soil classification & clay mineralogy

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ENGINEERING GEOLOGY & GEOMECHANICS CIVL2121 SOIL CLASSIFICATION Soil classification for Civil Engineering purposes is primarily on the basis of particle size – or more precisely, notional particle diameter – for coarser particles, but also on the basis of mineralogy (plasticity) for finer material. See http://geotech.uta.edu/lab/Main/sieve and http://geotech.uta.edu/lab/Main/Hydrometer Classification Systems vary from country to country, but most are based on the US system (The Unified Soil Classification System, USCS), or the British Standard Soil Classification System. The Australian Standard Soil Classification System is similar to the British Standard, but the USCS is widely used in Australia and the SE Asia region. The USCS system is set out on Page 3. The size definitions in the BS system follow a simple 2-6-2-6 pattern, and is easy to remember (these sizes give almost even spacing on a logarithmic plot, as shown below): Clay Silt Sand Gravel Cobbles Fine Medium Coarse Fine Medium Coarse Fine Medium Coarse < 0.002 (< 2 μm) 0.002– 0.006 (2–6 μm) 0.006– 0.02 (6–20 μm) 0.02–0.06 (20–60 μm) 0.06–0.2 (60–200 μm) 0.2–0.6 0.6 – 2 2 – 6 6 – 20 20 – 60 > 60 Size Range (mm) In the USCS system, the divisions are slightly different, but given the wide range of particle sizes, these differences are not important (for example, in the BS system, “fines” includes silt and clay, and is defined as being < 60 μm, but in the USCS, “fines” is < 75μm). The information on particle sizes is generally presented in the form of a particle size distribution curve, examples of which are shown right. (Note: in US practice, size reduces left-to-right on the grading plots). Exercise: Use the USCS chart to describe the soils represented by curves A–E. Characteristic Sizes: D 10 : Maximum size of the smallest 10% D 30 : Maximum size of the smallest 30% D 50 : Maximum size of the smallest 50% D 60 : Maximum size of the smallest 60% D 10 : Effective size (eg for permeability) C u : Coefficient of Uniformity = D 60 /D 10 C c : Coefficient of Curvature = (D 30 ) 2 /(D 60 *D 10 ) (also called C g : Coefficient of Gradation Note: Hazen suggested that the soil permeability (k, m/s) is related to the D 10 size: k(mm/s) = C k (D 10 ) 2 (with D 10 given in mm) C k Soil type D 10 range (s -1 .mm -1 ) (mm) 8 – 12 Uniform sands (C u < 5) 0.06 – 3.0 5 – 8 Well-graded sands and) 0.003 – 0.6 silty sands (C u > 5)

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Page 1: Soil Classification & Clay Mineralogy

ENGINEERING GEOLOGY & GEOMECHANICS CIVL2121 SOIL CLASSIFICATION

Soil classification for Civil Engineering purposes is primarily on the basis of particle size – or more precisely, notional particle diameter – for coarser particles, but also on the basis of mineralogy (plasticity) for finer material. See http://geotech.uta.edu/lab/Main/sieve and http://geotech.uta.edu/lab/Main/Hydrometer Classification Systems vary from country to country, but most are based on the US system (The Unified Soil Classification System, USCS), or the British Standard Soil Classification System. The Australian Standard Soil Classification System is similar to the British Standard, but the USCS is widely used in Australia and the SE Asia region. The USCS system is set out on Page 3. The size definitions in the BS system follow a simple 2-6-2-6 pattern, and is easy to remember (these sizes give almost even spacing on a logarithmic plot, as shown below):

Clay Silt Sand Gravel Cobbles Fine Medium Coarse Fine Medium Coarse Fine Medium Coarse

< 0.002 (< 2 μm)

0.002–0.006

(2–6 μm)

0.006–0.02

(6–20 μm)

0.02–0.06 (20–60

μm)

0.06–0.2 (60–200

μm)

