Question 5 of 6: Underground Round Design – Burn Cut, Drop Raise and Ring Blast

Question 5: Underground Round Design – Burn Cut, Drop Raise and Ring Blast (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) Burn cut design for a 42 mm tunnel round

A burn (parallel-hole) cut is the standard choice when all holes must be drilled parallel to the tunnel axis (as opposed to an angled fan/wedge cut) – it relies on one or more large, empty (or lightly-loaded) relief holes at the centre of the cut, into which the surrounding loaded holes break in successive, expanding rings.

Design. Drill a single central relief hole left uncharged (a common, economical burn-cut variant when a larger-diameter relief bit is not available uses one of the same 42 mm holes, left empty, as the relief hole). Arrange the charged 42 mm holes in 2–3 expanding squares/rings around it, with the first ring's burden approximately 1.5× the relief hole diameter and each successive ring's burden expanding by roughly 1.5–2× the previous ring's burden (the standard parallel-hole-cut progression, Holmberg's design method) – giving first-ring holes at ≈65 mm from the relief hole, second-ring holes at ≈150 mm, and a third ring (if the round depth/face size calls for it) at ≈300 mm, growing the cut to roughly 0.6–0.8 m across before the surrounding stoping/lifter/rib holes take over the rest of the face.

Sequence and timing. Fire the innermost ring first (into the single empty relief hole's void), then each successive ring in order, outward, on progressively longer delays (e.g. millisecond delays of roughly 0, 25, 50, 75 ms for rings 1–4) so each ring has time to break and relieve into the growing cavity left by the ring before it – firing two rings simultaneously, or out of the centre-outward order, chokes the cut (no void to break into) and is the most common cause of cut failure.

Problems that may arise. Hole deviation is the dominant risk at small (42 mm) diameter and typical tunnel round lengths (2–4 m) – even a small angular error closes or widens the gap to the relief hole enough to choke the cut or blow out prematurely; insufficient relief-hole volume for the charge broken by the first ring (a burn cut needs roughly 1.5–2× the relief hole's own volume of swell space per ring) chokes the round and can leave a "bootleg" (unbroken cut, requiring re-drilling); sympathetic desensitisation between closely-spaced, near-simultaneous holes (Q1(c)) can dead-press an unfired hole in the next ring before its own delay arrives; and delay scatter on pyrotechnic detonators can fire two rings close enough together to choke each other even when designed sequentially.

(c) 2 m × 2 m drop raise, 102 mm holes, emulsion 1.25 g/cm³

Given. Raise cross-section 2 m×2 m, blasthole diameter 102 mm, explosive: emulsion, ρ_e=1.25 g/cm³.

Find. A per-lift round design (pattern, loading, sequence, timing) for a drop raise, where broken rock falls by gravity to a drawpoint on the level below (no mechanical mucking inside the raise).

Approach. Same burn-cut principle as part (a), scaled to the 102 mm diameter and the small 2×2 m section: one central empty relief hole plus one expanding ring of loaded holes at the four corners (and, section permitting, mid-side holes), fired centre-out on delays; charge each loaded hole from its own column length and the given explosive density.

1. Pattern. One 102 mm empty relief hole at the section centre; four loaded 102 mm holes at the corners of the 2 m×2 m section (burden to the relief hole ≈1.0–1.2 m, i.e. roughly the corner-to-centre half-diagonal), plus four mid-side loaded holes between the corners and the raise perimeter for full-face breakage – nine holes total per lift, a standard small-section burn-cut layout.
2. Charge per hole. Adopting a practical per-lift round length of 3 m (a conservative advance for 102 mm long-hole drilling without excessive deviation risk in a raise): $$Q_{hole}=3.0\times\frac{\pi}{4}(0.102)^2\times1250=\boxed{30.6\ \text{kg per fully-coupled hole}}$$ Corner holes closest to the relief hole may be decked (reduced charge in the lower ≈1/3 of the column) to avoid overbreaking the small relief void prematurely; perimeter holes carry the full 30.6 kg column.
3. Sequence and timing. Fire the four corner holes first (shortest delay, ≈0–25 ms) into the empty relief hole, then the four mid-side/perimeter holes on a longer delay (≈50–75 ms) once the corners have opened a full-section void – the same centre-out logic as the tunnel burn cut, adapted to a square raise section. Muck is allowed to fall clear through the drawpoint below before the next lift's round is drilled and charged.

(d) Ring blast – firing two rings per blast

Given. Ring-blast fan pattern (Figure 3) drilled from a drawpoint/access drift, fan height 18 m; firing two rings per blast.

Loading. Load each fan hole from its toe (the deepest, most confined point, farthest from the drawpoint) with a full or near-full emulsion column, leaving the collar ≈1–1.5 m unloaded/stemmed for control – the shorter, more acutely-angled holes near the fan's edges may be lightly decked to avoid over-breaking near the drift back/walls.

Initiation and timing. Within Ring 1, initiate the toe of the longest, most central hole first and sequence outward/upward toward the collar holes on progressively shorter delays (e.g. 17, 25, 42, 67, 84 ms as marked in the figure, symmetric about the central hole) – each hole breaks into the void created by the one fired immediately before it, giving a clean retreating break toward the drawpoint. Once every hole in Ring 1 has fired, initiate Ring 2 as a whole on a longer delay (≈100–150 ms after Ring 1's last hole) so the full burden between the two rings has time to relieve before Ring 2 fires; Ring 2's own holes are timed toe-to-collar in exactly the same progression as Ring 1. Firing two full rings (rather than one) per blast advances the stope by two ring-burdens per round, improving productivity, at the cost of a larger single-blast tonnage/vibration event that must itself be checked against any nearby vibration limit (Q6).