As open pit mines grow deeper, it becomes necessary to have more inclined slopes to ensure profitability and avoid excessive wastage of material. However, steep highwalls increase the geotechnical hazards, such as slope instability, crest destruction, and accidental rock falls (Tasoren et al., 2016). Pre-splitting blasting is a technique used to protect the final pit walls from deterioration due to blasting operations. Pre-split blasters make a series of holes near the planned boundary line in a linear pattern, charge them lightly and blast them prior to blasting the main charge. Pre-split blasting creates a continuous tensile crack line which acts as a boundary buffer zone. This artificial crack line will reflect the shock waves and release the explosive gases away from the rock mass (Hustrulid, 2006). Proper optimization of the pre-split blasting technique will create stable and unharmed steep highwalls.
Wall protection requires tailored geometrical configurations and controlled explosive energy distribution. The traditional industry recommendations state that the hole spacing should be within 8-to-12-hole diameters, while modern thermodynamic models have shown the existence of a direct correlation between hole spacing and Blast-Induced Damaged Zone (BiDZ), with estimated hole spacing between 1.5 to 3.0 times the BiDZ radius (Yim et al., 2026). Fully coupled explosives result in detonation pressures higher than rock compressive strength, thus leading to local crushing of the walls. The use of decoupled explosive charges to maintain the diameter ratio of 25% to 33%, as well as air decks, lowers borehole wall pressure under the threshold of compressive strength while keeping hoop stress enough to break rock web (Hustrulid, 2006; ERG Industrial, 2025). The split factors vary between 0.30 to 0.60 kg/m² for usual rock formations and can even reach up to 0.90 kg/m² for hard rocks. Boreholes inclined at 15° to 30° angles from the vertical are aligned with the final bench slopes, which prevents overhanging crests.
The timing and control of adjacent charges impact the effectiveness of the pre-split fracture plane. Synchronized initiation of pre-split holes with no delay results in the synchronization of the stress wave and pressure gases along the split plane (Rouse, 2021). Electronic detonators reduce timing variation, while group delay spacing or millisecond dogbone relays decrease Peak Particle Velocity (PPV) without affecting the split plane in vibration-sensitive areas (Verma, 2008; ERG Industrial, 2025). Besides, pre-split planes need special buffer hole rows to protect the highwall from backward production energy. Buffer holes need to be spaced away from the pre-split boundary at about 10 hole diameters. Decrease in the charge weight in buffer holes by 30% to 50% along with the absence of sub-drilling and delay spacing reduces horizontal energy, thereby directing the explosive energy towards the open pit wall (Verma, 2008; Narayan et al., 2026).
The optimum field performance is achieved through perfect execution combined with specific adaptations at the site. Collar misalignment or borehole deviation beyond ±150 mm leads to design spacing being disrupted resulting in overbreak or undersplitting (ERG Industrial, 2025). Geological discontinuities aligned along the wall lead to fracture stopping, requiring closer spacing and lighter charges in case of jointed rock masses. Boreholes filled with water improve energy transfer to the rock mass; field crews are advised to reduce powder factor by 25% to 50% or use water resistant decoupled charges to prevent radial cracking (Chiappetta, 2014). Field quality control involves audit of half barrel trace percentage, photogrammetric bench profile, and seismic PPV. Feedback loops allow for improvement of blasting parameters with increasing pit depths.
The optimal design of the pre-split blast at deep open pits is an exercise that requires a combination of geometry, timing and quality control in the field. The use of charge decoupling, borehole spacing and buffer-row sequence delays helps to reduce back-break. It is important for there to be strict controls in drilling and geotechnical audits to maintain stable benches, reduce scaling costs and facilitate increased sloping.
References
Chiappetta, R. (2014). Presplitting and Highwall Controls: 12 Rules to Guarantee Results. Proceedings of IQPC Drill & Blast Africa, Johannesburg, South Africa.
ERG Industrial (2025). Pre-split Blasting in Mining, Quarrying and Construction. Technical Guide & Pre-Split Blasting Calculator.
Hustrulid, W. (2006). A Practical, Yet Technically Sound, Design Procedure for Pre-Split Blasts. Spokane Research Laboratory, NIOSH / CDC.
Narayan, K., Prasad, S. S., Kumar, G., & Kumar, S. (2026). Evaluation of Controlled Blasting Techniques for Reducing Ground Vibrations in Surface Mining. Indian Journal of Modern Research and Reviews, 4(5), 125-128.
Rouse, N. / Strayos (2021). Guidelines for Perimeter Control & Pre-Split Design. Strayos Technical Webinar.
Tasoren, K., Gardhouse, G., & Ran, J. (2016). Lessons in slope stability management from Kinross’ Tasiast mine, Mauritania. Proceedings of 3rd International Symposium on Mine Safety Science and Engineering (ISMS 2016), Montreal, Canada, 115-120.
Verma, H. K. (2008). Controlled Blasting Techniques for Development of Road Infrastructures in Hilly Terrain. CSIR-Central Institute of Mining and Fuel Research (CIMFR).
Yim, J., Jung, J. H., Kang, H. B., & Shin, Y. J. (2026). Design and Field-Scale Demonstration of Pre-Fracturing with a Practical Model for the Size Estimation of Blast-Induced Damaged Zone. Archives of Mining Sciences, 71(2), 199-218.
