Geological and hydrogeological environment with geohazards during underground construction in Hangzhou: a review

The urban area of Hangzhou is located in the northeast region of the Hangzhou Bay, beside the estuary of the Qiantang River. Three main ancient rivers are present in the case study area. The ancient rivers, together with the influence of three transgressions, influence the geological and hydrogeological formations of the urban area of Hangzhou. Soft clay is widely deposited in the study area and the Quaternary strata are rich in pore water, karst water, and bedrock fissure water. Confined aquifers located in the case study area have high pressure, high permeability, and rich water, and their levels fluctuate with tide. Karst strata, soft clay, ancient confined aquifer, and shallow gas deposits are potential geological features, which may threaten underground construction in the urban area of Hangzhou. As such, corresponding pre-treatments should be adopted to control the potential geohazards. Karst caves are proposed be infilled or reinforced before the commencement of underground construction works. Ground improvement methods can be adopted to enhance the strength of soft soil. Foundation dewatering in foundation pits with pensile curtains can be adopted to control ancient confined aquifers. Pre-exhaustion of shallow gas prior to commencement of construction works is an effective measure to control shallow gas with high pressure. Moreover, the impacts of the Qiantang River tide on the groundwater level and the deformation of underground structures should be considered.