United States Department of Transportation
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Continuous flight auger pile bridge foundation : Implementation Phase I : Texas Gulf Coast Region : final report.
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2004-01-01
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Abstract:Continuous flight auger (CFA) piles better known as Augered, Cast In-Place (ACIP) piles have been widely used in the United States for several decades because of relatively rapid installation and minimum environmental impact during installation with comparable cost to other foundation systems. However, their use in transportation projects has been limited to small secondary structures such as sound barrier walls and overhead signs that exert relatively very small bearing loads.
In this implementation project, a new bridge was designed and constructed entirely with 64 ACIP piles by Texas Department of Transportation at the intersection of Krenek Road and highway U. S. 90 in Crosby, Texas. This study was undertaken to verify whether ACIP piles could be used as an alternative to driven piles with acceptable in-service performance for bridge foundations in the Pleistocene soils of the Texas Gulf Coast region. In the top 65 ft of soil profile, the site had mainly clay with two thin layers of sand and groundwater table 5 ft below ground surface. ACIP piles were designed using the drilled shaft design procedure established by TxDOT. The grouting ratio (grout volume pumped / theoretical volume of borehole) and grout line pressure at the ground level were monitored during the construction of every ACIP pile. Before installing production piles the design capacity of the instrumented 18 in. (0.46 m) diameter ACIP pile was verified by performing a full-scale load test on site and was compared it to a driven prestressed concrete (PC) pile on an adjacent bridge (at Runneburg site) in nearly similar geotechnical condition. The failure load for the ACIP pile, which was designed for an axial capacity of 90 tons with a factor of safety of 2, was 215 tons. Greater load transfer was measured along the ACIP pile shaft as compared to the designed value. The axial capacity of the 16 in. square driven pile was 165 tons.
The length of the ACIP piles varied from 57 to 62 ft and each pile was installed within 15 minutes. Total of 12 ACIP production piles (plumb central bent and battered abutment piles) were instrumented with calibrated vibrating wire sister bars to measure strains to determine the long-term load transfer characteristics of the piles. Residual axial tensile strains developed in the ACIP piles. The central bent footings and abutments were instrumented with contact pressure cells to determine the load sharing between piles and pile cap. Initially there was load sharing but with time all the loads were transferred to the ACIP piles. All the aspects of bridge construction were monitored for the purpose of quality control, fill settlement, and footing and abutment settlements. The performance of the bridge has been monitored for 6 months under service loads.
Measured settlements in various components of the bridge were less than 0.12 in (3 mm). Most of the load transfer was along the shaft of the ACIP piles and no tip resistance was measured for pile tipped in sand. Based on the ultimate load capacity, ACIP pile was cost-effective compared to a driven prestressed concrete pile. Finite element analysis was performed to investigate the group action within the central bent piles in a pile group.
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