Blasting Design Without Subdrilling on Jointed Limestone to Optimize Fragmentation and Blasting Cost

Bench blasting operation on limestone opencast mining at PT. Semen Padang (Persero), Indonesia performed by electric blasting using coloumn loading system. Limestone deposit consist of three type of rock characteritics that are hard limestone, sugary limestone, and jointed limestone. In general, a good blasting design should implementing subdrilling to avoid toe. In this case of study the subdrilling is not need due to the present of many cracks caused by basalt intrusion. Blasting geometry that applied to blast the jointed limestone resulting blasting fragmentation that commonly relatively small in size (<60 cm) but still has some boulders (>60 cm) that caused by coloumn loading system where energy was concentrated in the bottom. Based on field observation of limestone blasting operation and productivity of excavator being used that is Excavator Hitachi EX 1100 with 5.4 m3 bucket capacity, can be optimized. Especially from the view of drilling and blasting efficiency by modifying blasting geometry and explosive usage to optimize fragmentation to the productivity of excavator being used. Modification that performed is eliminate the sub drilling and change the explosives loading system from coloumn loading to deck loading (include amount of explosives and detonator being used). Based on result of modification, achieve decreasing of boulder percentage as much as 50.25% that is from 9.71% to 4.83% and increasing of excavator productivity up to 36.10% from 1,479 ton/hour to 2,013 ton/hour as well as decreasing blasting cost up to 17.18% from Rp 3,456/ton to Rp 2,862/ton.