Evaluation On Surrounding Rock Stability Of Emergency Rescue Station Caverns In A Tunnel Of Chenglan Railway

Chenglan railway is the“Heaven Road”built by our highland over 3000m above sea level,rising from the Chengdu plain of 500m to the eastern edge of Qinghai Tibet Plateau in 3400m.A tunnel emergency rescue station caverns is located in the middle of Chenglan railway,located on the left bank of the Minjiang River,with a depth of430m.The caverns is mainly composed of No.4 horizontal tunnel,left rescue horizontal tunnel,main tunnel,right rescue horizontal tunnel,evacuation transverse passage and tunnel bottom evacuation contact channel.The caverns are high4.0～12.5m and span 4.5～15.0m.The lithology of the surrounding rock is mainly phyllite and carbonaceous phyllite,folds and joints are developed,rock mass is broken.The development of groundwater in some tunnel sections,and the stability of surrounding rock is poor.Under such complicated geological conditions,the stability of surrounding rock will directly affect the safety,duration and cost of tunnel excavation.Therefore,on the basis of the study of the engineering geological conditions,rock mass quality and monitoring data of the caverns,the two-dimensional numerical simulation software Phase and the three-dimensional numerical simulation software FLAC^3D are used to study the stability of the surrounding rock of the emergency rescue station caverns,and the following main understandings are obtained:（1）The surrounding rock of the caverns is the upper Triassic Juris group（T₃zh）,and the lithology is mainly phyllite and carbonaceous phyllite,partial sandstone and slate,the joint fracture is developed,and the rock mass structure is mainly thin and interbedded.By combining the qualitative and quantitative methods,the quality classification of rock mass and the dynamic classification index are integrated.The result shows that all the caverns areas of surrounding rock is V level.（2）The deformation and failure of the surrounding rock mass are mainly include the collapse of the surrounding rock of the palm,the extrusion of the inner wall and the uplift of the floor,the initial shotcrete layer peeled off the block and the distortion of the steel arch.The main factors affecting these deformation and failure are the lithology and strength of the strata,the structural characteristics of the rock mass,ground stress,underground water,cavern layout,supporting measures and construction technology.（3）The time history curve of the surrounding rock deformation is“step”,and generally experienced four stages:rapid growth,temporary stability,slow growth and gradual stability.The surrounding rock mass of the caverns is large deformation and stable deformation takes a long time,especially in the cross section of the caverns and the groundwater development area increase the deformation of the surrounding rock.Combined with the geological conditions of the rescue station,the deformation mechanism of surrounding rock can be divided into three types:stress dilatancy mechanism,structural deformation mechanism and groundwater softening mechanism.（4）According to the analysis of the D5K222+470 stress monitoring data of the typical section of the caverns,it is found that the contact pressure of the surrounding rock and support,the axial force of the bolt,the internal force of the secondary lining is small,and the internal force of the steel arch is large shows that the steel arch frame plays the main role in the initial support.（5）The results of two-dimensional numerical simulation of the three step method:the construction of the upper step has the greatest influence on the deformation of the surrounding rock of the vault,which the deformation of the vault accounts for about80%of the total deformation;there is a obvious stress relaxation zone around the hole,the deformation of the rock column between adjacent caverns tends to main tunnel,and the excavation disturbance of the main tunnel has a“traction”effect on the plastic zone between the two caverns.（6）The finite element software Ansys is used to establish the numerical model of the caverns in the emergency rescue station.Through the FLAC^3D simulate the stress,deformation and plastic zone characteristics of the caverns after the actual excavation,the stability of the caverns surrounding rock and the influence of different excavation sequence on the stability of the caverns are analyzed.The first excavation of the right side to horizontal tunnel and re excavation is relatively favorable to the overall stability of the caverns.By using this order of excavation,the effect of different spacing of the caverns on the displacement field,stress field and plastic zone of the caverns is analyzed by Phase.When the distance between the caverns L is less than1.5B（B is tunnel span）,reducing spacing will have a great influence on the stability of surrounding rock of caverns.