Many gentle dip landslides have taken place in Wanzhou, located in the Three Gorges Reservoir area. In order to study the mechanism of the gentle dip landslides, the authors selected the Anlesi landslide as a typical gentle dip landslide to study in detail. Field investigations show that the slip zones of the Anlesi landslide formed from a white mudstone in Jurassic red strata by compressive stress. The X-ray diffraction and infrared ray analysis reveal that the main mineral components of the slip zone are composed of montmorillonite, illite, feldspar, and quartz. A set of tests were conducted on the slip zone specimens to obtain the physicomechanical characteristics. Test results show that the slip zone soils are silty clay, of medium swelling potential, as shear strength becomes very low once the slip zone attracts water to saturation.
The main factors contributing to the gentle dip landslide mechanism are incompetent beds, recent tectonic activities, and intensive rainfall. Several stable, continuous, and thick incompetent beds exist in Jurassic red strata in Wanzhou. The integrity of incompetent beds was compromised under tectonic stress. The recent tectonic activities caused shear failure along the incompetent beds and joints in the sandstone. With the effect of intensive rainfall, water permeates to the incompetent beds along tectonic fissures, resulting in swelling of the soil material and high hydrostatic pressure in fissures of the strata. Therefore, the slopes are prone to slide along the incompetent beds.
Creep tests have been conducted to study the rheological properties of slip zone soils of the Anlesi landslide. The stages of creep attenuation and even rapid creep can be seen clearly from the creep curves. The rheological properties of the slip zone soils have nonlinear characteristics, and the nonlinear Burgers model formula was obtained successfully. FLAC3D software was used to simulate the Anlesi landslide in Jurassic red strata in Wanzhou city. The simulation results show that the stress, displacement, and plastic area change with varying creep time. The maximum displacement at X-direction reaches 7.59 m after a 200-year creep. Therefore, the Anlesi slope failed completely indicated by the rheology effect of Jurassic red strata.