Materiały konferencyjne SEP 1992

458 Underground Exploitation School '92 An area is called outburst-prone if, under simłlar circum- stances In the same seam, an outburst has occurred previously. The most important fact in unknown areas is the geo logi cal structure: the regulations specify that a working place may be dangerous with respect to outbursts» if there is a layer of sandstone or materiał with equivalent strength at least 5 m thick, less than 10 m above the seam. Together with this condi- tion» one of the following special mining or geological condi- tions also must be present: 1. start up of a face before settiement of the main roof, 2. face, or any excavation approaching a waste area, or another excavation, or a fault that is less than 40 m away, 3. extraction of residua! pillars, 4. residual pillars or working boundaries, less than 200 m above the excavated seam, 5. thick layer of sandstone, or any other materiał of equivalent strength, less than 5 m below the seam, 6. variable seajn thickness. These are the main features of the regulations. Whenever possible the mining is planned so that none of the ment 1 oned cr i ter i a exist. If this is not poss ible, the working area is defined as possibly outburst-prone. The mining office then sets up a special test-dri11ing procedure to obtain control data about the stress distribution and to define areas where destres- sing is needed. 3. Test-Drilling and Destressing Meihods The test-dri11ing-holes have a diameter of 42 to 50 mm. They are sited in the middle of the seam and depth drilled depends on t he t hi ckness of the seam (M) and t he dai 1y face advance (A). The hole depth is defined as (3M+A) in areas with mining or to (4M) in other areeis. The figures 1 and 2 show typical test- dri 11 ing-schemes for heading locations and for longwal1-faces. A test dril1ing is called critical if, within the defined depth, the amount of the coal dri 1led per meter exceeds a special threshold. This threshold is 15 l/m (liters per meter) without any seam-reactlon, such as audible cracking, or 6 to asaeas