Diverting a river with trench boxes.
On three occasions, the district has needed to rehabilitate utility pipes that run underneath the river because the swift current keeps eroding the river bed, exposing the sewer lines and potentially causing leaks and cracks.
On the third occurrence of sewer pipe trouble--this time in Riverdale--the CWSD let the $600,000 Weber River Sewer Crossing Rehabilitation project to Whitaker Construction Company of Brigham City, UT. Whitaker Construction had previously repaired a similar sewer line in a different area of the Weber River, and was familiar with the challenges of accessing under-the-river sewer pipes. The engineer for the project was Greg Poole from Hansen, Allen, and Luce, LLC; and the project inspector was Paul Higgins.
The first step for the CWSD's project engineer was assessing the condition of the 18-inch pipe encased in four-feet of poured-in-place cement. A diver was called in to inspect the pipe and make a recommendation. Whitaker decided to dam the river to allow unencumbered access while repairing the pipe.
Damming the river also gave them the opportunity to work at controlling the river's flow. Boulders and large rocks were placed just below the natural waterline of the river to act as a "waterfall" for the current, decreasing its speed and slowing the erosion.
Trench boxes as a cofferdam
Rather than utilize an expensive cofferdam system, Whitaker used a much more cost-effective system to dam the river--trench boxes manufactured by Efficiency Production Inc. According to Whitaker Construction Supervisor Brian Hamson, trench boxes also created less dirt and silt in the water.
After a bypass channel was dug and shored, four, 8 by 20 foot trench boxes with 84-inch spreaders were set upstream in the river one at a time, and then filled with dirt to create a solid cofferdam. Downstream of the work area, three, 8 by 20 foot dirt-filled trench boxes, also seven-feet wide, dammed off any backflow of the river. Once the river was diverted through the bypass channel, any remaining water in the dammed section was then pumped out with three, 6-inch electrical pumps, each with a capacity of pumping 1,600 gallons of water per minute. In total, 525 feet of the river was closed off and diverted.
Whitaker then dug four feet into the riverbed to get to the pipe, and shored that cut with steel sheeting driven 17 to 18 feet below the encasement. The pipe and encasement were checked for cracks and repaired where necessary. The encasement was then covered and built back up with boulders to add to the protection of the sewer pipe.
Damming the river was only part of the challenge. The river's swift waters needed to be diverted to a point downstream of where they were working on the sewer pipe. Normally, an open bypass channel would be excavated on one side of the dammed river, and the work would be performed from the other riverbank. This section of the river, however, had a railroad yard bordering tightly to the east side of the river's edge, completely eliminating access from that direction.
Whitaker had no choice but to run the diversion channel around the west bank of the river, and bring all their equipment and supplies onto the "island" that was created between the diversion channel and the dammed river section.
To get to the island, Whitaker built a bridge over the diversion channel using a trench box wall supported by 20, 16 by 16 foot I-beams. Their equipment included three excavators, a Volvo 360, a CAT 330 and an Hitachi 300; two Volvo 120 front loaders, and two seven-yard Efficiency Production Stone Mizers.
Whitaker had more concerns with the bypass channel. "There is so much development in that area that we had to keep the channel relatively close to the dammed section of the river," said Hamson. "We were concerned that the current could expand the channel so quickly, that we'd have no room to work."
To control the flow of water through the diversion channel, Whitaker lined the channel with 33 trench boxes.
The eight-foot high trench boxes were set with 16-foot-wide spreaders and lined with 1/32-inch Polypro-Olin sheets. A combination of 10-, 12-, 16- and 20-foot-long trench boxes were used for a total length of over 300 feet of "shielded" channel.
The channel was designed so that five feet of water would pass through the boxes, which in normal conditions would carry about 600 cubic feet per second (cfs) of water flow within the inside of the trench box walls. Because of increased water flow due to rain and flood waters, however, the actual water flow pressure was greater than 1,350 cfs, well past the rating of the boxes; yet the boxes withstood the flows with no problems.
In took Whitaker just one week to cut the bypass channel and dam the river, and then seven weeks to complete the rehabilitation to the pipe and riverbed.
Restoration was a simple reversal of the set-up process. The downstream cofferdam was removed first, one trench box at a time, and then the upstream cofferdam was pulled out. For a period of time, water flowed down both the river and bypass channel. Whitaker then closed off the upstream mouth of the diversion channel, returning the flow of the river to its original course. After pulling the trench boxes from the bypass channel, it was backfilled to complete the restoration of the area.
Whitaker Construction Company has been in the underground utility business for more than 50 years and are still family owned by brothers Bob, Dennis and Rick Whitaker. Brian Hamson has worked for the Whitaker family for more than 33 years.
For more information:
Trench boxes, shoring equipment:
James McRay, Efficiency Production Inc., (800) 552-8800,
Brian Hamson, Whitaker Construction,
Trench safety rentals:
Jeff Day, United Rentals Trench Safety,
(801) 886-0586, www.unitedrentals.com