Custom-Built Trench Shields On Pipe Project


Underground Construction

Charlotte-Mecklenburg Utilities recently let a $9.6-million bid to install 20,000 linear feet of high capacity 48-inch steel spiral water main pipe in 50-foot joint lengths. Taking on the challenge of installing the behemoth sticks of pipe over that long run is Sanders Utility Construction, Inc. of Charlotte, North Carolina.

"Before we started the project, we debated on whether to install 25-, 30- or 50-foot lengths of pipe," says Freddie Young, project manager for Sanders Utility. "Because so much of this pipeline is going cross country, from a production aspect the most cost-effective option was installing 50-foot joints as much as possible."

Shoring the installation of such long pipe is no easy task. To build one trench box to accommodate that length would require extremely high-capacity machines just to move the trench shield. So, Steve Sanders, project superintendent, contacted local equipment dealer MSP Rents in Charlotte, North Carolina, for possible shoring options.

"I talked with Freddie about a couple of different shoring options, including building a 40- or 50-foot trench shield," says Robbie Belk of MSP Rents. "Together, we called our trench shielding supplier, Safe-T-Shore, to get some more ideas on how to shore the pipeline."

Safe-T-Shore — a leading manufacturer of trench shields and shoring headquartered in Arizona — has a strong reputation for custom engineering trench shielding systems to meet unique shoring challenges. Mike West, Safe-T-Shore's vice president of engineering, came up with a plan for a long in-line trench shielding system that incorporated two "trained" trench shields with abutted high-clearance spreader arches.

Explains West, "We finalized a custom-designed system that combines a 34-foot and a 30-foot-long trench box into one 64-foot modular unit. The boxes can quickly and easily be put in and pulled out of the trench as the pipe installation progresses, and it doesn't require extraordinarily heavy equipment."

Each trench shield is a custom-built Safe-T-Shield™ HDSD8 model with sidewalls measuring 9 inches thick and 10 feet high. Pin-in-place spreader arches with 60-inch cross beams are set at the adjoining ends of each box to allow greater clearance for the pipe joints. The outer ends of the two trench shields are equipped with 7-foot spreader pipes.

The average depth of the new water main is between 12 feet and 16 feet. About 60 percent of the pipeline runs across country, with 40 percent installed in the right-of-way of existing roads. In places, Sanders uses only one trench box to install 25-foot joints of pipe.

This pipeline installation is part of Charlotte Utilities' Southwest Water Main Project, Phase B-1. When completed, the new transmission main will provide continued capacity to the city-wide water distribution system.

Undoubtedly, heavy machinery is necessary to manage the pipe installation and the two-shield combination, so Sanders is using a Komatsu PC750 and Komatsu PC400 excavators. The job started in January 2008 and will be completed in January 2009.

"We've actually built a couple of these 'super-arch' boxes for other projects that have needed the extra length for long pipe installs," explains West. "In each situation, the contractor has been very happy with the trench shields, and they have worked very well."

Sanders agrees. "We're real pleased with how the boxes are working. We're getting very good daily pipe production, and the shields are working exactly the way we expected them to work." Sanders recently ordered two more Safe-T-Shore trench shields to be primarily used for installing the pipe at greater depth, where stacking the trench shields is the optimal technique for shoring.