Geotechnical: Soil Classification, Compaction, and Strength
Geotechnical items reward fluency with index properties and the standard formulas. Know the Atterberg limits (LL, PL, PI = LL − PL), the Unified Soil Classification System symbols (GW, SP, CL, CH, etc.), grain-size distribution (D10, D60, Cu, Cc), and compaction (Standard vs. Modified Proctor, optimum moisture, relative compaction). For strength and seepage, know effective stress (σ' = σ − u), Terzaghi bearing capacity terms, Mohr-Coulomb (τ = c + σ' tan φ), and Darcy's law (q = kiA).
Traps
- Using total stress where the problem gives a water table - always subtract pore pressure.
- Confusing Cu (uniformity coefficient, D60/D10) with Cc (coefficient of curvature, D30²/(D10·D60)).
- Assuming higher moisture always weakens soil - compaction peaks at optimum moisture, and both dry and wet of optimum reduce dry density.
- Mixing unit weights: γsat vs. γdry vs. γ' = γsat − γw for submerged conditions.
Worked example
A saturated clay at 10 ft depth with γsat = 120 pcf and water table at the surface: total σ = 120 × 10 = 1,200 psf; u = 62.4 × 10 = 624 psf; σ' = 576 psf. Rule of thumb: for any strength or settlement calculation below the water table, work in effective stress.