Load Transfer in Piles
A pile transfers structural loads to competent soil or rock deep below the surface. As load is applied at the pile head, it progressively transfers to the surrounding soil through skin friction along the shaft and bearing pressure at the tip. The simulation visualizes this load transfer mechanism — at the top, the pile carries the full applied load; with depth, skin friction strips away load until only the remaining portion reaches the tip. This distribution determines whether a pile behaves as an end-bearing or friction pile.
Skin Friction
Shaft friction develops from the interaction between the pile surface and surrounding soil. In granular soils, unit friction f_s = Kσ'_v·tan(δ), where K is the lateral earth pressure coefficient (typically 0.7-1.5 for driven piles), σ'_v is the effective vertical stress, and δ is the pile-soil interface friction angle. Because σ'_v increases with depth, skin friction increases linearly — until a critical depth (typically 15-20D) where it plateaus due to arching effects in the soil.
Tip Resistance
Base bearing capacity follows the same principles as shallow foundation theory but at the pile tip depth: Q_b = A_b·N_q·σ'_v. The bearing capacity factor N_q for deep foundations is much larger than Terzaghi's shallow foundation values because the failure mechanism is fully contained within the soil mass. For φ = 32°, N_q can range from 20 to 80 depending on which correlation is used — Berezantsev, Meyerhof, or Vesic — reflecting uncertainty in deep bearing behavior.
Design & Testing
Static analysis provides an estimate, but pile capacity in practice is confirmed by load testing. Static load tests remain the gold standard: a pile is loaded incrementally to failure while settlement is measured. Failure is defined by criteria such as Davisson's offset method (settlement exceeding elastic compression plus a specified offset). Dynamic methods using PDA during driving offer rapid estimates by analyzing stress wave reflections, though they require correlation with static tests for reliability.