Open Sesame

Are we ignoring a great alternative to bridges?

Watercourse crossings are often seen by municipalities and their engineers as “bridge vs. culvert.” Because of this, several less traditional types of structures are often overlooked that could provide a cheaper way to get traffic over a watercourse. Take open bottom culverts, for example. Although seen as a niche product that is only viable when fish habitat requires it, perhaps there is an opportunity for greater utilization of the limited resources in County bridge replacement budgets. Take, for example, the Etherington Creek Campground access road in the Kananaskis Country, Alberta.

Etherington Creek
The 2013 flood made quick work of the
existing non-bridge sized culvert

During the 2013 floods, the existing 1200 mm diameter CSP was washed out and ended up in the downstream trees. This was not the only time it had been washed out, and it was clear to Alberta Parks that a replacement bridge or bridge-sized culvert was necessary at the site. The existing culvert was put back in as a temporary solution.  The replacement, a 7301 mm x 1743 mm corrugated aluminum structural plate culvert was manufactured by Canada Culvert and Roseke Engineering was the project engineer.

Structurally, the dimensions of the open bottom culvert were beyond the limits of stability, that is, the structure was too wide and too flat to handle standard highway loading.  However, the manufacturer designed steel “ribs” that spanned each corrugation and effectively converted the structure from flexible to rigid.  Because of this, it is safe to say that these “aluminum box” structures can be designed to any size and shape, and can be a viable alternative virtually anywhere that a bridge is being considered.

Due to the high bedrock a bridge sized CSP was not advisable. The significant fish presence also necessitated a structure that didn’t have an invert.  The height of the road above the streambed was low, and the only feasible option using traditional engineering considerations was a bridge. However, the installation of an open bottom culvert was investigated, and since the costs were similar it was eventually chosen as the preferred alternative by Alberta Parks.

Environmental Issues

Etherington Creek
High fish presence ensured that turbidity
concerns were extremely high on the
priority list

In the Kananaskis country, environmental issues are at the forefront of everyone’s mind.  In fact, on numerous occasions members of the public would show up on site, asking questions about how the environment was being handled (trees, fish, etc.).  Thus, it was clear that if any environmental issue was not handled with the utmost concern for the environment, it could turn political very quickly.  Since the minister of Environment & Parks is mandated to respond to every inquiry the public makes about the environment, some weeks this consumed most of the project sponsor’s time.

The initial fish habitat assessment observed approximately 50 fish over a two hour period (without electrofishing), including bull trout, a provincially threatened species.  Therefore, full time turbidity monitoring was necessary during construction. A fish was even observed within the isolation berms during construction in an area that had never had any connectivity with the stream.

Turbidity events proved very difficult to contain because the site was situated in a marshy area and significant springs were present throughout the excavation.  This resulted in two changes to the site isolation system, each of which resulted in their own turbidity spikes.  The work was forced to stop both times due to elevated turbidity in the downstream watercourse, and the fish biologist had to submit a report to Alberta Environment & Parks (the regulatory division).  However, each time the work was able to continue the following morning as the turbidity had subsided to acceptable levels.

In addition, the cast in place concrete foundations had to be sealed to contain the materials within the concrete, which leach through the soil and into the stream channel and are toxic to fish.

An Alternative to Bridges

Etherington Creek aluminum box culvert
The structural ribs were installed above the
corrugated aluminum structure, allowing
for a greater span

Although the fish habitat was a major consideration at Etherington Creek, in hindsight we cannot help but notice that this structure would have been a viable alternative to a bridge even if the fish presence was minimal.

The tendered price of the aluminum box structure was probably about 25% lower than an equivalent bridge.  If the structural ribs weren’t necessary it would have been even cheaper.  But because of the ability to place I-shaped ribs on the structure’s corrugations, effectively converting the structure from flexible to rigid, the aluminum open bottom box culvert is an option at almost any site where a bridge is being considered, even when the environmental issues are less than at Etherington Creek.  The aluminum structure is significantly more corrosion resistant than steel, and a 75 year life span can be assumed, equivalent to a bridge.

20160511_143247

The finished product is economical and
very aesthetically pleasing

The cast in place concrete footings are covered with clay and riprap, ensuring that undermining would only occur in a flood of a magnitude that would inflict significant damage to a bridge as well.

Municipalities would be well served to investigate aluminum box culvert at most sites where bridges are normally built.  We hope the lessons of Etherington Creek will result in an expansion of the knowledge of bridge structures and an overall increase in the effective use of limited funds for bridge replacement by municipalities.


Bernie Roseke, P.Eng, PMP, is the founder and president of Roseke Engineering Ltd., based in Lethbridge, AB.  He specializes in rural bridges, culverts, and drainage structures. He has designed every type of rural bridge structure including CSP, Structural Plate CSP, Open bottom culverts, horizontally ellipsed CSP, concrete pipe, concrete boxes, and single and multiple span bridges.

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