
close up to existing mature trees or landscape planting. I
these situations itmust be used with caution especially if th
ground Within and adjacent to the house walls is in a desic
cated state. If the clays are desiccated the followingrecommendations should be adopted to prevent foundation
heave should the clays rehydrate.
1. Provide a fully suspended voided ground-floor construc
tion using full-span timber joists orjoists on sleeper wall
on their own foundation. In deep foundations the use o
sleeper walls can be uneconomic. Alternatively a precas
beam and block floor can be used.
2. Provide a low-density polystyrene material of suitable
thickness on the internal face of the trench fillto all foun
dation walls affected by desiccation of clays. The density
of the polystyrene should be no greater than 11 kg/m''. A
product such as Claymaster or Claylite would be suitable
3. Provide a slip membrane by using I200g polythenesheets on the external face of the trench fill. If th
existing trees are within a distance less than twice th
foundation depth then a suitable thickness of low-density
polystyrene should be provided on both sides of th
trench fill. The density of the polystyrene should no
exceed 11 kg/m",
4. Ensure that the width of the trench fillis maintained with
out any projections occurring at higher levels. If the clay
rehydrate, any projections are going to be subjected to
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Foundation design
0.1048
Fig. 2.14 Vertical pressure under a rectangular footing.
Polytheneslip membrane Claymaster
or similarlow-densitypolystyrene
If . Eoomin
' l I \ "(a )
Fig. 2.15 Trench-fill foundations: (a) correct; (b) incorrect.
34
uplift pressures; these have been known to separat
upper wider sections of concrete from the lower sec
(Fig. 2.15).
2.5 RAFT FOUNDATIONS
Raft foundations are most suitable for use in ground
ditions such as soft clays and filled ground, in old mareas which may have a potential for instability, and in a
mining localities. The raft foundation has that inherent
ness, not available in strip footings, which by virtue
load-spreading capacity is more resistant to differential s
ments.
If the fill is old, well layered and does not contain s
ficant voids or organic matter, a pseudo edge beam raf
be used placed on a layer of granular fill which should ex
under and beyond the raft edge for about 0.5 m (Fig. 2
The raft should have edge beams about 450 mm deep
sufficient width under the load-bearing walls to suit
ground conditions and be capable of spanning 3.0 m
cantilevering 1.5 m at the comers.If the fills are variable and contain mixed materials
advisable to remove the poor fill and proof-roll the forma
prior to either replacing the fill if it is suitable for recom
tion or using imported granular materials of a suitable g
ing. This method is obviously only economic when depth
replacement fill are less than 1.5 m.
This foundation method is most suitable for use on
where old cellars are encountered and the minimum th
ness of stone fill over the old cellar walls should be 1.0
prevent a hard spot from developing.
The pseudo edge beam raft is also suitable for sites w
trees and heavy vegetation have been removed below pote
Upward heave onsides and projectiqnscan separate concreteatX-X
6,
_ i x
t t..
x
\ Avoid ledges inexcavatedtrench
"'. 0'
~~ l>
(b )
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rate the
sections
nd con-
mining
in active
stiff-
ue of its
al settle-
signi-
raft can
extend
. 2.16).
ep with
suit the
m and
it is
mation
le grad-
pths of
on si tes
n thick-
1.0 m to
es where
potential
Raft foundation
house plots and where the upper clays have become desiccated
and could be affected by trees which are to remain. NHBC
Chapter 4.2 recommends that the thickness of stone fill below
the underside of the raft slab should be 50% of the depths
required from their tables, up to a maximum of 1.0m. Upfill in
e x c e s s of 1.0 m is only permitted if the NHBC can be sat-
isfied that the correct fill is being used and it is being
compacted properly. This generally requires the builder to have
the works supervised by qualified engineers who may carry out
various tests to confirm the adequacy of the compaction.
There are situations where pseudo rafts can be placed at a
lower level and the poorer fills removed can be replaced on
top of the raft. This type of construction is often used on
sites where a band of peat is present below firm soils and the
peat is underlain by soft alluvium or soft silty clays. The ad-
vantages of this form of construction are that no materials
require to be taken off site, and that the amount of imported
stone is reduced. Where the formation is very soft it is good
practice to place a layer of geotextile fabric down prior to
placing the stone cushion (Fig. 2.17).
Rafts are also used in areas where old shallow mine
workings are present and require grouting up. Where the old
workings have rock cover of less than six times the seam
thickness and have been grouted up there is still a risk
residual subsidence; a raft is better able to withstand loca
ized crown hole subsidence.
Example 2.1 Semi-rigidraft design
Consider a typical semi-detached dwelling of brick and blocconstruction to be built of a raft foundation placed on to wea
clayey sands which have a high water table. The ground floor pla
isshown in Fig. 2.18.
DESIGN INFORMATION
Loading: to BS 6399
Reinforced concrete design in accordance with BS 8110
Imposed loads on roof: 0.75 kN/m2 for 25°pitch
Imposed loads on floors: 1.50 kN/m2
Concrete: 35.0 N/mm2 at 28 days
Reinforcement: high-yield bars and high-yield fabric.
Face of edgebeam given twocoats of bituminouspaint
Two course clear cavity
Finished floor level
Construction jointif desirable
Fig. 2.16 Stepped-edge beam raft.
TImber joist orprecast concrete floor
Fill level orconcrete
oversite levelto be at orabove outsideground level
600mm min.to clayformations
l ~ p . . . . . . : : . t . . - 'r--...o.<~___;,"::""'--,,",--,,_,,:,,'---~_....:.</ No dpm required
_)":";"";"';'::"~II:,-'-r-L---,Y unless forprotection fromsulphates in theground150 mm minimum
stone bed
Fig. 2.17 Voided raft.
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Foundation design
The site investigation reveals that the upper stratum of shallow fills
overlies weak clayey sands. In view of the high water table it is
considered prudent to adopt a pseudo or semi-rigid raft foundation
with edge beams designed to span 3 m and cantilever 1.5 m.
Total unfactored floor loads = 2.40 kN/m2
Total factored floor loads = 3.66 kN/m2
LOADINGS
Roof
External walls
(kN/m2)
2.25
1.25
0.25
3.75 x lAO
Service loads
(kN/m2)Tiles 0.55
Battens and felt 0.05
Trusses 0.23
Ceiling board 0.15
Insulation 0.02
1.00 x lAO
Imposed loads
Snow 25' pitch 0.75
Storage 0.25
1.0 x 1.60
Factored loads
(kN/m2)
Brick
Block (100 mm)
Plaster
1.40 Spine walls (kitchen/Ioonge)
100 mm blockwork
Plaster
1.25
0.50
1.75 x lAO
1.60
Total unfactored roof load =2.0 kN/m2
Total factored roof load =3.0 kN/m2Staircase wall
First floor100mm blockwork
Plaster
1.25
0.50
1.75 x lAO
(kN/m2)
0.10
0.15
0.15
0.50
0.90 x lAO
Imposed loads = 1.50 kN/m2 x 1.60
Factored
loads
(kN/m2)
Boarding (22mm)
Joists 225 x 50
Ceiling board
Partitions (stud)
Party wall
2 skins blockwork 2.50
0.50
3.0 x lAO
1.26
2.40
2 coats plaster
8.28 (12.10)kNlrn
First floor joists
( ) EE
4.900m
Lounge
100mm
Fig. 2.18 Example 2.1: ground-floor plan. Figures in brackets are factored loads.
36
= 5
2
2
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WALL LOADINGS
Ignore windows and doors.
Front walls
Unfactored
loads
(kN/m)
5.25Roof
1.00 x 10.52
1.0 x 10.52
Walls
3.75 x 5.202.45
Raft foundations
Staircase wall
First floor 0.90 x 4.~0 2.20 x 1.40 3.08
100 mm block 1.25 x 2.4 = 3.00 x 1.40 4.20
Plaster 0.50 x 2.4 1.20 x 1.40 1.6
Factored6.40 8.96
loads
(kN/m)
Gable wall
5.25 x 1.40 7.35
5.25 x 1.60 8.40
19.50 x 1.40
30.00
Rear walls
(kN/m)
2.45Roof
I00 X10.5
. 2 5.25 x 1.40
1.0 x 10.52
5.25 x 1.60
4.20
Firstfloor
0.90 x 3.402
1.50 x 3.40-2- 2.55 x 1.60
1.53 x 1.40
. .Walls
3.75 x 5.20 19.50 x 1.40
34.08
Party walls
(kN/m)
Average height 6.50 x 3 = 19.50 x 1.40
First floor 2 x 0.90 x 2.5 = 4.50 x 1.40
First floor 2 x 1.50 x 2.5 = 7.50 x 1.60
31.50
Spine wall
First floor 0.90 x 32 4(kN/m)
1.53 x 1.40
1.50x 3:/ 2.55 x 1.60
100mm block 1.25 x 2.4 = 3.00 x 1.40
Plaster 0.50 x 2.4 1.20 x 1.40
8.28
27.30
43.05
(kN/m)
7.35
8.40
2.14
4.08
27.30
49.27
(kN/m)
27.30
6.30
12.00
45.60
(kN/m)
2.14
4.08
4.20
1.68
12.10
3.75 x6.5
(kN/m)
= 24.37 x 1.40
(kN/m
34.12
Total load (unfactored)
Front wall: 30.00 x 12.75
Rear wall: 34.08 x 12.75
Gable wall: 24.37 x 2 x 10
Spine wall: 8.28 x 2 x 6
Stair wall: 6.40 x 2 x 6.50
Party wall: 31.50 x 6.5
Party wall: 19.50 x 3.50
Total
(kN)
383
435
487
100
83
= 205
68
1761
4.8kN/m2
= 1.6kN/m2
6.40kN/m2
Self-weight of raft = 0.2 x 24
Edge thickenings
Area of raft
Weight of raft
= 10.90 x 13.150 = 143m2
143 x 6.40 915 kN
1761+915143
round pressure 18.71 kN/m2
Plus imposed load 1.50
20.21 kN/m2
Maximum line load = 34.08 kN/m
Consider edge beam and raft slab as composite with slab actin
as a tie. If overall width istaken as 900 mm:
Line load = 34.08 kN/m
Edge beam = 6.85 kN/m
40.93 kN/m
Pressure under edge strip = 40.93 = 46 kN/m2 unfactored0.9
This is less than the allowable ground bearing pressure o
50 kN/m2.
With the pseudo raft the centroid of loading on the walls a
foundation level generally coincides with the centroid of the edg
strip. The raft slab is therefore not subjected to any torsiona
moments. In practice, the ground pressure under the edge strip i
more likely to approach a triangular distribution with a higher edg
pressure. The slab therefore needs to have sufficient reinforcemen
in the top section to cater for this rotating force. Its main function i
to act as a structural t ie while atthe same time enhancing the edg
beam stiffrless.Where tHied ground occurs it is prudent toprovide
layer of bottom reinforcement to enable the raft slab to span ove
any soft spots.
To produce a theoretical edge pressure twice the allowable
would result in an equivalent eccentricity o f 150 mm. The slab wil
therefore haveto cater for an ultimate moment of 0.15 x 40.93 x 1.
=9.20 kN/m.
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Corner cantilever
. 58.180 X 1.502
6545 kNUltimate design moment = =. m
2.0
M
bd2feu
65.45 X 106
=0.025
450 X 4002 x35
I.factor = 0.95.
&
65.45 x 10
62
Therelore A.= 0.87 x 460 x 400 x 0.95 = 430 mm
. . 252x500 2A252 Fabric 500 mm wide = =126 mm
1000
Two T16 bars in top =402
528mm2
Provide A252 Fabric plus two T16 bars in top and bottom of edge
beams at comers.
Place raft on a 1200 gauge polythene dpm laid on sand-blinded
stone f i l l .
Toe design
Design as a cantilevered slab, to carry shear loading from outer leaf
of cavity wall. Take height of wall as 6.8 m using a 150 mm thick
toe reinforced with A252 fabric mesh in bottom.
Consider 1.0 m length.
Therefore b;= 1000 mm
d = 150 - (40 + 3.5) = 106.50 mm
Loading / metre run = 110 mm brickwork= 2.30 x 6.8 =·15.64 kN/m
Toe self-weight = 23.6 x 0.15 x 0.45 = 1.60 kN/m
Therefore gx = 17.24 kN/m
Therefore design shear V = 1.40 x 17.24 = 24.136 kN/m.
' "Shear stress
V 24.136x103V=-= 0.23 N/mm2
b; d 1000 x 106.50
Reference BS 8110 clause 3.5.5 (shear resistance of solid slabs) and
clause 3.4.5.2 (shear stress in beams).
Permissible concrete shear stress
Reference BS 8110 Table 3.9:
100/A.= loox252 =0.236
bv d 1000 x 106.5
v is less than Ve• therefore from BS 8110 Table 3.17 no shear steel isrequired.
Check toefor bending
Upward design load from OBP = 1.50 x 46 x 0.45 = 31.05 kN/m
Downward design load (wall) = 1.40 x 15.64 = 21.89 kN/m
(toe self-weight) = 1.40 x 1.60 2.24
24.13
Raft foundation
Bending
31.05 x 0.45 _ 21.89 x 0.10 _ 2.24 x 0.45
2.0 2.0
=6.98 - 2.819 - 0.504 3.65 kN m
Reinforcement
Vsing design formulae method (BS 8110 Cl. 3.4.4.4):
By inspection k is greater than 0.95. therefore z = 0.95 d.
Therefore
M 3.65x106 2
A.=--= =90mm0.87/yz 0.87x46OxO.95xl06.5
Minimum percentage =0.13% = 195mm2, therefore use A25
Fabric in bottom of toe.
Internal ground beam (party wall)
Factored wall line load
Self-weight of beam 23.6 x 0.45 x 0.80 x 1.40
45.60
11.90
57.50kN/m
Ground beam considered to act as a partially fixed element:
VI· desi 57.50x3.02
5175 ktimate esign moment = . Nm10
Vse two layers of A252 in slab with one layer of A252
thickening.
Consider A252 in bottom plus two T16. Therefore:
Area of fabric = 700 176 mm21000
Two T16 bars 402mm2
578mm2Total
d = 450 - (40 + 12) = 398.0 mm
Mb = 800 mm, z = 0.95 d . Therefore: As = ---
0.87/yz
Therefore: Design moment of resistance = As 0.87/y Z
M = 578 x 0.87 x 460 x 0.95 x 398.0 x 10-6= 87.46 kN m.
OK> 51.75
Consider both layers of A252 in slab when checking cantileve
mode.
Ultimate design moment = 57.50 x 1.502
= 64.68 kNm2.0
Moment of resistance of two layers ofmesh
= 176 x 0.87 x 460 x 0.95 x415 x 10-6= 27.76kN m
= 176x 0.87 x 460 x 0.93 x 300 x 10-6= 19.65 kN m
Moment of resistance of two T16 bars = 402 x 0.87 x460 xO.95
405 x 10-6= 61.89 kN m
Total moment of resistance = 109.30 kNm
Use one layer of A252 in bottom of raft thickening with two layer
in slab with two T16 in top and bottom of beams (Fig. 2.21).
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Foundation design
Table 2.3. Summary of ground beam loadings
Wall location Ground beam type Maximum
factored
wall line load
(kN/m)
External front
wall
43.05
External rear
wall
900 mm wide edge beam
with two T16 bars in
top and bottom with A252
fabric mesh in toe beam.T10 links at 250 mrn centres
900 mm wide edge beam
with two T16 bars in
top and bottom with A252
fabric mesh in toe beams.
T10 links at 250 mm centres
800 mm wide slab thickening
with A252 in base and two
layers of A252 in 200 mm
slab with two T16 top and
bottom
Party wall
Gable wall 750 mm wide edge beam
with two T16 bars intop and bottom with A252
fabric mesh in toe beam
T10 links at 250 mm centres
600 mm wide slab thickening
with A252 in base and two
layers of A252 in 200 mm
slab
Spine wall
Staircase
wall
As spine wall 8.96
42.27
45.60
34.12
12.10
Example 2.2 Structural calculations for three-
Morey flats
DESIGN INFORMATION
Design codes
• BS 6399 Part ILoading
• BS 648 1964 Schedules of weights of building materials
• BS 5628 Structural use of masonry
• BS 8110 Structural use of concrete
Foundation concrete: Fco = 35 N/mm2
Reinforcement: fy 460 N/mm2
Tf = 1.50 for dead plus imposed loads.
The si te investigation has revealed that the upper firm clays are
underlain at about 2 m below ground by a 100 mm band.of peat. In
addition, the site is in an area known to be affected by subsidence
arising from solution features in gypsum strata at depth. In view of
this, a stiff edge beam raft will be used.
LOADINGS
Roof
Concrete tiles
Battens and felt
40
Self-weight of trusses at 600 mm centres
Insulation
Plasterboard
Imposed load:
Dead load
Snow
Ceiling
0.23
0.02
0.15
1.00
0.75
0.25
1.0
2.00k
(kN/m2)
2.25
1.20
0.15
0.50
4.10
1.50
5.60kN
(kN/m2)
2.0
2.0
0.25
4.25
(kN/m2)
2.0
0.50
2.50
(kN/m2)
4.20
0.30
4.50
3.00
7.50kN
(kN/m)
38.25
33.60
71.85
External wall B
Wall 4.25 x 7.80 33.15
(kN/m2) Roof 8.13 x 2.08.13
0.552.0
0.05 Total 41.28
Therefore total roof load taken as 1.00 + 1.0
Floors
150mm deep beam and block floor
50 mm concrete screed
Plasterboard
Stud partitions
Dead load
Imposed load
Therefore total floor load taken as 4.10 + 1.50 =
External walls
100mm blockwork
102mm brickwork
12.50 mm plaster
Internal walls
l00mm blockwork
Two coats of plaster
Staircases
175 rnm in-situ concrete
Finishes
Imposed load
Total
Loadings to walls
External wall A
Wall 4.25 x 9.0
3 x 5.60 x4.02.0
Three floors
Total
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1.00
m2)
2
2.0
2.0
2.0
m2)
Raft foundation
275 mm----+-i~cavity 22 mm chipboard on
vapour barrier on38 mm polystyrene
A252 mesh
- , , . '
'----1200 gauge polythenedpm on sand-blinded crushedstone fill252
mesh 900mm
(a )
275mmblockworkparty wall
A252 mesh
.,. 0
A252mesh
300mm
mesh
800mm~--~-- __
(b)
Fig.2.21 Example 2.1: reinforcement details to raft foundation. (a) External edge beam; (b) party wall thickening.
