Geotechnical Engineering and Foundations
Core concepts
Know soil classification (grain size, Atterberg limits, USCS), effective stress (total stress minus pore pressure), consolidation settlement, shear strength (Mohr-Coulomb, c and φ), and bearing capacity (Terzaghi and general forms). Understand shallow foundations (spread footings, bearing pressure distribution), deep foundations (pile capacity from skin friction and end bearing), and lateral earth pressure (Rankine and Coulomb).
Common traps
- Effective vs. total stress: forgetting to subtract pore pressure in saturated soils leads to wrong settlement and strength.
- Water table position: changes unit weight from moist to submerged and alters effective stress.
- Settlement components: immediate, primary consolidation, and secondary - questions often ask which dominates for a given soil.
- Pile capacity: distinguishing ultimate from allowable, and skin friction from end bearing.
Worked rule of thumb
For a saturated clay, undrained shear strength relates to consolidation history: normally consolidated clays have strength increasing with depth, while overconsolidated clays can have a crust of higher strength near the surface. For bearing capacity, the general form is qu = c·Nc + q·Nq + 0.5·γ·B·Nγ; the terms weight differently for sands (friction dominates) versus clays (cohesion dominates).
Always compute factor of safety against bearing failure and check settlement separately - a foundation can be safe against bearing failure yet fail serviceability.