Almost every access machine overturn traces back to what was under the wheels or outriggers, not to what was happening in the air. An elevating work platform concentrates a heavy, top-loaded structure onto a small footprint, and the ground has to carry that load without shifting, sinking or collapsing. Because the ground is easy to take for granted and hard to see below the surface, it deserves a deliberate assessment every time the machine is set up somewhere new. This guide covers how to read the ground, how packing and pads spread the load, what slopes do to stability, and why the biggest hazards are often the ones you cannot see.
Assess the ground before you commit
A ground assessment is a habit, not a one-off. The surface that carried a machine last week may have been dug up, backfilled, rained on or driven over since, so each new set-up starts fresh.
The questions worth asking on every set-up:
- What is the surface — compacted hardstand, asphalt, concrete, gravel or soft natural ground — and how much load will it actually bear?
- Has the area been recently excavated or backfilled, where loose fill can look solid but give way under a point load?
- Are there trenches, pits, basements, cellars or voids nearby whose edges could collapse under the machine's footprint?
- Is the surface hiding something below, such as a covered drain, a service pit lid or a soft patch from a leaking pipe?
- Will the ground stay sound for the duration, or is rain, traffic or thaw likely to change it while the machine is set up?
Packing and pads spread the load
An outrigger or wheel presses down through a small area, and if that pressure exceeds what the ground can bear, the leg sinks and the machine tips. Packing and outrigger pads solve this by spreading the load over a larger area, lowering the pressure on the ground beneath.
The pad has to be sound, of adequate size for the load and the ground, and set on a prepared, level footing. Undersized or damaged packing, timber that is rotten or split, or a pad perched on an uneven surface can fail suddenly. The principle to hold onto is that soft ground needs a bigger footprint, and the softer it is, the larger and more solid the pad must be — the sizing follows the load and the ground's bearing capacity rather than a fixed habit.
Slopes and levelling
Access machines are designed to work within a defined slope limit, and that limit is part of what keeps the centre of gravity inside the stable footprint. Set up beyond it, or let the machine sit unlevel, and raising the boom or platform can walk the centre of gravity toward the tipping line.
On uneven ground the machine is levelled within its allowances using its own outriggers or levelling system, checked against its indicator rather than eyeballed. Building up one side with unstable packing to fake a level, or working across a grade steeper than the machine permits, is how a routine set-up becomes an overturn. Where the ground cannot be made good enough, the right move is to reposition to firmer, flatter ground.
Underground services and voids
The most dangerous ground hazards are the ones out of sight. A machine can be set up on a surface that looks perfect while a shallow drain, an old cellar, a service trench or a collapsing void sits directly beneath a leg, waiting for the load to arrive.
- Underground drainage, pits and disused structures can collapse under a concentrated outrigger load with no warning at the surface.
- Recently trenched services leave backfill that has not consolidated and behaves like soft ground even when the surface is graded flat.
- Buried tanks, culverts and utility ducts create voids and weak zones that a walk-over will not reveal.
When the ground is beyond judgement
For most firm, well-drained hardstand, an experienced operator's assessment and correctly sized packing are enough. Where the ground is questionable — reclaimed land, unknown fill, proximity to excavations, or a load heavy enough that the margins are thin — the sensible step is to get the site's bearing capacity confirmed by someone qualified to judge it, and to have services located before anything is set down.
Recording the assessment and the set-up used also builds a defensible history if the ground is ever queried. If you want the outrigger system, levelling function or stability devices on your machines verified as part of an inspection, EWP Australia is reachable on 0416 689 689.
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.