Lateral Earth Pressure and Retaining Walls

A retaining wall is a structure that supports a vertical or near vertical face of soil and is used to facilitate a major change in the ground surface level. The resulting horizontal stress from the soil on the wall is commonly referred to as lateral earth pressure . In the evaluation of the magnitude and distribution of this lateral earth pressure, it is assumed that the soil behind the wall (backfill) is on the verge of failure and obeys either the Tresca or Mohr-Coulomb failure criterion. Failure of the backfill occurs by two mechanisms depending on the direction of the wall displacement. If the displacement of the wall is away from the backfill soil the resulting failure is called active . A passive failure occurs if the wall is displaced towards the backfill until the limiting displacement is achieved. When the wall has no displacement, the lateral earth pressure at-rest can be estimated from elastic equilibrium or field measurements. In general there are basic theories used to estimate lateral earth pressures, viz., the Rankine method and the Terzaghi and Peck method. Rankine’s theory assumes a long and smooth vertical wall where the lateral pressure increases linearly with depth. This analysis, although convenient, imposes some limitations and consequential errors. For a wall with significant friction, or if there is an irregular ground surface, a Coulomb wedge analysis, in conjunction with the charts produced by Terzaghi & Peck (1967 ), is applied. In this method the actual lateral pressure distribution is not defined but, based on experimental data, the location of the resultant of these (lateral) pressures is defined. However, for long frictional walls such as sheet piles, Rankine’s method is applied using the earth pressure coefficients obtained from a Coulomb wedge analysis. This combination, although theoretically incorrect, appears to give reasonable estimates of the magnitude and distribution of the lateral soil pressure.