By Hannu Lehtinen, Rovakaira Networks Construction Ltd
|A creosote treated pole showing cracks breaching the outer preservative protected sapwood.|
The North American Wood Pole Council estimates that there are nearly 130 million wooden utility poles in service in North America. As a result of ground line decay around 3 to 4 million of these poles are replaced every year, costing American utilities more than $3 billion dollars a year. For many utilities the cost of replacing these rotten poles is one of their single largest annual maintenance costs.
The life span of a wooden pole can vary greatly, depending on the climate in which it is installed, the type of wood, the utility’s inspection and maintenance program and other factors. One of the main reasons wood poles must be replaced stems from degradation due to decay in the near-ground line zone. The use of composite ground line barrier technology can double pole life thus reducing costs and preventing early failures while also improving system reliability. The following case study explains the causes of ground decay and provides an example of the use of this technology in Europe.
Rovakaira distributes power in northern Finland to 37,000 users over an area of 28,800 square kilometres (km). The population is widely spread amongst many small hamlets and villages. With 6,500 km of network and 2,500 substations to maintain, Rovakaira has a major challenge in meeting both the low distribution costs and outage targets set by the regulator.
This challenge is made worse because most of the distribution area is in the Arctic Circle with some of the harshest conditions on earth, including a recorded lowest temperature of -51 C (-60 F), strong arctic winds, heavy and deep snow for six months of the year and long periods without daylight.
In these conditions, maintaining a reliable supply of power to users can literally be a matter of life or death.
To meet the requirements of both reduced long-term maintenance costs and improved network reliability, Rovakaira looked at a range of options, including ground line barrier sleeves, to prevent ground rot in its wooden utility poles.
For Rovakaira, wooden pole failure, as a result of ground rot, is a likely occurrence due to severe winter conditions, which result in heavy ice loading of the conductors. A solution to prevent decay was seen as a way to improve reliability and reduce long-term maintenance and repair costs, as well as reduce the need for line repairs in bad weather.
|Example of pole failure as a result of core rot.|
Rovakaira contacted its pole supplier, which led to the pole supplier proposing the use of in-ground barrier sleeves to prevent decay and pole failure. The pole supplier identified three possible products that could be used. Before proceeding further, Rovakaira decided to get a better understanding of the types and causes of pole decay so that it would be in a good position to set criteria and evaluate the different barrier sleeve products to ensure it selected the best solution for its needs.
Understanding Ground Line Decay
All wooden poles suffer from ground line decay. When combined with high mechanical loading, ground line decay results in the ground line being the normal point of failure for wooden power and telecom poles.
For pole decay to occur, all of the following criteria must be present:
- Moisture content greater than 25 percent
- Wood decaying fungi
- Temperature above 10 C
- Depleted preservative protection
If any one of these criteria is not present, decay will not occur.
Looking at the ground in more detail, it soon becomes clear that deeper in the ground, (typically 12″ or 30 cm or more below ground level), the factors necessary for decay to occur are greatly reduced or not present at all. This means that the risk of decay below this level is non-existent.
Wetting and drying
The ground line section of the pole is surrounded by all of the factors necessary for decay to occur, making wood preservative the only defense against decay and pole failure. The ground line section is subjected to major variations in moisture content throughout the year, however, as the topsoil becomes seasonally wet and dry. Subsequently, every time there is heavy rain, decaying organisms and nitrogen are absorbed into the wood as the dry pole becomes saturated. As the pole dries, moisture and preservative leach out of the pole and into the soil. While these effects are small, over time they lead to depletion of the wood preservative, making the pole vulnerable to attack by wood decaying fungi present in the soil at the ground line section.
Deeper in the ground, the moisture content of the soil tends to remain constant so this effect is almost non-existent and higher above the ground the moisture content of the wood is generally below the 25 percent level necessary for decay to occur.
ROT and CRACKING
Two types of decay cause ground-line failure in wooden utility poles: soft rot and core rot.
Soft rot is where the outer part of the pole is attacked by decaying organisms present in the soil. Core rot (or brown rot) attacks and decays the core of the pole.
In a preservative-treated pole, the outer sapwood is receptive to preservative treatment; the inner core wood is not. In practice, this means that the outer part of the pole is protected from decay while the core is not. This is not a problem until the pole cracks, creating an open access point to the core of the pole allowing airborne brown rot fungal spores or soilborne fungi to access the unprotected core of the pole. If the moisture content is greater than 25 percent, then the fungi will thrive and core rot will occur, leading to pole failure.
Moisture from the ground
Wood is a cellular material that absorbs moisture. Over time, if one part of the wood becomes wet, the moisture will move through the wood to ultimately create an equilibrium state where the moisture content is uniform throughout the wood.
