The first LowSpan prestressed, post-tensioned clamshell culvert installed in North America meets stringent criteria while providing an affordable solution.
By Shari Held
Precast concrete is chosen for projects for a variety of reasons, whether that’s strength, its durability, speed of installation or more, and one project in Michigan required precast for all those reasons to ensure success.
The Chippewa County Road Commission (CCRC) in Michigan needed an economical solution for replacing the failing Charlotte River crossing at 12 Mile Road. The metal pipes in the existing twin-tube structure were corroded, and it was inadequate for the size of the water course. The crossing often flooded during spring run-off, creating erosion issues. Plus, the Michigan Department of Environmental Quality specified that replacements meet the bankfull widths of the water course.
“In this case, it forced us to go from a 20-foot span structure to a 30-foot span minimum,” said Rob Laitinen, superintendent manager for CCRC.
A traditional reinforced culvert couldn’t meet the bankfull width specification, leaving a free-standing bridge as CCRC’s only option. But it wasn’t a good one. The area’s corrosive clay soil offered poor weight-bearing capacity. A bridge would have required multiple vertical steel piles in excess of 100 feet in length and expensive lightweight geofoam backfill. Deep piling could potentially penetrate a nearby artisan aquifier, causing groundwater leakage. And, the road would be closed an estimated 60-plus days.
Lowspan Mega-Box: An alternative solution
A brainstorming session between CCRC and Escanaba, Mich.-based Upper Peninsula Concrete Pipe Co. (UPCPC) led to the design and creation of a LowSpan clamshell-style culvert called Mega-Box. The innovative, three-sided precast prestressed design provides added structure and span lengths of up to 70 feet to conventional LRFD HL-93 loading and 50 foot spans lengths to MDOT’s enhanced HL-93 modified load (72 kips/axle), as was required for 12 Mile Road. The bottom slab is placed below the stream bed and acts as a foundation, making it stable even in poor soil and eliminating the need for piles and costly backfill. In fact, Laitinen calculated the Mega-Box was approximately one-third of the price of the bridge for this project.
Designed for strength
“This was the first of its kind,” said John Kloet, president of Gladstone, Mich.-based LowSpan LLC, a UPCPC spin-off. “Gaining familiarity with efficiencies in design, casting and installing was the main project challenge.”
LowSpan culverts achieve the strength to span large widths by prestressing the top and bottom slabs. The concrete is poured into the form over high-strength, pre-tensioned steel strands. The structure has a life expectancy of 100 years.
A DIY installation
The installation process was smooth from start to finish partly because of the size and weight of the Mega-Box components.
The 11 main components were 30-feet-long, 6-feet-wide and 4-feet-tall and weighed 33 tons each. Everything could be delivered via conventional truckloads, which was a big plus.
“The clamshell design reduced the weight of each piece by half (compared to a full-box section),” Laitinen said. “That also reduced the size of the crane necessary for installation. The design just adds a lot of benefit.”
LowSpan supplied a crane operator to operate the Grove 275-ton mobile crane and an overseer to guide the assembly. The installation of the Mega-Box took just two days and the road was closed only 30 days during the entire project.
Mega-Box proves its value
Initially, CCRC monitored the structure for stability weekly, then quarterly, and once this year. Laitinen said it has been rock solid.
“I would consider this project a success in every way,” Laitinen said. “It meets all the modern highway load ratings, it met our needs for reducing costs, and we were able to utilize our own people and excavating equipment to install it. I take a particular amount of pride in having accomplished that.”
Kloet sums it up this way: “Mega-Box provided the right product for the conditions and loading criteria.”
Shari Held is an Indianapolis, Ind.-based freelance writer who has covered the construction industry for more than 10 years.