Analysis of certain factors affecting the unstable behaviour of saturated loose sand

The factors affecting the onset of unstable behaviour in saturated loose sand loaded under drained stress-controlled conditions are not as well known as those involved in its undrained behaviour, which has been analysed in detail.

This paper is intended as a first step towards a better understanding of the onset of instability in drained stress-controlled tests and reports on the analysis of certain factors affecting the unstable behaviour of saturated loose sand samples subjected to drained stress-controlled triaxial compression tests. The samples are loaded continuously at a constant loading rate and the results are compared with those obtained by other authors who discontinuously applied a load using finite increments.

A special experimental apparatus was designed so that the axial load could be applied in various proportions by a dead weight and by a pneumatic cylinder in a triaxial cell with no friction on the loading ram.

The paper focuses first on the influence of the load application method (i.e. by dead weights and by a pneumatic cylinder), which proves important, since the sample always collapses at a more or less constantly low mobilized friction angle when loaded with dead loads, whereas it may have unstable strain jumps starting from a high value of the mobilised friction angle, but may even be stable up to the steady state of deformation, when the axial load is applied by a pneumatic cylinder.

Then the influence of the loading rate and loading path is examined and found to be negligible, at least in a first approximation. On the other hand, the effects of preshear and initial density are shown to have a major role in the onset of collapse: both the preshear and the increase in density lead to an increase in the resistance of the loose sand to collapse under drained stress-controlled conditions. Copyright © 2000 John Wiley & Sons, Ltd.