Ground protection for tracked plant near trees

Ground protection for trees on a construction site is a load-spreading problem, and the specification changes completely between foot traffic and tracked plant. BS 5837:2012 treats the two cases separately for good reason: a single layer of scaffold boards over a compressible layer is adequate for people and wheelbarrows, and useless under a 5-tonne excavator. If a route for plant has to pass inside a root protection area, the protection needs designing for the actual load rather than copied from a standard detail.

Most damage to trees on building sites happens in the first fortnight, before anyone has read the arboricultural method statement, and almost none of it is visible at the time.

Why compaction is the main risk

Tree roots need air. A healthy soil has a network of pores that hold oxygen and allow carbon dioxide to escape, and the fine absorbing roots sit in the top 600 mm where that network is best developed. Compaction collapses it.

The consequences arrive slowly. Reduced gas exchange, standing water, root death, then crown dieback two or three growing seasons after the machine passed over. By then the connection between cause and effect is hard to prove and impossible to reverse, which is why the planning system deals with this through prevention rather than remedy.

Severance from excavation is more obvious and easier to argue about. Compaction is the quiet one, and it is the reason a construction exclusion zone exists at all.

Pedestrian traffic compared with tracked plant

The distinction that matters is contact pressure and the number of passes.

A person applies a low pressure over a small area and does little harm to a soil already covered by a load-spreading layer. A tracked excavator applies a lower ground pressure than a wheeled machine of the same weight, but it applies it repeatedly along the same line, and the dynamic loading from slewing and digging concentrates stress at the track edges. A wheeled dumper is worse again, because the load sits on four small contact patches.

BS 5837:2012 reflects this. For pedestrian movement it describes a single thickness of scaffold boards laid on a compressible layer over a geotextile membrane, or a proprietary ground protection system. For wheeled or tracked plant it expects an alternative system designed in consultation with an engineer, usually a geotextile separation layer, a granular layer of specified depth and interlocking load-bearing panels on top.

The specification you submit should name which case applies to which route, with both shown on the tree protection plan. A method statement that specifies boards and then shows a plant route through the same area will be picked up by any competent tree officer.

Scaffold board systems and their limits

The scaffold board detail works when it is built properly and used for what it is for.

A correct build-up is a geotextile membrane laid directly on the existing soil surface without stripping, a compressible layer of woodchip or similar spread evenly over it, then scaffold boards laid tight and secured so they cannot shift underfoot. The compressible layer does the work. Boards laid straight onto soil transmit point loads at the board ends.

Its limits are real. It is for pedestrian traffic, it is not a wheelbarrow route for a full working day, and it does not survive being driven over by a telehandler at the end of a wet week. If the site needs materials moved across a root protection area by machine, say so at method statement stage and design for it, rather than hoping that boards will cope.

The fencing question sits alongside this. Where ground protection replaces fencing inside the root protection area, the boundary of the protected area still needs marking so nobody parks a van on it.

Proprietary systems and geotextile build-ups

For plant loading the usual answer is a proprietary system, either interlocking plastic panels or a three-dimensional cellular confinement layer filled with granular material.

A typical engineered build-up has a geotextile separation layer on the existing ground, a granular fill layer designed to spread the wheel or track load to an acceptable bearing pressure, and a running surface of load-bearing panels or road plates. The depth of the granular layer is the design variable, and it depends on the machine, the number of passes and the soil. On a soft clay in winter it can be substantially deeper than the generic detail in a method statement template.

Two details get missed. The separation geotextile has to be continuous and lapped, or granular material migrates into the soil and the protection becomes permanent damage. And levels matter: building up 300 mm of granular fill against a stem changes the soil atmosphere at the root collar and can cause bark decay, so the build-up should be held off the stem with a clear zone.

Specification by an engineer is what BS 5837 asks for here, and the arboriculturist's role is to set the constraints rather than to design the pavement. Both names should appear on the method statement, as we discuss in the context of other reports in scaffolding and crane oversail over a protected tree.

Load cases and duration

Three questions decide the specification, and a Tree Survey carried out to BS 5837 should give you the information to answer them.

1. What is the heaviest machine that will use the route, laden, and what is its ground pressure?
2. How many passes over how many weeks, and in which season?
3. What is the soil, and what is its condition when wet?

A single delivery across a dry summer sward is a different proposition from a daily muck-away route in February. Duration also governs aftercare: protection left in place for 18 months creates its own problems with light exclusion and soil temperature, and anything semi-permanent should be designed as such.

Winter working on clay is where most schemes come unstuck. If the programme puts groundworks in the wet season and the only site access runs past a mature oak, that is a design problem to solve at planning stage and not a matter for the site manager.

Where the protection must extend to

Protection covers the root protection area, which BS 5837:2012 derives from stem diameter and which may be modified in shape where soil conditions or existing structures justify it, subject to the area being maintained.

The practical errors are consistent. Protection is laid out from the trunk by eye rather than from the plotted root protection area. Off-site trees are left out because their stems are on the neighbour's land, even though their root protection areas cross the boundary. And the area is sized for the tree alone, with no allowance for the 2 metres of working space the operative needs outside it.

Plot it, dimension it, and mark it on the ground before anything arrives on site. The pre-commencement site meeting with the tree officer is the right moment, and it is often a condition of the permission.

Removal and aftercare

Removal is part of the specification. Panels and granular fill come up by machine working off the protected surface, never by a machine standing on the exposed soil, and the geotextile is lifted with the fill above it.

Then the soil is assessed. Where compaction has occurred despite the protection, decompaction by air spade or by careful fracturing with hand tools, followed by mulching, is the normal remedy, and it should be specified in advance rather than negotiated afterwards. Mulch to a depth of around 75 mm over the previously protected area helps re-establish the surface soil structure.

Aftercare also means watering through the first dry summer after works, because a tree whose rooting environment has been disturbed loses the buffer it had. Where new planting forms part of the scheme, tie that into the wider Landscape Strategy rather than treating it as a separate exercise. And keep the monitoring reports: a tree that declines four years later will be the subject of an enforcement conversation, and the record of what was done is the only defence available. The risk of acting before permission is covered in clearing a site before an application and the risk.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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