By Paul Robbins
Wood pole structures are used widely by electric utilities with an estimated 130 million throughout North America. WAPA has around 68,000 of them, supporting thousands of miles of transmission lines. They are the backbones of the organization.
The Pacific Coast Douglas fir, western red cedar and southern yellow pine are the three primary types of wood used in poles throughout the United States. WAPA has some older western red cedar poles in service, however, for more than 40 years, Pacific Coast Douglas fir has been the most widely used type.
With several alternative materials available such as concrete, steel and fiberglass, why use wood? Because it offers an excellent balance between structural capabilities and cost competitiveness related to installation and maintenance.
Wood poles are cost effective because they are easier to install than other materials. They are directly embedded into the ground, usually 10% of the pole length plus two feet, and require no concrete or special foundation design. They are also easier to maintain, climbable and more accessible in an emergency. Various lengths are stocked in WAPA warehouses, making them readily available for emergency rebuilds and Shoo-flies, which are temporary lines that bypass construction or other activities.
While WAPA uses single poles for some of its transmission lines, most are suspended by wood H-frame structures. Civil Engineer Karen Rowe says that the lateral strength obtained with two poles, crossarms and bracing is an important element of H-frame design.
“Being made of a renewable natural resource, the most interesting thing about wood H-frame structures is the inherent strength and durability they have,” said Rowe. “The H-frame construction provides more lateral stability than single pole construction. One of the poles in the H-frame may have damage or decay, however that does not mean the structure will fail.”
To prevent decay, poles are generally treated with oil-borne pentachlorophenol, a preservative used for repelling fungal decay since the 1930s. Creosote and copper naphthenate preservatives are also used to treat WAPA’s poles, crossarms and braces. Some treated structures have stood for more than 70 years.
Although treated structures can stand for decades, they are still vulnerable to decay, most starting at the groundline.
The groundline is literally the point at which the ground meets the pole. After being embedded in the ground, a pole is backfilled with soil, also called “native fill.” The groundline is most vulnerable to decay because of several factors including moisture, oxygen, fungi, nitrogen, temperature and reduced preservative protection. Just like a dead tree stump in the back yard that sprouts mushrooms in spring, the wood pole is a food source for fungi and the groundline is the buffet.
Decay can affect poles in different ways.
“Some poles have a strong outside shell and a center more susceptible to internal decay or ‘heart rot,’” said Rowe. “Others have a strong center and tend to decay externally, called ‘shell rot.’”
Whether it is shell or heart rot, WAPA has programs that inspect poles for decay and provide treatment if needed.
Shell rot is identified in several ways, including visual inspection, scraping the surface of the pole or penetrating its shell with a dull probe. The purpose is to find signs of decay such as a soft, spongey surface; an underlying brittle, crumbly texture; or even a musty smell due to water damage.
Finding heart rot uses more intensive inspection methods. Sounding, also called “hammer testing,” involves striking a pole with a hammer and listening to the result. Healthy wood sounds solid and a hollow thud signals rot.
Another test involves incremental boring into wood, then pulling out a core that can be examined for rot. Groundline drilling is also conducted on older poles to identify decay at the groundline or below. The inspector gauges resistance on the drill bit as it goes in; less resistance indicates decay. Groundline drilling also emits bits of wood that are examined for rot.
In addition, there are forms of non-destructive evaluation testing that include X-ray evaluation and sonic testing that estimates fiber strength using sound waves.
Groundline decay can be a long, slow process and therefore can be delayed by regular inspection and maintenance activities. Shell rot is treated with a paste or wrap that contains fungi-killing agents and highly concentrated preservatives.
Sometimes damage is not as extreme and requires what is called a “pole bandage,” which is applied to the damaged area for protection against further decay. On the inside, heart rot creates a void within a pole that is treated with chemicals that impede decay and kill insects.
During application, a hole is bored into the area, chemical fumigant is injected into the space and the hole is plugged. Treatment can dramatically increase a pole’s lifespan.
Since 1988, all new Douglas fir poles at WAPA are through-drilled prior to preservative treatment. During this process, a series of small holes are drilled into the pole above, below and at ground level so that there is 100% preservative penetration in the groundline zone to inhibit decay.
This process is so effective that during later inspections, no groundline drilling is required on newer through-drilled poles.
For more than 175 years, wood poles have stood the test of time because of their strength, stability, cost effectiveness and structural capabilities.
Note: Robbins is a technical writer who works under the Cherokee Nation Strategic Programs contract.
Last modified on September 12th, 2023