1 Soil Formation and Classification

8/8/2019 1 Soil Formation and Classification

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GEOTECH 1 CIV2039S / 3034S

1. SOILS AND THEIR CLASSIFICATION

1.1 Definitions

Soil

The word 'soil' has different meanings for different professions.

To the agriculturist, soil is the top thin layer of earth within which organic forces are

predominant and which is responsible for the support of plant life.

To the geologist, soil is the material in the top thin zone within which roots occur.

From the point of view of an engineer, soil is:

uncemented or weakly cemented accumulation of mineral and organic particles

and sediments found above the bedrock, or


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Soil Mechanics: (ASTM) the application of the laws and principles of mechanics and

hydraulics to engineering problems dealing with soil as an engineering material.

Geotechnical Engineering:

Geotechnical engineering is concerned with the engineering properties of earth

materials. en.wikipedia.org/wiki/Geotechnical_engineering

The application of engineering geology, hydrogeology, soil mechanics, rock

mechanics and mining seismology to the practical solution of ground controlchallenges. www.minesafe.org/training_education/terms.html

1.2 Origin of Soils

Soil is a three phase system of:

solid particles (S)

pore fluid (W)

pore gas (A)


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Transported by the action of:

Glaciers (glacial)

Moving water (fluvial)

Wind (aeolian)

Settling out in salt water (marine)

Settling out in fresh water (lactustrine)

Due to gravity movement downslope (colluvial)

(most common in temperate regions)

Table 1: Naturally occurring soils.Type of soil Mode of Formation Terms

In-situ weathered soils Mechanical and/or chemical weathering of in-situ rocks

depending on the climatic environment.

Residual

Tropical

In-situ peat Decomposition of organic deposits. Cumulose

Gravitational deposits Gravitational action. Colluvium

Water-borne soils River sediments AlluviumLake sediments (Fresh or saltwater) Lacustrine

Formed in an Estuary Estuarine

Formed at the mouth of a river Deltaic

Marine sediments of terrestrial, volcanic or cosmic sources Lithogeneous

Remains of marine organisms Biogeneous

Precipitates from marine or groundwater Hydrogeneous

Glacial deposits Material deposited by ice Till

M i l d i d b l O h


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correlating the results from different boreholes and this information is used to build a

picture of the sub-surface profile.

An indication of the engineering properties is determined on the basis of particle size.

This crude approach is used because the engineering behaviour of soils with very

small particles, usually containing clay minerals, is significantly different from the

behaviour of soils with larger particles. Clays can cause problems because they are

relatively compressible, drain poorly, have low strengths and can swell in the

presence of water.

1.5 Particle Size Definitions

The precise boundaries between different soil types are somewhat arbitrary, but the

following scale is now in use worldwide.

Gravel Sand Silt Clay

C M F C M F C M F C M F60 20 6 2 0.6 0.2 0.06 0.02 .006 .002 .0006 .0002

where C, M, F stand for coarse, medium and fine respectively, and the particle sizes

are in millimetres.

Note

th l ith i l M t il t i i t f d ilt d l ti l


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Organic These may be of either clay or silt sized particles. They containsignificant amounts of vegetable matter. The soils as a result are

usually dark grey or black and have a noticeable odour from decayingmatter. Generally only a surface phenonomen but layers of peat may

be found at depth. These are very poor soils for most engineering

purposes.

1.7 Procedure for grain size determination

Different procedures are required for fine and coarse-grained material. These will bedemonstrated in a laboratory demonstration session.

Coarse Sieve analysis is used to determine the distribution of the larger grainsizes. The soil is passed through a series of sieves with the mesh size

reducing progressively, and the proportions by weight of the soil

retained on each sieve are measured. There are a range of sieve sizes

that can be used, and the finest is usually a 75 m sieve. Sieving can be

performed either wet or dry. Because of the tendency for fine particlesto clump together, wet sieving is often required with fine-grained soils.

Fine To determine the grain size distribution of material passing the 75 msieve the hydrometer method is commonly used. The soil is mixed with

water and a dispersing agent, stirred vigorously, and allowed to settle

to the bottom of a measuring cylinder. As the soil particles settle out of

i th ifi it f th i t d A h d t


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Figure 5 Classification Chart

Important observations from figure 3 are that any soil containing more than 50% of

clay sized particles would be classified as a clay, whereas sand and silt require 80% of

the particles to be in that size range. Also any soil having more than 20% clay would

have some clay like properties.

