Analysis of undrained cyclic response of saturated soils

Some previous constitutive models introduce many sophisticated concepts and new parameters to characterise cyclic behaviour of saturated soils, while other simpler models show incomplete theoretical developments which limit their applications. This study aims to improve previous studies by revising the way the yield surface responds to cyclic loading under undrained conditions, i.e., the yield surface is assumed to change its shape while maintaining its size during unloading. The proposed hypothesis is used to examine a series of undrained cyclic tests where various cohesive soils are examined with an acceptable agreement. Based on this process, empirical relationships between the model parameters and given load and soil characteristics are formed, thus enabling the input parameters to be determined rigorously. The analysis shows that soil can reach an unstable state where the excess pore pressure (EPP) and axial strain develop rapidly when the cyclic stress ratio reaches a critical degree. A higher loading frequency requires a larger number of cycles but in shorter timeframe to initiate instability of the soil. The lower the plasticity of the soil, the larger and faster the accumulation of EPP and axial strain whereas increasing the initial shear stress can make soil unstable earlier.