Classification of Soils

HAZZAD BIN YAHYAKEA10012

CLASSIFICATION OF SOILS

IMPORTANT STATEMENT: These lecture notes are based upon BS5930:1999 and BS1377:1990 but, where appropriate, reference is made to “Eurocode 7 related documents”. These are namely BS EN ISO 14688-1:2002, BS EN ISO 14688-2:2004 and BS EN ISO 14689-1:2003). During this transitional stage as the full recommendations of the Eurocode are being implemented during 2010 students are advised to be aware that published text books are likely to make little reference to the Eurocode 7 (ie. EN 1997) and there are some major differences in the way that soils are described.

Purpose of soil classification

1. Provides a concise and systematic method for designating various types of soil. 2. Enables useful engineering conclusions to be made about soil properties. 3. Provides a common language for the transmission of information. 4. Permits the precise presentation of boring records and test results.

Object of soil classification

Is to provide a soil NAME and symbol, e.g. GRAVEL is G, based on the results of simple and quick to perform (therefore economic) key tests; 1. Particle size distribution (P.S.D.) or sieve analysis. 2. Plastic properties; Liquid limit test Plastic limit test

Soil is initially classified into either coarse or fine soil on the basis of particle size.

Coarse soil (Granular) Physical characteristics and appearance are influenced by the distribution of particle sizes within the soil, i.e.>0.063mm (1/16mm)

A granular soil is classified according to its Particle Size Distribution.

Fine soil (Cohesive) Physical characteristics and appearance influenced by cohesion and plastic properties (plasticity) associated with mineral composition and water content.

The fine soil is sub-grouped according to its plasticity.

CLASSIFICATION OF COARSE SOIL

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It is classified on the sand (symbol S) and gravel (symbol G) content plus the amount of fines present.

Particle size distribution by sieve analysis

This test involves passing soil through a series of sieves of decreasing mesh size and recording the weight of soil retained on each. Soil which passes through the finest sieve (63 microns or 0.063mm) will be collected in a base tray and be weighed.

The results are processed and plotted on a semi-log chart of cumulative percentage passing (y-axis) verses log of particle size in mm (x-axis).

Where a soil contains fines, a relatively small proportion may cause the soil to form a cohesive mass and dominate the properties of the soil. It is considered that a soil containing 35% fines or more will behave as a cohesive soil (ie silt or clay).

The fines content is critical and is indicated with an extra symbol;

Symbol % Fines Terminology Fines contentG 0 - 15 Clean gravel

Clean sandLittle influence

SG-F 5 – 15 Gravel with some fines

Sand with some finesSome influence

S-FGF 15 - 35 Gravel with much fines

Sand with much finesConsiderable influenceSF

Grading

The shape of the Particle Size Distribution curve indicates the range of particle sizes within a soil. Coarse soils are sub-grouped on whether a soil is well graded or poorly graded.

A well graded (symbol W) soil has approximately equal proportions of particles sizes and the curve is usually smooth. Note the Till is a well graded soil and the well graded gravel GW in the PSD chart below.

A poorly graded (symbol P) soil may contain a high proportion of material within a limited particle size band or bands. Poorly graded soil may be further sub-divided into uniform soil and gap graded soil:

A poorly graded soil (Pu, uniform or closely graded) has a major proportion of the particles lying between narrow size limits. As shown by the Estuary Sand, SPuF, in the PSD chart.

Gap graded soil has particles of both large and small sizes but with a relatively small proportion of particles with intermediate size, see clayey gravely sand, SPg in the PSD chart.

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Soil description

Four elements can be deduced on the basis of the grading curve:

Grading Secondary constituent Sub-divisions Soil name(IN CAPS)Well graded sandy fine and medium GRAVEL

Other physical characteristics can also be included, for example overall colour, strength, presence of root inclusions, layering/banding or organic odours.

Some examples of soil descriptions are:

Dense, reddish-brown, sub-angular, well graded, gravelly SAND

Firm, grey, laminated CLAY of low plasticity with occasional silt partings 0.5-2.0mm

Dense, brown, heterogeneous, well graded, very silty SAND and GRAVEL with some COBBLES: Till

Stiff, brown, closely fissured CLAY of high plasticity: London Clay

Spongy, dark brown, fibrous PEAT

NOTE: A soil description is NOT a soil classification.

Analysis of particle size distribution curves

Analyse each of the curves A, B and C shown below using the table provided;

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Soil Compositions: Soil A: Gravel…..…% Sand…..…% Fines……..%

Soil B: Gravel……..% Sand……..% Fines……..%

Soil C: Gravel……..% Sand……..% Silt……..% Clay……..%

Soil classifications: Soil A: ______ Soil B: ______ Soil C: ______

Soil Descriptions Soil A: _____________________________________ Soil B: _____________________________________ (Soil C requires further classification of the fines faction according to plasticity)

CLASSIFICATION OF FINE SOIL

The classification of fine soil (F) is based on the property of soil plasticity. Plasticity is a function of a fine soils capacity to absorb water and remain in a cohesive state.