0.2–0.6 0.6 – 2 2 – 6 6 – 20 20 – 60 > 60

Size Range (mm) In the USCS system, the divisions are slightly different, but given the wide range of particle sizes, these differences are not important (for example, in the BS system, “fines” includes silt and clay, and is defined as being < 60 μm, but in the USCS, “fines” is < 75μm). The information on particle sizes is generally presented in the form of a particle size distribution curve, examples of which are shown right. (Note: in US practice, size reduces left-to-right on the grading plots). Exercise: Use the USCS chart to describe the soils represented by curves A–E. Characteristic Sizes: D10: Maximum size of the smallest 10% D30: Maximum size of the smallest 30% D50: Maximum size of the smallest 50% D60: Maximum size of the smallest 60%

D10: Effective size (eg for permeability) Cu: Coefficient of Uniformity = D60/D10 Cc: Coefficient of Curvature = (D30)2/(D60*D10) (also called Cg: Coefficient of Gradation

Note: Hazen suggested that the soil permeability (k, m/s) is related to the D10 size: k(mm/s) = Ck (D10)2 (with D10 given in mm)

Ck Soil type D10 range (s-1.mm-1) (mm) 8 – 12 Uniform sands (Cu < 5) 0.06 – 3.0 5 – 8 Well-graded sands and) 0.003 – 0.6 silty sands (Cu > 5)

Page 2: Soil Classification & Clay Mineralogy

CIVL2121 (“Geomechanics”) Page 2

Atterberg Limits These refer to water contents at certain ‘changes of state’ of clayey soils. Liquid Limit (wL): Consistency changes from a viscous liquid to a plastic solid (at this stage, consistency is like soft spreadable butter) Plastic Limit (wP): Consistency changes from a plastic solid to a brittle solid (at this stage, consistency is like stiff putty) Shrinkage Limit (ws): Water content at which soil stops shrinking as it is being air-dried. From, these, the Plasticity Index (Ip) is defined as: Ip = wL – wP. Liquid Limit Apparatus “Casagrande Cup” apparatus (traditional method:used in many countries, not the standard method in Australia) See: http://geotech.uta.edu/lab/Main/atrbrg_lmts/ for a description of how to do the test, and set of photographs of the test procedure (scroll to the right in the thumbnails to see additional photographs)

“Fall Cone” apparatus (current method) (right) also used (both methods specified in the Australian Standard). Soil in container; cone rested on soil surface, locked in place; cone released; when penetration = 20 mm, soil is at wL (see Whitlow). Plastic limit test: Roll “threads” of soil between hand and glass plate. When “threads” of 3 mm diameter or so crumble, soil is at wP. Note the website http://geotech.uta.edu/lab/Main/atrbrg_lmts/ shows this being done between two Perspex plates, but this is not the standard method. Shrinkage limit test: Soil at the liquid limit is placed in semi-circular container (below), and allowed to dry, first in air, and then in the oven. At the end, the length of the soil is measured. Linear shrinkage = reduction in length / original length (expressed as %).

Apparatus and grooving tool Groove cut in sample prior to the test

Groove closed over 12.5 mm – soil at wL if this requires 25 “blows”

Page 3: Soil Classification & Clay Mineralogy

CIVL2121 (“Geomechanics”) Page 3

Unified Soil Classification System Laboratory Criteria

Major Divisions Group Symbol Typical Names

Fines (%) Grading Plasticity

Cu > 4 GW Well-graded gravels and gravel-

sand mixtures, little or no fines 0-5 1 < Cc < 3 Clean Gravels

GP Poorly graded gravels and gravel-sand mixtures, little or no fines 0-5

Not satisfying GW requirements

GM Silty gravels, gravel-sand-silt mixtures >12 Below A-line or Ip < 4

Gravels

50% or more of course fraction

retained on the 4.75 mm (No. 4) sieve Gravels

with Fines GC Clayey gravels, gravel-sand-clay mixtures >12 Above A-line or Ip > 7

