Geotechnical: Effective Stress, Strength, and Settlement
Soil behavior
Geotechnical questions test effective stress, consolidation, shear strength, and bearing capacity. Always separate total stress from pore pressure: effective stress is total stress minus pore pressure. Use effective unit weight below the water table; a frequent trap is applying saturated unit weight to the full depth and forgetting hydrostatic uplift. Shear strength of saturated fine-grained soil is expressed with effective-stress parameters c′ and φ′; for long-term conditions use drained parameters, and for rapid loading in low-permeability soil consider undrained strength.
Classification and compaction
Know the USCS sieve boundaries and plasticity criteria: gravel >4.75 mm, sand 0.075–4.75 mm, fines <0.075 mm. Liquid limit, plastic limit, and plasticity index drive CL/CH/ML/MH distinctions. Compaction curves show optimum moisture content and maximum dry density; a question may ask whether adding water increases or decreases density on the dry or wet side of optimum. Relative density and standard Proctor versus modified Proctor energy are common traps.
Worked example
A 10 ft sand layer has γ = 120 pcf above the water table, which is 5 ft deep; below it γsat = 125 pcf. Total vertical stress at 10 ft = 120(5) + 125(5) = 1,225 psf. Pore pressure u = 62.4(5) = 312 psf. Effective stress = 1,225 − 312 = 913 psf. Using 62.4 for water and keeping the water table depth explicit avoids the usual error.
Walls and slopes
For retaining walls, active pressure is lower than at-rest, and passive resistance is higher; do not mix them. Slope stability questions usually ask for factor of safety against shear strength, and liquefaction screening considers loose saturated sands and seismic loading.