Trenchless Technology Key Factor in Pipe Project : CEG

Photo courtesy of SSC Underground

SSC Underground used trenchless technology to build an underground pipeline to provide backup water supplies from two rivers in Phoenix.

To combat the effects of a drought in the southwestern United States, the City of Phoenix commissioned SSC Underground to install an underground pipeline to provide emergency water from the Salt and Verde Rivers.

The Phoenix Drought Pipeline Project included 12 miles of new pipeline and four booster stations. SSC Underground provided tunneling services and worked with the prime contractors. Sundt Constructionsix tunnels over four miles; PCLthree tunnels in a water treatment plant; Kiewita tunnel at a single location.

Photo courtesy of SSC Underground

SSC Underground's mission was to use trenchless technology to dig through the earth so that traffic and daily life above ground could continue with minimal disruption.

Trenchless technology allows for the installation of casing under highways, canals, streets, railroad and light rail tracks, and anywhere where open cutting or breaking the surface is not possible. These options install steel casing in sizes from 8 inches to over 120 inches to support a variety of carrier line types and sizes.

This approach used small skid steers to dig through rocks and dirt, and workers then used drilling rigs to push the steel casing into the newly created hole. In the Sundt project, the enormous casings were 10 feet long and 7 feet in diameter, tall enough for the entire Phoenix Suns professional basketball team to walk through without bending over.

Once tunnel construction began, precise excavation was required. In the Sundt section of the work, workers had to dig a tunnel to allow casings to connect the pipes laid by Sundt in open trenches to a tolerance of about one inch.

SSC Underground began work on the project in 2019 and completed it in 2022. SSC's consulting division worked closely with designers and other contractors to develop an underground roadmap for the work. Using its underground excavation fleet, the crew located existing utilities so they could be relocated or pipeline routing could be planned around them.

“Every dollar spent on preventive shaft excavation/SUE saves $7.82 in construction costs,” said Arvid Veidmark, President of SSC. “When tunneling, we often encounter previously unknown infrastructure, even gas pipelines, which of course can be quite dangerous.”

Tunnel workers installed the steel linings beneath major arterial roads and intersections, residential streets and an active water treatment plant. Soil conditions ranged from limestone, cemented cobblestone, alluvium and hard soil. Each day seemed to bring its own challenges as workers changed their strategy according to soil requirements. The job required more than 12 trenchless installations. Workers managed each operation to minimize disruption to the public, taking into account time, cost and risk.

SSC began tunneling by installing 105 feet of 66-inch casing for a 48-inch water main under Glendale Avenue at the base of Piestewa Peak Mountain. A Robbins motorized drilling unit was used for this crossing because the ground was solid rock. On behalf of Kiewit, this crossing was a critical link because this section passed through residential areas and had significant impacts on local residents.

The second section included six 86-inch tunnels under intersections along 32nd Street, totaling 834 feet for a 66-inch wide water main. These tunnels were built using an S70 Bobcat skid steer equipped with either a hammer attachment or rotary head to perform the excavation work in hard soil. Production varied between 3 and 9 feet of installation per shift. As a subcontractor to Sundt Construction, SSC worked as a partner on the sequencing of the tunnels to minimize roadway closures and keep access as open as possible for the many businesses along this corridor.

Photo courtesy of SSC Underground

“Our deepest dig was 10 meters underground,” Veidmark said. “That was inside the water treatment plant. Our work crews had to keep the equipment running and still remain alert as they dug under roads and major structures. We stayed away from a reservoir and a traffic intersection. Often they came across previously undiscovered underground structures. They had to decide whether to remove them, dig deeper or go around them.”

The final section of SSC installations was for PCL Construction. The excavation crew built three 96-inch tunnels at the pipeline's origin site, the 24th Street Water Treatment Plant. Using the same skid steer method as previous excavations, workers encountered solid rock conditions and achieved an average of 18 inches of tunnel advance per shift.

These three tunnels – 35 feet, 115 feet and 101 feet long – connect four pits for the installation of a 72-inch-long water main. Three of the four pits were built as semi-permanent structures to allow for the angles and heights required to complete the connections.

The work crews not only had to deal with hard rock, but also with unexpected weather conditions. “Several times our crews arrived to work and found that the pit had been flooded during the night. We simply pumped out the water and got to work.”

methodology

SSC utilizes its equipment and resources to complete crossings using a variety of trenchless methods, including:

  • Horizontal earth drilling;
  • Tunnel construction by hand;
  • Small Drilling Unit (SBU);
  • Tunnel attachment for drilling machines (BMTA);
  • Pneumatic pipe ramming.

All of these options follow a similar process of digging a bore or tunnel horizontally through the earth to a receiving pit from a borehole while the steel casing is pushed into place where the earth was removed. The carrier line (utility line) is then pushed or slid into the steel casing once it is fully installed.

The choice of method depends on a variety of factors including soil conditions, access to the site, length and size of the enclosure, time constraints and type of carrier line to be installed. SSC has the experience with these and many other methods to make your project successful. CEG


Chuck MacDonald

Chuck MacDonald is an editor, blogger and freelance feature writer whose writing adventures have taken him to 48 states and 10 countries. He has been an editor for highway, chemical, insurance and missions magazines. Chuck enjoys biking, kayaking and reading. He has a journalism degree from the University of Missouri. Chuck lives in Annapolis, Maryland with his wife, Kristen. They have seven grandchildren.

Read more from Chuck MacDonald here.


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