Fourth Terzaghi Oration 1 1883-19631963
Soil Clay Mineralogy and Physico- chemical Mechanisms Governing the Fine-Grained Soil Behaviour Presented by A. SRIDHARAN Honorary Research Scientist, INSA New Delhi (Formerly Professor of Civil Engineering INDIAN INSTITUTE OF SCIENCE BANGALORE). 2
INTRODUCTION Understanding the engineering behaviour of clays and its prediction is of immense importance in Geotechnical Engineering Practice. The complexity associated with the engineering behaviour of fine-grained soils can be attributed to the clay mineralogical composition of the soils. The engineering behaviour of fine-grained soils is controlled by physico-chemical mechanisms governed by the clay mineralogy of the soils. In the early history of soil mechanics, this fact has not been properly recognised and addressed. 3
Fundamental questions: - What are clays and what are the associated surface electrical forces? - How do the soil clay mineralogy, the pore medium chemistry and the associated electrical force characteristics contribute to: (i) the soil fabric? (ii) the water holding capacity of the soil? (iii) the intrinsic effective stress associated with the soil? (iv) the resulting volume change and strength behaviour? 4
The natural soils are composed of many clay minerals. Montmorillonite and kaolinite are the extreme clay mineral types present in the soil. 5
Typical features of kaoliniteand and montmorilloniteclay minerals Distinctive features Unit cell Kaolinite clay mineral Silicon tetrahedral sheet connected with aluminum octahedral sheet G Montmorillonite clay mineral Aluminum octahedral sheet sandwiched between two silicon tetrahedral sheet G
Typical features of kaolinite and montmorillonite clay minerals contd Distinctive features Kaolinite clay mineral Relatively strong hydrogen bond Montmorillonite clay mineral Very weak van der Waals force of attraction. Bonding between unit cells G G H bond G van der Waals forces G
Typical features of kaolinite and montmorillonite clay minerals contd Distinctive features Kaolinite clay mineral Montmorillonite clay mineral Isomorphous in silica sheet in Gibbsite sheet substitution Very less Extensive Cation exchange capacity 3 15 meq./100g 80 150 meq./100g Specific surface 5 20 m 2 /g 400 800 m 2 /g
Typical features of kaolinite and montmorillonite clay minerals contd Distinctive features Kaolinite clay mineral Montmorillonite clay mineral Liquid limit (%) 30-50 300-800 Plastic limit (%) 20-30 40-60 Shrinkage limit (%) 20-30 6-14 Activity 0.5-1 6-12
The development of charges on the edges of kaolin clay particles together with the resulting distribution of charges on the particles 10
The large variations in the specific surface, charge characteristics of clay minerals, pore medium chemistry result in a variety of clay particle arrangement termed clay/soil fabric. The clay fabric together with the inter particle forces determine the: Consistency limits Sediment Volume Volume change behaviour Shear strength Permeability behaviour. The amount and type of clay mineral present in the natural soil can influence its behaviour to an extent much greater than percent clay size fraction in the soil. 11
Electrical Forces of Attraction and Repulsion 12
ATTRACTIVE FORCES Many complex factors are responsible for net attractive forces Primary valence bond Secondary valence forces Hydrogen bond van der Waals forces Hamaker attractive forces Attractive forces (Coulombic attraction, hydrogen bond, ion-dipole, ion induced dipole, dipole-dipole interaction) are inversely proportional to the dielectric constant of the pore medium. 13
ATTRACTIVE FORCES Contd Hamaker's force between parallel plates F = A 6πdπ 3 A - the Hamaker's constant varies quite widely (5 X 10-14 to l X l0-12 ergs). 14
ATTRACTIVE FORCES Contd Variation of Hamaker s A coefficient with degree of saturation Variation of Hamaker s A coefficient 15 with dielectric constant
