Timber is used extensively throughout the network and is found in many critical applications including track bearing longitudinal waybeams (wheel timbers), structural applications and decking soffits.
Exova BM TRADA provides specialised timber inspection services which provide an enhanced level of detail and expertise which complements regular maintenance examinations of structures carried out by Network Rail framework consultants. Our services provide a level of detail that enable asset managers to co-ordinate decisions and actions across timber containing assets which can deliver enhanced service life and more efficient use of resources and more effective risk management.
Our condition-based approach to supporting asset management has two purposes. The first immediate purpose is to detect timber defects such as decay and/or mechanical failure that could endanger safety and reliability of railway traffic. The second purpose is carry out periodic inspections, usually at 5 year intervals, which, using evidence-based knowledge on how timber deteriorates, can deliver accurate condition-based monitoring. This in turn provides insight into the asset’s behaviour over time which can enable condition forecasting and planning of maintenance or renewal.
A range of softwoods and more durable, stronger hardwoods are used throughout the rail network. It is the variety of timber species, type of wood preservation and natural variability of timber that can present asset managers with difficult decisions. It is this variability that makes it difficult for asset managers to predict performance of timber components. Furthermore, regular examinations take into account the whole structure and do not provide the level of detail required to make accurate predictions relating to condition and service life of timber with the result that, often, timber components are removed prematurely from the network.
There are two factors to consider. These are condition and strength and both are inextricably linked.
Timber and timber-based products become vulnerable to fungal decay if the moisture content exceeds the decay threshold, which is nominally 20%. The service environment of timber in railway infrastructure results in these being exposed periodically to wetting for prolonged periods.
Timber surveys are carried out using non-destructive techniques which include decay detection drilling with microprobes. Core samples are not removed as this can breach any protective preservative envelope and can provide a wetting pathway.
As decay develops, it usually progresses at an increasing rate with the result that the subsequent loss in strength will not be linear and cannot be predicted accurately. However, it is possible to evaluate if the extent of decay has reduced the cross section of the member to the extent that strength has been affected. If strength has not been affected then implementing a remedial preservative treatment regime can halt further deterioration and extend service life.
Condition surveys are complemented with visual strength grading and species analysis. The strength characteristics of the structural timbers are determined by undertaking indicative in situ visual strength grading of the members. This in situ grading is carried out using the requirements of the relevant British Standards as guidance. These standards are:
- British Standard BS 4978: 2007+A1 2011 ‘Visual strength grading of softwoods. Specification’.
- British Standard BS 5756: 2007+A1 2011 ‘Visual strength grading of hardwoods. Specification’.
Small specimens (matchstick size) are removed to confirm species using microscopic techniques so the correct strength class for the species/grade combination can be assigned using British Standard BS EN 1912: 2012 ‘Structural timber. Strength classes. Assignment of visual grades and species’.
A recent example of work carried out by Exova BM TRADA was to support AECOM in the examination and assessment of over 70 timber containing structures throughout Scotland. The output from this work was to confirm the condition and strength of these timber assets and to support Network Rail in developing an inventory of these assets ranked by condition and priority for interventions ranging from interventions to maintain performance and to extend service life, to prioritising renewals on evidence-based condition assessment.
Exova BM TRADA consultants are PTS AC/DCCR certified.