Soil Composition Classification Notes

Geotechnical Engineering 268 − Dr Mohamed Shahin Curtin University of Technology −

Soil Composition & Classification


Soil Composition Soil is a particulate material that consists of individual particles assembled together

and the engineering properties of soil rely on the interaction among these particles.

A soil consists of rock particles of different sizes and shapes and these are referred to as solids. The spaces between the solid particles are known as voids, or pores. These voids can contain air, water or both. If the voids contain only air, the soil is called dry soil, whereas if voids contain only water, the soil is called saturated soil. Figure 1 shows the three phases of a soil including the solids, water and air.

If the voids are completely full of water, the soil is said to be saturated. If, on the other hand, there is some air in the voids, the soil is referred to as unsaturated or partially saturated.

Figure 1: Soil components including solids and voids


Phase Relationships In order to understand the engineering properties of a soil, the volumes and

weights of the main three phases of soil should be quantified. The soil three phases can be schematically represented as shown in Figure 2, which is known as the phase diagram.

Air

Water

Solids

Weight (or mass)Volume

Wa = 0

Ww

Ws

Wt

Va

Vw

Vv

Vs

Vt

Where;Wa, Ww, Ws = weights of air, water and solids, respectively;Va, Vw, Vs = volumes of air, water and solids, respectively;Wt, Vt = total weight of soil, total volume of soil; andVv = volume of voids.

Figure 2: The phase diagram of soil


Basic Definitions Geotechnical engineers usually need to relate several volume-weight parameters

of the soil components shown previously in Figure 2 to obtain important information on the composition of a particular soil. The basic definitions needed are explained below.

Moisture Content: is the weight of water which can be removed from the soil, by drying to a constant mass at 105oC, expressed as a percentage of the dry weight, as follows:

100(%) ×=s

wc W

Wm (1)

Degree of Saturation: is the volume of water in the voids, expressed as a percentage of the total volume of voids, as follows:

(2)100(%) ×=v

w

VVS

Porosity: is the ratio of the volume of voids (containing air and/or water) in a soil to the total volume of soil, as follows:

(3)100(%) ×=t

v

VVn


Bulk Unit Weight or Total Unit Weight: is the total weight of soil (including solid particles and any contained water) to the total volume of soil, as follows:

Dry Unit Weight: is the weight of dry soil to the total volume of soil, as follows:

Saturated Unit Weight, γs is the total unit weight, γt, when soil is fully saturated under the ground water table.

t

tt V

W=γ (4)

t

sd V

W=γ (5)

Unit of Water, γw = 9.81 kN/m3 (6)

The dry unit weight is also related to the total unit weight and moisture content as follows:

(7)c

td m+=

1γγ


Void Ratio: is the ratio of the volume of voids (containing air and/or water) in a soil to the volume of solids, as follows:

(8)s

v

VVe =

Air Voids (content): is the amount of air in a soil mass, as follows:

(9)100(%) ×=t

av V

VA

Specific Gravity: is the ratio of the density of soil grains (solids) to the density of water. It tells us how many times the soil grains are heavier than water, and it can be calculated as follows:

(10)ws

s

w

ss V

WGγ

=γγ

=


If we know any three of the previous properties, we can evaluate the other ones by applying the equations derived from the phase relationships. The Australian Standards Methods of Testing Soils for Engineering Purposes (AS 1289) detail a large number of laboratory and field procedures for measuring all the previous parameters. Of these parameters, it is only necessary to measure the moisture content, bulk unit weight (or dry unit weight) and the specific gravity. The section that follows outlines these tests.

Moisture Content is determined by taking a representative sample of the soil and weighing it, giving Wt. The sample is then placed overnight in a constant temperature oven set at 105oC, or for a short time in a microwave oven, and then re-weighed, giving the weight of solids, Ws. The weight of water is determined by subtracting Ws from Wt. The moisture content, mc, can then be calculated using Equation (1).

Bulk Unit Weight is determined by taking a representative sample of the soil and weighing it, giving Wt. The volume, Vt, of the sample can be measured in a number of ways. One technique is by the sand replacement method, in which an excavated soil is replaced with sand of known unit weight. Once Wt and Vt are known, γ can be calculated using Equation (4).


Dry Unit Weight is determined in much the same way as the bulk unit weight, except that the sample is placed in an oven, in a manner similar to that for the measurement of the moisture content. The sample is weighed on removal from the oven yielding Ws. The volume, Vt, of the sample can be measured using the sand replacement method. Once Ws and Vt are known, γd can be calculated using Equation (5).

