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Abstract

This article presents values of porosity and compression strength of hard coals from the area of the Upper Silesian Coal Basin. The change of the stage of carbonification, which results from conversion of coal substance in the process of coalification, is a source of many changes in the structure of coal. These changes exert influence on values of physical parameters, including the values of porosity and strength. Porosity and compression strength change with the degree of carbonification, a result of the depth of deposition. This study determined the values of effective porosity of coals and their strength considering the age chronology of coal seams and the depth of their occurrence. It examined coals of the Cracow Sandstone Series, the Mudstone Series, the Upper Silesian Sandstone Series, and the Paralic Series from depths ranging from about 350 m to 1200 m. The authors have shown that effective porosity of the Upper Silesian coals changes for particular stratigraphic groups and assumes values from a few to a dozen or so per cent, while compression strength from several to several dozen megapascals. It has been observed, from a chronostratigraphic perspective, that there is a shifting of the upper and lower limits of intervals of porosity variations towards higher values for younger coals. With the increase in compression strength, value of porosity in particular stratigraphic groups generally decreases. However, no regular changes were observed in mean, uniaxial compressive strength with the increase in the age of subsequent stratigraphic groups. On the other hand, for bright coal and semi-bright coal, a visible decrease in compression strength with the depth of deposition of strata was observed.

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Authors and Affiliations

Mirosława Bukowska
ORCID: ORCID
Urszula Sanetra
Mariusz Wadas
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Abstract

In the Carboniferous rock mass of the Upper Silesian Coal Basin, large changes in the geomechanical conditions often occur over relatively short distances. These conditions relate to rock properties that are primarily responsible for the occurrence of geodynamic phenomena in the rock mass. The main factor influencing the manifestation of these phenomena is tectonic stress developed during Variscan and subsequent Alpine orogenesis. This stress contributed to creating tectonic structures in the Carboniferous formations and influenced the properties of the rocks themselves and the rock mass they form. As a result of the action of the stresses, compaction zones (main stresses were compressive) were formed, along with zones in which one of the main stresses was tensile. For the compaction zones in the Carboniferous rocks, the following geomechanical parameters have been calculated: uniaxial compressive strength, Young’s modulus and post-critical modulus. The local stress field was determined according to the focal mechanism in selected areas (Main and Bytom troughs) to characterize changes in geomechanical properties of the rocks that are responsible for high-energy tremors (E ≥ 106 J, ML ≥ 2.2).

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Authors and Affiliations

Józef Dubiński
Krystyna Stec
Mirosława Bukowska
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Abstract

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.
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Authors and Affiliations

Mirosława Bukowska
1
ORCID: ORCID
Przemysław Bukowski
1
ORCID: ORCID

  1. GIG Research Institute, 1 Gwarków Sq., 40-166 Katowice, Poland

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