Failure of a utility pole can be catastrophic. Besides cutting power to a potentially significant number of customers, the impact of a 500+ kilogram pole falling in some random direction can lead to significant damage, injuries, or even death. The impact of a single pole failure can have a disproportional impact on customers depending on the location of the pole. A pole in a hard-to-reach area, such as a steep slope in a wilderness setting, will take significantly more time to replace. Pole failures can damage or disrupt other infrastructure, such as when a pole lands on a highway, train track, or structures. Then of course, are the shock hazards created when energized equipment and conductors make contact with whatever happens to be unlucky enough to be underneath.
The reasons a pole can fail can be many and pole failure is often the combination of multiple factors. Some of these factors include neglect/oversight/abandonment, pole decay, alteration of the pole by third parties, natural events, interaction with vegetation, vehicle impacts, overload, improper use, alteration or removal of guy wires or their anchors, or improper pole setting, erosion, or other changes to the soil near the pole.
Some failure mechanisms of poles are difficult to detect and therefore more difficult to anticipate. For example, with pole decay, the deterioration of the pole is often hidden from sight, typically occurring at the ground line or below. In the case of wood utility poles, they can be treated with chemical preservatives to extend their life expectancy. However chemical preservatives degrade over time and utility poles must be regularly inspected and maintained. These treated poles will decay over time from moisture exposure that promotes wood fungal decay, reducing the structural capacity of the pole from being directly embedded in the ground.
To the public, a pole may appear to be simply stuck into the ground. In reality, the utility pole design and the engineering to site a pole are often complex. The depth to which a utility pole is embedded in the ground is typically dictated by the forces it must carry. Design variabilities such as regional weather conditions, soil composition, clearance requirements, and the spacing between poles all potentially affect the anticipated forces that each pole needs to support; each pole’s design parameters are unique.
As an example, in certain regions, utility poles must be designed to support ice or dust loads on the conductors. In both cases, moisture condenses on the conductor. Ice or dust accumulation creates an enlarged surface area, which increases wind resistance as the wind blows on that enlarged area of wire.
This additional resistance may lead to stresses the pole was never designed to withstand. The loads, naturally, will vary depending on the region, but the problem is more complex than simply the weight of the accumulating load. Forces are imposed near the top of the pole, and those forces must be safely transferred to the ground.
Clearance of the conductors with the ground, buildings, vehicles, and other wires defines the placement of the pole. The spacing between poles will affect a pole’s design; poles with differential wire sag on either side will experience unbalanced forces. If the forces are unbalanced, then those forces must be restrained at the ground. Guy wires may be used to stabilize poles when they are subjected to unequal forces. All of these factors need to be monitored and understood by the utility.