Outrigger Setup Over an Unshored Utility Trench
A 47 m truck-mounted boom pump is scheduled for a two-story addition. The only staging area is a 9 m by 12 m gravel pad. The pump's load chart for the planned boom position requires a 6.5 m outrigger spread. One front outrigger would sit 1.2 m from the edge of a 1.8 m deep, unshored utility trench. Utility locates show a 200 mm gas main at 1.1 m depth running parallel to the trench. The general contractor says the pad was compacted in 300 mm lifts and passed a plate test, but no geotechnical report is available. Each outrigger pad is 600 mm by 600 mm. The manufacturer's maximum outrigger load for this configuration is 18,000 kg. The foreman proposes using the existing pads because the trench is only for storm drainage. Wind is 12 km/h, and the boom will swing over the trench side for half the pour.
1. Is the proposed setup acceptable? Include the ground-pressure check and identify the controlling hazard.
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No. The nominal outrigger pressure is 18,000 kg x 9.81 / (0.6 m x 0.6 m) = about 491 kPa. That is a very high bearing pressure, and a plate test does not establish an allowable bearing capacity for a 1.8 m deep unshored trench with a gas main nearby. The controlling hazard is trench-wall collapse or soil movement under the outrigger, which could cause sudden settlement, boom movement, or tip-over. A 1.2 m setback is less than the trench depth and is not adequate for heavy equipment without an engineered assessment. The gas main at 1.1 m depth makes excavation or surcharging near the trench especially risky. Reject the setup as proposed. Relocate the pump so the outrigger is outside the trench influence zone, or obtain an engineered foundation design and trench setback from a competent person. Do not pump over that side until the foundation and setback are verified.
2. What documents and field verifications should be required before this pump is set up?
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Require the manufacturer's outrigger load chart for the actual boom length and swing position, a geotechnical or competent-person evaluation of allowable bearing pressure, and confirmation that the compacted fill is not disturbed near the trench. Verify utility locate markings and the gas main's horizontal and vertical clearance. The trench must be inspected by a competent person, with the required setback and protective system determined for heavy equipment. Confirm outrigger pad size and any mat design, check slope and drainage, and mark the outrigger positions. Document wind limits and the planned boom path. The operator should also confirm that the pump's load chart includes dynamic and swing loads and that the pads are on firm, level contact, not spanning loose fill.
3. If the contractor insists the pump must stay in that location, what load-spreading solution and monitoring plan would you require?
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I would require an engineered load-spreading system, not just larger timber. For example, a steel or engineered mat sized so the outrigger load is spread over an area that the competent person or engineer approves for the actual soil and trench condition. A 1.5 m by 1.5 m mat would reduce nominal pressure to about 78 kPa, but the mat must also span the trench influence zone and be designed for bending; the number alone is not approval. The trench may need shoring or backfill before setup. Monitor the outrigger with a survey mark or dial indicator before and during pumping; any settlement over about 10 mm, soil cracking, or trench movement is a stop-work condition. Keep personnel out of the trench and the boom swing zone, and re-check after the first concrete arrives and after boom movement.