Interaction between a rigid retaining wall and unsaturated soils

Abstract

Retaining walls are widely used for stabilising earth slopes and supporting excavations. They often interact with soils which are unsaturated. However, retaining wall-unsaturated soil interaction has not been investigated in experiments and treated adequately in theories. In this research the very first experimental testing program to investigate aspects of how a rigid retaining wall interacts with unsaturated soils is conducted. Another first is the extension of slip line theory to unsaturated soils for the retaining wall problem.

The research characterises the unsaturated silty sand soil used in the experiments, with mechanical properties investigated using triaxial compression tests, and hydraulic properties using pressure plate tests and mercury intrusion porosimetry. Observed strength gains due to suction are described successfully using the effective stress concept. Also, water retention and hydraulic hysteresis are successfully represented by a fractal-based water retention model. The research details the development and validation of a new testing facility and set of procedures for carrying out retaining wall model tests in unsaturated soils. This includes a new compaction technique employing a hand-held electric percussion hammer to prepare samples at targeted moisture contents and densities. Also, a system is developed to saturate commercially available vibrating wire piezometers for reliable suction measurements during testing. Furthermore, soil displacements are successfully determined using particle image velocimetry.

Results of five retaining wall model tests are presented in full. For each a rigid retaining wall is rotated about its toe into a sample, compacted at different combinations of moisture content and density. Both density and moisture content are found to affect the loads the soil exerted on the wall, the earth pressures mobilised, the depths of passive thrusts, and suction variations as wall rotation occurred. Guidelines are given on how these aspects may be incorporated into retaining wall design. The slip line theory demonstrates that suction increases the limiting passive earth pressures. Suction also influences soil displacements as the wall undergoes a rotation increment. Although the theory can not capture discontinuities in the measured soil displacements, it does predict average values of earth pressures to a reasonable accuracy.