Study On The Reasonable Buried Depth Of Metro Tunnel In The Two Element Strata Of Soil And Rock

Buried depth is an important technical index of metro project construction.Geological environment is the material base of metro tunnel.When the metro tunnel is excavated in the urban area where the buried depth of the bedrock is shallow and the thickness of soft stratum is thin,the tunnel is placed in the hard rock stratum and maintains a reasonable thin rock cover thickness,which plays an important role in lowering construction costs,reducing engineering risk and construction difficulty,improving the comfort and convenience,and reducing the impact on surrounding environment,etc.However,up to now,there is no uniform understanding and standard on the question of the reasonable buried depth of the metro tunnel.With more and more metro tunnels built in the the two element strata of soil and rock,such as the cities of Qingdao,Dalian,Xiamen,etc,the research on the metro tunnel reasonable buried depth has important theoretical and practical significance.This dissertation is closely combined with the practice of Qingdao metro construction,based on the Qingdao metro line 3 and line 2,the metro tunnel reasonable buried depth is analyzed by methods of statistical analysis,theoretical research,numerical calculation,engineering analogy and feedback analysis of field monitoring data.The main conclusions and research results are obtained in this paper as following:?1?Based on the statistical analysis of the distribution of the thickness and the physical and mechanical parater of the strata along the 49.2 km of Qingdao metro line,the quaternary stratum and the strongly weathered stratum were classified into the soft soil stratum;Meanwhile the moderately and the slightly weathered stratum were classified into hard rock stratum.The spatial distribution diagram of the unique characteristics of the dualistic structure of soft soil stratum underlying hard rock stratum along Qingdao metro line was constructed.?2?The conceptions of the stability region decomposition of the development and utilization urban underground space was proposed;and systematical analysis was conducted on the stability region decomposition of metro tunnel under various thicknesses of soft stratum and excavation span.Combining with the stratum distribution characteristics,the spatial distribution diagram of the stability region decomposition along the metro lines was constructed.The boundary buried depth of the stability decomposition along Qingdao metro line is generally shallow,and the average value of 12.0m,50%of them within 11.7m,80%of them within 17.3m,90%of them within 20.0 m,95%of them within 23.0m.The metro buried depth is placed properly,most of metro line can be placed in the easy stability region where the geological condition is good,avoiding the difficult stability region where the geological condition is poor.?3?Based on taking advantage of the self-stability capacity of hard surrounding rock,the conceptions of the optimum thickness of rock cover H_ropt and the minimum thickness of rock cover H_rmin in the the two element strata of soil and rock were proposed;the change characteristics of H_ropt and H_rmin of 4 types of metro tunnel cross section under different thickness of soft soil stratum were discussed,which were single hole single line running tunnel,single hole double line running tunnel,single arch large span station tunnel and supper large span metro tunnel.The reasonable buried depth of metro tunnel should be placed between the H_ropt and H_rmin,and the spatial distribution diagram of reasonable buried depth based on self-stability of surrounding rock along Qingdao metro lines was constructed.The acerage value of buried depth is 19.5m,50%of them within 19.0m,80%of them within 28.0m,90%of them within33.0 m,and 95%of them within 36.2m,which meets the requirement of optimum overburden thickness along Qingdao metro line.The buried depth which meets the requirement of minimum overburden thickness is almost the same as the boundary buried depth between the easy and the hard stability region along Qingdao metro line.?4?The construction cost of the mined underground metro station in Qingdao is about97.78¹82.77 million yuan RMB,and the average is 136.37 million yuan RMB;the cost of the metro running is about 79¹40 thousand yuan RMB per double line meter,and the average is 106.6 thousand yuan RMB per double line meter.When the metro tunnel enters the lower-hard rock stratum from upper-soft soft stratum,the construction cost reduced suddenly.When the metro is placed in the lower-hard rock stratum and maintained a reasonable minimum rock cover thickness in the two element strata of soil and rock,the best economic results can be achieved.?5?Combining with the project examples of Qingdao metro line 3 and line 2,the spatial distribution diagrams of the safety risk grade were drawn along the 2 metro lines.The length proportionality coefficient of the low,moderate and height risk section and its variation characters were analyzed along the 2 metro lines.The length proportionality coefficient of the low,moderate and height risk section were 60.42%,1.29%and 38.26%respectively along Qingdao metro line 3;when the design metro buried depth increased 6m and 10m,the length proportionality coefficient of the low risk section increased to 81.90%and 93.72%;meanwhile,the length proportionality coefficient of the height risk section reduced to 17.24%and 5.89%.The length proportionality coefficient of the low,moderate and height risk section were 65.10%,2.69%and 32.21%respectively along Qingdao metro line 2;when the design metro buried depth increased 6m and 10m,the length proportionality coefficient of the low risk section increased to 82.24%and 93.36%;meanwhile,the length proportionality coefficient of the height risk section reduced to 16.87%and 5.05%.The existed design buried depth was increased properly along Qingdao metro line,which could reduce geological risk greatly,improve the comprehensivebenefits of safety and economy,and achieve the good and fast goal of metro construction.