Why 80% of Crane Failures Start Before the Boom Moves
A crane lift is either won or lost on the ground long before the hook takes weight.
According to SafeWork NSW incident investigations, the majority of crane overturns do not happen because the boom snaps or the winch cable shears. They happen because of ground failure—an outrigger punches through compromised soil, an operator short-rigs without an engineered derate, or dunnage timbers fail under point loading.
Operating slewing mobile cranes around the Illawarra presents a unique set of geotechnical headaches that standard textbook scenarios don’t cover:
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Port Kembla & Industrial Strips: Reclaimed industrial land, unpredictable slag backfill, and hidden service trenches right beneath hardstand gravel.
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Kembla Grange & Calderwood: Heavy reactive clays that turn slick on top while remaining spongy underneath following South Coast rain events.
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Escarpment & Mount Ousley Corridors: Sloping bench cuts, variable sandstone strata, and water seepage zones that silently weaken subgrade bearing capacity.
Here is the exact pre-lift setup protocol we drill into every trainee on our Liebherr mobile crane at AOTA’s Unanderra training yard before the key ever turns.
1. Sizing Your Dunnage: The Ground Bearing Pressure Equation
Never drop an outrigger steel foot directly onto asphalt, compacted base, or bare ground. Asphalt softens in 30°C Wollongong summer heat and will punch through under high slewing loads.
Every operator must understand the basic ground contact relationship:
Pressure = Outrigger Load\Pad Area
If a 40-tonne Liebherr swings a load over a single outrigger corner, that individual outrigger jack can see up to 70%–80% of the combined crane and load weight.
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A standard steel outrigger float has a surface area of roughly 0.2 to 0.3 square metres.
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Dropping 30 tonnes onto that tiny footprint delivers over 1,000 kPa of pressure directly into the dirt.
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Typical compacted soil or hardstand holds between 150 and 250 kPa before failing.
Without heavy timber dunnage mats or engineered composite pads underneath the float to distribute that force across a larger surface area, the ground simply caves, and the machine tips. We teach operators how to inspect timber cribbing: tight grain, hardwood, zero splits, laid perpendicular with no gaps.
2. The 100% Extension Trap (Short-Rigging vs Full Span)
Modern cranes feature computerized Rated Capacity Limiters (RCL) that allow partial outrigger extension. However, relying on partial extension charts on tight residential or civil sites across Wollongong introduces zero margin for error.
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Always aim for 100% outrigger beam extension on all four corners whenever site boundaries allow.
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Visually confirm the mechanical lock pins have seated—never rely purely on cab sensor lights.
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If a site requires an intermediate outrigger position, you must verify that the crane’s computer is explicitly programmed for that specific outrigger footprint configuration before the hook leaves the headache ball rack.
3. Tires Clear of the Deck
A mobile crane is designed to lift on outriggers only when the entire chassis weight is supported by the jacks.
During our practical assessments in Unanderra, we check for complete tire clearance. If tire rubber remains compressed against the bitumen, the suspension can oscillate during slewing. That slight bounce transfers dynamic shock-loading back through the boom, tripping the load limiter or introducing crane sway. Jack the machine until all tires are fully clear of the ground, verify the bubble level in the carrier cab, and double-check level indicators inside the crane superstructure.
4. The 1:1 Trench Rule
Working close to storm-water drains, utility trenches, or escarpment edges is common across local civil sub-divisions. SafeWork NSW mandates minimum setback distances:
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In standard firm soil, the distance from the edge of the outrigger pad to the bottom edge of an open trench must be at least equal to the depth of the trench (a 1:1 ratio).
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In soft ground, sand, or loose backfill (common around Shellharbour and coastal zones), that safety buffer doubles to a 2:1 ratio.
Master the Machines That Build the Region
Holding a C6 slewing mobile crane ticket opens the door to top-tier civil and port infrastructure projects across NSW. At All Onsite Training and Assessment in Unanderra, our training is delivered by career operators who have rigged and lifted on real job sites for decades.
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Course Location: 13-15 Doyle Avenue, Unanderra NSW
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Phone: (02) 4206 8378
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Tickets Covered: C2, C6, Dogging (DG), Basic Rigging (RB), Intermediate Rigging (RI), Advanced Rigging (RA)
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Remember, your dream career is within reach. With AOTA by your side, you can build the skills, confidence, and qualifications you need to succeed. So, take the first step today and start building your future!
Dedicated to your success
Nigel
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