Study On Permeability Characteristic And Hysteresis Of The Landslide Mass In The Three Gorges Reservoir Area

China is a country with frequent geological disasters,and the Three Gorges Reservoir Area has always been a region with frequent geological disasters.According to the survey,there are 4664 collapse and landslides in the whole reservoir area,among them,there are 2619 Wading landslides.Since 2003,nearly 700 landslides and accumulations have been deformed.It seriously threatens the safety of people's life and property in the reservoir area,and puzzles the local normal production and life and economic construction.During the normal impoundment and operation period of the Three Gorges reservoir,the water level rises and falls between 145-175 m,and changes of 30 m every year,especially in the stage of large-scale decline of the water level,will directly cause changes in the seepage field of the landslide mass on both sides of the bank slope,and even changes in the permeability.It can be seen that the stability of landslide accumulation is closely related to its permeability and seepage.Therefore,the study on permeability characteristic and hysteresis of the wading landslide mass is of great significance and scientific value for geological disaster prevention and civil engineering construction in the Three Gorges Reservoir Area.In this paper,through the engineering geological analysis of several typical landslides,three main factors affecting the permeability of landslide bodies were recognized.In view of these three factors,a large number of experiments were carried out to study the permeability of the hydro-fluctuation belt of landslide bodies in the Three Gorges Reservoir Area by the double ring permeability test.Then,the spatial permeability of single landslide was analyzed from the point of view,taking the Shiliushubao landslide as an example.Based on the large-scale physical model test in the laboratory,the change characteristics of permeability of landslide bodies under the periodic rise and fall of reservoir water level were analyzed in terms of time.Finally,the analytical solution of groundwater saturation line was modified,and the seepage hysteresis was studied by numerical simulation.The main works and achievements are as follows:(1)Based on the analysis of the engineering geological characteristics of five typical landslides,it is concluded that the genetic mechanism,material lithology and material structure composition of landslides are the three main factors affecting the permeability of landslide bodies,the genetic mechanisms of landslides in the Three Gorges Reservoir area can be divided into five types: collapse type,bendingfracturing type,sliding-bending type,sliding-fracturing type,plastic flowing-fracturing type(translational type),and the structural characteristics and macro permeability of landslide bodies of different genesis are put forward.(2)Based on the in-situ seepage tests of 86 landslides and the sorting of the existing seepage data,1188 seepage test data of fluctuating zone of 396 wading landslide in the Three Gorges Reservoir Area were obtained,and the permeability characteristics of hydro-fluctuation belt of different types of landslide bodies in the Three Gorges Reservoir Area were found out.The permeability coefficient of landslides in the Three Gorges Reservoir Area is positively correlated with the rock strength and the content of gravel stone.However,the permeability of fractured rock mass is lower than that of crushed rock soil.The permeability of the landslide mass in the Three Gorges Reservoir Area has certain regional distribution characteristics.The permeability of the landslide mass is mainly medium and good.The permeability of the hydro-fluctuation belt is different from that of its macro permeability,which is mainly manifested in the weakening of permeability.(3)Through the detailed exploration work on the Shiliushubao landslide,the comprehensive observation of the landslide was carried out by installing various monitoring equipment,and the multi-element permeability test was carried out on the landslide.The spatial permeability,seepage characteristics and groundwater dynamic characteristics of the landslide were found out.The results show that the permeability of every part of the landslide body is different,and the variation is large.The main factors influencing the spatial variability of permeability of landslide mass are genetic mechanism,material composition and structure,material lithology,consolidation pressure,stress history and water rock action.The front of the landslide is mainly pipe network seepage,the change of groundwater is synchronous with that of reservoir water,and the middle and back of the landslide is mainly interface network seepage combined with pore seepage,the groundwater lags behind the change of reservoir water level.The change range of matrix suction in the surface layer(within 1 m)of landslide mass is large,but in the shallow and deep part,the matrix suction is very small,the change is also small.(4)Three groups of large-scale physical model tests were carried out with the different of material lithology and composition of landslide body to study the change characteristics and mechanism of permeability of the landslide mass under the multiple periodic rise and fall of reservoir water level.The main performance is that the permeability of the front increases and the middle and rear decreases.The main factors affecting the permeability change are collapsible compaction,seepage,front bank collapse,internal deformation and the rate of reservoir water level rise and fall.The permeability of the landslide body mainly composed of hard rock is more easily affected by the reservoir water than that of the landslide body mainly composed of soft rock.The permeability of the landslide body composed of soft rock changes slowly,and the deformation also has a long-term nature.(5)The analytical solution formula of the groundwater saturation line on the bank slope under the action of the rise and fall of the water level of the reservoir was modified from the following three aspects: the change of the rate of the rise and fall of the reservoir water level,the slope of the reservoir bank,and the calculation of the specific water yield.Finally,the modified analytical solution was applied to the solution of the phreatic line of Shiliushubao landslide,and compared with the numerical analysis results and monitoring results,it is considered that the solution of the phreatic line of the modified analytical solution in the part of the reservoir water level fluctuation zone is closer to the actual situation,which can meet the needs of the actual project.(6)Based on the seep / W program software of Geo-studio,a typical generalized model of landslide in the Three Gorges Reservoir Area was established.The hysteresis characteristics of landslide in the reservoir area under the condition of reservoir water level decline are systematically analyzed according to the four influencing factors of different saturation permeability coefficient,reservoir water level decline rate,landslide front slope and landslide thickness.The research reveals that the hysteresis coefficient and the slope of the water level fluctuation zone are the important factors affecting the hysteresis of the landslide seepage,and the influence of the landslide thickness is small.According to the saturated seepage parameter range and slope characteristics of the landslide in the Three Gorges Reservoir Area,the hysteresis coefficient 35 is taken as the boundary value of seepage hysteresis type and synchronous type,and the hysteresis grade is divided.