2. Materials and Methods
2.1. Geological and Mining Conditions in the Investigated Area
2.2. Objectives of Seismic Activity Evaluation
- Total number of registered events, N (-);
- Total energy release, As (J);
- Average energy released in a single tremor, As/N (J);
- Average rate of working face advance (m) per a single tremor, W1 (m/1 tremor); and
- Average energy generated per 1 m of the working face advance, W2 (J/m).
2.3. Main Objectives of Geomechanical Assessment of Seismic Activity and Rockburst Hazard
- Vertical component of the primary stress (σz), as a superposition of gravity-induced stresses (pz), stresses caused by the presence of residues of previous mining operations (pk) and by impacts of geological disturbances and bedding irregularities (pt):
- Stress concentration factor (k), expressed as the quotient of primary vertical stresses (σz) and lithostatic stresses (pz):
3.1. Seismic Activity during the Drifting Operations
- Heading Y#10 (coal seam 409-3/4); direction of drift advance: eastwards; working method: mechanical working (roadheader) and traditional methods (use of explosives); progress status: fully completed (839 m, 100%);
- Downcast gate Y#2 (coal seam 409-3/4); driftage direction: eastwards; working method: mechanical working (roadheader) and traditional methods (use of explosives); progress status: fully completed (486 m, 100%);
- Top heading Y#4 (coal seam 409-4); driftage direction: southwards; working method: mechanical working (roadheader) and traditional methods (use of explosives); progress status: fully completed (550 m, 100%);
- Bottom heading Y#4 (coal seam 409-4); two driftage directions: the northward segment; working method: mechanical working (roadheader) and traditional methods (use of explosives); progress status: partially completed (507 m, ~93%); and the southward segment; working method: traditional methods (use of explosives); progress status: partially completed (5 m, ~13%);
- Bottom heading Y#2 (coal seam 409-4); driftage direction: southwards; working method: traditional methods (use of explosives); progress status: partially completed (21 m, ~4%).
- Sixty-one events (including seven blasting-induced tremors) with the total energy release 4.21 × 105 J are assumed to have occurred in conjunction with the roadheading operations in the northward direction;
- Twenty-two events (including five blasting-induced tremors) with the total energy release 1.13 × 105 J are assumed to have occurred in conjunction with the roadheading operations in the southward direction.
- Drifting in the northward direction, including only the seismic events in the energy categories 102–103 J (three tremors with the energy rating 104 J were all spontaneous)—11.5%;
- Drifting in the southward direction, covering seismic events in all registered energy categories (102–104 J)—22.7%.
3.2. Seismic Risk Evaluation in the Light of Stress Forecasts
- Variability range of the vertical stress component (σz) in the region of mine workings is wide, ranging from 19.5 to 41.4 MPa (Figure 8);
- There are both destressed zones where the stress concentration factor (k) is in the range 0.79–1.0, alongside stress concentration zones where the value of k becomes as high as 1.68 (Figure 9).
- The most unfavourable stress conditions (of 30 MPa or more) prevail in the belt-shaped section of intact coal rock abutting the edge coinciding with the boundary of mined-out goafs in the coal seam 409-3; the impacts of remnants of previous mining operations in the coal seam 406-1 (and overlying) are relatively insignificant;
- In terms of critical stress analysis, the location of the heading Y#10 (coal seam 409-4) and the downcast gate Y#2 (in the coal seam 409-3/4) is quite favourable because the actual values of the vertical stress component and stress concentration factor along the face range only slightly differ from the lithostatic stress conditions (of the order of 24.2 MPa);
- Segments of the top headings (Y#4, Y#2) and of the bottom heading (Y#2) from the crossing with the heading Y#10 to the southern end of the pillar protecting drift F experience most unfavourable stress conditions due to the transverse edge of a goaf in coal seam 409-3.
- Heading Y#10, the forecasted maximal value of vertical stress component (σzmax) 27.5 MPa (stress concentration factor kmax = 1.11);
- Downcast gate Y#2, the forecasted maximal value of vertical stress component (σzmax) 26.3 MPa (stress concentration factor kmax = 1.07).
- Top heading Y#4 (the initial segment ~70 m in length, in the southward direction), the forecasted maximal vertical stress value (σzmax) 38.7 MPa (stress concentration factor kmax = 1.65);
- Bottom heading Y#4 (the final segment ~25 m in length, in the northward direction and the initial part ~5 m in length, in the southward direction), the forecasted maximal vertical stress value (σzmax) 39.0 MPa (stress concentration factor kmax = 1.64);
- Bottom heading Y#2 (the initial fragment ~20 m in length, driven in the southward direction), the forecasted maximal vertical stress value (σzmax) 39.6 MPa (stress concentration factor kmax = 1.62).