ATTRACTIVE FORCES Contd The clay-water system is not an ideal system. The nature of inter-particle contacts are not well understood. Predominant contribution of any specific attractive force to the net attractive force cannot be firmly stated. The system is so complex that individual effects cannot be readily separated. The attractive forces vary inversely with the dielectric constant of the pore medium and distance between the particles. They increase with concentration of cation and its valency. 16
ATTRACTIVE FORCES Contd As the hydrated size of cation decreases, the attractive forces increases. In view of the negative charges present on the surface of the particle, cations accommodate themselves in the vicinity of clay particles and hence, their influence is predominant. Information on the effect of anion type and its concentration on the attractive forces is scanty and is a potential area of further work. 17
Helmholtz model (1853) REPULSIVE FORCES A layer of tightly bound ions remains near the surface (Stern layer) and a region of diffuse ions follows ( Gouy layer ).The Stern layer is few angstroms in thickness and the boundary between the two layers is outer Helmholtz plane. This was the basis for all the present diffuse double layer theories describing clay-water-electrolyte interactions in Geotechnical engineering. The double layer is a simple plate condenser and the excess charges on the metal surface are balanced by an equal number of opposite charges at one plane on the solution side, forming an electric double layer. This is usually called the Helmholtz double layer. 18
REPULSIVE FORCES Contd Interaction between diffuse double layers Gouy-Chapman diffuse double layer theory is well recognised to be used in understanding the volume change behaviour of clays. The important equations of the theory: p = 2nkT(cosh u -1) e = Gγ w Sd z u ( ) ( 1/ 2 2cosh y 2cosh u ) dy = dy d = B S ξ x = 0 ( ) 2π εnkt d 0 dξ = Kd K 8π 2 e' v εkt n 2 = 19
REPULSIVE FORCES Contd u = 2.35-4.375 log10 Kd where u = mid plane non-dimensional potential between clay platelet separated by a distance 2d.. S = specific surface G = specific gravity γ w = unit weight of water e'= 4.8 x 10-10 esu v =valence of cations ε = dielectric constant of the fluid 20
REPULSIVE FORCES Contd 21
REPULSIVE FORCES Contd Relationship between u and kd for various values of (dy/dξ) x=0 22
REPULSIVE FORCES Contd νd plotted against log p using Bolt s (1956) data 23
REPULSIVE FORCES Contd Comparison of experimental and theoretical relationship between 24 log d n and log p/n
REPULSIVE FORCES Contd Relaionship of u and log 10 Kd for sodium montmorillonite clay 25 water electrolyte systems using experimental results
REPULSIVE FORCES Contd Theoretical and experimenal u-kd relationship for MX80 bentonite 26 (weighted average valency, v = 1.14) Tripathy et. al. (2004)
REPULSIVE FORCES Contd u = 2.35 4.375 log 10 K d Theoretical, for all clays (Sridharan & Jayadeva, 1984) u = 2.10 4.583 log 10 K d Theoretical equation for Na-montmorillonite (Sridharan and Choudhury, 2002) u = 2.81 3.375 log 10 K d Experimental equation for Na-montmorillonite (Sridharan and Choudhury, 2002) u = 2.77 3.804 log 10 K d Theoretical equation for MX 80 Bentonite (Tripathy et. al., 2004) u = 2.91 7.277 log 10 K d Experimental equation for compacted MX 80 Bentonite (Tripathy et. al., 2004) 27
Soil structure, Pore Spaces and Fabric 28
SOIL STRUCTURE, PORE SPACES AND FABRIC The term soil structure refers to inter-particle forces operative in a clay water system as well as the geometric arrangement of clay particles (i.e., soil fabric). Two kinds of pores or pore spaces can readily be identified in clayey soils. The pore spaces between fabric units are larger than the pore spaces between particles constituting the fabric units. The pores between the fabric units are termed as macro pores, and the pore spaces between particles within the fabric units as micro pores. 29
