Numerical modeling of destress blasting for strata separation

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 93e100 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 a 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.

Geomechanical effects of stress shadow created by large-scale destress blasting

This study aims to determine if large-scale choked panel destress blasting can provide sufficient beneficial stress reduction in highly-stressed remnant ore pillar that is planned for production. The orebody is divided into 20 stopes over 2 levels, and 2 panels are choke-blasted in the hanging wall to shield the ore pillar by creating a stress shadow around it. A linear-elastic model of the mining system is constructed with finite difference code FLAC3 D. The effect of destress blasting in the panels is simulated by applying a fragmentation factor(α) to the rock mass stiffness and a stress reduction factor(β) to the current state of stress in the region occupied by the destress panels. As an extreme case, the destress panel is also modeled as a void to obtain the maximum possible beneficial effects of destressing and stress shadow.Four stopes are mined in the stress shadow of the panels in 6 lifts and then backfilled. The effect of destress blasting on the remnant ore pillar is quantified based on stress change and brittle shear ratio(BSR) in the stress shadow zone compared to the base case without destress blasting. To establish realistic rock fragmentation and stress reduction factors, model results are compared to measured stress changes reported for case studies at Fraser and Brunswick mines. A 1.5 MPa immediate stress decrease was observed 20 m away from the panel at Fraser Mine, and a 4 MPa immediate stress decrease was observed 25 m away at Brunswick Mine. Comparable results are obtained from the current model with a rock fragmentation factor α of 0.2 and a stress reduction factor α of 0.8. It is shown that a destress blasting with these parameters reduces the major principal stress in the nearest stopes by 10-25 MPa.This yields an immediate reduction of BSR, which is deemed sufficient to reduce volume of ore at risk in the pillar.

Seismic evaluation of the destress blasting efficiency

In this paper, selected methods of destress blasting efficiency assessment are presented, and novel quantitative methods based on in situ seismic measurements are proposed. The newly formulated solution combines two different approaches. The first, which is useful mostly for the near-field seismic analyses, is based on the analysis of seismic amplitude characteristics, and the second, relevant for farfield evaluation, is extended by the duration and frequency of the seismic wave. Both approaches are based on the seismic analyses of the waveforms generated by blasting recorded by the local seismic network. The proposed solutions are tested and validated in deep underground mines in Poland in which the room-and-pillar mining method is applied. Based on performed analysis, it is shown that both methods may be used as a rockburst hazard control in underground mines. However, developed methods may also be successfully implemented in other engineering practices, including the assessment of seismic vibrations in open pits and quarries.