Experimental Study On Progressive Instability Mechanism And Evolution Of Retrogressive Landslide Based On Sliding Zone Softening

With the rapid development of China's economic construction,especially since the implementation of the"Western Development"strategy and the national hydropower development plan,a large number of large-scale water conservancy and hydropower projects have been built in the frequent occurrence of geological disasters in Southwest China.The construction of these reservoirs has greatly alleviated the flooding problem and achieved great economic benefits.However,behind these advantages are the occurrence of disasters such as reservoir bank landslide revival and even instability caused by reservoir storage,and the cost of treatment is also huge.Therefore,this paper takes the old landslide on reservoir bank in the southwestern area as the research object,combines theoretical analysis,model test and field investigation,and makes a thorough study and discussion on the progressive instability mechanism and stability evolution law of the retrogressive landslide based on sliding zone softening under the action of reservoir water,so as to provide theoretical basis and technical support for the treatment of reservoir bank landslide.(1)The calculation method of the trailing edge inclination angle of retrogressive landslide grading slider is proposed.Based on the retrogressive landslide instability mechanism and failure mode,the mathematical mechanics model is established.The limit equilibrium analysis method is used to calculate the most dangerous rupture inclination angle of each grading slider,and the trailing edge rupture surface is taken as the division type of the limit equilibrium slice method.This method provides a new idea for the stability analysis of the progressive failure process of the retrogressive landslide.(2)A landslide stability evalution method suitable for progressive failure mechanism of retrogressive landslide is proposed.Based on the determination of the type of slice divison,the method in this paper is improved in the basic principle of the Sarma method.The?-?constitutive model that can describe the strain softening characteristics of the sliding surface and a shear-slip displacement model along the sliding surface are introduced,the deformation characteristics of the slider are related to the progressive failure process of the retrogressive landslide.A stability calculation method the can consider the relationship between the shear strength and shear displacement of the sliding-zone soil is establilshed,and the quantitative evaluation relationship of the safety factor and the displacement of the sliding body is realized.(3)A test device is developed to simulate both the softening effect of the groundwater on the sliding zone and progressive failure characteristics of the landslide.The sectional sliding surface,which is a part of this device and composed of several permeable boxes,can simulate sliding surfaces with different geometric shapes.The sliding zone can be simulated to be saturated and softened in stages by injecting water into different permeable boxes,so that the progressive failure process of the retrogressive landslide is realized.(4)Using the new test device and test system of“Segmental sliding surface bottom penetration method”,ten groups of indoor model tests under different conditions were carried out,mainly considering the influences of the condition of rising reservoir water level,different initial reservoir water levels,different sliding zone soil and landslide body soil,and the evolution characteristics of landslide body displacement,the moisture content of sliding zone soil and the change of pore water pressure and spatial evolution characteristics and the typical failure modes of landslide body were obtained.The test process includes the entire process from the initial start of the landslide body to complete instability.The research shows that the strength degradation of the sliding zone soil has a good correlation with the deformation of the landslide body.It is an important condition for landslide occurrence that the shear strength of sliding zone soil decreases greatly after saturation.When the reservoir water level rises,the old landslide on the reservoir bank is liable to cause high pore water pressure in the sliding zone soil,and the pore pressure variation at the sliding surface stimulates the revival of the old landslide.Pore water pressure plays a leading role in the revival process of reservoir bank landslide and is an important inducement for the revival of old landslide.(5)The old landslide on reservoir bank has retrogressive sliding characteristics under the condition of rising water level.As the water level increases,the deformation area of the landslide body expands continuously,the degree of slope damage increases,and the displacement significantly increases.Different initial water level heights have an important impact on the instability mode of the landslide body.When the water level is higher,the softening range of the sliding zone is enlarged,the pore water pressure is increased,the shear strength is reduced gradually,the deformation range of the slope is larger,and a large-scale overall landslide with a high slip rate is likely to occur;when the water level is lower,the slope deformation area is small,and the local instability mainly occurs.Due to the different properties of the sliding body soil and the sliding-zone soil,the landslide body has different failure modes.The landslide body with the clay-sand sliding zone firstly occurs local failure at the foot of the slope,and then the progressive failure process caused by the strength degradation of the sliding zone soil is dominant.The landslide body with the sand-clay sliding zone is the synchronous action of the double-effect mechanism of the strength degradation of the sliding zone and the instability of the toe of slope.The landslide body with the clay sliding zone is mainly unstable due to the strength degradation of the sliding zone.Under the action of reservoir water rise,the landslide body with sand-clay soil is prone to the overall slip failure mechanism along the sliding surface.The instability mode of the fine-grained clay sand landslide body is decomposed and integrated,forming a multi-level secondary slider,no longer retaining the original slope shape.The coarse-grained clay sand landslide body usually undergoes a collapse-type instability and has no specific slope failure morphology.(6)The model tests of 21 groups of retrogressive landslides in small deformation stage were carried out.The effects of different factors such as length of unstable sliding zone,slope shape,sliding surface shape,thickness of landslide body and water content of sliding body on the trailing edge rupture surface of retrogressive landslide and the evolution characteristics of the cracks in the process of progressive instability of sliders were studied.The results show that the spatial shape of the trailing edge cracks in the landslide body is not single,which is affected by various factors,mainly manifested as fold line,(inverted)arc type and linear type.In the soil with high water content,the cracks on the trailing edge show backward slope type.Among them,the fold line cracks account for about 1/2 of the total number.The thickness of the slope has an effect on the shape of the trailing edge.When the thickness increases,and the crack at the trailing edge of the slope is“inverted arc”;when the thickness is reduced,and the crack at the trailing edge develops into“fold line type”,and the“straight line and semi-arc”type of the vertical slope is finally formed.(7)Assuming that the possible formation mechanism of the trailing edge of retrogressive landslide is general slide failure,sliding-tension failure,sliding shear failure,the inclination angles of the trailing edge of various failure mechanisms are calculated respectively.Comparing with the measured values,the calculated values of sliding-tension failure mechanism are closest to the experimental values,and the relative error is less than10%.It can be concluded that the formation mechanism of the trailing edge of the retrogressive landslide is as follows:the bottom sliding surface is unstable and sliding,and there is a tension failure zone at the end of the unstable sliding zone.The trailing edge tension failure surface gradually develops from the slip surface to the slope surface,and the direction of the tension fracture surface is consistent with the direction of the maximum principal stress.(8)By means of experiments,various factors affecting the instability range of the first section of straight-line landslide are analyzed,including the instability length of the sliding zone,the thickness of the sliding body and the slope angle.The variation law of the instability length of the first section of the slope affected by various factors is revealed,and a formula for calculating the instability length of the first section of the local slope is proposed preliminarily:(?)Through the comprehensive analysis of two typical engineering examples,the formula for calculating the local slope instability range of the first section of linear landslide is verified.The results show that the theoretical formula in this paper is reasonable and reliable,and has a certain degree of safety.