Custom-Built Trench Box

The Anchorage Water and Wastewater Utility let a $7-million contract last year to install 4,200 linear feet of high-capacity 48-inch-diameter steel spiral water main pipe, in 40-foot joint lengths.

Taking on the challenge was Pruhs Construction Inc. of Anchorage. The new pipeline is part of a larger undertaking, the Anchorage Water Loop project.

The Anchorage Water Loop project is helping to produce an alignment change to a segment of the original transmission main. Consisting of installation of approximately 5,700 feet of 48-inch water main and related appurtenances, it is necessary to avoid conflicts with existing utilities. When completed, the new transmission main will provide continued capacity to the citywide water distribution system.

"Normally, we use trench boxes for pipe installation, but with the extra long sticks, just one box wasn't going to work; it would be too heavy for our equipment," said Lee Darbous, project superintendent for Pruhs Construction Inc. "(We) called Efficiency Production in Michigan to get some ideas on how to shore the pipeline. They suggested two joined boxes with abutted arches."

A Modular Unit

Efficiency Production Inc., headquartered in Michigan, is a manufacturer of trench shields and shoring with a reputation for custom engineering trench shielding systems to meet unique shoring challenges. Mike West, Efficiency's vice president of engineering, came up with a plan for a long in-line trench shielding system that met the soil and pipe requirements.

"We finalized a custom-designed system that combines a 30-foot and a 24-foot-long trench box into one 54-foot modular unit," West explained. "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 Efficiency HT8 model with sidewalls measuring 8 inches thick and 9.5 feet high. They include a specially engineered, connecting-pin assembly that locks the boxes together on the ends of the adjoining sidewalls. 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 end of one of the boxes was equipped with 7-foot spreader pipes, while the other box had a specially designed inverted connected pipe system that pins to both pairs of spreader collars.

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

Darbous agreed: "It worked really well, exactly how we had planned it. It definitely met our expectations. Though it was heavy, our machines were able to handle the two shields without a problem." Darbous added that he is very happy with the progress his crew is making.

Good in Tight Spaces

The trench boxes were also extremely helpful in the conditions experienced along the pipeline install.

"There are two advantages to using a trench box rather than open cut. The first is that with limited space, you cannot cut back all you want. There were several electrical poles nearby, and open cut would have undermined the poles," Darbous explained. "Second, it's very helpful controlling costs associated with bedding and trench backfill by limiting the amount of disturbed soil, because there is less dirt to take out, and less material and backfill to put back. Pea-gravel is expensive around here and hard to come by."

Undoubtedly, heavy machinery is necessary to manage the pipe installation and the two-shield combination, so Pruhs Construction used a Hitachi 800 and a Hitachi 500 excavator in addition to a Volvo 220 loader with forks and a John Deere 650 dozer. The job started in the cold of April 2007 and was completed in July.

Pruhs Construction specializes in major civil construction work. Founded by Dana Pruhs some 13 years ago, the company has grown from bidding $500,000 contracts to $40-million contracts.

Efficiency Production Inc., which calls itself "America's Trench Box Builder," provides a wide selection of standard and custom trench shielding and shoring systems. Efficiency's versatile products are designed specifically for safe and cost-effective installation of utility systems and infrastructure improvements. All products are P.E. certified to meet OSHA and MIOSHA standards.