Soil Mechanics (Breadth)
Concepts
Soil mechanics questions test classification (USCS and AASHTO), index properties (LL, PL, PI, gradation), compaction (Standard and Modified Proctor, relative density, field density tests), permeability, effective stress, consolidation and settlement, shear strength (c, φ), bearing capacity, and lateral earth pressure. Always separate total stress from effective stress; water carries no shear stress but reduces effective stress.
Traps
Using total unit weight below the water table without subtracting pore pressure; confusing dry, moist, saturated, and buoyant unit weights; using PI when the question asks for liquidity index; assuming compaction increases strength without checking water content; applying Rankine active pressure to a rigid wall that cannot move. For retaining walls, active pressure is lower than at-rest, and passive is highest; use the correct condition.
Worked example
A 20-ft soil profile has 10 ft of moist sand (γ = 120 pcf) above the water table and 10 ft of saturated sand (γsat = 125 pcf) below. At 20 ft, total vertical stress = 120(10) + 125(10) = 2,450 psf. Pore pressure u = 62.4(10) = 624 psf. Effective stress σ′ = 2,450 − 624 = 1,826 psf. Using 120 pcf below the water table would overstate effective stress.
Rule of thumb
For normally consolidated clay, settlement is computed from Cc, initial void ratio, and the change in vertical effective stress. For granular soils, use SPT or CPT correlations with caution and local verification.