Grading and Excavation Contractor

By James McRay

The Anchorage Water and Wastewater Utility recently let a $7 million bid to install 4,200 linear feet of high-capacity, 48-inch steel, spiral water main pipe in 40-foot joints. Taking on the challenge is Pruhs Construction Inc. of Anchorage. The new pipeline is part of the larger project, the Anchorage Waterloop project.

Pin-in-place spreader arches with 60-inch crossbeams are set at the adjoining ends of each box to allow greater clearance for the pipe joints.

“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,” says Lee Darbous, project superintendent for Pruhs. “[We] called Efficiency Production in Michigan to get some ideas on how to shore the pipeline. They suggested two joined boxes with abutted arches.”

Efficiency Production Inc., based in Michigan, is a leading manufacturer of trench shields and shoring and has a strong 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 explains. “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.”

“Super Arch” Boxes Save the Day
Each trench shield is a custom-built Efficiency HT8 model with sidewalls measuring 8 inches in thickness and 9.5 feet in height. Included is 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 crossbeams 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 is equipped with 7-foot spreader pipes, while the other box has 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,” says West. “In each situation, the contractor has been very happy with the trench shields, and they have worked very well.”

Darbous agrees, “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 adds that he is very happy with the progress his crew is making.

One advantage to using a trench box, rather than open cut is that with limited space, you cannot cut back all you want.

The trench boxes were also extremely helpful in the conditions experienced along the pipeline installation. “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,” explained Darbous. “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.”

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

The Right Tools for the Job
Undoubtedly, heavy machinery is necessary to manage the pipe installation and the two-shield combination, so Pruhs 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 recently completed in July.

Pruhs specializes in major civil construction work. Founded by Dana Pruhs 12 years ago, the company has grown from bidding $500,000 contracts to $40 million dollar contracts.

Efficiency Production Inc. 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.

James McRay is marketing and media manager for Efficiency Production Inc.

GEC - May 2008

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