Effective Stress Concept 30
EFFECTIVE STRESS CONCEPT Terzaghi s effective stress principle σ ' = σ u where σ' = effective stress σ = total stress u = pore water pressure 31
EFFECTIVE STRESS CONCEPT Contd Modified effective stress principle (1968) C = σ am = σ u + A R where C σ = effective contact stress or the inter-granular stress = mineral to mineral contact stress a m = fraction of the total inter-particle contact area that is mineral to mineral contact (non-dimensional) A R = net attraction 32
EFFECTIVE STRESS CONCEPT Contd When the external stress (σ) is zero and u = 0, then i.e., C = σ am = A σ = A R R where σ, the net attraction, represents the intrinsic effective stress. when u is negative and the external stress is zero then the effective contact stress or the inter-granular stress is C = σam = u + A R 33
EFFECTIVE STRESS CONCEPT Contd Positive pore water pressure and repulsive pressure tend to separate the particles away from each other. then, C decreases External pressure (σ) and attractive pressure tend to bring the particle closer to each other. then, C increases 34
Liquid Limit 35
LIQUID LIMIT Liquid limit is the limiting water content between the viscous liquid and plastic states of consistency of fine-grained soils. Liquid limit is the water content at which the shear strength of the soil is zero. 36
LIQUID LIMIT Contd FACTORS AFFECTING LIQUID LIMIT Dielectric constant of pore medium. Electrolyte concentration of pore medium. Valency of exchangeable cations in the pore medium. Hydrated cationic size. Soil clay mineralogy. 37
LIQUID LIMIT Contd Basic Mechanisms: I. Mode of particle arrangement as determined by the interparticle forces. II. Thickness of the diffuse double layer. Kaolinitic soils: Factors affecting the attractive force and fabric influence the liquid limit (Mechanism I). Montmorillonitic soils: Factors affecting the thickness of diffuse double layer influence the liquid limit (Mechanism II). 38
LIQUID LIMIT Contd Effect of dielectric constant on liquid limit 39
LIQUID LIMIT Contd Comparison of liquid limits in water and CCl 4 40
LIQUID LIMIT Contd Effect of exchangeable sodium ions on the liquid limit of montmorillonitic soils 41
LIQUID LIMIT Contd Effect of exchangeable sodium ions on the liquid limit of kaolinitic 42 soils
LIQUID LIMIT Contd Effect of salt concentration on liquid limit of montmorillonitic soils Sl. No. Description of soil 1 Na-montmorillonite 2 Ca-montmorillonite 3 Bentonite 4 Black cotton soil 5 W-179-3: Na-soil 6 W-179-3: Ca-soil Salt concentration (N) Liquid limit (%) 00 950 0.01 N NaCl 870 1.00 N NaCl 350 00 360 1.00 N CaCl 2 310 00 332 0.5 N NaCl 94 00 92 0.5 N NaCl 85 0.01 N 90 1.00 N 74 0.01 N 78 1.00 N 73 Reference Yong and Warkentin (1975) Sridharan and Prakash (1999b) Sridharan et al, (2000b) 43
LIQUID LIMIT Contd Effect of salt concentration on liquid limit of montmorillonitic marine clays (Data 44 Source: Author s file)
LIQUID LIMIT Contd Effect of salt concentration on liquid limit of kaolinitic soils Sl. No. Description of soil Salt concentration (N) Liquid limit (%) Reference 0.01 N NaCl 34 1. Na-kaolinite 1.00 N NaCl 40 Yong and Warkentin (1975) 1. Kundara clay 00 38 0.50 N NaCl 55 Sridharan and Prakash (1999b) 1. E-17: Na-soil 1. E-17: Ca-soil 0.01 N 89 1.00 N 147 Sridharan et al, (2000b) 0.01 N 135 1.00 N 140 45
LIQUID LIMIT Contd Effect of sodium ion concentration on liquid limit for Isahaya (Kaolinitic) clay 46 (Data Source: Sridharan et al, 2000a)
