Heavy precast concrete foundations resting on a 6-inch layer of gravel-filled geocell wrapped with geotextile fabric anchor the stout, high-elevation boardwalk on the Snake Creek Trail at the Inyo National Forest in California. An additional 6,000 pounds of uplift force resistance is provided by two Manta Ray earth anchors attached through each concrete foundation. This area is subject to heavy seasonal flooding. The concrete sills resist uplift during the spring runoff. The geocell base helps spread the load of the structure, eliminating the loading differential that might occur with single posts. The boardwalk is designed to handle snow loads of 250 pounds per square foot, which is considerably greater than the American Association of State Highway and Transportation Officials (AASHTO) required pedestrian live load of 90 pounds per square foot.

Precast concrete foundations have some advantages and disadvantages when compared with conventional techniques.

Advantages

• Heavy concrete resists movement during floods.
  
  
• Geocell provides a broad base to spread loads.
  
  
• Precast concrete foundations do not leach wood preservative into the surrounding environment.
  
  
• Precast concrete foundations are well suited for areas with rocky or thin soils where helical piles or wood posts would be unable to penetrate the ground.
  
  
• Precast concrete foundations provide better quality control and faster field construction.

Disadvantages

• Precast concrete foundations are heavy and impractical in remote areas without nearby road access.
• Machinery is required to move the concrete foundation and preassembled substructure into place.

The following steps illustrate the installation process.

Step 1—A trench 8 feet by 3 feet by 1 to 2 feet (depth depends on
the required substructure ground elevation) is excavated to
accommodate the wrapped geocell and precast concrete
foundation.

Step 2—Geosynthetic fabric is placed and the geocell is staked and
filled with gravel. Geosynthetic fabric is folded over the top.

Step 3—The substructure consists of a 6-foot by 18-inch by 12-inch
precast concrete foundation. The top of the concrete foundation is
9 inches wide and has a 6-inch by 6-foot by 3⁄8-inch flat steel plate
with 7-inch shear studs cast into it. The posts (3-inch diameter
galvanized steel pipe with ¼-inch walls) are welded to the
steel plate. Rebar hooks embedded in the curb allow the curb to
be moved by machine.

Step 4—The substructure is placed on top of the wrapped geocell
filled with gravel so it is flush with the final ground elevation. Two
Manta Ray earth anchors are installed for each precast concrete
foundation to restrict uplift and lateral movement.

Step 5—The trench is backfilled with suitable excavated material.

Step 6—Saddle brackets are bolted to the tops of the pipes and
1⁄8-inch weep holes are drilled at the bottoms to prevent water
from collecting, freezing, and breaking the pipes.

Step 7—Pipe sleeves are used to bolt saddle brackets to the pipes
and header beams to form a bent.

Step 8—Two-inch-wide galvanized steel C-channel cross bracing is
added where the structure is 3 feet or higher above the ground.

Step 9—Steel angles bolted to the stringers hold them in place.

Step 10—Where steel pipes are not needed, a ¼-inch thick neoprene
pad is placed between the foundation block and beams to keep
moisture away from the wood beams.

Step 11—The finished structure features curbs with scupper blocks
for this ATV/motorcycle trail.

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