Spatial Deformation And Load Release Mechanism Of Surrounding Rock In Tunnel Construction With Super-Large Section And Small Spacing And Its Engineering Application

With the development of economic and social development and acceleration of urbanization,traffic congestion is increasingly restricting urban and regional development.The original two-lane and three-lane highways have been unable to meet the needs of increasing traffic flow.The construction of some mountainous highways and urban expressways has entered a new period of development,as a result,the construction of super-large cross-section tunnel is also increasing rapidly.But there are little engineering practice for super-large cross-section highway tunnel at home and abroad,without mature construction experience to learn from,reliable calculation method to use,or specifications and standards of design and construction to refer to.Analysis can only be done by comparing several simple projects.Based on research status and problems such as complicated press and construction process of super-large cross-section highway tunnels,uncertain standard to judge stability of surrounding rock,unclear calculation method for]oad and difficulty in support structure and parameter selection,this paper deeply studies spatial deformation and load release law of the surrounding rock in the construction of super-large cross-section tunnel,through theoretical analysis,indoor test,model test,numerical simulation and field test,by combining the constructing port channel tunnel engineering of east and south second ring project in Jinan.Besides,detailed studies has been conducted on comparison and selection of construction methods for super-large cross-section tunnel and distance optimization and a series of research results with theoretical value and engineering significance are obtained,which include:(1)The mechanical characteristics of the surrounding rock of tunnel with super-large cross-section and small distance are analyzed and summarized from the point of view of stress concentration,respectively,for factors affecting the stress characteristics of the surrounding rock of the tunnel,such as distance,rise span ratio and lateral pressure coefficient.Based on the existing general pressure theory and design standard of highway tunnel,the surrounding rock pressure and load mode of the super-large cross-section tunnel are discussed.Considering the interaction effect of the first hole and the rear hole in the construction of super-large cross-section tunnel with small distance,calculation method and formula derivation of surrounding rock pressure for shallow-and deep-buried super-large cross-section tunnel with small distance are respectively carried out.According to the analysis of the formula,the pressure of inner and outer surrounding rock of the super-large cross-section small-distance tunnel is asymmetrical.The pressure of the vertical rock in the middle wall is larger than that of the outside of the tunnel.Due to the impact of disturbance,the unsymmetrical pressure of first hole is more serious than that of the rear hole.Calculation is conducted based on the model,the design parameters of the tunnel support structure,which are safer and more rational,can be obtained,providing great significance for the design of similar projects.(2)The dynamic grading evaluation and the point load test of the surrounding rock are carried out,and the indoor experiment is carried out on the dolomitic limestone,which provide not only dynamic guidance to the site construction,also reference to select model test and selection of numerical simulation parameters.Based on the experience of similar materials preparation,the new fluid-solid coupling similar materials were prepared under the premise of"orthogonal test design and sensitive factor analysis",and successfully applied in the model test.Under the premise of saving the experimental expenses,a thick-walled square tube test bench and a rainfall simulation system were developed,and a multi-information real-time monitoring and acquisition system was constructed.Based on the laser phase measurement technique,the spatial dynamic measurement method and the virtual measurement technique,the flexible point cloud data measuring instrument suitable for the deformation test of the model test is carried out to realize the advantages such as high automation high adaptability,flexibility,non-contact and high efficiency,of geophysical model test excavation contour measurement.Three-dimensional orthogonal decomposition can be carried out simultaneously on scan path,and the best measurement trajectory is automatically obtained through calculation and distance measurement,achieving rapid and precise measurement for a variety of tunnel models and various shapes of excavation.Post-processing software is developed for the instrument based on MATLAB,which can conduct noise reduction,thinning and fitting for point cloud data of tunnel model collected by the flexible point cloud data measuring instrument.Then deformation information of tunnel model is extracted with spatial visualization.(3)Typical section of the engineering site is chosen as prototype to carry out model test study on the construction process of shallow-buried super-large cross-section tunnel with small distance and mechanical model test of the construction process of super-large cross-section tunnel with complex stratum.Besides,the influence of the construction process of shallow-buried super-large cross-section tunnel with small distance and rainfall infiltration on the spatial deformation of surrounding rock is studied,and the spatial deformation and the load-release law of surrounding rock under different excavation methods before and after going through fault fracture zone are discussed in the paper.Corresponding numerical simulation experiment is carried out for engineering prototype construction section selected in the model test,and reliability of the result is increased by mutual verification of the results.The results reveal the spatial deformation mechanism of the surrounding rock during the construction of shallow-buried super-large cross-section tunnel with small distance,ensure construction safety of the complex stratum and provide support for smooth passage of the tunnel.(4)The existing Moore-Coulomb yield proximity is modified.Elements affecting stability of the surrounding rock of super-large cross-section tunnel with small distance are analyzed evaluated based on the elastic-plastic finite difference numerical simulation and reasonable construction proposals for tunnels with different grades of surrounding rock.This paper studies mechanical characteristics of the super-large cross-section tunnel with small distance,analyzes and discusses deformation law of the surrounding rock of tunnel with small distance and spatial deformation effect of the construction,so as to establish reasonable optimizing criterion of small distance and to propose reasonable value of the minimum distance for tunnels with different grades of surrounding rock.(5)The mechanical characteristics and]aw of the system of surrounding rock and supporting structure during the construction process are monitored and analyzed based on the four-lane super-large cross-section ditch tunnel with small distance,and the time and space effect in the construction process is analyzed and summarized,so as to propose support parameters and scheme shallow-buried and small-distance section based on the project,which is of great significance to improve the construction quality and schedule,ensure the safety of construction and ensure the smooth passage of the tunnel.