Clays, Clay Minerals and Soil Shrink/Swell Behavior

1. Hillel, pp. 75-100 Clays, Clay Minerals and Soil Shrink/Swell Behavior

2. Introduction • Volume and pore space of swelling clayey soils vary with hydration state. • Shrink-swell phenomena affect many mechanical and engineering properties of soils and clay liners. • Constitutive transport properties for swelling soils are complicated by hydration-dependent soil attributes (pore space, strength, etc).

3. Clay shrink/swell damage to structures & roads • Changes in soil water content or solution chemistry of clayey soils induce swelling pressures sufficiently large to fracture and damage structures and roads. • Estimated damage in excess of $7 billion/yr in the US.

4. Clay Minerals – building blocks • Distinguish between “clay” size <2mm and clay minerals • Basic building blocks of clay minerals: • Silica centered tetrahedra • Al3+ (+ other cations Mg2+ ) centered octahedra

5. Formation of Silica and Alumina Sheets • The tetrahedra are joined (share oxygen) at their basal corners in a hexagonal pattern forming flat sheets ~ 0.493 nm thick. • The octahedra join along their edge to form triangular array 0.505 nm thick. • Stacked sheets form lamellae.

6. Isomorphic substitution • The space occupied by silica in a tetrahedron can accommodate atoms <~0.4 times O2 radius (Si4+ & Al3+). • Octahedra - 0.732 times O2 radius (accommodates iron, magnesium, aluminum, manganese, titanium, sodium, calcium) • Substitution of central atoms with valence < +4 (tetrahedron) or <+3 (octahedron) during crystallization is known as isomorphic substitution and results in net negative charge that must be balanced externally by adsorption of cations. • These cations are not permanent and can exchanged by other cations in soil solution.

7. Cation Exchange Capacity • The cation exchange capacity (CEC) describes the amount of exchangeable cations per unit soil mass: CEC = cmol of positive charge/kg • CEC values range from 2-15 cmol+/kg for Kaolinite; 20-40 illite, and 60- 100 for montmorillonite.

8. Formation of a Diffuse Double Layer (DDL) • Some of the exchangeable cations are bounded to surfaces whereas others may be dispersed in the aqueous solution – hence a “double layer”… • The distribution of cations (and associated anions) in solution reflect a balance between electrical and thermal forces resulting a diffuse “cloud” of cations with concentration diminishing with distance from clay surface. • The extent of this diffuse layer is not constant and varies with solution concentration, clay hydration, cation valence and clay type.

9. Different Clay Minerals • Distinguished by number and order of layering of basic tetra & octahedral sheets • Amount of isomorphic substitutions. • Types and amounts of cations bound to surfaces.

10. Exchangeable cation Montmorillonite • 2:1 - one octahedral sheet sandwiched between two tetrahedral sheets • Many isomorphic substitutions: Mg+2, Fe+2, & Fe+3 for Al+3 in octa • High surface area (600-800 m2/g) • Large CEC • Very active shrink/swell behavior

11. Kaolinite • 1:1 alternating octa/ tetrahedral sheets. • Few isomorphic substitutions • Thicker and stable stacks • Relatively low surface area: 5-30 m2/g • Not much swelling

12. - - - - - - - - + + + + + + + + + + + + Swelling and changes in lamellar Spacing H2O osmosis from bulk soil solution due to DDL cations

13. Swelling and Lamellar Spacing • Clay lamellar spacing increases with increasing potential (less negative/ wetter) resulting in swelling. • Interacting diffuse double layers (DDL) dominate swelling behavior. • Reasonable agreement exists between measured lamellae spacing and DLVO-theory: approaching DDLs develop a repulsive force proportional to excess ions relative to bulk (giving rise to osmotic pressure). Low [1980]; Warkentin et al. [ 1957]

14. Interacting DDL and swelling pressure • When two DDLs approach each other they develop a repulsive force that is proportional to excess ions relative to bulk (giving rise to osmotic pressure). • A convenient point for evaluation is midplane where dy/dx=0 (due to symmetry for equal surfaces). • Langmuir [1938] calculated the swelling pressure as: which simply van’t Hoff relations. • For short separation distances Langmuir obtained: h = separation distance [m] c0 = bulk electrolyte concentration [mol m-3] e = electron elementary charge [1.60218x10-19 C] k = the Boltzmann constant [1.38066x10-23 J K-1] R = universal gas constant [8.3145 J mol-1K-1] y1= y(h/2) mid-plane electric potential [V] z = signed ion valence. Scale electric potential The “trick” is how to determine the mid-plane electric potential y1 ?