3 x 7.500 x 2
2.0
Spine wall C Three landings 22.50
Wall 2.50 x 8.0 20.0
Three floors 3 x 5.60 x 7.058.80
2.0
Total 78.80
Staircase wall
Wall 4.25 x 9.0 (average height) 38.25
Three floors 3 x 5.60 x 325.20
2.0
Total 85.25
G R O U N D FLO O R BLO C K W OR K D E SIG N
Maximum line load on spine wall = 78.80kN/m. Using ION
crushing strength blocks with mortar designation (iii). /k =·8.2.
Design vertical resistance of wall / unit length:
n = {3tAW t'm
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floor to
)
erleaf to
level.
ature and
t of some
3.5 N blockwork
Precast floors
000
7.0 N blockworkE
Provide 35 mm x 5 mm ggalvanized steel N
restraint straps at 2.0 m C f c
(I .' •
10 N blockwork
8.00 ill 250mm
fixity. Maximum line load = 85.25 kN/m. With load factor of 1.50
for dead plus imposed loads the ultimate line load equals 1.50 x
85.25 = 127.875 kNm. Therefore
Ultimate moment (sagging) = 127.8;g x42
204.60 kNm
Ul· . 127.875x22
2557 kNnmate moment (hoggmg) = = 2 -. 5 m
Try 600 mm x 600 mm edge beam, d = 600 - 40 - 10 = 550 mm,From Fig. 2.11:
k
= -255.75 x 10
6
_ 0.04- bd2 f e u 600 X 5502 x35
Therefore lever arm factor =0.945, and therefore z = 0.945 x 550 =519mm.
MA.==-
0.87/z
255.75 x 106
= 1231mm2 -
0.87x460x519
Therefore use four T20 high-yield bars top and bottom (1256 mm-),
Shear
V = 85.25 x 2.0 x 1.50 = = 255.7 kN
v = = . I . . = 255.75 x 103=0.775 N/mm2 looA, =0.38
bd 550x6QO 6QOx550Vc = 0 .45 N/mm2
Therefore minimum links required
s=250mm
A s v > MObs 0.40 x 600 x 250 = = 150 Il1Ill2
0.87fy 0.87x 460
Therefore minimum two leg TI0 links at 250mmcentres. A s v =157mm2
Maximum spacing of links = 0.75 d = = 0.75 x 550 = 412mm.
Raft foundations
F.g. 2.23 Example 2.2: typical cross-section ofthree-storey flats.
Eo<D
N
EE
~-
Slab design
Total load on raft (kN)
8 x 71.85 x 1.0 575
8 x 78.80 x 1.0 630
8 x 85.25 x 1.0 = 682
7.50 x 41.28 x 2.0:: 620
2507
Load spread = 8,73 x 8.315 = 72.60 m2• Therefore average bearing
pressure below slab- = = 2507/72.60 = = 34.50kN/m2• Assume slab
spans simply supported. Maximum span = 4.0 m,
(kN/m2)51.75
6.72
45.03
Therefore net uplift pressure =45.03 kN/m2• Try two-way spanning
slab, simply supported at edges.
Provide spine beam across centre of raft.
Ix=4.0m, ly =4.0
1 1 l s . , = a . . x n I}msy= a.yn Ii
Ultimate design load
Less weight of slab
1.50 x 34.50
1.40 x 0.20 x 24 :::
From Table 3.14:
Iy= 4.0 =1.0
i, 4.0a s . r "=0.062
as, =0.062
m s x =0.062x45.03x42=44;66kNm
msy= 0.062 x 45.03x 42= 44.66 le N m
Iy =4.0 = 1.33 a s x = = 0.093t, 3.0
asy =0.055
m~,= 0.093x 15.03x32 =37.69 kNm
msy =0.055 xA5.03x32=22.28 kNm
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Foundation design
4T20D • ~.
Fig. 2.24 Example 2.2: raft edge detail.
Forfell = 35 Njmm? andfy = 460 N/mm2 .
M 44.66 x 106k=--= 0.053
bd2 fe u 1000 X 155x 155x35
From Fig. 2.11, z = 0.93. Therefore:
.. 44.66 X 106
A . 77 5 mm? 1m= 0.87x155xO.93x460
Use fabric A393 supplemented with T10 at 200mm centres in top
of slab both ways with A393 in bottom of slab.
For the 4.0m x 3.0 m bay, moment = 37.69 kN m. Therefore:
M 37.69 x 106
=0.044
bd2 fe u = 103 x 155x 155X 0.935
Therefore z=0.935
Th f A 37.69xW6
650 mm'ere ore s = 0.87x460x155xO.935
Use fabric A393 supplemented with TIO at 300mm centres in top
in both directions with A393 in bottom of slab.
2.6 PADANDPIERFOUNDATION
This type of foundation (Fig. 2.25) can be used in situations
where piling is being considered. If only one or two dwell-
ings are affected the cost of piling can be prohibitive because
of the initial cost of getting the piling rig to the site. It is used
in situations where very soft clays, peat and fill materials
overlie firm or stiff strata at depths up to 3-4 m. On sites
where foundations are on rock strata and a deep face is
44
E
1LEEoIi)
.r250mm
150 mm min well-consolidatedgranular fill
encountered because of past quarrying or geological faultin
depths of up to 6 m are still more economic than piling an
construction delays can be reduced.
The piers can often be constructed using manhole rin
sections placed on a concrete pad foundation and filled wi
mass concrete. The top section of the pier can be reinforce
to form a connection for the reinforced concrete ring beamplaced at or close to ground level.
This method of construction can also be used where exis
ing drainage is too close to a proposed wall foundation. In
cases the pad foundations must be wholly on similar bearin
strata.
2.6.1 Disused wells
Quite often old disused wells are encountered on housin
sites, usually during excavation for the foundations. Wh
such wells are found it is wise to make the well safe witho
altering the water source which supplies the well. Filling
well with. mass concrete is not a recommended solution:could be very expensive and changes in the groundwate
regime could occur. The most suitable method if the well
deeper than 2 is to fill the well with 150mm single-size ston
and, if in a garden area, provide a reinforced concrete ca
twice the diameter of the well. If the well is under or ve
close to a foundation then, after filling, a beam system w
be required to span over the well. The beams should extend
sufficient distance beyond the well; this minimum distance
generally taken as the well diameter each side.
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faulting,
iling and
ole ring
lled with
inforced
beams
e exist-
n. In all
r bearing
housing
When
without
Filling a
lution: it
e well is
ze stone
rete cap
or very
tem will
extend a
stance is
Pad and pier foundation
50mmC10Poversite concreteon 1200'gauge polythene
dpm
Timber suspended
ground floor
NoteAll foundations tobe on similar strata
Fig. 2.25 Padand pier foundations,
Example2.3 Disused well level. The well-was approximately 2.0min diameter and the to
2.0 m was brick-lined s :The well was positioned below the junctio
of the rear walland party wall. I t was notpossible to reposition th
dwelling so abeam system in the form of-a tee configuration w
adopted.
During excavation for a house footing a well was discovered
following removal of a large sandstone cover (Fig. 2.26). The well'
was found to be 6 mdeep and water was within 1.0m of the ground
E 494legTS
~L ~. centres
40 mm min._...jI - - - I 'cover W
Fig. 2.26 Example 2 .3 : . disused well - foundations. Concrete mix 30 N/mm2 at 28 days; reinforcement to be high-tensile bars; allowable
ground bearing pressure 80kN/m2 minimum.
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while
y clause
should be
kN/m
with three
Therefore
100As _ 100 x 3220
/;;i- 600x540 0.99
Therefore Vc = 0.63, and hence A s v is less than Vc + 0.4 and
minimum links are required.
Hence
= 0.4 x 600 x 300 =180 mm'A s v 0.87 x 460
Provide four leg T8 at 300 mm centres.
PAD FOUNDATIONS
Maximum un f actored line load on party wall = 48 + 8 = 56 kN/m.
With a 600 mm wide footing, bearing pressure = 56/0.60 =
93kN/m2.
Maximum unfactored line load on external wall = 26.80 + 8 =
34.80 kN/m.
With a 450 mm wide footing, bearing pressure = 34.80 = 77 kN/m
0.45
In order that differential settlements between the pad foundation
and the strip footings can be kept within acceptable limits, a
maximum allowable bearing pressure of 100 kN/m2 should be
adopted under the pad foundations even though the natural stiff
clays are good for about 200 kN/m2. In addition some section of the
ground beam between the bearing pads and the well will be sup-
ported on natural ground, reducing ground bearing pressures even
further.
Party wall pad
. 6 3.50 98Unfactored reaction = 5 x -- = kN
2
Self-weight of base = 0.4x 24 = 9.6 kN/m2
Are.quired 98.0 = 1.084 m2.
100-9.60 'say 1.05 m x 1.05 m x 400 mm thick pad foundation.
External wall pad
Unfactored reaction = 34.80 x 7.0 + 98.0 = 170.80 kN2 2
A. d 170.80
rea require =100-9.60
1.88 m 2 = 1.40 m x 1.40 m x
400mm thick pad foundation
Base reinforcement
d::: 400-40 -10 = 350mm
Maximum ult imate moment
0402:::(100 - 9.6) x 1.5 x _._ = 10.848 kN m
2.0
_!:!__= 10.848xI06
0003
bd 2 fe u 103 X 3502 X 30 .
Therefore l. = 0.95d = 0.95 x 350 =332 mm
Th e & .d 1O.848xl06
82 2e re ro re • - s = = mm
0.87 x 460 x 332
M· . 0.13 3 4 5 2i rnmum percentage =-- x 10 x 350 = 5 mm100
Piledfoundation
Therefore use TI2 at 200 mm centres both directions in bottom.
Party wall pad
Maximum ultimate moment
0.2252= (100 - 9.6) x 1.5 x -- = 3.43 kN m
2
By inspection, nominal steel will be required.
Minimum percentage = 0.13 x 103 x350 =455 mm?
100Therefore use 12 mm at 200 mm centres each way.
2.7 PILED FOUNDATIONS
Where a lot of dwellings require special foundations and t
fills or weak:ground are not suitable for ground improveme
techniques, piling can be carried out at very little extra co
provided the design details are carefully considered. In mo
situations driven piles are used, driven to a predetermined s
and subjected to a random load test of 1.50 times t
working load.
Driven piles should be avoided in situations where tbedrock profile can vary over a short distance, as in infill
railway cuttings and backfilled stone quarries, for exampl
In these situations driven piles are prone to drifting out
plumb during the driving stage and often end up bein
damaged because of the eccentric loads applied.
If piling is being used then the ground beams should
kept as high as possible. It is often more economic to desig
a scheme based on large piles, especially if the piles a
taken down on to rock or hard clays. One of the problems
using small mini-type piles in yielding strata, such as cla
and sands, is that the end bearing component of the workin
load required is usually of a low magnitude and the pil
need to be driven deeper to pick up sufficient skin frictioWhat may seem to be an economic piling scheme based o
estimated driven lengths may, on final remeasure, end u
being very costly.
Where piles are driven or bored through filled ground st
settling under its own weight, due allowance must be mad
for the additional load arising from negative skin friction
Also, on sites where highly compressible strata such as pe
are likely to be loaded, due to the site levels being raise
additional loads will be transferred to the pile shaft.
2.7.1 Bored piles
These are generally formed using a simple tripod rig. Whe
there are groundwater problems, or very soft clays whic
may cause necking, then temporary casings or permanen
steel sleeves should be used. Great care must be exercise
when withdrawing temporary casings, and a sufficient hea
of concrete should always be maintained in the pile shaft
prevent necking or concrete loss.
Bored piles usually rely on end bearing and skin friction
support the pile loads. They are best suited to clay site
where no groundwater problems exist and the upper stra
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Foundation design
are strong enough to maintain an open bore. Once steel
casings have to be considered, the bored pile can be uneco-
nomic compared with other faster systems.
Their main advantage is that they can be installed quietly
with minimum vibration; ideal when piling close to existing
buildings.
Where end bearing is required it is important to ensure
that the stratum below the pile toe remains competent for a
distance of at least 3 m. If no site investigation is available
this can be checked by overboring several piles.
Figure 2.27 shows the sequence of operations for
installing a bored pile. Different cutting tools are required for
the various soil conditions.
Safe loads for bored piles in clay soils are generally
calculated using the Skempton formulae which combine both
end bearing and frictional properties of the pile.
For varying soil strengths the skin friction is considered for
the separate elements of the pile shaft with negative skin
friction being considered when passing through filled ground.
2.7.2 Design of a bored pile
Adopt a factor of safety of 3 for end bearing and a factor ofsafety of 2 for skin friction. Qu = the ultimate resistance of the
pile = Qs + Qb. Qs is the ultimate value for skin friction =shaft
area X 0.45c = 1t dh x 0.45c. Qb is the ultimate value for end
bearing = base area x 9c = ~ 1t c f 2 x 9c where c isthe cohesionvalue of the clays determined by laboratory testing or by the
use of field tests: i.e. penetrometer or shear vane tests. Thus
Allowable working load for a pile = ~s + ~
I Ind2xge)
= -(ltdh x 0.45e) +--'-----'-2 3 4
ndc(2.7h+9d)
= . , 12
Where the clay cohesions vary, the pile shaft is split into
vertical elements using the appropriate values down the pile
length.
2.7.3 Design of bored and driven piles
Estimation of approximate working loads is as follows.
The ultimate load capacity Q u of a pile is
Qu=Qb+ Q s
where Qs = ultimate shaft resistance, Q b = ultimate base arearesistance, q = ultimate unit end bearing resistance, Ab =effective cross-sectional area of the base of the pile, /
=ultimate unit shaft resistance on sides of pile, As = effectivesurface area of pile shaft considered as loaded.
Base resistance: clay strata
The ultimate base resistance of a bored or driven pile in
cohesive strata is given by
q=N; Cb
48
where N; = bearing capacity factor, Cb = undrained she
strength of the cohesive strata at the pile base. Values of
for cohesive strata can be variable and are dependent on t
angle of internal friction, ¢. For estimating purposes a val
of 9 is generally used for pile diameters up to 450 m
diameter.
Base resistance: granular strata
The base resistance in a granular stratum is given by
q= rDNq
where Nq is the bearing capacity factor obtained from t
Berezantsev graph (Berezantsev, 1961) based on the angle
shearing resistance ¢ for the stratum. The value of ¢
usually determined from the standard penetration test resu
carried out in the field. For bored piles an Nq val
appropriate for loose soil conditions is recommended a s- t
boring operation loosens the strata, and ¢ values of 28-30
can be used. T = average effective unit weight of so
surrounding the pile, and D = depth to the base of the piThe value of q should not exceed I I MN/m2.
Shaft resistance in clay soils
The shaft resistance / is given by
f=aC,
where a = 0.45 for bored piles. Cs = undrained she
strength. a is taken as 1.0 for driven piles in contact wi
strata with C« < 50 kN/m2.
For strata with Cs > 50 kN/m2 the value of a lies betwe
0.25 and 1.0 and is dependent on the depth of penetratio
into the clay strata and the prevailing ground conditions.
The shaft resistance/in granular soils is given by
f=! reD + If) K, tan 0
where D = depth to base of pile or base of the granular stratwhichever is the lesser; d ;:: depth to the top of the granul
strata; (j = angle of friction between the granular strata athe pile shaft; K; = earth pressure coefficient dependent
the relative density of the soil.
Broms (I966) related the values of K s and (j to the e
fective angle of shearing resistance of granular soils ¢ f
various types of piles and relative densities. For driven pile
Pile
type
Low relative
density
High
relative
density
Steel
Concrete
0.5
1.0
1.0
2.0
¢is generally taken to be the value of ¢as obtained from t
SPT tests. For bored piles. values of (j =220 and K, = 1
should be used to cater for the loosening effect when borin
out.
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shear
s of Nc
on the
a value
50 mm
om the
angle of
of tp is
t results
q value
as the
28-30°
of soil
the pile.
shear
ct with
etween
strata,
granular
rata and
den t on
the ef-
ls tp for
n piles:
rom the
= 1.0
boring
The expression! t(D + d) K, tan 0 can only be used for
penetration depths up to 10--20 times the pile diameter.
Between 10 and 20 times the pile diameter a peak. value of
unit skin friction is reached and this value is not exceeded at
greater depths of penetration. It is prudent to adopt a peak
value of 100 kN/m2 for straight-sided piles.
Tomlinson (1980) suggested that the following approx-
imate value can be adopted for f (kN/m2):
Relative density
<0.35
0.35--0.65
0.65--0.85
>0.85
Loose 10
Medium dense 10--25
Dense 25-70
Very dense 70 but < 110
A factor of safety of 2.5 should be adopted to these ultimate
values to obtain the allowable or safe working load on the pile.
Example 2.4 Bored piles
A site investigation has revealed that loose colliery waste fills about
4m thick overlie firm-to-stiff clays which are underlain by
weathered mudstones. The maximum pile loading is 300 kN but an
additional load resulting from negative skin friction has to be
catered for as the fil ls have only been in place for one year and are
still consolidating under their own weight. The strata are described
in Figs 2.28 and 2.29.
An exist ing culvert passes under the proposed dwell ing and its
condition is not known. It has been decided therefore to provide
bored piles down to the strong mudstones at about 6.50 m below
ground level.
Based on information from borehole no. I(Fig. 2.28), no test
result"are available for the firm/stiff brown clays and weathered
mudstones. Assume a value for c of 50 kN/m2 which is veryconservative (Fig. 2.30).
unit negative skin friction
Po = = effective overburden pressures
¢e = = the effective angle of shearing resistance
"act = = k Po tan ¢e
From Bjerrum, raet = = 0.20 po for clays of low plasticity.
The negative skin friction factor a==0.20
Skin friction ultimate == rhaAS2
Required pile capacity = = 300 kN.
NEGATIVE SKIN FRICTION
This is approximately = = ~"ac,poAs
0.20(18x3.80)= = 2 x(1tx0.45x3.80)=38 kN
Therefore total pile capacity required ==300 + 38 = = 338 kN.
END BEARING
For N values of 49 the unconfined compressive strength of the
mudstone equates to 13.30 x 49 = 652 kN / m2 (soft rock).
Piled reinforcement
652The shear strength = = - = = 326 kN/m2
2
Allowable end bearing pressure
N e Xc 9x326 =978 kN/m2Factor of safety 3.0
End bearing capacity = 978 X 1tx 0.62
= = 277 kN4 .
