Question 4 of 6: Copper-Ore Final-Wall Control Program

Question 4: Copper-Ore Final-Wall Control Program (21 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) Wall control program – buffer row and final (presplit) row

Given.

Find. Burden, spacing and charging for a buffer row plus a final (wall control) row that limit overbreak/backbreak as the pit approaches its final wall.

Approach. Adopt the minimum permitted diameter (165 mm) for BOTH rows – a smaller final-wall diameter gives tighter spacing/burden and hence finer control, and sharing one diameter with the buffer row lets both be drilled with the same rig. Scale the buffer row's burden/spacing down from the production pattern by the diameter ratio (same similarity argument as Q2/Q3). Design the final row as a presplit line (the standard wall-control method for hard, massive rock, UCS 140 MPa) using the established smooth-blasting spacing rule and a lightly decoupled charge.

1. Buffer row (transition from production to wall control). Scale the 9 m production pattern down by the diameter ratio: $$\frac{De_{wall}}{De_{prod}}=\frac{165}{311}=0.531$$ $$B_{buf}=S_{buf}=9.0(0.531)=\boxed{4.77\ \text{m}\ (\approx4.8\ \text{m})}$$ This preserves the production powder factor at the smaller diameter, giving the buffer row a normal (not heavily reduced) charge that still breaks its own rock cleanly while beginning to step the pattern down toward the tighter final-row geometry.
2. Final (presplit) row spacing. Standard smooth-blasting/presplit guidance sets perimeter-hole spacing at 10–12×De: $$S_{final}=10\text{–}12\times0.165=1.65\text{–}1.98\ \text{m}\ \Rightarrow\ \boxed{S_{final}\approx1.8\ \text{m}}$$
3. Final-row burden from the buffer row. A presplit line is fired with little or no relief (often ahead of the whole round), so its own "burden" toward the buffer row is kept smaller than the buffer row's own burden to avoid over-stressing the presplit plane before it has done its job: $$B_{final}\approx0.8\times B_{buf}=0.8(4.77)=\boxed{3.82\ \text{m}\ (\approx3.8\ \text{m})}$$
4. Final-row (presplit) charge. Use a lightly decoupled column (small-diameter cartridge or detonating cord load well inside the 165 mm hole, decoupling ratio d_c/d_h≈0.2–0.3) rather than a fully-coupled production charge – enough energy to create a continuous shear/tension crack along the row without the overbreak-driving borehole pressure of a full charge: $$P_{borehole}=P_{detonation}\left(\frac{d_c}{d_h}\right)^{2.4}$$ Air-deck or leave the top ≈1–1.5 m uncharged to control collar fracturing (same mechanism discussed in Q1(e)).

Timing: fire the presplit (final) line FIRST – simultaneously or on a very short interval (0–3 ms via detonating cord) along its own length, before or with minimal lead-time ahead of the rest of the round – so the crack plane forms while the rock is still fully confined and undisturbed by adjacent blasting. The buffer row then fires as part of the main sequence (25–42 ms after the presplit line), with its charge and timing chosen so shock transmitted toward the already-formed presplit plane is kept low, protecting the final wall from further damage. The rest of the production pattern fires in its normal diagonal echelon sequence, retreating away from the wall toward the pit's open face.

(b) Sketch of loading and design

See Fig. Q4(b) above: three drilled rows are shown in cross-section approaching the final 80° pit wall – the existing 311 mm production pattern (full column charge, B=S=9 m), a 165 mm buffer row (full column, De-scaled burden ≈4.8 m) that steps the pattern down, and a 165 mm final row fired as a presplit line (tight 1.8 m spacing, decoupled charge with a top air-deck) directly against the intended final wall. The presplit row is drawn with a dashed fire line to indicate it is initiated first/independently of the main round.