Engineering Geology and Geohazards
What to know
Index testing begins with grain size, Atterberg limits, and plasticity index (PI = LL − PL). The Unified Soil Classification System groups soils by grain size and plasticity, which informs compaction, compressibility, and strength. Compaction tests (Standard and Modified Proctor) define maximum dry density at optimum water content. Consolidation analysis uses compression index Cc, coefficient of consolidation cv, and preconsolidation pressure to predict settlement.
Shear strength follows Mohr–Coulomb: τ = c′ + σ′n tanφ′, in terms of effective stress. Undrained conditions (φu ≈ 0) are used for short-term loading of saturated clays; drained parameters apply long-term. Effective stress σ′ = σ − u controls volume change and strength.
Slope stability compares resisting to driving forces: FS = resisting/driving; FS < 1 indicates failure. Common failure surfaces include planar (infinite slope), circular (rotational slump), and wedge (rock). Rock mass characterization uses RQD, discontinuities, RMR, and Q. Geohazards include liquefaction of loose saturated sands, seismically induced settlement, rockfall, debris flows, and karst subsidence.
Traps
Using total instead of effective stress; applying drained parameters to rapid loading of saturated clay; confusing PI with liquidity index; assuming high porosity means high strength; using FS < 1.5 as acceptable for permanent slopes in practice (often 1.3–1.5 minimum).
Rule of thumb
Infinite slope with seepage parallel to the surface: FS = [c′ + (γ − γw) z cos²β tanφ′] / (γ z sinβ cosβ). At c′ = 0, FS = [(γ − γw)/γ]·(tanφ′/tanβ), so β must be below φ′ for stability.