Do earthworks fail if trees are removed?
The claim that the removal of trees makes earthwork failure more likely is often made by those opposing tree clearance adjacent to the railway. GARETH DENNIS unpicks the scientific and practical realities to understand if there is any truth behind it.
A version of this article also appeared in Issue 946 (15 December 2021) of RAIL magazine.
Network Rail figures indicate that the organisation and its contractors cleared as much as 500 hectares of tree covered land in 2020/21. It’s a significant volume of felling, particularly if you consider the noise being made by conservationists over the comparatively meagre 30 hectares of clearance being undertaken for the construction of High Speed 2.
The responsibility of an organisation with such a vast range of habitats under its stewardship is immense, and that is before we start thinking about the implications for railway safety and reliability. Because this felling isn’t done for reasons of cost-cutting or convenience — trees (or their branches) fall or can be blown across tracks, onto overhead wires or into trains. Dropped leaves and their detrimental effect on rail adhesion impact drastically on performance and safety, with the frightening Salisbury collision showing vividly what the consequences of that can be.
Balancing the need to clear vegetation to keep trains moving (or not moving) with the potential ecological benefits of its retention is consequently challenging. With a father who is an ecologist, and as someone who spends a lot of time out in the wilderness, I am deeply attached to wild spaces. As an urbanist, I also put a lot of value on the pockets of green space that the railways often provide in otherwise built-up areas. So I understand and share the pain that comes from seeing trees being felled.
Every year, tree felling in one or more invariably suburban locations raises a local community’s hackles and Network Rail comes under pressure to limit or alter its programme of clearance. Bear in mind that infrastructure budgets continues to be squeezed, and in the last year staff numbers have been hard hit. Limited funding and staffing rarely improve performance, and this is as true for communication with local communities as it is with the appropriate administration of the chainsaw.
The raised voices often (rightly) cite the habitat value of railway corridors for flora and fauna of all types — but there are also those who attempt to make claims about the necessity of trees for earthwork stability, and it is these claims that are problematic. By sounding sufficiently technically convincing, and with very few people familiar enough with the subject to challenge them, such claims can propagate quickly.
There are 10000 kilometres of earthworks (both cuttings and embankments) under Network Rail’s charge. I’ve spoken more broadly about maintaining earthworks in a previous Analysis following the tragic derailment at Carmont (RAIL912), but the number of annual earthwork failures has increased from as few as 65 a decade ago to nearly 150 failures in 2020/21.
The changing climate has a significant bearing on this figure, but how does the presence of trees impact on the likelihood of an earthwork failure? In RAIL912 I explained how soils are a mixture of solids, water and air, and how increased pore water pressure pushes soil particles apart reducing friction between them, which in turn reduces earthwork stability.
Trees impact on pore water pressure primarily through two mechanisms: shrink/swell and root suction.
Mature trees can extract large quantities of ground water via transpiration through their leaves. In clay soils (soils can generally be split between clay and sandy types), this removal of water can result in significant reductions in soil volume as decreased pore water pressures pull soil particles together. Winter rainfall without the corresponding removal of water via leaf transpiration results in an increase in soil volume, as pore water pressures are increased pushing soil particles apart. This cycle of shrinkage and swelling can result in movements measured in centimetres, which can impact heavily on track condition and ride quality, and can stress earthworks towards failure.
Root suction also results from water extraction from the deep root systems of mature trees, creating a steady suction of water through cohesive, saturated soils. This can act to reduce or even eliminate the pore water pressure variations resulting from seasonal rainfall, providing a measurable benefit to earthwork stability. Removing these trees can therefore impact on slope stability.
But what of the balance between these two mechanisms? The science suggests that retaining trees over the lower third of the slope can help hold soil together at the base of the earthwork whilst also limiting pore water pressure variations in the upper parts of the slope. This in turn can reduce the shrink/swell associated with seasonal rainfall (clearly this can only apply for embankments given that for cuttings the track is located adjacent to the toe!).
To understand the impact of these mechanisms in practice, I spoke to Network Rail asset managers with experience of looking after a huge range of earthwork types across the country.
It quickly became clear that tree felling is a last resort measure. Not primarily because of environmental concerns (though I was pleasantly surprised by how much biodiversity was a personal passion of everyone I spoke to) but because felling trees is dangerous, challenging, cost and time intensive, and can involve closing the railway.
The value of retaining trees where possible was clear. The root suction benefits, particularly at the toe of embankments, were well understood and known to improve the stability of earthworks against slips. Trees that form a reasonably dense canopy also reduce the growth of dense scrub such as bramble and bracken.
It is worth pointing out that Network Rail clear as much as 1000 hectares (double the volume again of the tree felling they do) of scrub a year, much of which is monoculture with little habitat value and is a menace for inspection, maintenance and enhancement teams. When it comes to earthworks, it prevents access for maintenance and it blocks out newer forms of automated inspection such as using UAVs or front-facing cab cameras.
The need for tree removal was explained — particular problems for earthworks occur in clay soils and on rocky slopes, and where roots or leaves block or disrupt drainage systems. But the picture was not clear cut at all, so to speak. Far from “do we or do we not remove trees”, input from experts across disciplines looked at each location in detail to understand what the most useful mix of vegetation including trees would be to optimise earthwork health, railway safety, access and biodiversity.
This said, the asset managers I spoke to explained that earthwork failure events resulting from the removal of vegetation were rare, and indeed the only examples they could recall resulted from last-minute changes preventing the intended mitigations to be applied, or from extreme rainfall occurring part-way through vegetation clearance works, again before final mitigations had been applied.
There are general rules that Network Rail now follows in relation to foliage cover on its slopes — in the case of a cutting face, the diagram accompanying this piece shows an approach that is common. Within 6.5 metres of the nearest rail, all woody vegetation is to be removed. In a band 6.5 metres to 13 metres away from the nearest rail, only trees with an average mature height above 6 metres shall be removed. Beyond these limits and within the railway boundary, a judgement is made on trees that represent a risk to the operational railway, and those which could impact on safety are removed.
The implications and ways to achieve “no net biodiversity loss”, a requirement by 2024 introduced under Jo Johnson during his tenure as Minister for Rail in 2018, is not yet entirely clear, but bringing in more ecological expertise is helping demystify things. Certainly, there is a much greater understanding of the value of marginal habitats now than there was a few years ago — biodiversity is generally at its greatest in heterogenous habitats or at the margins between habitat types. This can map well onto the general rules for vegetation management above.
As part of our fight against climate change, the railways need to double their capacity by the middle of this century — and climate change is by far the biggest long-term threat to Britain’s habitats. Diseases that would not have reached our shores even a decade ago are now ravaging woodland stock to the tune of tens of thousands of trees a year, according to the National Trust. Increasingly common extreme weather events accelerate the loss of diseased but recovering trees and divert resources away from habitat renewal.
To achieve this growth in rail capacity, we need a railway system that is safe, resilient and reliable. We must therefore empower our infrastructure manager — in Wales, Scotland and England that’s Network Rail for the most part — to remove trees where it is necessary.
Whilst that means appropriately funding Network Rail so that it can make proper assessments of environmental and engineering factors in relation to trees on earthworks, and resourcing teams such that they can carry out works and communicate with communities throughout the process, it also means pushing back when misinformation is being spread.
The picture is immensely complex, and needs must be determined on a site-by-site basis; as we’ve explored, mature trees can serve quite a few useful functions including in providing additional earthwork strength, but to say that they are a necessary component for stable earthworks is not borne out by the evidence. Let’s hope claims to the contrary no longer go unchallenged.