Question 3 of 7: Underground Drift Cut Design

Question 3: Underground Drift Cut Design (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) Cut design for three successive rounds

Given. Drift 6.0 m (W) × 4.7 m (H), all holes 51 mm diameter, advance ≥5 m per round.

Approach. With only 51 mm holes available (no larger relief-hole diameter is specified), the round uses a classical expanding-square parallel (burn) cut: one 51 mm hole left uncharged as the relief void, with charged 51 mm rings expanding outward around it, each ring breaking into the void the previous ring created.

1. Hole depth for ≥5 m advance. Practical drilling/charging efficiency for a parallel cut is typically ≈90-95% of drilled depth; drilling $$L_{drill}=\frac{5.0}{0.95}\approx5.3\ \text{m}$$ gives a realised advance of ≈5.0 m per round.
2. Ring 1 (4 holes). First-ring burden for a single 51 mm empty hole, b₁≈1.5×D_empty: $$b_1=1.5\times51=76\ \text{mm}$$ – tight enough to reliably break into the empty hole rather than crater around it.
3. Ring 2 (4 holes). $$b_2\approx2\times b_1=153\ \text{mm}$$, breaking into the enlarged Ring-1 void.
4. Ring 3 (4 holes). $$b_3\approx3.4\times b_1=260\ \text{mm}$$, growing the cut opening to roughly 0.5-0.6 m square – a sufficient free face for the stoping/relief holes.
5. Remaining round. Stoping (relief) holes at a standard tunnel burden/ spacing (≈0.7-0.8 m for 51 mm charge holes) break the bulk of the 6×4.7 m face outward from the enlarged cut opening; perimeter (contour) holes trim the profile using smooth blasting (light, spaced/decoupled charge, S≈10-12×D_h≈0.5-0.6 m); floor lifters (angled slightly downward/outward) are the most heavily loaded to throw the muck clear and break the floor cleanly to grade.

All three successive rounds re-drill the identical cut geometry once the previous round's muck is cleared and ground support (if required) is installed – the cut design itself does not change round to round since the face geometry and rock conditions are assumed unchanged; only the ring/stoping/perimeter hole collars are re-marked at the new face.

(b) Delays and explosive selection

Explosive of choice: emulsion cartridges (small diameter, e.g. 25-32 mm, in the 51 mm holes) for the cut and stoping holes – emulsion's small critical diameter and water resistance make it reliable in tightly spaced, potentially wet underground holes, unlike ANFO which is vulnerable to water and has a larger critical diameter at small charge sizes. Modern electronic detonators are used for full programmable control of the many closely spaced delay periods a burn cut requires.

The empty relief hole is never charged. Firing proceeds strictly centre-out so each ring always breaks into an already-open void: Ring 1 on the earliest delay (≈0-25 ms), Ring 2 next (≈25-50 ms), Ring 3 (≈50-75 ms); stoping/relief holes follow on progressively later delays working outward from the enlarged cut toward the drift walls (≈100-250 ms); perimeter/contour holes fire on a late, closely spaced delay group (≈250-300 ms), lightly charged for smooth blasting so they trim the profile without over-breaking already-relieved rock; floor lifters fire last (≈300-350 ms+) so the whole face has already broken and can be thrown forward, clear of the round's own muckpile. A short, consistent delay interval (≈25 ms) between successive rings/holes is used throughout to keep each stage's rock movement well separated from the next, minimising choke and cut-off risk in the tightly spaced cut.