Water Permeability of Rock

the ability of rock to permit the passage of water. The degree of water permeability depends upon the size and quantity of interconnecting pores and cracks, as well as upon the uniformity of the rock grains. Among the highly permeable rocks are gravel, shingle, large-grained sands, and intensely karsted and fissured rock. Virtually impermeable (waterproof) rock includes clays and compact loams, as well as unfissured crystalline, metamorphic, and compact sedimentary rock.

The water permeability of rock can be determined by the filtration rate, which equals the quantity of water that passes through a unit of area of the cross section of the filtering rock. This relation is expressed by the Darcy formula: v = kl, where v is the filtration rate, k the filtration coefficient, and I the pressure gradient, which equals the ratio of the pressure drop h to the length of the filtration path L (I = h/L). The filtration coefficient is measured as a flow rate (cm/sec, or m/day). Thus, a filtration rate with a pressure gradient equal to one is identical to the filtration coefficient.

Because rock water can move under the effect of various forces (such as hydraulic pressure, the force of gravity, temperature gradient, and capillary, adsorption, and capillary-osmotic forces), the quantitative characteristics for the water permeability of rock can be expressed, aside from the filtration coefficient, also by water conductivity and piezoconductivity coefficients. In hydrogeological research and calculations, the water conductivity coefficient (the product of multiplying the filtration coefficient by the thickness of the aquifer) is the index for the filtration capacity of rock.

Depending upon geological structure, the water-bearing rock in filtration terms can be classified as isotropic when water conductivity is the same in any direction, and aniso-tropic when there is a regular change in water permeability in different directions.

A study of the water permeability of rock is essential in searching and prospecting for underground water for water supply purposes, in building hydraulic structures, in exploiting various types of underground waters, in calculating the tolerable drops in the water level and the seepage area of water-supply wells, and in designing and carrying out drainage and irrigatipn measures.