Moisture capacity

MOISTURE CAPACITY OF ROCKS (EN: rock's specific retention, rock's moisture capacity; DE: Wasseraufnahmevermogen der Gesteine, Bergfeuchte; FR: capacite hygroscopique des roches; ES: capacidad higroscópica de las rocas; RU: влагоемкость) is the ability of the rocks to retain the water within the voids (pores, cavities, and cracks).

The moisture capacity is evaluated according to the relative or voluminous content (in terms of the percents) of the moisture, using the method for the weighing of the samples of the rock, which have been saturated with the water, and which have been dried to the constant mass. According to the character of the distribution of the water within the voids of the rock, they distinguish the hygroscopic (which is characteristic for the soils, which are embedded near to the surface), molecular, capillary, and complete moisture capacity. The maximal hygroscopic moisture capacity is determined as the amount of the moisture, which the rock is able to absorb from the air with the relative humidity of 94%. The hygroscopicity according to the mass is approximately 1% on average for the sands, 5-10% for the loesses and silts, 15-20% for the clays. The molecular moisture capacity (the amount of the water, which is retained on the account of the molecular interactions on the surface of the grains of the minerals) depends on the characteristic of the wettability of the surface of the grains, and on the degree of their dispersivity. For example, the quartz sands have the least molecular moisture capacity, namely, approximately 1.5%, the loesses have approximately 14%, the clays have up to 40% (according to the mass). The capillary moisture capacity corresponds to the amount of the water, which is retained within the voids of the rocks on the account of the action of the capillary forces. The value of the capillary moisture capacity grows with the decreasing of the average size of the voids, mainly of the pore channels, and amounts to several percents for the sands, and from 18% to 50% for the clays. The complete moisture capacity is the maximal amount of the water, which the water saturated rock is able to retain under the natural conditions of its embedment; the limiting value of the complete moisture capacity is equal to the open porosity of the rocks, and ranges from 0.5% to 60% and more (for the loose rocks, tuffs, brown coals, clays, and limestones) on average.

The moisture capacity is taken into account during the calculations of the systems for the drying of the water, of the parameters of the installations and technology for the injection of the water into the rocks and layers of the coal with the aim of their weakening, during the development of the methods for the fight against the sudden outbursts, during the determining of the requirement for the water during the underground leaching of the ores, during the calculation of the process for the underground melting of sulfur, during the plugging, and so on.