Materiały konferencyjne SEP 1992

Szkoła Eksploatacji Podziemnej * 92 459 8 l/m, if there is any seam reaction. A test drilling also is called critical if very strong seam reactions are noticed. The test dri 11 ings usually have to be made each day. For example, in a longwal1-face the holes are dri1led every 10 to 30 m. Spec i al test i ng programs are preseri bed for each area where there is a possible danger of outbursts. If the critical thresholds are exceeded or other criteria are fulfilled during a test drilling, then drilling operations are stopped immediately, The drilling site is then designated an "area of recognized danger of outbursts" and destressing opera- tions have to be carried out. The idea of the test drilling is to find out the position of the highly stressed zone. A situation is believed to be critical if this highly stressed zone lies very close to the face or the sidewall. Destressing operations are designed to move this zone deeper into the seam. In German coal mines, destressing usually is done by drill- ing. Only in special cases are high pressure infusion or destre- ssing blasting used. Destressing drilling is carried out with a crown of at least 95 mm in diameter. The depth dri 1 led is such as to ensure that the hole extends at least 3M beyond the face position after the next advance, where M is the seam thickness. The distance between destressing holes must not exceed 10 m. A number of special regulations are promulgated for destressing drilling operations, because they themselves could be dajigerous. For example, the workers have to stay at least 20 m away from the hole being drilled. The success of destressing is partially controlled by measuring the volume of coal dri 1 led out and by noting seam reactions. The decision whether the destressing was successful mainly depends on the results of subseąuent test dri11ings. 4. Measurements of Seismic Seam Reactions during Test Drillings The assessment of test drilling and the degree of success in destressing is partially based on the observed "seam reactions", such as sudden movements of the drilling rods, audible cracking noises, vibrations of the ground, etc. Obviously these seam re-