Undrained monotonic and cyclic simple shear behaviour of carbonate sand

The paper presents a study of the undrained behaviour of an uncemented carbonate sand (Quiou sand) under simple shear loading conditions. The experimental study was conducted through cyclic and monotonic undrained/constant volume simple shear tests carried out on reconstituted specimens prepared by using the sedimentation in water (WS) method. Tests were carried out on specimens reconstituted at two void ratios (i.e. loose and dense) and different effective consolidation stresses. Furthermore, to account for the effect of non-zero mean shear stress level, cyclic simple shear tests were performed under both symmetrical and non-symmetrical cyclic loading. Two types of failure modes have been observed in cyclic tests, that is 'cyclic liquefaction' or 'cyclic mobility', depending on whether or not they were conducted under shear stress reversal conditions. A unified framework seems to exist whereby undrained monotonic and cyclic response can be comparatively analysed. In particular, normalising the cyclic liquefaction resistances obtained from symmetrical tests by phase transformation strengths determined in corresponding monotonic tests, provides a cyclic liquefaction resistance curve which was found to be unique, irrespective of initial void ratio and vertical effective stress. Undrained cyclic shear strength of the tested sand appears to be affected by the presence of a non-zero mean shear stress, following a pattern of behaviour, which is similar for both loose and dense specimens. Furthermore, the normalised stress strain curves of the cyclic tests show back-bone curves that are practically coincident with the equivalent monotonic curves.