Quick answer: the ground must support the machine, its load and its point loads — before it travels, while it sets up, and for the whole time it’s elevated. Most EWP overturns are ground failures: soft fill, hidden voids, trench edges, saturated soil and overloaded slabs. Surface appearance proves nothing; you check what’s under it.
Why good-looking ground fails
A crusted surface can hide months-old backfill. A neat lawn can hide a septic tank. Pavement can hide a service trench compacted in a hurry. The machine doesn’t care what the ground looks like — it cares what the ground carries, and the highest wheel loads happen when the platform is up and off-centre.
The eight hazards to walk for
- Backfilled ground — trenches and service runs re-compacted poorly; give them the machine-width berth
- Excavation edges — load near an edge can collapse it; keep set-up outside the zone of influence (rule of thumb: at least the trench depth away, further in soft soil — get engineering input if you must work closer)
- Voids and covers — pits, tanks, pipes, basements, pit lids not rated for wheel loads
- Soft and saturated soil — yesterday’s rain changes today’s bearing; ruts from delivery trucks are a warning
- Slopes — check the machine’s rated limits (travel vs operating slope are different numbers)
- Suspended slabs and mezzanines — the slab rating must beat the machine + load + dynamic effects; ask the building owner or engineer
- Kerbs, ramps and transitions — tipping risk while travelling, especially elevated (don’t)
- Surface protection — pavers, tiles and finished floors may carry the load but not survive it
Machine weight and contact pressure (real numbers)
What the ground must carry varies enormously across one hire fleet:
- JLG ES1932 19ft electric: 1,550kg, ground bearing pressure 8.0 kg/cm²
- LGMG S1212E-2 40ft electric: 2,940kg on four small tyre patches
- LGMG SR1018D 32ft RT: 4,330kg including outriggers
- LGMG AR11J boom: 5,400kg, loads shifting with reach
- Zoomlion ZS0612C tracked: 1,820kg spread across full-length crawlers — the low-pressure option
Add platform load (230–454kg) and remember: raising and reaching concentrates weight on one wheel or one outrigger foot.
Spreader plates and mats
Outrigger and wheel loads punch through surfaces that would carry a spread load happily. Use spreader plates/pads where the manual calls for them, on any soft or unproven ground, and on surfaces you can’t afford to mark. Plates must be sized for the load and bear on solid material — a pad on mud is just tidy mud.
Slopes: two different limits
Gradeability (e.g. 35% stowed on the SR1018D, 45% on the AR11J) is what the machine can travel up. The operating slope limit for elevating is far smaller (3°–4.5° classes on the boom; tracked machines level themselves on outriggers to work on up to 12°/7° ground). Never elevate on a slope beyond the plate — the tilt alarm is the last warning, not the first check.
When to get engineering input
Ask for a geotechnical report or engineer’s advice when: working on suspended slabs without a documented rating; near excavation edges; on deep or recent fill; or when a machine at the heavier end (4–5.5 tonnes) must sit on anything other than proven ground. On commercial sites the report often already exists — ask for it.
Re-check when conditions change
Rain, new excavation, repeated wheel loads and long static set-ups all change bearing capacity mid-hire. Outrigger feet settle in soft ground over hours. If the machine has sat loaded in one spot through weather, re-level and re-check before elevating again.
FAQs
Why can good-looking ground still be unsafe? Because bearing failures start below the surface — fill, voids, services and saturation don’t show until loaded.
What should I check near excavations? Edge distance and the zone of influence: keep at least trench-depth clearance in good ground, more in soft ground, and get engineering input to work closer.
Are spreader plates always required? Use them where the manual requires them and wherever ground is soft, unproven or precious. They spread point loads — they don’t create bearing capacity that isn’t there.
Does rock mean safe support? Usually good news, but surface rock can be a floater over softer material — context still matters.
Why is backfilled ground risky? Re-compacted soil rarely matches undisturbed ground; recent trench lines are the classic EWP wheel-drop.
Should I ask for a geotechnical report? If one exists, read it. Commission advice for suspended slabs, deep fill or heavy machines on uncertain ground.
Are tracked machines automatically safe on soft ground? Safer, not safe — tracks lower contact pressure, but outrigger feet still need solid bearing and edges still collapse. See the Tracked Scissor Lift Guide.
What happens if the machine sits in one place too long? Wheels and outrigger pads settle, especially after rain — re-check level before elevating again.
Can I judge support just by walking the site? Walking helps you spot the warning signs, but it can’t prove bearing capacity — history (what’s buried), ratings (slabs) and weather finish the picture.
Whose job is the ground check? The PCBU controlling the work must ensure support is adequate; the operator confirms it at set-up and stops when it changes. Tell us the ground honestly at quote stage and we’ll send the right machine — that’s the cheapest control of all.
Machine figures from current manufacturer spec sheets — specifications vary by model; confirm before you book.