COMPARATIVE ANALYSIS OF CREEP SHEAR BEHAVIOR OF COHESIVE SOIL UNDER DIFFERENT STATES

It is well recognized that the characteristic of slow-moving landslides is controlled by creep shear behavior of its slip zone's materials. However, it is obvious that creep behavior of the slip zone's materials of the new-born and reactivated slow-moving landslides belongs to creep shear behavior under pre-peak state and at residual state, respectively. Up to now, neither certain answer has been given nor insight has been disclosed yet by professionals about the differences when the slip zone's materials experience creep shear under different states. To investigate this issue, creep behavior of the slip zones' materials of two giant slow-moving landslides in Zhouqu county of Gansu province, called the Suoertou landslide and the Xieliupo landslide respectively, were studied experimentally. Slip zones of the two landslides are composed of cohesive soil with various amount of gravel. The soils were sheared under the pre-peak state and at residual state respectively with direct shear creep test and that following reversed direct shear. It was found that the soils experienced various stages of creep under both states, and that the critical shear stresses at each creep stage linearly increased with both normal and shear stresses, and gravel content as well. However, there was no second creep stage when the soil was re-sheared at the residual state, while the soil experienced a significant second creep stage under the pre-peak state. Moreover, it was observed that for the soil experienced creep shear under pre-peak state its critical shear stress at the tertiary creep was greater than that at the residual state. Significantly, under pre-peak state, the soils' critical shear stress at the tertiary creep was much less than its peak strength and greater than its residual strength, whereas at the residual state, its critical shear stress at the tertiary creep was very slightly greater than the residual strength. It was further discussed that for the cohesive soil mechanism behind the different creep behavior of the cohesive soil under different state should be largely attributed to its structures, specifically no structural defect under pre-peak state versus occurrence of well-defined structural defect at the residual state. These findings give an important implication that the slopes composed of cohesive soils may be more prone to occur the reactivated slow-moving landslides than the new-born slow-moving landslides.