Fracture characterization using hydrogeological approaches and measures taken for groundwater inrush mitigation in shaft excavation

Abstract

Plentiful groundwater in fractured rock masses adjacent to a construction site of a shaft in a highway tunnel project has been notified, which might cause groundwater inrush during its excavation. Hydrogeological approaches were used before the construction of the shaft to investigate the hydraulic features of the site, and thus to characterize the site and propose corresponding measures for groundwater inrush mitigation. This study further determines the fractures and the Q-values of rock masses in the bare walls following excavation of the study shaft, and the hydraulic conductivity of surrounding rocks, based on the measured variations of groundwater level and pressure in four observational wells around the shaft, to check the predicted results using the hydrogeological approaches and conventional Q-logging. The applicability of the proposed mitigation measures to groundwater inrush is also assessed. The results of comparisons indicate that the spatial distributions of three preferential flow paths that were predicted using hydrogeological approaches based on fractures in rocks are consistent with those revealed after excavation of the shaft. The depths at which groundwater inflow is high and the direction of the source of the groundwater can be predicted more precisely from hydrogeological approaches than using Q-values. Mitigation of groundwater inrush involves a grouted curtain that is applied to the regolith stratum from the surface, grouting for sealing that is carried out in the shaft, and aspirating pumps within the shaft and by pumping wells around the shaft. These methods effectively reduce the impact of groundwater on shaft construction. However, the aspirating pump takes time to control the water level inside the shaft. Hydrogeological approaches predicted the hydraulic conductivity of regolith and fractured rock masses nearby the study shaft over four orders of magnitudes, yielding results that are consistent with those obtained by excavation of the shaft, also confirming their reliability in elucidating the hydrogeological characteristics and groundwater inrush potential at a shaft site in a fractured rock mass.