Shallow and Deep Foundations
What to know
For shallow foundations, know the bearing capacity equation in general form - qu = cNc + qNq + 0.5γBNγ - with Terzaghi, Meyerhof, or Vesic factors, plus shape, depth, and inclination corrections. Confirm drainage: undrained clay uses su with Nc = 5.14 (plus depth and shape terms), drained sand uses c' = 0 and φ'. Separately address settlement, which usually governs on sand: elastic (immediate) settlement, primary consolidation, and secondary compression.
For deep foundations, distinguish skin friction and end bearing for driven piles and drilled shafts, the α (total stress) and β (effective stress) methods, and group efficiency with block failure checks. Know the static load test as the benchmark, plus Osterberg cells and dynamic testing with PDA and CAPWAP. Consider setup (friction gain with time), relaxation, downdrag on piles in settling fill, and lateral capacity by Broms' method.
Traps
Gross versus net allowable bearing pressure - always subtract the overburden removed at the base. Using total unit weight below the water table in the surcharge and γBNγ terms. Applying a pile group's capacity as simply n times the single-pile value without checking block failure or group settlement.
Worked example
Square footing, B = 5 ft, Df = 4 ft, γ = 120 pcf, c = 0, φ = 30° (Nq = 22.5, Nγ = 19.7). qu = qNq + 0.4γBNγ = (4 × 120)(22.5) + 0.4(120)(5)(19.7) = 10,800 + 4,728 ≈ 15,500 psf. With FS = 3, gross allowable ≈ 5,180 psf; net allowable ≈ 5,180 − 480 ≈ 4,700 psf.