Settlement of footings

Every footing settles. The question is whether settlement, and particularly the difference in settlement between neighbouring footings, stays within what the structure can tolerate. Excess total settlement breaks pipes and services; differential settlement tilts walls, cracks brickwork and frames, and jams doors. In many designs on clay, settlement rather than bearing capacity decides the footing size.

Settlement has three parts. Immediate (elastic) settlement happens as load is applied, from soil distortion without volume change; it dominates in sands and is estimated from elastic theory or from SPT and CPT correlations. Consolidation settlement occurs in saturated clays as water drains out over time, and is computed from oedometer data. Secondary compression continues slowly after that, mainly in organic soils.

For sands, empirical methods based on SPT N values or cone resistance are widely used, because undisturbed samples cannot be taken. Plate load tests can be scaled to footing size, but the plate stresses only shallow soil, so a weak layer at depth may be missed. For clays, divide the compressible zone into layers, find the added stress at the middle of each using a stress distribution method, compute each layer's settlement and add. Codes give permissible total and differential settlements and angular distortions depending on soil type and structure type; framed buildings on sand typically tolerate less total settlement than those on clay.