SKIN FRICTION
a ==adhesion factor = = 0.40.
u = cxA, == 0.40 x 50x (trxO.45 x 2.45)
S Factor of safety 2.0
==34kN
Therefore pile capacity = 277 + 34 ==311 kN.
With a 1.0 m penetration into the mudstone this value isconsidere
adequate for borehole no. 1.
Based on soils strata in borehole no. 2 (Figs 2.29, 2.31):
Required pile capacityNegative skin friction = == = 300 kN
0.2(18x 7.2) ( 5 72)2 x trxO.4 x .
+ 0.1(18 x 7.2 + 0.65 x 20) (z x 0.45 x 1.3)
= = 132 kN
26kN
Therefore pile capacity required
For N ==152, unconfined compressive strength
= 13.3 x 152
2022Shear strength =--
2
Allowable end bearing capacity
9 xlOll x(tr x 0.452)/4
3
== 458 kN
= = 2022 kN/m2
= lOll kN/m
=482 kN
This is greater than the 458 kN required. The piles must therefore
penetrate at least 1.0 m into the shaley mudstones, ideal ly below
the weathered zones. The minimum cement content in piles shoul
be 370 kg/m3 sulphate-resisting cement with a free water ratio no
exceeding 0.45 to cater for Class 2 sulphates in soils.
PILE REINFORCEMENT
N = 1.60 x 458 = 732 kN
f e u =40 N/mm2
With 75 mm tolerance on pile position the bending moment on the
pile shaft equates to
75-3 x (1.60 x 458) = 54.96 kN(ult)10
N 732 X 103
== 0.090d2f = 450x450x40
~ == 54.96 x 106 0.015.
d3 ! C U 450
3X 40
Therefore use 'nominal steel only. Provide seven R12 vertical bar
with nominal R6 helical binders at 150 mm pitch.
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Foundation design
HUDDERSPIELD
Sheet 1. Borehole
I";S:;"o"""'ri:-n-g-:-M:-e""th-o"""d:-------------------------l of 1. No. 1
L. . /GHT CABLE PER.CuSSION AT 150 MM DIAfrtETE:R.. ~S;::;i:;:te:---=--...L.---==----I
Drillingcommenced GroundLevel 122·0 AoD
DescriptionfStrata
MADFi ! GlbUND: Co/IIU. j fAP..Sbe
w·ci1t 6(JMQ. c oa l: / bn c.ks
c/~ and: t-Lmh£r
MADE GR.oVND
DepthLegend(m )
Remarks
2.7.4 Driven piles
Fig. 2.28 Example 2.4: borehole 1.
In the housing field these usually consist of steel tubes filled
with concrete, precast concrete segmental piles, concrete
segmental shell piles or other similar types.
The main advantage of these types of pile is that they
be placed through weak or water-bearing strata without
changes occurring in their cross-section. They are gene
end bearing piles but additional loads resulting from
friction (adhesion) can be developed when driven thro
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they can
ut any
om skin
through
Piled reinforcement
Sheet L I BoreholeBoring Method of 1 No·2
LIGHT CABI.E PEra:.USSION AT ISO NM DIAmETER. Site
HUDOERSF/cLDDrilling commenced Ground Level 12/·00 lor>
Samples/Tests StandI, ReducScale
ing edDescription of Strata Depth
Legendample Insitu Water Level (m)Type Test Level (m) (m)
f l l /AOr= GROUND Shak a/ld. /
= - Mudsh:mM, brick. froe~s,C U ' I d timr:w-
_g;Q_
~r = -
>(.-= -~~ '2 ·qo,6.0
~MADE 6ROVNO: ISrick.s J sfoI1es
~ tVand c..~s
~(4-)
~ 3·!i"C!
rS'o mADE GRoUND ; 8hA.ck . a.6h and~ brick fr"$~5~-
=. . 1 Q ; Q _
=~r : -....!bQ_ (,'0
- mADe GROc.JND : SCu1d..st-oNl. ~~
: aJK l b~c/( CA'J,t
= -...illL
7';ZO
= 6of~ d.a.rk c r~~v~ ~
.. . : _ : .
;-:::::-:-= - f!~~ ~5~~ rtrat:-;--
- - = - - - = -=16.0
<. n s;V(JrtS . ~ _ I
f{o..3~ -:-=---
:.. ~
2·50 ~
= - Oark: e " " ~ and. bto.Gk 1 . U € C A k ~;"'IAj~~ S'.<;;>n =
= uJ~ed. t ' t - r . i . n } : t _ b t Z - d d . u 1 .18.0
;: IVE;~
Mt.JCk;1-o
t (152)&r~~ SU$p~ ti1, sero":)= - ( ' IWcL~ !LeS ai: q.30 M.
-20.0
Remarks
Tn:u.e-s o f wa..ter enbered: bore~ cW~tnj drliW'lj cd 770 j\.(
Borehole c i r ; J onWtfk:J.rr:iwa)
o r GaSCM.J
Fig.2.29 Example 2.4: borehole 2.
the clays or gravels overlying the bearing strata.
These piles are usually driven to a predetermined set based
on the Hiley formula which is a dynamic criterion related to
the weight of the driving hammer and height of the drop.
When using this concept it is essential that a random pile b
load tested using kentledge or by jacking against tension
piles to confirm that the pile driving assumptions and
established set criteria are valid.
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Foundation design
Ground level 122.00AOD
~".-, (?" '.f :0/'\\ 0.'
0" ~ 0,
t , . . . : - -- '' ' ' . . . .450mm "-..
dia ---to 1/ f--I I . . . . . . .
~
~r5')'0 ,~? V-- r ~ ~ " . : . . ' :...D_:_-+=-!;2~,8~0
t--
k : : ; . ~ \ r _M;;d;grc~~md •. < E
I:-- ",Clays, and coal D'...:....~
V F . 0 0 " b.. o_:" -T--" ,. ..D~~,80. . . . : : . _ _ : ; , _ ~? - - : - - - - . -Assume c
r--:' _ . . -----. --- =50kN/m2
~ 1°./, -===-Firm to stiff brown:==- . -=~~--=- 'V..· .' _clays C ' : 1-- r-.. ~ - __ ... -~----.
- -= V ..- --'., -==---=~--L5.10
_--...._ I/o, ====Weathered grey ..__ - - = = - - ~S;~~/~2
C / ~ - . _~:_~~ud_~t~n~ .. ' . E
~,i;;,---'- " : - + . . .~~-I. ' I>.~N =49 blows 0,2.5
-_.......___J
'YW= 18 kNlm3
E
Made groundcolliery shale, bricks
clay, timber etc.(N values = 4)
8CD
N
EoIi)
< D
~,50
Fig. 2.30 Example 2.4: ground conditions at borehole 1 .
«' ~n450mm dia.
'J'
1l
Made ground. ,
ash, claysE
'Yw= 18 kN/m30
'"C\l
, ,0 o r - : .
J •.
s:0.c~III
£c0
c0
~
E~c
0 ~Ii) III
.,; CD>iOlCI l
Z
o() .
d '
.0
-7.2
Eo
' C ' : 1
-8.5
/N=152
f
52
Fig. 2.31 Example 2.4: ground conditions at borehole 2.
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loads are
h to which
no access
) is to be
o access is
.
d is zero.d load is
2 for a 3 0°
are taken
ve single
corridors,
kN/m2.
which they
Assessment of wall loading
30 4010 20 5 0
o
A . . . . . . .~ i = 1 l. .. i . i l i a 4 . : _ ~ · · : " . ·.! , . - ·. _ · . .• · _ t · · = _ · · . · . · . ; ; L ~.~ · · , · · ~ = : t ? d _ : . · _ T : · . .L · · . . . _ · L ; . • • . .· · · _ · · · ·. . . . . . ; . . .N!:":::~~"'_;_""i.-...I'''''''i'''''' .. L;- ;. i .
-2 ,-r- ;E· .. · · · . L , · · · ·.· - i . . · . . . . I ; ?~.
i~! - · ~ · : - · · ·· . · · : · - - : · :. ~ ' . · · · · - -. . .: · · ; ·- · · · -· · · · + :r : _ : . . : _ : : : : ~ ( _ : : : : ~ J ~ · · ~ : _ : : : : ~ : : I _. . . _ . . . + · I _ _ + . · - - · ~ · · · · ~ ·.. . - - · · · ~ · · ! - - · r · - · . .· - ~ : ~ _ ~ · i - - · ·.· . - . . . . . . .-.~.-..--+-~ ..
~ . , ..·1· .. + · + · + · · . 1 . . · · . . . . . . . I '~ . .. .. ! l· ....'.·.·....· ·. . -!- ..· · · -! - -! - - - ! - .
..... ··1 ..·.·.·...... .: 1 ' . . 1 · . ; . . . .
.+-~~--~-+~r-+--+~~+--r~~+-~~--+-~~~+-~-7--~~~--~;
. . .. · .. . . .. . . .. . . + ' : . .. : : : i ! : : , .. ! ! : . . : : : · ~ : : : : : : : : : r · : : : : : r :: : : : " r :: : : .: : : : . : : : : : : : . . . . . : . . . : · · · · I : : r . . . . . . ! · . . ..... .. -!- ·..·' ·I j j + + . . . . -! ' , . ' '.. " r : "j .
-8 ~ __~_. __~ __._~ __~j~~i __~' '~-L__~ __~~ __~~L-_. __~ __~~ __~ __~~ __~~ ___,
Load (tonnes)
Fig. 2.33 Load-sett lement plot.
;.- , .····-r·
.. ~.:::~::.'"
.... ; ..····1·······; ,.,.
' . : ! : "
r ' : ' "
1
2. . . 3
E 4g
5
E 6(I)
E 7
~ 8(J)
9
10Time (h)
Fig. 2.34 Graph of load and settlement versus time.
Roof Imposed loads
Dead loads(kN/m2)
0.75
0.25Factored 25° pitch snow load
Ceil ing access loads(kN/m2)
0.55
0.05
0 .23
0.15
0.02
1.00 x lAO = 1.40 kN/m2
Tiles
Battens and felt
Trusses
Plasterboard ceiling
Insulation
1.0 x 1.60 = 1.60 kN/m2
Total unfactored dead load + imposed load = 2.00 kN/m2
Total factored dead load + imposed load = 3.00 kN/m2otal =
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!,""
Pile loadings
First floor2.4x4.0 4.80 7.32 Ground floor
2.4 x 4.04.80 7.32
2.0 2.0
shown in 2.4 x4.0 4.80 7.32 Walls 1.75 x 2.6Ground floor2.0
4.55 6.37
Walls 3.75 x 5.5 20.62 28.86Wall (F) total 14.15 21.01
Wall (Ct) total = 36.82 53.40 WallGFactored
(kN/m)Wall C2
100 mm blockwork = 1.75 x 2.6 = 4.55 x 1.4 = 6.37
1.50
7.3236.82 - (2 x 4.80) = 27.22 38.70
Wall H (as wall F)
Wa/lD=14.95x 1.4 =21.01
7.32
Unfactored FactoredThe calculated wall line loads are summarized in Fig. 2.36.
34.125 (kN/m) (kN/m)The pile layout is shown in Fig. 2.37.
Roof: nominal 0.5 m width =0.5 x2.0 1.00 1.50Ground beams: 600 mm x 400 mm wide
50.265 Self-weight 5.76 kN/mWalls = 3.75 x 6.5 24.375 34.125
Tie beams: 300 rom x 300 mm wideWall (D) total 25.375 35.625 Self-weight 2.16 kN/m
FactoredWallE
(kN/m) PILE LOADINGS
9.90 Unfactored Factored
2.4 x2.0(kN/m) (kN/m) Pile 1
3.66 First floor 2.40 3.662.0 Unfactored Factored
3.66 First floor 2.4 x4.0 4.80 7.32 (kN) (kN)2.0 34.975 x 2.875 50.27
50.625 x 2.87572.25
28.86 Ground floor2.4 x 2.0
2.40 3.662.0 2.0
2.0 27.22 x 4.175 56.8238.70 x 4.175
46.08 2.4 x 4.0 2.080.78
Ground floor 4.80 7.322.0
2.0 5.76 x (4.175 + 2.8751 20.30 8 x 3.525 28.20
Wall: 100mm blockwork = 1.75 x 2.6 4.55 6.372.0
38.70--
Total 127.39 = 181.23Wall (E) total 18.95 28.33
WallF Pile 2
Factored Unfactored Factored(kN/m)
Unfactored Factored
9.90(kN/m) (kN/m)
(kN) (kN)
First t\jpr2.4 x4.0 4.80 7.32 34.975 x
2.875= 50.27 50.265 x 2.875 = 72.25= =
2.0 2.0 2.0_. l~ ~ _.0 61 32.02 (46.08) 6227.22 (38.70)
T:c
I._.~C o(J1
~ »c. >
E 18.95 (28.33) ~ ~C o
_. 0>~
_. (J1
G 4.55 (6.37) A
~.(J1 I\) 3
0I\)
I~Oi
(J1
i : . > 2 10
~~ !> C!l(J1 0-
,EJw I~<0(J1
(J1
~J
I\)
~
I1
N IC1 36.82 (53.40) T e o C!l C2 27.22 (38.70) Co)
I
Fig. 2.36 Example 2.5: wall line loads (kN/m). Factored lineloads sho~n in brackets.
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75 = 30.20
0 - 14.045 -
=239.07
Factored(leN)
13.29
16.70
30.20
= 36.50
25.40
43.62=
= 165.71
Factored
(leN)
80.78
5 =84.77
= 24.58 21.01 x 3.475= 36.502.0
4 15X 3.475I. 2.0
x(4.175 + 3.175 + 3.475) 31.175.76 2.0
8 x 5,41 = 43.28
=171.02otal =245.33
Pile 7
Unfactored Factored
(kN) (kN)
32.02 x 3.175 50.83 46.08 x3.175 =
73.152.0 2.0
32.02 x .l:2_ 48.03 46.08 x3.0
= 69.122.0 2.0
5.76x (3+3.175)
17.78 8 x 3.0875= 24.702.0
Total 116.64 = 166.97
PileS
Unfactored Factored(leN) (kN)
18.953.175
30.08 28.33 x3.175
44.972.0 2.0
18.953.0
28.42 28.33 x__lQ__
42.502.0 2.0
5.766.175
17.78 8 x6.175
24.70=2.0 2.0
2.164.175
4.50 3 x4.175
6.26--2.0 2.0
Total 80.78 118.43
Pile t j I J
Unfactored Factored
(kN) (kN)
36.826.175
113.68 53,406.175
164.87x2.0 2.0
5.766.175
17.78 86.175
24.70-- x --2.0 2.0
2.164.175
4.50 34.175
6.26x2.0 2.0
Total 135.96 195.83
Pile 10
3.0
2.0
2.175
2.0
5.76 x (3 + 2.175)
2.0
Unfactored
(kN)
48.03 46.08 x
Factored
(kN)
69.122.02 x3.0
2.0
35.625 x 2.175 38.745.375 x 27.592.0
8 x 2.584.90 20.70
Total 90.52 128.56
Ground beam analysis
Pile 11
Unfactored Factored
(leN) (kN)
25.375 x(2.175 +4.175)
80.56 35.625 x 3.175 = 113.12.0
18.95 x3.0
28.425 28.33 x3.0
42.502.0 2.0
5.76 x(2.175+4.175+3.0) =
26.92 8.0 x 4.67 = 37.362.0
Total = 135.90 192.96
Pile 12
Unfactored Factored
(kN) (kN)
25.375 x4.175
52.97 35.625 x4.175
74.362.0 2.0
36.823.0
55.23 53.403.0
80.10x2.0 2.0
5.76 x(4.175 + 3) =
20.66 8 x 3.58 = 28.702.0
Total = 128.86 = 183.1
The calculated pile loadings are summarized in Table 2.4.
Maximum unfactored pile loads have not taken into account th
additional load due to elastic shears. If these are to be considered
then it is appropriate to multiply the working loads by 1.25:
Maximum working load based on pile 6 = 171.02 x 1.25
. = 213.77kN
Use 165mm diameter steel tube piles driven to a predetermined se
to give a maximum working load of 225 kN.
A ll piles to be subjected to a re-strike and one random pile to b
load-tested using kentledge. The test load applied to b e 1.50 x 225
= 337.50kN.
GROUND BEAM ANALYSIS
This analysis assumes the ground beams are continuous and
designed to cater for bending moments top and bottom of wf2 / 10 .
Some engineers use a simple supported design philosophy with
the use of anti-crack reinforcement over the pile supports. This i
very conservative for the bottom steel but problems of service-
ability cracking over the supports may result which could affect the
durability of the concrete beams.
On sites where clay heave is unlikely the ground beams can be
cast against the earth face using 75 mm cover to the links or the
beams can be poured in shutter moulds (BS 8110 Clause 3.3.1.4)
A ll design in accordance with BS 8110 Part I (1985).
C 30 P mix feu = 30 N/mm2
High-yield bars fy = 460 N/mm2
b=400mm, h=600mm
Figure 2.38 shows a typical beam section.
From BS 8110 Table 6.1, Minimum cement content to be 370 k g /m?
with a water/cement ratio of 0.45. This gives Class 2 sulphate
protection (concrete exposed to sulphate attack).
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Foundation design
As = area of steel in mrn?
d = effective depth of beam =600 - 40 - 8 - 1 { ! 1 2 = 552 - 12 =
540mm
z is limited to O.95d
Design ultimate moment, M = As x 0.87 xfy x Z
= 380.20 As d x IQ 6 kNm
z is less than O.95d
From BS 8110 Clause 3.4.4.4 :
Therefore
(.:. - 0.5)2 = 0.25 _ ~d 0.9
z2 z k
df"-d=- 0.9
Table 2.4. Calculated pile loadings for Example 2.5
Pile Service load
number (kN)
•127.39
2 150.84
3 139.58
4 166.97
5 114.39
6 171.02
7 116.64
8 80.78
9 135.96
10 90.52
II 135.90
12 128.86
Ultimate load
(kN)
181.23
214.95
198.71
230.07
165.71
245.33
166.97
118.43
195.83
128.56
192.96
183.16
D -EE E0 EL() 0
0CD
-.
\ . 400mm
Fig. 2.38 Example 2.5: typical beam section.