Specific Gravity is determined using a simple apparatus called a pycnometer. Detailed description of the test procedure can be found the laboratory manual. In this test, a known weight of oven-dried soil, Ws, is immersed in water contained in special glass jar, giving Vs. Gs can then be evaluated using Equation (10).


Worked Example (1)A cylindrical sample of moist clay has a diameter of 38 mm, height of 76 mm and mass of 174.2 gm. The sample was kept in the oven at 105oC for about 24 hours and the clay mass was reduced to 148.4 gm. The specific gravity of this clay is 2.7. Determine the bulk unit weight, dry unit weight, water content, degree of saturation and void ratio. [Answer: γt = 19.82 kN/m3; γd = 16.89 kN/m3; mc = 17.4%; S = 82.4%; e = 0.57]


Soil Classification Soils can behave differently depending on their geotechnical properties and

classification. Soils can be classified into two main types: coarse grained soils and fine grained soils.

Coarse grained soils (also called granular soils) are those where the grains are larger than 0.075 mm, and their engineering behaviour is mainly governed by the grain size, density of packing, shape and distribution of solid particles.

Fine grained soils are those where the grains are smaller than 0.075 mm, and their behaviour is greatly influenced by the mineralogy (minerals present in the soil mass) and water content. The borderline between the coarse and fine grained soils is 0.075 mm, which is the smallest size one can distinguish with the naked eye.

Based on grain sizes, the Australian Standards AS 1726 groups soils into clays (<0.002 mm), silts (0.002-0.075 mm), sands (0.075-2.36 mm), gravels (2.36-63 mm), cobbles (63-200 mm) and boulders (>200 mm). The relative proportions of each group within a soil mass significantly influence the engineering behaviour of soil, and the two main tools that enable the geotechnical engineer to classify soils are the grain size distribution and Atterberg limits, as will be explained later.


Grain Size Distribution The grain size distribution is a graphical plot of the cumulative percentage by

weight below any nominated size. It is obtained by means of two tests: sieve analysis for coarse grained soils and hydrometer analysis for fine grained soils.

Sieve Analysis: is carried out on soil particles less than 0.075 mm using a standard particle size distribution test, and detailed description of the test can be found in the laboratory manual. The test involves mechanically shaking an oven-dried soil through a series of woven-wire square-mesh sieves with successively smaller openings (Figure 3). Since the total mass of the sample is known, the percentage retained or passing each sieve can be determined by weighing the amount of soil retained on each sieve after shaking. The grain size distribution can then be plotted on a logarithmic graph, as shown in Figure 5.

Hydrometer Analysis: since it is not practical to manufacture sieves that are smaller than 0.075 mm in size, thus, the hydrometer test (Figure 4) is used to determine the grain size distribution of fine grained soils. Detailed procedures of the test can be found in the laboratory manual. This test relies on sedimentation (the rate at which particles settle) and Stokes’ law. Stokes’ law states that particles in a suspension settle at a rate varies with their size, consequently, it is possible to calculate the size of particles that have settled a known distance in a suspension at any time from the beginning of sedimentation. The obtained grain size distribution can then be plotted on the logarithmic graph, as shown in Figure 5.


It should be noted that very often a soil contains both coarse and fine grains and it is necessary to do both sieve and hydrometer analyses to obtain the complete grain size distribution (see for example Figure 5).

Figure 3: Sieve analysis test Figure 4: Hydrometer test


Figure 5: Typical grain size distribution curve

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90

100

0.001 0.01 0.1 1 10 100Particle size (mm)

% P

assi

ng

clay cobblessilt sand gravel

fine medium coarse fine medium coarse fine medium coarse

0.002 0.075 2.36 63.00.006 0.02 0.2 0.6 6.0 20.0

Hydrometer test Sieve test


A soil is known to be well-graded when a wide range of particle sizes is present in which smaller grains fill the voids created by larger grains thus producing a dense packing. The soil that is not well graded is poorly-graded in which there is a deficiency or an excess of a certain particle size. Uniform soils and gap-graded soils are special cases of poorly graded soils. A uniform soil is the one where all grains are about the same size, and a gap-graded soil is the one that has a range of grain size is absent from the soil mass. Typical examples of different grain size distribution curves are shown in Figure 6.

