Pile capacity

A pile carries load in two ways: through its base, by end bearing on a firm stratum, and along its shaft, by friction or adhesion with the surrounding soil. Long piles in clay are mostly friction piles; short piles to rock are mostly end-bearing. The ultimate capacity is the sum of both, and a factor of safety (commonly 2.5 on static formulas) gives the safe working load.

In clay, shaft resistance is the adhesion factor α times undrained strength times shaft area, with α falling as clay gets stiffer. Base resistance is about nine times cu times the base area. In sand, shaft friction depends on effective overburden, an earth pressure coefficient and the interface friction angle; base resistance uses a bearing capacity factor Nq times the effective overburden at the toe. Beyond a critical depth these values level off. Driven piles densify sand and gain capacity; bored piles may loosen it.

Because formulas carry uncertainty, codes require load tests. Initial tests on trial piles confirm design; routine tests on working piles check construction. Static vertical load tests measure settlement under increments; dynamic tests with strain gauges and accelerometers, and integrity tests by low-strain sonic methods, are faster. Negative skin friction, where consolidating fill drags the pile down, can cut capacity sharply and must be allowed for.