In assessing the local stability of simple retaining walls, conventional design methods require that independent assessments are made of sliding, overturning, and bearing capacity, with the width of the wall base being determined by the most critical of these considerations. It is shown that these three considerations can be interrelated by using an integrated design method. This method indicates that the selection of high values of angle of base friction, as permitted by the current Code of Practice for earth retaining structures, can lead to unacceptably low factors of safety on bearing capacity when due account is taken of load eccentricity and inclination. An introduction is made to design using partial coefficients, or load factors, and it is shown that a partial coefficient of 1.4 to 1.5 is required on the tangent of the internal angle of shearing resistance to obtain conventional factors of safety of the required value. Adoption of the Danish Code factor of 1.2 appears to lead to conventional factors of safety less than 2. Finally, wall design using compaction theory is introduced where some account is taken of the high lateral earth pressures induced by compaction plant. It is shown that the resulting high lateral thrust has a potentially dramatic effect on factors of safety against sliding and bearing capacity. Under these circumstances, there is a likelihood of the wall sliding, in which case the lateral thrust is reduced to the active value. Despite this, bending moments at the base of the wall stem are still higher than those obtained using a quasi-hydrostatic distribution of lateral earth pressure, since bodily wall translation is associated with a parabolic pressure distribution. This leads to an elevation of the line of active thrust which can readily account for a 50% increase in the design bending moment.
Professor T.S. Ingold
The conventional process of wall design involves