OLDCASTLE PRECAST CHANNELS SAFETY, WATER CONSERVATION THROUGH NEARLY TWO MILES OF PIPE, CUSTOM BOX CULVERT
Special precast box culverts were used for one of the most significant segments of the $25.8 million Cache Water Restoration Project (CWRP), an emergency upgrade of approximately six miles of mostly open, unlined channels and canals. The Logan, Utah, project incorporates new precast pipeline, box culverts, a section of pressurized pipe, metering systems, turn-outs, head gates, and improved maintenance access.
CWRP stemmed from a 2009 landslide that killed three people in a home below an irrigation canal hugging a steep Logan hillside. The tragedy prompted a total canal system reconfiguration over a three-year schedule, during which shareholders and farmers along the waterways were without a prime water source. Irrigation water was restored to the canals by mid-2013.
One of the key CWRP components is piping for the open channel section of Logan Canyon to alleviate leakage in steep terrain. Engineers were uncertain about fitting a precast, reinforced concrete box culvert into the existing canal section, carved into cliffs over 100 years ago. At its widest, the open canal was approximately 14 feet, with steep cliffs rising above the canal and sharp drop-offs to the state highway below. After considerable investigation, project designers confirmed the precast box culvert specification.
Oldcastle Precast was contracted to supply approximately 10,000 linear feet of box culvert and precast pipe for the canal’s upper portions. Nearly 4,300 feet of 5- x 5-ft. and 6- x 5-ft. boxes was used according to canal hydraulics for the Logan Canyon section. After emerging from the canyon, water is transferred from the box culvert to a northward, 66-in. concrete pipeline.
Precast structures proved unfeasible in some canyon sections—those narrowing to 8-ft. widths, with overhanging cliffs—due to the impracticality of being able to fit heavy construction equipment into the canal. While crews tackled cast-in-place culvert construction on target, the precast box culvert portions were placed six times faster.
|Precise fabrication methods enabled Whitaker Construction to enclose much of the winding Logan Canyon canal in precast box culverts, narrow waterway width requiring cast-in-place methods at limited points.
Designing a box culvert that would fit within the channel without impacting the steep slopes on either side was the challenge, designers beveling the majority of segments to “snake” along the alignment, without elbows, transition vaults or closure pours. Oldcastle Precast designed new form “headers” that could bevel—or angle one side of the box culvert so that a series of bevels would form a curve—up to an 8-in. drop in length when measuring opposing sides of the box culvert. The joint at the end of each bevel was the same as straight box culvert sections, so that there were no extra requirements at the joints.
To fit the numerous curves in the canal alignment, beveled precast box culverts were designed in ½-in. increments with bevels ranging from ½ to 8 inches. The bevels had to be specifically designed as “curve to the left” or “curve to the right;” each piece had to be numbered and installed so the box culvert did not deviate from the alignment.
Whitaker Construction Co. crews installed each section so the inner joint gap measured ½ inch. Once the alignment was established, the installation progressed rapidly with the contractor averaging over 15 sections per day. Work rate was impressive when considering winter conditions and the presence of only one access to the upper section of the project. Crew had to haul the box culvert and its bedding materials 3,000 feet along the canal at the start of the section.
When operators opened the new facilities, they were amazed by how much water savings had been achieved with the addition of concrete box culvert and pipe. Early reports indicated that flow metering showed almost no water loss over two miles of new structures. While 2013 was a difficult water year, with many areas of Utah on water restriction, the Cache Valley irrigators were supplied throughout the growing season.
The project was designed and constructed according to the Construction Management General Contractor method, which utilizes an integrated team approach and spans pre-construction and construction phase services. The CWRP team included Cache County, owner; J-U-B Engineers Inc., program manager; Montgomery Watson & Harza and Hansen, Allen & Luce, design engineers; and, Whitaker Construction, contractor. — adapted from a report in the American Concrete Pipe Association’s Concrete Pipe News by Randy Wahlen, P.E., Marketing Engineer, Oldcastle Precast Inc., North Salt Lake, Utah.