Apparent Lateral Earth Pressures below the Lowest Support Level

Abstract

A considerable number of authors and authorities have come to propose their own set of apparent lateral earth pressure diagrams for braced excavations. When stiff clays dominate, all diagrams reduce the apparent earth pressure to zero at the subgrade level. In the author's experience this practice is likely both unconservative and unsubstantiated for modern urban excavations and may lead to unsatisfactory wall performance. While the actual equivalent earth reaction at subgrade has rarely been directly measured it is still incorporated in apparent earth pressure diagrams. Hence, current practice diagrams truly cover earth pressures only down to midway between the lowest support level and the excavation subgrade. Furthermore, little guidance is offered in terms of defining the appropriate lateral driving earth pressure below subgrade and misguide designers to make questionable design assumptions or even assume zero lateral driving earth pressure below subgrade. A series of controlled finite element simulations were performed to examine the behavior of lateral earth pressures near and below the lowest support level. Finite element results from both the controlled cases and from benchmarked excavation models are compared. Last, results are condensed into simplified modifications and diagrams that can be applied to any apparent earth pressure diagram.