- Seismic hazard associated with roadheading operations in respective headings would vary, both in quantitative terms and in terms of energy potential. Depending on the progress status of the driftage, the number of recorded seismic events would range from 43 to 480. Energy release during the tremors was of the order of 105 J.
- According to the analytical forecasts, the state of stress in the investigated coal seam section was non-uniform, revealing the presence of both relieved (destress) zones and the stress-concentration zones. The actual location of anomaly zones was to a large extent associated with the prevailing mining conditions, including the impacts of previous mining operations (exploitation edges, old workings) in the adjacent seams. The horizontal impacts of the edges of coal seams 409-3 and 406-1 extend to about 70 m, covering the area between the downcast gate Y#2 and the southern boundary of the pillar protecting the drift F.
- In terms of totalled values of seismicity parameters, the most extensive seismic activity was registered during the driftage of the downcast gate Y#2. Alongside the entire working range 486 m in length (the shortest fully completed working) 480 seismic events were recorded with the energy potential approaching 2.5 × 106 J. Similar to other workings, the tremors of the order of 103 J would be predominant (76.5%) whilst a single tremor of 106 J was registered, too.
- Recalling the criteria based on the unit (averaged) calculation parameters, the highest levels of seismic hazard were registered during the driftage of top heading Y#4 (the initial segment ~70 m in length, driven southwards), bottom heading Y#4 (the ultimate segment ~25 m in length, driven northwards and the initial segment ~5 m in length driven southwards), as well as bottom heading Y#2 (the initial segment ~20 m in length driven southwards). Seismic events were few and far between (their energy potential averaging 2.9–13.1 × 103 J), registered with a frequency of less than 0.8 m of the working face advance, whilst the energy release per one running metre of the driven heading would range from 6.1 to 18.3 kJ.
- Locations of those sections of mine workings where the unit seismicity indicators proved to be the least favourable seem to coincide with stress concentration zones within the coal seam obtained by analytical forecasts, whilst roadheading operations in zones forecasted to be destressed gave rise to decidedly lower seismic activity, both in quantitative terms and in terms of their energy potential.
- Locations of registered rock mass tremor epicentres and foci seem to verify and confirm the adequacy of analytical forecasts predicting locally unfavourable state of stress in the vicinity of mine workings of concern. A lower degree of correspondence between forecast results and seismological data is found for workings driven along the belt-shaped elevated stress zone (heading Y#10, downcast gate Y#2).
- Unfavourable stress conditions in the coal seam are revealed by the rock strata response to active rockburst control measures. Concussion and destress blasting operations in the coal strata triggered the incidence of about 16% seismic events with energy potential in the range 103–104 J (79% of the entire population of recorded tremors). Tremors were found to have been more readily triggered in the stress concentration zones determined by analytical modelling.
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
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|Date||H||M||S||x||y||z||Energy||Panel Section||Coalseam||Excavation||Additional Remarks|
|1 May 2018||7||36||21||−49,690||−16,560||−620||2.2 × 104||Y||409-4||bottom heading Y#2|
|17 July 2017||0||45||52||−49,760||−16,950||−660||1.7 × 103||Y||409-4||top heading Y#4||SU|
|29 June 2017||23||44||11||−49,780||−16,950||−640||6.3 × 104||Y||409-4||top heading Y#4|
|8 April 2018||11||31||52||−49,640||−16,580||−710||3.4 × 102||Y||409-4||bottom heading Y#2|
|10 March 2018||4||17||28||−49,740||−16,880||−670||5.0 × 103||Y||409-4||heading Y#10|
|30 April 2018||10||57||19||−49,750||−16,760||−660||9.4 × 102||Y||409-4||bottom heading Y#4||ST|
|Heading Y#10||839.0/E||142 |
|Downcast gate Y#2||486.0/E||480 |
|Top heading Y#4||550.0/S||101 |
0.8 (70 m)
|Bottom heading Y#4||512.0|
507/N + 5/S
0.5 (30 m)
|Bottom heading Y#2||21.0/S||43|
|Site||N (Nw) with Given Energy|
|102 J||103 J||104 J||105 J|
|Heading Y#10||34 (2 SU)||106 (14 SU)||2||--|
|Downcast gate Y#2||88 (1 SU)||367 (36 SU)||24 (15 ST)||1 (1 ST)|
|Top heading Y#4||12 (1 SU)||65 (27 SU)||24 (9 SU, 5 ST)||--|
|Bottom heading Y#4/N||17 (3 SU)||41 (4 SU)||3||--|
|Bottom heading Y#4/S||11 (1 SU)||10 (3 SU)||1 (1 ST)||--|
|Bottom heading Y#2/S||13 (3 SU)||30 (14 SU)||--||--|
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