Study On Failure Mechanism Of The Qianjiangping Slope

The Qianjiangping landslide is the first large scale rock slide in Three Gorges Reservoir(TGR)Area,China after the impoundment of the TGR.Previous studies on the landslide showed that most researchers agreed that reservoir impoundment and rainfall were the two main triggering factors of the landslide.However,there were different views about the influence degrees of the two factors on the failure of the landslide:(1)The influence of reservoir impoundment was greater than that of rainfall:(2)The influence of rainfall was greater than that of reservoir impoundment.In order to clarify this question,the rock near the rock bridge that was part of the slip zone after sliding was selected.The triaxial compressive tests were carried out to investigate the effects of reservoir water immersion on strength and deformation of rock bridge in the front of the slope.Based on fluid-solid coupling theory,seepage field of groundwater and stresses of the slope under the three conditions(reservoir impoundment,rainfall,and combined effects of reservoir impoundment and rainfall)were calculated by using the ABAQUS software.Based on the stress-strain relation measured in experiment,failure approach index was improved which was adopted and improved based on constitutive relationship of the rock obtained by the triaxial compressive tests to analyze the failure characteristics of the landslide under each of the three conditions.(1)The peak compressive strength(0 MPa,2 MPa,4 MPa)decreased by 47.22%,49.18% and 42.31%,respectively;the cohesion and internal friction angles decreased by 44.85% and 9.15%,respectively;the elastic modulus(0 MPa,2 MPa,4 MPa)decreased by 55.98%,59.04% and 54.41%,respectively;the deformation modulus(0 MPa,2 MPa,4 MPa)decreased 57.10%,61.79%,58.40% respectively.(2)The stress-strain curves of the rock mass near the rock bridge measured by the tests accord with strain hardening-softening model.According to the strain hardening-softening model,the failure approach index is improved theoretically.The numerical tests verifie that the improved failure approach index can correctly and reasonably evaluate the danger degree of stress and damage degree of rock material in loading / unloading process can be correctly and reasonably evaluated.(3)The effect of rainfall on the stability of Qianjiangping slope is greater than that of reservoir impoundment.However,any single factor of reservoir impoundment and rainfall can not cause landslide.(4)Qianqiangping landslide was induced by the combined effect of reservoir impoundment and rainfall.The rock bridge in the gently dipping discontinuous structural plane of the bank slope is weakened by the influence of reservoir water so as to reduce the anti-slide force of the bank slope.In addition,the rainfall before the sliding increased the sliding force of the slope-promoting section.The rock bridges in the gently dipping discontinuous structural plane were gradually cut off.The sliding surface was formed along the interlayer staggered zone and the gently dipping discontinuous structural plane,and the failure of the slope occurred.