Estimation of permeability for the rock mass around the shiplocks of the Three Gorges Project, China

Abstract

Changes in the bulk permeability of the fractured rock mass around the shiplocks of the Three Gorges Project, China, during excavation have been investigated using a numerical modelling technique. The numerical model took into account both the effect of fractures on the in situ bulk permeability and the increase in permeability resulting from the stress relief and opening of the fractures caused by excavation. Fracture patterns were estimated on the basis of field exposures in three orthogonal planes. The numerical model was calibrated for the in situ condition by varying the fracture stiffness until a close match (in terms of both magnitude and direction) was obtained with the equivalent permeabilities estimated on the basis of field pumping tests. Excavation of the shiplocks both with and without the use of temporary supports was modelled, and the resulting changes in stress and permeability in the surrounding rock mass calculated. In both cases, the opening of fractures subparallel to the shiplock channels resulted in an increase in the permeability of the ground, but when the vertical sidewalls were supported during excavation the increase in permeability was less marked. The results of the analyses highlight the effects of engineering activities on the permeability of fractured rock masses, and demonstrate that these effects can be controlled through the judicious choice of construction techniques and procedures.