Geotechnical Engineering and Foundation Engineering
Geotechnical Engineering and Foundation Engineering tests soil classification, phase relationships, compaction, effective stress, consolidation, shear strength, bearing capacity, earth pressure, slopes, and pile foundations. Numerical questions often hinge on effective-stress concepts.
Core rules
- Effective stress is σ′ = σ − u; use submerged unit weight below the water table.
- The Mohr-Coulomb envelope is τ = c′ + σ′ tan φ′ for effective stress, and τ = c_u for undrained clay.
- Terzaghi bearing capacity: q_u = cN_c + qN_q + 0.5γBN_γ for a strip footing.
- Consolidation settlement depends on C_c, initial and final effective stress, and layer thickness.
- Active earth pressure is lower than at-rest pressure; passive pressure is higher.
Worked example
A sand layer 5 m deep has γ = 18 kN/m³, with the water table at 2 m. At 5 m, total stress σ = 5 × 18 = 90 kPa. Pore pressure u = (5 − 2) × 9.81 = 29.43 kPa. Effective stress σ′ = 90 − 29.43 = 60.57 kPa. If submerged unit weight is used below the water table, verify the same result: 2 × 18 + 3 × (18 − 9.81) = 36 + 24.57 = 60.57 kPa.
Traps that cost marks
- Using total unit weight below the water table without subtracting pore pressure.
- Treating φ = 0 in a UU test as a drained parameter.
- Confusing compaction (densification) with consolidation (expulsion of water).
- Forgetting skin friction and end bearing both contribute to pile capacity.
Exam focus
Prioritise effective-stress profiles, Atterberg-limit classification, bearing-capacity terms, and one-dimensional consolidation settlement.