Study is conducted based on the summative results of 19 landslides in the middle route of the South-to-North water transfer project, Nanyang section. A series of geological survey has been done on the characteristics and evolution mechanism of expansive soil landslide in the area by excavating trench slides. The results show that most landslides in the area occurred in the quaternary Middle Pleistocene alluvial-diluvial strata. Slope stability is jointly controlled by vertical joints in the upper layer soil and the gently dipping large fissures at the toe of the slope. Sliding surface is composed by the steep tilting fissures at the trailing edge of slope and large gently dipping fissures at the slope toe. Excavation unloading effect causes the vertical joints open, which destroys the integrity of the slope soil and serves as the main channel for the water. There is a high humidity area 4-8 m below the surface of slope. Soil in the area is weak and the sliding surface develops in it. Wetting and drying cycle caused by climate and excavation unloading leads to the vertical fissures to open and propagate deep into the slope. This effect causes a dramatic attenuation of soil strength. The vertical fissures connected with the large gently dipping fissures at the toe of the slope eventually. The slope slides after these fissures are filled with water after the occurrence of strong lasting precipitation.
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