60
SubstitutingM M
0.S7fAforzand -?- fork:
bd-feu
M -M
0.87fyAsd 0.9bd2feu0.87fAd)2
Therefore
M
(0.87fA4 0.87fAd 0.9bd2feu
Therefore
0.9bfeu
Whenfy =460,[eu = 30 and h = 400, then
(0.87 x 460 , s ) 2M =0.87x460x As d '':i
. 400x30xO.9
Ths: M 400A,d -14.83A,l
ere, ore = 6 kN m10
This formula only applies when z is less than 0.95d.
Maximum span
Deflection criteria are as follows.
Clause 3.4.6.3 Table 3.10:
Span = 20 simply supportedEffective depth
Clause 3.4.6.5 Table 3.11: Modification factor for tension s
equal to
0.55+ 477-5fy/8
120(0.9+Mlbd2)
but no greater than 2.0.
Therefore
Maximum span = (0.55 + 1.57 2 )20d0.9+MI400d
Shear reinforcement
. (I00A Ibvdt\400ld)1/4Design concrete shear st ress, ve = 0.79-'--.::.'---'--'---....::.....-
'Y m
where 'Y m =1.25.
(0.25A )1/3(400)1/
4
v =0.63 --' - N/mm2e d d
Clause 3.4.5.2 Table 3.8:
Minimum shear steel using TS links
A = .0.4b,.s,.sv 0.87!.",
where by = 400 mm,[yy = 460 N/mm2, Asv = 100.5 mm-. Theref
100.50 x 0.S7 x 460 2s; = = 50 mm
0.4 x 400
Minimum links T8 at 250 mm centres.
With minimum links v = Ve+ 0.4 with v =V/byd. Therefore
V=vb d'= (vc+0.4)400dv 103
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steel is
(50.625 + 8.0)t ' I [ 1 1 1 1 1 1 1 1 1 [ 1 1 1 1 1 1 1 1 1 1 1 1 \
2.875 m 1 3.475 m123
Beam 1-3
(46.08+ 8.0)
Loadings
f 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
10 3.00 m f7 3.175 m
fllllllllllllllllllllf [
Beam1-4-7-10
(53.40+ 8.0)
Beam3-6-9-12
(35.625 + 8.0)
1 t , [
(38.70+ 8.0)
I I I I I I I I4.175m
(38.70+ 8.0)
I I I III [ II I I I III I I I I f2.175m f11 4.175m12
Beam10-11-12
(28.33+ 8.0)
f i I II II1 I II f II II I I Illl11 3.0 m 8 3.175 m (4,5)
Beam 11-8-{4,5)
57.76 (21.01+ 8.0)
4 P I I I I II 7 0 :5
1 1 1 1 I I I I I I f2.875 m 3475 m 6• •••
Beam 4-5-6
Fig.2.39 Example 2.5: beam-loading diagrams.
Shear steel T8 links
A = bvsv(v-vJ
sv 0. 87 fy v
v = A sv x O. 87 fy v +v c
bv sv
Therefore
V A S Y xO .87fyvd vb d-----;;-'--+ -..£.._L_ kNSy X103 103
s, V
100 D Ad + O .4ved
125 D Ad + O Aved
150 D Ad + OAved
175 0.23d + OAved
200 0.20d + OAved
225 O. I S d+OAved
Beam design
Beam 1-3 (Fig. 2.39(a))
Maximum ultimate moment = 5S.265x 304752
70.358 kNm10
Beam 1-4-7-10 (Fig. 2.39(b))
M· 1· 46.7x4.175
2
ax imurn u nmate moment = SIAO kNm10
Beam 3-6-9-12 (Fig. 2.39(c))
. I· . 46.7 x 4.1752
Maximum u timate moment = 81040 kNm10
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Foundation design
Ventilated void
'." .... ~. c" ,,· 50mmconcrete
r=-----.,..., '_":":"~~-__:.----~-----; oversite on1200 gauge polythenedpm
1+----40 mm min.cover to l inks
, . ."
2T16 '.
T10300 mm
centres
EE0
.. ',~.
0<0
~.
i -2T16 .1:.
.
~40mm cover
to l inks
400
I+----+-f----Reinforcement frompile
Fig. 2.40 Example 2.5: ground beam detail.
Beam 10-11-12 (Fig. 2.39(d))
M· I' 43.625x4.l75
27
aximum u trrnate moment = = 6.04 kN m10
Beam 11--8-(4, 5) (Fig. 2.39(e))
-M I' 36.33 x 3.1752
aximum u tim ate moment = 36.62 kN m10
Beam 4-5-6 (Fig. 2.39(f))
R. = 29.01 x 2.875 = 41.70 kN2
57.67 x _Q2_ =14.04 kN2.875
R. = 55.74kN
Rb = 41.70+(57.67 x 2.175)=85.32 kN2.875
1922Moment 4 - 5 = 55.74 x 1.92 - 29.01 x -._ = 53.55 kN m
2
Maximum shear = 101.23 kN
Using C30 N/mm2 concrete with 40 mm cover and Design Chart
No. 1 (Fig. 2.11):
k = __!!__ = 81.40 X 106
0.023
bd2fcu 400 x 540
2x30
Therefore lever arm factor = 0.95
A.=~= 81.40xl06
=397mm2
0.87 fy z 0.87 x 460 x 0.95 x 540
62
75mmprojection
t
Concrete or steeltube pile filledwith concrete
Use two TI6 mm bars top and bottom in all beams.
. 101.23xl03
Ma x im um shear stress = 0.468 N/mm2400x540
Vc = 0.36 N/mm2; provide minimum links throughout beams.
A = 0.40 x 400 x 300 =119 mrn?sv 0.87 x 460
Use T1 0 links at 300 mm centres throughout.
Example 2.6
Poor ground conditions on part of a housing site require
plots to be built off piled foundations. Maximum pile load
approximately 450 kN. Piles to be 200 mm square precast co
driven to a predetermined set and this set to be checked
striking the piles. Minimum factor of safety on piles to be 2.2
piles will be driven with a 4 tonne hydraulic hammer (Banut
with a 400 mm drop (Table 2.5).
Using the modified Hiley formula:
R = Eu S+C/2.0
Where R; = total load (450 kN) x factor of safety (21012.50kN;
E = transfer energy at top of pile = 0.85 x 104 kNmm;
C = temporary compression of pile and ground per blow (a
lOmm);
S = set per blow.
Therefore:
E C 8500 10S=---=-----=3.39 mm/blow
Ru . .2.0 1012.50 2
Therefore set for 10 blows = 34 mm or less.
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several
ading is
concrete
by re-
2.25. The
nut type)
(2.25)
(assume
Pile loading
Table 2.5. Hammer transfer energy table. Rig type: hydraulic
hammer (Banut type)
Suspended in-situ concrete ground floor
200mmslab 4.70
Stud partitions 0.50 1.50
Total 5.20
External walls
Unfactored dead loads
(kN/m2)
102.5 mm brickwork 2.25
100 mm blockwork 1.25
Plaster 0.25
Total 3.75
Hammer weight
(tonnes)
Transfer energy
(tonne m)
Hammer drop
(mm)
300 400 500 600 700
0.25 0.35 0.450.55 0.70 0.90
0.85 1.10
1.05 1.40
1.503.0
4.0
5.0
LOADINGS
Floor plan is shown in Fig. 2.41.
RoofParty walls
215 mm brickwork 4.50
Plaster two sides 0.50
Unfactored Total 5.00
imposed loads
(kN/m2)
Walls A and D
Dead load Imposed load0.75 (kN/m) (kN/m)0.25
Roof and ceil ing1.00 x 8.50
4.251.0 x 8.50
= 4.21.00 2.0 2.0
First floor nominal 0.75 x 1.0 0.75 1.50 x 1.0 = 1.5
30° pitch
Unfactored dead loads
Concrete tiles
Battens and felt
Trusses
Insulation
Ceiling: plasterboard
(kN/m2)
0.55
0.05
0.23
0.02
0.15
Total 1.00
First floor
Tongued and grooved boarding 0.10
Plasterboard and skim 0.15
Stud partitions 0.50
Ground floor 5.20 x 5.0 x 0.33 = 8.58 1.50 x 5 x 0.33= 2.50
3.75 x 5.0 = 18.75
= 10.00
1.5 Wall
. . Total 0.75 Underbuild and ground beam
Total
II "
5.0m
" I WallB
A------1- Roof and ceil ing 1.00 x 0.60
I0.75 x 5.0I First floor
2.0
+~ 1 II Ground floor 5.20x2:Q
B C '0 E 2.0
I 0010a: ~
I Wall 3.75 x 6.0
First f loor span I Underbuild and ground beam
Ground floor
I Total
___ J.
42.33 8.25
0.63 1.0 xO.6pO = 0.60
1.50 1.50 x 5.0 = 3.752.0
= 13.00 1.50 x 5.0 = 3.752.0
= 22.50
= 10.00
47.97 8.10
oWall C (party wall)
a : . Party wail
IFig. 2.41 Example 2.6: floor plans.
Roof and ceiling 1.00 xO.6 x2
0.75x5.Ox22n.0
1.20 1.0 x 0.6 x 2 = 1.20
3.75 1.50 x 5.0 x 2 =7.502.0
First floor
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Foundation design
5.20 x 5.0 x 2 = 24.50 1.50 x 5.0 x 2 =7.502.0 2.0
Ground floor
Wall 5.00 x 6.0 = 30.00
Underbuild and ground beam
Total
= 10.00
70.95 16.20
PILE LOADINGS
Pile I
(42.33+ 8.25)x 5.0 +(47.97+8.25) x 4.252.0 2.0
=245.60 kN
Pile 2
( 8 25)5.0 x 2.0 ( ) 4.25
42.33+. x---+ 70.95+16.20 x- =438.00 kN2.0 2.0
Pile 3
(47.97 + 8.10) x 4.25 = 238.30kN
Pile 4
(70.95 + 16.20) x 4.25 = 370.38kN
Pile 5
As Pile I = 245.60kN
Pile 6
As Pile 2 = 438.00kN
BEAMS
jearn moments
BeamsAandD
42.33 x lAO x 5.02
(kNm)
Ultimate moment 148.15105.02
33.00.25 x 1.60 x-10
Total 181.15
BeamB
47.97 x lAO x 4.252
(kNm)
Ultimate moment 121.3010
8.10 x 1.60 x 4.252
2304110
Total 144.71
BeamC
70.95 x lAO x 4.252
(kNm)
Ultimate moment 17904110
64
16.20 x 1.60 x 4.252
10
Total
Beam shears
Beams A andD
Ultimate shear 42.33 x lA O x~2
8.25 x 1.60 x ~2
Total
BeamB
Ultimate shear 47.97 x lAO x 4.252
8.10 x 1.60 x 4.252
Total
BeamC
4
22
14
1
(
1
1
Ultimate shear = (70.95 x lA O + 16.20 x 1.60)x 4.25 = 266.152
Beams A and D
Ultimate moment = 181.15 kNm Ultimate shear = 181.15 k
k=.....!!._= 181.15xl06
0.119bd 2 fe u 400 X 3902 x 25
Therefore lever arm factor =0.83
18U5 x 106A. - 1398 mm '- 0.87x460x390xO.83
Use five T20 mm bars (1571 mm-)
Fig. 2.42 Example 2.6: pile layout.
EL)
C\I
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Beams A and D
2Tl0
I -
1.2Sm _ I e 1.2Sm
- I
2Tl0
1 i 14T2O
1.,
T8-200 J T8-2S0 I T8-1200 I T8-2S0
I I I I I
tT20
I14T20I
~ 4.2Sm ~::; 4.2Sm
IBeamB
2Tl0
Ell)
C\J
-.i
46.818
226.23
(kN)
148.155
33.00
181.15
(kN)
142.71
27.54
170.25
.15 kN
Beam
18U5x103Shear stress v = 1.16 N/mm2
400 x 390' tW~·'~1~ r '3 ; r - ' P4+ .---.-+~ ~(J) . ~
~ o.Z. t:.
I
- + - ( " . , . , ~ -1--
looA, = 100 X 1571
b,d 400 x 390
Therefore v, = 0.64
1.0
Because (v c + OA) is less than v provide nominallinks to beam
Using T8 links (Table 2.6) :
s = loo.50xO.87x460 =251 mm
, O AO x 400
Provide T8 links at 250 mm centres for full length of beam.
Fig. 2.43 Example 2.6: line loads (kN/m).
2T101.50 m I 1.50 m
I ! 5T20 I_2T10
TS-200 TS-250 \ TS-200 TB~ 200. J TS - 250
I 1.0 m .I .1 _ I 1.0m 1.0 m 'I I
I I
. ,
I ~ . . . I
l d ST20 ~ ~ 5T2DP P
. .p5.00m 5.DOm
BeamC
Fig. 2.44 Example 2.6: reinforcement details.
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m from
length of
tion of
on Soil
ing soils
bearing
31.
erials,
1: Unre-
de of
andards
Code of
arts for
4th
r Site.
f!
I!i
I
t
3.1 INTRODUCTION
Foundations transmit the total load from a building on to the
ground by direct contact pressure. The foundation must dis-
tribute the building loads in such a way that it ensures that
the bearing stratum is not overstressed and that total settle-
ments are within acceptable limits. The foundation designer
therefore needs to have some knowledge of the type of strata
present below the foundations.
Site investigations seldom reveal an allowable bearing
capacity in simple terms. This is because the various strata
are composed of many different soil types, each having
different properties.
Cohesive soils are subjected to plastic deformation when
they are loaded. If the pressures applied to such soils are
just sufficient to cause shear failure, this pressure is de-
scribed as the ultimate bearing capacity of the soil. By
applying a factor of safety to this value, we have a reducedbearing pressure which is referred to as the safe bearing
capacity.
The allowable bearing pressure is the maximum net
loading that a soil can sustain, taking into account the safe
bearing pressure and the magnitude of settlement that the
building can accommodate safely.
The net bearing pressure is the difference between the
actual pressure below the foundation and the pressure from
the removed overburden. This principle is often adopted
when designing buoyant foundations ..
3.2 SETTLEMENTS IN COHESIVE SOILS
Soil conditions can change considerably from before to
during and following construction of foundations. Most
cases of excessive settlement arise because of unforeseen soil
conditions which suddenly arise. It is therefore useful to
examine the types of ground movement mechanism which
are potential causes of settlement in cohesive soils.
(a) Consolidation settlements. In cohesive soils which are
saturated, the effect of loading the soils is to squeeze out
1
Chapter 3
Foundations in
cohesive soils
some of the porewater. This is called consolidation. A
change of loading is required for this consolidation t
take place and it may take several years before it finishe
settling. The most susceptible strata are the normally
consolidated clays and silts, and organic clays such a
soft alluvium and clayey peats.
(b) Moisture movements. Some types of clay show
marked volumetric change as their moisture content i
changed. Clays which fall into this category are referred
to as shrinkable or expansive clays. They are usually
found in southern and eastern counties of the UK, bu
they can occur in other areas in localized pockets.
(c) Effects of trees and vegetation. A major factor which
can affect cohesive soils with medium-to-high plasticity
is the effects of trees and vegetation. The tree-roo
system abstracts water from the clays, resulting in
surface subsidence. If trees and vegetation are removed
the clays are allowed to rehydrate with the result thaswelling takes place. This subject is dealt with in more
detail in Chapter 6.
(d) Groundwater lowering. Clays containing a high wate
table can be affected if this water table level is drawn
down, by pumping for example. First, the resulting
reduction in moisture will cause the clay to shrink and
settle and, second, the weight of the overburden wil
increase as the soils are no longer buoyant. With sof
clays and peat strata this could result in further
consolidation stresses occurring because of the increase
in effective stresses.
(e) Temperature changes. Frost can cause severe ground
heave in sustained low-temperature conditions. Mossilts, fine sands and chalks are frost-susceptible. Grea
care must be taken when designing foundations for cold
storage buildings.
(f) Lateral displacement. This is often caused by deep
trenches being excavated parallel and too close to
existing foundations. In effect the clays are subjected to
a shear-slip type of failure similar to that experienced on
sloping sites.
(g) Mining subsidence. Settlement can occur at the surface
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a bear-
the dif-
e limits.
ia may
ulated
soils
on case,
design
can be
ere the
e shear
normal
internal
ground
r a soil
where c = undrained cohesion,
N « =bearing capacity factor,
p = = total overburden pressures at foundation level.
When ~ = 0, N, = 2 + 1t = 5.14.In 1943 Terzaghi produced an equation for q« which
allowed for the effects of cohesion and friction under the
foundation base; this was applicable to shallow foundations,
i.e. where zl B is less than 1.0. For a strip footing Terzaghi's
equation isqu = c N; + 'Y z N q + 0.50 'Y B Nl
The values of Nc, Nq and Nl for various values of ¢can be
obtained from Fig. 3.1. The value of Nc increases to 5.70 for
a surface foundation due to the frictional allowance.
Therefore
quit = 5.70 C + 'Y z
The coefficient N q allows for the surcharge effects arising
from the overburden, and Ny allows for the size of the
foundation, B. When ~ = 0, Ny = 0, N; = 5.70 and Nq = 1.0.Skempton showed that N; increases with foundation depth
increase for ¢ = 0 soils and these values can be obtained
from Table 3.1.
Table 3.1. Nc values for depth factor in soils with ~ = 0 (after
Skempton)
Foundation Depth foundation width ratio, Z/B
type
0 0.50 1.0 2.0 4.0
Circle or square 6.2 7.1 7.7 8040 9.0
Strip footing 5.1 5.9 6.40 7.0 7.50
Using these coefficients, the ultimate net bearing capacity
of a strip footing is given byqn u = C Nc + p o (N q - 1 ) + 0. 5 'Y B Nl
For a square or circular foundation
qn u = 1.2 c N; + Po (N - 1) + 00 4 'Y B Ny
where y = the bulk density of the soil below the foundation,
c = the undrained shear strength of the soil,P o = the effective overburden pressure at foundation level,and B = the foundation width (or diameter).The ultimate bearing capacity Puis given by
Pu=Pnu +p
where p = the total overburden pressure at the foundation
level. If the water table is at or above the foundation levelthen the value for the density must be the submerged density.
When calculating p o, a similar density must be used when
the water table is at or above the foundation level.
Meyerhof (1952) modified the Terzaghi equations to make
allowance for the foundation shape, depth and roughness of
the base.
These values for N c, Nq and Ny are shown in Fig. 3.2.