Figure 6: Typical grain size distributions

0

10

20

30

40

50

60

70

80

90

100

0.001 0.01 0.1 1 10 100Particle size (mm)

% P

assi

ng

Uniform

Gap graded

Well graded


The grain size distribution curve can be described through two simple coefficients, including the coefficient of uniformity, Cu, and coefficient of curvature, Cc, which are calculated as follows:

10

60

DDCu =Coefficient of Uniformity,

6010

230)(DD

DCc =Coefficient of Curvature,

where; D10, D30 and D60 are the grain diameters (in mm) corresponding to 10%,30% and 60%, respectively, soil passing.

A sand is well graded if Cu > 6 and Cc = 1–3, whereas a gravel is well graded if Cu> 4 and Cc =1–3. For the grain size distribution curve shown earlier in Figure 5, D10 = 0.01 mm, D30 = 0.075 mm, D60 = 0.2 mm, then Cu = 20 and Cc = 2.8. Therefore, it is a well graded soil. It can also be seen that this soil contains 69% sand, 7% gravel and 24% fines.

(11)

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Worked Example (2) The results of a sieve analysis for a dry soil are given in the table below. Plot the

grain size distribution curve and calculate Cu and Cc, and describe the soil grading. [Answer: Cu = 3.5 , Cc = 1.9; poorly graded]

Sieve size (mm) Mass Retained (gm)4.75 0.0

2.36 2.6

1.18 12.5

0.6 57.7

0.425 62

0.3 34.2

0.15 31.4

0.075 13.1

Pan 3.9


Sieve size (mm)

Mass Retained (gm)

% Retained % Cumulative Retained

% Cumulative Passing

4.75 0.0 0.0 0.0 1002.36 2.6 1.2 1.2 98.81.18 12.5 5.8 7.0 93.00.6 57.7 26.5 33.5 66.5

0.425 62 28.5 62.0 38.00.3 34.2 15.7 77.7 22.30.15 31.4 14.4 92.1 7.9

0.075 13.1 6.0 98.1 1.9Pan 3.9 1.8 100 0.0

Total = 217.4 gm

Calculation of worked example (2)


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90

100

0.001 0.01 0.1 1 10 100Particle size (mm)

% P

assi

ng



0.002 0.075 2.36 63.00.006 0.02 0.2 0.6 6.0 20.0

Grain size distribution of worked example (2)


Atterberg Limits The consistency (degree of firmness, i.e. soft, firm, stiff) of fine grained soils

relies on the water content of soil. As the water content is increased from zero, soil consistency varies from the solid state (when dry) to the semisolid, plasticand liquid states, as shown in Figure 7. Atterberg limits are the border line water contents between such states. These border line water contents are termed the liquid limit, plastic limit and shrinkage limit, which will be defined next. Atterberg limits are very valuable in soil classification

Figure 7: Soil consistency and Atterberg limits


Liquid Limit (LL or wL): is the water content over which the soil behaves like a viscous mud, flowing under its own weight and having a very little strength. It is the transition water content between the plastic and liquid states. The liquid limit is determined by either one of two widely accepted laboratory test methods: Casagrande device or fall-cone penetrometer.

Casagrande test involves taking approximately 250 gm of soil that passes the 0.425 mm sieve and curing it overnight in water. The cured soil is then thoroughly mixed and a portion is placed in the Casagrande liquid device, as shown in Figure 8(a). The device consists of a brass cup which is connected to a handle and cranking mechanism so that when the handle is rotated, the cup is lifted 10 mm and falls to the hard rubber base. Casagrande defined the liquid limit as the moisture content at which a standard groove cut in the remoulded soil sample by a grooving tool (Figure 8a) will close over a distance of 13 mm at 25 blows of the liquid limit device. In practice, it is difficult to add the exact water necessary to achieve this, however, Casagrande found that one could plot the moisture content and the blow counts for a number of tests (about four), and thus plot a curve, as shown in Figure 8(b), from which the liquid limit can be obtained.

The fall-cone penetrometer, on the other hand, consists of a 30o stainless steel cone, 35 mm long, having a mass of 80 gm (Figure 9a). The soil is prepared as above and once cured, a cylindrical metal cup, 55 mm internal diameter and 40 mm deep, is filled and levelled with the soil sample. The cone is lowered so that it just touches the surface of the soil in the cup and the cone is then released. The depth of penetration is measured and the test repeated at least four times. A curve (see Figure 9b) is obtained by plotting the cone penetration against moisture content. The liquid limit is the moisture content at which the cone penetrates the soil sample by 20 mm.


