Geotechnical Engineering and Foundations
Domain overview
This section tests foundation design from site investigation through bearing capacity, settlement, earth pressure and pile capacity. The exam favours effective-stress reasoning, clear drainage assumptions and checks for both ultimate and serviceability limit states.
Core rules
- Effective stress σ′ = σ − u governs strength and compression; use submerged unit weight below the water table.
- Bearing capacity equations give ultimate capacity; apply a factor of safety or code resistance factor, then check settlement.
- Eccentric loading: for e≤L/6, qmax,min = P/A(1±6e/L); for larger e, part of the base may lift.
- Pile capacity combines shaft friction and end bearing; group effects, negative skin friction and settlement must be considered.
Worked example
A 2.5 m × 2.5 m footing carries a 1200 kN column load and 150 kN self-weight, with eccentricity 0.2 m. P=1350 kN, A=6.25 m², so average pressure is 216 kPa. Since e=0.2 < L/6=0.417 m, qmax=216(1+6×0.2/2.5)=216×1.48=319.7 kPa and qmin=216×0.52=112.3 kPa. Compare qmax with allowable bearing pressure and check settlement; if qmax exceeds allowable, enlarge the footing or reduce eccentricity.
Traps
Using total stress below the water table, ignoring eccentricity, comparing ultimate bearing capacity directly with service pressure, and forgetting that settlement may control on clay even when bearing capacity is adequate.