Study On The Mechanical Properties Of Pipe Roof And The Ground Settlement During Excavation In Shallow Undercutting Construction Method

With the rapid development of urbanization in our country, the speed and scale of the subway construction in city are in the leading level in the world and it is becoming the world’s biggest subway construction market now. As shallow undercutting construction method is widely used in subway tunnel engineering, its impact on the environment must be strictly controlled within the allowable range, especially the ground settlement during excavation. Therefore, the related problems about how to predict and control the ground settlement and what the law of ground settlement during excavation are always the hot spot in subway tunnel engineering.The thesis is based on the tunnel engineering between Yuntai Garden Station and Dajinzhong station, in Guangzhou Metro Line eleven. And it starts from the study on mechanical properties of pipe roof to the law of the ground settlement during excavation. The main contents that the thesis studies can be shown in the following aspects:The main influencing factors that lead to the ground settlement during excavation in shallow undercutting construction method: construction excavation method, engineering properties of surrounding rocks, groundwater, stress release of stratum, thickness span ratio. Then based on the Peck empirical formula, it discusses the related law of ground deformation caused by excavation, and calculate the value of the ground settlement about the tunnel engineering.Through the comparison of several tunnel advanced support techniques currently used, it proves the advantages of the advanced support technique of pipe roof applied in this tunnel engineering. Then, starting from the construction technology of pipe roof, it analyzes mechanical properties of pipe roof and put forward the concept of equivalent area of pipe roof reinforcement. Subsequently, relied on the relevant conditions, it derives the formula how to calculate equivalent elastic constants of the reinforcement area. Meanwhile, the finite element method is used to demonstrate the formula’s correctness. The related factors affecting the equivalent elastic constants of the reinforcement area are discussed in detail: grouting material, the size of steel pipe, the arrangement of steel pipe and so on.Combined with the project overview and preliminary design of the tunnel engineering between Yuntai Garden Station and Dajinzhong station, it uses Midas GTS to establish the corresponding numerical model and simulate the construction process of the tunnel engineering. Then based on the results of finite element calculation, it discusses the law of ground settlement. And through the comparison with theoretical formula, it comes to the conclusion that the calculation coefficient K of the settling tank can be calculated by 0.76 when the stratum is sticky soil, sand or weathered rock in Guangzhou region, and the loss rate of stratum can be calculated by 0.135% when pipe roof advance support technique, the central heading-tunnel bench cut method and the left-right-tunnel CD method are applied in the double-arch tunnel excavation. Meanwhile, it also simulate the tunnel excavation in different cases to study on the difference of ground subsidence in different cases.