Soil Mechanics Fundamentals - A Review

1. Soil Mechanics Fundamentals - A Review John Sturman, Rutgers University 2003

2. Introduction Soils are three-phase materials consisting of solid matter (usually rock with some organic matter), water, and air. The engineering properties of soils are either determined by direct testing representative samples under conditions (stress, saturation, etc.) of concern or through index properties.

3. Index Properties and Soil Classification Many engineering properties can be correlated with the index properties and classification of soils. Key index properties include: • Grain size distribution, shape, and uniformity • Void ratio/porosity • Water Content and Degree of Saturation

4. Key Index Properties (continued) • Specific Gravity • Unit weight or Density • Relative Density (sands) • Liquid and Plastic Limits • Unified Soil Classification Type

6. Unified Soil Classification • A system to group soils by similar constituents to predict behavior. • Primarily sorts by granular (sands and gravels) and fine-grained (silts and clays). • Uses grain size distribution, plastic limit, and liquid limit mainly. • In-situ water content and density is independent of soil classification.

8. Plasticity • An index property used to estimate the behavior of a soil under varying moisture conditions, especially shrink-swell • Tested in the laboratory using the Atterberg Limit Tests • The term Plasticity Index is defined as the difference between the liquid and plastic limits PI = LL - PL

11. Shear Strength • A function of normal stress for most soils under typical conditions • We use the Mohr-Coulomb Failure envelope to model the upper bound of stress that can be imposed on a soil • The formula S = C + σ (tan Ø) expresses shear strength of soil

12. Cohesion • The interparticle force binding a soil mass together • The inherent shear strength of a soil under no confining stress • Cohesion is assumed to be zero for clean sands and normally consolidated clays (tested under CU conditions)

13. Internal Angle of Friction • Relates shear strength to normal stress • Found from direct shear tests or triaxial compression tests in the laboratory • Correlated to density and relative density by various field tests including the SPT and CPT • Greater for granular than cohesive soils • Increases with density & angularity

14. Relationship of Ø v. DR

15. Types of Triaxial Testing • Consolidated-Drained (CD) • Consolidated-Undrained (CU) • Unconsolidated-Undrained (UU) and Unconfined Compression (UC), a special type of UU

17. Seepage and Permeability Laminar flow through saturated porous media explained by Darcy Equation v= ki v = velocity k = coefficient of permeability i = flow gradient = - Δ h/ΔL

19. Seepage and Permeability (continued) • For most soils, k is directly proportional to grain size & inversely proportional to density • k is best correlated with D10 Example: Hazen Eqn k= C D10 where k is expressed in mm/sec, D10 is in mm and C ranges between about 10 and 15

20. Seepage and Permeability (continued) • In cohesive soils, permeability is often anisotropic due to the structure of clays • Remolded (recompacted) soils can have “engineered” permeability based on the type of compactive stress and the moisture content

21. Effective Stress • We are concerned with the forces experienced by a soil over a cross-sectional area • The soil bears the loads through solid-to-solid contacts • In a saturated soil mass, the pore water carries the load in the void spaces

22. Effective Stress (continued) • In a static water situation, pore water pressure u is defined as γwater. Hwater (or ρgH) • In a seepage condition, the pore water pressure may be up, down, or horizontal • Assuming a static condition, the total stress σ is equal to the pore water pressure u plus the reduced solid-to-solid stress σ’

23. Effective Stress (continued) So, σ = u + σ’ and σ’ = σ - u We define σ’ as the effective stress (a derived term) And we can express γ in saturated conditions as γ’ where γ’ = γSAT - γWAT = the effective or submerged unit weight

24. Effective Stress (continued) We notethat u can be negative due to capillary forces in unsaturated soils Capillary force is inversely proportional to grain size. D10 again is a good correlation

25. One-Dimensional Consolidation • Saturated Clays, Silts, and Silt-Clay mixtures exhibit a time-dependent stress-strain behavior in response to changes in effective stress. • We must consider the change in effective stress at the depth of concern, and know the one-dimensional stress-strain behavior of the soil at that depth

26. Consolidation • The compressibility of a cohesive soil is characterized by laboratory consolidation testing. After adding loads, the additional pore water is slowly expulsed. • Measurements are taken to characterize the void ratio, e, and density state of the soil at equilibrium with that load. The time required to reach equilibrium is also an important measurement.

27. Consolidation • The pressure-soil volume relationship is not a simple linear relationship. We approximate the relationship as two lines joining at a single point on a graph of the void ratio v. the log (base 10) of the pressure. The point of inflection is termed the preconsolidation pressure.

28. Consolidation • The slope of the e-log P graph for pressures less than the preconsolidation pressure is termed the recompression index. • The slope of the e-log P graph for pressures greater than the preconsolidation pressure is termed the compression index. • The compression index is greater as the recompression index represents the previous stress states of the soil.

29. Consolidation • When a soil has a preconsolidation pressure, Pp, greater than it’s current effective overburden stress, Po, the soil is termed oversonsolidated. Overconsolidation results from a variety of factors including previous glacial loading, removal of previous layers, rise in the ground water table. • Overconsolidation ratio OCR = Pp/Po