The Bench That Moved Overnight
You are the geotechnical competent person for a hard-rock quarry. A 12 m high bench on the north face was scaled and inspected three weeks ago and passed. Overnight there was 35 mm of rain. This morning the face survey shows a new 4 m long tension crack running parallel to the crest, set back 2.5 m, with about 80 mm of vertical displacement. The toe of the bench shows fresh rockfall debris and a seepage line that was not there before. A loading shovel and two haul trucks are working the bench below. The bench above carries the main haul road used by all site traffic. The shotfirer has a blast scheduled on the adjacent bench at 14:00.
1. Interpret the observations and decide the immediate action for the bench below and the haul road above.
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The combination is a classic pre-failure signature:
- Tension crack parallel to the crest with 80 mm displacement indicates the upper part of the face is dilating and moving outwards.
- Fresh toe debris and new seepage indicate water pressure has built up in discontinuities and is driving the movement; the seepage reduces effective stress and lubricates surfaces.
- Rainfall of 35 mm is the trigger.
This is not a stable face. Immediate actions:
- Stop work and withdraw the shovel and trucks from below the bench; establish an exclusion zone at least the height of the bench (12 m) plus a margin, using cones and barriers.
- Close the haul road above and divert traffic; a failure could take the road with it.
- Notify the quarry manager and record the time and observations.
- Arrange a geotechnical assessment - survey monitoring points, crack gauges, and a face inspection by a competent engineer before any reopening.
- Do not scale the face until the movement is understood; scaling a moving face is itself a fatality risk.
2. How should the 14:00 blast on the adjacent bench be handled?
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The blast should be postponed until the instability is assessed. Reasons:
- Ground vibration from the blast could trigger the failure that is already incipient. A face with an open tension crack and active seepage has very little margin.
- Air overpressure and flyrock are secondary concerns but the primary risk is slope failure.
- The shotfirer must be told of the changed conditions and the blast design reviewed by the geotechnical competent person.
If, after assessment, blasting is permitted, controls would include reduced charge weight per delay, electronic timing to lower peak particle velocity, vibration monitoring at the crack, and exclusion of all personnel from the affected bench and haul road. The decision and its basis must be recorded. Postponing a blast is a normal supervisory judgement, not a failure.
3. What monitoring would you put in place before allowing the bench to reopen?
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A defensible monitoring regime:
- Crack monitoring: tell-tales or wire gauges across the tension crack, read daily and after rain.
- Survey prisms on the face and crest, with baseline readings and a defined trigger level (e.g. 5 mm/day movement) that closes the area again.
- Piezometers in the seepage zone to track water pressure.
- Visual inspection each shift by a competent person, with a written record.
- Rainfall log linked to trigger levels - e.g. any 24-hour total above 25 mm triggers a re-inspection.
Reopening only when movement has stabilised and the geotechnical engineer signs off. All records retained.