Cu > 6 SW Well-graded sands and gravelly sands, little or no fines

0-5 1 < Cc < 3 Clean Sands

SP Poorly graded sands and gravelly sands, little or no fines 0-5 Not satisfying SW

requirements

SM Silty sands, sand-silt mixtures >12 Below A-line or Ip < 4

Course-Grained Soils

More than

50% retained on the 0.075

mm (No. 200)

sieve

Sands

50% or more of course fraction passes the 4.75

(No. 4) sieve Sands

with Fines SC Clayey sands, sand-clay mixtures >12 Above A-line or Ip>7

ML Inorganic silts, very fine sands, rock four, silty or clayey fine sands Use plasticity chart

CL Inorganic clays of low to medium plasticity, gravelly/sandy/silty/lean clays

Use plasticity chart Silts and Clays

Liquid Limit 50% or less

OL Organic silts and organic silty clays of low plasticity Use plasticity chart

MH Inorganic silts, micaceous or diatomaceous fine sands or silts, elastic silts

Use plasticity chart

CH Inorganic clays or high plasticity, fat clays Use plasticity chart

Fine-Grained Soils

More than

50% passes the 0.075 mm

(No. 200) sieve

Silts and Clays

Liquid Limit greater than 50%

OH Organic clays of medium to high plasticity Use plasticity chart

Highly Organic Soils Pt Peat, muck, and other highly organic soils

See Notes on Hydrometer Test for determining sizes < 60 μm

Page 4: Soil Classification & Clay Mineralogy

CIVL2121 (“Geomechanics”) Page 4

Atterberg Limits and Shear Strength: Shear strength (how strong, stiff, is the soil) depends on water content – as it dries, soil gets stronger. The Atterberg Limits tests relate approximately to certain strengths – i.e. we find that at the liquid limit, the shear strength is about 2 kPa, while at the plastic limit, the strength is about 100 times greater (about 200 kPa). If plotted as water content against logarithm of shear strength (w – log su), each soil plots as a unique straight line. Liquidity Index: Liquidity Index (IL) is define as:

pL

pL ww

wwI

−=

IL is a measure of where the actual water content of the sample, w, lies between the liquid and plastic limits: IL = 1.0 at w = wL and IL = 0.0 at w = wp. Soil wetter than the liquid limit has IL > 1. If we now replot the w – log su data, using IL rather than water content (w), we get all soils plotting on the same line. From similar triangles, we can show:

( ) kPaI6.4exp200s

6.4s

200log

2200log

s200log

2log200logslog200log

wwww

I

Lu

ue

e

ue

ee

uee

pL

pL

×−=⇒

⎟⎟⎠

⎞⎜⎜⎝

=⎟⎠⎞

⎜⎝⎛

⎟⎟⎠

⎞⎜⎜⎝

=

−−

=−

−=

This equation gives an indication of the shear strength of the soil as a function of liquidity index (IL). Sensitivity: The shear strength indicated by the above equation is equivalent to the remoulded shear strength. A natural soil will have an undisturbed shear strength greater than this (due to the soil developing structure over geological time). The ratio of the undisturbed strength to the remoulded strength is known as the sensitivity of the soil:

)remoulded(s)dundisturbe(s

strengthshear remouldedstrengthshear dundisturbeySensitivit

u

u==

For example, the Norwegian ‘quick clays’ have a liquidity index of >>1.0, and hence have a remoulded shear strength <<2 kPa, but they have an undisturbed shear strength of ~20 kPa – so the sensitivity is > 10. A similar situation is found with many offshore clays, but most onshore clays have low sensitivity (2 – 5). Example: In the table below, it is shown that, assuming the wL and wp values are as shown for kaolin and montmorillonite, then the remoulded shear strength of a sample of kaolin with water content of 45% would be the same as the remoulded shear strength of a sample of montmorillonite with a water content of 330%. (These water contents are “half way” between the liquid and plastic limits, and hence have a liquidity index of 0.5, and therefore their shear strengths are “half way” between 200 and 2 kPa in a logarithmic scale – i.e. a shear strength of 20 kPa).

