Estimating the permeability of fractured rock masses for design of construction dewatering systems

Abstract

A comprehensive data on dewatering and pumping-well systems installed in fractured rock is compiled and analyzed to estimate the permeability of fractured rock masses. The data covers construction activities of different nature and dewatering depths, and considered the richest of its kind. Due to the relatively small volume and the nature of rock masses involved in the analyzed systems, the effect of having predominant oriented fracture sets, and consequently different directional permeabilities (i.e., scale-dependent permeabilities) are minimal. As a result, the current study is a clear advancement on state-of-the art given that the vast majority of the related studies available in the literature are presented to estimate the directional permeability of rock masses. The analyses showed a significant discrepancy between the coefficients of permeability estimated from field packer and falling-head tests and those back-calculated from the actual discharge pumped out of rock masses using dewatering or pumping-well systems. Back-calculated permeabilities can be up to five orders of magnitude larger than the measured values. Such discrepancy increases with increasing permeability of rock mass. The sensitivity of the estimated permeability to the difference in rock formation, utilized fracturing index, test type, and depth of test is studied. A data-driven approach is used to develop a novel correlation between the measured and back-calculated coefficients of permeability. Accordingly, a procedure is proposed to estimate the permeability of fractured rock masses involved in construction dewatering as a function of the measured permeability and rock degree of fracturing.