Characterizing fractures to mitigate inrush of water into a shaft using hydrogeological approaches

Abstract

The inrush of water during excavation is one of the most difficult challenges in modern underground engineering. Studies of the permeability of a rock mass, and especially potential paths for the flow of groundwater, are important for mitigation. This study considers a ventilation shaft for a twin-bore four-lane highway tunnel, and applies hydrogeological approaches to characterize the site, and to propose corresponding plans for mitigating water inrush. Based on detailed descriptions on outcrops, a scanline survey, and the interpretation of scanned images of four boreholes with a total length of 480m that were obtained using a televiewer, more than 3400 data concerning the attitudes and locations of fractures are acquired to classify the strata close to the study site as terrace alluvium, regolith and fractured rock masses. Five predominant fracture sets, with corresponding parameters of their respective spatial distributions that are determined by stereographic projection analysis and statistical analysis, are characterized to establish a model to characterize the fractures of fractured rock masses. Taking into account the locations where high equivalent hydraulic conductivity are identified, this study presumes a model that provides potential paths of groundwater that flows to the shaft, including specific locations, scopes and directions of groundwater flows. Another model that is inferred based on the conventional Qlogging method is also provided for comparison. Two plans for mitigating water inrush during shaft construction are proposed based on the models of the spatial distribution of fractures and potential paths of flowing groundwater to the shaft. The adopted plan involves grouting in the regolith stratum to form a water-proof curtain that is applied in the ground surface or inside the shaft, radial grouting in the fractured rock masses that is applied inside the shaft, and the installation of pumping wells at the ground surface to draw down the groundwater table.