LIQUID LIMIT Contd Influence of valency and size of the adsorbed cations on the liquid limit of bentonite (Sridharan et al, 1986b) Bentonite type Specific gravity Liquid limit (%) Plastic limit (%) Hydrated ionic o radius (A) Lithium 2.61 675 49.1 7.30-10.30 Sodium 2.81 495 49.2 5.60 7.90 Ammonium 2.59 223 55.8 5.37 Potassium 2.72 233 57.8 3.80-5.32 Magnesium 2.65 129 49.9 10.80 Calcium 2.65 125 40.6 9,60 Barium 2.73 108 45.8 8.80 Aluminum b 2.43 108 60.5 - Iron b 2.70 120 63.5 - b Owing to the hydrolysis of the Al 3+ and Fe 3+ ions in the presence of water, the hydrated radii of these ions cannot be evaluated. 47
LIQUID LIMIT Contd Effect of hydrated radius of absorbed cations on the liquid limit of montmorilonitic 48 soils (Data Source: Sridharan and Prakash, 2000c)
Plastic Limit 49
PLASTIC LIMIT Mechanisms controlling Plastic limits of fine grained soil are not clear. Very less work on the effect of soil clay mineralogy on plastic limit of soils. Observed variation of plastic limit with cation concentration of the pore fluid is similar to that of variation of liquid limit of soils. 50
PLASTIC LIMIT Contd Influence of cation concentration on the plastic limit of soils (Sridharan and Prakash, 1999a) Soil Water 0.5N NaCl Montmorillonitic soils: Bentonite 50 48 Black cotton soil 52 43 Kaolinitic soils: Kundara clay 34 39 51
Shrinkage Limit 52
SHRINKAGE LIMIT Shrinkage limit of natural fine-grained soils has been observed to be affected by many factors, out of which the effect of relative grain size distribution appears to be more dominant. Soils with well graded particle distribution would exhibit lesser shrinkage limits, and soils with poor gradation of particles would have higher shrinkage limits. Shrinkage limit of Kaolinite mineral has been observed to be much higher than that of pure montmorillonite mineral. Kaolinitic and montmorillonitic clays show increase in Shrinkage limit with increase in electrical attractive forces and decrease with increase repulsive forces. 53
Sediment Volume 54
SEDIMENT VOLUME FORCES EXISTS IN THE SOIL WATER SYSTEM Forces due to self weight. Electrical forces of attraction (i.e., distance forces). Electrical forces of repulsion (i.e., distance forces). 55
SEDIMENT VOLUME Contd FACTORS AFFECTING SEDIMENT VOLUME Dielectric constant of pore medium. Electrolyte concentration of pore medium. Valency of exchangeable cations in the pore medium. Hydrated cationic size. Soil clay mineralogy. 56
SEDIMENT VOLUME Contd Position of kaolinitic and montmorillonitic soils on the plasticity chart (Data 57 Source: Sridharan et al, 2005)
SEDIMENT VOLUME Contd Effect of dielectric constant of the pore medium on the equilibrium sediment 58 volume of the soils (Data Source: Sridharan et al, 2005)
SEDIMENT VOLUME Contd Plot of sediment volume of clays and soils in water versus their sediment 59 volumes in carbon tetrachloride
SEDIMENT VOLUME Contd Variation of equilibrium sediment volume of montmorillonitic soils with dielectric constant (Data Source: Sridharan and Prakash, 1999a) Variation of of equilibrium sediment volume of kaolinitic soils with dielectric constant (Data Source: Sridharan 60and Prakash, 1999a)
SEDIMENT VOLUME Contd Variation of the equilibrium sediment volume of montmorillonitic and kaolinitic 61 soils with electrolyte concentration (Data Source: Sridharan and Prakash, 1999a)
SEDIMENT VOLUME Contd Effect of valency and hydrated radius of cation on equilibrium sediment volume of the soils (Sridharan and Prakash, 1999a) Salt solution Valency of cation Hydrated cationic radius (A) Equilibrium sediment volume (cm 3 /g) Bentonite Black cotton soil Kundara clay Potassium chloride 1 3.80-5.32 3.0 2.2 4.7 Ammonium 1 5.37 3.5 2.3 5.8 chloride Sodium chloride 1 5.60-7.90 3.8 2.8 2.9 Lithium chloride 1 7.30-1030 7.4 3.3 2.5 Barium chloride 2 8.8 3.3 2.7 3.6 Calcium chloride 2 9.6 3.4 2.7 3.8 Magnesium chloride 2 10.8 3.4 2.9 3.7 Iron oxide b 3-2.8 2.6 3.6 b Due to hydrolysis of Fe 3+ ions in the presence of water, the hydrated radius of these ions 62could not be evaluated.
