Slope Deformation Failure Mechanism And Excavation Deformation Response Of Tibet Zara Hydropower Station At Yuqu River

The plant site slope of Zhala Hydropower Station in Tibet is a Counter-tilt slope with rigid layers on the soft.In the course of long-term slope evolution,the slope has a strong toppling deformation and unloading deformation.Due to the unique dual slope structure of the slope,the slope presents complex deformation and destruction characteristics,and the stability problem is prominent.The construction of the factory building inevitably involves the issue of slope excavation.The stability of the slope after the excavation of the slope is related to the construction and operation safety of the hydropower project.On the basis of field investigation,this paper combines the physical simulation test with numerical simulation system to study the characteristics of deformation and failure of the slope with rigid layers on the soft and the deformation response of the slope under the condition of excavation slope.It has great engineering application value and theoretical research significance.The main research results obtained are as follows:（1）The Zhala plant site slope has the characteristics of“rigid layers on the soft”in the slope structure.The upper hard rock is a thin layered crystalline limestone and marble in the middle of the Triassic Wapu Formation（T₃wp）,and the lower part is softer.The rock is a thin layered calcareous slate sandwiched by carbonaceous slate in the Triassic Wapu Formation（T₃wp）.Normal plates are pour into the northeast and have an inclination of 70°to 80°.The overall deformation characteristics of the slope are summarized as the unloading tension deformation of the upper hard rock and the toppling deformation of the lower soft rock.According to the rock unloading strength,the upper hard rock is divided into a strong unloading zone（0²1.5m）,a strong unloading zone（21.5⁷9.6m）,and a weak unloading zone（79.6¹85m）;according to the rock mass The degree of collapse deformation of the lower slate is divided into Class A extremely strong toppling deformation（0²2m）,Class B strong toppling deformation（22⁶9m）and C type weak toppling deformation（69m^）.（2）The discrete element software was used to simulate the evolution of the site slope under the undercut of the river.The process of deformation and failure of the slope was divided into the development stage of unloading deformation of hard rock,the initial deformation stage of soft rock toppling,and the deformation stage of unloading-toppling combination.And its possible deformation failure mode is the combination of the collapse deformation of the lower soft rock and the upper unloading tension zone,ie the collapse bending-unloading expansion-shearing slip.（3）The physical test results of the slope excavation show that:1)The good air condition（excavation slope foot）is a necessary condition for re-deformation of the slope toppling deformation,when the lower toppling deformation develops to a certain extent,that is,the bending zone gradually passes through to form a continuous sliding surface,slipping rock masses are subject to slip damage.At this time,if the shear force generated by the deformation of the upper rock mass controlled by the unloading fracture can break through the locking section at the boundary,a toppling-unloading combination slip failure will occur.2)The removal of rock mass in the extremely strong toppling area has little effect on the stability of the slope,but when the excavation involves the slope foot rock mass in the strong toppling area,the strong slumped area in the lower slate will quickly generate larger deformation and promote the middle and upper parts.Shear failure of hard rock unloading rock mass occurs.（4）The deformation and destruction of the slope with rigid layers on the sof is caused by the further toppling deformation of the lower soft rock,and gradually extends toward the rear of the slope.At the same time,the stretched fractures on the trailing edge of the slope expand or appear tensioned cracks and follow the development of the original structural plane.If the trailing edge cracks are connected with the gradually shifting tilting and bending belt to form a continuous sliding surface,the slope will be unstable.Dump-shear-slip damage.The large deformation of the slope is often completed instantaneously or in a short time,but its early deformation or energy accumulation is a long process.After adapting to the new state of the slope,it will maintain long-term transient stability and accumulate energy.When the next breakthrough exists,the energy will be rapidly deformed.（5）The slope of the plant site under the combined action of the slate toppling deformation and the unloading deformation of the hard rock will generate stress concentration at the lithological boundary,forming an area similar to the locking section.The strength of the locking section is very important for the overall stability of the slope,so the strength of the locking section should be paid more attention when reinforcing the slope.The strong toppling area responds most fiercely to the deformation of the excavation,and it is necessary to reinforce the rock mass in the strong toppling area.Protection.Considering comprehensively,the anchoring of the lower toppling area and the solidification of the middle locking section are more effective targeted support measures.（6）The finite difference numerical simulation was used to study the stress and displacement changes after excavation and reinforcement of slopes,the results show that:In the natural state,the deformation and failure of the site slope mainly comes from the unloading rock mass controlled by the upper unloading zone,and the deformation of the lower soft rock is smalle;After the excavation of the slope,especially the second excavation,the rock mass of the lower slate toppling deformation area has a large deformation,in which the rock deformation in the strong toppling area is the most severe and needs attention;The failure mode of the slope under the excavation condition is:the combination of the lower soft rock toppling and deformation and the shear deformation of the unloading rock mass controlled by the upper unloading zone form a shear-slip failure;Adopting the“excavation surface anchorage+solid waist”reinforcement scheme can effectively prevent the further development of the lower toppling deformation and the shear deformation of the upper unloading rock mass,and ensure the stability of the slope after excavating the slope foot.