From the Ground Up: How to Assess Site Ground Conditions

Stability is fundamental to the safe use of powered access platforms. Understanding and accurately assessing the ground conditions is therefore absolutely crucial. In this guide, we answer some of the most commonly-asked questions about ground conditions.

Do I only need to assess ground conditions if I am using a platform that relies on jacks, stabilisers or outriggers?
It is good practice to assess the ground conditions, no matter what type of platform you are using. We would advise you to carry out a site assessment even before taking an access platform across the terrain, never mind setting it up or attempting to operate it.
Most self-propelled machines, such Genie booms, Genie scissors and MEC rough terrain scissors can be driven while elevated. If you don’t check the terrain before driving over it, you are risking a loss of stability that could result in the platform toppling over. For example, moving from hard ground to much softer ground can cause a self-propelled lift to overturn.

How can I spread the ground pressure of outriggers, jacks or stabilisers?
The surface area of the “foot” of an outrigger is relatively small, and consequently generates high pressures on the ground. A lot of surfaces are not capable of supporting these high pressures, including most types of soil and unmade ground, as well as some paved and tarmac-covered areas. Outrigger pads – also known as spreader plates - help to reduce the pressure to an acceptable level.
IPAF’s Spread the Load campaign advises that “spreader plates should always be used with boom-type mobile elevating work platforms (MEWPs) when fully supported on their outriggers. Spreader plates should be used with all other MEWPs that have outriggers unless a risk assessment indicates they are not necessary”. In addition, very poor ground conditions may require additional foundations, such as ground mats, timber mats, proprietary mats, steel grillages or concrete pads.

What if I am driving a self-propelled machine in the stowed position?
Again, industry best practice is to assess the ground conditions before driving, even in the stowed position. Driving from stowed significantly reduces the risk of toppling over; however you can still get stuck in unexpected soft or boggy ground. This can cause delays on site while your platform is freed - and you may end up having to pay recovery costs.

What are the most common ground condition hazards?
The most common ground condition hazards that you might encounter include the following:

1. Uncompacted fill
Soil or other fill material can be piled into a backfilled trench without being compacted. A sure sign is visible cracks along the line of the trench.

2. Nearby excavations
You should not position powered access platforms close to the edge of a trench or other excavation, as these are likely to collapse without warning. If the machine needs to be used in such a way that its outriggers or wheels will be close to the edge of such a feature, IPAF recommends that a competent geotechnical engineer carries out an assessment before allowing the platform to be positioned, set up and operated.

3. Floors, cellars and basements
Many floors, cellars and basements are simply not designed to bear the weight of a powered access platform. It is vital that you take into account the strength of floors and location of cellars/basements before positioning a platform in this type of area.

4. Paved Areas
Paved areas can look deceptively strong but might have been laid on weak ground underneath. You should consider footpaths as suspect, because there could be weaker ground or shallow services underneath the surfacing.

5. Underground Services
Sewers, drains, manholes, and gas and water mains could all be damaged by the weight of a platform. They could even collapse, causing the platform to become unstable or overturn.

6. Weather Conditions
As ever in the UK, the weather can be a major factor. Heavy or prolonged rain can alter ground conditions and cause sinking of outriggers or wheels. If you suspect that the ground supporting a platform is getting softer, you should make regular checks on the machine level and make any appropriate adjustments to outriggers, spreader plates and pads. Also make regular checks when frozen ground is thawing out, since it can appear to be much firmer than it actually is.

How are the different types of ground classified?
The following industry classifications apply
1. Greenfield sites
2. Brownfield sites
3. Pavement and roads
4. Town and city centres
5. Beaches

What should I look out for on a greenfield site?
Areas of high-risk on Greenfield sites include fields next to rivers, estuaries and flood plains. These areas often have soft ground and high water tables.

What dangers can the ground pose on a brownfield site?
There are huge variables on brownfield sites. In particular, problems could arise from hidden basements and cellars, underground storage tanks, and backfilled pits.

What should I look for when working on paved areas or tarmac?
A main road that HGVS regularly drive over – and still shows no major signs of distress – should be of less concern than a car park or a route through an estate. Footpaths are definitely worth close inspection, as the surface can be thin – and there could be services or weak ground beneath. The edges of paved areas are also a weak point and you should take extra care around them.

What special measures should I take when working in urban areas?
The nature of urban living means that there will be a high prevalence of underground hazards. These can include live services, manholes, drains, cellars, basements, sewers, and even poorly backfilled trenches.

What conditions should I check for on a beach?
Beaches can be tricky places on which to work. Patches of sand with low density can easily shift under an outrigger or a wheel. The variable water table can also create challenges.

How do I monitor changes in ground conditions from the platform?
All modern powered access platforms are fitted with level sensors. Pay attention to the indicators and act immediately if any alerts or warning lights are activated. If the level indicator indicates that you are exceeding the machine’s operating limits, you should lower the platform, then reset the machine until the indicators confirm that it is in a level position. If you are on soft ground and fear that outriggers or stabilisers could sink during operation, make regular checks and adjust accordingly.

Where can I find further information?
The IPAF website has a superb resource library, including advice on assessing ground conditions:

Outriggerpads provides a comprehensive range of ground mats, outrigger pads and spreader plates for powered access platforms, cranes and other mobile plant:

The Outriggerpads calculator also helps you to work out which spreader plates are most suitable for your fleet:

For detailed advice on assessment of ground conditions, calculation of bearing pressure and selection of spreader plates, read Crane Stability on Site, C703, 2003, published by the Construction Industry Research and Information Association (CIRIA).

You can also find additional guidance in the British Standards publication
BS 8460:2005 Safe use of MEWPs — Code of practice.