SEDIMENT VOLUME Contd 3.5 Equibrium se ediment volume (cc/g 3 2.5 2 1.5 1 0.5 0 0 20 40 60 80 100 120 Cation exchange capacity (meq/100g) Effect of cation exchange capacity on equilibrium sediment volume in water. 63
SEDIMENT VOLUME Contd Effect of pore medium chemistry on the sediment volume and liquid limit of fine-grained soils Increase In Sediment Volume / Liquid Limit Kaolinitic soils Montmorillonitic soil Dielectric constant Decrease Increase Concentration of ions in pore medium Increase Decrease Valency of cation Increase Decrease Hydrated size of ions in pore medium Decrease Increase 64
USE OF Eq. SEDIMENT VOLUME OF SOILS Prediction of liquid limit of soils Relationship between liquid limit (cup method) and equilibrium sediment volume 65 (Data Source: Prakash and Sridharan, 2002)
USE OF Eq. SEDIMENT VOLUME OF SOILS Contd Prediction of liquid limit of soils Contd 66
USE OF Eq. SEDIMENT VOLUME OF SOILS Contd Prediction of swell potential and clay mineralogy of soils 67
USE OF Eq. SEDIMENT VOLUME OF SOILS Contd Prediction of swell potential and clay mineralogy of soils Contd Expansive soil classification based on FSR (Prakash and Sridharan, 2004) Free swell ratio Clay type Degree of soil expansivity Dominant clay mineral type 1.0 Non-swelling Negligible Kaolinitic 1.0 1.5 Mixture of swelling and non-swelling Low Mixture of Kaolinitic and Montmorillonitic 1.5 2.0 Swelling Moderate Montmorillonitic 2.0 4.0 Swelling High Montmorillonitic > 4.0 Swelling Very High Montmorillonitic 68
USE OF Eq. SEDIMENT VOLUME OF SOILS Contd Prediction of swell potential and clay mineralogy of soils Contd 80 V d, cc 70 60 50 40 30 20 10 III C 4 1 III B II I 1 III A 1 1 2 1.5 1 Soil Group I II III A III B III C Soil Type Kaolinitic (Kaolinitic + Montmorillonitic) Moderately Swelling Montmorillonitic Highly Swelling Montmorillonitic Very Highly Swelling Montmorillonitic 0 0 10 20 30 40 V k, cc Classification of soils as Montomorillonitic and Kaolinitic types (Prakash and 69 Sridharan, 2004)
A NOTE ON GOUY CHAPMAN THEORY Satisfactorily explains the effect of dielectric constant and electrolyte concentration on liquid limit / sediment volume behaviour of montmorillonitic soils. Cannot explain satisfactorily the effect of dielectric constant and electrolyte concentration on liquid limit / sediment volume behaviour of kaolinitic soils. Cannot explain the effect of hydrated cationic size on the liquid limit / sediment volume behaviour of both montmorillonitic and kaolinitic soils. 70
Volume Change Behaviour 71
VOLUME CHANGE BEHAVIOUR BASIC MECHANISMS CONTROLLING VOLUME CHANGE BEHAVIOUR Terzaghi (1923) High compressibility is due to the presence of 'scale shaped' particles. Adsorbed water - reason for low permeability and secondary compression. Leonards & Altschaeffl (1964) Sliding between particles results in volume changes. Bond and shear strength between particles control the volume changes. Kenny et al (1967) Shearing resistance at contact points controls the deformation. 72
VOLUME CHANGE BEHAVIOUR Contd BASIC MECHANISMS CONTROLLING VOLUME CHANGE BEHAVIOUR Contd Bolt (1956) Long range repulsive forces control the volume changes. Diffuse double layer repulsion is the primary cause for swelling. Guoy-Chapman theory was examined for the prediction. Olson & Mesri (1970) Physico-chemical mechanism controls volume changes in montmorillonite clays. 73
VOLUME CHANGE BEHAVIOUR Contd Compression curves of Na-montmorillonite and Ca-montmorillonite in NaCl 74and CaCl 2 solutions (theoretical and experimental curves)
VOLUME CHANGE BEHAVIOUR Contd Compression curves of Na-illite in NaCl solutions (theoretical and experimental 75 curves)
VOLUME CHANGE BEHAVIOUR Contd Relationship between interparticle spacing and pressure for montmorillonite 76 (from Warkentin, Bolt and Miller, 1957)
VOLUME CHANGE BEHAVIOUR Contd FACTORS AFFECTING VOLUME CHANGE BEHAVIOR Dielectric constant of pore medium. Electrolyte concentration of pore medium. Valency of exchangeable cations in the pore medium. Hydrated cationic size. ph of the pore medium. Soil clay mineralogy. 77
VOLUME CHANGE BEHAVIOUR Contd Effect of dielectric constant (ε) on e-log σ' curves for montmorillonite (Data 78 Source: Sridharan and Rao, 1975)
VOLUME CHANGE BEHAVIOUR Contd Effect of dielectric constant (ε) on e-log σ' curves for kaolinite (Data Source: 79 Sridharan and Rao, 1975)
VOLUME CHANGE BEHAVIOUR Contd Effect of dielectric constant (ε) on e-log σ' curves for Ariake clay (Sridharan 80 et al, 2000a)
VOLUME CHANGE BEHAVIOUR Contd Effect of replacement of moulding pore fluid on e-log σ' curves of black cotton soil (1 kg/cm2 = 98.1 kn/m2) (Data Source: Sridharan and Rao, 1975). 81
VOLUME CHANGE BEHAVIOUR Contd Effect of replacement of moulding pore fluid on e-log σ' curves of kaolinite (1 kg/cm 2 = 98.1 kn/m 2 82 ) (Data Source: Sridharan and Rao, 1975).
