Study On The Deformation Mechanism Of Anti Slide Pile Of Ma Jia Gou Landslide In The Three Gorges Reservoir Area

After the completion of anti-slide pile construction on Majiagou landslide in the Three Gorges Reservoir area,penetrating cracks emerging between pile and sliding mass behind the pile due to their disharmonious deformation under water level fluctuations of the Three Gorges Reservoir.Previous studies on the deformation mechanism of the anti-slide piles in Majiagou landslide mainly focused on the deformation of the landslide under the influence of inducing factors(rainfall,water level fluctuating,and so on),and these studies show that thrust is the main reason for deformation of landslide anti-slide pile.However,this view of point cannot explain how the cracks develop.In order to study the development of the cracks,red sandstone samples from embedded section of the anti-slide piles in Majiagou landslide were taken for triaxial and splitting tests under long-term immersion conditions to obtain compressive and tensile strength of the sandstone.The groundwater seepage fields during the period between the moment of initial impoundment and the time of cracks appearing are simulated by GEO-STUDIO software based on the theory of saturated-unsaturated seepage.The deformation of the landslide during development of the cracks are calculated using FLAC3 D software based on the theory of fluid-solid coupling.At last,the developing mechanism of the cracks is discussed based on the results of above tests and numerical simulations.The main study results are summarized as follows:(1)In order to simulate the conditions of the sandstone samples from the embedded section in Majiagou landslide undergoing wetting-drying cycles more reasonably,the test system for rock tensile strength is developed independently to test tensile strengths of the sandstone samples during the period of water level fluctuations of Three Gorges Reservoir.s.(2)During the period of the cracks developing,the physical and mechanical parameters of the sandstone samples degraded obviously after a long time immersion.The peak compressive strength of the sandstone samples immersed for 75 days under confining pressures of 0 MPa,2 MPa and 4MPa decreased by 67.96%,60.89% and 59.34% respectively;the tensile strength decreased by 58.95%,the cohesion and internal friction angles decreased by 61.06% and 13.71%,respectively.(3)During the period of the development of the cracks,the saturation line at the toe of landslide is greatly affected by the decrease of reservoir water level.In the early stage of water level decline,the angle between the saturation line and horizontal direction is small.As the decreasing of reservoir water level,the angle increases continuously,and so do the difference of water head between the underground water table in the landslide and the reservoir water level,which make a large seepage force pointing outside landslide.(4)On the one hand,macroscopic mechanical strength of the sandstone in embedded section of the anti-slide piles degraded obviously due to its long-term immersing,which make the resistance of sandstone in the front of the pile reduced and the effects of embedding weakened during the period of the development of the cracks.On the other hand,the descending speed of underground water table in landslide is obviously slower than that of reservoir water level due to small permeability coefficient of the sliding mass,resulting in a great difference of water heads inside and outside of the landslide,then a large seepage force pointing outward the landslide is produced.The deformation of the sliding mass in the front of the piles increases under the action of the seepage forces.Finally,the supporting effect of the anti-slide piles greatly weakened.Cracks were gradually developing with the combined effects of strength deterioration of the sandstone in embedded section and decline of reservoir water level.(5)The strength deterioration of the sandstone in embedded section of the anti-slide piles after long-time immersion is the main reason for the development of the cracks,and the deformation of sliding mass in the front of the pile promoted the development of this progress.Hence,the effect of the long-time immersion on the strength of rocks in embedded section of anti-slide piles should be taken into account for anti-slide pile design.In other words,we should not only determine the strength parameters in saturated state of the rocks in embedded section,but also need to obtain the strength parameters of the rocks after long-time immersion in reservoir water.