shield tunnel underneath an operational railway in sand pebble formation

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Now, all countries in the world are in the era of vigorously developing infrastructure, especially urban rail transit, the engineering conditions and the construction environment are getting more and more complex, and similar construction works within proximity also widely exist. In this paper, relying on an engineering example, the construction risk control problems of the shield underpassing the existing railroads are discussed in detail, and a set of feasible risk control methods are established under the basic ideas as below: ① to conduct the detailed investigation and analysis on the engineering background and the construction environment condition, and to find out the key control indicators; ② to carry out the risk assessment analysis, and to identify the key factors influencing the engineering and environmental safety; ③ in view of key risk factors, to bring up purposeful risk control measures; ④ to establish a numerical model, and to predict in advance the feasibility of risk control measures by simulation; and, ⑤ to establish strict field construction monitoring systems, to dynamically adjust the construction parameters at real time and evaluate the engineering safety according to the monitoring results, and to ensure the risk control effect. The main research results and conclusions of specific studies on the engineering example are as below: Using the analytic hierarchy process (AHP), the risks for the tunneling shield underpassing the existing railroads are evaluated and analyzed. The results show that, the key risk sources are sand pebble stratum, delayed grouting and earth pressure in the chamber set incorrectly, and without proper control, these 3 factors may lead to large-area settlement and deformation during the construction, thus affecting the safe operations of the existing railroads.

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