Blasting Methods to Minimise Rockfall in Open Pit Mines
Most of the large open pit mines employs only one type of blasting method: This is known as Production Blasting. Mines that have a high consequence for rockfall usually employ three additional blasting methods: Pre-split and Trim Blasting.
Production blasting: the main aim of this method is to break material in large volumes for extraction. In this type of blast the fragmentation of the rock is a key design objective and higher energies are generated when compared to limit blasting.
Trim and pre-split drill and blasting methods: the primary objective is to ensure a stable slope and minimise the occurrence of rock falls. Trim blasts are taken near the pit limits and have a reduced charge when compared to a production blast which helps to minimize the energy that is directed back into the sidewall. Pre-split blasts are taken on the design limit and prevent the gases and compressive energy from the trim blast from moving into and disturbing the rock mass that will make up the future pit slope.
During the short term planning process, limit blasts (pre-split and trim) must be anticipated. This is done by leaving approximately 20m between the limit line and production block. Pre-splits are to be blasted on the design limit. Pre-splits should always blasted before trim blasts and where possible before a production block is within 70m of the highwall. This is done to minimise the effect of blast energy on the highwall. The timing of each blast hole in the trim blast pattern is to be initiated individually to minimize energy going back to the highwall. Trims must be blasted within atleast 20m from the design limit. Trim blasts are designed in such a way that it has a reduced charge. This ensures that minimal energy is directed back into the sidewall. The slope angles were designed on the basis that trim blasting and pre-splitting will be applied.
No Sub-drill is to be allowed on trim blast holes and pre-split. This is to ensure that the crest of the next bench below the trim is not damaged. Loose material (overspill) or muck pile should not be left in front of trim blasts since this will influence the effectiveness of the energy distribution and will increase the potential for back break. Pre-split and trim blasting is an effective method to minimise the energy going back in the highwall and consequently minimises the potential for rockfall events.
Ernest Rankhododo (MSc. Eng.)
