Construction Plant Loadings: Why Ground Pressure Varies by Plant Type

Tracked plant — excavators, crawler cranes, a lot of piling rigs — runs on a pair of continuous steel or rubber tracks that span the full length of the undercarriage. That geometry carries the machine's weight along a long, unbroken strip on each side: two long, continuous rectangles beneath the machine rather than a handful of separate points. Because that contact area is continuous and comparatively large for the size of the machine, the same total weight is spread more thinly over each patch of ground than it would be if it were pinched into small points.

Wheeled plant — telehandlers, wheeled excavators, and mobile cranes travelling on their tyres rather than rigged on outriggers — works on the same underlying idea of weight over contact area, but the shape of that area is entirely different. Instead of two long strips you have three, four or more separate, comparatively small tyre contact patches, each only as big as the section of tyre actually deforming against the ground at that moment. The same overall weight is divided among fewer, smaller, discontinuous points rather than run along a continuous line.

None of this fixes the ground pressure of any particular model — that depends on the machine's real weight, its exact track or tyre specification, and its load state at the time, and it can only come from that machine's own manufacturer data (more on that below). What we're describing is a structural fact about running gear: tracks and tyres change the shape and continuity of the contact area, and contact area is one of the two things — alongside total force — that decide ground pressure. Two machines of identical weight, one tracked and one wheeled, simply will not load the ground the same way, purely because of that geometry.

Mobile cranes rigged for lifting, concrete pumps, and some mobile elevating work platforms sit in a third category of their own. Rather than travelling on the ground the way tracked or wheeled plant does, they deliberately lift much or all of the machine clear of its wheels and drive the entire reaction down through a small number of extending legs, each ending in a comparatively small foot or float. Instead of spreading load continuously like tracks, or splitting it across several tyre patches like wheeled plant, an outrigger machine concentrates its whole weight — plus whatever it's lifting or pumping — through as few as four discrete points.

This is exactly why outrigger-based loadings are usually the highest point-pressure category on a typical site, even when the machine generating them weighs considerably less than a large tracked excavator working alongside it. That's no contradiction — it's the direct consequence of concentrating a given total force through a handful of points instead of distributing it along a track run or across several tyres. How a mobile crane's total reaction resolves into individual leg-by-leg loads, and how that shifts with boom position and radius, is where the real detail lives — we cover it fully in our mobile crane outrigger mats guide rather than repeat it here.

Contact area and geometry aside, there's a further variable sitting on top of all three categories: whether the plant is static or actively working. A stationary dead weight on tracks, tyres or outriggers is the easy case. Plant that's working is a different matter — a concrete pump mid-pour with the boom out and vibration running through the legs, a crane slewing a suspended load out towards full radius, an excavator swinging a full bucket at reach — all of these can add loading effects on top of the static weight, from vibration through to dynamic and shock loading as mass moves around.

We set out the vibration case for a working concrete pump in more detail in our concrete pump outrigger pads guide. The point to carry away here is a general one: dynamic effects are real, and they need assessing for the actual working condition of the machine — not assumed away by reading a single static weight off a spec sheet and stopping there.

We see plant hire companies group machines into general size or tonnage classes all the time, and that's a useful shorthand for picking a machine with the right lifting capacity, reach or bucket size for the job. But it's a poor stand-in for an actual ground-loading figure when you're making matting decisions, because it bands plant by rough weight — not by how each machine puts that weight into the ground.

A "20-tonne class" tracked excavator and a "20-tonne class" mobile crane on outriggers can sit in the same hire bracket while loading the ground in completely different ways — one down a long continuous track run, the other through four small concentrated feet. The tonnage class tells you which machine to book. It tells you next to nothing about what mats or ground protection that specific machine, in that specific configuration, actually needs, which is why we always push past the tonnage class and ask for the real figures.

The trustworthy source for a specific machine's loading data is that machine's own manufacturer technical data or operator's handbook — not a generic guide, and not a rule of thumb based on its size class. Every handbook or spec sheet states the operating weight in its various configurations. For outrigger-based plant the load chart goes further, giving the reaction at each individual leg across the boom positions and radii the machine can work at — and that leg reaction, not the machine's overall weight, is the figure that matters for matting design.

For tracked and wheeled plant the ground contact pressure is sometimes published directly, and where it isn't, it can be derived from the documented weight together with the stated track or tyre contact area — again, figures that come from the manufacturer's data for that model, not a generic estimate. Because the figure moves with configuration, attachment and load state, the right move is always to ask the plant hire company or the manufacturer for the specific data on the specific unit and set-up coming to site, rather than guessing from a general sense of "what excavators are usually like".

Once you have those real figures in hand, choosing matting becomes concrete. ALIMATS®, our modular aluminium crane mat system built from 6005A alloy, is configurable from 1.34m2 up to 8.07m2 per set-up and rated to a safe working load of up to 80 tonnes — comfortably covering cranes up to 150 tonnes, and independently verified with a Fellow of the Institution of Structural Engineers. Matching that configuration range to a piece of plant is straightforward arithmetic once you have the machine's real reaction figures to work with. Getting hold of the right figures in the first place is the part worth being disciplined about.

Whatever category the plant falls into — tracked, wheeled, or outrigger-based, static or working — the same question from our load-spread calculation guide still governs it: force divided by allowable bearing pressure gives the required contact area. That guide walks through the arithmetic in full, and our ground bearing capacity guide covers how to pin down the allowable bearing pressure for the ground itself.

What varies so much in character across plant types is the force side of that equation, which is why the answer to "how heavy is it" is so rarely the whole answer to "what ground protection do I need". Get the real figures for the specific machine, in its actual working configuration, from the people who hold them. After that, the calculation is the easy part.