55

60

65

70

75

10 15 20 25 30 35 40

Number of blows

Moi

stur

e co

nten

t (%

)

wL

Figure 8: Determination of liquid limit by the Casagrande method device

(a) Casagrande liquid limit device

(b) Casagrande liquid limit curve


Figure 9: Determination of liquid limit by the fall-cone penetrometer

(a) Fall-cone penetrometer (b) Fall-cone liquid limit curve


Plastic Limit (PL or wp): is the lowest water content at which the soil exhibits plastic behaviour. It is the transition water content between the semi-solid and plastic states. The plastic limit is determined by a test that involves preparing and curing a sample of soil in the same way as for the liquid limit test. Portions of the soil are rolled into threads on a glass plate. The plastic limit is the moisture content at which the threads of the soil crumble at a diameter of 3mm, as shown in Figure 10. If the threads can be rolled to diameters less than 3 mm before crumbling, the soil is too wet, and conversely, if the threads crumble before 3 mm, the soil is too dry. This test requires some experience before consistent results can be obtained.

Figure 10: Determination of plastic limit


Shrinkage Limit (SL or ws): is the water content below which the soil will not shrink when dried. It is the transition water content between the solid and semi-solid states. The shrinkage limit is determined by a test that involves placing a soil sample in a shrinkage mould, as shown in Figure 11. The mould is completely filled with the soil, levelled and placed in an oven until the soil dries. The length of the sample is measured upon removal from the oven and the shrinkage limit is obtained as the decrease in length expressed as a percentage of the original length, as follows:

Figure 11: Determination of shrinkage limit

1001 ×

⋅

⋅⋅−=

lengthinitialdryingafterlengthSL (13)


Atterberg limits can also be used to obtain some consistency parameters from which the behaviour fine grained soils can be distinguished. These parameters include the Plasticity Index (PI), liquidity index (LI) and activity (A).

Plasticity index is the range of water content over which the soil remains plastic and is calculated as follows:

PLLLPI −= (14)

PIPLmLI c −=

Liquidity Index is defined as follows:

where, mc is the in-situ moisture content of the soil.

if LI < 0 the soil will behave in a brittle fashion when sheared;if 0 < LI < 1 the soil will behave plastically when sheared; andif LI ≥ 1 the soil will behave like a viscous mud when sheared.

(15)


Activity is a good indicator of soil potential to swell-shrink. Higher the activity means higher the swell-shrink potential. Activity is calculated as follows:

clayofPIA⋅⋅

=%

if A < 0.75 the clay is said to be inactive;if 0.75 < A < 1.25 the clay is said to be normal; andif A ≥ 1.25 the clay is said to be active.

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The Unified Soil Classification System

Major soil group Descriptor

Gravel (G)Sand (S)

Well-graded (W)

Poorly-graded (P)

Silty (M)

Clayey (C)

Silt (M)Clay (C)Organic (O)

Low plasticity (L)

High plasticity (H)

Table 1: The USCS soil groups and their descriptors

The Unified Soil Classification System (USCS) is the most widely used soil classification system in geotechnical engineering. It divides the soils into major groups of coarse-grained soils (i.e. sand and gravel) and fine-grained soils (i.e. silt, clay and organic), and describes each soil group using standard descriptors, as shown in Table 1. Each soil is given a two-letter code, the first letter refers to the major soil group and the second letter refers to the descriptor. For example, a GW is a well-graded GRAVEL, an SP is a poorly-graded SAND, a GM is a silty GRAVEL, an SC is a clayey SAND, a ML is a low plasticity SILT, a CH is a high plasticity CLAY, an OL is a low plasticity ORGANIC soil.


To use the USCS, follow the following steps:

1. Determine if the soil is highly organic. Such soil is composed primarily of organic matter, has dark brown/grey or black colour and has a definite rotten odour especially when wet. If the soil has these characteristics, then classify it as Peat, Pt. This soil is very problematic because it has high compressibility and low strength. On the other hand, if the soil is inorganic, then go to Step 2.

2. Conduct a sieve analysis (and a hydrometer test if the soil has fines) to determine the grain-size distribution curve, and then follow the instructions on Table 2, with the aid of grading equations and plasticity chart.

3. The U-line of the plasticity chart shown in Table 2 indicates the upper range of plasticity index and liquid limit coordinates. Where the Atterberg limits of any soil are found to plot above the U-line, it is suggested that the results be treated with caution and re-checked.


