Question 2 of 5: Support pressure for the Excavation Disturbed Zone (EDZ)

Question 2: Support pressure for the Excavation Disturbed Zone (EDZ) (20 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.

Drilling-and-blasting excavation always damages the rock beyond the intended profile: the detonation stress wave and expanding gas pressure open new radial and circumferential fractures in an annular zone of rock around the opening, and where those fractures cross pre-existing joints they isolate discrete blocks that are no longer keyed to the surrounding rock mass. This blast-fractured annulus is the Excavation Disturbed Zone. Inside it, a block bounded on all sides by open, unfilled, blast-induced or pre-existing fractures has effectively zero cohesion and (once separated) very little frictional resistance along those fracture surfaces — it is held in place only by its interlock with neighbouring blocks, which the blast itself has just destroyed. Once that interlock is gone, the block's only remaining equilibrium condition is gravity versus whatever support pressure is applied at the excavation boundary.

Given. Rock unit weight $\gamma=25\ \text{kN/m}^3$; the EDZ is defined by the blast having fully disaggregated a layer of thickness $t$ around the opening into a loose, cohesionless, gravity- loaded block (no interlock, no arching once separated).

Find. The support pressure required to hold the loosened EDZ block in place.

Approach. Take a unit plan area of the crown block. With zero cohesion across the bounding fractures and no arching (the block has already separated from its neighbours), the block's only load path to failure is free-fall under gravity; the support pressure needed is simply the block's own weight per unit plan area, applied at the excavation boundary before the block can move.

1. Free-body equilibrium of the loosened block. For a block of thickness $t$ and unit plan area $1\ \text{m}^2$, self-weight $W=\gamma \cdot t \cdot 1\ \text{m}^2$. With no cohesion or friction available along the bounding fractures (they are open, blast-created surfaces with no confinement holding them closed), the support pressure $p_s$ must balance this weight directly: $$p_s = \frac{W}{A} = \gamma\, t$$ This is the classical Terzaghi “dead-weight” or “loosening-pressure” design load — the lower-bound, most conservative case for roof support in blocky ground, since it assumes no arching redistributes any of the load to the surrounding rock.
2. Illustrative numeric value. The EDZ thickness $t$ is not stated in this question (it depends on blast design — hole spacing, delay timing, presplitting/smooth-wall blasting practice, and the intact joint spacing); a commonly adopted design value for a controlled drill-and-blast excavation is $t\approx 0.5$ m (flagged explicitly below as an assumption, not given data). $$p_s=\gamma t = 25\ \text{kN/m}^3\times 0.5\ \text{m}=\boxed{12.5\ \text{kPa}}$$