Outrigger settlement on a backfilled excavation
You are setting up a 100 t slewing mobile crane to lift precast culverts weighing 5.5 t each from a stockpile to a trench. The chosen position places the crane's four outriggers on a surface that includes a 1.2 m deep backfilled excavation, filled eight months ago with un-compacted spoil and capped with 150 mm of compacted crushed rock. The outrigger pads are 600 mm square steel plates. The load chart at the working radius of 16 m gives a rated capacity of 8.2 t. The crane is level within 0.5 degrees. Ground bearing pressure has not been assessed. The site engineer says the crushed rock cap is 'good enough' because it was compacted. A worker has already walked across the fill and left boot prints 40 mm deep.
1. Assess the ground conditions and determine whether the set-up is acceptable.
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The set-up is not acceptable. The boot prints are direct evidence that the surface is yielding under a person's weight, which is a fraction of the outrigger load. If 150 mm of crushed rock deforms under a pedestrian, it will not distribute the outrigger load.
The outrigger load is not simply the load divided by four. The crane's own weight plus the load, multiplied by the load moment, concentrates force on the outrigger nearest the load. For a 100 t crane at 16 m radius with a 5.5 t load, the near outrigger can carry a substantial share of the total, often well over half the combined crane and load weight. That force is applied through a 600 mm pad, giving a high bearing pressure at the pad. The crushed rock cap spreads that load only if it is structurally competent and the material beneath it can carry the pressure. Un-compacted fill over a 1.2 m deep excavation cannot be assumed to do so.
A geotechnical assessment is required. Options include matting or steel plates to spread the load, moving the crane to competent ground, or excavating and re-compacting the fill under engineering supervision. The engineer's verbal assurance is not a substitute for a bearing capacity check.
2. What monitoring and operational controls would you apply if the crane were repositioned to the edge of the fill?
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Repositioning to the edge of the fill still requires verification. The outrigger must be far enough from the excavation edge that the load does not sit on the slope's failure wedge; a common rule is to keep the outrigger at least the depth of the excavation away from the edge, but the actual distance must come from the geotechnical assessment.
Controls: use load-spreading mats sized for the calculated outrigger load and the ground's bearing capacity; set the outriggers fully extended and confirm the crane is level; mark the outrigger pads and photograph them before the lift; assign a person to watch the pads and the ground for movement during the lift; and stop immediately if any pad sinks, the crane tilts, or the level indicator moves. Keep the boom over the outrigger base where the chart permits, and avoid slewing the load over the fill. Re-check the level after the first lift, because settlement often appears under the first application of load. Record the set-up, the mat calculations and the monitoring results.