Outrigger Over a Buried Storm Drain
A 50 t rough-terrain crane is set up in a newly landscaped area behind a retail unit to lift steel beams onto a mezzanine floor. The site plan shows a 600 mm storm drain running 1.2 m below finished grade, crossing beneath the right-hand outrigger position. The asphalt was laid three weeks ago over 200 mm of compacted granular fill; the geotechnical report gives an allowable bearing pressure of 150 kPa for that fill. You are using 600 mm x 600 mm outrigger floats sitting on 1.2 m x 1.2 m timber mats. For the planned configuration (21 m boom, 14 m radius, 360 degree chart) the chart gives 11.5 t and the manufacturer's notes state a maximum outrigger reaction of 380 kN at the most heavily loaded corner. The ground is level, but after the first beam was landed the left front outrigger had sunk about 40 mm into the asphalt and you can see a short crack in the surface near the culvert.
1. Calculate the average ground bearing pressure under the most heavily loaded outrigger and assess it against the site information.
Read the worked answer
The mat footprint is 1.2 m x 1.2 m = 1.44 m2.
Pressure = 380 kN / 1.44 m2 = 264 kPa.
Allowable bearing pressure of the fill = 150 kPa.
Utilisation = 264 / 150 = 176% of allowable. The mat is roughly half the area it needs to be. A quick check of the required area: 380 / 150 = 2.53 m2, so at bare minimum a 1.6 m x 1.6 m pad, and that is with no safety factor and with the assumption that the mat spreads the load perfectly uniformly, which a single timber mat on soft new asphalt will not. Peak pressure directly under the float will be higher than the average.
Two further points matter: the 380 kN is the maximum corner reaction for the configuration, and the actual reaction moves around as the boom slews, so the corner nearest the drain will not always be the lightest. And the 40 mm of settlement already observed proves the assumption of a uniform, elastic bearing surface is wrong.
2. What do the settlement and the surface crack tell you, and what is the correct immediate action?
Read the worked answer
Settlement of 40 mm under a loaded outrigger is not normal bedding-in and must be treated as a bearing failure in progress. The likely mechanisms are the new fill punching under the mat, or the culvert below acting as a void into which the load is being shed. The crack near the culvert points toward the second. If the pipe has begun to deflect, the surface can drop further and suddenly once the pipe shell or the fill over it gives way.
The correct action is to stop the lift before the next beam is picked. Land and secure any suspended load, then take the load off the outriggers and boom down. Do not simply re-level with the outrigger jack and carry on: re-levelling chases the failure downward, and a crane that has taken a set may also have twisted the chassis or damaged an outrigger beam that needs inspection before further use.
Then re-plan the setup so no outrigger loads the drain. Move the crane so the outrigger mats and their load-spread zone stay clear of the pipe, or bridge the drain with steel plate or beams designed by a competent engineer for the actual reaction. Re-check the mat size against the fill's 150 kPa allowable and confirm the crane manufacturer's outrigger and mat requirements are met.
3. What must be verified before work resumes, and who is responsible for what?
Read the worked answer
Before resuming: a competent person re-assesses the ground at the new position, including any buried services, using the site drawings plus a scan or trial pit if the drawings are uncertain. Mats are re-sized so the pressure under each float is comfortably below 150 kPa, and steel plate is added where the fill is soft. The outrigger beams, floats and chassis are inspected for damage from the settlement event. The crane is re-levelled with the bubble on the chassis and the beams fully extended per the manufacturer, then the level is re-checked after the first lift and any further settlement stops the job again.
Responsibility is shared but not interchangeable. The site or principal contractor provides accurate ground and services information and a suitable working area. The appointed person or lift planner designs the setup around that information. The operator's duty is to verify what can be verified, to recognise when the information is missing or contradicted by what the ground is doing, and to refuse the lift until it is resolved. 'The drawings said it was fine' is not a defence once the outrigger has gone down 40 mm while the operator was watching.