| In the next few years,our country will get rid of poverty and build a well-to-do society in an all-round way.Rural road construction also ushered in great developments and technical standards will be greatly enhanced.Rural highway tunnels will be upgraded from‘single lane’to‘two-lane and two sidewalks’large span and section tunnel.Under the special circumstances that budget of rural highway is not enough,construction technology and equipment level is relatively low,and construction period is urgent,how to construct a large span and large-section tunnel more economically and safely becomes the research background of this paper.Based on the project of Mopanpo Tunnel in Shuangjiang Ancient Town,Tongnan,Chongqing,this paper carry out research on optimization of long-span and large-section tunnel excavation schemes.The main research work done in this paper is as follows:①Introducing the engineering geology,engineering designs and original plans as well as the different tunnel excavation methods and the evolution of tunnel full-face or large-section excavation method with the progress of construction technology.This provides new ideas and theoretical basis for the later optimization of the Mopanpo tunnel excavation program.②According to the structure characteristics and mechanical characteristics of large section and large span tunnel,this paper will select single side heading method,three-bench seven-step excavation method,double side heading method and two-step method respectively and use MIDAS GTS NX to stimulate three-dimensional data of the entire excavating process.It is found that double side heading method has the best effect when considering safety.However,results to vertical and horizontal displacements of the tunnel structures and key points which correspond to the four methods all meet requirements within the specified scope.③Under conditions of construction safety,construction quality,construction cost,construction schedule and construction technical requirements as standard level,according to AHP in theory of operations research,numerical simulation results of four different methods are used to conduct quantitative comparison.Ultimately weights of each program are sorted in descending order:two-step method is 0.4312,three-step seven-step excavation method is 0.2163,single side heading method is 0.1918,double side heading is 0.1607 and it is determined that two-step method is optimal excavation plan for Mopanpo tunnle.④This subject optimizes the form of excavation section which corresponds to selected optimal tunnel excavation program--two-step method.Drawing on experience from predecessors and feedback from technicians worked in construction sites,two-bench new four-step excavation method is extraced.In comprehensive comparison with results of three-dimensional numerical simulation analysis about traditional two-step method,reserving core soil step method and two-step instep arch excavation method,two-bench new four-step excavation method is finally selected to excavate.⑤The excavation step height and step length of the two-bench new four-step excavation method are optimized.Through the numerical simulation analysis of six different excavation step heights and seven different excavation step lengths,the conclusion about widest boundary of the hole width is that 0.68H and L=9m～L=12m range are considered as the step height and length of the step respectively.⑥Presentation of the optimal scheme-the practical application of the two-bench new four-step excavation method in the Mopandao tunnel excavation process and the purpose and significance of the monitoring measurement in the tunnel project are introduced in this paper.Compared the actual monitoring data with the previous numerical simulation results,this paper finds that the numerical simulation results are in agreement with the settlement value of actual monitoring measurements in some extend.The actual feasibility of the optimization scheme and the true accuracy of the three-dimensional numerical simulation were verified. |
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