Face stability of shallowly buried large-section EPB box jacking crossing the Beijing-Hangzhou Grand Canal

The mechanical response of conditioned soil in an earth chamber is crucial for maintaining the face stability of earth pressure balance (EPB) box jacking. This study presents the project of a shallowly buried large-section box jacking crossing the Beijing-Hangzhou Grand Canal in Suzhou, China. The characteristics of tunneling parameters, such as the earth chamber pressure pe, total jacking force Pf and frictional force Ffric, were revealed by field measurements. A refined three-dimensional finite difference model was developed using the fluid-solid coupling analysis mode to evaluate the tunnel face stability, in which the geometric configurations of box jacking machine and the contact interfaces between different components were considered explicitly. In addition, the differences between driving and stoppage of box jacking machine were also considered. Upon the successful calibration of the numerical model, the transfer mechanism and the face support efficacy of conditioned soil are evaluated. In the end, a parametric study was conducted to discuss the influences of conditioned soil permeability, shear strength and thickness. The results indicate that the permeability of conditioned soil has no significant influence on its mechanical response, and a larger shear strength can obviously enhance the stress level at the lower part of the earth chamber. In addition, a larger thickness of conditioned soil can reduce its transfer coefficient and the fluctuation of earth chamber pressure.