Large deformation characteristics and the countermeasures of a deep-buried tunnel in layered shale under groundwater

Abstract

Deep-buried tunnels constructed through layered soft rocks undergo large deformation, which threatens their safety and leads to construction delay. When a soft rock tunnel is excavated below the water table, the rock mass will deteriorate due to the presence of groundwater, aggravating the large deformation issue. In this work, the Baozhen tunnel excavated in layered carbonaceous shale was taken as a case study to investigate the mechanical behavior and support countermeasures of deep-buried tunnels with a high geostress considering the effect of groundwater. First, laboratory tests were performed to study the groundwater effect on the mechanical properties of the shale. Subsequently, the geological and large deformation characteristics of selected study areas using the original support scheme were analyzed. Using the tested constitutive parameters combined with the field-monitored deformation, a UDEC numerical model was established to investigate the tunnel deformation behavior under different bedding angles, groundwater depths, and shear stresses. Finally, the optimized support scheme was verified through numerical and on-site monitoring results. The tunnel responses at three typical monitoring cross-sections, such as the deformation distribution and normal pressures acting on the primary support and secondary lining, were compared to investigate the effect of groundwater and the optimized support scheme on the tunnel responses. An asymmetric large deformation was induced by the combined action of the soft layered rock and shearing action under the effect of groundwater. The groundwater reduced the rock strength and increased the tunnel deformation, and it also strengthened the unsymmetrical loading effect, which was reflected in the distributions of the tunnel deformation and support pressures. As a countermeasure, asymmetrically arranged rockbolts could help reduce the full-ring tunnel deformation and support pressure by 50.3 % and 22 %, respectively, which could also alleviate the unsymmetrical loading effect, thus enhancing the synergistic performance of the support structures. The insights gained in this study can help improve the effectiveness of countermeasures against layered rock instability while tunneling in the presence of groundwater.