Table 2: Unified Soil Classification SystemSoil Classification Criteria for assigning Group Symbols and Group Names using laboratory tests

Group Symbol Group Name Cu ³ 4 and Cc = 1–3c GW Well-graded Gravel < 5% finesa Cu < 4 or Cc ≠ 1–3c GP Poorly-graded Gravel Fines classify as ML or MH GM Silty Gravel

Gravel > 50% retained on 2.36 mm sieve size > 12% finesa

Fines classify as CL or CH GC Clayey Gravel Cu ³ 6 and Cc = 1–3c SW Well-graded Sand < 5% finesb Cu < 6 or Cc ≠ 1–3c SP Poorly-graded Sand Fines classify as ML or MH SM Silty Sand

Coarse Grained Soil > 50% retained on 0.075 mm sieve size

Sand < 50% retained on 2.36 mm sieve size > 12% finesb

Fines classify as CL or CH GC Clayey Sand PI plots on or above A- lined CL Low plasticity Clay Inorganic PI plots below A- lined ML Low plasticity Silt PI plots on or above A- lined OL Low Plasticity Organic Clay

Silts and Clays Liquid Limit < 50

Organice PI plots below A- lined OL Low Plasticity Organic Silt PI plots on or above A- lined CH High Plasticity Clay Inorganic PI plots below A- lined MH High Plasticity Silt PI plots on or above A- lined OH High Plasticity Organic Clay

Fine Grained Soil < 50% retained on 0.075 mm sieve size

Silts and Clays Liquid Limit ³ 50

Organice PI plots below A- lined OH High Plasticity Organic Silt

Highly Organic Soil Primarily organic matter, dark in colour and rotten odour Pt Peat a Gravel with 5–12% fines requires dual symbols as follows: GW–GM Well-graded Gravel with Silt GW–GC Well-graded Gravel with Clay GP–GM Poorly-graded Gravel with Silt GP–GC Poorly-graded Gravel with Clay b Sand with 5–12% fines requires dual symbols as follows: SW–SM Well-graded Sand with Silt SW–SC Well-graded Sand with Clay SP–SM Poorly-graded Sand with Silt SP–SC Poorly-graded Sand with Clay

c

10

60

DD

Cu = and 6010

230 )(

DDD

Cc ×=

d CL–ML Atterberg limits plot in the hatched area, soil is classified as Low Plasticity Clay and Silt

e 75.0)mod(

).(<

ifiedunLLdriedovenLL

Plas

ticity

inde

x, I p

Liquid limit, wl (%)

Plas

ticity

inde

x, I p

Liquid limit, wl (%)


Worked Example (3) A grain size distribution analysis is carried out on a soil and gave the results shown in the

figure below. Atterberg limits were performed on the fine fraction of the soil and gave a liquid limit of 30% and plastic limit of 14%. Classify the soil using the USCS and describe all the necessary steps for your classification. [Answer: GP-GC Poorly graded GRAVEL with Clay]

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0.001 0.01 0.1 1 10 100Particle size (mm)

% P

assi

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0.002 0.075 2.36 63.00.006 0.02 0.2 0.6 6.0 20.0


Visual Identification of Soils

Test Method Result

Grittiness Rub soil sample between fingers Gritty − SILTSticky − CLAY

Plasticity Try to roll a moist sample to a thread in your hand Crumbles − SILTRolls to a thread − CLAY

Dry strength Allow soil to dry, then crush the dry soil between the fingers Crush to powder − SILTHard to break − CLAY

Shaking Squeeze a moist soil sample between the two palms several times Water film comes to the surface − SILTNo moisture film − CLAY

A soil in the field can be identified simply by feeling it. Coarse grained soils are easy to identify due to their large grain size. Fine grained soils are identified on the basis of the simple tests given in Table 3.

Table 3: Identification of fine grained soils


Shape of grains in a coarse grained soil is important as it affect the soil strength and stiffness. Coarse grained soils can be angular, subangular, rounded or subrounded. When soil grains are angular, there would be interlocking between the grains and therefore the strength and stiffness of soil increase.

The term Relative Density is commonly used to indicate the in-situ denseness or looseness of coarse grained soils and it is defined as follows:

(18)100(%)minmax

max ×−−

=ee

eeDr

where; Dr = relative density;e = in-situ void ratio;emax = maximum void ratio (void ration in the loosest condition); and emin = minimum void ratio (void ratio in the densest condition).

