LATERAL EARTH PRESSURE AT REST
Description:
The ratio of the horizontal principal effective stress to the vertical principal effective stress is called the lateral earth pressure coeffi cient at rest (Ko), that is,
The at-rest condition implies that no deformation occurs. We will revisit the at-rest coeffi cient in later chapters. You must remember that Ko applies only to effective principal, not total principal, stresses.
To fi nd the lateral total stress, you must add the porewater pressure. Remember that the porewater pressure is hydrostatic and, at any given depth, the porewater pressures in all directions are equal. For a soil that was never subjected to effective stresses higher than its current effective stress (normally consolidated soil), Ko 5 Ko nc is reasonably predicted by an equation suggested by Jaky (1944) as
.
The value of Ko nc is constant. During unloading or reloading, the soil stresses must adjust to be in equilibrium with the applied stress. This means that stress changes take place not only vertically but also horizontally. For a given surface stress, the changes in horizontal total stresses and vertical total stresses are different, but the porewater pressure changes in every direction are the same. Therefore, the current effective stresses are different in different directions.
A soil in which the current effective stress is lower than the past maximum stress is called an overconsolidated soil. The Ko values for overconsolidated soils are not constants. We will denote Ko for overconsolidated soils as Ko oc. Various equations have been suggested linking Ko oc to Ko nc. One equation that is popular and found to match test data reasonably well is an equation proposed by Meyerhof (1976) as
where OCR is the overconsolidation ratio (see Chapter 9 for more information), defi ned as the ratio of the past vertical effective stress to the current vertical effective stress