VOLUME CHANGE BEHAVIOUR Contd Effect of exchangeable sodium on the percent swell of 83 montmorillonitic soils
VOLUME CHANGE BEHAVIOUR Contd Effect of electrolyte concentration on e-log σ' curves for Na-montmorillonite at ph 7 (1 kg/cm 2 = 98.1 kn/m 2 ) (Data source: Mesri and Olsen, 1971). 84
E-17: Reclamation area, Ariake Megeurie section, Ariake Kantaku W-179-3: Paddy field of the Saga Agricultural Expt. Stn. Void ratio consolidation curves for samples E-17 and W-179-3 85
VOLUME CHANGE BEHAVIOUR Contd Effect of cationic size e-log σ' curves for homoionised bentonites (1 kg/cm 2 = 98.1 kn/m 2 ) (Data Source: Sridharan et al, 1986b) 86
VOLUME CHANGE BEHAVIOUR Contd DiMaio (1996) has studied exposure of bentonite to salt solution with respect to consolidation and residual shear strength. Studies with NaCI, KCI and CaCl 2 solutions with bentonite have shown that NaCI effects were reversible when the specimens were reexposed to water, while KCI,CaCl 2 effects persisted even after some months of continuous testing These results support the irreversible effect of divalent / trivalent ions on the diffuse double layer thickness. 87
VOLUME CHANGE BEHAVIOUR Contd Comparisons of consolidation and swelling for a specimen in water, a specimen in saturated NaCl solution and two specimens with replacement of the pore fluid (Data source: Di Maio, 1996) 88
VOLUME CHANGE BEHAVIOUR Contd Effect ph on virgin consolidation curves for sedimented specimens of sodium 89 kaolinite (Data source: Olsen, 1974).
VOLUME CHANGE BEHAVIOUR Contd Mechanisms controlling the volume change behavior of clays Mechanism 1: Volume change is primarily controlled by the shear resistance at the particle level as dictated by inter-particle attraction valid for kaolinitic soils. Mechanism 2: Volume change is primarily controlled by the long range double layer repulsive forces valid for montmorillonitic soils. 90
VOLUME CHANGE BEHAVIOUR Contd Secondary Compression Coefficient The non - dimensional secondary compression coefficient, Cs, is defined as the ratio of secondary compression per unit of log time to the thickness of the sample. 91
VOLUME CHANGE BEHAVIOUR Contd Secondary compression coefficient Contd Effect of dielectric constant on secondary compression coefficient Cs for: (a) black cotton soil; (b) sodium kaolinite (1 kg/cm2 = 98.1 kn/m2) (Data Source: 92 Sridharan and Rao, 1982)
VOLUME CHANGE BEHAVIOUR Contd Secondary compression coefficient Contd Irrespective of the clay mineralogy of the soils, the non - dimensional secondary compression coefficient, Cs, is related to the strength at the particle level, which is a function of the modified effective stress. Void ratio level has a significant influence on Cs. As the load increment increases, Cs increases at the same void ratio level; but, the load increment ratio has no definite relationship with Cs. 93
Shear Strength 94
SHEAR STRENGTH DRAINED SHEAR STRENGTH Strength envelops for statically compacted and saturated kaolinite (Data 95 Source: Sridharan and Rao, 1979)
SHEAR STRENGTH Contd DRAINED SHEAR STRENGTH Contd Strength envelops for statically compacted and saturated montmorillonite (Data 96 Source: Sridharan and Rao, 1979)
SHEAR STRENGTH Contd DRAINED SHEAR STRENGTH Contd Shear resistance vs pressure relationships for homoionised montmorillonites 97 and kaolinites (Data Source: Grim, 1962)
SHEAR STRENGTH Contd DRAINED SHEAR STRENGTH Contd The strong influence of fluid composition on the mechanical behaviour of clays has been well brought out by Di Maio (1996). Ponza bentonite exposed to saturated NaCl, KCl or CaCl 2 solutions caused deformation due to depression of diffused double layer and a large increase in the effective residual shear strength. For KCl and CaCl 2 treated clays, the increase in residual strength is permanent and irreversible because of higher values of valency of calcium and ionic size of mono-valent potassium. Treatment with higher concentration of CaCl 2 is reversible when concentration is reduced. 98
