Load-Out Bay Widening in Weak, Jointed Ground
During widening of a 5.0 m x 5.5 m decline into a 9 m wide load-out bay, a 1.5 m thick slab of crown rock fell between existing bolts in altered andesite (UCS about 25 MPa). Two joint sets control the failure: one smooth, clay-coated set striking across the drive at 45 degrees, and a second set at 70 degrees with 0.3-0.6 m spacing. The excavation is 240 m below surface in a modest stress regime. Current support is 2.4 m fully resin-encapsulated bolts at 1.2 m spacing with 50 mm fibre-reinforced shotcrete; the shotcrete cracked but did not hold the slab. Convergence pins across the bay read 0.6 mm/day at day 3, 0.7 mm/day at day 7 and 0.8 mm/day at day 10, all in the same direction. Production needs the bay in six weeks.
1. Do you continue widening the bay? Support your answer with the monitoring data.
Read the worked answer
No - stop widening.
- A stable excavation shows decelerating convergence that flattens out within one to three weeks. Here the rate is constant to slightly rising (0.6 to 0.7 to 0.8 mm/day) ten days after a fresh 1.5 m slab fell.
- The stress regime is modest (240 m), so this is not stress-driven squeezing that might be tolerated; it is progressive, structurally controlled loosening of blocks defined by the joint sets.
- The fall itself is evidence: the installed pattern cannot retain the block sizes the 0.3-0.6 m joint spacing produces in a 9 m span.
Immediate actions
- Barricade, sign and post the area; no personnel under the unsupported span.
- Scale from a protected position or with a remote scaler - not by hand.
- Install temporary support (steel sets, forepoling/umbrella arches, mesh) before any further widening.
- Tell production the six-week date is not achievable until the design changes. Do not let a schedule pressure ground-control decisions.
2. Redesign the support for a 9 m span in this rock mass. Explain why the existing pattern failed.
Read the worked answer
Why it failed
- 2.4 m bolts at 1.2 m spacing partially anchor within the loosened zone in a 9 m span of 25 MPa rock. Short bolts retain individual blocks; they cannot clamp a 1.5 m slab.
- 50 mm fibre-reinforced shotcrete is a surface retention layer. It cracked because it was asked to carry load, and clay-coated joints give it nothing to bond to.
Redesign principles
- Change the shape. A flat 9 m span in this rock is the root cause. Arch the back and shoulders so the rock mass works in compression.
- Anchor beyond the yield zone. 6-8 m cable bolts (or self-drilling anchors) on a 1.5-2.0 m grid at the crown and shoulders, with the 2.4 m bolts kept for surface block control between cables.
- Retain the surface. Weld mesh or straps over the whole crown and shoulders, then 75-100 mm steel- or synthetic-fibre shotcrete over the mesh. Mesh takes the slab; shotcrete stops ravelling.
- Manage the clay. Do not wash the clay seams; drain and cover them. Consider a sealed liner or shotcrete skin on the clay-coated set.
- Verify. Extensometers through the yield zone plus a convergence array, with a written trigger-action-response plan before re-entry.
- Question the geometry. If 9 m cannot be supported economically, relocate the bay or leave a pillar - the span may simply exceed what this rock mass can carry.