15. Measurement of swelling pressure

16. Large spacing weak interaction approximation • A very useful approximation for swelling pressure at large spacing and weak interactions was developed by Derjaguin [1987]: • Note that this expression is dependent on surface potential 0 (and not on mid-plane 1) Low [1980]; Warkentin et al. [ 1957]

17. Calculation of swelling pressure - Example • Consider two DDLs separated by a distance of h=5 nm with bulk monovalent electrolyte concentration of [NaCl]=0.001 M; surface potential y0 =55 mV. Find the swelling potential. • Using the approximation: • Simplified as: • Approximating k: • We find that Pe(5 nm)=22.5 kPa • Changing the concentration to 0.01 M, we obtainPe(5 nm)=73.2 kPa • Changing the distance to 1 nm ([NaCl]=0.01) Pe(1 nm)=273 kPa

18. Lamellar swelling – the disjoining pressure • A more general treatment considers the various interactions between charged clay surfaces and aqueous solutions using the disjoining pressure formalism (P), or the so-called DLVO theory. • The equilibrium potential (m) as function of water film thickness (h =half lamellar spacing) is comprised of three primary components: Where: = van der Waals forces (attractive, “short” range) = hydration force (short range, repulsive) = electrostatic force (long range, repulsive)

19. The disjoining pressure at equilibrium van der Waals forces (attractive) hydration force (repulsive) electrostatic force (repulsive)

20. Mesopores and their role in volume change • Lamellar swelling alone cannot explain volume changes and water retention in clay fabric. • SEM images show a lamellar network with micropores separating tactoids (quasi-crystalline stacks of lamellas). • Important for modeling clay fabric response.

21. Hydration effects on clay fabric geometry 0.03 bar • SEM images support bulk volume measurements and reveal a lamellar network with micropores between quasi-crystalline stacks of lamellae. • The SEM images show simultaneous evolution of microstructure and bulk volume of Greek Na+ montmorillonite during first drying [Tessier, 1990]. • A strong orientation of lamellar structure and micropores (1-2 mm) occurs during drying anisotropy. 1.0 bar 10 bars

22. Interacting DDL different electrolytes • Ion distribution between two clay surfaces – different electrolytes.

23. Electrolyte effects on microstructure • SEM images (+ scheme) for influences of Ca2+ and Na+ montmorillonite microstructure prepared with dilute solutions [Tessier, 1990]. • Electrolyte type and concentration affects: • Arrangement and spacing between layers (smaller for Ca2+), between ordered stacks, and between tactoids. • Number of layers and apparent length of quasi-crystals(tactoids) - more lamellae for Ca2+ (dilute) solutions.

24. Evolution of clay fabric - mesopore formation • Images of uniform Glass beads (40 um): • mixed with 10% dry Na+ Bentonite. • wetting resulted in complete filling of skeletal pore space by jell-like clay fabric. • upon subsequent drying, mesopores are formed between glass beads. • The mixing and distribution of clay domains among other soil textural components remains an open question. Mesopores Silt-clay Sand-clay [Fies and Bruand, 1998]

25. Clay barriers for waste isolation

26. Clay Liners Clay layers (Bentonite) to prevent leaching Geotextile layers for mechanical stability

27. Clay Liners Geotextiles are permeable fabrics (polypropylene, polyesters, etc.) which, when used in association with soil, have the ability to separate, filter, reinforce, protect or drain.Geomembranes are impermeable membranes used widely as cut-offs and liners.

28. Clay Liners

29. Effect of shrink-swell on soil pore volume

30. Shrink-swell affects soil pores at all scales Microscale (clay fabric) Macroscale (cracks) Mesoscale (texture)

31. Effect of clay content on porosity & permeability • The critical clay content that completely fills sand-silt skeletal porosity is about 35-40% (and minimum overall porosity). • The saturated hydraulic conductivity decreases with increased clay content to critical value, and then rebounds to the value of clay fabric saturated hydraulic conductivity.

32. Modeling clay fabric geometry • Development of an idealized clay fabric representation: (a) SEM of montmorillonite; (b) approximated clay fabric structure; and (c) idealized clay fabric representation applied in the model • SEM observations and bulk clay behavior are used to derive and constrain parameter values for the idealized clay fabric.