4.6 Summary

The progress to failure of different systems of propping retaining walls in soft clay has been studied using centrifuge modelling. For an unpropped excavation, the cantilever wall rotates around a pivot near the wall toe and the retained soil exhibites typical linear slip surface accompanied by some tension cracks near the soil surface. Calculations based on Rankine¡¯s earth pressure theory show a reasonably good agreement between the predicted and measured critical excavation depth, confirming that the model excavation is essentially in an undrained condition.

By using an upper strut at ground level to support the wall, the soil movement issignificantly reduced as comparied to an unstrutted excavation. Due to insufficient wall

embedment, the wall fails by rotating around the support point of upper strut with a curved shear failure surface. Again, reasonable agreement on critical excavation depth between the tests and prediction using Rankine¡¯s theory is obtained.

For a two-level propped wall, the second strut is found to effectively control the excess deflection of the wall and the soil movement. A local failure mode for the toe kick-out is observed.

Based on the measurement of pore water pressure in the retained soil, the mechanism of time-dependent behaviour of an excavation is also examined. As the wall moves towards the cut, the lateral stress was reduced and the soil tends to swell. Due to the low permeability of clay, excess negative pore pressure build up and the dissipation of such excess pore pressure are observed to be slow. When the excavation is completed, the wall movement slows down and the pore pressures begin to rebound which reflects the dissipation of negative pore pressure in the long term. With this dissipation, the effective stress of soil would decrease and result in further movement of the wall towards the cut.

The results of test series 1 verify that centrifuge model is an effective way to simulate

the phenomena of an excavation up to the failures of the retaining system at a model scale. Such study is useful in providing some basic ideas and methodology for test series 2 to evaluate the prefailure behaviour of an excavation.