Experimental investigation on the monotonic and cyclic behaviour of a structured clay at high confinement

In this paper, the monotonic and cyclic behaviour of a structured clay is experimentally evaluated by means of anisotropically consolidated undrained triaxial tests. The material behaviour under high confinement pressure is analysed with a view to underground gas storage applications by imposing one-way fatigue loads characterised by large periods. The experimental results highlight the role of the material structure and the consequences of its progressive degradation, which implies a fragile stress–strain response. The results also show the relevance of time-dependent effects. Under monotonic strain-controlled conditions, such effects induce a strong strain rate dependence of the peak strength. Under cyclic stress-controlled loading, the fatigue life is shown to be influenced by the loading period and the characteristics of the sinusoidal history. Longer loading periods result in a lower number of cycles to failure. Similarly, larger maximum imposed deviatoric stresses also result in a reduction in fatigue life. Conversely, a somewhat counter-intuitive response is observed in tests carried out with the same maximum imposed deviatoric stress, since an increase in loading amplitude implies an increase in the number of cycles to failure. The reasons for the observed results are discussed in detail in the light of the peculiarities of the material response.