Every new boardwalk project is unique.
To a casual observer comparing two similar boardwalks, the finished walkways could seem to be indistinguishable. Alike in both color and texture, perhaps even having the same length and width, the boardwalks may appear identical.
Despite these similarities, most landscape architects and designers would agree: no two boardwalk projects are the same.What is oftentimes the key differentiating factor? A boardwalk’s foundational system. It is one of the most important variables to consider when starting a new project. How do I know what type of foundation system is right for my next boardwalk project?
Four Crucial Factors to Consider When Planning Your Next Boardwalk Project
Several factors should guide the planning and decision-making process for engineers and landscape architects.
1. Site Geographical Factors
The design phase should start with a geotechnical report of the site to provide soil characteristics and other important site conditions that will directly impact the foundation system used.
A boardwalk planned over water or through wetlands will almost certainly need a deep foundational structure.
If the site is in an upland location or it is designed over plains, sand or stone, designers could utilize a shallow foundation.
The geotechnical report may indicate sufficient soil bearing pressure. In this case, it may be best to take advantage of the good soil and use a shallow precast concrete foundation.
2. Dead and Live Load Factors
Once a geotechnical engineer has conducted a geotechnical report of the boardwalk site, dead and live loads of the project should be analyzed.
The “dead” load of the boardwalk project is a more permanent load resulting from the total weight of the structure itself. This includes the PermaTrak boardwalk, the foundational materials, and any other construction attachments that will always sit securely on the foundation.
For PermaTrak boardwalk projects, the designers determine the dead load of a timber or composite foundational structure. When utilizing the precast concrete pier system, PermaTrak engineers calculate the dead load and forward on to the designer.
The “live” loads of the boardwalk project are the more temporary loads which will be applied to the boardwalk on and off over its entire lifetime. This includes pedestrian usage of the walkway and any pressure caused by lateral loads, scour, and uplift (described below). The PermaTrak live load should be the same regardless of the type of foundation, whether timber, composite, or precast concrete.
3. Lateral Load, Scour, and Uplift Factors
Calculated and estimated as a result of the boardwalk site’s geography and the “dead” and “live” load factors, designers should also consider lateral load, scour, and uplift factors.
Lateral loads can be defined as any horizontal forces (side to side) that are acting on the boardwalk structure. Common lateral loads include wind loads, possible seismic loads, and pressures from both water and soil at the boardwalk’s site.
A boardwalk encounters “scour” when the sentiment, soil, or riverbed substrate surrounding the boardwalk erodes or is removed in some way from the site’s overall infrastructure. The right boardwalk foundation will mitigate and greatly reduce the risk of damage in environments more prone to scour.
Similar to horizontal lateral load forces, a boardwalk experiences “uplift” forces when something pushes up from the underside of the construction. Common uplift forces include rising floodplain waters, coastal storm surge, wind loads, stream debris, stream flow, and seismic forces. Boardwalk designers should consider potential uplift forces in their choice of foundation.
4. Life-Cycle Cost Analysis of Foundation Material
As part of the planning and design process, engineers and landscape architects will conduct a life-cycle cost analysis of the foundation material.
Depending on the geotechnical report, it will be determined whether timber, concrete, steel, or a composite foundational material could be used. Based on the recommended materials (considering the geo-factors), a life-cycle cost analysis is analyzed.
Will a timber foundation deteriorate over a certain period of time in this wetland environment? Or would it be better to utilize a virtually no-maintenance composite pile foundation?
The total life-time cost of the various materials are factored into the overall decision-making process.
Major Types of Foundational Systems
Once the geotechnical report has been conducted and the four crucial factors described above have been analyzed, landscape architects, engineers, and designers can make an informed decision about which foundational system will be best for the project.
A. Deep Foundations
Deep foundations should be utilized in environments and geographical sites where the boardwalks will run over water or wetlands.
Timber Piles
One of the most common deep foundational materials is timber. For sandy soil and water geographical conditions, such as in the Gulf Coast region, Pressure Treated timber piles are the most common commercial option. Industry specifications will guide the designer in determining the right Class of Piling.
