Numerical modeling of destress blasting for strata separation

Abstract

Destress blasting (DB) implemented along the perimeter of safety pillars is a special application of destressing in coal longwall mining. The goal is to separate relatively more deformed mined areas from safety pillars, such as shaft pillars or cross-cut pillars, to reduce the transfer of high stresses to the protective pillar. This case study aims to numerically simulate selected destress blasts in the Czech part of the Upper Silesian Coal Basin and examine its impact on stress transfer to the safety pillar area. To separate the area between the protective pillar and the longwall (LW), two fans of five 93-mm blast holes (length of 93–100 m) were drilled from the gate roads into the overburden strata. Each set of blast holes was fired separately in two stages without time delay. The explosive charge (gelatin-type of explosive) of each stage is 3450 kg. The two DB stages were fired when the longwall face was approximately 158 m and 152 m away from the blast. A 3D mine-wide model is built and validated with in situ stress measured with hydrofracturing. Mining and destressing in three 5-m thick coal seams are simulated in the region. Numerical modeling of DB is successfully conducted using a rock fragmentation factor α of 0.05 and a stress reduction/dissipation factor β of 0.95. Buffering of transfer of additional stress from the mining area into the safety pillar is evaluated by comparison of yielding volume before and after DB. It is shown that yielding volume drops after DB by nearly 80% in the area of the destressing panel and near the safety shaft pillar.

Keywords

• Rockburst hazard
• Destress blasting (DB)
• Strata separation
• Safety pillar
• Numerical modeling
• Fragmentation factor