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Plant hire depot surfacing - why depot yards fail in the same three places

In short

PKB Civils are plant and equipment depot yard specialists. We build and repair the concrete surfacing and drainage that keep plant hire depots operational. Plant hire depot surfacing does not wear evenly. It fails in three places, and it is the same three places on every depot: along the routes the machines drive, at the point where they are loaded onto low-loaders, and around the wash bay. All three fail for the same reason, and it is not the concrete. It is the sub-base underneath it. Which is why patching the surface buys a season and not a solution.

The assumption is usually that heavy plant is simply too heavy for the yard. The numbers say otherwise. A 20 tonne tracked excavator puts around 47 kilopascals through the ground, because that weight is spread across two large track footprints. A loaded HGV tyre puts roughly 700 kilopascals through a contact patch the size of a dinner plate (Caterpillar 320 GC specification, 2022). Standing still, the excavator is gentler on your slab than the wagon that delivers to you.

This article explains what actually causes the three failure points, what happens underneath the slab once they start, how to check your own depot, and what good specification looks like on a site that cannot stop for it.

Why plant hire depot surfacing fails in three places, not evenly

A yard looks like one continuous surface, but it never takes one continuous load. Traffic in a depot is channelised. Machines come off the low-loader, follow the same line to the same standing area, and go back out the same way. Loading happens in the same bay because that is where the space and the access are. Everything gets washed in the wash bay, because that is where the drainage went in.

So while the yard as a whole sees moderate use, three narrow zones absorb a hugely disproportionate share of the work. If a yard were simply old and worn you would expect even deterioration. You do not get that.

You get three concentrated failures with sound concrete in between them, which tells you the surface is not the problem.

The three failure points behind almost every depot yard breakdown

  • The routes the machines drive. Not the weight, the way the load is applied. Tracks do not steer, they slew. One track drives while the other brakes, dragging steel grousers sideways across the surface, and concrete is far weaker in surface shear than it is in compression. Add telehandlers running high axle loads on comparatively small tyres, and outrigger pads concentrating several tonnes through a few hundred square centimetres, and you get fretting and spalling in narrow bands that follow the running line.
  • The low-loader loading point. The most demanding square metre on the site. Every machine out and every machine back crosses the same spot, and a machine coming off a beavertail transfers weight abruptly, so the slab takes repeated impact rather than the static load it was designed for. Loading points also tend to sit at a slab edge or hard against a joint, where capacity is roughly half to two thirds of the middle of a bay (Concrete Society TR34). The heaviest loading in the yard lands on the weakest part of the slab.
  • The wash bay. A Type 1 sub-base works because it is well graded, compacted and able to drain. Keep it saturated and it stops behaving like sub-base. Settlement then flattens the falls, so water that used to reach a gully sits in a depression instead and the saturation accelerates. Meanwhile silt washed off the plant blinds the gullies and clogs the separator, so the water goes across the yard rather than down the drain.

What happens underneath once the surface breaks

The damage you can see is the last stage, not the first. Two things are going on below it.

First, the sub-base starts pumping. Water gets in through a joint or a crack, and repeated loading pumps it back out again carrying fine material with it. Over months that migration of fines creates voids under the slab. The concrete has not got weaker, but it has lost its support, so it starts failing under loads it carried without complaint for years. There is a simple test for this: watch a joint as a loaded machine crosses it, and if water or grey slurry squeezes up out of it, the sub-base underneath is voided.

Second, patching makes it worse. A concrete slab is a load-spreading layer, and it only spreads load if something solid is holding it up. Break out four square metres of failed concrete, pour new concrete into the same hole, and you have replaced the part that was working and left the part that had failed. The patch is now a small isolated slab with four free edges and no load transfer to the concrete around it, which is the weakest thing you can build. That is why patches at a loading point often go faster than the original surface did.

What good specification looks like

  • A proved formation. Tested before anything goes on top, with soft spots dug out and replaced rather than bridged over. A soft spot left in place will find its way to the surface within two winters.
  • A geotextile separation layer. Stops fines from the subgrade migrating up into the sub-base and stops the sub-base punching down. This is the layer that resists the pumping mechanism directly.
  • A sub-base matched to real point loads. 300mm of Type 1 granular to Specification for Highway Works Clause 803, laid and compacted in layers rather than a single deep lift, with the loading point treated as its own design problem rather than covered by a yard-wide spec.
  • A slab designed to the machines. 200mm C40 with A252 mesh is typical for depots handling tracked plant and low-loaders, with polypropylene fibre in the mix to control early age shrinkage cracking, and a thickened edge where machines load. Thickness set by loading, not by habit.
  • Joints on a designed layout. Saw-cut induced joints so cracks go where you put them, dowelled where load transfer matters so adjacent bays work together instead of independently.
  • Drainage sized for the work it does. Designed falls to an adequate number of gullies, with silt traps where plant is washed or tracked mud is likely, and kerb restraint at the slab edges to contain both the concrete and the water.
  • A wash bay built to contain. An impermeable surface, contained so wash water cannot escape and clean water cannot get in, drained through a silt trap and an appropriately sized separator to BS EN 858. Enforcement is real: Winvic Construction Ltd was fined £160,000 plus costs at Nottingham Magistrates Court in March 2023 after silt-laden run-off entered a watercourse.

