Trim blasting is a control technique which is used to clean up a final wall after production blasting has taken place. The production blasting may have taken place many years earlier or could have taken place on an earlier delay within the same blast.
Trim blasting has relatively few requirements for successful implementation. Trim blasting is used in conjunction with large-diameter cartridge charges taped on detonating cord at predetermined intervals. These trim blast holes are often fired simultaneously or in groups to maximize the amount of charge per eight millisecond (8 ms) delay intervals. The final requirement for trim blasting is the drilling and initiation of the blast. Typically, trim blasts are drilled and initiated after the primary production blast has occurred. This makes trim blasting unique when com pared to other controlled blasting techniques since it allows for full observation of the geology of the excavation line prior to drilling and loading of the holes. This reduces the number of assumptions that are required to design the trim blast when compared to other controlled blasting techniques (Eades & Perry, 2019).
Manyofthe limitations of trim blasting are shared with smooth wall blasting. Because trim blasting uses decoupled explosives, drill hole accuracy is critical to the success of the operation. Bore hole deviations can cause excessive burdens throughout the exca vation line, which reduces the overall effectiveness of the trim blast. Trim blasting is also dependent upon the ‘‘pseudo final exca vation line” that was left after the primary blast. The excavation line will have variable burdens and will require a unique design each time to ensure a controlled excavation line after the trim blasting operation is complete (Konya & Walter, 1990). In addition to these concerns, the primary limitation for trim blasting is related to the production and scheduling of blasts. Because trim blast is conducted after the primary production blast, additional time is required to move the drill rigs back on to the bench and drill holes for the controlled blast shot (McKenzie & Holley, 2004). This creates a delay in preparations for the next pro duction blast. This fact raises concerns with work and equipment safety. There will be a period of time where the personnel and equipment are working and operating near a highwall or an under ground heading where no measures have been taken to reduce or control overbreak. Due to these issues, trim blasting is rarely cho sen as the controlled blasting technique for many operations (Eades & Perry, 2019).
Reference
Eades, R. Q., & Perry, K. (2019). Understanding the connection between blasting and highwall stability. International Journal of Mining Science and Technology, 29(1), 99–103. https://doi.org/10.1016/j.ijmst.2018.11.016
Konya, C. J., & Walter, E. (1990). Surface Blast Design. https://www.semanticscholar.org/paper/Surface-Blast-Design-Konya-Walter/1f5ad0b76b760559079338b7c89053406a3a5755
McKenzie, C., & Holley, K. (2004). A study of damage profiles behind blasts. Proceedings of the Annual Conference on Explosives and Blasting Technique, 2, 203–214. https://miningandblasting.wordpress.com/wp-content/uploads/2009/09/a-study-of-damage-profiles-behind-blasts_mckenzie_holley.pdf
