Research On Mechanical Effect And Settlement Control Technology Of Subgrade Under Subway Tunnel

In recent years,the expansion of urban area and the rapid development of urban subway,there are more and more new subway under the existing structures of the tunnel underpass project.The construction risk of tunnel underpass project is high and causes serious accidents.For cities with multiple backfills subject to geographical constraints,urban subway tunnel construction will inevitably go through backfill lots,and its construction risks and accident severity are even greater than those of general tunnel underpassing projects.In this thesis,we analyze and study the tunnel excavation method,overstrengthening measures and factors affecting overstrengthening measures through literature,theoretical analysis and numerical simulation,and optimize the original overstrengthening plan based on the findings of the study.The main research results of the thesis are as follows:(1)By collating and summarizing the experience of similar tunnel underpass projects,relevant domestic and international codes,and also according to the relevant requirements of ground structure stability and road levelness,it is found that the standard value of 30 mm for surface settlement control of most tunnel underpass projects can meet the engineering requirements.Considering the complexity of the stratum environment and the uncertainty of the actual construction,the control value of the ground settlement of the filling subgrade is a conservative 20 mm.(2)FLAC 3D software was used to simulate and analyze the influence of four excavation methods: two-step method,two-step reserved core soil method,three-step method and CD method on the displacement of tunnel surrounding rock,surrounding rock stress and plastic zone distribution.The results show that: the four excavation methods of the surrounding rock deformation,stress distribution law is more or less the same,CD method on the surrounding rock deformation control effect is the best.(3)Combined with the actual excavation method of the dependent project(two-stage reserved core soil method),numerical simulation of three commonly used overstrengthening measures,namely single-layer small conduit,overstretched pipe shed and double-layer small conduit,was carried out respectively,and the stress and strain distribution laws of the surrounding rock,primary support structure and overstrengthened structure were derived.Through comparative analysis,it is found that the effect of overrun support of pipe shed mainly relies on long beam effect,and the effect of overrun support of small conduit mainly relies on grouting reinforcement effect;the overall reinforcement effect of three overrun reinforcement measures in order is:double-layer small conduit,overrun pipe shed,and single-layer small conduit.(4)From the two factors that mainly affect the effect of grouting reinforcement,namely,the circumferential reinforcement range of double-layer small conduit and the outer insertion angle of outer conduit,a single variable control method is used to analyze the degree of influence of these two factors on the deformation of the tunnel surrounding rock and the stability of the initial support structure.The results show that the influence of the circumferential reinforcement range of the small conduit on the surrounding rock and the initial support structure is greater than that of the outer insertion angle,and the final determination of the overrun support scheme of the relying project is 180° of circumferential reinforcement and 30° of outer conduit insertion angle.