Development of an Optimum Forepole Spacing (OFS) determination method for tunnelling in silty clay with a case study

Phase III of Harbin Metro Line #1 was constructed in low to medium plasticity silty clay. In the primary design, forepoles with a spacing of 0.8 m were used as pre-support. However, it was found in the practice that the drilling rate was low and the grouting diffusion range was limited when the water content was low in the silty clay. In order to determine the optimum forepole spacing in relation to water content of the silty clay subject to tunnelling, a method combining laboratory tests, numerical simulations with field monitoring was developed in this study. A series of laboratory tests were performed to relate the deformation and strength parameters of the silty clay with water content. The silty clay has been sub-classified according to liquid index the mechanical parameters at different sub-classes were suggested. The parameters were used in numerical simulations for the determination of the optimum forepole spacing for tunnels in soil mass with different water contents. Field monitoring was performed to calibrate the water content-dependent deformation and strength parameters used in the numerical simulations. Numerical simulations were performed to obtain the ground surface settlement, crown settlement and steel rib axial stress for the cases without pre-support, and with forepores in different spacing. Finally, a scheme for the determining the optimum forepole spacing in relation to water content of the silty clay was obtained. In the scheme, for the silty clay with liquid limits in the range of 0.05-0.20, the tunnel could maintain stability without forepoles for the silty clay with liquid limits in the ranges of 0.20-0.45, and 0.45-0.8, the tunnel could maintain stable with forepoles in spacing of 0.8 m and 1.6 m, respectively. This scheme was used in both the phase III of Harbin Metro Line #1 and Harbin Metro Line #3. The practices proved that the scheme was applicable to the pre-support for safe and economic tunnelling in the low to medium plasticity silty clay for Harbin Metro construction.