When using Meyerhof values a shape factor ')..must be
applied; this can be obtained from Fig. 3.3. As for the
Consolidation settlemen
104
-Values of Nc -- "-ValuesofNq ---
. I) Ii~
(I
r/.
VIW /
V ~ A
/ /J / ./
/."/ 'it.V/? - -
.;~
~,.L
ff. V
~'./
~
Deep foundation(0)58)
Shallowfoundatio(0=8);E
"0C
to 10 3
;l
~o-0. l ! !
~ 10 2
li l0.tooOJ
.E 10to
~ 86
4
2
10
Surfacefoundation
(a )10 300 50 60
Angle of shearing resistance, 4l
104 h (
V,3 il
fI 1
/'I I
1 # V/1 1/
II/! J J
v/
{I I
Deep foundatio
0>58)
10 Surfaceoundation
;[
.9 10 2~~'0
§o 10
g > 8'm 6OJ 4
2
1. 00.80.6
0.3
0. 2
0.11 10 20 30 40 50
Angle of shearing resistance, 4l(b )
Fig. 3.2 (a) Values of N « andN q for strip footings; (b) values ofN
for strip footings (after Meyerhof, 1952).
Terzaghi formula, the submerged density must be used if th
water table is at or above the foundation level. For pur
cohesive soils, i.e , ~= 0, N q = 0 and Ny = O.Therefore valueof N; canbetaken from Fig. 3.4.
Using the Skempton formula, the allowable bearing
capacity qa is given by
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4.0
Fig. 3.4 N, values for strip footings on soils with ~ = 0 (Meyerhof,
1952).
Foundations in cohesive soils
oShape factor. A
1 2 3
~~1~~~ \~\~ ~~-+--t---l
2~~--+-~~~~~~--~
~ 3 ~ + - - + - - + - t I - l - t t l + - l - H - I \ . . . . . . . . - t - - - t~ \ \ \ \e 4~+-~~-;;-~H-\~
~ 5~~--+--H~-'~~~--~
~ 6~~--+--H-r-a~~~~~s:
a .~ 7~~--+--H-+-H-r~-i+-~
9~~--+--H~-H-+~~~~
10
.p = 45° 20 10 5 2 1uriedfootings
.p = 40° 10 5 2 1 0
.p =35° 5 0 (circle)
.p =30° 1
Length/width ratios ofrectangular foundations
Fig. 3.3 Values of shape factor Afor strip footings (afterMeyerhof, 1952).
5
Bearing capacity factor, Nc
6 7 8-_~~
\
r -1\
Theoretical \Experimental --- \.
~ 1.0
ao
~~ 2.0"0
~a~ 3.0
o
9
where Nc = bearing capacity factor, Cu = average undrainedshear strength of soils below the foundation, Po = vertical
pressure applied to the soil at footing level, F = factor of
safety (usually a minimum of 3.0). Therefore
qne t=Nc Cu
F
For a strip footing, Nc = 6.50 for zlB = 0.75/0.60 = 1.25;
therefore
6.50Cu 215 CqUl t=~=. u
Since the bearing capacity factor N; is never less than 5-{)
then a reasonable net allowable bearing pressure for shallow
70
strip footings can be obtained by using the undrained shea
strength Cu x 2.0.
3.3.2 Vertical stress distribution
When a foundation load is applied to a soil a pressure bulb i
generated. The stress on the ground decreases with depth and
by using graphs the values of vertical pressure can be
obtained from Table 3.2.
Table 3.2. Vertical pressure factors
BIZ Factor for Factor for vertical
shear stress pressure
0 0 0
0.1 0.032 0.065
0.2 0.063 0.127
0.5 0.15 strip 0.30
0.6 footings 0.358
0.8 0.22 0.46
1.0 0.25 0.55
1.50 0.30 0.71
2.0 0.32 0.82
2.2 0.31 0.88
3.0 0.29 0.92
3.50 0.27 0.94
4.0 0.25 0.96
4.50 0.23 0.97
5.0 0.22 0.978
105.50 0.20 0.985
6.0 0.19 0.988
6.50 0.18 0.99
7.0 0.17 0.991
8.0 0.15 0.994
9.0 0.13 0.996
10.0 0.12 0.997
100.0 0.01 1 .0
q =contractpressure;B =foundationwidth;Z=depthof soilelementbelowfoundationbase.Shearstress=q x shearfactorVerticalpressure=q x verticalpressurefactor
3.3.3 Construction problems on clay sites
Some clay soils are very variable. They often contain water-
bearing lenses of sands, gravels and silts as a result of past
glaciation. When these are encountered in an excavation,
many building inspectors ask the groundworks foreman to
excavate deeper in the hope of finding clays at a lower level
Quite often, excavating deeper can lead to a costly founda-tion. If the clays are not encountered within a reasonable
distance and the sands are water-bearing, or contain perched
water, the sides of the trench will collapse, and a large, sof
'h{)lewiIl result. The only solution left is to pump out the
excavation and fill it with mass concrete to within 900 mm of
the ground level.
It will be then be possible to compact granular fill in
discrete layers over the mass concrete and provide a raft
foundation or wide reinforced stiff ground beam.
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shear
bulb is
th and
can be
water-
past
an to
und a-
nable
soft
ut the
fill in
a raft
To avoid this situation occurring on site, always excavate
a trial hole about 3 m away from the foundation trench to
determine whether good clays are present at shallow depth
below the sand. If they are not present at a depth of 1.2 m
then consideration should be given to using either a raft
foundation or a wide reinforced stiff ground beam.
When excavating for deep trench-fill foundations, pre-
cautions should be taken to prevent collapse of the trench
sides due to heavy ingress of water from wet sandy lenses.Generally, such flows can be controlled by pumping from a
lower sump at one end of the excavations.
When constructing foundations in clays with medium to
high plasticities the base of the excavation should be con-
creted immediately following excavation to reduce the risk
of swelling from seepage or rainwater. Failure to protect the
base of the excavation can result in the clay's bearing capa-
city being reduced as the water makes it more compressible.
Trench excavations which are left unconcreted in dry
weather for long periods can be subject to swelling as the wet
concrete restores the moisture levels. If the soil's moisture
content increases, the soil will heave and upward movement
of the concrete will occur. It is therefore prudent on clay sites
to install as much as possible of the main drainage so that in
wet weather the working conditions are improved.
Clay sites can give rise to problems of slope stability,
especially if major cut-and-fill operations are taking place on
the site. The placing of fills on to clay slopes should be
avoided as the natural- slope drainage is blocked, and an
unstable slope could result.
3.3.4 Foundation designs on clay soils
Where firm or stiff clays overlie soft clays, soft silty
alluvium which reduce in strength with depth, then it is
recommended that a detailed trial pit or borehole
investigation be carried out and shear strengths obtained for
the various strata at depths likely to be affected by the
proposed foundation loads. In trial pits the use of a hand
shear vane tester with extended rods is recommended, as
trial pits in excess of 1.20 m should not be entered,
especially if soft ground or peats are suspected.
When using the shear vane it is most important to take a
range of readings at each level so that an average value can
be obtained. This is particularly important in the soft to very
soft clays. In such clays account may need to be taken of the
adhesion of the clay on the barrel of the vane tester and this
value can be determined by using the dummy vane and
deducting the amount obtained.Once these in-situ undrained shear values are obtained the
allowable bearing capacities can be determined from
Terzaghi's equations. These can then be compared with the
calculated stresses at foundation level and in the stressed levels
below the foundation base within the pressure bulb (Table 3.4).
A correction factor 11 can be used depending on the
plasticity index of the clays. These fall within the range
shown in Table 3.3. For a strip footing q. = 1.90SIl,
neglecting overburden pressures.
Consolidation settlement
Table 3.3. Correction factor, J l
Clay category Undrained shear strength J l(kN/m2)
Very stiff ,/-;_150"'\.r '110' e l l 1.0
Stiff 100-150 r;~ 0.90
Firm to stiff 75-100 0.75
Firm 50-75 0.65
Soft to firm 40-50 0.60
Soft 20-40 / 0.55Very soft <20 0.50
Table 3.4. Proposed allowable bearing values for clays (after
Terzaghi and Peck (1968»
Description N c qrn
of clay
Square Strip,
Very soft <2 < 13.50 <32 <24
Soft 2-4 13.50-27 32--M 24-48
Medium 4-8 27-54 64-128 48-96
Stiff 8-15 54-107 128-260 96-190
Very stiff 15-30 107-215 260-515 190-385Hard > 30 >215 > 515 >385
N=numberof blowsperfootin standard penetration.est; .c =cohesion(kN/m2);qm = propo s ed normalal lowable bear ing value ( l eN/m2);F = factorof saf e ty withrespectto ba s e failure
Example 3.1
f ' A v - - t ( : < t l~(IV'Vt,-y {
Strip footing on clay soil
A strip footing 1.00m wide isplaced at a depth below ground level
of 1.00 m. Vane test readings down to 3.0 m show that the clays are
firm down to 2.0 m with shear strengths of 60 kN/m2 changing to
30 kN/m2 from 2.0 m down. Determine the allowable bearing
capacity of the soils at foundation level, and check that the widthused is acceptable to carry a line load of 60 kN/m along its length.
Using Meyerhof values for Nc, Nq for I'l=0:
N « = 5.14, Nq = 1.0, Ny = 0
Therefore net ultimate bearing capacity = 5.14 Cll = 5.14 x 60 =308kN/m2
Using a factor of safety of 3.0:
Allowable bearing pressure =~~~= 103kN/m2
Actual pressure = l ~ g=60 kN/m2 < 103 OK.Check at 2.0 m depth:
Allowable bearing pressure = 5.1~~ 30 51.50 kN/m2
From Table 3.2, vert ical pressure factor for zlB = 1.0/1.0 = 1.0
equals 0.55.
Actual pressure =60 x 0.55 = 33 kN/m2, < 51.50.
The safe bearing capacity = Cll NJ3.0 + overburden pressures yz .
Therefore at 2.0 m, safe bearing capacity = 51.50 + 18 x 2 = 87.50kN/m2. Should' groundwater levels rise these pressures should be
halved. The width of the foundations is therefore satisfactory.
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kN/m2
h, total
ost irn-
settle-
of such
ess of
posed
tables
and
soil).
c, the
t
ading.
elow a
r each
of the
ly over
such
value
the in-
nsoli-
cor-
lue if
p:= J 1 Pood
Values of 1 1 are given in Table 3.3 for the different types of
clay.
Because of the variations which can occur in soils, settle-
ment calculations can only be considered an approximate
guide.
Differential settlements are generally taken as one half of
the total settlements calculated. Though this is only a rule of
thumb, it is adequate for simple structures on fairly uniform
strata.
For sites where the strata consist of soft uniform clays to
depths in excess of twice the foundation width, there are
quick approximate methods available for calculat ing the total
consolidation settlements. Consider a simple strip footing
loaded to give a contact pressure of p kN/m2 at the base of
the footing. The vertical normal stress beneath the centre of
the footing can be considered to be in the form of a tri-
angular dispersal as illustrated in Fig. 3.6.
f(
1.58
Fig. 3.6 Strip footing on uniform soils: vertical pressure
distribution.
1.678
Directly below the footing the contact pressure will be p,
which equals O'z . At a depth of 1.50 B below the footing, O' z
is approximately equal to 0.1 p. Therefore the maximum
depth of stressed soil is equal to
1.50B = 16 7 Bm0.90 .
For an average pressure O' z of 0.50 p the maximum total
settlement is given by
P o e d = 1.67 B x 0.50 p x m y = 0.835 my B mm
This is an approximation. On a soil of infinite thickness with
m y decreasing with the total normal vertical pressures, it
would not be realistic to consider settlements deeper than 2B
below the footings.
Consolidation settlement
Flexiblesquarefoundation
; ; - . 1
2.56 2.08 8
58'~---+----4-----L---~--~28 8 0
I
Fig. 3.7 Vertical pressure under a uniformly loaded square
foundation.
(b) Influence line/actors method
Where a footing is on an infinitely thick layer of soft com-
pressible stratum which has a constant strength it is possible
to calculate the total settlements using depth influence
factors.
As m; decreases with depth (as the vertical pressures
decrease) the settlements between ZI and Z2 (variable depths)
can be obtained by using the coefficients obtained from
Fig. 3.8:
Pood =m y B p (h-[I)
where IIand li are the influence factors for depths z , and Z2
respectively.
Example 3.2 Settlements on clay soil
A house foundation 600 mm wide supports a three-storey gable
wall . The applied line load equals 65 kN/m run. Vane test results
taken at various depths in the soils directly below the footing indi-
cate that there is a stiff desiccated clay crust for 1.0m below ground
level underlain by a soft silty clay alluvium. The values of the vane
shear strengths are 75 kN/m2 down to 1.0m depth and 30 kN/m2
down to a depth of 3.0 m below ground level . Determine the total
settlements under the gable foundation.
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Foundations in cohesive soils
o020 040
Influence factor, I
060 080 1 0 120 140
- - ~ r--r-- . _ _~
~ ~
r-;~ ~ <
\'\1 \ ' \ .~ ~ ~t,.,;,,~)
\ \\ -, <,r-,
~
\ r -
K \r- ~ -,
1\ II
tIJ- . . . .-
~'iii' '& \" E
I f \ \ ]
1.0
~~ 2.0Ql
o
3.0
4.0
Fig. 3.8 Influence line factors I at the centre of a foundation.
Stress applied at footing level:
65x103 = 108 kN/m2600
Consider the loading to be on an infinite footing length. The vertical
stress distribution graph is shown in Fig. 3.10. The values of pv are
obtained as follows:
!...= 0.25 = 0.41B 0.60
Therefore
Pv =0.75P
pv = 0.75p = 0.75 x 108= 81 kN/m2
!...= 0.75 =1.25 r, =0.50p=0.50x108=54kN/m2
B 0.60
!.. .= 1.25 = 2.08 P: = 0.30p = 0.30xl08 = 33 kN/m2B 0.60
!...= 1.75 =2.91 Pv =0.20p=0.20x108=21 kN/m2B 0.60
!...= 2.25 =3.75 Pv =0.14p=0.14xl08=15 kN/m2B 0.60
The stiff clay has an undrained shear strength of 75 kN/m2.
Therefore an approximate value of Evl (the modulus of
compressibility, = lImv) is given by
Ev l = 130Cu = 130 x 75 = 9750kN/m2
The soft clay has an undrained shear strength of 30kN/m2• Therefore
Evl = 130 Cu = 130 x 30 = 3900 kN/m2
The total settlement is equal to the area of the pressure diagram in
Fig. 3.10. For an approximation take a triangular distribution. Then
74
108+81 0.25xl03Cil=--2- x-
9-7-5-0-
81+54 0.50X!03
Ci 2 =-2-x 3900
54 + 33 0.50 x !O 3Ci 3 =--2-x 3900
33+21 0.50xI03
Ci4 =-2-x 3900
21+15 0.50x103
Ci5 =-2-x 3900
=2.42 mm
=8.65mm
=5.57mm
=3.46 mm
=2.30 mm
Total settlement =22.40 mm
This is less than 25 mm and is considered acceptable. A corr
factor J 1 could be applied where J 1 = 0.55-{).6 which would re
a settlement of about 14mm.
Using the quick approximate method:
P =0835B =0.835xO.60xIXI08X103
=138~ . ~P ~oo .7mm
3.4 MOISTURE MOVEMENTS
One of the commonest problems with cohesive soils
effect of the soil's drying out as a result of extreme
weather or from moisture abstraction by roots of large tThe slow volume changes which occur when mo
evaporates from a clay soil can be predicted by assumin
lower limit of the soil's moisture content to be the shrin
limit. Desiccation beyond this value will not bring abou
further reduction in volume.
Many clay soils in the UK, especially in the southe
England, possess a large potential for slow volumetric ch
However, the mild damp climate which generally pre
means that any significant deficits in soil moisture co
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ult in
s is the
e dry
trees.
ing the
rinkage
out any
ast of
change.
content
Moisture movements
qlQ
o 1 02 03 04 05 06 07 0.8 0.9 10Or-~--~--~~--'---r-~--~~~~~--.-~~~--'---r--T--.-~---r--.
0.51--t_-+_-t-_t--f'+_t-;. e~~/ I ./
/o v V V/ ~\~e'l'l'V~-+--t_+--t-+---l--I
J i _ jV' / )::'; /1.01---+-+---i-['_~++--+-..y___'-+V~""-+-+-I--+-+--+-+--l---1
UJ V .ffL .,I / ij~--h./~~-f--+--+--+--+--t--t--t----i,.---J
~~---+-~~~~+-~~~~--~~--+-~---t---+---t--t---t--~--+---t--;I If 0°fLV
zlB I V
J
1 J I2.01--+---+---I+/I-I---+-/+,-lI-+--I--I--t--+----l-+---+---+--I---I--t---t----t
V I
1 / / /
/ 1
4.01--4-+~-~-4-~--1--~--+_-;--~-+_-;~-+-+--4~~-+__t-_r__i
Fig. 3.9 Distribution of vertical stress beneath a long strip footing.
0.75
Soft silty
dayalluvium
which occur in the summer months are generally limited to
the top 1.0--1.50 m below ground level, and these soils gen-
erally recover over the winter period. However, it is recog-
nized that deeper permanent deficiencies can be caused by
large high water demand trees.
Driscoll states that the moisture content of a clay at valuesof suction eF equal to 2 and 3 are 0.50 liquid limit and 0040
liquid limit. These provide a crude estimate of the moisture
content at the beginning of the desiccation process and when
it becomes significant.
In addition to shrinking, these types of clay are also prone
to swelling when the clays rehydrate. Poor or inadequate
drainage can introduce excess water into the soils and weaken
them. Expansive clays can be identified from their plasticity
characteristics. One of the soil properties most widely used
2.0m
~.
-.k~:::!'!..
Fig. 3.10 Example 3.2: vertical stress distribution.
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s-
~»>
->_,
~
./'->
->v
~~
. . - - -
52 53 54Moisture content{%)
Fig. 3.12 Liquid limit graph using cone penetrometer test. The liquid limit is the'water content corresponding to 20 mm penetration.
55%.
Foundations in cohesive soils
to predict swelling potential is the activity of the clay. This
was researched by Skempton in 1953. A clay's activity is
defined as
Plasticity indexActivity = .:...__
Clay percentage
Clays with large activities are referred to as active clays; they
show plastic properties over a wide range of moisture content
values. Figure 3.11 (After Skempton) indicates the rela-
tionship between the plasticity index and clay percentage.
