As a landscape architect or engineer, you may have faced this situation before: where your client is looking for a solution to widen a road shoulder that has a steep slope off its side - in order to provide access for pedestrians and cyclists.
Which option best fits your project? Building a retaining wall, adding fill on the downhill side and pouring a sidewalk or installation of a new boardwalk, which would allow the slope off the side of the road to remain untouched?
This design challenge, a retaining wall + sidewalk vs. a boardwalk on the side of the road slope, comes up often in our meetings with design teams. We recently produced a PermaTrak boardwalk for such an application - called Beech Road Boardwalk in New Albany, OH.
I wanted to share some insight after going through the design process with the civil engineering consultant, EP Ferris and Associates, and supporting them with construction documents for PermaTrak's concrete boardwalk system.
In case you're looking into a similar application, here are a few points to keep in mind as you evaluate the options for your project site.
1. Cost of the two design optionsAssuming cost is one of the primary factors in making this design decision (as it always is), keep in mind that the big drivers to boardwalk costs are railing and foundation system.
On several recent boardwalk cost estimates, I've seen our boardwalk system (footings + PermaTrak + railing) be installed for under $50/sf. Recent bids have come in around $60-$70/sf. Deep driven piles for foundations and customized railings can drive this cost upward. A very typical footing may consist of a Sonotube formed concrete pier as you can see in the drawing below. The cross section below shows a concrete boardwalk that is running parallel to Beech Road in New Albany, OH. This boardwalk is sitting on the slope of the road shoulder. This design option provides an alternate to building a retaining wall and adding fill on the downhill side.
A low cost railing (aluminum picket for example) can keep your overall installed boardwalk cost manageable. There are some things we can do to help lower this boardwalk construction cost, while custom colors or textures cold push it upward. However, this information and other cost information on PermaTrak should help you get started with an order of magnitude - at least for this type of concrete boardwalk system.
2. Site disturbance
Perhaps your project site involves a lake, wetland or otherwise sensitive area. One option to address this road shoulder widening challenge is to apply for a permit, fill in the wetland area, build a retaining wall, mitigate the impact to the wetland area to fulfill the requirements of the approving environmental agency, and move on. But is that the most environmentally responsible decision? What does the construction process look like for this option - is it overly invasive? Most likely there are other other less intrusive design options.
3. Permitting and time
In addition to the potential site disturbance, if you are looking to extend the road shoulder out with additional earthen fill, this may end up pushing the slope out 10-20' from the road. This would eliminate the drainage swale and require a pipe to be installed underground, running the entire length of the project. More engineering time is required for this type of effort, and most likely, additional permitting work. More permits = more time. When we provide design support on projects that involve bodies of water and/or the Army Corps of Engineers, the estimated time for finalizing a permit can go from 6 months to 18 months very quickly. You'll also be accounting for the cost of the earthen fill, which can make this option of retaining wall + sidewalk cost-prohibitive. We saw this recently on a project in Arkansas, where the project designer ran cost estimates for a boardwalk installation vs. earthen fill and a retaining wall. He presented to his client that the boardwalk option was the cheaper and more aesthetically pleasing option.
4. Construction requirements
Another important consideration for designing a retaining wall + paved sidewalk vs. a boardwalk on the side of the road is construction feasability. PermaTrak is the primary design concept right now for a project running along the Hudson Bay in NY. Extending the road shoulder out into the Hudson Bay simply is not an option, for many reasons! On a project site like that, the retaining wall option may be eliminated simply due to the difficulty of the construction.
Does your project have a similar challenge? Schedule a quick phone call with me and we can discuss whether an elevated boardwalk would be a good fit for your project site.