Liquid Limit, wL

Plastic limit, wp

Natural water content, w (%)

Liquidity Index, IL

Estimated remoulded shear strength, su(remoulded), (kPa)

Kaolin 60 30 45 0.5 20 Montmorillonite 600 60 330 0.5 20

Undrained shear strength, su (kPa) 2 20 200

• su at wL ≈ 2 kPa • su at wp ≈ 200 kPa • w – su relationship is about linear when

plotted on w-log su axes • each soil has unique line on such a plot

Water content, w (%)

600

500

400

300

200

100

0

700

Soil 1

Soil 2

wL(1)

wp(1)

wp(2)

wL(2)

Undrained shear strength, su (kPa)2 20 200

• All soils plot on the same line • At IL = 0.0, su = 200 kPa (plastic limit) • At IL = 1.0, su = 2 kPa (liquid limit)

Liquidity Index (IL)

1.0

0.8

0.6

0.4

0.2

0

Soil 1

Soil 2

wL(1)

wp(1)

wp(2)

wL(2)

Page 5: Soil Classification & Clay Mineralogy

CIVL2121 (“Geomechanics”) Page 5

Importance of Atterberg Limits The previous discussion on clay mineralogy showed that clay minerals are characterised (among other properties) by the strength of the bonding between 2-layer sheets for the 2-layer minerals (such as kaolin) or between the 3-layer sheets for the 3-layer minerals (such as montmorillonite). For kaolin, the hydrogen bonding between the sheets is quite strong, so that particles consisting of many layers of 2-layer sheets remain intact when placed in water. However, for montmorillonite, the bonding between the 3-layer sheets is very weak, such that when placed in water, the water can penetrate between individual sheets, thereby exposing an enormous surface area per unit weight of solid clay (this is expressed by the very high ‘specific surface area’). This explains why one form of montmorillonite (so-called Wyoming bentonite, which is a sodium montmorillonite) has a liquid limit of around 600% – adding 600 g (0.6 L) of water to 100 g of this montmorillonite, after thorough mixing, produces a paste with a shear strength of about 2 kPa (i.e the consistency of ‘spreadable butter’). To achieve the same with kaolin (assuming wL = 60%), would require only 60 g of water to be added be added to 100 g of the soil to achieve the same paste consistency (i.e. 10 times less capacity to absorb water than montmorillonite). Therefore, the Atterberg Limits give an indication of the mineralogy. If we have a pure clay, low values of the Atterberg Limits (wL, wp, Ip) indicate a relatively ‘inactive’ clay (probably mainly kaolin), while high values indicate an ‘active’ clay (either illite if moderately high, or montmorillonite, if very high). If we have clay mixed with non-clay minerals (silt and/or sand), we can still obtain the Atterberg Limits, but must take account of the percentage of clay mineral present when interpreting the values. This is defined by the Activity, A:

m2%

Ifraction clay%

wwA,Activity ppL

μ<=

−=

So, for example, if montmorillonite has Ip of 540% (say), a sandy-silty-clayey soil with only 10% montmorillonite might have an Ip of only 54% (only the montmorillonite can absorb the water). If we took that at face value, it would indicate a (clay) soil of low activity (perhaps kaolin), but if we realise that there is only 10% clay fraction in the sample, the calculation of Activity would show A = 54/0.1 = 540, and we would infer that the clay fraction probably consisted of montmorillonite. The ‘activity chart’ shown below shows dashed lines for various clay minerals (the gradient is equal to the Plasticity Index), and some data points, representing results obtained for some WA tailings materials. (These dashed lines, if extrapolated to 100% clay size, would give the Ip of the pure clay mineral. Note that most natural kaolin soils contain perhaps only 60% < 2 μm, rather than 100%.). Swell/Shrink behaviour, and soil compressibility: The Atterberg Limits give a good indication of the degree of swelling/shrinking that will occur as the soil wets/dries. For example, when montmorillonite is exposed to sufficient water, the water penetrates between individual sheets, forcing them apart – lots of swelling – while this does not happen to anything like the same extent with kaolin. We can also think of it like this:

