Study On The Failure Mechanism Of The Slope At Section K0+100 To K0+876 Of The Traffic Way Of A Hydropower Station In Xinjiang Province

Stability of slopes,especially which at reservoir banks,is an issue frequently being encountered in hydropower engineering.Since the hydropower engineering are generally situated in alpine and gorge regions,such potential slope hazards as large rock slopes and artificial high slopes failure,revive of the old landslides or collapse will be inevitably faced during engineering construction and operation.In particular,once failures occur in slopes at the banks adjacent to the dams,the high-speed slide mass could give rise to large ground swells and therefore endangering the safety of the dam and associated facilities.To launch an intensive study on the boundary conditions,failure mechanism,present and future stability of bank slopes is therefore key to propose appropriate engineering measures for controlling slope hazards.Taking the instable slope at right bank of Laxiwa Hydroeletric power station as case study,this thesis has thoroughly surveyed the engineering geological conditions of the slope,and then investigated basic features of the slope,factors contributing to slope instability,and the failure mechanism of the slope.Findings are listed below:?1?By means of field surface investigation,the boundaries of unstable area at the slope before and after impounding water are both obtained.According to the landform,slope structure,and failure characteristics,the slope is split into two sections:Section I and Section II.?2?Influenced by regional tectonic fissures,weathering and unloading,a severe epigenetic and superficial reconstruction of the slope occurs,and the apparent stress-release cracks make the rock mass less integrated and cataclastic.And the rock mass at inner slope are mostly block-shaped.Utilizing the adit exploration,we found three dominant discontinuities steeply inclined to free face,steeply inclined to inner slope and gently inclined to free face,respectively.?3?It is found by field survey before impounding water that rock deformation and failure mainly occurs at middle to rear part of the slope,and that Section I undergone severe slope deformation than Section II.While in field survey after impounding water,it is observed that a large tensile fracture gradually developed with the settling reaches 15m,and rock deformation at Section I is more significant than that at Section II too.?4?Analyzing the monitoring data,it is obtained that the slope was creeping continuously before impoundment,the rate of deformation at Section I is greater than that at Section II,and the former is worse than the latter in terms of stability.While after impoundment,the rate of deformation at different positions largely varies.Section I witnessed a severe slope sliding in January 16^th,2013,and after that,the slope gradually arrived at a relative stable state.Influenced by the slope failure occurred at Section I,Section II undergone a slight slope deformation with the accumulated displacement and deformation rate are both not significant,illustrating that Section II is stable as a whole.?5?Factors contributing to slope instability can be categorized into two categories:intrinsic factors and external contributor.The former consists of severe superficial weathering and unloading,well developed joints,deep unloading zone and three unfavorable discontinuities.And the latter are composed of engineering blasting disturbance and reservoir water level variation.The failure mechanism of the slope at reservoir bank can be divided into two categories based on the factors inducing slope failure.First,the failure mode of bank slope result from internal factors is unloading rebound deformation.The failure mode result from blasting is mainly rock cracking and toppling at rear slope,while that introduced by water level variation is slope slipping along the sliding surface formed by the steep inclined discontinuity and tensile fracture at rear slope.?6?To consider the influence of blasting and impounding on slope stability,the numerical calculation code GeoStudio is employed.The results obtained are:?1?the impact of blasting exerted on Section I is greater than that on Section II.The vibrating load induced by blasting in slope excavation could intensively influence slope deformation.One side,because of elevation amplification effect of vibration,rock deformation at rear slope can be aggravated.For another,the continuousvibrating load not only decreased mechanical strength of the discontinuities,but could also accelerate the creep deformation in slope.?2?after blasting excavation,stress redistribution occurs in both Sections I and II.For section I,the value of stress in the deformation zone is decreased to some extent as a whole,but the major and minor principal stress increase at some local area.The minor principal stress in low stress zone of slope reaches 0.4MPa.For section II,the major and minor principal stress is 2MPa and 0.4MPa,respectively.Tensile stress is observed at slope crest.?3?rock deformation in Section I is not obvious when water level in reservoir rises from 1415m to 1475m,indicating that water level exertes little impact on the slope stability;while water level rises from 1475m to 1490m,sliding-tension type of slope failure occurs with the front of slide above river valley.As for Section II,rock deformation is comparatively smaller during the whole impounding process.The plastic zone in deep area get larger but not connecting until the water lever rise to1490m.Thus,it reveals that water level has little influence on the slope stability at Section II.