Question 5 of 6: Final-Wall Control – Buffer and Presplit Rows

Question 5: Final-Wall Control – Buffer and Presplit Rows (15 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Buffer zone and final (presplit) row design

Given. Production pattern 7 m × 9 m at 311 mm; copper ore ρ=2.7 g/cm³, UCS=140 MPa, V_p=5 km/s; buffer hole De=175 mm; final row De=100 mm; 80° face slope; dry.

Find. Burden/spacing, explosive and timing for a buffer row (transition from full production charge) and a final presplit row (controls the 80° wall) that together minimise damage to the remaining pit wall.

Approach. The buffer row absorbs and dampens the stress wave from the last full production row before it reaches the wall, using a reduced (not full-production) charge; the final row is a true presplit line, fired ahead of the buffer/production blast so its crack forms in undisturbed rock and then acts as a shear plane that the subsequent blast breaks up to – both rows use the same S=12D_h smooth-blasting spacing guideline as Question 3(a), scaled to their own (larger) diameters.

1. Buffer row (175 mm) – reduced production charge. $$S_{buf}=12(0.175)=2.10\ \text{m},\qquad B_{buf}=\frac{S_{buf}}{0.9}=\boxed{2.33\ \text{m}}$$ Load with the same emulsion/ANFO blend as the main production pattern but at ≈70–80% of the full production powder factor (a reduced-charge, fully-stemmed column, not decoupled) – enough to break and displace the buffer rock cleanly while transmitting materially less peak stress toward the final wall than a full-strength charge would.
2. Final (presplit) row (100 mm) – decoupled trim line. $$S_{fin}=12(0.100)=\boxed{1.20\ \text{m}},\qquad B_{fin}\approx S_{fin}=1.20\ \text{m}$$ (burden ≈ spacing is the standard presplit proportion, unlike the buffer/production rows). Load with small-diameter decoupled emulsion cartridges (as in Question 3a) at a low linear charge concentration, lightly stemmed, sufficient only to propagate a shear crack between adjacent presplit holes.

Sequence, timing and justification. The presplit (final) row fires first, well ahead of (or as the leading delay period of) the same round, with all presplit holes on the same delay so the crack forms simultaneously along the whole final-wall line while the rock is still fully intact and undisturbed – this is what lets the crack propagate cleanly along the drilled line rather than following whatever stress concentrations an already-blasted buffer zone would introduce. The buffer row fires next, breaking normally but at reduced charge, with its own stress wave now attenuated by the presplit crack acting as a partial reflective boundary, so materially less energy reaches beyond the final wall line than an unmodified production blast would deliver. The production rows behind the buffer fire last, progressing away from the wall in the usual echelon sequence. Reduced buffer charge (not full production charge) is justified because the buffer row's job is displacement/breakage of rock that is ALREADY isolated from the final wall by the presplit line, not final-wall protection itself – protection is the presplit line's job; the buffer row's only remaining requirement is to avoid re-loading the presplit crack with excessive stress from behind.

(b) Design sketch