Identifying hidden decay in highways fencing

Posted on 22/06/2017

Research by Exova BM TRADA’s Timber Investigations Team has shown that visual inspection of common roadside open-stock fencing, acoustic barriers and anti-glare barriers does not produce reliable results it terms of identifying fugal decay.

Severe fungal decay can be present in posts that visually appear sound, but because the decayed area is below ground, a broken post is usually the first and only visible sign of the problem, which is too late. Leaning posts could be a sign of decay, but much more frequently rotten posts remain upright until they fail. This makes assessment of fencing by way of visual inspection very unreliable, bordering on worthless. Given that potential consequences of fence post failures include debris on a carriageway, or uncontained livestock, it is clearly preferable to identify decayed or partially decayed posts before failure occurs. Specialist inspection techniques make it possible to determine whether a fence post is sound, or partially or extensively decayed, in the ‘high-risk’ zone below ground.
Wood-rotting fungi need the right mix of oxygen and water to develop. Wetting from rain dries relatively quickly in the wind, so although fence post (or panel/barrier) elements which are above ground are regularly exposed to oxygen and water, and airborne fungal spores, for the majority of the time the wood is too dry for fungal decay to develop. Deeply embedded parts are exposed to soil-organisms and are permanently wet, but there is a lack of available oxygen, so fungal decay is arrested. The ‘high risk’ zone is the part of the post that is embedded, but not deeply. In this zone there is a perfect recipe of oxygen, water and soil organisms (including spores of numerous wood-rotting fungi), which is an ideal environment for decay to develop.
In light of the NHSS Sector 4 specification for a 30 year desired service life it might be expected that the age of a fence would be the main factor in its condition (this assumes that accurate records for fencing installation dates are available), with older fence-posts tending to be most affected by fungal decay. This is broadly the case. However, premature failures occur in some fences, while others retain a high percentage of sound posts well beyond the desired service life. Decay rates can also vary greatly between posts of the same age in the same stretch of fencing in the same soil conditions. From an asset management perspective, simply using the age or visible appearance of a fence as a basis for repair/replacement strategies therefore risks unnecessary replacement, or, in the worst case, fencing failures in-service.
The ability to identify individual posts affected by fugal decay and those which are not, and provide data for runs or lengths of fencing to be evaluated as a whole, is a big step forward. It is now possible for our technical experts to carry out physical inspections of the ‘high risk’ zones of individual posts using decay detection probes to monitor their condition without breaching the preservative treatment envelope in that zone. Posts can then be marked on site based on their expected service life. This allows decisions on repair/replacement strategies to be based on empirical data. The cost savings of avoiding unnecessary replacements outweigh inspection costs, and enable future budget planning with greater certainty.
The condition assessment of a timber fence or barrier of any age can now be achieved with a precision that far exceeds simple visual inspection. Posts can be categorised and marked on site based on their condition and expected service life, which allows for easy identification of individual ‘at risk’ posts, and the wider assessment of stretches or runs of fencing.  Advanced warning of fungal-decay related failures in service is now available from Exova BM TRADA, while also providing potential cost savings and greater control of financial planning.  

Photograph 1. The first sign of a decay problem in this acoustic barrier was failure.

Photograph 2. Open-stock fencing with posts extensively decayed below ground (marked red) that remained upright without leaning over.

Photograph 3. Visual inspection does not identify decay to the embedded part of the post.

Photograph 4. Below ground, fungal decay can be severe (red), moderate (yellow) or absent (unmarked) in adjacent posts that have a similar appearance above ground. 

Fig 1. The high risk zone for fungal decay in fence posts