Effective stress response of clay to undrained cyclic loading

No information exists in the current soil mechanics literature on the behaviour of saturated clays under slow cyclic reversal in stress and strain, that will allow an effective stress interpretation of their undrained response. This study fills such a gap in our knowledge of clay behaviour. The undrained response of normally consolidated Cloverdale clay to slow constant stress amplitude cyclic reversal in stress and strain follows two distinct patterns. Under low cyclic stress levels (less than half the undrained strength), residual strain and pore water pressure development with loading cycles tends toward equilibrium. Under high cyclic stress levels (higher than half the undrained strength), strains at peak of cyclic stress eventually accumulate at an accelerated rate per cycle. A single threshold principal effective stress ratio signals the onset of strain acceleration for all cyclic stress levels. A step increase in cyclic stress level may shift the clay response from equilibrium type into instability type within the same cyclic loading event. During cyclic loading the effective stress states for a given number of cycles form a collapsing stable state boundary surface common to all cyclic stress levels. The relationship between principal effective stress ratio and strain at peak of cyclic stress depends on both cyclic stress level and loading phase. Cyclic stress level also affects the correlation between cumulative hysteretic work and residual pore water pressure. However, damping ratio remains essentially constant for all cyclic stress levels. Upon cyclic instability, the clay fails on the same effective stress strength failure envelope as under monotonic loading. Undrained cyclic loading clay response depends on the initial loading direction. Cyclic loading initiated in the direction opposite to natural clay deposition is more damaging than loading initially congruent with natural direction of material deposition. Constant stress ratio amplitude cyclic reversal in stress and strain results in limited strain and pore water pressure development with loading cycles. The collapsing stable state boundary surface found for constant stress amplitude cyclic loading is however also a bounding effective stress state for constant stress ratio cyclic loading. Postcyclic undrained monotonic loading results in strength and stiffness degradation directly related to the maximum strain developed during cyclic loading and to the effective stress stability threshold found for constant stress amplitude cyclic loading. A unique relationship exists between normalized undrained strength and overconsolidation ratio regardless of the type of overconsolidation history, provided that the overconsolidation ratio is defined with respect to the equivalent consolidation stress.Applied Science, Faculty ofCivil Engineering, Department ofGraduat