LAUSR

Abstract Recent laboratory investigations conducted on soils prone to mud pumping have shown that the subgrade soil experiences softening associated with an internal redistribution of moisture when the cyclic stress ratio (CSR) exceeds a certain critical level. This article aims to evaluate the stiffness degradation of these problematic subgrade soils subjected to a wide range of loading conditions. The test results revealed that the instability of the specimens is caused by the early softening behaviour, and this is accompanied by a sharp reduction in the specimen stiffness. As high as 70-80% reduction in the initial small-strain stiffness was observed even for densely compacted specimens (ρd = 1790 kg/m3). In this regard, a new method for predicting the threshold number of cycles, Nthr and residual axial strains required for the onset of subgrade instability is proposed. As a useful practical guide, a quasi-linear relationship between the threshold strain and Nthr is also determined.