Summary

During the rapid loading of clay soils there is often no opportunity for the dissipation of any porewater pressure that may be generated. It is largely for this reason that heavy clay fill is not currently specified for use in reinforced soil construction where this short-term, or undrained, condition might be associated with instability. The response of reinforced clay to undrained loading will be a function of the nature of the reinforcement, and to investigate possible responses a pilot study was undertaken using two different reinforcing materials. These were installed in cylindrical clay samples which were subject to either rapid shear or shear at constant volume, both of these regimes being consistent with the notion of undrained loading. It was found that impermeable ‘reinforcement’ in the form of aluminium foil caused a consistent and substantial decrease in strength compared to that of an unreinforced sample. For clay reinforced with porous plastic reinforcement the strength of the soil improved as reinforcement spacing decreased and ultimately exceeded that of the unreinforced clay. This response reflects the fact that rapid loading is not necessarily associated with undrained loading if the reinforcement, which also acts as a drain, is installed at a sufficiently small spacing. In truly undrained tests, where constant volume conditions are imposed, a strength decrease was anticipated. That this was not the case, is thought to be due to the low porewater pressure response of the reinforcement which has the effect of depressing the otherwise high deleterious porewater pressure generated in the clay.

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