As a consequence of geomechanical and geotechnical conditions and also anthropogenic activities in mines of the Ostrava-Karviná Coal Basin (Czech Republic) rockbursts have been recognized to be a serious seismic risk. Therefore, local as well as regional monitoring arrays were deployed there in order to investigate the actual seismic activity continually. One of the essential parameters which was observed at the seismic stations situated in-situ was the particle velocity of P- and S-waves. The aim of this paper is to derive some empirical dependences by determining the decrease of seismic wave amplitudes with distance from rockburst foci. Seismic energy, local magnitude, co-ordinates of seismic stations, rockburst foci and positions where damage to workings occurred were employed as additional imput. Moreover, the degree of this damage was tested based on the values of particle velocity and, simultaneously, stress due to the loading of the workings by the stress wave was also calculated.
This paper considers the fundamental role of seismic monitoring in the eastern part of the Ostrava-Karviná Coal Basin (hereinafter referred to analysis as OKCB), where rockbursts have occurred almost two hundred years [14], while instrumental detection of these phenomena started in the 1980s of the last century [22] and [26]. The origin of these mining-induced events is closely related to coal-extraction processes and is also affected by other factors, e.g., by specific mining and geological conditions, physical and mechanical properties of the rockmass, the mining technique used in the past and at present, stability of all types of pillars, changes in stress–strain state of rock mass, etc. [12]. In layers underlying our coal deposit, induced seismic events occur very often, and moreover, some of them have the character of damaging rockbursts. Therefore, in areas affected by these dangerous seismic events both the passive (e.g., mine design, choice of mining equipment, selection of suitable mounting, etc.) and active preventive measures (e.g., destressing drilling and blasting, water infusion of coal seams, large-scale non-productive blasting operations in the roof, etc.) have been applied in order to mitigate their devastation of the workings underground [11] and [6].
In the scope of the geomechanical service of the Ostrava-Karviná mines, a rockburst is characterized as a seismic event, in consequence of which workings are permanently destroyed so that they can no longer fulfill their operational function. For the purpose of this study, a series of rockbursts was selected, recorded in the years 1993–2009 simultaneously at three underground seismic stations where triaxial sensors were deployed. The particle velocity of P- and S-waves was chosen as the basic observed parameter, while the focus location, released seismic energy and local magnitude, were calculated subsequently also using the first-arrival times of these seismic waves. The subject of our interest was the determination of the coefficient of decay of the maximum P- and S-wave particle velocity in dependence on scaled distance in the rockmass and estimation of the extent of damage to workings after the rockburst.
The following conclusions can be drawn:
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The rigid interlayer between coal seams nos. 33 and 37 can be considered a very dangerous zone, namely for mining at lower horizons due to concentration of strain energy.
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Considering that occurrence of rockbursts is very probable, the application of active and passive preventive measures in the endagered areas underground are obligatory.
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The decay factor for P- and S-waves b=−0.893 and −1.16, respectively.
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The d*=d/√E was introduced to describe the dependence of the particle velocity on scaled distance.
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Some rockbursts, the foci of which are seated in the higher roof, generated mining shocks felt by inhabitants from time to time. They are seldom followed by surface waves recorded at regional seismic stations.
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The degree of workings damage after rockburst is usually estimated by supervisors from Czech Mining Bureau by visual inspection. The supervisors also paid attention to length of damaged roads and entries which are measurable. As opposed to other manifestations, e.g., floor lift, reinforcement failure, shift of stone–dust barrier, etc., these cannot be measured directly, e.g., with a measuring tape.
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Lengths of the devastations were compared either with particle velocity or with acting stress σs
