Soil Mechanics and Foundation Engineering
Soil mechanics and foundation engineering covers index properties, classification, permeability, effective stress, compaction, consolidation, shear strength, bearing capacity and shallow/deep foundations. BTSC JE questions often use phase relationships and effective-stress calculations.
Core rules
- Phase relations: e = Vv/Vs; n = e/(1+e); γd = γ/(1+w); γsat = (G + e)γw/(1+e); submerged unit weight γ′ = γsat − γw.
- IS 1498 classification uses liquid limit, plastic limit and plasticity index; PI = LL − PL. A soil with PI > 17 is highly plastic clay.
- Darcy’s law: q = kiA. Effective stress σ′ = σ − u; capillary rise increases effective stress, upward seepage reduces it.
- Terzaghi bearing capacity for a strip footing: qu = cNc + qNq + 0.5γBNγ. For square and circular footings, shape factors are applied.
- Compaction is immediate densification; consolidation is time-dependent expulsion of pore water.
Worked example
A saturated clay deposit has γsat = 20 kN/m³ and the water table is 2 m below ground. At 5 m depth, total stress σ = 20 × 5 = 100 kPa. Pore pressure u = 9.81 × (5 − 2) = 29.43 kPa. Effective stress σ′ = 100 − 29.43 = 70.57 kPa. Using 9.81 × 5 would overestimate u and understate σ′.
Traps
Using total stress in settlement or shear-strength calculations; forgetting water-table depth; swapping void ratio and porosity; applying Terzaghi factors without shape correction; confusing compaction with consolidation.