Ground Bearing Under an Outrigger on Made Ground
A 25 t mobile crane is set up to lift a 4 t package at a 12 m radius. One outrigger sits on a filled area that was a service trench 18 months ago. The crane's load chart gives a maximum outrigger load of 18 t per leg for this configuration. The outrigger pad is 0.9 m by 0.9 m. The site engineer says the fill has been 'compacted and is fine'. There are no records of the fill, and the trench runs diagonally under the outrigger. The lift is a single pick onto a low trailer; the ground is dry and the weather is calm.
1. Calculate the bearing pressure under the pad and explain why the calculation alone does not settle the question.
Read the worked answer
Pad area = 0.9 × 0.9 = 0.81 m². Bearing pressure = 18 t / 0.81 m² ≈ 22.2 t/m², which is roughly 222 kPa (using 1 t ≈ 9.81 kN, 18 t ≈ 176.6 kN; 176.6/0.81 ≈ 218 kPa).
The calculation gives the pressure the crane imposes, not the pressure the ground can carry. The critical unknown is the bearing capacity of the fill over the trench. Compacted fill over a trench is a classic void and differential-settlement hazard: the trench backfill may be looser than the surrounding ground, and the diagonal run means the pad straddles two materials. The engineer's 'it's fine' is an opinion without records. A pad spreads load but does not bridge a soft zone if the zone is wider than the pad or extends below it. The calculation therefore sets the demand; it does not prove the ground can meet it. That requires investigation - trial pits, plate load testing, or records - and a designed mat or crane mat sized for the actual capacity.
2. State the actions you would take before allowing the lift, and the alternative if the ground cannot be verified.
Read the worked answer
First, stop and treat the outrigger as unsupported until proven otherwise. Request investigation of the fill: trial pits or probing along the trench line under and around the pad, and plate load testing or dynamic probing to establish capacity. Get the result in writing from a competent person, not a verbal assurance. If capacity is confirmed adequate with a sensible factor of safety, use a designed crane mat sized to spread the 18 t to below the required pressure, and check the mat is rated for that load. Re-level the crane, re-check the load chart for the actual radius and configuration, and confirm all four outriggers are on competent ground.
If the ground cannot be verified, do not set up there. Options: reposition the crane so no outrigger sits over the trench; use a larger crane at a shorter radius to reduce outrigger loads; or use a piled or structural mat designed by an engineer. The lift is a single pick, so the temptation is to rush - but a crane overturn from an outrigger punch-through is catastrophic, and the dogger has the authority to refuse the set-up.