Impact of excavation technique on strength of oil shale pillars

Summary The load carrying capacity of oil shale pillars excavated by conventional blasting can be increased significantly by presplit blasting and mechanical mining. Comparisons ofin situ vertical stresses and fractures obtained from overcoring horizontal holes in the Colony Mine, Piceance Creek Basin, Colorado indicate that conventional blasting causes a strength loss in a zone of damage approximately 3 m (10 ft) thick. Presplit blasting reduces damage significantly, and increases the load carrying capacity in the 3 m (10 ft) thick zone by 5.93 MPa (860 psi). Mechanical mining causes little or no rock damage, and an increase of 9.83 MPa (1425 psi) in strength in the same 3 m (10 ft) thick zone. Pillar design using presplit blasting and mechanical mining techniques can increase the extraction ratio by at least 3% and 5%, respectively, as compared to conventional blasting. It is speculated that comparable increases in extraction should also occur due to increases in span dimensions.