What Ground Conditions Decide About a Lift You Are Standing In

Why this matters

Before anyone goes up, the setup is a ground question, not a machine question: park on ground whose bearing capacity somebody has established, or do not elevate. A boom lift that tips does not tip slowly. The platform arrives at the ground travelling faster than the machine did, and the occupant's harness and lanyard, which were sized to arrest a fall from the platform, are still attached to a platform that is now moving. The machine did not fail. The rated capacity was not exceeded. What failed was three feet of ground under one outrigger pad, and the load that broke it was several times the number most people would guess.

The load path does not stop at the tires

A lift is a machine sitting on top of a structural member nobody designed: the ground. Every other element in the load path has a published rating. The boom has one, the turntable has one, the platform has one, the outrigger leg has one. The soil under the pad has one too, and it is the only one on the site that is invisible, varies within a few feet, and changes with last night's rain.

Two Parts reach this work and they are not interchangeable. Construction is 29 CFR 1926.453, aerial lifts, reaching vehicle-mounted elevating and rotating work platforms; general industry is 29 CFR 1910.67. A scissor lift falls under neither in OSHA's own reading, because it does not rotate: it is treated as a mobile scaffold under 29 CFR 1926 Subpart L in construction and 29 CFR 1910 Subpart D in general industry, and a reader hunting a ground requirement in 1926.453 for a scissor lift will not find one.

Neither Part gives you a number for the ground. There is no federal allowable bearing pressure, because the answer is a property of the site. The ANSI/SAIA A92 suite covering mobile elevating work platform safe use is where the industry's ground-support practice is written down, and it binds you through your employer's programme or a contract, never on its own, in the edition your programme has adopted.

The number you need is not the machine's weight divided by its legs

This is the error that produces most undersized pads. A machine on four outriggers puts a quarter of its weight through each one only when the boom is stowed and centred, which is the one configuration in which nobody is working.

Swing the boom out over a corner and the machine rotates about the line between the two legs nearest it. Load shifts off the far legs onto the near ones, and at full extension the governing outrigger can approach the entire weight of the machine plus everything in the platform. That is why manufacturers publish a maximum leg load or a ground bearing pressure chart rather than leaving you to divide.

The governing leg load is the maximum across the whole swing envelope you intend to use, not the value at the position where you set up. A setup checked with the boom stowed and then worked at full reach over the back corner has been checked in the one orientation that does not govern.

What the published figure already contains

Before you correct a manufacturer's leg load, write down what is already inside it. A published maximum outrigger load or ground bearing pressure figure is normally stated for the machine at its rated platform capacity and normally already carries the manufacturer's allowance for the dynamic effects of normal boom motion. If that is what your chart says, adding the platform's rated load on top is not a correction, it is charging the same weight twice.

So this is a re-basing question, not an addition question. Read the chart's stated basis: if the figure already includes rated platform load and normal dynamic allowance, your job is to confirm your intended use sits inside that basis, not to add terms. If it sits outside - a load suspended below the platform, wind above the machine's rated limit, travel while elevated where the machine permits it - the published basis does not apply and the amount to add is the manufacturer's to state, not yours to estimate.

The one relationship you can run yourself

Pressure under a pad is leg load divided by bearing area, and it is the only piece of this arithmetic that belongs to the person in the field.

Bearing area is the area actually in contact and stiff enough to spread the load. A thin pad under a small round foot dishes rather than spreading, so the pressure concentrates near the foot. That is a stiffness question owned by the pad manufacturer: use the published effective bearing area, and where none is published use the smaller area you can defend rather than the outside dimension.

At constant leg load, pressure falls with the square of the pad's linear dimension. Double the side of a square pad and you quarter the pressure. That relationship holds only while the leg load is held constant, which is exactly what changes when the boom swings, so run it against the governing leg load and not the one you measured.

Round pressure up and round the ground's allowable capacity down. Both of those are the conservative direction, and both are free.

What the surface tells you and what it hides

A finished surface is evidence about the top inch and nothing else. The failures happen underneath.