SHEAR STRENGTH Contd DRAINED SHEAR STRENGTH Contd Residual shear resistance vs pressure relationships for various concentrations 99 of NaCl solution (Data Source: Di Maio, 1996)
SHEAR STRENGTH Contd DRAINED SHEAR STRENGTH Contd Mechanism Controlling the drained shear strength is the same for both kaolnitic and montmorillonitic soils. Drained strength behavior of soils is governed by modified effective stress concept. 100
SHEAR STRENGTH Contd UNDRAINED SHEAR STRENGTH Short term stability problems Saturated soils with high void ratios Clays and silts of low permeability Electrical forces assume importance 101
SHEAR STRENGTH Contd UNDRAINED SHEAR STRENGTH Contd COHESION/COHESION INTERCEPT: Independent of Effective Stress. Physico -Chemical Component of Shear Strength. Basically Two Concepts are identified: Cohesion is due to the layer of adsorbed water, which can be considered as the inner layer of the diffuse double layer. Clays posses an apparent structural viscosity - contributing to viscous resistance. 102
SHEAR STRENGTH Contd UNDRAINED SHEAR STRENGTH Contd Cohesion is due to manifestation of the net inter particle attractive forces. c = σ" tan φ' = (A - R) tan φ' van der Waals' forces of attraction are of sufficient magnitude to account for cohesion. 103
SHEAR STRENGTH Contd UNDRAINED SHEAR STRENGTH Contd Undrained shear strength equivalent water content void ratio relationship 104 for kaolinitic soils (Data Source: Sridharan and Prakash, 1999b).
SHEAR STRENGTH Contd UNDRAINED SHEAR STRENGTH Contd Undrained shear strength equivalent water content void ratio relationship for montmorillonitic soils (Data Source: Sridharan and Prakash, 1999b 105
SHEAR STRENGTH Contd UNDRAINED SHEAR STRENGTH Contd Variation of undrained shear strength with the moulding water content for (a) black cotton soil (montmorillonitic soil) (b) red earth (kaolinitic soil) (Data 106 Source: Sridharan, 2001).
SHEAR STRENGTH Contd UNDRAINED SHEAR STRENGTH Contd The undrained shear strength of kaoilinitic soils - is mainly dependent on the net attractive force and the mode of particle arrangement as determined by the inter-particle forces. A decrease in the dielectric constant or an increase in the electrolyte concentration of the pore fluid or an increase in the valency of the exchangeable cation increases the inter-particle attractive forces while reducing the repulsive forces. This leads to an increase in the net attractive force in the system [i.e. net (A R)] and in turn in an increase in the shear strength at the particle level, which favours the development of more flocculent fabric. This gets manifested in an increase in the undrained shear strength. Modified effective stress concept supports this behaviour. 107
SHEAR STRENGTH Contd UNDRAINED SHEAR STRENGTH Contd The black cotton soil, on homo ionisation with higher valency ions, gives lower undrained shear strengths. Any increase in the valency of exchangeable cation reduces the diffused double layer thickness. This results in a reduction in the viscous shear strength and hence, in the undrained shear strength of the montmorillonitic soil. The red earth, on homo ionisation with higher valency ions, gives higher undrained shear strengths at all water contents. Any increase in the exchangeable cationic valency favours an increase in the level of flocculation, which in turn results in higher undrained shear strength of kaolinitic soils 108
SHEAR STRENGTH Contd UNDRAINED SHEAR STRENGTH Contd Components of undrained shear strength. Viscous shear strength. Frictional shear strength. 109
SHEAR STRENGTH Contd UNDRAINED SHEAR STRENGTH Contd The two basic concepts about cohesion are: (i) "Cohesion" is due to the viscous nature of the double layer / adsorbed layer of water (ii) Cohesion is due to the net inter-particle attractive forces 110
