Stability analysis of multiple pipe-jacking tunnels construction under seepage-stress coupling effect

Abstract

When three parallel pipe-jacking tunnels with large cross-sections and small spacing are arranged in water-rich muddy gravel pebble strata, the combined action of strong coupling effects of groundwater seepage and soil stress field around tunnels often makes it tedious for traditional single-domain analysis to reasonably evaluate construction safety. Besides, the random dynamic load of vehicles on the roads in the construction area further complicates the assessment of ground surface deformation. To better understand such coupling effect in pipe-jacking tunnel construction, this paper combines the Strain-softening model with the FLAC^3D seepage-stress coupling module to simulate the whole process of pore water pressure migration, stress redistribution, grouting, and jacking. The accuracy of the numerical simulation results is verified through comparison with on-site monitoring data. The results show that the jacking process causes rapid dissipation of pore water pressure at the boundary of tunnels. This consequently induces a symmetrical distribution pattern of stress and strain fields about the boundary between extrusion and unloading zones. The maximum principal strain concentrates at the tunnels’ waistline. The seepage-stress coupling effect increases the settlement of the vault by 8.3 mm, while the uplift and horizontal displacement of the bottom arch decrease by 4.1 mm and 8.0 mm respectively. The sensitivity analysis of vehicle dynamic load reveals that the peak ground surface settlement increases by 12 % when the vehicle speed decreases from 80 km/h to 40 km/h. The rise of groundwater level significantly increases the value of ground surface settlement. A quantitative relationship is proposed to quickly predict the ground surface settlement under different groundwater levels, and the Peck’s settlement formula is revised in this paper. The error between the peak settlement and monitoring data is only 2.5 %, which is significantly better than the traditional superposition method. The contour map of safety factor shows that the soil overlying the left pipe-jacking is the weakest (FOS = 1.22), but the whole structure is still in a safe state.