Some Problems On Metro Shield Tunneling

Shield tunneling method is becoming the most primary method in undergroundexcavation with the minimum soil disturbance, but because of the limit of geologyconditions and construction technique, it is inevitable that shield constructions resultsin soil disturbance and ground movement. The the ground settlement consists of twoparts: firstly, construction settlement induced by ground loss, secondly, long-termsettlement incurred by consolidation and second consolidation. When the groundsettlement exceeds a certain range，it would destroy the buildings, endanger the safetyof underground pipe network, and cause a series of environmental geotechnicalproblems. It is the main point in the dissertation as to consolidation settlement causedby shield tunneling and its related problems. The paper developed the following workand research:(1) The level obliquity of the vertical ground formation region played animportant role in the extent of soil disturbance. The extent of soil disturbance causedby shield tunneling construction is analyzed．Based on the principle of limitequilibrium, the level obliquity of the vertical ground formation region formula ispresented. Using the finite difference method to determine the plastic zone, therelationships between the gap parameter, the depth of the tunnel and the verticalground formation region are analyzed.(2) Due to stress release by unloading of soil excavation, the excess pore waterpressure caused by ground disturbance will be generated.With respect to gapparameter, the excess pore water pressure formula during the tunnel construction inelastoplastic soil is presented based on Henkel pore water pressure theory, and it canbe calculated by means of numerical analysis. The gap parameter and the tunnel depthare used to consider excess pore water pressure.(3) Following a tunnel excavation in low-permeability soil, it is commonlyobserved that the ground surface continues to settle with time, as the dissipation of theexcess pore pressure leads to the consolidation settlements. Based on the research ofthe excess pore water pressure, the consolidation settlement of soil has been studiedby means of numerical analysis. This approach has been applied to the engineeringcase of the Shanghai metro tunnel-line 2, and the calculation results of theconsolidation settlement are in good agreement with the field data. (4) The finite element model of cutter head is established for the cutter headvibration in complex strata during the shield tunneling. Based on the modal analysistheory, the modal property of the cutter head is analyzed. According to the cutter headvibration in shield tunneling, the results of the modal analysis are effective. Besides,by changing the value of the flange diameter and flexibility, suggestions forimproving the structure of cutter head are put forward. These suggestions can beapplied to the design of other cutter heads, and have greater practical value.