Top down construction for wetland boardwalks
Designers working on wetland boardwalks often ask similar questions at the beginning of the project: how can we simplify the permitting process, and how can we provide access to the area without harming the wetland itself?
Both of these questions should be discussed in the concept phase of the design. While many wetland boardwalks are permitted and built with marsh buggies or cranes placed on large mats in the wetlands, invasive machinery like this can be avoided by designing the boardwalk to be installed from above. This method is called top down construction.
The preferred construction methods can be determined during the concept or project design phases. Boardwalk components can then be designed accordingly to support the construction equipment loads during installation. As a result, there is a reduced negative ecological impact, and the permitting process is usually simplified.
How does top down construction work?
Top down construction is a method where a small, efficient machine with specific attachments drives on top of the existing boardwalk surface to place the next section of foundation components and then beams and treads.
Once that section is finished, the machine moves forward on the newly placed walking surface to continue installing the next portion. The process continues incrementally until the boardwalk is complete.
Throughout the process, the machine doesn’t drive directly on the wetland floor, reducing the footprint.
See the video below for an example of top down construction for a concrete boardwalk.
Comparing boardwalk materials for top down construction methods
Below we’ll take a look at the pros and cons of three of the most common commercial boardwalk materials used for top down construction:
Top down construction on a PermaTrak precast concrete boardwalk
If your project requires top down construction to minimize impacts to the wetland area, PermaTrak’s engineering team can assist with a structural analysis that defines the construction live load. It is well known that precast concrete is a stronger and much more durable material than timber or composite decking.
As an inherently strong building material, precast concrete gives the installing contractor flexibility when it comes to choosing their equipment for construction, lowering construction costs. With over 400 boardwalk projects installed to date, we’ve seen massive cost savings passed on to the project owners when contractors have the opportunity to be cost-efficient by using either the equipment they already own or equipment that can be easily rented.
Something to be aware of however, is that as a heavier material, precast concrete may not be ideal in rare boardwalk installation scenarios when the boardwalk material must be hand carried onto the site location.
To begin construction, foundations are installed first and are often spaced 10’ to 15’ apart. PermaTrak precast concrete boardwalks can be installed on timber piles (shown below), helical piles, concrete piles or cast-in-place caissons. Next, the longitudinal concrete beams can be placed (shown in second photo below with concrete caissons).
The beams are placed with the same machine that installs the foundation. Following these load-bearing elements, the walking surface treads are set with the same piece of equipment. A steady stream of boardwalk components is brought to the end of the structure as it is built, usually by a skid-steer type of implement.
Top down construction on a timber boardwalk
Timber boardwalks can be constructed using the top-down method. Specialty contractors are well equipped to design and install timber boardwalks using this kind of construction method.
In addition, special boardwalk applications in remote areas may require the boardwalk components to be hand carried into the project site. While this may require more individual laborers to move and transfer the material to the section being built, it can be done easily with timber planks, as the components are relatively light.
However, pressure-treated timber bends easily, and its shear capacity is much lower than concrete and would require many more stringers (runners) and possibly shorter longitudinal spans between piles to support the same load. In most cases, this results in more material, longer installation times and higher installation costs overall.
Similar to installing a concrete boardwalk with top down construction, for a traditional timber boardwalk, the foundations are installed first, typically spaced at 8’ to 10’ feet apart. Next, the headers and runners can be placed.
The headers and runners are placed with the same machine that installs the foundations, and the timber planks are individually screwed down. In the same way, the boardwalk components are moved to the end of the structure as it is built.
Top down construction with composite decking
One of the biggest drawbacks to using a composite decking material for wetland boardwalks is that the boardwalk is less likely to be built via the top down approach. Most composite decking products are manufactured for residential uses and simply are not designed for heavy vehicle loading, whether that is a piece of construction equipment used during installation or a 10,000 lb. truck used by the Parks & Rec maintenance staff in the future.
In rare cases, boardwalk contractors have shared a more creative way to build a wetland boardwalk with composite decking. The boardwalk is built first with timber planks before the decking is removed as sacrificial material and replaced with composite decking. This, in turn, increases the cost of materials, the time it takes to complete the boardwalk construction, and the cost of the installation overall. These factors often combine to eliminate composite decking from the conversation when designing a wetland boardwalk that must be built via top down construction.
Advantages of top down construction
An obvious advantage to top down construction is the time and money saved on mitigation efforts. Additionally, undue damage to the fragile wetland will be avoided and the wetland itself will experience a faster self-healing period, recovering quicker, as most of the work was performed from above.
The wetland boardwalk design and installation method can be specified so contractors use top down construction techniques. There are three primary benefits of top down boardwalk construction:
- permitting should be eased (in most cases)
- the construction footprint is reduced
- the wetlands will remain healthier
Do you have a specific question about top down construction or wetland boardwalk design? Send us a note!
