The large deformation mechanism of Xietan sewage-treatment plant slope in the Three Gorges Reservoir, China

A large deformation of the Xietan sewage-treatment plant slope happened on November 11, 2008, when the water level of Three Gorges impounded to 172.79m; it had resulted in the abandonment of the newly built sewage plant due to foundation cracking. To characterize the large deformation mechanism owing to the interaction between underground water changing and gravelly soil in the front of the slope during reservoir impounding, elaborated laboratory permeation and triaxial tests combining with numerical simulation were carried out. With the rise of reservoir water, continuous transport and loss of fine particles were observed in the course of the permeation test. Tested permeability of soil sample showed a general increasing trend and could reach 20 times the initial value after the permeation test. Triaxial tests exhibited significant deterioration of soil mechanical properties under permeation action. When the reservoir water impounded to 172.79m, the secant modulus E 1.5% averagely decreased 50%, while the cohesion and friction angle of soil sample respectively reduced 50.4% and 21.4% after 13 days of permeation. The large deformation of the Xietan sewage-treatment plant slope was triggered by the dramatic reduction of soil shear strengths and enhancement of soil deformability in the front of the slope under permeation erosion action in the course of reservoir impoundment.