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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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