Designers had to bevel most of the box culvert segments so that the box culvert could “snake” along the alignment, without elbows, transition vaults, or closure pours.
Special precast box culverts were used for one of the most significant segments of a $25.8 million emergency water restoration project in Cache County, Utah. Known as the Cache Water Restoration Project (CWRP), assisted by the Natural Resources Conservation Service (NRCS), the project involved reconstruction and improvement of approximately six miles of mostly open, unlined channels that make up the Logan and Northern canals, as well as the Hyde Park and Smithfield canals. The project incorporated new precast pipeline, box culverts, a section of pressurized pipe, metering systems, turn-outs, head gates, and improved maintenance access.
The CWRP is the result of a landslide in 2009 that killed three people living in a home below an irrigation canal that hugged a steep hillside in Logan. The tragedy prompted a total reconfiguration of the canal systems. It has been a three-year process to reconfigure the canals, meaning shareholders and farmers along the waterways have gone without water since the breach. These canals are the heart of the community’s economy and water security — their lifeblood. The new improvements to the canals will deliver water more safely and efficiently to a range of individual, agricultural, and institutional users.
With irrigation water restored to the canals by May 2013, one of the key components of the CWRP was design of the piping for the open channel section of Logan Canyon to alleviate leakage. There was uncertainty about fitting a precast, reinforced concrete box culvert into the existing canal section. This section of the canal, constructed more than 100 years ago, was basically carved into the cliffs along the canyon. At its widest, the open canal was approximately 14 feet wide with steep cliffs rising above the canal and sharp drop-offs to the state highway below. After considerable investigation, project designers decided on precast box culvert to solve part of the problem.
Oldcastle Precast was contracted to supply approximately 10,000 linear feet of specially designed, precast concrete box culvert and precast pipe that was used in the upper portions of the canal project. Of this, nearly 4,300 feet of precast box culvert was used (some 5 feet by 5 feet and some 6 feet by 5 feet to match the hydraulics of the canal) for the Logan Canyon section.
Box culvert bevels had to be specifically designed as “curve to the left” or “curve to the right,” and each piece of box culvert had to be numbered and installed in a series so that the box culvert did not deviate from the alignment.
The challenge was to design a box culvert that would fit within the channel without impacting the steep slopes on either side. To accomplish this, designers had to bevel most of the box culvert segments so that the box culvert could “snake” along the alignment, without elbows, transition vaults, or closure pours.
There were some sections of the canyon where precast was unfeasible because of the impracticality of being able to fit heavy construction equipment into the canal (the canal narrowed to 8 feet wide with overhanging cliffs in some sections). While the cast-in-place construction was extremely well performed, the speed and efficiency of precast box culvert allowed for six-time faster construction. Some of the cast-in-place construction slowed to a halt during January 2013, as the temperatures were too cold to pour concrete for several weeks.
To manufacture bevels, Oldcastle Precast designed and purchased new “headers” that could bevel — angle one side of the box culvert so that a series of bevels would form a curve — up to an 8-inch difference 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 there were no extra requirements at the joints.
To fit the numerous curves in the canal alignment, beveled precast box culverts were designed in 1/2-inch increments, with bevels ranging from 1/2 inch to 8 inches. The bevels had to be specifically designed as “curve to the left” or “curve to the right,” and each piece of box culvert had to be numbered and installed in a series so that the box culvert did not deviate from the alignment. After emerging from Logan Canyon, the water is transferred from the box culvert to a 66-inch concrete pipeline that heads north.
Whitaker Construction installed each section so that the inner joint gap measured 1/2 inch. Once the alignment was established, the installation progressed rapidly with the contractor averaging more than 15 sections per day. This was a record rate of installation when considering winter conditions and that only one access to the upper section of the project existed, so the box culvert and its bedding materials had to be transported 3,000 feet along the canal at the start of the section.
The box culvert was backfilled with 6 inches of road base to provide for a maintenance access road and a recreational trail. The top of the box culvert is now gated at both ends and can be used for maintenance as well as access by the U.S. Forest Service.
“The canal operators used to have to drive a truck in the channel to do maintenance like removing rocks. The box culvert eliminates that problem. We’ve built an access road on top of the box culvert with openings for access,” said Zan Murray, from the Logan office of project manager J-U-B Engineering, in the June issue article of Utah Construction & Design.
While the project requirements were difficult enough, the construction schedule started in October 2012 with the irrigation facilities required to be operational by May 2013. When operators opened the new facilities in 2013, 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 concrete box and concrete pipe. While 2013 was a difficult water year, with many areas of Utah on water restriction, the Cache Valley irrigators were able to have water throughout the growing season.
The project was designed and constructed according to a contracting method called Construction Management General Contractor (CMGC), which utilizes an integrated team approach. The CMGC method includes both preconstruction and construction-phase services. The CWRP CMGC team included the owner (Cache County, NRCS), program manager (J-U-B Engineers, Inc.), design engineers (MWH and Hansen, Allen & Luce), and contractor (Whitaker Construction Co.).
The revamped Cache Valley canal system came in under budget and ahead of schedule. This project received an outstanding project award from Utah Construction & Design.
Information provided by Oldcastle Precast (oldcastleprecast.com)