Undrained monotonic and cyclic simple shear behaviour of carbonate sand

The paper presents a study of the undrained behaviourof an uncemented carbonate sand (Quiou sand) undersimple shear loading conditions. The experimental studywas conducted through cyclic and monotonic undrained/constant volume simple shear tests carried out on reconstitutedspecimens prepared by using the sedimentationin water (WS) method. Tests were carried out on specimensreconstituted at two void ratios (i.e. loose anddense) and different effective consolidation stresses.Furthermore, to account for the effect of non-zero meanshear stress level, cyclic simple shear tests were performedunder both symmetrical and non-symmetricalcyclic loading. Two types of failure modes have beenobserved in cyclic tests, that is ‘cyclic liquefaction’ or‘cyclic mobility’, depending on whether or not they wereconducted under shear stress reversal conditions. A unifiedframework seems to exist whereby undrained monotonicand cyclic response can be comparatively analysed.In particular, normalising the cyclic liquefaction resistancesobtained from symmetrical tests by phase transformationstrengths determined in corresponding monotonictests, provides a cyclic liquefaction resistance curve whichwas found to be unique, irrespective of initial void ratioand vertical effective stress. Undrained cyclic shearstrength of the tested sand appears to be affected by thepresence of a non-zero mean shear stress, following apattern of behaviour, which is similar for both loose anddense specimens. Furthermore, the normalised stress–strain curves of the cyclic tests show back-bone curvesthat are practically coincident with the equivalent monotoniccurves.