| In the process of site investigation,it is found that the rock mass in adit and slope surface of the dam site is relatively broken due to strong weathering and unloading,and the stability of rock mass in the local area of the slope is extremely poor.The failure of these rock masses is cut by the large unloading cracks caused by unloading,and these unloading cracks are often the controlling structural planes controlling the stability of the slope rock mass and even the whole mountain,which play an important role in the stability of the slope at the dam site and the safe construction of the project.In order to provide reference for the safe construction of the project,this paper in the detailed engineering geological conditions of the dam site area after the investigation,combined with the geological background of the study area,the slope rock structure characteristics,weathering characteristics and unloading characteristics are analyzed and summarized,the unloading zone of the dam site area for the scientific division.Then combined with the indoor comparative test of unloading conditions of rock mass and the numerical simulation of the slope at the middle dam site,the unloading failure mechanism of rock mass at the dam site was analyzed in detail,and the following results were obtained:(1)The structure of the surface of dam site area on the left bank slope statistics,classification and grading,found that after the slope rock mass in the left bank of the dam site and no levels structural plane and structure development sign below.The other structural planes are mostly the cracks outside the dip slope,and the dip Angle is usually steep.Structural planes at all levels mainly develop in the NW-order NE direction,and only a few develop in the SE direction.The rock mass in the adit is generally cut into block,Mosaic,fracture,loose structure and rock structure types between these structures by these structural planes.(2)The rock mass in the dam site is subjected to different degrees of weathering and unloading,and the weathering is mainly chemical weathering.However,the strength of rock mass subjected to weathering generally depends on the nature of the rock mass itself,the topography of the region,the distribution elevation and other factors.Generally speaking,the resistance to weathering of rock with better properties is stronger.The weathering of rock mass in gully is weaker than that in ridge.The rock mass at high elevation is more affected by weathering than that at low elevation.At the same time,according to the field investigation on the unloading effect of rock mass in the dam site area,the unloading zone of rock mass on the left bank is divided scientifically by combining the crack opening of rock mass in the adit and the acoustic velocity of rock mass in the adit.(3)Based on the indoor comparative test of rock mass properties and creep characteristics under unloading conditions of slope rock samples,it is found that the deformation and failure modes and creep characteristics of rock mass under reunloading conditions are significantly different from those of ununloaded rock mass.Physical properties such as elastic modulus and poisson's ratio of rock mass also change with the unloading process.(4)After the geological environment of rock mass in the dam site is generalized,the variation trend of stress field,displacement field and plastic zone in the unloading process of rock mass in the dam site is studied by means of numerical simulation.It is found that the maximum principal stress and the minimum principal stress of rock body decrease gradually during unloading.The slope rock mass will move towards the direction of the air face,and the displacement generated by the movement is generally positively correlated with the elevation.Accordingly,the coverage area of plastic zone caused by rock mass displacement will gradually expand and the influence depth will gradually deepen.(5)In the long-term geological history process,with the continuous evolution of river valley,the topography and landform finally formed the present form.Most of the existing deformation and failure of the slope in the dam site area is directly related to the unloading of the slope rock mass.The structural plane formed by unloading of rock mass often cuts itself so that the resistance to deformation and failure of rock mass is reduced and the rock mass moves towards the direction of the air plane.Some structural planes even directly cause the failure of rock mass.(6)The deformation and failure modes of slope rock mass under unloading mainly include toppling deformation,sliding deformation,shallow surface sliding and block instability.Among them,toppling deformation,sliding deformation and block instability can be regarded as deformation and failure directly caused by rock mass fracture caused by unloading,while shallow surface collapse is mainly secondary failure of rock mass under the influence of material,topography and other factors formed after the first deformation and failure. |
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