Monotonic fast residual strength of clay soils.

A laboratory study on the fast residual strength of clay soils is present in this paper. Laboratory tests were carried out at the Imperial College of London with the IC/NGI ring shear apparatus, modified by Parathiras to reduce soil loss during the very large displacements achieved under fast sliding. The tests were carried out for two different natural plastic soils, the less plastic Cowden till and the more plastic London clay, by using the two shearing velocities of 100 mm/min and of 1000 mm/min, in both cases. Depending on soil type, confining stress and rate of displacement, three possible rate effects might occur during fast shearing: positive, negative and neutral, which mean a steady state fast residual strength above, below or equal to the slow value, the latter evaluated at a rate of displacement not exceeding 0.05-0.01 mm/min. Initially, all samples showed a growth in strength; therefore, as displacements increased, fast residual strength might either stabilise or decrease to a minimum. In the more plastic London clay, fast residual strength never dropped below the slow residual value. Otherwise, depending on the applied normal stress, fast residual strength of the less plastic Cowden till might drop well below the slow residual, so giving rise to a negative effect of fast shearing. Due to the lack of pore pressure data within the sliding surface, test results were analysed in terms of total stress, to highlight any rate effect upon the fast residual strength when compared to the slow residual value. The different features in the steady state fast residual strength were also explained by soil fabric modifications nearby the sliding surface during fast sliding.