Effects of groundwater geochemistry on the permeability of grouted fractures

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Abstract
A granite core containing a natural fracture, oriented parallel to the core axis, was collected, prepared, and its permeability measured in a hydrostatic test cell at different confining pressures. After the permeability hysteresis had been removed by several cycles of increasing and decreasing confining pressure, grout was placed within the fracture plane and allowed to cure for 40 days. Distilled water was forced through the sample until a significant flow rate developed. The influent solution for the sample was changed twice during the study to examine the effects of differing concentrations of Ca(HCO3)2 solutions on calcite precipitation within the fracture plane. The interaction of the Ca(HCO3)2 solution with the grout material resulted in a decrease in the permeability of the fracture plane over several months. At the close of the experiments, the sample was removed from the hydrostatic cell and opened along the fracture plane. The fracture exhibited two discrete braided channelways each with an average width of 2 mm. Precipitated calcite formed a rind along the walls of the channelways averaging 0.25 mm in thickness. The quantity of calcite precipitate determined from physical measurements was in reasonable agreement with that calculated using reaction rate expressions for calcite precipitation. The results of this study suggest the possibility that strategically located geochemical material could induce a geochemical evolution of groundwater within poorly grouted fractures, so as to decrease the permeability of the fractured rock system. Keywords: fracture permeability, grouted fractures, geochemistry, laboratory experiments, calcite precipitation.

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