Investigation of geomechanical properties of carboniferous rocks for evaluating the possibility of energetic use of water and methane from hard coal mines

This study aimed to indicate the variability range of parameter values describing the geomechanical properties of Carboniferous rocks depending on the moisture content of the laboratory sample. We assumed that the moisture content in the tested rock samples corresponds to various water saturation states in the rock mass. The states could be caused by complete and long-term drainage, water inflow, or the position of the rock sample to the ventilation ducts or the water table in flooded mine workings. In line with this assumption, measurements were made on samples of accompanying rock using two water saturation states of rock pores – moisture of samples, i.e., air-dried and capillary saturation states. Laboratory surveys were also made for the state of moisture of the coals obtained in the process of immersion of the sample in water. The air-dried state of rocks as standard in geomechanical tests in laboratories was compared with the surroundings of mining excavations, mostly ventilated ones, located within a long-term preserved depression cone, especially in hydrogeological covered areas. We used the capillary saturation state to demonstrate significant changes in the values of basic geomechanical parameters under the influence of the water from the surface and higher aquifers, circulating in the rock mass near groundwater reservoirs. Capillary saturation was the closest to natural moisture in the rock mass drained from free water. The coefficient of changes in the geomechanical properties of rocks associated with the change in moisture content and the transition of rocks from the air-dried state to the capillary saturation state was determined. The parameter was suitable for simulating probable changes in the values of geomechanical parameters of rocks and approximating the laboratory moisture content to the conditions occurring in the rock mass. Linear relationships were also developed with very good or good, and sometimes satisfactory coefficient determinations.