• Recent backfill. A trench crossing a lot, a utility service to a building, a pole base, a repaired water main. Backfill that has not been engineered and compacted to a spec has no bearing value anyone can state, and asphalt laid over it hides it completely.
• Buried voids. Utility vaults, septic tanks and lids, cisterns, basement and crawlspace extensions past the wall line, storm structures. A lid rated for pedestrian traffic is not rated for a concentrated outrigger load.
• Edges. Ground near the top of a slope, a retaining wall, a curb, a dock or an open cut has less lateral support than the same soil mid-lot. A lift set near an open excavation is a surcharge on that excavation's wall and makes it worse, not better. The 2 ft minimum for spoil and equipment at 29 CFR 1926.651(j)(2) is a floor for loose material at an edge, not a setback that makes a lift acceptable next to a cut; that distance belongs to the excavation's protective system design.
• Water. Standing water, a wet spot in dry weather, a downspout discharge, seepage at a wall base. Water reduces bearing capacity, and the mechanism is the same one that downgrades soil in an excavation, so the direction is consistent: wetter is always worse. Frozen crust behaves the same way in reverse - ground that carried the machine at 7 a.m. may not carry it at 2 p.m.

The same machine on two setups

Both use the same rule: governing leg load from the chart, divided by defensible bearing area, rounded up, compared against an allowable the site's own information supports. Every figure below is illustrative and labelled as such. Use your machine's chart and your site's data.

Setup A, an established parking area with a soils report on file.

The fix is area, and the arithmetic is direct: at 2,000 psf allowed and an 11,000 lb leg load, the required area is 5.5 sq ft, so a 2.5 ft by 2.5 ft pad giving 6.25 sq ft lands at 1,760 psf and passes. Note what the relationship above predicts: going from a 2 ft to a 4 ft pad would take the area to 16 sq ft and the pressure to about 690 psf, a quarter of the original, and that holds only because the leg load was held at 11,000 lb across the comparison.

The failure mode on Setup A is not the arithmetic, it is the habit of setting the same pads under every leg. Three legs at 4,000 lb and one at 11,000 lb take the same pad in most crews' practice, and the one that governs is the one nobody sized.

Setup B, the same machine and the same pads, 6 ft from a trench that was backfilled two weeks ago and paved over.

The published leg load has not changed. The pad has not changed. The pressure demanded is the same 2,800 psf. What has changed is that there is no allowable bearing figure that applies, because nobody characterised the backfill and the soils report describes undisturbed ground the trench removed. The comparison cannot be made, which means it fails: an unknown allowable is not a large one.

Three outcomes are legitimate and none of them is a bigger pad chosen by eye. Move the machine outside the influence of the backfill, a distance somebody has to establish rather than pace off. Bridge onto undisturbed ground with mats or plates sized by someone competent to size them, noting that a mat only helps if it is stiff enough to span, which is a published property and not a guess. Or have the ground characterised and, if needed, remediated.

Setup B is the one that kills people, and the reason is that it looks better than Setup A. It is paved, flat and dry, and the two weeks it has stood are not evidence of anything, because nothing has yet applied a concentrated load to it.

Who owns each number

You own the pressure calculation and the observation. You do not own the rest, and pretending otherwise is where the injury comes from.

• Maximum leg or wheel load, allowable slope for the machine, wind limit, and whether the chart includes platform load: the manufacturer's operating manual and load chart for that machine.
• Allowable bearing capacity of the ground, and the influence distance from an edge, a backfilled trench or a buried structure: a geotechnical engineer, the site's soils report, or the party who owns the site's utility and structure records.
• Whether a protective system near an open cut can take the surcharge of your machine: the excavation's protective system designer, which under 29 CFR 1926 Subpart P is a registered professional engineer for anything outside the standard's own tabulated options.
• Whether this particular setup is acceptable today, and the authority to stop it: a competent person as defined at 29 CFR 1926.650(b), where the authority half is not optional. A sibling card covers why.

Catching it before it matters

Watch the pad, not the machine. A pad that is level at setup and has one edge sunk half an inch an hour later is telling you the ground is consolidating under a load it is close to failing at, and the correct response is to bring the platform down and re-set, not to level the machine and continue.

That is why the level indicator is the wrong instrument here. An operator who trusts the bubble and never looks at the pads will re-level a settling machine once, then twice, and the third correction is the one that runs out of leg travel at full boom extension.

References

• 29 CFR 1926.453, aerial lifts, and 29 CFR 1910.67, vehicle-mounted elevating and rotating work platforms, for the construction and general industry homes of this work.
• 29 CFR 1926 Subpart L and 29 CFR 1910 Subpart D, scaffolds, which is where OSHA places scissor lifts.
• 29 CFR 1926.651(j)(2) and 29 CFR 1926.650(b), for the excavation edge minimum and the definition of a competent person.
• ANSI/SAIA A92 series on mobile elevating work platforms, in the edition adopted by your employer's programme or your contract.
• Manufacturer's load chart and operating manual for the specific machine, which owns every capacity figure in this article.
• See related: the library's cards on fall protection systems and on what a personal fall arrest system has to do, for the occupant's protection once the setup is sound.