Blast Design for a Drop Raise: 24-MMP-B2 Rock Fragmentation, December 2015, Q7

Question 7: Blast Design for a Drop Raise (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.

Given. Raise 30 m long, 2.4 m × 2.4 m cross-section, accessed and drilled from the top level; borehole diameter 89 mm; emulsion ρ=1.25 g/cm³.

Approach. Because the raise is drilled from the top down and no bottom-level access is described, the recommended method is a long-hole, retreat-fired drop raise (closely related to Vertical Crater Retreat practice): parallel 89 mm holes are drilled the full 30 m length from the top, then fired in short deck increments starting from the bottom (the free end, wherever it breaks into an existing opening/drift below) and retreating upward, so each successive deck always has a free face to break to once the deck below it has already fallen away.

Hole pattern. A 2.4 m×2.4 m opening drilled with 89 mm parallel holes is well served by a 3×3 grid at ≈0.8 m spacing (fits the 2.4 m side exactly in three 0.8 m increments), giving 9 parallel holes for the full raise length.
Linear charge concentration. $$q_L=\frac{\pi}{4}(0.089)^2(1250)=\boxed{7.78\ \text{kg/m}}$$
Deck length per retreat increment. Following standard VCR-style design practice, deck length is set approximately equal to the burden for this hole size, ≈1.0-1.2 m, giving a charge mass per deck per hole of $$m_{deck}\approx7.78\times1.1\approx8.6\ \text{kg}$$, with stemming (drill cuttings or fine sand, ≈0.8-1.0 m) separating each deck from the next above it.
Number of retreat increments. Over the full 30 m length at ≈1.1 m/deck (plus stemming), roughly 13-15 sequential retreat blasts are required to fully develop the raise, each mucked out (where access allows) or allowed to gravity-fall into the opening below before the next increment is fired.

Quantity	Value
Hole pattern	3×3 parallel 89 mm holes, ≈0.8 m spacing
Linear charge concentration	7.78 kg/m
Deck length / charge per deck-hole	≈1.1 m / ≈8.6 kg
Retreat increments over 30 m	≈13-15

Delays and sequencing. Each retreat increment fires the lowest not-yet-broken deck across all 9 holes essentially simultaneously (or on a very short delay progressing from the centre hole outward, to help draw the broken rock toward the raise centreline), so it breaks freely into the void left by the increment fired below it; the next increment up is not fired until the broken rock from the current one has cleared (by gravity fall down the raise, or by mucking, depending on access), preserving a free face for every successive blast – this is the same working principle as Vertical Crater Retreat mining applied to raise development rather than stope production.

Check: the 3×3/0.8 m hole pattern, ≈1.1 m deck length and 9-hole count are reasonable design choices for an 89 mm-hole raise of this cross-section and are not uniquely specified by the source, which gives only the raise geometry, hole diameter and explosive. No rock strength/RQD data are given, so the deck length follows the standard VCR rule-of-thumb (deck≈burden) rather than a site-specific optimisation.

Topic: Long-hole (VCR-style) drop-raise development from a single top-level access.

Key relations: $$q_L=\frac{\pi}{4}D^2\rho$$; VCR deck length ≈ burden; retreat-from-a-free-face sequencing.

Why this works: without a bottom-level opening to blast toward directly, raise advance must be built in retreat increments so every blast, including the very first, has a genuine free face – the first increment breaks toward whatever opening exists at the raise's bottom terminus, and every subsequent increment breaks toward the void the previous one created.

Common pitfall: attempting to fire the entire 30 m column in one blast, which has no free face for the upper decks to break toward and would produce severe confinement, poor fragmentation, and likely charge failure/sympathetic detonation problems.

Source: Hustrulid & Bullock (eds.), Underground Mining Methods, Ch. on Vertical Crater Retreat and long-hole raising; SME Mining Engineering Handbook, 3rd ed., Ch. Raise Development.

Question 7 of 7: Blast Design for a Drop Raise

Question 7: Blast Design for a Drop Raise (15 marks)