• su at wL is 100 times less than su at wp

• su depends on the pressure (stress) on the soil (more specifically the ‘effective stress’ – later)

• Therefore, if we have a soil at the liquid limit, the effective stress is about 10 kPa (explained later)

• If we increase the effective stress by a factor of 100 (i.e. to 1000 kPa), we can ‘squeeze out’ sufficient water to bring that soil to the plastic limit

• This means, an amount of water equivalent to the plasticity index is ‘squeezed out’ by increasing the stress by 100 times

• For kaolin, this stress increase would ‘squeeze out’ about 30% water (the Ip for kaolin), but for montmorillonite, it would ‘squeeze out’ up to 540% water (the Ip for montmorillonite).

• Therefore, we can see that Ip gives an indication of the volume change that would occur due to a stress increase – i.e. it gives an indication of the soil COMPRESSIBILITY (the inverse of STIFFNESS).

So, we can say that Atterberg limits give an indication of soil mineralogy, shear strength, compressibility, shrink-swell behaviour. The limits are very important in choosing soils for various construction activities – e.g for road base, or for the ‘impermeable’ central ‘core’ of an earth dam.

0

20

40

60

80

0 20 40 60 80

% clay-sized particles (< 2 μm)

Pla

stic

ity In

dex

IpSodium

montmorillonite

Weald clay

Illite

Calcium montmorillonite

Kaolinite

Various gold tailings from WA

Various mineral sands tailings

from WA

ACTIVITY CHART

Sodium montmorillonite simply means montmorillonite where the adsorbed cation is sodium (Na+). It can be converted to calcium montmorillonite if water with calcium ions (Ca++) is passed through it, since, as can see in Point 12 on the following page,, Ca++ will replace Na+ as the adsorbed cation.

