Elevated walkways, boardwalks, and pedestrian bridges are essential infrastructure for providing safe and accessible pathways across challenging terrains. When constructed on steep cross slopes, these structures require careful planning to ensure stability, durability, and minimal environmental impact.
This article explores key design considerations for constructing boardwalks on steep cross slopes, covering stability methods such as rip-rap, retaining walls, foundation stability techniques and construction methods. By integrating these strategies, designers can create safe and resilient walkways while preserving the surrounding landscape.
Understanding Steep Cross Slopes
A cross slope refers to the transverse slope or gradient of the groundline, measured perpendicular
to the direction of travel and is typically notated as a ratio of horizontal to vertical distance (H:V).
A cross slope steeper than 5:1 (H:V) is generally considered to be a ‘steep cross slope’.
In flat terrains, cross slopes are minimal, allowing for relatively straightforward foundation and boardwalk installation. However, a steep cross slope presents unique engineering challenges, including slope stability, foundation design, and constructibility considerations while installing the structure that must be considered as part of the design process.
Geotechnical Stability Considerations
A. Rip-Rap for Erosion Control
Rip-rap consists of large, angular stones strategically placed to stabilize slopes and prevent soil erosion. Some examples of rip-rap include hand-placed riprap, machine-placed riprap, gabions, and wire-enclosed riprap.
Example of machine placed rip-rap that was utilized as a slope stabilization method at Hickory Riverfront in Hickory, NC
B. Retaining Walls for Slope Support
Retaining walls provide lateral support by holding back soil and preventing slope failure. Common types include gravity walls, cantilevered walls, and anchored walls (with tie-backs).
Example of PermaTrak tread being supported by retaining wall on one side and PermaTrak
beam on the opposite side at HUB RTP in Raleigh, NC.
Example of PermaTrak tread being set adjacently to retaining wall on one side (self-supported
separately from retaining wall) at Glen Road Pedestrian Bridge in Ontario, Canada.
Example of completed PermaTrak structure installed adjacently to retaining wall on one side
(self-supported separately from retaining wall), at Chain of Lakes in Frisco, Texas.
Foundation Design Considerations
When designing foundations on a steep cross slope, foundation stability is an additional consideration since one foundation will have very little stickup height, while the adjacent foundation will have a much taller stickup height. The most common way to provide lateral stability for a foundation constructed on a steep cross slope is to add lateral bracing.
Example of lateral bracing used to stabilize helical pile foundation for PermaTrak boardwalk on steep cross slope on the Ashland Trolley Line Trail in Ashland, VA.
Constructibility Considerations
Building the Boardwalk from Adjacent Access Area:
Constructing a boardwalk on a steep cross slope presents unique challenges, particularly when the terrain restricts access for construction equipment. At PermaTrak, the most common installation method we've encountered involves utilizing the adjacent access area to construct the boardwalk from the side. This approach allows for greater efficiency and minimizes disruption to the surrounding environment. However, in cases where access is extremely limited, alternative methods such as top-down construction may be necessary to ensure a safe and effective installation.
Example of foundation being installed from adjacent access area alongside boardwalk in
Matthews, NC.
Example of PermaTrak treads being installed from adjacent access area alongside boardwalk in
Norwalk, CT.
Example of PermaTrak treads being installed from adjacent access area alongside boardwalk in
Ontario, Canada.
Building the Boardwalk Via Top-Down Construction
When a site features a steep cross slope and has little to no access adjacent to the boardwalk, the 'Top-Down' construction method may be the most practical solution. This approach allows for the structure to be built from above, minimizing environmental impact and reducing the need for extensive groundwork. By working from the top down, construction teams can maintain stability on challenging terrain while ensuring efficient progress in areas with limited accessibility.
Example of the Top-Down construction method being performed to install a PermaTrak
boardwalk on a steep slope at Hickory Riverfront in Hickory, NC.
Conclusion
Designing elevated walkways, boardwalks, and pedestrian bridges on steep cross slopes requires meticulous planning to address slope stability, foundation support, and construction feasibility. By incorporating techniques such as riprap, retaining walls, foundation bracing, and strategic construction methods, designers can create durable, safe, and visually harmonious structures that integrate seamlessly with the natural landscape. With thoughtful planning, pedestrian pathways on steep slopes can provide long-term access to scenic and otherwise hard-to-reach locations, enriching outdoor experiences while preserving the integrity of the terrain.