SHEAR STRENGTH Contd UNDRAINED SHEAR STRENGTH Contd Controlling Mechanisms: Mechanism 1: Undrained shear strength of kaolinitic soils is mainly dependent on net attractive force and fabric Modified effective stress concept is valid. Mechanism 2: Undrained shear strength of montmorillonitic soils is mainly due to viscous shear resistance of diffuse double layer water to shear deformation Modified effective stress concept not valid. 111
Permeability 112
PERMEABILITY Determination of permeability of clays is required in many situations related to waste containments. It is well known that highly plastic clays like montmorillonites are affected significantly by the pore fluid characteristics. It is very difficult to assess the relative importance of factors affecting the permeability since many of them are interdependent. 113
PERMEABILITY Contd FACTORS AFFECTING PERMEABILITY Dielectric constant of pore medium. Electrolyte concentration of pore medium. Valency of exchangeable cations in the pore medium. Hydrated cationic size. Soil clay mineralogy. 114
PERMEABILITY Contd Effect of cation, cation size and pore fluid characteristics on coefficient of permeability of montmorillonites (Rao and Sridharan, 1987) Pore medium chemistry Soil Void ratio (e) Montmorillonite 2.50 Ratio of coefficients of permeability Valency effect Bentonite 2.00 2.00 Cation size effect Electrolyte concentration Organic Solvent Bentonite 2.00 Montmorillonite 2.00 Montmorillonite 2.00 115
PERMEABILITY Contd EFFECTIVE VOID RATIO It is the void ratio devoid of equivalent diffuse double layer thickness. The equivalent diffuse double layer thicknesses due to various pore medium system based on the diffuse double layer theory can be determined and hence the equivalent void ratio. This results in an unique relationship between the effective void ratio and permeability coefficient irrespective of the type of pore medium chemistry. 116
CONCLUSIONS Fine-grained soil behavior is highly complex. It depends upon many inter-related factors. Soil clay mineralogy and pore medium chemistry affect the fine-grained soil behaviour appreciably. 117
CONCLUSIONS Contd Liquid limit, sediment volume, Compressibility and undrained shear strength behaviour of kaolinitc soils are mainly controlled by net attractive force and soil fabric as determined by inter-particle forces. Liquid limit, sediment volume, Compressibility and undrained shear strength behaviour of montmorillonitic soils are mainly controlled by diffuse double layer related factors. 118
CONCLUSIONS Contd Drained strength and secondary compression coefficient of both kaolinitic and montmorillonitic fine grained soils are controlled by modified effective stress. Permeability of fine-grained soils is significantly affected by pore medium chemistry. 119
CONCLUSIONS Contd There is a definite need to classify the fine-grained soils into kaolinitic and montmorillonitic soils. 120
ACKNOWLEDGEMENT The author wishes to place on record his sincere thanks to: his former Ph D, MSc (Engg.) and ME students his professional colleagues in India and abroad Dr. K. Prakash, Professor and Head, Department of Civil Engineering, S.J.C.E., Mysore Mr. H.V. Naveen Kumar, Department of Civil Engineering, S.J.C.E., Mysore for the their immense contribution to the work presented today. 121
ACKNOWLEDGEMENT Contd The author started his academic career more than five decades ago under the guidance of Prof. N.S. Govinda Rao, the then chairman of the department of civil engineering, Indian Institute of Science, Bengaluru, who could be considered as the Father of Civil Engineering Research in India. The author is highly grateful to Prof. N.S. Govinda Rao for the motivation and whole hearted timely support. The research atmosphere and the freedom of carrying out independent research the author enjoyed at IISc cannot be described in simple words. 122
ACKNOWLEDGEMENT Contd Prof. N S Govinda Rao FNA, FASc (1907-1995) Founder Head, Department of Civil Engineering (1950-1967) 123 Father of Civil Engineering Research in India, President, IGS (1965-67)
Fourth Terzaghi Oration 124 1883-19631963