Page 6: Soil Classification & Clay Mineralogy

ENGINEERING GEOLOGY & GEOMECHANICS AMEC2121

SUMMARY OF CLAY MINERALOGY 1. Clay minerals are made up of stacks of basic 2-layer or 3-layer sheets. 2. Linkage between the sheets (to form the clay particles) may be: ♦ very strong – e.g. the K+ linkage in muscovite (mica) due to the replacement of 1 in 4 of the Si4+ ions in the outer tetrahedral layers by Al3; ♦ strong – e.g. the hydrogen bonding in many of the 2-layer materials such as kaolin (due to the presence of OH– in the top of the octahedral layer in top of one sheet and O2– in the bottom of the tetrahedral layer in the bottom of the adjacent sheet); in this case, water has difficulty separating sheets, so that particles are quite thic ♦ weak – e.g. between the sheets of some 3-layer minerals, such as montmorillonite (where substitution is in the centre of the central octahedral layer – Mg2+ for Al3+ – which is too remote from the sheet boundary to give a potassium linkage such as in mica); in this case, water can penetrate between individual sheets, practically, resulting in a tremendous capacity to absorb water (and hence to swell when wet, and shrink when dry). 3. In basic form, sheets may be electrically neutral. However, substitution (for Si4+ in tetrahedral layer, or for Al3+ or Mg2+ in octahedral layer gives net negative charge if the substituting ion is of lower charge than the one it replaces (e.g. if Al3+ replaces Si4+). 4. Particle broken edges may also result in net negative charge (depending on how the layer sequence terminates; this depends on pH – pH < 7 tends to give + edges; pH > 7 gives – edges). 5. Even if soil particles (i.e. a stack of sheets) are electrically balanced (neutral), the distribution of charge on the particle is uneven. The particle surface is therefore almost always negatively charged. Also, extra negative charge comes from substitution and from broken particle edges (3 above) 6. For particle in suspension in water, cations (positively charged ions) are adsorbed onto the particle surface – attracted to the particle surface to balance the negative surface charge. These are hydrated (surrounded by water molecules). b. Thus, particles are surrounded by a layer of adsorbed water. The thickness depends on: ♦ the charge on the cations in the water (i.e. monovalent or divalent); fewer 2+ cations than 1+ cations are required to neutralise a given charge, thus 2+ cations result in a thinner adsorbed water layer; ♦ the physical size of the cation; small cations (particularly H+) result in a thinner layer; ♦ the concentration of cations; high concentration results in a thinner layer. 8. In order of decreasing layer thickness, the cations can be ranked as: Na+, K+, Ca2+, H+, Mg2+, Fe3+. 9. Depending on the thickness of the adsorbed water layer, particles in suspension tend to flocculate or remain dispersed. Flocculation occurs if the adsorbed water layer is thin enough for Van der Waal’s attractive force to overcome the repulsive electrical force at all spacings; the particles remain dispersed if the adsorbed water layer is thick enough to prevent flocculation. 10. Particles tend to flocculate in an “edge-to-face” arrangement, resulting in an open “house-of-cards” structure. This tendence is accentuated when the pH is low (i.e. where the adsorbed cation is H+), because the charge distribution tends to be non-uniform (the negative charge is on the face). 11. A flocculated clay settles faster, but can occupy a greater volume after settling (because of open-voided structure of the flocs – the “house-of-cards” structure. 12. The cation in the adsorbed layer can be replaced by another cation, if the chemistry of the pore water changes. This can give some clays a tremendous ability to attenuate (“lock up”) undesirable pollutants. The “cation exchange capacity” (CEC) is a measure of this ability(see columns 7 and 8 of table). In the following list, cations earlier in the series will be replaced by ones later in the series (if they are available): Na+ < Li+ < K+ < Rb+ Cs+ < Mg2+ < Ca2+ < Ba2+ < Cu2+ < Al3+ < Fe3+ < Th4+. 13. In terms of cation exchange capacity or base exchange capacity, the kaolins have a poor capacity, while – a montmorillonite and illite have a high capacity.

Page 7: Soil Classification & Clay Mineralogy

CIVL2121 (“Geomechanics”) Page 7

The “Building Blocks” of clay minerals: The basic building blocks of clay minerals are tetrahedral (Si+4) elements, and octahedral (Mg+2, or Al+3) elements. The tetrahedral elements form a planar layer called a “tetrahedral layer”, while the octahedral elements form either a Brucite (Mg+2) octahedral layer, or a Gibbsite (Al+3) octahedral layer. All clay minerals consist of either a tetrahedral layer and an octahedral layer combined (“two-layer” minerals), or two tetrahedral layers combined with an octahedral layer (“three-layer” minerals). In basic format, the sheets are electrically neutral, but around the edges, where the sequences terminate, there may be unbalanced charges (sign depends on pH). However, substitution (for Si4+ in tetrahedral layer, or for Al3+ or Mg2+ in octahedral layer gives net negative charge if the substituting ion is of lower charge than the one it replaces (e.g. if Al3+ replaces Si4+). In three-layer minerals, substitution in the central (octahedral) layer, being farther from the boundary, has less effect than substition in the tetrahedral layers Tetrahedral and Octahedral Elements: Two-layer minerals:

Even where the sequence leads to a neutral charge, the sequence has to terminate somewhere. Precisely how this termination occurs depends, for kaolin, on pH, as shown in the diagram (left); at low pH, giving a positive charge to the edge, and at high pH, giving a negative charge to the edge.

Three-layer minerals:

Page 8: Soil Classification & Clay Mineralogy

CIVL2121 (“Geomechanics”) Page 8

Clay Minerals: Micrographs of various clay minerals. Kaolin particles are thick (consisting of many layers of the two-layer sheets shown earlier, bonded together quite strongly by hydrogen bonding. Montmorillonite (bentonite): Three-layer mineral, with bonding between the sheets being very weak, so that water can penetrate between individual sheets. The particle thickness is therefore equivalent to just a single three-layer sheet.

