EarthPressure

Earth pressure is the lateral stress exerted by soil on adjacent structures such as retaining walls, excavation faces, and underground spaces. It depends on soil properties (unit weight, internal friction angle, cohesion), moisture state, backfill geometry, and wall movement. The pressure is described in terms of active, passive, and at-rest states, corresponding to different wall displacements relative to the soil.

When a wall moves away from the soil, the soil mobilizes shear and the lateral pressure reaches the active state, the minimum pressure for that soil. If the wall moves toward the soil, a higher passive pressure develops. If the wall remains stationary, the soil is in an at-rest state with a moderate, non-mobilized lateral pressure.

The most common analytic approaches are Rankine and Coulomb theories. Rankine theory provides simple expressions for non-cohesive backfills. Active earth pressure Pa = 0.5 γ' H^2 Ka, with Ka = (1 − sin φ)/(1 + sin φ). Passive pressure Pp = 0.5 γ' H^2 Kp, with Kp = (1 + sin φ)/(1 − sin φ). γ' is the effective unit weight (γ − γw when submerged). For cohesive soils, cohesion reduces Pa, and Coulomb’s theory yields more general results by accounting for friction along the failure surface.

At-rest earth pressure on a rigid vertical wall is p(z) = γ z K0, with K0 ≈ 1 − sin φ (Jaky’s formula). The resultant acts at depth 2H/3, and the total force is (1/2) γ K0 H^2 (using γ′ if submerged).

Groundwater and seepage influence the effective stresses and pressure magnitudes. The theories assume homogeneous, isotropic soils and do not capture nonlinear, time-dependent, or seismic effects. Modern practice uses field measurements and numerical methods to handle complex conditions. Earth pressure theory informs the design of retaining walls, braced excavations, and tunnel or basement structures.