Study On Mechanical Characteristics Of Prefabricated Secondary Lining Of Single-tube Double Track Railway Tunnel Constructed By Drill And Blast Method

At present,the construction method of railway tunnel in China is still based on drill and blast method,and the common lining structure is composite lining.However,the cast-inplace secondary lining is prone to crack,insufficient thickness and other quality defects,which affect the safety of railway tunnel operation.The prefabricated lining structure has the advantages of fast construction speed,load-bearing capacity and guaranteed lining quality,therefore it can become the development direction of the secondary lining of tunnel construction by drill and blast method.Based on a single-tube double track railway tunnel project,the ratio of surrounding rock pressure shared by the secondary lining of the deep-buried section of Ⅳ grade and shallowburied section of Ⅴ grade of surrounding rock is studied.According to the results of internal force calculation of cast-in-place secondary lining structure,six kinds of secondary lining assembly schemes are put forward.The beam-spring model was established to calculate the structure of each assembly scheme,and the optimal partitioned scheme was determined.The shell-spring model was established to study the mechanical properties of the precast secondary lining structure,and the three-dimensional finite element model of the stratumstructure was established to calculate and analyze the mechanical properties of the prefabricated secondary lining structure by the full section method and the two-step method.The main research contents and conclusions of this thesis are as follows:(1)Based on the engineering geological conditions of railway tunnel engineering,three dimensional finite element models of deep-buried tunnel of Ⅳ grade and shallowburied tunnel of Ⅴ grade of surrounding rock are established respectively,and the simulated tunnel excavation method is two-step method.The contact pressure between layers of lining structure was calculated and analyzed under different step lengths and safety step spacing of secondary lining.According to the contact pressure relationship between layers of composite lining structure,the ratio of surrounding rock pressure shared by secondary lining under the condition of two kinds of rock grade and buried depth is calculated respectively.Based on engineering Ⅳ grade of surrounding rock of deep-buried section of measurement of contact pressure between lining structure layer and highway tunnel design specification advice the proportion of surrounding rock pressure shared by secondary lining in Ⅳ grade and Ⅴ grade of surrounding rock tunnel,It is comprehensively determined that the proportion of secondary lining in the deep buried section of Ⅳ grade surrounding rock to share the surrounding rock pressure is 33%～46%,and the proportion of secondary lining in the shallow buried section of Ⅴ grade surrounding rock to share the surrounding rock pressure is 70%～80%.(2)A homogeneous annular calculation model is established to calculate and analyze the internal forces of the secondary lining structure in the shallow buried section of Ⅴ grade surrounding rock of the single tunnel double track railway tunnel project.According to the principle of the joint were set at a smaller bending moment,six types of secondary lining block schemes were proposed,and the stiffness of the assembled secondary lining joint was calculated by referring to the theoretical calculation formula of the stiffness of the prefabricated segment joint in shield tunnel.A beam-spring calculation model was established to analyze the structural mechanical properties of each block scheme,and 9 block schemes were comprehensively selected from the aspects of the stress,transportation,installation and waterproof characteristics of the assembled secondary lining structure,namely,the side wall and vault were divided into 5 pieces,the arch foot was divided into 2 piece,and the invert was divided into 2 pieces.(3)A shell-spring model was established to analyze the effect of the recommended thickness,width,assembling method and connecting bolt grade of prefabricated secondary lining on the internal force and deformation of the lining structure.Study shows that the precast lining structure internal force increases with the increase of secondary lining thickness and width,lining deformation decreases with increasing the thickness of the lining structure,along with the width increases considering calculation results and the method of large diameter shield tunnel design and construction experience,and suggest prestressed assembly secondary lining thickness of 0.55 m,width 2.0 m,bolt use M36 grade,The prefabricated secondary lining of the tunnel with good surrounding rock condition can be assembled by through or staggered joints,and the prefabricated secondary lining of the tunnel with poor surrounding rock strata can be assembled by staggered joints.(4)A three-dimensional numerical model of a Ⅴ grade surrounding rock shallowburied single tube double track railway tunnel is established.The effects of the full-section method and the two step method on the mechanical properties of the tunnel lining structure and the contact stress between rings of the precast secondary lining are calculated and analyzed.The results show that the influence of the two excavation methods on the mechanical properties of the lining structure and the contact pressure between the rings and the annulus of the assembled secondary lining blocks is basically the same,and their value are also basically the same.Therefore,the prefabricated assembled secondary lining can be used in the two excavation methods.It is suggested that the waterproof measures of precast assembled secondary lining structures should adopt two waterproof layers,namely laying waterproof plates between the initial support and the secondary lining and improving the waterproof performance of assembled secondary lining itself.It is necessary to carry out special design when prefabricated secondary lining is used for tunnels in special geological areas.The purpose of using prefabricated secondary lining is realized by choosing reasonable excavation way,optimizing lining structural parameters and choosing suitable material type.