Details such as the type of timber, length, diameter, and taper will factor into the decision. Timber piles tend to be more inexpensive compared to other types of driven piling like concrete and steel. The access and size of the required equipment should be considered when selecting piling type.
Other deep foundation options include fiberglass composite piles, drilled shaft or concrete caissons, driven steel piles (H-piles or pipes), steel helical piles (screw piles), and driven concrete piles.
Fiberglass Composite Piles
Fiberglass composite are light weight with a high capacity, making them ideal for locations with difficult access or requiring top-down construction methods. Fiberglass also has a significantly longer service life than timber in most environments.
Drilled Shaft or Concrete Caissons
Drilled shaft or concrete caissons have commonly been used as foundational systems for PermaTrak boardwalks in northeastern states like Connecticut or Massachusetts. Drilled shafts are also often used in Texas due to the expansive clay soils.
While offering long-term performance and strength, drilled shaft or concrete caissons are more expensive than timber piling. Due to their bulk and handling characteristics, they also require larger equipment and more space to maneuver and install.
Driven Steel Piles (H-Piles)
Driven steel piles, or H-Piles, are manufactured in a variety of lengths and sizes. They are driven to reach deeper strata for high capacity vertical support. In contrast, most timber and concrete piling foundation systems are driven for friction capacity.
H-Piles are fairly easy to drive into the ground, but they tend to be expensive and may require large equipment for installation. Long-term corrosion can limit performance compared to concrete options; however, H-Piles are a hardy, durable choice for foundation systems.
Steel Helical Piles (Screw Piles)
Steel helical piles are not driven, but drilled to a specific depth calculated for the required boardwalk capacity. Helical piles, or screw piles, are common on clay or any situations where low geographical disturbance is preferred. This makes them a great deep foundational option for a sensitive wetland or any ecosystem needing top down construction and as little environmental stress as possible.
The helical piles’ strength is achieved through soil bearing at each helix, which is attached to the lead section of helical pier. The lead section of a helical pier is typically a steel tube or square bar. They are installed quickly with light construction equipment, and oftentimes they are a cost-effective solution for both PermaTrak and timber boardwalks.
Driven Concrete Piles
Driven concrete piles offer durability and longevity as a deep foundation system; however, they are less common than some of the other options presented above.
Driven concrete piles are typically manufactured in circular or square shapes. The PermaTrak cap attaches similarly to other concrete options with a steel dowel and epoxy connection. They usually are at a higher cost than most foundation systems.
B. Shallow Foundations
Shallow foundational materials can be used in upload locations, often designed over plains, sand or stone. Shallow foundations can be used when high load-bearing capacity is needed, but they should not be used in wetlands or water geographical areas.
Shallow foundation options include precast concrete piers and cast-in-place concrete shallow footings.
PermaTrak Precast Concrete Pier System
When geographical and soil conditions allow, the best choice for the boardwalk foundation is the PermaTrak precast concrete pier system. It is the most cost-effective foundation option, can be manually handled, and enjoys the smallest environmental footprint.
The PermaTrak pier system provides a complete precast concrete foundation that is totally compatible with the 50-75 year design life of the overall PermaTrak system.
Cast-In-Place Concrete Shallow Footings
Cast-in-place concrete shallow footings can be circular or square. These concrete footings are best used for medium load requirements, a standard for most boardwalks
The cast in place concrete system uses a drilled shaft, lined with a tube that will maintain the shape of the shaft.
The contractor then lowers a rebar cage into the tube and pours concrete into the shaft. The pilings can be cast to the exact site elevations, can be installed quickly, fairly inexpensively and with relatively small construction equipment.
The Right Foundation for your Next Boardwalk Project
Engineers and landscape architects have a variety of material options to choose from in designing a foundation system.
The question is: what is the best option for your next boardwalk?
The PermaTrak precast concrete boardwalk system is unique in its long-term durability and maintenance free performance over a 50-75 year design life.
It deserves a foundational option that can match its durability and strength, but there are so many different factors to consider.
Are deep foundations required, or will the PermaTrak precast concrete pier system work with your geographical and soil conditions?
Let us walk you through a complimentary consultation to start the planning process. We will take you through the process from start to finish.
