Abstract：There are many approximately horizontal slip surface landslides in Jurassic red strata in Wanzhou city of the Three Gorges Reservoir area. However，the formation mechanisms of these landslides are not clear. In order to explore the mechanism of these landslides，triaxial stress relaxation experiments with GDS unsaturated triaxial testing systems are carried out to investigate the stress relaxation properties of the sliding zone soil of Anlesi landslide. Test results show that the stress relaxation speed changes linearly with initial strain variation. However，it is independent of confining pressure. The soil volume changes little；and it can almost be ignored during the relaxation tests. Based on the stress relaxation tests，a new constitutive model for the stress-strain-time behavior of the sliding zone clay is introduced by assuming a hyperbolic stress-strain relationship and a logarithmic time-tress relationship. The comparison between the rheological model and experimental results reveals that the new rheological model presents the relaxation characteristics of the sliding zone clay nicely. And that also provides the model and parameters for the further researches on approximately horizontal slip surface landslides existing in Jurassic red strata in the Three Gorges Reservoir area. The new relaxation model indicates that the shear stress of the sliding zone clay will decrease to zero. The result is significant for researching and controlling the approximately horizontal slip surface landslides.
