Application of advanced blasting technologies for large scale de-stress blasts at Brunswick mine
Mining in highly stressed ground conditions has been an increasing challenge at Brunswick mine. After over 30 years of mining, there were two large regional pillars established on the 1000 m Level to support the ground for safe extraction of the surrounding ore. The South Regional Pillar contains about 5 Mt of ore and the North Regional Pillar (NRP) has about 3 Mt of ore.
In the fall of 2000, mining activities were suspended following a series of ground movements around the South Regional Pillar along weak geological structures in the West Ore Zone (WOZ). After extensive geotechnical investigations, a decision was made to carry out a mass de-stress blast in the WOZ to cut the principal stress across an area of 135 m along strike and 86 m high. To get enough void, stopes at both extremities (north end and south end) were blasted and mucked out prior to the mass blast. The mass blast itself had six blocks covering two sublevels with downholes and one section with upholes along the strike of the WOZ orebody.
To ensure the success of this critical blast of over 350,000 tonnes of ore, two key blasting technologies were applied: 1) DMC-blast (distinct motion code) simulation for delay timing design; and 2) electronic detonators (Orica’s i-kon detonators) to execute the designed initiation plan. Due to the complexity of this mass, DMC-blast had to be used to determine two important parameters: the firing speed between rings with ring width of only 6 m and the delay timing coordination between blocks to prevent any possibility of freezing the blast. To accomplish the designed blast delay sequencing, the best technology available was the new electronic detonators (i-kon) which gave 8,000 ms in total delay with a 1 ms increment. The final design for the mass blast called for 6,400 ms total delay time for the 2 m (burden) by 2.5 m (spacing) pattern with mostly 4.5 in. (114 mm) diameter blastholes.
After eight months of extensive preparation of rehabilitation and drilling, the WOZ mass blast was loaded in four weeks for 232,000 kg of bulk emulsion explosives and successfully initiated at the end of July 2001 with over 1,500 electronic detonators. The figure shows the firing sequence of the WOZ mass blast. Following this success, Brunswick mine decided to take a similar approach to de-stress the North Regional Pillar. For the NRP mass blast, one unique challenge was to muck out the high sulphide zones as fast as possible to prevent self-heating. For this purpose, DMC-blast code was again used to locate the high sulphide muck after blasting. A mucking strategy was then established based on the simulation results. The NRP final mass blast was also initiated with electronic detonators in early February 2003.
This paper describes the process in which advanced blasting technologies played an important role in solving a strategic problem for the mine operation. The experience gained from these two large-scale de-stress mass blasts can definitely be helpful when mining at depth to handle highly stressed ore zones while producing ore at the same time.