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The water transmission characteristics of rock formations are far more variable than those of most soils. The flow conduits include (1) primary porosity, the voids between mineral grains or fragments; (2) genetic porosity, voids which developed within the rock during its formation; and (3) secondary porosity, the joints, shear zones, and other cracks that formed subsequent to deposition. The porosity changes over time as the voids both fill up and become enlarged.
The flow patterns are complex, with tortuous paths between the different forms of porosity. Laminar and turbulent flow occur simultaneously, depending on void size and energy gradients. The flow can be characterized by a variable pseudopermeability coefficient, kp, in the expression q = kpiNA, where N varies from 1 for laminar flow to 0.5 for turbulent flow. Laboratory tests are useless unless the sample size is an order of magnitude larger than the secondary porosity spacing. Field measurements in bore holes or an evaluation of subregional discharge is necessary for making a realistic evaluation of kp of rock aquifers. There are likely to be large variations in kp depending on location, time, and changes in the groundwater environment.
rock, groundwater, permeability, seepage well tests, pumping test
Senior geotechnical consultant and senior vice president, Law Engineering Testing Co., Marietta, Ga.