Soil Mechanics and Laboratory/Field Testing
What to know
You must connect index properties to engineering behavior. Know Atterberg limits and plasticity index, specific gravity (typically 2.65–2.70), and gradation from sieve and hydrometer data. For compaction, separate Standard (ASTM D698) from Modified (D1557) effort and know that the compaction curve shifts up and left with increasing energy. For consolidation, know Cc from the slope of the e–log σ' curve in the virgin range, Cr on rebound, cv from time-rate data, and OCR from the preconsolidation pressure by Casagrande's construction.
Shear strength requires knowing the drainage condition of each test: UU gives su with φ = 0, CU gives effective-stress parameters with pore pressure measurement, CD gives drained c' and φ'. Direct shear, triaxial, and field vane all measure different things - the field vane gives undrained strength in soft clays and overpredicts in fissured or layered soils. Permeability follows Darcy's law; falling-head for fine soils, constant-head for coarse, and field tests (slug, packer, pumping) for the mass permeability.
Traps
Using total stress parameters in a drained problem (or the reverse), confusing Cc with Cr on an unload–reload cycle, and forgetting that sample disturbance lowers the apparent preconsolidation pressure. Watch unit consistency - tsf versus psf versus kPa.
Worked example
Void ratio drops from 1.00 at σ' = 1 tsf to 0.82 at 4 tsf: Cc = Δe / log(σ'2/σ'1) = 0.18 / log(4) = 0.18 / 0.602 ≈ 0.30. Settlement of a 10-ft layer with e0 = 1.00 is ΔH = Cc·H/(1+e0)·log(4) = 0.30 × 10/2.0 × 0.602 ≈ 0.90 ft.