Lateral earth stress theory is used to estimate how much soil can exert pressure perpendicular to gravity. This is the pressure on the retaining walls. The lateral earth stress coefficient, K, is defined as the ratio of the lateral (horizontal) stress to the vertical stress for cohesionless soil (K = σ h/σ v). There are three coefficients: at rest, active and passive. Rest stress is the lateral stress in the soil before any disturbance.
A state of active stress is reached when a wall is displaced from the soil under lateral pressure, and shear failure results from the reduction in lateral pressure. A state of passive stress is reached when a wall is pushed into the soil enough to cause massive shear failure due to increased lateral stress. There are several theories for estimating lateral earth pressure; Some are empirically based and some are derived analytically.
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