Application of Horizontal MJS Piles in Tunneling Beneath Existing Twin Tunnels

This paper presents a case of a single tunnel excavated beneath closely spaced existing twin tunnels with reinforced MJS piles in Changsha, China. Due to soil disturbance and stress re-distribution induced by tunneling, the safety of overlying tunnels during excavating becomes a serious issue. To solve this problem, the Metro Jet System (MJS) method was used. The response of stratum and overlying tunnels are studied during the installation of horizontal MJS piles and shield tunneling, by monitoring pore water pressure of surrounding stratum, additional stress and vertical displacement of tunnel in the water-rich sand layer. It indicates that the pore water pressure increased rapidly during the period of jetting-grout and the maximum excess pore water pressure reached 35 kPa. The distribution of additional stress at monitoring cross-section is studied in different construction stages. The vertical displacement of the two existing tunnels presents distinct characters due to temporary road load on the tunnels. A satisfactory match between the proposed fitting curves and the measured settlement data of north tunnel is presented. Overall, MJS horizontal pile can effectively reduce the settlement of existing tunnel.