Most EWP incidents involving tip-overs or instability are not caused by mechanical failure. They are caused by inadequate site assessment before setup: soft ground, underground voids, slopes that exceed the machine's rated limits, or outriggers set on surfaces that cannot carry the load. Understanding ground bearing pressure and what contributes to stability gives site supervisors the tools to prevent these events before they happen.
What is ground bearing pressure?
Ground bearing pressure is the load applied to the ground per unit area — expressed in kilopascals (kPa) or tonnes per square metre. When an EWP deploys its outriggers, the machine's weight (plus the platform load and the overturning moment from the extended boom) is concentrated through the outrigger pads onto the ground beneath them.
A large machine on small outrigger pads concentrates a high pressure onto a small area. If the ground cannot support that pressure, it compresses, the pad sinks, the machine tilts and the stability margin is reduced — potentially to the point of tip-over.
Factors that affect ground conditions
- Soil type and moisture — saturated clay or fill can lose bearing capacity rapidly under load, especially after rain.
- Underground services and voids — stormwater pipes, cable conduits, old footings and tree roots create voids that can collapse under concentrated outrigger load.
- Recent excavations and trenches — even backfilled trenches have lower compaction than undisturbed ground and can settle under load.
- Slopes — a machine set up on a grade beyond its rated slope limit has a reduced stability margin in the downhill direction.
- Paved surfaces — concrete and asphalt vary enormously in thickness and support quality; thin paving over soft fill is a common hazard.
Outrigger pads and load spreading
Outrigger pads — timber, rubber, or proprietary composite boards — are placed under the outrigger feet to spread the point load over a larger contact area, reducing ground bearing pressure. A larger pad area means lower pressure for the same machine weight.
The pad must be solid, flat, level and large enough for the outrigger foot to rest fully on it. A pad that rocks, tilts or is set on uneven ground gives false stability: the outrigger appears deployed but is not fully transferring load safely.
Before setting up: the site check
- Check the machine's rated maximum slope and verify the ground incline is within that limit.
- Inspect the ground beneath each outrigger position for recent disturbance, patches, drains and service markers.
- Use appropriate outrigger pads — sized to match the machine's requirements and the ground conditions.
- Do not set up over stormwater grates, inspection covers, or suspected voids.
- On soft ground, seek ground bearing pressure data from the machine's documentation and compare against the known site capacity.
This guide is general information prepared by EWP Australia and is grounded in publicly available Australian Standards and Safe Work Australia material. It is not a substitute for the current published edition of the relevant standard or for site-specific engineering advice. Always work from the current standard and consult a competent person for your specific equipment and application.