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Post-critical properties of Carboniferous rocks in the Upper Silesian Coal Basin under varying strai |
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Post-critical properties of Carboniferous rocks in the Upper Silesian Coal Basin under varying strain rate and confinig pressure conditions Author: M. Bukowska
The influences of the strain rate ranging from 10 sup -5 -10 sup -1s sup -1 and of the confining pressure ranging from 0 to 50 MPa on certain geomechanical properties of rocks such as critical stress, [sigma] sub cr residual stress, [sigma] sub r, and the moduls of softening M, have been determined. The tests were carried out using the MTS-810 NEW testing machine with a kinematic control by means of longitudinal strain rates of 10 sup -5 -10 sup -1s sup -1. Confining pressures in the range of 10-50 MPa vcre applied. The following Carboniferous rocks have been tested: - fine-grained sandstone ? 72 specimens; - siltstone ? 39 specimens; - semi-bright coal ? 51 specimens. A total of 162 rock specimens were tested. To determine the effects of the strain rate and confining pressure on the post-critical parameters, a statistical analysis allowing for the sample population of a particular rock type has been performed. Relations between a given parameter and the following factors: - strain rate or logarithm of the strain rate, - confining pressure were determined. These relations and their correlation coefficients are shown in Fig. 1-7. The following arc the conclusions from the analysis of the results of a study performed to investigate the influence of the strain rate and confining pressure on the post-critical parameters: - The magnitude of critical stress as a function of the strain rate for sandstone, siltstone and coal increases in accordance with the power function for all the confining pressures used. Relations between the critical stress and the logarithm of the strain rate for different values of confining pressure are of the rectilinear shape with very high values of correlation coefficient. - The values of residual stress for the confining pressure of p = 0 MPa are low, which is typical of the uniaxial compression, and decrease with the increase in strain rate. However, the residual stress for the values of confining pressure ranging from 10 to 50 MPa increases with the increase in strain rate. The relation between residual stress and confining pressure in the range of 0-50 MPa at strain rates of 10 sup -5 -10 sup -1s sup -1, can best be defined by the power function having the correlation coefficients indicating very high values of correlation at confining pressures ranging from 0 to 10 MPa for siltstone and coal and from 0 to 20 MPa for sandstone. For the higher values of confining pressure (in excess of 20 MPa), lower values of the correlation coefficient can be distinguished. The relations between the strain rate for different values of confining pressure have been described by the linear functions. - There is a relationship between the critical stress and the confining pressure and between the residual stress and the confining pressure. The relation [sigma] sub cr = f(p), [sigma] sub r = f(p) for sandstone, siltstone and coal can best be defined by a linear function. - The investigation of the influence of strain rates of 10 sup -5 -10 sup -1s sup -1 on the values of modulus of softening M for Carboniferous rocks have not indicated any regular variation of modulus of softening M with strain rate. However, the relation between the modulus of softening and the confining pressure is clear, i.e. the modulus of softening decreases with the increase in confining pressure for sandstone and coal according to the polynomial of the second degree or the exponential function. At the same time, a greater scatter of the values of modulus of softening M for coal relative to strain rates for the lower values of confining pressure can be observed. For the higher values of confining pressure in the range of 20-50 MPa, the differences between values of modulus of softening M for the corresponding strain rates decrease. For siltstone, no relation between modulus of softening M and confining pressure p in the range of 0-50 MPa, at strain rates of 10 sup -4 -10 sup -1s sup -1, has been found. The extensive studies performed to investigate the influence of the strain rate and confining pressure on the values of modulus of softening for the Carboniferous rocks have confirmed the validity of the test results presented in papers by Bukowska (1996, 2000, 2002a). - Taking into account the influences of the strain rate in the range of 10 sup -5 -10 sup -1s sup -1 and of the confining pressure in the range of 0-50 MPa can be of use for analysing the stability of underground mine workings and for predicting the loss of stability leading to rockburst potential and occurrence.
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2011