Th h d i ll i d h h f l i d i l

1009080706050403020100

100

90

80

70

60

50

40

30

20

10

0100

90

80

70

60

50

40

30

20

10

0

Silt Sizes (%)

Sand

Siz

es(%

) ClaySizes

(%)

SandSil ty Sand Sandy Sil t

Clay-Sand Clay-Silt

Sandy Clay Silty Clay

Clay

LOWER MISSISSIPPI VALLEY DIVISION,U. S. ENGINEER DEPT.


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Figure 6. Moisture content versus volume relation

(SL) The Shrinkage Limit - This is the moisture content the soil would have had ifit were fully saturated at the point at which no further shrinkage occurs on drying.

moisturecontentweight of water

weight of solids

w

w

w

s

(1)

In the shrinkage test the soil is left to dry and the soil is therefore not saturated when

the shrinkage limit is reached. To estimate SL it is necessary to measure the totalvolume, V, and the weight of the solids, ws. Then

SL mV

w G

w

s s

1(2)

where w is the unit weight of water, and

Gs is the specific gravity

(PL) The Plastic Limit - This is the minimum water content at which the soil will

deform plastically

(LL) The Liquid Limit - This is the minimum water content at which the soil will

flow under a small disturbing force


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1. To determine the suitability of different soils for various purposes

2. To develop correlations with useful soil properties, for example, compressibility

and strength

The reason for the large number of such systems is the use of particular systems for

certain types of construction, and the development of localised systems.

1.10.1 PRA (AASHO) system

An example is the PRA system of AASHO (American Association of State Highway

Officials), which ranks soils from 1 to 8 to indicate their suitability as a subgrade for

pavements.

1. Well graded gravel or sand; may include fines

2. Sands and Gravels with excess fines

3. Fine sands4. Low compressibility silts

5. High compressibility silts

6. Low to medium compressibility clays

7. High compressibility clays

8. Peat, organic soils

1.10.2 Unified Soil Classification


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First determine the percentage of fines, that is the % of material passing the 75 m

sieve.

Then if % fines is < 5% use W or P as suffix

> 12% use M or C as suffix

between 5% and 12% use dual symbols. Use the prefix from

above with first one of W or P and then with one of M or C.

If W or P are required for the suffix then Cu and Cc must be evaluated

CD

Du

60

10

CD

D Dc

30

2

60 10( )

If prefix is G then suffix is W if Cu > 4 and Cc is between 1 and 3

otherwise use P

If prefix is S then suffix is W if Cu > 6 and Cc is between 1 and 3

otherwise use P

If M or C are required they have to be determined from the procedure used for fine

grained materials discussed below. Note that M stands for Silt and C for Clay. This is

determined from whether the soil lies above or below the A-line in the plasticity chart


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Figure 5 Plasticity chart for laboratory classification of fine grained soils

The final stage of the classification is to give a description of the soil to go with the 2-

symbol class. For a coarse grained soil this should include:

th t f d d l

0 10 20 30 40 50 60 70 80 90 100Liquid limit

0

10

20

30

40

50

60

Plastic

ity

index

CH

OH

or

MH

CLOL

MLor

CL

ML

"A"

line

Comparing soils at equal liquid limit

Toughness and dry strength increase

with increasing plasticity index


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Give typical names: indicate ap-proximate percentages of sandand gravel: maximum size:angularity, surface condition,and hardness of the coarsegrains: local or geological nameand other pertinent descriptiveinformation and symbol inparentheses.

For undisturbed soils add infor-mation on stratification, degreeof compactness, cementation,moisture conditions and drain-age characteristics.

Example:

Well graded gravels, gravel-sand mixtures, little or nofines

Poorly graded gravels, gravel-sand mixtures, little or nofines

Silty gravels, poorlygraded gravel-sand-silt mixtures

Clayey gravels, poorly gradedgravel-sand-clay mixtures

Well graded sands, gravellysands, little or no fines

Poorly graded sands, gravellysands, little or no fines

Silty sands, poorly gradedsand-silt mixtures

Clayey sands, poorly gradedsand-clay mixtures

GW

GP

GM

GC

SW

SP

SM

SC

Wide range of grain size and substantialamounts of all intermediate particlesizes

Predominantly one size or a range ofsizes with some intermediate sizesmissing

Non-plastic fines (for identificationprocedures see ML below)

Plastic fines (for identification pro-cedures see CL below)

Wide range in grain sizes and sub-stantial amounts of all intermediateparticle sizes

Predominantely one size or a range ofsizes with some intermediate sizes missing

Non-plastic fines (for identification pro-cedures, see ML below)

Plastic fines (for identification pro-cedures, see CL below)

