MEASURING THE VERTICAL PERMEABILITY OF HORIZONTALLY STRATIFIED FRACTURED ROCK

In many horizontally stratified rocks, groundwater flow is predominantly in the horizontal direction, governed by the presence of horizontal fracture features. The degree of interconnection between these features is limited by the presence of vertical or sub-vertical fractures. Thus, to assess the potential degree of contaminant spreading with depth in an aquifer of this type, an estimate of the hydraulic properties of the vertical fractures is required. To investigate techniques for measuring vertical permeability, a study site was constructed at which nine boreholes were drilled at an inclined orientation into a horizontally stratified carbonate rock of Silurian age. Contiguous measurements of transmissvity using a packer spacing of 1.5 m were obtained with depth in each borehole. Four horizontal fracture zones in the upper 30 m, were found to pervade the carbonate rock at the study site location. The transmissivity of the fracture zones is approximately 10^-4 - 10^-6 m²/s, and the T of the un-fractured rock is as low as 10^-9 m²/s. To measure the vertical hydraulic properties of the rock, four pumping tests of 12-hr. duration were conducted. In each test, the most permeable horizontal fracture zone was pumped and drawdown was observed via pressure transducers in 19 isolated intervals located in other boreholes. Pumping was conducted at a flow rate of approximately 4.5 L/min for each test. The test results were interpreted using a multiple-aquifer analytical model, the ratio method, and FRAC3DVS (a discrete-fracture numerical model). The results of the interpretations were compared to the results of the direct measurements of T obtained from identified vertical fractures. A good agreement was found generally between the pumping test interpretations and the direct measurements; however, use of the analytical models was found to overestimate the vertical permeability of the rock mass in some cases.