Behavior of Amorphous Peaty Soil under Long-Term Cyclic Loading

A series of one-way and long-term (10,000 cycles) cyclic triaxial tests was carried out to investigate the cyclic behavior of peaty soil from Kunming, China. The organic components of the peaty soil were generally highly decomposed and amorphous. The cyclic behavior of the peaty soil under long-term cyclic loading was found to be highly dependent on the ratio between cyclic stress amplitude and shear strength [cyclic stress ratio (CSR)], loading frequency, drainage conditions, and consolidation stress conditions. The cyclically induced accumulations of permanent strain and excess pore pressure at a given number of cycles were found to increase with an increase of CSR and decrease with an increase in loading frequency. The permanent strain accumulation rate was greater for the peaty soil than that for the inorganic soils reported in the literature. In addition, the permanent strain during cyclic loading was always larger for anisotropically consolidated specimens than for isotropically consolidated specimens. A correction factor of 1.44 is proposed for the isotropically consolidated specimens to reflect the anisotropic consolidation condition. The permanent strain was greater in undrained specimens than that in partially drained specimens for a high CSR. The strain difference was greatly reduced with a decrease of CSR, and more permanent axial strain presumably would be accumulated in a partially drained specimen if CSR further decreased. Finally, an advanced model for the peaty soil subjected to long-term cyclic loading under the undrained condition is proposed by combining classical elastoplastic theory and empirical equations. This model is implemented in finite-element formulations and validated through simulations of element tests and a shallow foundation subjected to long-term repetitive loading.