Question 4 of 7: Open-Pit Blast – Delay Pattern and Loading Design

Question 4: Open-Pit Blast – Delay Pattern and Loading 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) Delay pattern for shovel-loadable muckpile with wall protection

Approach. Rope shovels dig most efficiently into a tall, well-cast, loose muckpile (unlike a hydraulic excavator, which prefers a flatter, less-cast pile), so the design goal is to progressively cast rock toward the free face while isolating the final wall from the main blast's stress wave.

1. Wall control (100 mm) row fires first, well ahead of the rest of the round, as a light presplit/trim line – this forms a continuous shear plane along the final wall before the main event, so later stress waves from the buffer and main blast are reflected/attenuated at this pre-formed crack rather than propagating into the remaining pit wall.
2. Buffer (160 mm) row fires next, on a moderate delay, acting as a cushion between the wall-control line and the main blast – it relieves toward the (now presplit) wall side while beginning to move rock toward the free face.
3. Main blast (254 mm) rows fire progressively toward the free face, each successive row initiated slightly after the row ahead of it (a V or echelon-style delay pattern moving away from the wall control line, toward the open pit floor/free face), so each row always breaks into the void the previous row has already opened.

Net muckpile movement: rock is progressively displaced away from the (protected) final wall and toward the free face, building a taller, well-cast, loosened pile on the loading side that suits rope-shovel digging, while the pre-split final wall behind the blast sees only the much-reduced stress wave that has already been decoupled by the presplit crack.

(b) Loading design for the three hole diameters

Given. Main D=254 mm, buffer D=160 mm, wall control D=100 mm; explosives ANFO and ANFO-emulsion blends for main/buffer, free-face slope 80°.

Approach. Main holes take full-strength, fully coupled ANFO for maximum production energy; buffer holes step down to a denser ANFO-emulsion blend for a controlled transition; wall-control holes use a decoupled emulsion cartridge for smooth-blasting perimeter control, since ANFO cannot be radially decoupled the way a cartridge product can.

1. Main blast, ANFO, ρ=0.85 g/cm³, D=254 mm – fully coupled linear charge concentration. $$q_{L,main}=\frac{\pi}{4}(0.254)^2(850)=\boxed{43.1\ \text{kg/m}}$$
2. Buffer, 50/50 ANFO-emulsion blend, ρ≈1.0 g/cm³, D=160 mm. $$q_{L,buffer}=\frac{\pi}{4}(0.160)^2(1000)=\boxed{20.1\ \text{kg/m}}$$ – roughly half the main hole's concentration, providing a graded transition in applied energy between the full-strength main blast and the light wall-control line.
3. Wall control, decoupled emulsion cartridge, 65 mm diameter in the 100 mm hole, ρ=1.2 g/cm³, VOD=4500 m/s. Detonation pressure (fully coupled reference): $$P_d=\frac{1.2\times4.5^2}{4}=6.08\ \text{GPa}$$ Decoupled borehole wall pressure ($$P_b=P_d(d_c/d_h)^{2.4}$$, decoupling ratio 65/100=0.65): $$P_b=6.08\times(0.65)^{2.4}=\boxed{2.16\ \text{GPa}}$$ – a roughly 3-fold reduction from the fully-coupled value, keeping the wall-control holes' energy well below the main/buffer rows so they trim, rather than shatter, the final wall.
4. Wall-control linear charge concentration (65 mm cartridge string). $$q_{L,wall}=\frac{\pi}{4}(0.065)^2(1200)=\boxed{3.98\ \text{kg/m}}$$ – more than 10× lighter than the main holes, consistent with standard smooth blasting practice.