ML

CL,CI

OL

MH

Dry strengthcrushing

character-istics

None toslight

Medium tohigh

Slight tomedium

Slight tomedium

High to very

Dilatency(reaction

to shaking)

Quick toslow

None to veryslow

Slow

Slow tonone

None

Toughness(consistencynear plastic

limit)

None

Medium

Slight

Slight tomedium

High

Inorganic silts and very fine sands,rock flour, silty or clayeyfine sands with slight plasticityInorganic clays of low to mediumplasticity, gravelly clays, sandyclays, silty clays, lean clays

Organic silts and organic silt-clays of low plasticity

inorganic silts, micaceous ordictomaceous fine sandy orsilty soils, elastic silts

Inorganic clays of high

Give typical name; indicate degreeand character of plasticity,amount and maximum size ofcoarse grains: colour in wet con-dition, odour if any, local orgeological name, and other pert-inent descriptive information, andsymbol in parentheses

For undisturbed soils add infor-mation on structure, stratif-ication, consistency and undis-

turbed and remoulded states

Field identification procedures(Excluding particles larger than 75mm and basing fractions on

estimated weights)

Groupsymbols

1Typical names

Information required fordescribing soils

Laboratory classificationcriteria

C = G re at er t ha n 4D

D----60

10U

C = Between 1 and 3(D )

D x D----------------------30

10c

2

60

Not meeting all gradation requirements for GW

Atterberg limits below"A" line or PI less than 4

Atterberg limits above "A"line with PI greater than 7

Above "A" line withPI between 4 and 7

are borderline casesrequiring use of dualsymbols

Not meeting all gradation requirements for SW

C = G re ate r t han 6D

D---- 60

10U

C = Between 1 and 3(D )

D x D----------------------30

10c

2

60

Atterberg limits below"A" line or PI less than 4

Atterberg limits above "A"line with PI greater than 7

Above "A" line withPI between 4 and 7are borderline casesrequiring use of dualsymbolsD

eterminepercentagesofgravelandsan

dfromgrainsizecurve

Usegrainsizecurveinident

ifyingthefractionsasgivenunderfieldidentification

Dependingonpercentagesoffines(fractionsmallerthan.0

75mm

sievesize)coarsegrainedsoilsareclassifiedasfollows

Lessthan5%

Morethan12%

5%to12%

GW,

GP,SW,S

P

GM,G

C,S

M,

SC

Bordelinecaserequiringuseofdualsymbols

The.0

75mmsievesizeisaboutthesmallestparticlevisibletothenakedeye

Finegrainedsoils

M

rethanhalfofmaterialissmallerthan

.075mmsievesize

Coarsegrainedsoils

Morethanhalfofmaterialislargerthan

.075mmsievesize

ndclays

dlimit

erthan

50

Siltsandclays

liquidlimit

lessthan50

Sands

Morethanhalfofcoarse

fractionissmallerthan

2.36mm

Gravels

Morethanhalfofcoarse

fractionislargerthan

2.36mm

Sandswith

fines

(appreciable

amountoffines)

Cleansands

(littleorno

fines)

Grave

lswith

fines

(apre

ciable

amount

offines)

Cleangravels

(littleorno

fines)

Identification procedure on fraction smaller than .425mmsieve size

Unified soil classification (including identification and description)

Silty sand, gravelly; about 20%hard angular gravel particles12.5mm maximum size; roundedand subangular sand grainscoarse to fine, about 15% non-plastic lines with low drystrength; well compacted andmoist in places; alluvial sand;(SM)

20

30

40

50

60

Plasticityindex

CH

OH

orOLCL

"A"

line

Comparing soils at equal liquid limit

Toughness and dry strength increase

with increasing plasticity index

CI


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Atterberg limits: Liquid limit LL = 32, Plastic Limit, PL =26

Step 1: Determine the % fines from the grading curve

%fines (% finer than 75 m) = 11% - Coarse grained, Dual symbols required

Step 2: Determine % of different particle size fractions (to determine G or S), and D10,

D30, D60 from grading curve (to determine W or P)

D10 = 0.06 mm, D30 = 0.25 mm, D60 = 0.75 mm

Cu = 12.5, Cc = 1.38, and hence Suffix1 = W

Particle size fractions: Gravel 17%

Sand 73%

Silt and Clay 10%

Of the coarse fraction about 80% is sand, hence Prefix is S

Step 3: From the Atterberg Test results determine its Plasticity chart location

LL = 32, PL = 26. Hence Plasticity Index Ip = 32 - 26 = 6

From Plasticity Chart point lies below A-line, and hence Suffix2 = M


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1.15

AASHTO Classification


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