Study On Failure Mechanism Of Double-O-Tube Shield Tunnel

Shield method has already been one of the most common methods to constructrailway tunnels in China. With the continuous utilization and development of theunderground space, for the DOT (Double-O-Tunnels) shield method can saveunderground space, shorten the construction period and save investment, it willbecome more and more popular in future. However, because of the special structureand construction technology of DOT shield, the failure mechanism of the excavationface and land subsidence law are different from that of the single circle shield tunnels,these problems need to be studied in depth.In this paper, the failure mechanism of the excavation face and land subsidencelaw were studied through theory analysis, numerical simulation and case study. Themain content includes the following aspects:(1) The failure mechanism of the excavation face was studied based on the limitanalysis method. A three-dimensional collapse failure mechanism was proposed, thecomputing formula of the limit supporting force acted on the excavation face wasderived. And the linear equation and charts based on the variables of φ，c/γDå'ŒÏƒc/γDwere presented, which can be used in practical engineering expediently.(2) In practical engineering, the supporting force acting on the excavation facewas conservative, and the collapse failure of excavation face was also avoided tooccur. In order to verify the theory mechanism proposed in this paper, with the DOTshield tunnel engineering as background, the construction progress was simulated bymeans of finite difference software FLAC3D, the collapse failure of excavation facewas simulated. The failure forms of the excavation face and land subsidence law afterthe failure were analyzed. At the same time, the deformation of ground surface whenthe supporting force is bigger or smaller than the limit supporting force was analyzed.(3) The rationality of the three-dimensional mechanism constructed in this paperwas verified by contrast with the monitoring data of real projects. With the DOTshield tunnel in Shanghai district as background, the construction steps such as tunneling excavation, lining segments assembling, grouting at the tail and groutconcretion and so on were considered, the deformation law of ground surface and theinfluence of supporting force on land subsidence during the construction of tunnelwere analyzed. In the analysis, keep other factors stable, decrease the supporting forcefrom220kPa (1.1γh) to140kPa (0.7γh), then to100kPa (0.5γh), the land subsidencecorresponding with different supporting force was analyzed. Through the contrast andanalysis, theoretically speaking, reducing the supporting force appropriately can alsocontrol the land subsidence and at the same time ensure normal boring of shield.Besides, the transverse settlement distribution and longitudinal settlement distributionof ground surface was very close, which demonstrated that the three-dimensionalmechanism proposed in this paper can simulate the influence of construction of DOTshield tunnel on land subsidence.