| Most of the diversion tunnels of large hydropower project are shallow buried large section tunnels.During the excavation process,interactions and mutual restrictions generate between the diversion tunnel and the inlet slope on the construction safety.Once the surrounding rock unstability occurred,it will not only cause the loss of life and property of constructer,but also seriously affect the construction progress.Thus,the stability of tunnel surrounding rock and the slope of tunnel entrance during the excavation process of the diversion tunnel of large hydropower project is often a key concern in the field of geotechnical engineering.For the diversion tunnel of large hydropower project,the design and construction that consider the interaction between the tunnel and the slope of tunnel entrance is a complex process influenced by many factors.There are no reliable rules to follow and few compatative studies between different projects can be found.The safety of the unfavorable geological section of tunnel is directly affected by the construction technology overall the construction process of the project.Therefore,it is necessary to conduct in-depth studies on the construction technology and stability of surrounding rock of the tunnel with large cross-section of typical engineering locating in the unfavorable geology,proposing a set of optimum construction methods and techniques to instruct construction.Based on a large hydropower project,in-depth studies has been conducted on the construction technology and stability of surrounding rock of the tunnel with large cross-section locating in the unfavorable geology,improving the tunnel supportting method of large cross-section tunnel.It can provide a reference for the design and construction of similar projects in engineering practice.The contents and results are as follows:(1)A systematic summarization and study has been conducted on the advance support measures,mechanism and effects of shotcrete and rock bolt support,and mechanism of steel mesh reinforcement support,etc.According to the difference of advance support technology,the main role of advance support measures can be summarized as two aspects.First is to improve the formation characteristics,namely improving rock-soil characteristics of the formation that surrounds excavation face by grouting,freezing and other methods.Second is the pre-supporting effect,namely improving the carrying capacity of the surrounding rock by improving the stability of overall surrounding rock due to the anchorage effects and the effects of the longitudinal beams.Numerical results show that advanced small pipe grouting can obviously improving the carrying capacity of the rock and it is well controlled for the deformation of surrounding rock and plastic zone in loose rock mass during construction,with the improvements of surrounding rock stability.Meanwhile,taking the shotcrete support for example,the comparative analysis of rock stability with or without shotcrete supports,shotcrete thickness,shotcrete support strength and other factors has been researched,providing an appropriate basis for the actual construction.(2)Considering the interaction mechanism between diversion tunnel and the inlet slope,the stability of the surrounding rock and slop of the tunnel with large cross-section locating in the unfavorable geology during construction is studied.Reasonable construction sequences and optimized support plans are discussed.Specific contents are consisted of three aspects.Firstly,the stability of surrounding rock of the tunnel with large cross-section locating in the unfavorable geology during construction is studied.Secondly,construction schemes of the tunnel with large cross-section locating in the unfavorable geology are optimized by protocoling multiple construction schemes,and the reasonable and safe construction orders are determined by analyzing the deformation of surrounding rock of transition section and the inlet slope.Thirdly,the stability of surrounding rock of transition section is studied in the condition of multiple supporting schemes.Meanwhile,the deformation of surrounding rock of transition section and the characteristics of plastic zone are analyzed between the support schemes of design,the support schemes of construction and no support schemes.The effects of support schemes on the stability of inlet slope are discussed.(3)Based on the summary of the construction methods of the tunnel with large cross-section locating in the unfavorable geology,the stability of surrounding rock and characteristics of support force during construction have been studied.Construction methods and excavation sequences of the tunnel with large cross-section locating in the unfavorable geology are optimized and studied.For the construction methods,the main studies are on the stability of surrounding rock and force characteristics of anchors of the tunnel with large cross-section locating in the unfavorable geology in the case of left-right excavation method and core-soil excavation method.For the construction sequences,the stability of surrounding rock and force characteristics of anchors in the case of bedding excavation and inverse excavation have been comparatively studied.The results show that left-right excavation method presents characteristics that faster construction,simpler process,smaller distribution range of plastic zone,and easy to meet the requirements of surrounding rock stability.So selecting left-right excavation method for projects is better.For excavation sequences,employing the inverse excavation method,the deformation of top arches,bottom arches and side walls is larger,and the range of plastic zones is wider,while the bedding excavation method is better for passing through unfavorable geology.(4)On the basis of the general calculation method of the numerical model,considering the dynamic geological information in the process of tunnel construction,the real-time analysis method of stability of long tunnel in the condition of unfavorable geology are put forward.According to plane design information and the engineering geologic information of tunnel engineering,the regular three-dimensional geometric model(3D model)is built.Based on the 3D model and considering the information of construction progress,a 4D information model of tunnel construction is proposed employinh the 4D information modeling technology.Based on the powerful search function of SQL database search engine,the calculation model element corresponding to the information changing area of tunnel engineering can be quickly found.With a secondary development of numerical calculation in ABAQUS software,the mechanical parameters of searched elements are changed automaticly based on the updated information,so as to achieve the real-time dynamic updating of tunnel numerical calculation model.Thus,a coupling 4D information-simulation model is achieved with the development of dynamic parameter inversion technique between current monitoring information and calculated results of conducted FEM model,providing the real-time rock mass safety assessment and mechanism prediction of the tunnel by numerical simulation. |
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