100
~80
0:
~ 60
~:5'40
~ , : ~ ~ ~ ; ; ~ ~ : : : = = = = = = = ~ ~ ~ i ~ n ~ : e : : ~ ~ ~20 40 60
% Clay fract ion> 2 ( . 1 1 1 1
Fig.3.11 Clay 'activity' graph (after Skempton).
80
To design foundations in cohesive soils. such that the
effects of moisture depletion or excess moisture do not result
in settlements or heave, it is essential to determine the Atter-
berg limits of the clays. These tests will enable the clays to
be fully classified. They were designed by Atterberg in 1911.
The tests determine the various values of moisture content at
which changes in a soil's strength characteristics occur; as a
26
25
24
23
22
1 2 1co~ 20
Q i
: Ii 19c.G)
§ 18o
17
16
15
14
50 51
76
silt or clay dries out its strength increases and it becom
compressible.
The moisture content at which the clays stop acting
quid and start acting as a plastic solid is known as th
limit. As further moisture is removed from the
becomes possible for the clays to resist large s
stresses. Eventually the soil simply fractures with no
deformation taking place. The limit at which plastic
changes to brittle fracture is referred to as the plastic
The plasticity index is the range of moisture conten
in which a clay is plastic. The finer the grain particles
soils the greater is its plasticity index.
Plasticity index = Liquid limit - Plastic limit
PI=LL-PL
100
3.4.1 Liquid limit test
BS 1377 specifies two methods for determining the
limit of a clay sample. Field samples for these tests
be a minimum of 2 kg or greater.
(a) Conepenetrometer
The clay sample to be tested is first kiln-dried and tho
ly mixed. 200 g of the sample are then sieved thr
425 micron sieve and placed on a glass slide. The sa
then mixed with enough distilled water to form a p
standard metal mould, approximately 55 mm in diame
40 mm deep. is filled with the clay paste and levelled
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s less
as a li-
liquid
lays it
plastic
t with-
s in the
liquid
should
rough-
a
ple is
ste. A
ter and
.off at
the surface. The cone penetrometer is placed at the centroid
of the sample and level with it. The cone is then released so
that it penetrates into the sample and the full penetration
depth over a period of 5 s is measured.
This test is repeated by lifting the cone clear, filling in the
first depression with more paste and allowing the cone to fall
again. If the difference between the two measurements is less
than 0.50 mm then the test is considered to be valid. The
average penetration is noted and the moisture content of thesample is determined in the normal way.
The procedure is repeated at least four times with increas-
ing moisture contents. The amount of water added to the
samples should be just sufficient to produce depths of pen-
etrations within the range of 15to 25mm.
The liquid limit is then found by plotting the variation of
cone penetration on the vertical scale against the various
moisture content values on the horizontal scale. A best-fit
straight line should be drawn through the points on the graph.
The liquid limit is taken to be the moisture content which
corresponds to a cone penetration of 20mm (Fig. 3.12).
(b) Casagrande apparatus
This method was superseded by the cone penetrometer, but it
is still widely used. The process of drying the sample and
making a paste is similar to that for the cone method. The
paste is then placed in a special brass cup and a 2mm wide
groove is cut in the top of the sample using a special profiled
grooving tool. The brass cup is then inserted into the
apparatus and the handle is turned at a rate of 2 rev/so This
actuates the cam which causes the brass cup to lift 10mm
and then fall on to the base plate. The number of blows to
close the 2 mm gap over 13mm is recorded and the moisture
content is determined in the usual way (Fig. 3.13).
Rubber base
(a )
13 mmt ~C
(b)
Fig. 3.13 Casagrande apparatus: (a) Casagrande liquid limit test
apparatus; (b) grooving tool.
The test is repeated at least four times and the moisture
contents are plotted on the vertical scale against the number
of blows on the horizontal scale, using a log scale. The mois-
ture content which corresponds to 25 blows is the liquid
limit, expressed as a whole number (Fig. 3.14).
Moisture movements
8 0
. . . . . . . .<
. . . . . . . . . . . . .
~Uquid limit r - . . . . . . . . .r--- rr-r-,I
~l <t - - . . .\t
l
~:::-70§c: :oo 60
~LLiii
~ 50
40o 10 15 20 25 30 40 50 60
Number of blows (log scale)
Fig. 3.14 Graph for l iquid limit determination using Casagrande
apparatus.
3.4.2 Plastic limit test
Take 20-25g of the clay sample after it has been kiln dried.
The sample is then placed onto a glass plate and sufficient
water is mixed with the sample to form a paste which can berolled out between the palm of the hand and the glass plate.
The sample is said to be at its plastic limit when itjust begins
to crumble at a thread diameter of 3 mm. The moisture
content of the sample is determined and the test is repeated
several times.
Once the soil plasticity characteristics have been found.
the clays can be classified by using the Casagrande plasticity
chart (BS 5930:1981) which will enable comparisons to be
made.
(a) Casagrande plasticity chart
To use the plasticity chart (Fig. 3.15) the coordinates forplasticity index and the corresponding liquid limits are plot-
ted. The sample can then be classified from its position on
the chart relative to the A line: an empirical boundary be-
tween inorganic clays which come above the line and
organic silts and clays which come below the line. The A
line is drawn through the baseline where the PI is equal to
zero and the liquid limit is 20%.
The main soil types are given specific designation letters
and additional designatory lettering is used to denote the
grading and plasticity (Table 3.6).
(b) The triaxial test (undrained compressive test)
This test, to determine the values of the total shear strength
parameters of a soil, is carried out in the triaxial test appar-
atus but the sample is prevented from draining during shear-
ing and is therefore sheared immediately after the application
of the cell pressure. The test is quick and the results are
expressed in terms of total stress.
The shear strength of a clay soil is made up of
two components: cohesion and frictional resistance. Samples
of the clay are subjected to quick undrained triaxial
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rate
circles
dc to
st gaveof
circles
on th e
drawn
Bibliography
03=70 _ I
J01 = 107
JI I
03 = 1401
I01 = 258
03 = 210
01 =340 ~I
Fig.3.16 Determination of c and j!j from Mohr stress circles .
Shear stress
[ : L _ _ _ _ _ . , L _ _ ~ ~ e ~Normal stress
I :
~ I
~ I1
Fig. 3.17 Construction of Mohr's circle of stress.
BIBLIOGRAPHY Carter, M. (1983) Geotechnical Engineering Handbook, Pentech
Press. London.
Casagrande, A. (1947) Classification and identification of soils.
Proc. American Society of Civil Engineers. No. 73.
Department of the Environment (1991) Approved Documents.
HMSO, London.
BSI (1999) BS 5930: Code of practice for si te investigations,
British Standards Insti tute.
BSI (1990) BS 1377: Method of testing for civil engineering
purpose, British Standards Institute.
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Foundations in cohesive soils
Meyerhof, G.G. (1952) The ultimate bearing capacity of founda-
tions. Geotechnique, 2 (4), 301-332.
Peck, R.B., Hanson, W.E. and Thombum, T.H. (1974) Foundation
Engineering, John Wiley, New York.
Powell, M.J.V. (ed.) (1979) House-Builder's Reference Book,
Newnes-Butterworth, London.
80
Skempton, A.W. (1951) The bearing capacity of clays. Bu
Research Conference, Institution of Civil Engineers, Div.
Terzaghi, K. and Peck, R.B. (1968) Soil Mechanics in Engin
Practice, 2nd edn, John Wiley, New York.
Tomlinson, M.J. (1980) Foundation Design and Constructio
edn, Pitman.
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E(') 0 ('\10100 E~ ~ (') o co~ 0 (')100 (;jg ~~ 0 ....
0..:;co -e-- E"":N c? 10P , .. .. , .. .. (\I N~ L COr-
01/
(3)/ 2) (1 )
0
/-
0"..
0/
/./
v - I
"../
o f . I . I T I ) 2 8 20 60 200 600 2 8 20 60 200(m
Clay:Fine I Medium Coarse r Rne IMedium r Coarse I Fine I Medium I Coarse I Cobbles
Silt I Sand I Gravel I
i/ding
,180.
4th
4.1 CLASSIFICATION OF SANDS AND GRAVELS
Sands and gravels are classified in the laboratory by carrying
out a sieve test. In this test the soil samples are washed, kiln
dried, and then run through a set of graded sieves. The soil
mass retained on each sieve is recorded and the results are
plotted on a particle size distribution chart as shown in
Fig. 4.1. From these grading curves it is possible to
determine for each soil sample the total percentage of a
particular particle size and the percentage of particle sizes
larger or smaller than any particular particle size.
• A sand or gravel is deemed to be well graded if the curve
on the chart is not too steep and is constant over the full
range of the soil's particle sizes with no excess or defi-
ciencies of any particular size of particle.
• A sand or gravel is deemed to be poorly graded if the
majority of the curve on the chart is too steep, the soils
10
9
8
2
Chapter 4
Foundations in
sands and gravels
have a limited particle size distribution and most of t
particles tend to be about the same size.
• If the curve on the chart shows a large percentage of larg
and smaller particles with only a small fraction of theinte
mediate sizes then the sample is deemed to be gap-graded
Figure 4.1 illustrates the curves for three samples usin
sieve analysis:
I . a gap-graded sandy gravel;
2. a uniformly graded sand;
3 a fine silty clay.
4.1.1 Composite sands and gravels
Gravels laid down in the form of alluvial deposits are usual
mixed with sands in various proportions. Table 4.1 lists t
required description for such mixed soils based on the
composition.
British Standard Sieve Sizes
m)
Fig. 4.1 Particle size distribution chart. Sample 1:gap-graded sandy gravel. Sample 2: uniformly graded sand. Sample 3: fine, silty sand.
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ere is
ty or
ed by
when
test
in thesands
trial
ushed
pick.
than
g into
a soft
). A
hand
of a
n the
after
sides
soils
Such
es of
t too
2. Was any groundwater evident? The level of groundwater
in granular soils can affect the allowable bearing capacity
and settlement criteria by a factor of 2. The level of the
groundwater lowers the effective stress parameters of the
soils, thus reducing their ult imate bearing capacity.
3. Did the sands flow laterally because of water ingress?
Fine sands in a wet loose condition are susceptible to
lateral movement when excavated. Major problems can
occur on sites where drainage excavations follow on afterfoundations are placed resulting in a loss of confinement
of the sands below the foundation base with resulting
subsidence.
4. Did the sands contain soft clay or soft silty lenses? Such
soils, if encountered, will require the bearing capacity of
the sands to be re-assessed and most likely reduced to
avoid excessive settlements.
In general, therefore, well-graded dry sands or gravels of
medium dense or dense composition have higher ultimate
bearing capacities than most cohesive soils. In addition, the
settlements under load take place quickly but these settle-
ments can increase if the groundwater levels rise to within a
distance equal to the foundation width.
4.2.3 Groundwater levels
It is most important to make sure that the full effect of arising water table due to seasonal movements is allowed for
in the foundation design. Trial pits excavated in a dry sum-
mer period may not be so dry during a wet winter period. If
there is any doubt, it should be assumed that groundwater
could rise and consequently the allowable bearing capacity
should be halved.
Table S in BS 8004:1986 gives the density classification
for granular soils based on standard penetration test results.
These are shown in soil reports as N values.
4.2.4 The standard penetration test
This test is the most widely used method of determining the
relative density of a granular soil. The test involves placing a
split spoon sampler with a bottom steel driving shoe on to
the drilling rods. When the borehole has been sufficiently
advanced the split spoon sampler is lowered down the hole
and driven into the soil by means of hammer blows on the
top of the drilling rods. The hammer weighs 63.S0 kg and is
dropped a distance of 760 mm. The number of blows re-
quired to drive the sampler through three ISO mm intervals is
recorded. The sum of the number of blows required to drive
the last two ISO mm increments is recorded as the N value.
The first ISO mm increment is a seating in allowance and is
disregarded (Fig. 4.2).
SPT values are usually obtained at 1.50-2.0 m intervals.
Though used predominantly for granular soils the test can be
used in granular fills, mixed soils and clays, but the results
Obtained must be tempered with caution.
Table 4.3 illustrates the relative densities of sands and
gravels based on SPT results.
Relative densities of granular soi
1"-- Drilling rod
Holes for tommy bar
~-- Split barrel
~H--- Open cutting shoe
50mm
I--l
~
60° cone shoeadded for takingSPT readings
Fig. 4.2 SPT apparatus, showing drilling rod, split barrel sampler
and SPT cone shoe.
Table 4.3. Relative densities of sands and gravels based on S PTresults
Relative density N, blow count/300 mm
Very loose
Loose
Medium dense
Dense
Very dense
<4
4-10
10-30
30-50
>50
The SPT is an empirical test, based on experience, and
suitable precautions need to be taken when carrying out the
test to ensure accurate results. The base of the hole must b
carefully cleaned out, removing any disturbed soil. When
drilling in sands below the water table, the positioning of the
casing can be critical to obtaining accurate results. If the
casing is not advanced far enough, the wet sands can surge
into the borehole, which will result in low N values. If the
casing is extended too far, the sands may be compacted and
high N values will result.
Interpretation of the test results is based on experience,
and- many researchers in soil mechanics have produced
correlations to be applied for various soil conditions. A
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Foundations in sands and gravels
rCorrection factor = corrected N value
measured N value2 3 4
Ie~~100~------~Yr----------t----------1~a.c<I>
'E:>
£~~ 150~--~L---1-----------~--------4>U
~
250L---------~--------~L---------~
Fig. 4.3 Depth-correction values for SPT N vaiues (Gibbs and
Holtz, 1957).
shallow depths the recorded N values tend to be under-
estimated, and correction factors were derived by Gibbs and
Holtz (1957). These can be obtained from Fig. 4.3 to take
account of the overburden pressures.
Silty sands and saturated silts usually produce an over-
estimation of the relative density and a modified N value, Nm ,can be obtained from the formula
Nm =15+.!_(N-15)2
where N> 15. IfN < 15then no correction is required.
4.2.5 Interpretation of SPT results
Terzaghi and Peck (1968) produced correlations for bearing
capacity factors based on the relative density of the soils
obtained from standard penetration results. Figure 4.4 illus-
trates the factors N q and N.., derived by Terzaghi, Peck and
Hanson using the angle of shearing resistance c p . In addition,the SPT results can be used in Fig. 4.5 to determine the
.allowable bearing pressures for foundations in excess of
1.0m width based on settlements not being greater than
25 mm as indicated in BS 8004 Table 1.
However, w h e r e foundations narrower than 1.0 mare
being used in sands the allowable bearing capacity must be\
checked, as research has shown that these reduce rapidly and 1bearing capacity failure becomes the criterion with a suitable.r
factor of safety applied.
A set of charts are indicated in Figs 4.6, 4.7 and 4.8 based
on different ratios of depth to foundation width and based on
84
14
13
12
0
0 ....... J Ny
<, I'0
" ' "0
r-, ,<,
I!Nq
0~ I J
0
II " '\.0
'J \0 I ,f\
1 /l l'
.Iy
A
1/'~.
.:./
11
;:(10""0c<II 9~~ 8
~~ 7>.
~ 6a.
~ 50Clc'iij 40<I>
ID 30
20
10
o28 30 32 34 36 38 40 42 44 46
Angle of shearing resistance, Ijl
o410
20
Very
Lo
Mede
De
V
de
Relden
30
40
50
60
70
Fig. 4.4 Correlation of values of e, N q and Ny with SPT testsPeck, Hanson and Thornburn).
.0 3Footing width (m)
Fig. 4.5 Allowable bearing pressures on sands based on SPT
N values (25 mm settlement criterion) .
5
a straight-line relationship for simplicity. These valu
for dry soils and based on a factor of safety of 2. Shou
water levels rise to within a distance equal to the foun
width, then these values should be halved.
4.2.6 Ultimate bearing capacities
For granular soils where the dissipation of pore
pressures is usually fairly rapid, the effective shear str
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e
y
s are
the
water
650~,--------.--------,--------.--------,
N=50
5401~-------+--~----~------~--------~
N=40
~4301~-------+~------A--------,--------~
}e.'iii0-
~3251~------~~----f-----+-~------~--------~
BC)
c
ss~ 215,~--~L---*---~-----h~-------+--------~
1'aQ ;
z
N=30
N=20
N= 15
300 60Q _ . '" 900
\ \.'£, 'foundation widfR, B (mm)
65,lli--------r-------.------~------~
N=50
N=40
'"4301r---------~--~----~------~--------~e . .
s>.:t:o
~3251r_------~~------_r~------~L-----~~
~
1£2151r_-----+~--~~--_hL-----~~------~
~O JQ ;
z N= 10
108J--I--J.~L~~4:~::=::==I=====t
N=30
N=20
N= 15
N=5
600 900 1200Foundation width, B (mm)
Fig.4.7 Net allowable bearing capacity for zl B =0.50.or
foundation settlements not exceeding 25mm. Factor of safety = 2.0.
Relative densities of granular soi
Fig. 4.6 Net allowable bearing capacity for zl B = 1.0. For
1200 I. foundation settlements not exceeding 25mm. Factor of
~ 1- . l , . , ( , ( ' safety = 2.0.
6501,----.----.------.------,---
N=50
108N= 10
N=5
300 600 900Foundation width, B (mm)
Fig. 4.8 Net allowable bearing capacity for zl B =0.25.or
foundation settlements not exceeding 25mm. Factor at safety =2.
1200
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·~
/~
/~
~
·~v
·
v/r--
).. .V -~
0. . . .
,/. . . . . . . ./
Ny, V.
_//
. , . , i - - " Vv-I- Nq/- /
Foundations in sands and gravels
45
40
-Go 35
mo& i 301 ii
8 i~ 25c:
~~ 20til
'0Q)
"Ol15
.& :
10
S
0.2 0.3 0.40.S 1.0 2
Fig. 4.9 Bearing capacity factors Ny and Nq for sands and gravel strata. Ny and Nq values based on SPT N values andj1j
relationship.
Table 4.4.
Value o f e
(degrees)
Relative Round grain Angular grain Silty Sandy Inorganic
density Uniform Well graded sands gravels silts
27-33 35
30-34 50
Loose
Dense
27.50
34
33
45
27-30
30-35
are used when considering the allowable bearing capacities.
Because of the difficulty in obtaining undisturbed samples in
the field for laboratory testing, the ground strength para-
meters are usually obtained from SPT results using the
various correlations.
