Guy anchor pull-out on a stiff-leg derrick after overnight rain
A 12 t stiff-leg derrick has been working for three days on a compacted gravel pad at a quarry. Its two backstays are anchored to buried concrete deadmen rated at 90 kN each, installed by the site contractor. The load chart shows a maximum backstay tension of 68 kN at the working radius being used. Overnight, 45 mm of rain fell. The morning operator walks the anchors and finds the gravel around one deadman is soft and a 20 mm crack has opened in the ground running away from the anchor line. The bubble in the mast plumb indicator is still within the marked tolerance, and the previous shift reported no problems. The rigger says the crack is just surface erosion and wants to start lifting because the concrete pour crew is waiting.
1. Should the crane be operated? Identify the hazard, the reasoning, and the immediate control you would apply.
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No lifting. The crack running away from the deadman, aligned with the backstay line, plus softened ground after 45 mm of rain, is a classic precursor to anchor pull-out. The deadman resists 68 kN of backstay tension only through the passive resistance of the soil ahead of it; saturated gravel loses a large part of that resistance, so the installed 90 kN rating no longer represents the actual capacity. A plumb mast proves nothing here, because the mast stays vertical until the anchor actually moves. The failure mode is sudden and the mast falls toward the load.
Immediate controls: stop work and barricade the area within the fall radius of the mast; do not load or boom the crane; notify the site supervisor and the person who designed the anchor system. Re-inspection by a competent person, with the soil condition assessed, is required before any lift. If the anchor cannot be verified, the derrick must be de-rigged or re-anchored to a designed system. Document the stop, the notification, and the basis for any return to service.
2. The rigger argues the load chart governs, and the chart says 68 kN, which is below the 90 kN anchor rating. Explain why this comparison is not a valid safety check.
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The load chart gives the maximum backstay tension the derrick can develop in service; the 90 kN figure is the anchor's rated capacity in the assumed design soil condition. Comparing them is only meaningful if both are valid at the same time and in the same condition.
Three problems arise. First, the anchor capacity is a function of soil strength, which has changed; the rated figure no longer applies. Second, the chart tension is a nominal value, and real tensions can exceed it through dynamic effects, wind, off-lead pulls, or a snatch load, so a small margin is not a guarantee. Third, the design assumes the load direction matches the backstay line; a crack indicates the ground is already yielding in that direction, meaning the resistance is being mobilised and reduced. The correct check is whether the anchor as installed, in its current condition, can carry the design tension with an adequate factor of safety, verified by a competent person. Until that is established, the chart is irrelevant to the anchor question.