Effect of soil consistency of sediments on the occurrence of the breakthrough of clay in mines of Kuzbass

Abstract

1. The clay soils of sediments on coal outcrops in the Prokop'evsk-Kiselevsk region are quite homogeneous in their grain-size distribution and petrographic composition over the entire area of the deposit and are characterized by a very high percentage of dusty-silty particles, which makes, all the indicated soils unstable and easily convertible to a quick state upon wetting. i.e., potentially dangerous with respect to the possibility of their breaking through into mine workings. 2. The water content of naturally occurring sediments is everywhere close to the plastic limits of the soils composing them however in areas where clays have broken through, the natural water content as a rule is somewhat higher than the plastic limits, whereas in areas where there were no breakthroughs it is below the plastic limits or equal to them in most cases. 3. Upon increasing to the plastic limits the water content, of soils with a disturbed structure, their shear strength markedly drops and at water contents exceeding the plastic limit by 5–7% they flow under a pressure of only 1–2 kg/cm2 (10–20 t/m2). Such pressures in the goaf can be created by the bulk weight of a pillar of clay soil 5–15 m high. 4. As a consequence of the percolation of rain into the goaf, a comparatively shallow zone of soil is formed under the bottom of sinks with a higher water content. In those cases where water has accumulated in sinks, the water content of the soil in this zone can appreciably (by 5–15% and more) exceed the plastic limits. The depth of this zone can reach 6–8 m from the bottom of the sink, so that the volume of the soil having a dangerous consistency concentrated in it can be from several hundred to several thousand cubic meters within the limits of one pillar. 5. Upon bypassing soil of dangerous consistency (with a water content exceeding the plastic limits by more than 3–5%) into the goaf of the subjacent level, this soil as a result of mining the latter can change to a fluid state under the effect of the weight of the pillar of fallen-in rock and under appropriate conditions can break through into the mine workings. 6. By providing a constant runoff of water from the sinks, the depths of the zone of wet soil under the bottom of the skins and the water content of the soil in it are appreciably less than in those cases where the water stands in the sinks for several years. Therefore, it is expedient when backfilling the skins, to break open the dams between them and level the bottom of the backfilled sinks, while forming a general slope for the runoff of water within one or several dugout sections, which enables the water to collect in, and be pumped out of, one place. 7. The presence of appreciable inflows of water into sinks at a period when the sediment soil that collapsed into the goaf does not have time to become consolidated (for example, during showers or preventive mud filling through recently formed skins), and also the backfilling of water-filled sinks, without, preliminary pumping of the water from them promote soaking of large volumes of soil in the goaf, which can be a cause for catastrophic breakthroughs of clay.