The Don Valley Intercepting Sewer Scheme, Sheffield, England: An Investigation of Flow Into Underground Workings

Abstract

Prediction of the movement of water into subsurface excavations, and surrounding ground-water pressures, is of vital importance as regards the control of the stability and flooding of such excavations, especially below the general water table. As such it forms an important aspect of ground investigations for excavation works. Predictions are usually based on laboratory and in situ determinations of permeability along with observations of the ground-water levels in the area concerned. Although established methods offer a means of assessing the results of permeability determinations, the actual ingress of water into excavations may depend on many factors. The objective of this paper is to consider the influence of geological controls, notably bedding planes and joints, on the permeability and ingress values in fractured rock masses. This is done with reference to tunnels and shafts excavated in Carboniferous Coal Measures strata as part of the Don Valley Intercepting Sewer Scheme in Sheffield, England. The paper includes a consideration of the determination of permeability in fractured rock masses, the geological and hydrogeological situation of the excavations in question, the interpretation of in situ permeability tests in terms of the rock mass intactness and the prediction of rock mass permeability from these data. A primary objective of the paper is to improve the quantitative prediction of water ingress into excavations using information that would usually be obtained in the course of a standard site investigation for a civil engineering structures. During the construction of the tunnels and shafts detailed records were kept of the ground conditions and the water ingress quantities. The variations in water ingresses are discussed in relation to the relevant hydrogeological and geological situation, including the presence of faulted and mine disturbed ground.