Study On Deformation,Failure Characterristics And Excavation Stability Of Right Abutment Slope Of Zala Hydropower Station In Tibet

The engineering geological conditions in the southwest valley area are complex,the anti-dip rock slope is widely developed and the frequent and intense internal and external dynamic geological action plays a decisive role in the long evolutionary history of the formation and development of the toppling deformational body.The toppling deformation is always in the state of dynamic equilibrium under natural conditions,and the stress and deformation are coordinated and stabilized continuously.Once the instability occurs,it usually starts from the slope surface fragmentation rock mass caving,gradually diffuses to the interior,and even forms the deep broken surface,which seriously threatens the construction safety of the hydropower station.The deformation and failure mode of toppling deformable body has always been a hot topic in academic research.When excavation changes its stress boundary condition,the reverse slope will produce more complex deformation feedback,which is an important research content of slope excavation scheme design.Taking the dump deformation slope of the right abutment of Zala Hydropower Station in Changdu region,Tibet,as an example,combining the relevant literature and detailed field investigation results,a reasonable geological conceptual model is established.The deformation and failure mode of right abutment slope,displacement deformation characteristics and excavation stability under different excavation schemes are analyzed by discrete element method and finite element method.The main results are as follows:（1）The right abutment slope is a steep reverse slope,the bedrock is mainly calcareous slate,the rock strike is NW,and the normal rock dip angle is 74°⁸0°.According to the investigation results of the adit,the depth of toppling deformation is large,about 61.5⁸5 m.At present,the surface of the slope reveals a very strong toppling area of 0¹9 m,and a weak dip area of over 60m deep.Many rock masses in the area show a cataclastic structure.Under the disturbance of rainfall and construction,these areas have the risk of large deformation and even instability,which is a huge security hazard for the excavation of dam site.（2）Through the record of PD10 and PD11 and the investigation of rock mass structure,it is found that there is no controlled large-scale fault in the slope of right abutment,and the structural plane of gradeⅣ₁^V is mainly developed,in which the weak structural plane（fault,compression surface）is mostly steeply inclined.Except for a few gently inclined slopes,most of the fillers are debris and mud,but the thickness is more than 1² cm.The deformational joint fractures are developed,especially the long joint J₁ can form the lateral boundary of slope and local block instability,and the external joint of inclined slope is less developed.Only sporadic exposure.（3）The toppling deformation slope of the right abutment is affected by gravity,the backpressure of the accumulation body at the foot of the slope and the control of the vertical joint,which gives birth to the unique toppling deformation phenomenon in the evolution process of the toppling deformation slope of the right abutment.The investigation of adit shows that the dip angle of the toppling deformable rock strata is fluctuating and the regional development of joints and fissures is weak so it is impossible to determine the toppling deformation zone directly according to the relevant indexes.According to the data of wave velocity test,field rock deformation test,shear test and indoor related test,the above indexes are selected,and the inclination angle,wave velocity and strength parameters are fitted directly and curvilinear by the method of data fitting in origin.It is found that the position of 25 m in depth of 10%PD10 and 45 m in depth of PD11 is the turning point of the variation trend of inclination angle and wave velocity,and the fitting results are in good agreement with each other.Finally,the toppling deformation zoning results of the right abutment slope are obtained according to the development characteristics of the structural plane.（4）According to the signs of deformation and failure,the deformation and failure of the right abutment are mainly the whole toppling deformation and local loosening caving,in addition,the deformation and failure of shallow surface layer are block instability and fall.According to the results of physical simulation of bottom friction,the development stage of toppling deformation includes four stages:front tipping deformation,front-middle toppling deformation,trailing edge deformation and broken zone failure.The process of deformation is accompanied by local caving of rock mass and shallow surface and filling process of voids formed by toppling and so on.In the natural state of the right abutment slope,with the development of the toppling deformation,the slope experienced the formation of obvious breakage zone in the already extremely strong tipping area,and the unsteady failure of the continuous expansion and transfixion.（5）According to the results of discrete element analysis,the toppling deformation body is significantly affected by the front accumulated body and the shape of the slope surface.Firstly,the deformation and failure mode of the deformation and failure model is the shear failure slip of the aggregate body of the gravel soil and the gradual expansion of the breakage zone of the extremely strong toppling area,and the local caving of the slope surface is the main one.After the instability of the gravel soil at the slope foot,the empty surface of the toppling deformation area increases,and the slip surface is basically formed through the bottom of the extremely strong toppling area.At first,a large tension crack is formed at the steep and gentle turning point of the slope surface of the upper slope body,and finally the whole downward slip is unstable.Due to the shear force,the toppling deformation process of the bottom of the strong dumping area is hindered,and the deformation of the strong toppling area is significantly aggravated after the unstable slip occurs in the direction of the egg drift rock toward the foot of the slope near the river.The reverse pressure of the accumulation body has obvious influence on the process and excavation sequence of the toppling deformable body.（6）The deformation-failure mode of toppling slope of right abutment by discrete element analysis The right abutment slope is numerically simulated by using finite difference principle.The characteristics of stress and deformation of the slope under natural and excavation conditions are analyzed.The results show that the slope is basically in a stable state under natural conditions.The deformation displacement is mainly concentrated in the accumulation body and the extremely strong dumping area of the slope foot,and the increment of shear stress is smaller in the lower part of the extremely strong dumping area.This is consistent with the analysis of the right abutment slope by discrete element,which provides the foundation support for the later excavation scheme.（7）According to the above,the excavation slope ratio of 1:0.75 is the most favorable for the overall stability of the slope.In conclusion,the excavation scheme of1:0.75 is the most favorable for the overall stability of the slope.At the same time,the slope reduction of the slope top is carried out according to the excavation scheme of1:0.75.The disturbance response of the dump body will be further reduced,and finally,reasonable support measures are matched to ensure the long-term stability of the dump body.