Undrained dynamic behaviour of peaty organic soil under long-term cyclic loading, Part I: Experimental investigation

To investigate the undrained cyclic behaviour of peaty organic soil under long-term cyclic loading, a series of undrained cyclic triaxial compression tests was conducted. The permanent axial strain, accumulated excess pore pressure as well as secant shear modulus during the loading cycles were studied and their dependencies on variables such as confining pressure, cyclic deviator stress and static deviator stress were summarized. Distinctive characteristics of the peaty organic soil under long-term cyclic loading were presented in comparison to a soft clay from Wenzhou. For a given stress state, the permanent strain over the initial limited loading cycles was smaller for the peaty organic soil than for the inorganic soft clay for the higher static strength of peaty organic soil. The peaty organic soil displayed distinct viscous nature and higher permanent strain rate under long-term cyclic loading. Due to the highly decomposed and often amorphous organic components in the peaty organic soil, which results in a weak structure strength, the development of permanent strain was found to be insensitive to confining pressure. In addition, for a given shear strain, the secant modulus of peaty organic soils increased with decreasing organic content and degree of decomposition or with increasing confining pressure. While the equivalent damping ratio was found to generally decrease as the confining pressure, organic content or degree of decomposition increased.