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Increasing the Service Life of Direct-Embedded T&D Assets

Premature failures in overhead structures can be reduced considerably by enhancing the portion of the pole that will be in direct contact with the ground

Overhead powerline asset management strategies should be in place even before the assets are in the ground, as the service life of a structure can be dramatically increased by implementing best practices during installation and selecting appropriate backfill. Even though the material cost of the poles is a small portion of the overall budget of the overhead line, the equipment they support and the replacement cost of these structures is comparatively high. Premature failures in overhead structures can be reduced considerably by enhancing the portion of the pole that will be in direct contact with the ground.

Many overhead poles supporting tangent transmission and distribution circuits are round wood or laminated wood poles. Decay in wood poles is most likely to begin close to the groundline if the conditions are favorable for decay causing organisms to colonize and thrive. If poles are installed so that the area around the groundline does not support these favorable conditions, the service life of the pole can be extended considerably.

Backfill Material

Backfill material should be selected carefully for all direct-embedded structures to provide adequate support and facilitate drainage. Decay in poles begins if the moisture content of wood exceeds 20%. To reduce the moisture content in the wood at the groundline backfill with good drainage characteristic should be selected so that the water is drains away from the pole base. This can be accomplished by selecting backfill material that has high permeability and can provide adequate support to the pole to resist the loads experienced by the structure.

Many utilities use the soil that is excavated with the auger as backfill, if the excavated soil has poor drainage characteristics, this practice may be reducing the service life of the structures. Some soil types such as heavy clay and hard silts can hold moisture for an extended period, exposing the wood fiber to high moisture content. In areas with soil unsuitable to be used as backfill, crushed rock or gravel should be used. In sandy soils, native soil can be used as backfill, if it is free of organic material and has good drainage properties.

Compacted gravel should be used at the bottom of the augured hole to provide a stable foundation and increase the vertical bearing capacity of the soil. This is an important requirement for guyed structures as vertical load experienced by guyed poles from the guy wires are high compared to self-supporting structures. To provide a stable footing, the embedment hole should be augured about 1’ deeper than the design embedment depth and crushed rock or gravel should be compacted to avoid poles from sinking or leaning. According to RUS bulletin 1728F-810, “Backfill must be placed around the pole in layers not exceeding 6 inches in depth, with each layer mechanically tamped before the next layer is added”, this practice will ensure that the backfill is tightly compacted and can transfer forces efficiently to the native soil. Using crushed rock at the bottom of the hole, provides support and also ensures that the water does not collect at the base of the pole, which increases the moisture content of the wood fiber.

Causes of Decay

Moisture content higher than 20% is one of the important components required for incipient decay, if high moisture content is combined with favorable conditions such as moderate temperature, oxygen and food (which is wood itself), decay causing organisms will grow and thrive. Decay is a major concern at and around the ground line, as we move away from the ground line, in either direction chances of decay reduces considerably as all the components required for decay don’t coexist. Moisture content reduces as we move up from the groundline, removing one of the important components required for the microbes. Similarly, chances of decay of the pole decreases as we move down the pole, as the amount of oxygen present is not sufficient for the decay causing organisms to sustain and grow. Decay causing fungus could die or become dormant if the conditions for its growth is not favorable.

Insects such as termites, beetles and carpenter ants can attack wood improperly treated wood poles and compromise the structural integrity of the structures. If decay is noticed during inspection, borate rods can be used to treat and prevent further damage to the poles. Borate rods effectively control termites, carpenter ants, beetles and decay causing fungus, but it is not poisonous to humans or other mammals.

Enhanced protection at the groundline will provide extra protection to the structures where it needs the most protection. Wood poles with enhanced ground line protection are through-bored with a small diameter drill bit usually in a diamond pattern that provides 100% preservative penetration. Through boring extends 2’ above and 3’ below ground line, the length of the through bored zone is usually included in the utilities wood pole specification. This ensures that the ground line area of the pole is treated throughout the entire cross-section and not just at the outer shell of the pole. Poles with enhanced ground line protection can be set in concrete.

These precautionary steps will extend the life of the transmission and distribution assets. Pole inspection programs per the decay zone should also be implemented to check the poles for structural integrity. As the cost of building or replacing structures is increasing every year we should do everything in our power to extend the service life of our structures.

 

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