Geotechnical Engineering and Foundation Engineering
Geotechnical Engineering and Foundation Engineering covers soil classification, compaction, permeability, effective stress, consolidation, shear strength, bearing capacity, slope stability and foundations. The exam rewards correct phase relationships and effective-stress calculations.
Core rules
- Phase relations: e=V_v/V_s; n=e/(1+e); γ_d=γ/(1+w); γ_sat=(G_s+e)γ_w/(1+e).
- Effective stress σ'=σ−u; capillary rise reduces effective stress above the water table.
- Darcy q=kiA; permeability depends on void ratio and temperature.
- Consolidation settlement S=H C_c/(1+e₀) log((p₀+Δp)/p₀).
- Shear strength τ=c'+σ'tanφ'; UU, CU and CD tests give different parameters.
- Bearing capacity follows Terzaghi or IS 6403; foundation choice depends on depth and settlement.
Worked example
At 5 m depth, 0–2 m is sand with γ=18 kN/m³; 2–5 m is saturated sand with γ_sat=20 kN/m³; water table is at 2 m. Total stress σ=2×18+3×20=96 kPa. Pore pressure u=3×9.81=29.4 kPa. Effective stress σ'=96−29.4=66.6 kPa. Using total stress would overestimate strength by about 44%.
Traps
- Using γ_sat above the water table.
- Forgetting buoyancy in the capillary zone.
- Mixing void ratio e and porosity n.
- Applying total-stress parameters in effective-stress analysis.
- Ignoring settlement when bearing capacity is adequate.