Post-Critical Mechanical Properties Of Sedimentary Rocks In The Upper Silesian Coal Basin (Poland)

Abstract

In this paper, we present the results of a study of the Upper Carboniferous sedimentary rocks of the Upper Silesian Coal Basin (USCB) in Poland. We examined the hard coals, which belong to various stratigraphic units of Upper Carboniferous coal-bearing strata, and waste rocks, i.e., sandstones, mudstones, claystones. We present the results of tests of their post-critical mechanical properties. These results are from tests of the post-critical modulus, residual stress and residual deformation from experiments using a servo-controlled testing machine (MTS) with uniaxial compression and conventional triaxial compression. We applied confining pressures of up to 50 MPa at a strain rate of 10 -5 -10 -1 s -1 (0.003-6.0 mm/ sec). The confining pressure applied in the triaxial compression tests reflected the conditions of current and future mining activities in the USCB at depths exceeding 1.300 metres. The strain rate applied in the tests reflected the values observed in the rockmass surrounding the mine workings and the rate of certain geodynamic phenomena occurring in the Carboniferous rockmass in the USCB, e.g., rock bursts. We present the values of the sub-critical modulus of coals and waste rocks, the functional relationships between the post-critical modulus and uniaxial compression strength, which are described using an exponential function of high correlation coefficients of the given rocks, and an exponential relationship between the post-critical modulus and the longitudinal elasticity modulus (Young's modulus). Based on the results of tests of the post-critical properties of the Carboniferous rocks under triaxial compression and at various strain rates, we devised the functional relationships between the properties of the rocks and the confining pressure. The dependence of the post-critical modulus of the sandstones and claystones on the confining pressure is described using a polynomial function of degree 2, and that of the coals is described using an exponential function. The relationship between the residual stress and residual deformation in the rocks and the confining pressure was described using a linear function. The obtained results of tests have a practical application in forecasting behaviour of rocks located deep, and designing safe exploitation of mineral deposits. Confining pressures of up to 50 MPa used in the conventional triaxial compression tests allowed us to predict the behaviour of the rock mass at large depths. These data provide general knowledge of the tendencies in behaviour of rocks at substantial depths and the ability to design safe methods of mining deposits of various raw materials, including energy sources. These deposits are mined from increasingly great depths as the reserves are gradually exhausted and collieries of the largest European coal basins are continuously reconfigured.

Investigation of Geomechanical Properties of Carboniferous Rocks for Evaluating the Possibility of Energetic Use of Water and Methane from Hard Coal Mines

“…Compressive strength is also one of the main strength parameters when selecting support for mine workings in underground mining of mineral deposits and assessing the possibility of geodynamic phenomena, e.g., rock bursts. determining the impact of water on the parameter's value might change rock mass classification due to its tendency to rock bursts [1,25]. The parameter is fundamental for assessing general, public-surface (sinkhole hazard) and mining safety, and the safety of methane and water resources accumulated in goafs near active mine workings, which are considered for possible energy purposes.…”

“…If the dissipating elastic energy is slow, the energy dissipation might be high, and the kinetic energy value tends to zero. Therefore, it is an essential parameter for assessing the rock mass tendency to rock bursts and the rock-burst hazard in mine workings [25]. • Critical strain is a geomechanical parameter obtained from the stress-strain curve of a compressed rock sample, determined for the maximum (critical) stress.…”

The Impact of the Strength of Roof Rocks on the Extent of the Zone with a High Risk of Spontaneous Coal Combustion for Fully Powered Longwalls Ventilated with the Y-Type System—A Case Study

“…This value depends on the type of roof rocks forming the caving. The maximum value of the tensile strength of rocks in Polish mines amounts to approximately 8 MPa [1,2]. In practice, however, such value is rare.…”

“…They include claystones, coal shales, mudstones, sandstones and, where the ongoing exploitation is divided into layers, also hard coal. All of these rocks have different strength properties [1,2]. So far, the strength properties of the rocks surrounding mine headings have been considered primarily in terms of their impact on the capacity to maintain the stability of mine headings [3][4][5].…”

Evaluation Of Water Hazard In Hard Coal Mines In Changing Conditions Of Functioning Of Mining Industry In Upper Silesian Coal Basin – USCB (Poland)

“…Rockburst phenomena and high energy tremors of the rock mass disturb stability of the pillars and lower their strength. Employing the knowledge of strength and deformation properties of rocks in USCB, including post-peak failure properties (Bukowska, 2013(Bukowska, , 2015, it is advised to select safety parameters individually to calculate width of safety pillars on the side of reservoirs of closed mines and other underground reservoirs. Safety coefficient, which on average increases by 1.5-2.0 times the width of pillars if high rockburst or tremor susceptibility of the rock mass is concluded, is considered to be safe (Bukowski, 2010b).…”