STUDY ON THE SURFACE DEFORMATION LAW OF SHIELD TUNNELING IN AIR PRESSURE ASSISTED EARTH PRESSURE BALANCE MODE

In soft ground, the surface deformation caused by shield tunneling easily results in the deformation and cracking of adjacent buildings, bringing safety hazards and adverse social effects. In order to effectively control the surface deformation during shield tunneling, the earth pressure balance(EPB)mode and the air pressure assisted earth pressure balance(APEPB)mode were successively adopted in Longzhen Exit and Entry Lines of Guangzhou Rail Transit Line 18,which were shallow-buried tunnels in soft ground. First, the measured surface deformation data were analyzed in the two tunneling modes. It indicates that the APEPB mode does better in controlling surface deformation when the stratum has good air tightness. Second, the surface deformation law in the two modes as well as the influence of construction parameters on the surface deformation were studied. It indicates that the development of the surface deformation can be divided into five stages and the deformation values in the third and fourth stages are the largest. In the third stage, the surface deformation is mainly subsidence in the EPB mode, but the surface will uplift when adopting the middle-shield grouting or the APEPB mode. Compared with the middle-shield grouting volume, the linear correlation between deformation value and air pressure value is higher. In the APEPB mode, the linear correlations between deformation value in the fourth stage and grouting pressure value, grouting volume, and air pressure value increase successively, whose Pearson correlation coefficients are 0.53, 0.62 and 0.84,respectively. It indicates that the surface deformation can be effectively controlled by adjusting the value of air pressure. The results can provide a theoretical basis and technical support for the control of surface deformation under similar conditions during shield tunneling in the APEPB mode.