Geological characteristics of strata in Chongqing, China, and mitigation of the environmental impacts of tunneling-induced geo-hazards

This study investigated the geological characteristics of strata in Chongqing and the tunneling-induced geo-hazards therein. The urban area of Chongqing contains six syncline–anticline pairs and is bounded by the Huayingshan basement fault to the west and by the Changshou-Zunyi basement fault to the east. The bedrock, containing sandstone and mudstone of the Upper Shaximiao formation resulting from Middle Triassic strata, is occasionally outcropped. During urbanization, many valleys in Chongqing require to be backfilled to create more flat ground. Thus, the extent and thickness of the loose backfill layer varied from place to place. While tunneling in Chongqing, the following geological characteristics may cause geo-hazards: (1) backfill with a loose structure, poor gradation, high permeability, and large void ratio, (2) highly, or moderately weathered mudstone and sandstone that can easily become softened once subjected to groundwater seepage, and (3) syncline folding containing perched water or confined fissure water. The potential geo-hazards include groundwater ingress, ground surface settlement, and strata collapse. While tunneling within backfill synclines, sandstone synclines, or weak fractured rock, significant amount of groundwater may be encountered, thus leading to a high risk of groundwater ingress. Ground surface settlement can be triggered due to the disturbances of tunnel construction to the backfill or consolidation effect after groundwater ingress. Strata collapse can be caused by the blasting-induced disturbances to poor surrounding geology or by inadequate overburden thickness or by an increase in the effective stress resulting from groundwater loss caused by leakage. To mitigate the environmental impacts of the tunneling-induced geo-hazards, a series of countermeasures; that is, advance geological drilling, advance ground improvement including full-face grouting, and grouting of the surrounding geologic media, and advance pipe umbrella use, are proposed in this paper. The results from a case study indicate that the proposed countermeasures can greatly decrease the ground surface settlement and amount of groundwater ingress during tunneling in Chongqing verifying the effectiveness of the proposed countermeasures.