Precise, accurate geotechnical data helps save over $1 million in transmission line construction

Client

SunZia Transmission, LLC

Location

SE Arizona – West and Central New Mexico

Challenge

Seemingly minor geotechnical recommendations on large projects can lead to significant construction savings. Such was the case for the SunZia Southwest Transmission line – one of the largest in the U.S. – which consists of more than 500 miles of two single-circuit transmission lines, delivering 3,000 megawatts of renewable energy to communities in the desert Southwest. As geotechnical consultant, our role was producing reports and advising on the foundation design and construction of 1,965 structures along the route of the line including guyed transmission structures, monopole structures, lattice towers, and three-pole structures. If we could find a way to shorten the more than 7,200 guy anchors involved without impacting structural integrity, our client could save significant costs on this massive project.

Solution

New challenges sometimes require familiar solutions. To achieve the goal of shorter guy anchor embedment depths for our client, our geotechnical engineers needed empirical data to prove that it could be done without negatively impacting structural integrity. We performed guy anchor load tests at multiple locations to improve the bond stress between the ground and the grout for the guy anchors. Based on the potential savings and our previous experience with solar pile load testing, Terracon was fairly certain the groutto-ground bond stress could be doubled or better based on performing in-situ testing.

Results

Our test results showed that for clay soils, more site-specific exploration was needed to better estimate the bond stress. Towers located in granular soils showed potentially significant increases in the bond stress. Upon further review, it was clear the bond stress in the young granular alluvium was just about right in our initial report, using good soil mechanics. While the bond stress for older granular alluvium could be increased by 50% or more, which would lead to a significant cost savings where older granular alluvium was mapped at specific tower sites. These results further emphasize the need for good geological reconnaissance performed for any transmission line project.

When the potential bond stress increase was applied to the older alluvium across the site, it added up to significant savings in construction costs. The benefits of the transmission line to both communities and the environment, are even greater than the cost savings. The completed line will deliver sustainable power for more than 1 million homes, while avoiding 4.5 million metric tons of carbon emissions per year – the equivalent of taking 890,000 cars off the nation’s highways.