Page 9: Soil Classification & Clay Mineralogy

CIVL2121 (“Geomechanics”) Page 9

Summary Table of “Sheet Silicate” Minerals. The amount and type and location of Isomorphous Substitution determines the Cation Exchange Capacity, which determines capacity to trap various ionic pollutants.

Linkage between the sheets For two-layer minerals, sheets are stacked on top of each other to form the final particle. The linkage between the sheets is strong, consisting of hydrogen bonding. (See the section on Hydrogen Bonding in www.chemguide.co.uk/atoms/bondingmenu.html#top ). Hydrogen bonding is common. For example, in water (H2O), the Oxygen atom has a higher electronegativity (3.4) than Hydrogen (2.1). Thus, in the water covalent bonds, where electon pairs are shared between the O and H, the O has a stronger “pull” on the electrons, so that the O “end” is negatively charged, and the H “end” is positively charged. Note that the H–O–H are not arranged in a straight line, but at an angle a bit larger than a right angle (104.5°). The positive H atoms bond to the negative O atoms, with this bond being about 1/20th the strength of the covalent H–O bonds in the water molecule. (Water has odd properties, such as high

Page 10: Soil Classification & Clay Mineralogy

CIVL2121 (“Geomechanics”) Page 10

boiling temperature and high melting temperature, due to this hydrogen bonding). In two-layer minerals, H-bonding occurs between the H of the hydroxyl group at the ‘top’ surface of the octahedral layer and the O at the ‘bottom’ surface of the tetrahedral layer. In three-layer minerals, which have a tetrahedral layer at both ‘top’ and ‘bottom’, and hence both faces have only O atoms, H-bonding is not possible. Therefore, in montmorillonite, the linkage between sheets is very weak, and water can penetrate between individual sheets, exposing an enormous surface area (a high ‘specific surface area’). However, in mica (muscovite – see previous page), substitution of an Al+3 for a Si+4 occurs in the outer tetrahedral layer. This negative charge is balanced by potassium (K+1) ions, which fit neatly into the “hole” in the base of the tetrahedron. These K+1 ions are attracted in this way to adjoining sheets, linking them quite strongly together. Clay surface charges, and adsorbed water Clay surfaces are negatively charged. This is the case even where overall charge is neutral, due to uneven distribution of charge within the particle (different electronegativities, as in water – see above discussion). This charged surface attracts positively charged ions (cations). These are the exchangeable cations discussed later. This negative charged layer on the clay surface, and the associated layer of cations, is referred to as the Electrical Double Layer. In a soil-water mixture, the distribution of cations and anions in the water is also affected by the negatively-charged clay surface, with a greater concentration of cations than anions adjacent to the surface. This effect reduces with distance from the clay surface. These cations are hydrated (surrounded by bound-on water molecules). We can therefore think of the clay surface as attracting hydrated cations, and think of this as forming a “bound on” water layer, or an “absorbed water layer”. The thickness of the this water layer depends on the size of the cation, the concentration of the solution and whether the cations are monovalent (+1)or divalent (+2). See diagrams. In order of decreasing layer thickness, the cations can be ranked as: Na+, K+, Ca2+, H+, Mg2+, Fe3+. Effect of concentration (“molarity) Effect of valency (whether monovalent or divalent)

