Study On Excavation And Support Of Soft Rock Tunnel In High Ground Stress Field

In recent years,the traffic tunnel project in China has been developed unprecedentedly and therefore the soft rock tunnels in the high ground stress field began to appear in large numbers,which brings new challenges to the design and construction of tunnel engineering.Large quantities of engineering practice show that section reduction,lining crack,arch distortion,and the collapse of the tunnel face are common in the construction of the soft rock tunnels in the high ground stress field,which seriously affects the safety and progress of tunnel construction,and then the construction cost is increased.In the paper,with Lanjia tunnel on Mianzhu-Maoxian road in ABa Prefecture,Sichuan Province taken as an example and several typical feature sections selected as objects,and based on methods including theoretical analysis,field test,numerical simulation,indoor model and so on,excavation and support of soft rock tunnels in high ground stress field was studied.The principal results are as follows:(1)Based on the initial ground stress data obtained from the hydraulic fracturing method at the site of the ultra-large depth and longest length of the tunnel at Lanjiayan,the multi-linear regression method and numerical simulation techniques were applied comprehensively to the entire length of the tunnel.The initial stress distribution law carried out an inversion analysis,and compared with the initial stress field results measured by the stress relief method at the construction site,and verified the rationality of the inversion regression calculation results.Based on the ratio of strength to stress as the index,a classification index of large deformation of surrounding rock of soft rock tunnel in high ground stress field is proposed.Based on this,the grade and zone of large deformation of surrounding rock around the project are predicted.(2)Numerical simulation method was used to study the influence law of the maximum horizontal principal stress and tunnel axis angle?on the stability of soft rock tunnel surrounding rock in different types of ground stress field,and the rock deformation around the cave was proved based on single factor sensitivity analysis.The degree of sensitivity of the included angle?,multi-factor sensitivity analysis was used to obtain the degree of influence of the included angle?and lateral pressure coefficients?_H and?_h on rock deformation around the cave.A large-scale three-dimensional geomechanical model test was conducted to ascertain the stability of the surrounding rock of the soft rock tunnel under the two conditions of parallel and vertical conditions of the maximum horizontal principal stress and the tunnel axis.(3)Combining with numerical calculation and model test,the influence of single factors such as the layout of pipe roof,circumferential spacing and grouting thickness on the stability of surrounding rock mass in soft rock tunnels with high ground stress field was ascertained.Comprehensive using of Latin hypercube sampling,genetic algorithm and penalty function theory and other mathematical means,controlling the deformation of surrounding rock as a constraint condition,saving the cost of materials as the optimize goal,a parameter optimization method for pipe roof considering comprehensively supporting effect and material cost was put forward.The optimization method can take into account the interaction between design parameters of pipe roof.A large-scale geomechanical model test was carried out to verify the optimization results of pipe roof parameters,which proved the rationality of the pipe roof parameter optimization method proposed in this paper.(4)The commonly used excavation methods for soft rock tunnels in typical high ground stress field at home and abroad were determined.The influence of different excavation methods on the stability of soft rock tunnel surrounding rock under high ground stress field was studied by numerical analysis and model test.Excavation method for soft rock tunnel suitable for high ground stress field was selected.The numerical simulation method was used to optimize the construction parameters of the benching tunneling method,and the optimal value of footage and step length was obtained.The research results of this paper will be of great theoretical significance and practical value to the massive soft rock tunnels to be built in the western part of China.