When there is sufficient space, the easiest and most cost-effective way to install fences above SRW walls is to place them several feet (0.5–1 m) behind walls. With sufficient fence post depth and setback, the soil can provide a stable foundation. Separating fence posts from a wall also keeps wall movement from affecting the fence. While a minimum post depth of 30 inches (760 mm) is suggested, the embedment and distance behind the wall needed to create a stable post foundation varies and depends on the soil conditions. One option is to create post holes during wall construction by placing cylindrical tube forms at planned post locations and backfilling soil around them. After completing the wall, the tubes are filled with concrete and the fence posts set in the concrete (Figure 1).
When there is not enough room to set fence posts behind walls, they can be installed within top wall units prior to backfilling behind the wall. Break off the backs of the top few units to create room for the post. Cut or core the cap units to neatly receive posts (Figures 2 and 3). The fence should be flexible enough to accommodate differential movement between the units and the fence.
Placing posts near the front of a wall decreases the fence’s foundation support. To improve stability to the post, the concrete foundation should be enlarged, extended behind the wall and reinforced with reinforcing steel (Figures 2 and 3). The needed depth, extension length and reinforcement placement will vary depending on conditions and loading.
With proper design, guard rails can be used behind SRW walls. For proper support, place guard rails several feet (m) behind the wall units (Figure 4). The setback and embedment depth of the guard rail will vary with conditions and loading. For highway loading, American Association of State Highway and Transportation Officials (AASHTO recommends an embedment depth of 5 feet (1.5 m). Like fence posts, guard rails can be placed in cylindrical concrete tube forms placed during wall backfill.
POSTS PENETRATING GEOGRID
For walls requiring soil reinforcement, fence and guide rail posts will often extend below the top layer or two of geogrid. Often the geogrid can be cut to fit around the planned post locations. Usually the top layers of geogrid can accommodate small intrusions while still maintaining overall tensile strength. However, the area cut from the geogrid should be no more than the minimum needed to fit the post. The wall design engineer must evaluate any planned post intrusions into geogrid layers to ensure they do not reduce strengths below needed minimums. Augering or driving through backfilled geogrid after wall construction is generally not suggested because it may excessively disturb or pull geogrid from the soil or the wall units.
CONCRETE TRAFFIC BARRIERS
When there is no room to set guide rails behind a wall, traffic barriers can be placed directly on top of a wall. These can be cast-in-place concrete or precast barriers (such as Jersey barriers) or a combination of both. Concrete barriers should be designed for stability, independent of the wall. The foundation can be extended behind the wall (moment slab) to act as cantilevered resistance to lateral and overturning loads (Figure 5).
A qualified engineer must design traffic barriers on a project-specific basis. Reinforcing steel, barrier size, and geometry will vary with site conditions and loading. Other design considerations include the need for control joints, expansion joints and bond breaks to address differential movement between the barrier and the retaining wall. During concrete placement for cast-in-place barriers, temporary bracing of the retaining wall may also be required.
SRW stairs can accommodate a variety of railings with proper design, including railings anchored just above and below steps, into side wall units, or into step risers. Solid SRW units allow use of several common techniques for attaching railings to concrete, including fasteners that embed in polymer, grout or mortar, or anchors that cut threads into the concrete. The appropriate fastener varies with loading and site conditions. Refer to the fastener manufacturer’s and wall design engineer’s recommendations.
When practical, spanning railings from landing to landing and placing posts directly into the soil is usually the easiest way to provide a stable foundation for stair railings. When stairs have numerous risers and spanning is not practical, railings can be attached to the units in the side walls. When there are no side walls, rail posts can be placed through the step units (Figure 6). Step units can be split or cut to extend post hole at least 30 inches (760 mm) deep (more depth may be needed depending on loading). The post hole should be filled with concrete.
Caps can be cored to receive the post neatly, if desired.
SRW units are often used to create attractive freestanding walls that extend above the top of retaining walls. While these freestanding walls provide excellent aesthetics and visual screening, they should not be relied on to resist lateral loads. If pedestrian or traffic barriers are needed, independent fences or railings designed for the anticipated loads should be installed behind the freestanding wall (Figure 7).
When clear views over the tops of walls are desirable, lower, wider barriers sometimes are allowed as alternatives to tall fences. Depending on local building codes, back-to-back SRW walls that are spaced far enough apart can act as a pedestrian barrier while providing room for plantings. With proper design and reinforced concrete within the double SRW walls, they can also sometimes function as traffic barriers. Check with local codes for application and required width.
This article provides a general discussion regarding the design and installation of fences and railings. However, conditions and loadings vary with each project and these guidelines are not intended as construction drawings for any specific project. The user is responsible for complying with all applicable building codes and obtaining a final, project-specific design prepared by a qualified professional engineer for a wall and any appurtenant structures. CMD