Using Terzaghi equations, the ultimate net bearing
capacity of a shallow foundation is given by:
Strip footings
_ ( ) yBNyPnu-eNc +r; Nq -1+-2-
Pad foundations
P nu = 1. 2 e N; +P« (N q - 1) + 0. 4 yB N y
where e = the shear strength of the soil;"(= the bulk density of the soil;
B = the foundation width;Po = the effective overburden pressure;N c, Ny and N q are Terzaghi bearing capacity factors.
When the soils are granular, and e =zero, eN c = O .
Figure 4.9 and Table 4.4 can be used to determine the Nq
and Ny based on l/ J values.
86
3 4 S 20 30 40S0 2000 100
4.3 CONSTRUCTION PROBLEMS IN GRANULA
SOILS
Granular deposits make good founding strata if they ar
medium or dense state of compaction. In fact, such sand
gravels perform better than firm clays in that the found
settlements occur immediately they are loaded.
The major construction problems are caused
granular deposits have water-bearing levels either in the
of wet lenses or as a standing water table. Any excav
below the water table will cause instability in the sides
excavation. In addition the base of the excavation can
if the sands are very loose and start to flow.
If the water ingress into excavations is not too hea
may be possible to control itby pumping from a lower s
Consideration must be given to the effects of tempo
lowering the groundwater level especially if there
existing buildings close by. Any groundwater low
could remove the fine particles from the surroun
locality and result in subsidence of existing buildings. I
be necessary to consider the use of chemical inje
methods which are suitable for fine-to-coarse sands
gravels.
On. open sites where deep drainage is to be insthrough water-bearing granular soils, well pointing c
adopted if the soils have a fine-to-coarse grading.
system can also be used closer to existing building a
filtering system does not remove as much of the
particles as occurs when pumping from open sumps. W
the soils have a grading less than 0.06 mm, such as
silt, well pointing is not a suitable method of groundw
lowering and it may be necessary to use electro-osmos
ground-freezing methods.
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in a
s and
hen
form
boil'
it
are
ding
may
and
n be
This
s the
finer
fine
or
If water levels on a site cannot be lowered owing to
various circumstances it will be necessary to consider using a
! /':""Iraft foundation or piles. If a raft is used the main housey .
;,_,...-; ;, d rainage should be installed prior to forming the raft
formation. If piling is used, it is preferable and more eco-
nomic to use a precast concrete driven pile or steel pile than
a bored pile with a temporary casing. Alternatively a con-
tinuous flight auger pile can be used if wet granular bands
are evident at depth.
4.4 FOUNDATION DESIGN IN GRANULAR SOILS
The allowable bearing capacity of granular soils is usually
limited by settlement considerations. The allowable bearing
pressures shown in Fig. 4.4 are based on a maximum
settlement of 25 mm with ,nofactoI:s gf safety included for.
.They a re also based ontheassumpti6n'that the wafer table is
ata depth of at least Bbelow the foundation base.
For foun(iations l:ss than' ~.O.~~~.peatj~gcapaCity should be""checkeci, wffii a factor of safety of
~eirigappUe4agamst'1)eaniig"Capacity"'Iarrure~~'"
' U e a r r n g " c a p a c i t y ,failure of soil below a foundation is
generally accompanied by high settlements and rotational
movements, It i§ differential settlements that give rise to
most structural failures, and to a~~~'ii~Cturr~';lcesiiief o u n : d a t i ; ; ~ o ( sh~~i(fbt'designed to keep total settlements
Iwithin acceptable limits. In situations where sands a re
" 'Yaterlo~<!!I..m2:!J2!!.Q4~~p.Qwg~gjx~~t.~L!LWi.r:!i'i~'·", capacity failure.
~ .. ..._.~ - _'_ '__ ' ' _ - -" ,_ ' " • ' - _:_ • c _'
T For most practical uses it is sufficiently accurate to
obtain the values of N q and N y from the SPTresults and
obtain c jl from Table 4.4 and Table 4.5 derived by Terzaghi
andPeck (1968). "
0.4
Foundation design in granular soils
Table 4.5. Factor Nq (from Terzaghi and Peck)
SPTblow Angle of internal Nqcount.N friction, l : ' l
(degrees)
lO 30 18
20 33 26
30 36 37
40 39 55
50 41 72
~ ~ _ \ ~ f ~ L · \ ~ 1 o
Example4.1 Stri~;g on granu.l~rsoil
" ",'),~'\A strip footing for a fact'\?ry,..i$o ~ 5 0 ' mm wide. The maximum
line loading on the wall ~ 5~kN Per"metre run. Field tests have
shown that the site is und'eFlain by medium dense sands with an
angle of internal friction of 350 with average N values of 15 at
1.20m depth. Determine the net allowable bearing pressures and
check the foundation width if the,depth of the foundation is 1.20 m
below ground. Bulk density of soil,iS 18.0 kNho3.
FrolI\ Fig. 4.4 , Nq'';' 30 and Ny '; ' 3 0 . Therefore
' [ N ' D }(netult)= 2 '1 + ( N q - l . O ) ; B
where Nq and Ny are bearing capacity factors;
D r = depth of foundation below ground level;
B =foundation width;
'Y =bulk density of soil below foundations.
Therefore'
q(net ult .)=[ 30 +(30-1.0) 0·60'18XO.752.0 0.75J.
=(15+23.20)x13.50=515 kN/m2
With a factor of safety 'of 3.0 the allowable bearing capacity =515/3.0 = 170kN/rIi2• " ,
Actual bearing pressure =55:010.75 =13kNho2•
D,
Water table
0.5O.3L---------~--__--~~---- __----__~~~_,---------------
o 1.0Dw
Dl+B
Fig. 4.10 Correction factor to SPT blow count for depth of water table (after Peck, Hanson and Thornbum).
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.: I t
73.0 690 ~p= 0.47x45xO.50 =. mm p
.' o,'ty. \ ,~ 1 - _ ;'\ll-" ( ( \ \ A f r rt(\!- \";H't'f Example 4.3\ ' ~~
When excavations on site result in variations in the formation
bearing stratum with sands alternating with f i r m clays it is only
necessary to provide a bottom layer of mesh reinforcement. say
B283 as required in BS 8004.
Foundations in sands and gravels
As a rise in the groundwater level could result in the allowable
bearing capacity being halved to 85 kN/m2 this is a suitably sized
foundation which will keep settlements within acceptable limits.
Calcul~ted settleIl}!;~:''''\ / ,,~'\ \:" ('/
, " P : ' ; ' ' f A A l,",".'v",~, - \\''''Y
settlement p=-- 't ' ( ,\0.47N' ) """ J . , \
where P = applied bearing pressure; N = SPT blow count.
Overburden pressure = 18 x 1.20 = 21.60 kN/m2
From Fig. 4.3, correction factor = 3; therefore modified blow count
N' isgiven by
N'=3 x 15 =45
Assume groundwater can rise up to foundation level, and apply a
correction factor of 0.50 from Fig. 4.10. Then
Example 4.2 Pad foundation on sand
A pad foundation 3.0 x 3.0 m is founded at a depth of 1.0 m in a
thick sand layer which has been assessed as being in the dense
category. The value of shearing resistance <I > = 35° and the sand's
in-situ bulk density is 19 kN/m3. Determine the safe bearing
capacity of the sands using a factor of safety of 3.0. .
Safe bearing capacity = qunet + r Z3.0
qunet=rZ(Nq -1)SqDq +0.5rBNySyDy
where Sq= shape factor = I + (B/L) tan < I> (from De Beer);
D q = depth factor = 1+ 2 tan < I> (1- sin < 1 > ) 2 (z /B) for z/ B = < 1.0;
Sy= shape factor = I - 0.4 B/L;
Dy= depth factor = 1.0 for zl B = < 1.0.Using Table 4.6 for < I> = 35°, Nq=33.30 and Ny= 48.03:
Sq =1+~tan35=I+lxO.70=1.703.0
Sy = 1- 0.4 x 3.0 = 0 .603.0
D q =1+2xO.70(1-0,573)2 x~B
= 1+ (0.427)2 x.!. =1.0863
quit = 19x 1.0(33.30-1)1.70x 1.086
+0.5 x 19x3.0x48.03xO.6xl.O
= 1133+ 821.31= 1954.31 kN/m2
S & be . . 1954.31 19 10ale anng capacity = --- + x.3.0
=651+19 = 670 kN/m2
This value would be halved if the water table rose to within 3.0 m
of the foundation base, i.e. 335 kN/m2.
88
Table 4.6. Typical bearing capacity factors
p
(degrees)
o5
10
15
20
2530
35
40
45
50
5.13
6.50
8.34
10.98
14.83
20.7230.14
46.12
75.31
133.87
266.88
1.0
1.57
2.47
3.94
6.40
10.6618.40
33.30
64.20
134.87
319.06
(~
Strip footing on sand, high
A continuous strip footing 1.0m wide is founded at a
1.0m in a well-graded angular sand which has a bulk de
18.50 kN/m3• The water table is known to fluctuate to
foundation level. Determine the ultimate bearing capacit
sands if the soil strength parameters are based on SPT resul
12at 1.0m depth and N = 15at 1.20m depth.
Consider that < jl = 30° for N = 12 and < I> = 31° for N = 1
continuous footing with < jl = 30, Nc = 30.14. Ny = 22.40 a
18.40:
= 18.50 x 1.0 x 17.40 + 0.5 x 18.50 x 1.0x 22.10
= 321.90 + 207 = 529 kN/m2
Applying a factor of safety of 3.0:
529Safe bearing capacity = - + r Z
3.0
= 176+ 18.50 x 1.0 = 194.50 kN/m2
As the groundwater level can rise to the foundation ba
prudent to halve this value and use a figure of93 kN/m2•
Example 4.4 Bearing pressure of granular
A granular stratum was tested at depths of 2.0 m and 3.0 m
N blow counts recorded were 18. Groundwater was measu
depth of 1.30m below ground level. The saturated sands ha
density of 19 kN/m3.
A strip footing is to be placed at a depth of 1.0 m and isreqbe 1.20 m wide. Determine the allowable bearing pressure.
Corrected N value:
n;=15+t(N-15)=15+t(18-15)
= 16.50
Depth correction factor:
Overburden pressure = 2 x 19= 38 kN/m2
From Fig. 4.2 the depth correction factor = 2.50. Therefore
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o0.10
1.22
2.65
5.39
pth of
of
within
of the
of N=
. For a
ndNq =
e it is
soil
nd the
ed at a
a bulk
red to
Steel beam grillage
Thick steelplate beddedon 25 mm sand
Fig. 4.11 Plate bearing test apparatus.
N' = 16.50 x 2.50 = 41
For N' = 41 and B = 1.20 using Fig. 4.4:
Allowable bearing pressure = 490 kN/m2
This value is for a dry sand and should be halved to 245 kN/m2 to
allow for the groundwater levels rising.
4.5 PLATE BEARING TESTS
The allowable bearing pressure of a granular soil can be
determined by carrying out a plate bearing test (Fig. 4.11).
However, such tests must be carried out with full knowledge
of the underlying strata as the plate will only stress a limited
depth of ground below the foundation level and it is most
important that the groundwater level is known.
The main drawback in attempting to determine settlements
with this method is the effect of the small plate stressing ashallower zone of stratum than that stressed by a wider
foundation. The plates therefore should be as large as
possible and never less than 300 mm.
The test plate should be rigid enough to avoid bending and
can be between 400 mm and 800 mm, square or circular. The
kentledge should be placed in incremental stages, each
increment of load being about one fifth of the proposed
bearing pressure. The loading is then increased up to two or
three times the proposed loading, and settlement readings
should be recorded for each stage. Where there is no defini-
tive failure point the ultimate bearing capacity is taken to be
the pressure which causes a settlement equal to one fifth of
the plate width. The results of settlement against load inten-sity should be plotted on a log scale to determine the failure
point.
Terzaghi established that the settlement of a 300 m square
plate at a given load can be related to the settlement of a
foundation by using the formula
(2B ) 2
S 2 =SI --I+B
where SI = the settlement of a foundation of width B, where
Piling into sands and gravel strata
. 1Dial gauges supportedclear of the test area
B is taken as the ratio of foundation width to plate width;
S2= the settlement of a test plate.The ultimate bearing capacity of the foundation can be
assessed from that of the plate by applying the following
formula for granular soils:
Q2 = B 2
Q 1 s ,
where Q2 and QI are the ultimate bearing capacines of
foundation and plate, respectively, and B2 and B I are their
respective widths.
Settlement predictions from plate bearing tests are often
inaccurate but Peck, Hanson and Thornburn (1974) deve-
loped analysis based on field experience of small diameter
plates and actual foundations and their conclusions were that,
for foundations on granular strata:
p
p= 0.47N
where P = applied bearing pressure in kN/m2; N = SPT blow
count; p = settlement in mm. The SPT values should be thetest results with correction factors for depth from Fig. 4.3
and for groundwater levels from Fig. 4.10.
4.6 PILING INTO SANDS AND GRAVEL STRATA
Piles in sands and gravel strata can be bored, in-situ cast in
place driven type, precast or steel driven or continuous flight
auger (CFA) piles. Generally the preferred system is thedriven pile, which in driving increases the density of the
granular strata. Inaddition the risk of having a poorly formed
pile is ruled out when steel sections or precast concrete piles
cast in a factory are used.
Bored piles and CFA piles can be very useful on sites
where there are existing buildings and vibrations need to be
kept to a minimum. CFA piles are generally used when
water-bearing or very soft strata are encountered and a bored
type of pile is needed.
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Foundations in sands and gravels
4.6.1 Bored piles
These can be formed using a conventional three-leg tripod
rig. For large-diameter piles a special rig design is generally
required, especially if the piles are to be under-reamed, to
enlarge the pile base.
Where the piles have to pass through very weak soils or
water-bearing strata, the piling contractor may need to use
temporary or permanent casings. If the casing is temporary
and the pile is an in-situ concrete system in which the casing
is extracted during the construction of the pile shaft, it is
important to ensure that a sufficient head of concrete is
maintained in the pile shaft to prevent 'necking' of the pile
during withdrawal of the casing.
Great care must be exercised when withdrawing the pile
casings to avoid lifting the pile reinforcement cage and
surrounding concrete up with the casing. To prevent this
from being a problem the concrete should be a rich mix and
have a high workability.
In extreme situations where groundwater inflows are high
it may be desirable to form the pile shaft using a tremie pipe
operation. Concrete placed by tremie operation should be
easily workable, have a slump between 100 and 175 mm,and should have a high cement content of at least 400 kg/m",
In some situations the casing can be replaced by using a
bentonite slurry. The use of a tremie pipe in these situations
requires that an adequate head of concrete is kept in the
tremie pipe to overcome the pressure of the bentonite mud
which has to be displaced by the outflowing concrete.
4.6.2 Continuous flight auger piles
These piles are very useful in soils such as soft alluvium, wet
sands and peat soils. They should only be used when a good
site investigation is available. The auger on the piling rig has
a central core down which a cementitious mortar or fineconcrete can be pumped prior to and during removal of the
pile spoil on the auger. If, on removal of the auger, it is
evident from the soils on the auger tip that the pile has not
been formed in suitable bearing strata, then it is necessary to
replace the auger and reform a deeper pile.
4.6.3 Design of bored piles
In 1976 Meyerhof determined bearing capacity factors for
deep foundations. Similar work was carried out by Bere-
zantsev in 1961 and by Hansen and Vesic, and these factors
are listed in Table 4.7.
Ultimate load capacity Qu = Qb + Q s
where Q b : :;: ultimate end bearing component, and Q s
ultimate skin friction component. Now
Q b = qbAb =a'; NqAb
where a'; = the effective overburden pressure at the pile toe;Nq : :;:the bearing capacity factor (Table 4.7); Ab = area of pileat base. Now
Qs=/sA,
90
Table 4.7. Bearing capacity factors (after Meyerhof)
p No Nq
(degrees)
0 5.14 1.0
5 6.49 1.60
10 8.34 2.50
15 10.97 3.90
20 14.83 6.40
25 20.71 10.70
26 22.25 11.80
28 25.79 14.70
30 30.13 18.40
32 35.47 23.20 2
34 42.14 29.40 3
36 50.55 37.70 4
38 61.31 48.90 6
40 75.25 64.10 9
45 133.73 134.70 26
SO 266.50 318.50 87
where I s : : ; :average value of skin friction developedembedded length of the pile shaft; As ::;:surface are
embedded pile length of the pile shaft. The average
Is is given by
Is =K,c 'v tan Ii
where K ; ::; :he coefficient of lateral earth pressure; (
of friction between the pile shaft and the surroundin
Values for K; and (jare listed in Table 4.8 (derived b
in 1966).
Table 4.8. Typical values for 8 and K, (Broms, 1966)
s,
Pile material Relative density o
Loose
Steel 20° 0.50
Concrete 0.75 p' 1.0
Timber 0.67 p 1.50
\!t' =Angle of shearing resistance in respect of effective stress valu
The values ofIs are limited for pile lengths between20 times the pile diameter or pile width. For practical
maximum is taken as 100 kN/m2. Meyerhof determin
qb is approximately equal to 14N D lB where N: : ; : SP
count; B ::;:pile diameter or pile width; D = embedded
of pile in the end bearing strata; Is is approximately0.67 N kN/m2•N = the average uncorrected N valueshaft length considered.
When constructing bored piles in sands and gra
granular strata will be loosened during removal of
material. In view of this it is prudent to adopt a c
approach and use values of ¢ and N q based on lo
conditions to determine the ultimate end bearing a
friction values. Adopting this approach will resul
ultimate bearing capacity lower than that achieve
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Ny
1.10
er the
of the
alue of
angle
soils.
Broms
soil
Dense
1.0
2.0
4.0
10 and
that
blow
length
ual to
er the
s the
core
soil
skin
in an
by a
driven pile in the same strata but the loosening effect of the
boring operations on the base and pile shaft in granular soils
can be quite significant.
Example 4.5 Bored piles
A dwelling is to be supported on bored piles which are required
because of the closeness of an old existing building. The maximum
unfactored line load for the three storey dwelling is 60 kN per metre
run. The soil conditions below the site consist of approximately
4.0 m of very loose to loose ash fill which is still undergoing
consolidation settlement and, below the fill, a medium dense sand
with recorded SPT values of 22. The density of the ash fill is
1300kg/m! and all the boreholes revealed dry conditions down to
15.0 m depth. The density of the sand is 1850kg/m>.