There are two sub-groups in fine soil;

Sub-group

Symbol Particle size

(mm)Silt M 0.06 –

0.002

Clay C <0.002

Water added to coarse soil will fill the voids and saturate the sample, and further water will simply drain off and not be absorbed by the soil.

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Water added to a fine soil will initially fill the voids. When the soil is saturated it will continue to absorb water due to the properties of the clay minerals and an increase in volume of the soil mass will occur. At the same time the soil is progressively softened by the water which increases the distance between the clay minerals and decreases the attracting forces between them, thus decreasing the cohesion.

As the soil gets weaker it becomes pliable and when it is sufficiently pliable to be rolled out into a thread, it is said to be in a plastic state. Eventually the soil loses all its strength and starts to flow under its own weight, when it is said to be in a liquid state.

The moisture content at which the soil becomes plastic is defined as the Plastic Limit (PL)

The moisture content at which the soil changes from plastic to liquid is defined as the Liquid Limit (LL)

The range of moisture content over which the soil remains in a plastic condition is defined as the Plasticity Index (PI);

PI = LL – PL

Silt and clay have very different mechanical properties; Silt -very low cohesive strength -absorbs very little water to become a “liquid” -dries to a powder

Clay -cohesive -behaves as a plastic material over a wide range of moisture content -becomes brittle on drying

It is important to determine which soil type a fine soil belongs to, i.e. silt or clay?

The property of plasticity is used to determine whether a fine soil is either silt or clay. Clay has a large range of moisture content in which it is in a plastic state (P.I. value) compared to silt;

Tests to determine liquid and plastic limits

i) Plastic Limit

The moisture content is determined at which a thread of soil can be rolled without breaking until it is only 3mm in diameter.

Mix a small amount of clay with distilled water to form a pat about the size of a marble. Roll out the pat on a glass plate until a thread is formed. The thread should be gently rolled until the diameter is about 3 mm. If a smooth un-broken thread is obtained, roll up the thread into a ball and gently mark in the palm of the hand and repeat the process until the clay just begins to crumble when the 3 mm diameter is rolled. Then carry out normal moisture content determination.

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Repeat a total of 3 times and average the results.

ii) Liquid Limit (Preferred method using the cone penetrometer)

Details of the apparatus are shown below. The soil to be tested is air dried and thoroughly mixed. At least 200g of the soil are sieved through a 425 micron sieve and placed on a glass sheet. The soil is then mixed with distilled water into a paste. N.B. 1 micron = 0.001mm

A metal cup, approximately 55 mm in diameter and 40 mm deep is filled with paste and the surface struck off level. The cone is next placed at the centre of the smoothed soil surface and level with it. The cone is released so that it penetrates into the soil and the amount of penetration, over a time period of 5 seconds, is measured.

The test is now repeated by lifting the cone clear, cleaning it and filling up the depression in the soil surface by adding a little more of the wet soil.

If the difference between the two measured penetrations is less than 0.5 mm then the tests are considered valid. The average penetration is noted and a moisture content determination (detailed in practical laboratory work) is carried out on the soil tested.

The procedure is repeated at least four times with increasing water contents. The amount of water used throughout should be such that the penetration obtained lie within a range of 15 to 25 mm.

To obtain the liquid limit the variation of cone penetration (plotted vertically) to moisture content (plotted horizontally) is drawn out (both scales being natural).

The best straight line is drawn through the experimental points and the liquid limit is taken to be the moisture content corresponding to a cone penetration of 20 mm (expressed as a whole number).

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Use of the ‘A-Line’ classification chart for fine soil

This classification chart plots Plasticity Index (%) against Liquid Limit (%), see below. The main aim is to determine if fine soil is either silt or clay. Clay plots above the A-line and silt below. Generally high Liquid Limit values correlate with high the plasticity.

Silts and organic soils have a low Plasticity Index (ie. a small range of moisture content over which they are plastic) compared to their Liquid Limit.

Clay soils have a high Plasticity Index in relation to their Liquid Limit. Clay minerals have the capacity to take in moisture and still retain some cohesion.

To classify a fine soil according to the “A–Line” chart:

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i) Determine Liquid and Plastic Limits, and therefore Plasticity Index.

ii) Using values of PI and LL, „plot the soil‟ on the chart.

iii) Observe which segment the soil comes into (CL, CI, ML etc)

iv) Write down the soil name, CLAY for „C‟ soils, SILT for „M‟ soils.

v) Follow the name with the plasticity ie. CI = CLAY of intermediate plasticity.

