Formation mechanism and stability analysis of the Houba expansive soil landslide

To uncover the formation mechanism of the Houba landslide in the Three Gorges Reservoir Area in China, macroscopic deformation characteristics, matter and structural characteristics, and slope stability were investigated using geological field investigations, indoor and outdoor tests, mineral identification, etc. The results show that since a large deformation occurred in the Houba landslide in 2003, there has been no obvious overall deformation and the whole slope has been in a basic steady state since January 2004. Slope deformation mainly exists in the expansive soil layer from 0 to 5 m (surface-layer sliding zone), and there are some evidences suggesting activity at 20 m (shallow-layer sliding zone) and nearby 35 m (deep-layer sliding zone). The expansive soil in the surface layer has a thickness of approximately 5 m, is 25–35% chlorite and illite, and has a free swelling ratio of 25–80%, making it a weak-moderate expansive soil. The expansive soil also has strong hydrophilicity, and it is more sensitive to saturation changes. Its shear strength could be easily reduced by saturation–unsaturation cycles with the change of water table or rainfall, so the expansive soil itself creates a good condition for a gently inclined landslide. The surface-layer slope, shallow-layer slope and deep-layer slope are in steady states under natural and rainstorm conditions. However, the surface-layer landslide body on the back edge of the III–III′ profile is in a basic stable–unstable state under rainstorm conditions, and a surface-layer landslide may occur under continuous rainfall conditions. Many people live on the Houba landslide, and the coupling effect of expansive soil and rainfall could cause many more local landslides from the surface-layer expansive soil with a slope-free face. Some measures should be taken to control the landslide, such as surface draining, local landslide supporting and retaining, and reinforcing the front existing retaining wall.