An innovative arched fibreglass culvert system for environmentally sensitive streams is getting some attention following a trial project in the BC Interior.
By Jim Stirling
The ancient Greeks understood the inherent strength and versatility of the arch and incorporated it into many of their structures. They would have appreciated a modular fibreglass application of the form as the centrepiece in an innovative culvert system on logging roads designed to protect environmentally sensitive streams. The Enviro-Span archway has the added advantages of being lightweight, quick and easy to install and is re-usable.
The system’s developer is Enviro-Span Technology Inc, based in Prince George, British Columbia. The system has garnered considerable interest from logging companies, contractors and the scientific community since it was officially launched at Forest Expo 2002 in Prince George.
The principals in Enviro-Span Technology—businessmen Bob Semotiuk and Barry Nybo—have taken a deliberately cautious approach to the development of their archway culvert system. They’ve taken the time to get things right and properly organized, says Semotiuk.
“We knew what we wanted to do, but how to get there was the challenge.” Semotiuk and Nybo went through years of work, including exploring the concept with miniature arches, before detailed engineering studies and test profiles led them to the first generation of prototype.
“The forest industry was looking for a method of crossing streams without disturbing the streambed,” recalls Nybo. Part of that concern was ushered in by the provincial Forest Practices Code. It emphasizes preserving the integrity of natural watercourses during the construction of logging roads and in timber harvesting practices.
“We came up with the idea of using a different type of material,” says Nybo. The partners briefly considered various alternatives, ranging from PVC to cement composites, before opting for the advantages of an open-bottomed arch culvert system using fibreglass.
Traditional corrugated steel pipe culverts are strong but have drawbacks, including the need for site preparation. Creating a foundation for the structure courts the risk of damage to surrounding riparian areas and sedimentation. In the long term, steel degrades in the natural environment and can leach harmful metallic elements into the water.
Fibreglass, however, is inert, non-corrosive and non-metallic. It provides no risk to fish or aquatic fauna. And because the Enviro-Span system doesn’t require special sub-grade preparation or foundation elements, it also bypasses that set of potential damage problems, says the company.
It’s appealingly simple. The modular fibreglass arched units are light—a 1.8 metre long section weighs about 77 kilograms—making it easily manoeuvrable by two people. Nybo says it’s feasible to have up to 3.6 metre wide archway openings. The fibreglass units are flanged, lending themselves to one of the more intriguing aspects of the system. Units are held together and secured by placing an interconnecting joint over the adjacent flanges. These floating caps, as Enviro-Span refers to them, require no bolts, clamps or tools to use. The identical, interchangeable fibreglass units join together like Lego.
“The uniqueness of the floating cap and the flanges, that help deflect the strains, are a very significant design factor,” says Nybo. The Enviro-Span system is protected by both Canadian and US patents. The modules can be angled to better follow the natural curve of a stream and their flexibility allows them to follow vertical changes without the need for special levelling, he notes.
“A big factor in the Enviro-Span design was quick installation,” says Nybo. “And the fact that it can be used many times over.” As with any type of culvert installation, engineering site assessments and hydrological studies to determine peak run-offs must first be completed. That helps to ensure the appropriate diameter Enviro-Span unit is used in each application.
The fibreglass arch units fit on set-in-place log base stringers positioned parallel to the streambed. The logs, typically acquired through right-of-way clearing, should be straight, sound and up to 30 centimetres in diameter. The modules have footing arches which are centred on the log. The stream’s channel is untouched while the modules are placed in position and secured with the floating cap joints. It’s easy to rip-rap the stream bank under each module if that process is deemed necessary.
When the arched culvert is in place, it is backfilled on both sides with native material and compacted by tamping with an excavator bucket. Machinery can cross a 1.8-metre-diameter culvert after 90 centimetres of compacted cover is achieved.
An independent verification of an Enviro-Span test installation by Firth Hollin Resource Science Corporation made the following observation: “Assuming road access is available to points adjacent to the installation, a 12-metre length of 1.8-metre modular crossing should be installed with four hours of tracked excavator time and eight man-hours or less of labour.”
The trial installation and testing program for the Enviro-Span units took place in January 2002. The subject stream was on an operational logging road in the Driscoll Creek area of the Prince George Forest District and used primarily by TRC Cedar Ltd, of McBride, BC. At the culvert site—located about three kilometres from the Fraser River—the stream was classified as S-5 but was S-4 (fish bearing) downstream.
The Enviro-Span arch was readily endorsed by Trent Gainer, TRC’s operations supervisor. “The product definitely has its place. We never had to touch the streambed. It’s very simple to install and very durable,” he says. He notes the arches are installed without a machine, which saves costs and removes the risk of collateral damage. “They require a minimal amount of fill.”
Gainer estimated about 4,000 cubic metres of timber had been moved across the culvert in five- and six-axle logging trucks under winter, summer and wet conditions. “We’ve had absolutely no problems with it. We’ll definitely use them in the future,” he adds.
Electrical resistance strain gauges were installed inside the culvert to assess the stress distributions under a loaded logging truck. Maximum strains recorded were significantly less than one half of allowable strains for the fibreglass material. The arch’s durability is no surprise to Semotiuk and Nybo—or, one suspects, the ancient Greeks.
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