With piles at 4.0 m centres, and using continuously designed ring
beams, the maximum working load on each pile will equal 4 x 1.20
x 60 = 288 kN. Ultimate skin friction on pile shaft, I s = K; 'Y d tano.
Since boring will loosen the sands, use Table 4.8 for K, and o .K,= 1.0, c P =33° and 0= 0.75 x 33 = 24.75. Therefore
Unit skin friction at top of sand = 1.0 x 1.30 x 9.80 x 4 x tan 24.75
= 23.49 kN/m2
Ifwe assume piles will be approximately 10m long:
Unit skin friction at pile toe = 1.0 x (1.3 x 4.0 + 1.85 x 6)x 9.8 x tan 24.75
= 73.64 kN/m2
Assuming 400 mm diameter concrete piles:
Total skin friction = (23.49 + 73.64) x 6 x 1£x 0.40 = 366 kN2.0
Assuming that during the boring the medium dense sands wil l be
loosened and the value of c p will be reduced to 32°. Using the
Berezantsev chart (Fig. 4.12):
Nq for DIB = 6.0/0.40 = 15 is 33
1£X0.402
)End bearing resistance = (1.30x4.0+1.85x6 x9.8x334
=662.50 kN
300r-----~-----r-----r----~----~250200
2~~LL~_LLi_LLl~LL~~~~LL~20° 25· 30· 35° 40° 45°
Angle of shearing resistance, 4 >
Fig.4.12 Values of Nq for pile formula (after Berezantsev, 1961).
Piling into sands and gravel strata
Therefore
Ultimate resistance = 662.50 + 366 = 1028.50 kN
Adopting a combined factor of safety of 3.0, the maximum allow
able working load = 1028.50/3.0 = 342 kN.
Because the fi lls are st ill set tling under their own weight, th
effect of negative skin friction must be allowed for in the pi
design. This value of negative skin friction must be added to th
pile working load.
Assume the negative skin fric tion acts over the top 4.0 m of th
pile. The peak value of negative skin frict ion will not at any tim
act over the whole length of the pile shaft embedded in the fill and
will therefore be necessary to make a reasoned assessment of th
magnitude of the drag-down forces to be used in the design.
Negative skin friction = el y K tan C P ' a-
where el y = effective vertical stress, and K tan c p ' . is assumed to bconstant for the pile's length.
Unit skin friction at the top of the sand was calculated
23.49 kN/m2. This value can be used as the peak value of th
negative skin friction at the base of the fill, as it will b
approximately equal. Therefore total negative skin friction on th
top 4.0 m of the pile shaft equals
(0+23.49)X1£X0.40X4.0=59.0 kN2.0
Therefore
. 1028.50 2 96Factor of safety on pile = ---- = .
288+59
This is slightly less than the recommended value of 3.0 but is accep
able because of the low values adopted for the density of the sands
Using a drivel' pile
Using a 275 mm x 275 mm precast concrete pile driven through th
fills into the medium dense sands (N = 25 blows) to a calculated se
a factor of safety of 2.50 can be adopted.
Maximum pile working load = 288 + 59 = 347 kN.
Therefore ultimate load capacity requires to be 347 X 2.50
867.50 kN. For a SPT blow count of 25, < p = 36°. Using th
Berezantsev chart (Fig. 4.l2), for a.1O m pile length:
! : : _ = 6000 = 21.80B 275
Therefore
Nq=60
Qs=lsxAs
Is = K, d y tan 0
where K; = 2.0 and 0= 0.75 x 36 = 27. Therefore
Qs = 2.0x(4.0rl +6.0rz )tan27x4xo.2752.0
= 2.0 x(4.0X 13.0+6 x 18.50)x 0.509 x 1.10 = 91.26 kN2.0
and
Qb=Pb(Nq-I)Ab
where Pb is the effective overburden pressure at the base of the pil
and Ab is the area of the pile base = 0.275 x 0.275 = 0.075 m
Therefore from the Berezantsev chart
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Foundations in sands and gravels
L 6000B = 275 =21.80 Therefore V, =60
Q b = 163(60 -l)X 0.075 = 727kN
Q u = 727 +91.26 = 818kN
Though this is slightly less than 867.50 kN, the pile will be driven
to a calculated set and will most likely penetrate the sands for less
than the 6.0 m available.
4.6.4 Set calculations
Using the modified Hiley formula for precast piles,
R; = ultimate load = 347 x 2.50 = 867.50 kN;E = Transfer energy at pile top = 0.70 x 104,
c = temporary compression of pile and ground per blow, say12mm;
s = set blow count.Therefore
E c 7000 12s=---=----
Ru 2 867.50 2
= 8.06 - 6 = 2 rum/blow
Therefore adopt a set of 20 mm for 10 blows or less. In
practice.j, is taken as 100 kN/m2 maximum.
For driven piles in granular soils there are approximate
formulae derived by Meyerhof in 1976 to calculate pile
capacities.
40ND ()qb =-B- but<400N kN/m2
where N = SPT blow count, D = embedded length of pile inbearing strata, and B =diameter or width of the pile.
Consider example 4.5:
Qb = q b X Ab = 400 x 25 X 0.2752 = 756 kN
andj, =N kN/m2 = the average uncorrected N value over theembedded length of the pile in the bearing stratum. Therefore
Qs = 25 A s = 25 x 6 x 0.275 x 4 = 165 kN.
For a bored pile in granular soils
qb = 14ND kN/m2
B
and Is =0.67N kN/m2
Example 4.6 Working load of precast concrete
piles
A 3.0 m thick layer of loose sands and gravels overlie a thicker
deposit of dense sands and gravels. SPT tests in the dense sands
produced values of 35 from the base of the loose sands to a depth of
12.0 m below ground level at 1.0 m intervals. Using a 275 mm x
275 mm precast concrete pile and adopting a factor of safety of 2.50
determine the maximum allowable working load for the pile.
Ul timate bearing capacity = Qu = Q b + Q s
For Q b, ignore the loose sands and use qb
400NkN/m2. Therefore
40ND/B or
92
Dqb =40x35x-- =400x35 max
0.275
D = 400 x 35 x 0.275 = 2.75 m penetration into the dense s40x35
Therefore
Q b = 400 x 35 X 0.2752 = 1058 kN.
Q s in loose sands is discounted. Q s in dense sands =I s x A
Is= N = 35 kN/m2. ThereforeQ s = 35 x 2.75 x 4 x 0.275 = 105 kN
Qu= 105 + 1058 = 1163 kN
. 1163Allowable working load = -- =465 kN
2.50
In granular strata the end bearing component is much
than the skin friction component on the sides of the p
mobilize this skin friction a significant movement
occur at the pile toe. In dense granular strata this mo
is very small and because of this the factor of safet
driven pile can be 1.50 for skin friction and 3.0 f
bearing. Applying these factors to Example 4.6 the all
working load would be
105 + 1058 = 70 + 352 = 422 kN
1.50 3.0
4.6.5 Dynamic pile formula
The ultimate static resistance of a driven pile can b
dicted from the dynamics of the driving operation itse
kinetic energy imparted by the piling hammer is equ
the work done by the pile in penetrating into the g
Therefore
Net kinetic energy =Work done during penetration of pile
For a hammer of weight W tonnes falling a drop heig
and causing a penetration or set of s mrn, the pile res
load R, can be obtained from the formula
R, = Wh - energy losses
The energy losses are due to the pile and pile cap co
ssion, hammer rebound, and frictional losses
equipment.
Driving a pile into sands and gravel strata will incre
relative density of the sands and gravels and this has a
ficant effect on the predictions of load-carrying capaci
For concrete piles the modified Hiley formula is
applied but the Dutch formula is also often used. Theformula should only be applied to piles which obtai
support in sands and gravels, stiff-to-hard clays or roc
not applicable to frictional piles which obtain their sup
soft clays by adhesion along their length.
Specialist piling contractors who rely on piling to
their living generally put their trust in the simple
hammer. These hammers are often considered to be
and old-fashioned but they are very reliable and
effective as sophisticated hammers at less cost. Some
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where
reater
e. To
as to
for a
r end
pre·
. The
d to
t of h
the
the
signi-
often
Hiley
their
It is
rt in
make
drop
crude
st as
piling
firms have developed their own equipment and use purpose-
built hammers not readily available on the open market.
The Hiley formula can also be adopted for steel bearing
piles and a factor of safety of 2.0 can be used, where
Working pile load = F R f l'actor 0 sa ety
For practical purposes the ultimate load on a pile can be
defined as that load which causes a settlement of one tenth of
the pile diameter or pile width (Terzaghi and Peck, 1968).
The accuracy of a given dynamic formula can be im-
proved by recalibrating it for a given site against the test load
data obtained from static load tests. The formula can then be
more confidently used as a guide for selecting final penetra-
tions of those piles which are neither near any tested piles or
near ay borehole locations.
Dynamic formulae can be grossly inaccurate: using the
Hiley formula, the actual ultimate load obtained by test load-
ing may be between 0.70 and 3.0 times the figure obtained
by applying the formula. Pile testing should always be
carried out to verify the dynamic formula.
4.6.6 Re-drtve tests
These should be carried out on one or more piles at a
reasonable frequency rate with a minimum time interval of
12 h. Only if the re-drive final set (mm/blow) is equal to or
less than the set of the initial drive can the dynamic formula
be adopted. If the re-drive set is greater than the set obtained
on the initial drive the formula does not apply, and it will be
essential to re-drive the piles until a tighter set is achieved, or
to carry out a static load test.
4.6.7 Base-driven steel tube piles
R = 290W(1.0+h)
u s+ 12.70
where Ru = ultimate driving resistance in tonnes; W = weightof internal drop hammer in tonnes; h = actual drop of
hammer at final set in metres; s = final set (mm/blow). This
formula is applicable for:
• drops between 1.20 m and 2.0 m;
• sets less than 5 mm/blow, i.e. 5 blows to 25 mm .
4.6.8 Top-driven steel piles
W 2 Hs = ( ) metres penetration per blow
RW+p
where W = weight of hammer in newtons;
p = weight of pile (unit weight x length) in newtons;
H = effective hammer drop in metres;
R = penetration resistance or ultimate load capacity in
newtons;
s = set (penetration per hammer blow) in mm.
Allowing for 30% loss of efficiency:
Piling into sands and gravel strat
where WH =kinetic energy. Therefore
W2H
sets= ( )RW+p
Example 4.7 Steel piles
Working load on pile = 295 kN with pile length = 9.0 m. Factorsafety = 2.50. Therefore
R = 2.50 x 295 = 737.50 kN
Mass of hammer, m = 3.06 kN;
weight of hammer, w = 30 kN;
weight of pile, p =0.648 L kN where L =pile length;
effective hammer drop, H = 0.35 m.
Velocity of hammer at impact = 2 g H = 2 x 9.81 x 0.35 = 2.62 m
K" . mv2 3.06 x 103 x2.622menc energy =2 2 10500.0 Nm
Reduce this value by 30% for losses =0.70 x 10500.0 =7351 Nm
Therefore
s = _ W . , .. ,( _ W _ H ' - ,- )
R(W +p)
30x 103(7351)8.34 mm/blow
103 x37.50(30+0.648x9)
Therefore, use three blows of 350 mm for a set of 25 mm.
Example 4.8 Driving precast concrete piles
A 275 mm x 275 mm precast concrete pile is to be used to carr
safe working loads of 350-500 kN' The pile is reinforced with eigh
12mm high tensile bars in pairs bundled in each comer. Concret
strength is 50 N/mm2. Fy = 590 N/mm2; Feu = 50; A's = 452
As=452.
Ultimate axial compression load:
N = 0.40 Fe u A e + 0.75 As c r;= 0.40 x 50 x 275 x 275 + 0.75 x 2 X 452 X 590
= (1512 500 + 400 020) X 10.3= 1912 kN
For working load of 350 kN, using modified Hiley formula
Ru =_E__ with factor of safety = 2.25 :s+c/2
R« = 350 X 2.25 = 787.50 kN
Transfer energy at pile top, E = 0.85 X 104kN m; temporary
compression of pile and ground, c = 10mm. Set per blow inmm, S
is given by:
8500 10s =-_ - - = 5.80 mm/blow
787.5 2
Ten blows of hammer give a 58 mm set; therefore use a 4.0 t banu
hammer with a 400 mm drop. For working load of 350-500 kN:
S = 8500 . ! . Q . =2.60 mm/blow500 x 2.25 2
Therefore ten 'blows of hammer give a 26 mm set; therefore use
4.0 t banut hammer with a 400 mm drop, see Table 4.9.
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Foundations in sands and gravels
Table 4.9. Hammer transfer energy x 104 (Rig: hydraulic Banut
type)
Hammer weight
(tonnes)
1.50
3.00
4.00
5.0
Transfer energy (tonne metres)
Hammer drop
(mm)
300 400 500 600 700
0.25 0.35 OA5
0.55 0.70 0.90
0.85 1.10
1.05 lAO
D UT CH F OR M UL A
This formula provides an alternative method of determining a pile
set using a dynamic formula.
W2KHs=
Ru(W+ p )
where s = set in mm/blow;
W =weight of hammer = 35 kN;
K =hammer efficiency = 0.70;
H = hammer drop =450 mm;Ru =working load x 10,350 x 10=3500 kN;
p =weight of pile = 18kN.Therefore
35 x35x 0.70 x 450s = 2.27 rnm/blow
3500(35+18)
For ten blows this equals 22.70 mm set.
94
BIBLIOGRAPHY
Berezantsev,V.O. (1961) Load bearing capacity and deform
piled foundations. Proc. Fifth International Conference
Mechanics, Paris, Vol. 2, pp. 11-12.
Broms, B. (1966) Methods of calculating the ultimate
capacity of piles: a summary. Sols (Soils), 5(18/19), 21-3
BSI (1986) BS 8004: British Standard code of prac
foundations, British Standards Institution.
Carter, M. (1983) Geotechnical Engineering Handbook,Press, London.
De Beer, E .E . (1965) Bearing capacity and settlement of
foundations on sand. Proc. Symposium on Bearing Capa
Settlement of Foundations, Duke University, pp. 15-33.
Gibbs, H.I. and Holtz, W.O. (1957) Research on determi
density of sands by spoon penetration testing. Proc.
ICSMFE Conference, London, Vol. I,pp. 35-39.
Meyerhof, 0.0. (1952) The ultimate bearing capa
foundations. Geotechnique, 2 (4), 301-332.
Parry, R.H.O. (1971) A direct method of estimating settlem
sands from SPT values. Midlands SMFE Society.
Powell, M.J.V. (1979) House-Builder's Reference Book,
Butterworth, London.
Terzaghi, K. and Peck, R.B. (1968) Soil Mechanics in Eng
Practice, 2nd edn, John Wiley, New York.Vesic, A.S. (1966) Tests on instrumented piles. Ogeeche
site. Journal of Soil Mechanics and Foundation D
American Society of Civil Engineers, 96, SM 2.
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ion of
Soil
for
tech
the
y of
in
nes-The following guidance is given for builders and engineers
involved with the planning and construction of housing on
sites previously undermined by mineral extraction and on
sites where future extraction of coal or other minerals will
take place after the development has been completed.
There are various techniques for investigating and
consolidating old mine workings, securing of old mine
shafts, adits etc., and various foundation design options are
available to cater for any ground movements likely to arise.
Past and current coalmining is the most common cause of
subsidence but there are other minerals, such as fireclay,
sandstone (EIland flags), chalk, ironstone, salt and gypsum,
which can give stability problems below a site. The effects
of subsidence from modern longwall extraction methods
now in use in the British coalfield can be predicted fairly
accurately whereas movements resulting from old shallow
mineral workings are not so easily defined and require
sound judgements by engineers and geologists experienced
in this field based on the available mining and geological
data collected.
Building houses on land which is underlain by known
shallow coal workings or other mineral workings can result
in very expensive development costs. The total costs are
difficult to quantify prior to consolidation being carried out
because of the lack of information on the volume ofmaterial
extracted. On some sites it may well be cheaper not to de-
velop certain areas of the site and place public open space
over the no-build zones. If shallow workings are discovered
in the final phases of a development there will be less
properties available to spread the costs.
In known mining areas it is prudent to consult British Coal
or other bodies such as the Brine Boards, mineral valuers,
and British Geological Survey, before purchasing any land
for development. In some localities, planning authorities may
lay down conditions in regard to old or future mineral extrac-
tion. This has become more frequent since the closure of
many mines has allowed mines to flood which results in a
diaphragm effect in pushing mine gases such as methane and
carbon dioxide to the ground surface. This alone could
render a site undevelopable.
River
Chapter 5
B u ild in g in m in in g
localities
Coal, lead, tin, ironstone, fireclay, sandstone, gypsum, salt
chalk; sand, anhydrite and other minerals have been ex
tracted by various methods over the years, but many of the
industries related to the minerals have gone into decline. A
present the minerals most frequently extracted are coal
gypsum, anhydrite and salt.
Gypsum and anhydrite mines are extensively worked i
Cumbria, Yorkshire, Nottinghamshire and Sussex, but the
nature of the workings results in very large pillars being lef
to provide support for the overlying strata. The seams ex
tracted are very thick; often up to 10m is removed from
30 m thick beds of material.
Chalk was mined in a similar fashion using pillar-and-stall
methods in areas of Kent, Bury St Edmonds, Suffolk and
Norwich. In some areas the overlying rock strata may have
collapsed into swallow holes in the chalk and this material i
usually a mass of loose voided material. In areas of swallow-
hole activity piling taken below the chalk base is generally
required, but if the holes are large and widespread then the
site may not be viable owing to the effects of renewed and
often unpredictable subsidence O I l the site infrastructure.
Salt mining can be carried out by mining or by brine
pumping and the design of foundations in such areas require
special considerations.
5.1 COAL MINING, PAST AND PRESENT
Originally used as a means of obtaining fireclay or ironstone
bell pits (Fig. 5.1) were in use from the thirteenth century up
to the early 18oos. They were usually found in areas where
the thickness of drift (superficial deposits) was relatively thin
and the seams were shallow and fairly level. The depth o
these pits rarely exceeded 12m. They consisted of a vertica
shaft taken down to the various minerals or coal seam; the
shaft was belled out at the bottom to maximize the coa
recovery. Quite often, bell pits sunk for coal extraction
would encounter bands of ironstone and this was often re
moved for use. Sometimes shafts were excavated in pairs
and interconnected at the bottom. To facilitate ventilation fire
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