Asset ownership and maintenance comes with many considerations regarding life span, durability and risk. This is especially true with natural assets like wood poles. Even when treated with preservatives, wood poles still decay and have an average expected life span of 40 to 65+ years, depending on the type of timber used and their environment.
This can lead you to question the status of the pole and its remaining life expectancy. Like most questions regarding assets, the answer is subject to a range of factors that cause degradation. In all networks around Australia the average wood pole age is increasing to concerning levels. This leads to a growing number of pole failures. This, combined with tight capital and maintenance budgets, makes the issue of durability even more pressing.
This blog will outline the major decay issues facing wood poles, the importance of inspections, and how to best test for durability issues in your network.
What are the major decay issues for wood poles?
Ignoring the impacts of wood damaging borers, termites, fire and other animals (like woodpeckers), there are two main types of durability issues encountered by wood poles. These are internal decay and external decay, and these are generally split into two regions; ground line critical zone (~1m above ground down to 500mm below ground), and everywhere else (typically focused on above ground, which is sometimes also split into 2m from the pole top to the tip and everywhere else).
Internal decay is the deterioration of the pole in the heartwood and commonly starts either in the pith (centre) or at the interface between the sapwood and heartwood. The chances of this type of decay occurring tend to increase if bore (drill) tests are conducted on the pole. Generally, the heartwood is untreated by preservatives, and doesn’t contribute a large amount to the strength of the pole, unless the treated sapwood is quite thin (as with some hardwood species). Hence, internal decay tends to be less critical than external decay.
External decay is a loss of sound timber from the external circumference of the pole. Factors that contribute can include weathering, biological decay and chemical degradation (leaching, evaporation and chemical change). Loss of external sound wood has a considerable impact on the remaining strength of the pole and depending on the type of wood, the preservative used, the preservative loading and the environment, decay rates will vary widely. In many ways this is the easiest decay to detect, which is why good wood pole inspection programs will pick up most of the major issues before failure.
Why is it important to test for these decay issues?
Testing for durability issues in a wood pole is vital in determining its strength and serviceability. When poles were first installed, most were only designed to support power lines and streetlights. Since then, telecommunications, cable television, internet and mobile data carriers have added more to the poles to connect the public with advancing technologies. This, combined with increases in frequency and severity of extreme weather events, has seen more stresses on wooden poles than originally expected.
By ascertaining the deterioration and degradation of the pole, predictions can be made as to the likelihood and timing of the next failure, and to prevent this through maintenance or by replacing the pole.
Due to the expensive nature of replacing a utility pole, it is more cost effective over both the short and long term to accurately determine the health and remaining serviceability of each pole through a regular inspection program. An accurate and effective asset inspection process helps to strike a balance between identifying poles that have some deterioration, but can remain in service for some time, and alerting you to the poles that put both the reliability of the network, and human life, at risk.
With a repeatable, reproducible and reliable, inspection program you can gain a greater understanding of your asset base. This leads to a cost-effective way of managing and extending the life of your utility poles, while minimising the number of still serviceable poles being replaced.
Types of decay
There are three broad categories for decay organisms: Brown Rot, White Rot and Soft Rot. Brown Rot organisms and Soft Rot are the two most common around ground line for poles. White Rot is more common in the above ground areas because it is more efficient at utilising available nitrogen in the environment and doesn’t need it from the soil.
Brown Rot is brown because the decay attacks the cellulose (white) and leaves the lignin (brown). When wet it is spongy but becomes brittle and blocky when dry.
Soft Rot can attack both the cellulose and lignin. However, its ability to primarily decay the s2 layer of the wood cells means that it can deteriorate treated timber that has had insufficient pressure, or time under pressure during treatment for the preservative to penetrate far enough into the cell wall. If you have softening of the surface fibres of preservative treated timber, then this is the most likely culprit.
There is another type of decay that appears to be a soft-rot style decay that attacks the heartwood of naturally durable hardwoods and causes carroty type fractures of otherwise sound feeling wood. That type of rot tends to occur in old poles and is becoming more of a concern as it is difficult to detect using traditional and newer NDT methods.
White rot attacks the natural glue (lignin) so leaves the white fibrous cellulose behind. Hence the name. It is the most common culprit for above ground decay and produces fruiting bodies (look like mushrooms) on the surface of the pole. The fruiting bodies only seem to occur after considerable loss of cross section. Before that it can be hard to detect as it is out of reach of most methods.
How to best test wood pole durability?
Traditional methods of inspections for wood poles are being proven inefficient in large ageing populations. The current methods of testing used are rudimentary and rely heavily on either subjective assessment techniques like hammer sounding or limited destructive testing where the pole is drilled into, assessing only a very small portion of the pole cross section. Most inspection programs require the replacement of a significant number of poles with adequate remaining strength to try and make sure they remove the ones that are highest risk.
As technology develops, testing for pole durability becomes easier, faster and produces more accurate results. Evaluating wooden poles using Non-Destructive Testing (NDT) is the method we recommend. NDT is an asset inspection technique involving detecting and evaluating flaws to test and analyse an asset without causing any damage.
For more information on what NDT is, check out our blog on the topic here.
There are a range of different NDT testing methods designed to ensure the wood pole can continue to perform effectively, reliably, and safely. At Revo Group, we specialise in Non-Destructive Testing across a range of different industries.
Noticing a gap in NDT tools for wooden assets, we have developed our own inspection device. Proudly made by us in Australia, the PortaSCAN CA is the latest model in our range of portable density gauges. The PortaSCAN CA is used to identify variations in timber density. This portable device is a simple and fast tool that assists inspectors in examining what lies beneath the surface of the timber under assessment. The lightweight nature of the NDT device makes it perfect for utility poles, as it can scan all the way up to and including the cross arms.
While the PortaSCAN CA is good at what it does, we recognise the role that many different devices can play in wood pole inspections. Each device has strengths and weaknesses in certain situations and we are happy to use devices that suit the task rather than trying to just push the one technology. When considering devices, we look at the type of wood to be inspected, the type of treatment, the age of the population, the environment, and the likely degradation mechanisms, including but not limited to the likelihood of internal and external decay.
Internal and external decay are key degradation mechanisms for wood poles and are the main reason why wood poles require replacement, and why they fail in Australia and New Zealand (and globally). Whilst there are several devices that have been developed to detect internal and external decay over the years, of which the PortaSCAN CA is one, they all still need to be combined with a thorough visual and sounding regime to ensure the inspection process is as reliable as possible.
For more information or to discuss your inspection techniques and how we can help with operational efficiencies, please contact us.