Cyclic behaviour and resistance of saturated sand under non-symmetrical loading conditions

This paper reports findings from an experimental study that aimed to investigate the undrained behaviour of sand in non-symmetrical cyclic loading, and to clarify the role of initial static shear in liquefaction resistance. The testing programme, conducted on a standard sand under triaxial conditions, covers a broad range of initial states in terms of relative density, confining stress and initial shear stress ratio (alpha). Three distinct failure modes have been identified from the tests: flow-type failure, cyclic mobility and plastic strain accumulation. Of these, flow-type failure, characterised by abrupt runaway deformations without any prior warning, is the most critical, and pertains to sand in the loose state. The tests also demonstrate that the presence of initial static shear stress is beneficial to the liquefaction resistance of loose sand at low alpha levels, but it becomes detrimental at high alpha levels. In this connection the concept of threshold alpha is proposed, together with the use of a no-stress-reversal line for better characterisation of the effect of initial static shear. Furthermore, in the conceptual framework of critical state soil mechanics, a fairly good linear relationship has been established between the threshold alpha and the state parameter psi that collectively accounts for the initial relative density and mean stress level. This relationship suggests that the threshold alpha decreases with increasing values of psi or with sand becoming looser than the critical state. It is further proposed that the concept of threshold alpha also applies to sand at high relative density, as long as the confining stress becomes sufficiently high. This proposal leads to a unified and consistent interpretation of the complicated static shear effect.