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Table 1 British Soil Classification System for Engineering Purposes

SOIL GROUPS SUB-GROUPSand in laboratory identification

GRAVEL and SAND may be qualified sandy GRAVEL and gravely SAND where appropriate

Group Symbol

Sub-group symbol

Fines (% less than 0.06 mm)

Liquid limit

COARSE SOILSless than 35% of the material is finer than0.06 mm

GRAVELSMore than 50% of coarse material is of gravel size (coarser than 2 mm)

Slightly silty or Clayey GRAVEL

GWGGP

GW

GPu GPg

0 to 5

Silty GRAVELClayey GRAVEL

G-MG-FG-C

GWM GPM

GWC GPC

5 to 15

Very silty GRAVELVery clayey GRAVEL

GMGFGC

GML, etc

GCLGCIGCHGCVGCE

15 to 35

SANDSMore than 50% of coarse material is of sand size(finer than 2 mm)

Slightly silty orclayey SAND

SWSSP

SW

SPu SPg

0 to 5

Silty SANDClayey SAND

S-MS-FS-C

SWM SPM

SWC SPC

15 to 35

Very silty SANDVery clayey SAND

SMSFSC

SML etc

SCLSCISCHSCVSCE

15 to 35

FINE SOILSmore than 35% of the material is finer than 0.06 mm

Gravelly or sandy SILTS and CLAYS 35% to 65% fines

Gravelly SILT

Gravelly CLAY

MGFGCG

MLG etcCLGCIGCHGCVGCEG

< 3535 to 7050 to 7070 to 90> 90

Sandy SILT

Sandy CLAY

MSFSCS

MLS etc

CLS etcSILTS and CLAYS

SILT (M SOIL)

M ML etc < 3535 to 50

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65% to 100% fines

CLAYFC

CLCICHCVCE

50 to 7070 to 90> 90

ORGANIC SOILS

Description letter ‘O’ suffixed to say group or subgroup symbol

Organic matter in significant amount e.g. MHO – organic silt of high LL

PEAT Pt – consists predominantly of plant remains (fibrous or amorphous)

Primary Letter Secondary letter G Gravel W Well graded S Sand P Poorly graded M Silt M With non-plastic fines C Clay C With plastic fines O Organic Soil L Of low plasticity (LL<50) Pt Peat H Of high plasticity (LL>50)

Rapid methods of classification

Rapid methods of classification are appropriate for identification in the field, or where laboratory facilities are not available. A combination of sound judgment (based on experience) and simple tests is used in conjunction with Tables 2 to 4 below. If soil sub-group symbols are quoted they should be written enclosed in brackets to indicate that they are based on a rapid (as opposed to laboratory) method.

Particle size

Gravel sizes (>2 mm) are apparent visually; sands (<2 mm) tend to cling together when damp and they feel gritty between the fingers; silts (< 0.06 mm) feel abrasive, but not gritty; clays (< 2 μm) feel greasy.

Grading

The grading of a soil refers to the distribution of particle sizes: a uniform soil consists of a very narrow range of particle sizes, whereas a well-graded soil contains a wide range.

For a rapid estimate of grading, a field-settling test may be carried out in a tall jar or bottle. A sample of soil is shaken up with water in ajar and allowed to stand for a few minutes. The coarsest particles settle to the bottom first, so that a subsequent examination of the layers in the jar will yield approximate proportions of the various size ranges.

If over 65% of the soil particles are greater than 0.06 mm, the soil should be described as coarse-grained, i.e. either a SAND or a GRAVEL. If over 35% of the soil is less than 0.06 mm, it is fine-grained, i.e. SILT or CLAY.

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Amount of fines.

The quick-settling test will give a rough indication of the amount of fine-grain material present. To distinguish between silt and clay, rub the wet soil between the fingers: clay tends to stick to the fingers, whereas silt only leaves them slightly dusty.

Plasticity and consistency

If the soil particles stick together when wet, the soil possesses cohesion; if the wet mass can be easily moulded it possesses plasticity. Grades of consistency ranging between soft and very stiff may be identified. NOTE: BS EN ISO14688 procedure is DIFFERENT.

Dilatancy (test for silt)

A pat of moist soil is placed on the open palm of one hand and the side of

this hand tapped with the other. Dilatancy is exhibited when, as a result of the tapping, a glossy film of water appears on the surface of the pat. When the pat is gently pressed, the water disappears and the pat becomes stiff. Very fine sands and inorganic silts exhibit marked dilatancy.

Toughness

A thread of moist soil is rolled by the palm of the hand until it dries sufficiently to crumble and break just as it reaches a diameter of 3 mm. The greater the plasticity of the soil, the more easily this can be done.

Dry strength If a pat of moist soil is dried, preferably in an oven, it will shrink and harden according to the silt or clay content. Its dry strength may be estimated by attempting to break the pat with the fingers. A high dry strength indicates a clay of high plasticity, whereas a crumbly powdery dry pat indicates a silt of low plasticity.

Penetration resistance (sands and gravels) At the surface or in trial pits, a spade, a pick or a small wooden peg driven into the soil, will give an indication of compactness.

Soil structure From observations in trial pits or at other exposures, the main structural characteristics may be determined; such as the spacing between beds or laminations, whether or not different materials are inter-bedded and whether or not the soil is fissured.

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Classification of Soils