Page 11: Soil Classification & Clay Mineralogy

CIVL2121 (“Geomechanics”) Page 11

Flocculation: The negatively charged faces of clay particles (or their positively charged faces if we consider the exchangeable cations and the adsorbed water layer as being part of the particle) tend to repulse each other (like similar poles of two magnets); these are the electrostatic repulsive forces. However, all small particles have a natural force of attraction, called a Van der Waal’s force (a dipole effect). The reach of the electrostatic repulsive force (i.e. the thickness of the adsorbed water layer) is very sensitive to pH, ionic concentration, etc, but the Van der Waal’s attractive force is relatively stable. The diagram below shows that if the adsorbed water layer is thin enough, the particles can approach each other sufficiently for the Van der Waal’s forces to dominate, resulting in the particles flocculating. Otherwise, the repulsive force keeps them apart, in a dispersed state. For a clay in suspension in water, these flocs settle much faster than the individual dispersed particles will settle (in some cases, the dispersed particles are too fine to settle at all). However, after settling, the volume occupied by the flocculated soil is greater than that occupied by the dispersed soil (see Figure below). The flocculation can be ‘face-to-face’, as in (a) below, which occurs when normal salts (e.g. NaCl) provide the cation (N+

in this case), with some ‘edge-to-face’ flocculation also. However, for low pH (e.g. if HCl is added to the water), the broken edges of the particles tend to be + (see diagram earlier), so that the edges are attracted to the faces, giving very strong ‘edge-to-face’ flocculation, as in (b) below.

Cation Exchange: Cation Exchange Capacity The cation in the adsorbed layer can be replaced by another cation, if the chemistry of the pore water changes. The “cation exchange capacity” (CEC) is a measure of this ability(see columns 7 and 8 of table). This can give some clays a tremendous ability to attenuate (“lock up”) undesirable ionic pollutants. In the following list, cations earlier in the series will be replaced by ones later in the series (if they are available): Na+ < Li+ < K+ < Rb+ Cs+ < Mg2+ < Ca2+ < Ba2+ < Cu2+ < Al3+ < Fe3+ < Th4+. On the right is a schematic representation of the exchange Pb+2 replacing Na+1.

(a) Salt flocculation – mainly ‘face-to-face’ (b) Non-salt (eg HCl) flocculation (‘edge-to-face’ flocculation (c) dispersed state.

Page 12: Soil Classification & Clay Mineralogy

CIVL2121 (“Geomechanics”) Page 12

Specific surface area (or “specific surface”) The behaviour of clay minerals (in suspension and in a “soil” state) is partly determined by the “specific surface area” (more commonly called “specific surface”), which is defined as the free surface area that can be measured for a gram of the material. The surface area of a sphere is πD2, and the mass is (volume × density) = ρ(πD3/6), and therefore the “specific surface area” is 6/(D.ρ). Typically, for soil grains, ρ = 2.6 t/m3, so that the specific surface for spherical soil grains would be 2.3/D m2/t. Thus, for spherical particles 1 mm in diameter, 1 tonne of the particles would have a combined surface area of 2.3/(1 × 10–3) m3 = 2.3 × 103 m3/t = 2.3 × 10–3 m3/g, and for spherical particles 1 μm in diameter, the equivalent value is 2.3 m3/g (a gram of such a soil would have a surface area of 2.3 m3). Clay particles, however, are not spherical – they have a flat ‘plate’ shape. There are different strengths of bonding between the individual sheets that make up the clay particles – in kaolin, the bonds are strong (hydrogen bonds) so that individual clay particles consist of many 2-layer sheets stacked together, and these stacks do not break down in water. Thus, kaolin presents a relatively small surface area. However, the individual 3-layer sheets that for a montmorillonite particle are not well bonded together, so that if water is present, the water can penetrate between the individual sheets, forcing them apart. In this case, the individual clay particles may be very thin (may be just one 3-layer sheet thick), and therefore the surface area presented per unit weight can be very large. The table (from Whitlow), which gives some of the same information as in the table on Page 9, also gives values for specific surface for clay minerals. This shows values from about 10 m2/g for kaolin, up to 800 m2/g for montmorillonite. The approximate particle sizes are also given as long dimension of the plate (l) and particle thickness (t). The equivalent table on Page 9 shows this as particle diameter d (the same as l) and the thickness/diameter (t/d) ratio, which ranges from 1/3 to 1/10 for kaolin, down to 1/100 for montmorillonite. Similar information is show in the table (below), also from Whitlow.

(Fig 1.5 from Whitlow)