Water permeability

WATER PERMEABILITY of the rocks (EN: water permeability of rock; DE: Wasserdurchlassigkeit der Gesteine; FR: permeabilite des roches; ES: permeabilidad de las rocas; RU: водопроницаемость) is the ability of the rocks to permit the water to pass through the voids (pores, cracks, and so on) under the impact of the gravitational forces, pressure, or capillary rise. The water permeability is quantitatively evaluated using the volume of the water, which is passing through the unit of the surface during the unit of the time, while the gradient of the pressure is equal to one (the coefficient of the permeability), or using the speed of the movement of the water within the rocks (the coefficient of the filtration) KF (metres per hour):

where Q is the volume of the water, which is passing through the area S of the cross-section of the sample or massif of the rock during the time t.

The water permeability is measured under the laboratorial and natural conditions; the value of the coefficient under the natural conditions is sometimes by several orders of magnitude higher than within the sample. Because of this fact, there is introduced into the calculations the coefficient of the filtration, which is determined under the natural conditions, using the pumping of the water from the central borehole, around which have been drilled the observational boreholes.

The water permeability depends on the size and quantity of the pores, which are communicating among themselves, on the voids and cracks within the rocks, on the diameter of the pores, on the openness of the cracks, on the tortuosity of the channels of the filtration, and on the mineral composition of the rocks. The water permeability of the loose rocks depends on the degree of the compactness and packaging of the particles of the rock, on their particle size distribution, shape and degree of the sorting, and on their wettability. Depending on the value of the KF coefficient, they distinguish the water-resistant rocks (KF < 0.1 metres per day; for example, clays), poorly permeable (0.1 < KF < 10 metres per day; loesses, loams), averagely permeable (10 < KF < 500 metres per day; porous limestones, sandstones), and easily permeable (KF > 1000 metres per day; coarse sands, pebbles, cracked massifs of the rocky rocks). The water permeability influences the choice of the technology for the conducting of the mining works, of the methods and schemes for the drying of the deposits (for example, during the drying of the massifs of the poorly permeable rocks, they increase their water permeability using the methods of the loosening, hydraulic fracturing of the layer, hydrochloric acid treatment, and electric osmosis), of the methods for the fighting against the sudden outbursts, and the water permeability is also used for the determination of the quantity and power of the pumps for the water table management, and so on.