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Evolving Thermal Grouts Aid Growth of Underground Power Transmission Lines
Grouts have a multitude of uses for many types of construction.
For utility construction, grouts are most often thought of for sealing leaks and joints in water and sewer pipe infrastructure, manhole rehabilitation and filling annular space between pipes and surrounding structures.
In addition, thermal grout is an essential part of the process of placing high-voltage power transmission and distribution lines underground, a trend that is becoming more common (see the June issue of Underground Construction). The use of plastics for insulation, advances in cable design and manufacturing, and other factors have made underground placement of high-voltage transmission lines a viable or necessary option in many situations.
The right thermal grout mix correctly applied is a critical element in constructing segments of underground power transmission lines. Thermal grout is used to transmit heat from underground electrical cables into the surrounding soil or rock, said Guy Dickes, president, Constellation Group LLC, who over the past four years has been responsible for planning and executing grouting for four major underground power transmission projects from 13.6 kV to 230 kV with segments from 100 to 2,000 linear feet in length.
Heat is the enemy of electric transmission, Dickes said. Heat increases wire resistance and shortens the lifespan of the cable insulation. Cross-country transmission lines suspended high in the air from steel towers are not insulated and heat is easily dissipated into the air. But for cable buried in a conduit under the ground, heat is confined.
“Thermal grout surrounds the individual conduits holding each cable and fills the casing completely,” Dickes explained. “This grouting technology is rapidly advancing.”
For example, Dickes said horizontal directional drilling and thermal grouting is used in underground applications where open-cut-and cover construction cannot be utilized. Some examples where open cut construction cannot be used are under creeks and rivers, expensive golf courses, developed properties, urban areas and many other conditions.
Of all the elements contributing to successful underground power transmission construction projects, grouting is arguably the least understood and has been considered by many contractors to be “difficult.”