WATER-STABILITY of the rocks (EN: water stability of rock, rock's resistance to water; DE: Wasserbestandigkeit der Gesteine; FR: resistance des roches à l'eau; ES: resistencia de las rocas à la accion del agua; RU: водопрочность) is the ability of the rocks to retain the strength during the interaction with the water.

The decreasing of the strength of the rocks during the saturation with the water is caused by the penetration of this water into the smallest voids (pores and cracks), by the wedging of the cracks with the water, and by the swelling of the individual minerals within the rocks, and these events are leading to the the weakening of these rocks (the more intensive weakening of the rock proceeds within the moving stream of the water). The water-stability is also determined by the lithological composition of the rocks (for example, the content of the clayish minerals, and the weak connection between the particles within the sandstones, and so on, lead to the decreasing of the water-stability).

The water-stability is evaluated using the coefficient of the soaking (softening), which is equal to the ratio of the limit of the strength during the compression of the rock, which has been saturated with the water as much as possible, to the limit of the strength of this rock in the dry state. According to the magnitude of the coefficient nP of the soaking, the rocks are sub-divided into: the water-stable, or water-steady rocks (nP >= 0.9), namely, basalts, granites; the rocks with the decreased water-stability (0.7 < nP < 0.8), namely, sandstones, marbles; the weakly-stable rocks (nP < 0.7), namely, limestones, marls, coal; the water-unstable rocks (nP is approximately 0), namely, the weak argillaceous limestones, mudstones, loesses.

The water-stability is taken into account for the evaluation of the quality of the building rocks, and influences the choice of the means and methods for the development. For example, in case of the decreased water-stability, there are used the hydraulic breaking, and also the preliminary weakening of the coals using the injection of the water into the layers, which action increases the productivity of the mechanical means for the excavation of the coal, and the lumpiness of this coal. They also decrease the water-stability of the rocks, using the addition of the surfactants into the water.