As the nation’s highway infrastructure continues to deteriorate, bridge deficiencies adversely impact mobility, safety and economic development. Many state departments of transportation (DOT) are struggling to maintain an acceptable schedule of bridge maintenance, repair and replacement. It recently was reported that 14 percent of all bridges spanning 20 feet or longer were considered structurally deficient. Restrictions on vehicle weights that result from these deficiencies may lead to certain vehicles using alternate routes, thus lengthening travel times and reducing efficiency. At the same time, traffic congestion continues to increase.
To mitigate the problems associated with deficient bridges and increased traffic, bridges throughout the country must be replaced or widened, with lane capacity added. Improving the nation’s infrastructure requires a significant investment. To mitigate congestion, construction schedules must be accelerated to reduce the impact on the public, while observing economic stringencies to conserve funds for other projects. To achieve these goals, transportation practitioners responsible for foundation design and construction must identify more efficient and cost-effective methods for supporting structures.
Continuous-flight auger (CFA) pile foundations offer a low-cost alternative. CFA piles (also called auger cast-in-place piles) are a deep-foundation element characterized by drilling a hollow-stem auger into the ground to form the pile’s diameter. Sand-cement grout or concrete is pumped into the hole as the auger is removed, eliminating the need for temporary casing or slurry. After the auger is removed, reinforcement is installed. Typically, CFA piles are grouped based on the type of equip ment used to install them. CFA piles generally are available in 12-inch to 36-inch diameters, and typically extend to depths of 60 feet to 70 feet. In some cases, CFA piles have been installed to depths of more than 100 feet. Drilled displacement piles also are commonly used.
Continuous-flight auger piles can be installed quickly and inexpensively, and are a viable foundation alternative to driven piles or drilled shafts for certain applications. CFA piles can support lateral earth and critical and noncritical structures, and can be used in ground improvement applications. Typical highway project applications for CFA piles include structure support for new bridges, bridge widening, sound wall foundations, column support for embankment construction, and secant walls for lateral earth support. CFA piles are a good deep-foundation solution in areas that are environmentally sensitive or require minimal disturbance to human activity.
Key benefits of CFA piles:
- Rapid installation accelerates foundation construction,
which reduces project schedules.
- Automated monitoring equipment provides real-time quality
control.
- Suitable for low headrooms or confined spaces.
- Limited installation noise and vibration for sensitive urban environments.
The Texas DOT uses CFA piles as foundations for sound walls in the Houston area. The state successfully completed construction of a bridge in the town of Crossley, supporting the abutments on 69 18-inch-diameter piles. Pile lengths were as long as 67 feet. To handle lateral loads, some of the piles were constructed on a 4-to-1 batter.
To reduce vibrations that might have caused potential damage to an active Metro subway line, the District of Columbia Department of Transportation used drilled displacement piles to construct the foundation elements for a portion of a replacement structure. The piles were installed under low-headroom conditions and created minimal vibrations, which reduced disturbances to the overhead Metro line.
This technology is characterized by drilling into the ground a hollow stem that forms the diameter of the pile. Sand-cement grout or concrete is pumped into the hole as the auger is being removed from the hole, thus eliminating the need for temporary casting. After the auger is removed, reinforcement is installed in the pile. In many situations, these foundation systems can be constructed more quickly and less expensively than other deep foundation alternatives.
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This article is provided through the courtesy of the U.S. Department of Transportation.
