Geotechnical and Foundation Engineering
Geotechnical and Foundation Engineering covers soil classification, compaction, consolidation, shear strength, bearing capacity, earth pressure and foundation types.
Core rules
- IS soil classification uses particle size for coarse soils and plasticity for fines; the A-line on the plasticity chart separates clays from silts.
- Compaction: increasing compactive energy lowers optimum moisture content and raises maximum dry density. Standard Proctor is light; Modified Proctor is heavy.
- Consolidation: compression index Cc = Δe/log(p2/p1); coefficient of consolidation cv controls time rate.
- Shear strength: τ = c + σ tanφ. In an unconsolidated-undrained test on saturated clay, φu is taken as zero and strength is cu.
- Terzaghi bearing capacity: qu = cNc + qNq + 0.5γBNγ for a strip footing; for φ = 0, Nc = 5.7, Nq = 1, Nγ = 0.
- Rankine active pressure coefficient Ka = (1 − sinφ)/(1 + sinφ).
Worked example
A strip footing, B = 2 m, Df = 1.5 m, rests on saturated clay with c = 50 kPa, φ = 0 and γ = 18 kN/m³. Find the net allowable bearing pressure at FOS = 3. For φ = 0: qu = 5.7c + γDf = 5.7×50 + 18×1.5 = 285 + 27 = 312 kPa. Net ultimate = qu − γDf = 285 kPa. Allowable net pressure = 285/3 = 95 kPa.
Traps
Reporting gross instead of net bearing capacity; forgetting to reduce γ below the water table; using φu ≠ 0 for saturated clay in UU tests; confusing compaction with consolidation; mixing Cc and cv.