Sub-horizontal reinforcement of weathered granite before tunneling beneath a spillway

Sub-horizontal jet-grouted columns and pipe roof are usually adopted to reinforce weak ground ahead of tunnel excavation face, but they bear a low tensile strength and poor waterproof capacity, respectively. A combination of sub-horizontal jet-grouted columns and pipe roof was proposed to reinforce weak ground before Jiangmen tunnel passed beneath a spillway for drainage of Yu-long Reservoir. Due to the pipe roof added below the jet-grouted columns, numerical simulation demonstrates that stress in the jet-grouted columns, ground surface settlement, convergence and stress of primary liner are reduced, and the stability of the excavation face is increased substantially. The umbrella formed by jet-grouted columns works as a shell, and the pipes in the pipe roof behave as beams in the longitudinal direction of the tunnel. Due to their respective features in the working mechanism, Jiangmen tunnel stayed safe when it passed beneath the spillway. The field measurements are comparable to the numerical results, although there are some differences in their values due to some influence factors.