The values of Dr may vary from 0 for very loose soil to 100% for very dense soil. AS 1726 recommends the following terms for Dr.

Coarse Grained Soils – Particle Shape & Relative Density

very loose loose Medium dense dense very dense

0 15 35 65 85 100


By using the definition of dry unit weight given in Eqn. (5), Dr can also be expressed in terms of the maximum and minimum possible dry unit weights as follows:

(19)100(%) max

minmax

min ×

−

−= −

−−

−

d

d

dd

ddrD

γγ

γγγγ

where; γd = in-situ dry unit weight (at a void ratio of e);γd-min = dry unit weight in the loosest condition (at a void ratio of emax); andγd-max = dry unit weight in the densest condition (at a void ratio of emin).

It should be noted that the void ratio, e, is related to the dry unit weight using the following relationship:

(20)1−=d

wsGeγγ


Fine Grained Soil – Clay Mineralogy Soils that consist of silt, sand and gravel are primarily the result of physical and

mild chemical weathering processes and retain much of the chemical structure of their parent rocks. However, clay experiences extensive chemical weathering which changes it into a new material that is quite different from the parent rocks. As a result, the engineering properties of clays are quite different from other soils.

The chemical weathering process of clays form sheet-like chemical structure of two types: silica or tetrahedral and alumina or octahedral sheets. The silica sheets consist of silicon atoms surrounded by four oxygen atoms at the corners (Figure 12a), and the alumina sheets have aluminium atoms surrounded by six hydroxyls (OH) atoms (Figure 12b).

The produced sheets are bonded together to form three main groups of clays, including kaolinites, montmorillonites and illites.

Figure 12: composition of clays: (a) silica sheets; (b) alumina sheets

(a) (b)


Kaolinite consists of repeating two-sheets of silica and alumina (Figure 13a). The chemical bond between these sheets is strong, so kaolinite is a very stable clay and non-expansive (i.e. they do not exhibit shrinkage or swelling upon the addition or removal of water). Kaolinite is a weathering product of igneous and metamorphic rocks by the alteration of feldspars.

Montmorillonite consists of repeating layers of three-sheets; one alumina sheet surrounded by two silica sheet (Figure 13b). The chemical bond between these sheets is weak and thus large quantity of water can easily enter between the sheets and separate them, causing the clay to swell up to several times of its own volume. Conversely, upon drying, a saturated montmorillonite shrinks and cracks. This property can be very problematic as it causes extensive distortions in lightweight structures and highways. Montmorillonite clay is formed by degradation of some igneous rocks and volcanic ash. Bentonite is a specific form of clay with a high montmorillonite content.

Illite consists of repeating layers similar to those of the montmorillonite except that the repeating layers are bonded by potassium ions (Figure 13c) and thus, illite clay is relatively stable with stronger bond between layers than that of montmorillonite but weaker than kaolinite. Illite is derived from the weathering of acidic igneous and metamorphic rocks.


SiAl

SiAl

SiAl

SiAl

Water

SiAlSi

SiAlSi

SiAlSi

K ions

SiAlSi

SiAlSi

SiAlSi

Figure 13: Atomic structure of clays: (a) Kaolinite; (b) Montmorillonite; (c) illite

(a) Kaolinite (b) Montmorillonite (c) Illite


Individual clay particles are extremely small and require scanning electron microscope (SEM). Examples of SEM for some clays are shown in Figure 14, which indicate that clays have sheet-like shapes that are quite different from the spherically-shaped particles of coarser soils. Clay particles have two structures: flocculated and dispersed. (see Figure 15). In the flocculated structure, the soil particles are edge-to-face and attract each other, whereas in the dispersed structure, the soil particles are face-to-face and repel each other.

Figure 15: Types of soil structure: (a) flocculated; (b) dispersed

Figure 14: SEM of some clays


References: Craig, R. F. (2004). “Craig’s Soil Mechanics”, Spon Press, London. Das, B. M. (2000). “Fundamental of Geotechnical Engineering” Brooks/Cole

Thomson Learning, CA. Holtz, R. D. and Kovacs, W. D. (1981). “An Introduction to Geotechnical

Engineering”, Prentice-Hall, New Jersey. Smith, G. N. and Smith, I. G. N. (1988). “Elements of Soil Mechanics”, Blackwell

Publishing, Oxford. Whitlow, R. (1995). “Basic Soil Mechanics”, Longman Science & Technical, NY

Soil Composition Classification Notes

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