A rebuild without closing the depot. An Exeter example

A plant hire depot in Exeter shows how this gets fixed on a site that cannot stop. The fleet still had to go out every morning, which is the real reason depot yards get patched rather than rebuilt. It is a sequencing problem, not a budget one.

The yard was sectioned and rebuilt one area at a time, with a defined route in and out maintained for low-loaders throughout the programme and each section handed back before the next one started. The finished build-up was 1,000 square metres of new concrete at 200mm C40 with mesh and polyfibre over 300mm of Type 1, with nine surface water gullies, a new wash bay and new kerbing throughout.

One thousand square metres rebuilt. Nine gullies. The fleet out every morning.

How to check your own depot yard

An hour with a camera will tell you most of what you need to know. Look for:

  • Water or grey slurry squeezing out of a joint as a loaded machine crosses it
  • Slabs that rock or step at the joints, particularly near the loading point
  • Fretting, polishing or spalling in narrow bands that follow the machine route
  • Damage clustered within ten metres of where machines come off the low-loader
  • A wash bay still holding water twenty minutes after the wash has finished
  • Gullies and silt traps that are full, or a separator nobody can remember servicing
  • Previous repairs that are failing at their own edges
  • A dated photo log of all of the above

The question a proper assessment then answers is whether each defect is surface-level or structural. Damage over sound support is a repair. Damage over a failed sub-base is the start of something bigger, and patching the surface only buys the failure a few months.

FAQ's

For yards handling heavy plant and HGVs, PKB Civils typically specify a minimum of 200mm reinforced concrete with A252 mesh over a 300mm Type 1 sub-base. The exact figure depends on your machine weights, ground conditions and how the yard is used, which is why we survey and design to the site rather than applying a standard slab.
Water enters the crack and washes out the fine material in the sub-base beneath, so the slab loses support where it is already weakest. HGV loading then widens the crack, spalls the edges and turns a hairline defect into a pothole. Water trapped in joints over winter freezes and expands, accelerating the damage.
Because turning tracked plant applies horizontal shear rather than vertical load. One track drives while the other brakes, dragging steel grousers across the surface. Concrete is strong in compression and much weaker in surface shear, so turning areas fret and spall long before the rest of the yard shows wear.
Trailer legs and axle groups concentrate weight into the same fixed points on every visit. Unless the sub-base and slab in those zones were specified for repeated point loading, they fatigue faster than areas that only see moving, distributed traffic.
Every machine out and every machine back crosses the same spot, and coming off a beavertail transfers weight abruptly rather than steadily. That fixed point usually sits at a slab edge or joint, where capacity is significantly lower than mid-bay, so the heaviest and most dynamic loading in the yard lands on its weakest area.
Environment Agency guidance is that wash water containing oil should pass through a silt trap and an appropriately sized separator to BS EN 858 before discharge, and that discharge to foul sewer requires trade effluent consent. The right class and size depends on your discharge route and volumes. We build the containment and install the system to specification. Confirm your own obligations with the Environment Agency and your water company.
Patching is cheaper once. It is rarely cheaper over five years, because a patch over a failed sub-base repairs the symptom and creates a small isolated slab with four unsupported edges. If the failure is localised damage over sound support, patch it. If the sub-base has gone, patching is a holding measure rather than a repair.
Yes, this is the normal way we work. We phase the programme in sections, agree and maintain a route in and out for low-loaders throughout, and hand each section back before starting the next. The programme is planned around your movements before we start rather than worked out on site.

What a plant hire depot specialist does differently

Surfacing on a live plant hire depot is not general groundworks. The constraints are specific: point loading from telehandlers and outriggers, turning shear from tracked plant, a fixed loading point taking repeated impact, a wash bay that has to contain what comes off the machines, and a fleet that goes out every morning regardless. A specialist approach looks like this:

  • Root cause, not symptom. Repairs that address the failed sub-base, lost falls or undersized drainage causing the problem, not just the surface it shows up on.
  • Live-site phasing. Sectioned programmes with a low-loader route maintained throughout, sequenced around your movements so the fleet keeps working while the yard gets rebuilt.
  • In-house teams only. Directly employed crews, the same team from site survey to handover, and one point of contact throughout. No subcontractors, no gaps in accountability.
  • Honest assessment, fixed price. A free site survey with no obligation, and a fixed-price quote based on actual site conditions rather than a walk-past estimate.
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Plant hire depot surfacing – why depot yards fail in the same three places PKB Civils News