When building a boardwalk over wetlands or an environmentally sensitive area, top-down construction might be something suggested, or even required by the municipality or agency spearheading the project.
At PermaTrak, we get a number of questions from designers about when top-down construction should be used, the different methods that would be available for contractors, and how top-down construction impacts the total cost of your project.
What does top-down construction mean?
Top-down construction can have slightly different (but critically important for cost implications) meanings to various invested team members. The decision to employ a top-down construction approach is typically driven by environmental permit conditions. It is up to the designer to tell the contractor which scenario applies so they understand what is allowed during construction and can then determine how best to meet those requirements.
Taken verbatim at its strictest meaning, top-down could mean both the foundations and boardwalk are installed by not putting any equipment in the wetlands. This process drives installation time and precast boardwalk design. A cost effective balance must be found between precast sizing and equipment sizing. This approach can also be a big factor in determining what foundation type can be used with your boardwalk.
Most boardwalk structures are not wide enough, or have sufficient structural capacity, to support large pile driving equipment (think falling weight and pneumatic hammers with long leads). For example, the equipment required to drive precast concrete or steel H-piles will likely not be permitted to operate on a typical boardwalk. If this foundation is required due to geotechnical conditions, a separate frame may need to be constructed for the pile driving operation. Other foundation options might include timber, or FRP piles which can be installed using vibratory means, or helical piers.
What are the specific methods for top-down boardwalk construction?
We’ve seen three different methods for top-down construction when building a boardwalk. The first two methods apply top-down in its strictest sense: the equipment stays out of the wetlands and on the boardwalk itself. The third puts equipment in the wetland, but only in the “footprint” of the boardwalk: allowing the foundations to be installed from mats on the ground.
Method 1: Helical Piers from Boardwalk
For this method, a smaller piece of construction equipment can install a helical pile foundation, the beams, and the treads while sitting on the boardwalk itself and minimizing disruption to the wetland environment. It should sit on matting or plywood to protect the treads.
The construction equipment in this example acts as a giant screwdriver, screwing in the helical piers in the ground.
Foundation Used: Helical Piers
Sample Construction Equipment Needed: Bobcat E 63 (13,800lb)
Cost Impact: In comparison with Methods 2 and 3 below, Method 1 is typically significantly less expensive than Method 2, and slightly more expensive than Method 3.
Installation Time Impact: In comparison with Methods 2 and 3 below, Method 1 is typically slightly faster than Method 2 and 2-3x slower than Method 3.
Method 2: Vibrate Piles from Boardwalk
Similar to method 1, the construction equipment can install the foundation, beams, and treads while sitting on the boardwalk itself. Like method 1, the construction equipment should sit on matting; however, this should be more intensive load distribution mats designed by an engineer. As the boardwalk progresses, the matting is expanded to move with the equipment.
The construction equipment in this example shakes and presses the pile foundation (usually timber or composite) into the ground, vibrating rapidly to loosen the soil and applying pressure to drive the pile into the ground.
Foundation Used: Piles, often Timber or Composite
Sample Construction Equipment Needed: CAT 312 C Excavator (28,000lb)
Cost Impact: In comparison with Methods 1 above and 3 below, Method 2 is typically more expensive than Method 1 and significantly more expensive than Method 3.
Installation Time Impact: In comparison with Methods 1 above and 3 below, Method 2 is typically slightly slower than Method 1 and 2-3x slower than Method 3.
Method 3: Foundations Built from the Ground
Unlike methods 2-3 above, the foundations are not installed from the boardwalk itself. Instead, the piles are allowed to be installed from mats on the ground. Mats can be placed for a pile drill rig to move along these mats to install deep foundations.
With this method, heavier heavier driving equipment and higher capacity piles can be used, provided the mats can support the equipment. Once the piles are in place the matting is removed, and the boardwalk can be installed from above “top-down” with a smaller piece of equipment.
Foundation Used: Deep Foundations
Sample Construction Equipment Needed: Grove GHC30 Telescopic Crawler Crane (75,000 lb.) to install piles, Bobcat E50 excavator (11,500 lb.)
Cost Impact: In comparison with Methods 1 and 2 above, Method 3 is typically slightly less expensive than Method 1 and significantly less expensive than Method 2.
Installation Time Impact: In comparison with Methods 1 and 2 above, Method 3 is 2-3x faster than both.
Total Cost Comparison for 3 Methods of Top-Down Construction
As we can see from the chart below, the least expensive top-down method is typically Method 3. Method 3 also produces faster installation times than the first two methods.
The main drawback of Method 3 is that the larger construction equipment does need to sit in the wetland to install the piles; however, the disturbance can be minimized by the equipment sitting on mats as the foundations are installed. The equipment mats can be installed within the boardwalk construction limits of disturbance further reducing impacts to the environment.
Top-down method matters and should be spelled out
Whichever method is chosen, notes regarding the method of top-down to be used, and allowable equipment should be included in the boardwalk drawing and specifications.
Here are examples of the types of notes and schematics that PermaTrak recommends incorporating into your project documents.