Study On Stability Of Rock Slope At Tunnel Entrance In Seasonal Frozen Zone

In most areas north of Qinling-Huaihe River and west of Sichuan Basin,seasonal frozen soil distributes about 50% of the total land area.With the rapid development of national economy,more and more large-scale projects are built in seasonal frozen areas.The frost damage problems faced by these areas in the process of engineering construction are prominent.Slope instability problems are often encountered in the fields of highway,railway and water conservancy and Hydropower projects.In the slope engineering constructed in seasonal frozen zone,the joint distribution of slope rock mass is caused by weathering or tectonic action.As a weak structural plane of rock mass,the strength of joint surface often plays a decisive role in the stability of slope engineering.For the rock slope engineering in the seasonal frozen area,one of the frequent seasons of failure is the alternating season of winter and spring.The main reason is that with the decrease of winter temperature,groundwater freezes in the slope structural plane,and about 9% volume expansion is caused by water-ice phase transformation.At this time,the structural plane is expanded and dislocated by significant ice pressure,resulting in the displacement of the slope towards the free surface;In the process of spring warming up,the infiltration joints of ice and snow on the surface of slope and ice melting in slope lead to the weakening of the cementation properties of structural planes in slope body,the decrease of shear strength and the instability failure of slope.Slope instability affects the construction and operation of basic engineering and threatens the safety of people's lives and property.Therefore,it is of great practical significance to study the stability of rock slope under the conditions of freezing and thawing in seasonal frozen area for the construction of slope engineering in seasonal frozen area.In this paper,the slope at the entrance of Quantai Tunnel of Jishuang Expressway in Jilin Province is taken as the research object.By investigating the engineering geological conditions of the slope,the dynamic factors affecting the slope stability and the structure form of the slope are obtained.On this basis,the failure mode of the slope at the entrance of Quantai Tunnel is analyzed,and a geomechanical model is established.At the same time,samples of slope structure are collected for indoor freezing shear test to study the shear mechanics properties of rock joints under freezing conditions,consult predecessors' research on shear mechanics properties of rock joints,and apply the shear mechanics parameters under freezing and melting conditions to the established geomechanical model,The stability of rock slope is calculated by limit equilibrium method,and the stability of rock slope is checked by FLAC3 D numerical simulation software.The comprehensive stability evaluation of rock slope is put forward.The main achievements of this paper are as follows:(1)According to the frozen shear test of the joint surface,the failure mode and failure mechanism of the frozen rock joint are analyzed.The influencing factors and calculation formulas of the frozen shear strength of the joint surface are proposed.At the same time,the failure mode with the least shear strength is obtained according to the failure mechanism and test consequences.And the conditions,calculate the mechanical parameters under this condition,and propose the shear displacement formula under this condition.(2)The failure mode of rock slope in seasonal frozen area and the internal and external conditions affecting slope stability are analyzed and summarized.The natural geography and geological environment of Quantai Tunnel on Jishuang Expressway are also investigated.Geological survey are adopted for the slope of the entrance.The core method is used to obtain the occurrence and extension of the joint surface of the slope,and the influencing factors affecting the stability of the slope are proposed.(3)According to the joint shear mechanical parameters obtained by the experiment,combined with the development of the joints of the slope,the failure mode of the slope is determined,and the geomechanical model under freezing and melting is proposed respectively.The stability of the slope is qualitatively analyzed by using the red-flat projection,and the limit equilibrium method is used to evaluate the risk of the slope under freezing and melting conditions.The results show that slope is stable under the condition of freezing in winter,the stability is rapidly reduced.(4)Combining the mechanical parameters of the joint surface obtained by different experiments,the FLAC3 D finite difference method is used to analyze the safety stability coefficient of the slope under different working conditions and compare with the theoretical analysis and calculation to verify the reliability of the geological model.The slope has a risk of instability in the event of melting.