Abstract: An advance jacking pipe drainage scheme is proposed to effectively address frequent mud and water gushing and the slow advancement of tunnel construction in highly water-rich fine sand strata. The scheme, “jacking pipe drainage and final expansion excavation”, is proposed to reduce the water pressure and groundwater level on the tunneling face. Particularly, a drainage scheme combining prior pipe-jacking connection, centralized in-pipe drainage, and groundwater discharge via vertical drainage pipes is proposed considering the engineering geological conditions, high water content, weak cementation, disturbance-induced particle loss, and difficulty in cutting off groundwater supply by conventional local drainage in the tertiary semi-formed silty fine sand strata. Field monitoring and a three-dimensional seepage-coupled numerical model are employed to analyze the ground response during pipe jacking and drainage. The results show the following: (1) The advance drainage scheme markedly reduces the groundwater level and pore water pressure in strongly water-rich semi-formed silty fine sand strata, effectively decreasing the risks of mud inrush, water inflow, and tunnel-face collapse. (2) After pipe jacking, an unloading-loosening zone and an asymmetric stress zone form around the pipe, and the simulated displacement and stress distributions are generally consistent with the monitored strain and contact pressure trends, indicating that the model can capture the main mechanical responses during pipe jacking. (3) After drainage, pore water pressures above and below the pipe decrease by approximately 97.4% and 94.6%, respectively, compared with the initial values. Furthermore, field piezometric monitoring shows a sustained pressure decline, indicating that jacking pipe drainage exhibits a favorable pressure-reduction effect and can provide a reference for optimizing tunnel construction schemes in similar strongly water-rich semi-formed strata.

Key words: strong water-rich tunnel, semi-formed rock, jacking pipe advance drainage, numerical simulation