Ground Bearing Under the Outrigger Pad
You are setting up in a container yard on a compacted gravel hardstand. The yard manager points to a painted rectangle and says 'this is where the reach stackers always work.' You notice the surface there is rutted and one corner of the rectangle sits over a patch of darker, softer-looking ground near a recently backfilled drainage trench. Your machine's operating mass is 48 t, and the manufacturer's ground pressure guidance for the front axle with a loaded spreader is 9.5 kg/cm² under the tyre contact patch; the manual requires that ground be assessed for bearing capacity before operation and that the machine not be operated on ground that cannot support the imposed loads. A nearby site plan shows the drainage trench was backfilled three weeks ago and not compacted to specification.
1. What is your assessment of the proposed operating position, and what would you do before lifting anything?
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The painted rectangle is not an engineering assessment. It records where machines have previously worked, not whether the ground can carry the load now. The ruts show the surface has already been yielding under previous machines, and the darker ground over the uncompacted backfill is a specific weak zone.
Load path: The machine's mass (48 t) plus the load is carried through the tyres into the surface. At 9.5 kg/cm² the contact pressure is high, and under a loaded front axle with the boom extended the load concentrates further forward. Uncompacted backfill over a trench can settle or collapse under that pressure, and a wheel dropping into a void will tip the machine - a catastrophic event with a raised load.
Actions before lifting:
- Do not position over the trench or the soft patch. Relocate to a firm area clear of the backfill, or have the ground assessed and proof-rolled/compacted by a competent person.
- Ask for the geotechnical or compaction records for the backfill; 'three weeks ago' is not a specification.
- Walk the area and look for further soft spots, voids, or buried services.
- Keep the machine's travel and lift paths clear of the suspect zone and mark it off.
- Record the decision and the reason in the site diary and notify the supervisor.
2. The supervisor says the trench was 'only a shallow one' and to proceed. How do you respond, and what is the basis for your position?
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Depth is not the issue - support is. A shallow trench that has been loosely backfilled still creates a low-bearing zone, and the failure mode is not gradual: a wheel can punch through a void and the machine can tip in seconds with a container on the spreader. The consequence is severe injury or death, so the risk must be controlled before work, not managed by hoping.
The basis for refusing is the manufacturer's operating requirement that ground bearing capacity be confirmed, and the general duty to provide a safe place of work and safe systems of work. Those duties sit with the person conducting the business, but the operator is the last control and must not operate equipment outside its permitted conditions.
Practical response: Offer alternatives rather than a flat refusal - a different slot on firm ground, a load-spreading solution if the manufacturer permits one, or a competent ground assessment of the backfill. Ask for the assessment in writing. If the supervisor still directs you to work over the uncompacted area, escalate to the site safety contact and record the instruction and your response. 'Only shallow' is an opinion; a compaction test or a relocation is a control.