Geological conditions of saturated soft loess stratum and influence of tunnel excavation and dewatering system on its groundwater environment

During tunnel construction in the saturated soft loess stratum, although the dewatering system is recommended to improve construction safety, its dewatering effect is unknown. In this study, the distribution and occurrence conditions of a saturated soft loess stratum were analyzed. Based on the understanding of the spatial relationship between the tunnel and the saturated soft loess stratum, variations in the groundwater environment of the saturated soft loess tunnel before and after the operation of the dewatering system were monitored. The results showed that the loess was in a soft plastic or flow plastic state when the dry density was less than 1.48 g/cm3 in the phreatic aquifer of the loess tableland, while the plastic state of the loess was closely related to the saturation when the dry density was 1.48–1.63 g/cm3. In the process of the tunnel construction in the saturated soft loess stratum, the dewatering system was in operation, causing the groundwater level to drop. When the groundwater level dropped below the section without excavation, the water content of the surrounding rock and the flow rate in the tunnel varied from decreasing trend to stable state. When the groundwater level dropped below the saturated soft loess stratum, the pumping capacity of the dewatering wells varied from decreasing trend to stable state. However, the water content of the surrounding rock increased under the action of fissure water during and after section excavation. Therefore, in tunnel construction, the dewatering system plays a key role in improving construction safety.