The Manning Drive Steam Plant and Utility Distribution Tunnel, a $100 million project currently in development at the University of North Carolina at Chapel Hill, will redefine how maintenance is done on the utilities that provide steam and chilled water to current and future campus facilities.
The new tunnel, stretching more than 3,000 linear ft. (914 m) underground, will allow crews to service the utilities without having to dig to access pipes for regular maintenance and emergency repairs. Following the project’s completion in June 2006, maintenance workers will be able to walk through the tunnel to monitor the systems the university depends on to supply many buildings — including the hospital system — with heating, cooling, electrical, phone and data.
“This is the deepest excavation we’ve ever done,” said Judd Hamson of Whitaker Construction, the firm developing the first phase of the project. “The trench is 20 feet wide by 25 to 30 feet deep.”
Designed by the engineering firm Carter and Burgess and contracted by Associated Brigham Contractors (phase one) and Frank Lill and Son (phase two), the project began in February 2004.
To make the endeavor possible huge shoring shields —steel plates connected by large steel spreaders that retain the tunnel walls — enable workers to work safely while preparing the site for pouring the concrete floors and walls. Coble Trench Safety (CTS) of Burlington, NC, is providing the trench safety equipment for the monstrous job, which is currently demanding more than 100 shoring shields to hold back hundreds of thousands of tons of dirt. The boxes will be used on the project for about a year.
The project demanded a specially engineered shoring system, which CTS designed in conjunction with Whitaker to meet their specifications.
“The difference between these stacked boxes and conventional stacked boxes is a specially engineered locking system that fastens the stacked boxes together,” said CTS General Manager Brett Sondergard. “They are locked together with engineered steel clips that are welded to the bottom box. These clips allow the contractor to remove the spreaders from the trench box on the bottom of the excavation once the tunnel slab is poured and the bottom box is braced against the slab with wood timbers.
“After the bottom spreaders are removed,” Sondergard explained, “we now have 14 feet of clearance from the slab to the next set of spreaders on the second box. The contractor can now form and pour the walls and top of the tunnel.”
Hamson said the unique design of the sign placed unusual demands on the trench safety subcontractor, and that he was pleased with how Coble Trench Safety worked closely with them to meet specifications.
“Being from Utah, we didn’t really know anybody out here,” he said. “None of the rental companies really had any faith in doing it our way, but Brett and the Coble guys were willing to team up with us on this, and it worked out well.”
Coble Founder and President Tom Coble said the equipment saved a lot of time and money.
“Without trench shoring equipment they would have had to slope the walls through intense, time-consuming excavation,” he said. “That gets expensive quickly when you’re talking about a job of this size.”
Beyond enabling the engineering marvel to be constructed, there is a very practical reason for shoring up the 20 ft.-high dirt walls: safety. One cu. yd. (0.8 cu m) of dirt weighs nearly as much a late-model Volkswagen Beetle. OSHA requires contractors and municipalities to follow stringent underground safety guidelines when working in trenches. One cave-in can cost a contractor more than $1 million in fines and downtime, let alone the lives of the workers buried alive and crushed under tons of dirt.
“It happens so quickly,” Sondergard said. “The soil may look compact and firm, but it can happen faster than you can blink — you’re covered up, unable to move, unable to breath.”
Phase two of the Manning Drive Steam Plant and Utility Distribution Tunnel began in October and is scheduled for completion in June 2006.
Headquartered in Burlington, NC, Coble Trench Safety specializes in trench safety equipment rentals, sales, service and training in North Carolina, South Carolina and Virginia.
For more information, visit www.cobletrenchsafety.com.