In practice, this means that a pole installed in normal ground conditions will absorb moisture from the ground, resulting in high moisture content. The above-ground section of the pole is exposed to sun and air movement and will have lower moisture content than the in-ground section of the pole. Therefore, like with the wick of a candle, the moisture will move up the pole toward the above-ground section to try and create an equilibrium state. This can result in the above-ground section of the pole having moisture content greater than 25 percent, allowing brown rot spores to germinate in cracks in the pole just above ground level and rot out the core of the pole from above.
FIGURE 1: Typical moisture flow and profile in a wooden pole
The red danger area indicates the area where moisture content is greater than 25 percent. If air borne brown rot spores or soil borne fungi enter the unprotected core of the pole via cracks in this red zone, core rot can occur.
Setting the Criteria
From the research, it was clear to Rovakaira that using additional protection against decay on the vulnerable ground line section of the pole would help reduce outages, while also significantly extending pole life to give excellent cost savings. A good understanding of the causes of pole failure allowed
Rovakaira, in partnership with the pole supplier, to set out a list of key performance requirements needed for an effective barrier system. They are:
1. Provide protection at the vulnerable ground line section of the pole. It was clear that protecting the whole in-ground section of the pole was not necessary to extend pole life.
2. Exclude all the factors necessary for decay to occur, including oxygen, decaying organisms, nitrogen and, most importantly, moisture, as this is clearly a key prerequisite for decay to occur.
3. Contain the preservative within the wood to maintain secondary protection over time.
4. Nontoxic; environmental concerns and the need to be outside the requirements of the European Biocidal products directive were seen to be key factors.
5. Low additional cost.
6. Easy to apply to the pole.
7. Allow continued use of standard pole test procedures.
8. Tough and durable to allow transport of the sleeved pole from the pole supplier’s pole production plant to site and installation without damage issues that could compromise the performance of the barrier system over time.
9. Must stand up to installation in stony ground conditions using dig and backfill installation.
10.Independently tested and proven in use.
Evaluating the options
The pole supplier then investigated the available barrier sleeve options and their suitability. Various products were identified including: Heavy-duty open topped bags lined with a metal barrier film made in the U.S. that are slid over the base of the pole and taped to the pole above ground level.
A German system was also evaluated where metal foil was wrapped around the pole then covered with a heavy gauge plastic sheet that was wrapped over the metal foil and heat shrunk in place. And, a UK made heat shrink thermoplastic ground line sleeve lined with a hermoplastic bituminous sealant that melts.
|The ground conditions in Lapland can be very stony, as shown here.|
The first two options did not form a seal to the pole’s surface, raising concerns over the ingress of water, oxygen, nitrogen and decaying organisms via the gap between the sleeve and the pole surface.
The bag system was very tough; however, there were concerns that the bag was open to water ingress if it was punctured. This could allow water into the bag which could over time create wet ground conditions and soak the post.
The final option met all the criteria. Once heat is applied, the bituminous liner inside the outer thermoplastic heat shrink sleeve melts to form a tough watertight seal to the pole surface. This seal effectively lowers the entry point for moisture to the pole keeping the zone inside the sleeve relatively dry. Even if punctured or damaged, the area of wood exposed to water ingress and decay is limited to the damaged area only.
The independent testing showed that this watertight seal kept the moisture content in most of the sleeved section below the 25 percent level necessary for decay to occur. Extensive long-term independent test data showed a typical minimum doubling of the pole life when fitted with this type of sleeve. This was backed up by a long-term track record of more than 20 years in volume use with many other utilities also using the product as standard on their poles. In addition, this option was easy to apply to the pole and took only a minute or so.
And, finally, it was, by quite a large margin, the lowest cost option.
Based on the assessment, Rovakaira implemented initial trials using the composite barrier bituminous ground line sleeve system in 2011. The pole supplier applied the sleeves to the poles at their yard prior to delivering the poles to the utility. Initial trials were promising. Some problems with low-level damage to the sleeves when installing the poles with a back hoe excavator were soon overcome with operator training.
Subsequently, more than 4,000 poles fitted with these Polesaver sleeves have been successfully installed by Rovakaira to date. Based on the relatively low unit cost, reduction in outages and cost savings, Rovakaira has adopted this form of ground line protection as standard for all its pole replacements.
|Composite barrier sleeved poles in the pole supplier’s yard.|
Hannu Lehtinen (firstname.lastname@example.org ) is President and CEO of Rovakaira Networks Construction Ltd with overall responsibility for the network including maintenance. Hannu has been in his current position for 8 years and prior to that held a senior position in Telecoms. With extensive long term experience of treated wooden poles used in both Power and telecoms applications Hannu has gained an in-depth understanding of wooden poles.