Geotechnical Engineering: Soil Mechanics and Foundation Engineering
Questions run from index properties to bearing capacity. ISRO favours short numerical items on phase relationships, effective stress, consolidation settlement and shear strength, with classification and compaction theory as supporting topics.
Core rules
- Phase relations: e = wG/s for a saturated soil; γsat = (G + e)γw/(1 + e); γd = γ/(1 + w).
- Effective stress σ′ = σ − u governs strength and settlement; capillary suction above the water table adds apparent cohesion.
- Terzaghi one-dimensional consolidation: cv = k/(mv γw) and Tv = cv t/d², with U reaching 100% at Tv ≈ 1.0 for double drainage.
- Mohr–Coulomb: τ = c + σ′ tan φ. Normally consolidated clays typically show φ′ of 20–30°; overconsolidation raises the apparent cohesion.
- Terzaghi bearing capacity: qu = cNc + γDf Nq + 0.5 γ B Nγ, with N factors read from φ.
- Compaction peaks at the optimum moisture content; the zero-air-void line bounds every compaction curve.
Worked example
Sand from 0 to 3 m is dry, γ = 16 kN/m³. Below 3 m the sand is saturated, γsat = 20 kN/m³, with the water table at 3 m. At 7 m depth: σv = 3 × 16 + 4 × 20 = 128 kPa; u = 4 × 9.81 = 39.2 kPa; σ′ = 88.8 kPa. Using bulk unit weight instead of submerged unit weight below the water table would overstate σ′ by about 18 kPa.
Traps
Using γsat below the water table in effective-stress sums; forgetting that a rising water table reduces bearing capacity; confusing degree of consolidation U with the time factor Tv; and applying Skempton's Nc where Terzaghi's shallow-foundation formula is expected.