United States Department of Transportation
i
Evaluating the Effect of Temporary Casing on Drilled Shaft Rock Socket Friction
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2018-01-01
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[PDF-16.85 MB]
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Abstract:Installation of large diameter steel pipes, called casing, is a common means of stabilizing open excavations needed for drilled shaft construction. While methods vary in both the installation and sequencing of their installation, an expected depth of casing embedment based on boring logs can vary from that which actually is observed in the field. When the casing is terminated in limestone formations deeper than anticipated there is concern that the resulting concrete bond with the limestone outside the casing may be diminished once the casing is extracted and the fresh still fluid concrete is permitted to make contact with the limestone. The primary objective of this study was to quantify the effects on side shear from the use of temporary casing in regions where the casing is embedded into the limestone. Both small and large scale field evaluations of rock socketed shafts were performed Small scale field tests involved casting 29, 1/10th scale shafts constructed with three different casing installation/extraction methods including driven casing, and two different rotated casing cutting heads. Pullout test results of the small scale rock sockets showed the temporary cased shaft could have as low as 60% of the capacity of the uncased controls used for comparison. Full scale field testing entailed casting s side-by-side pair of 2ft diameter rock socketed shafts in limestone where the SPT blow counts were on the order of 50-60. This strength of limestone is sufficient in strength to seat a casing, but is also weak enough to allow for a casing to be embedded well within or even pass through. The results of the full scale tests showed the temporary cased shaft exhibited 83% of the uncased control. While often not necessary, the casing can be driven through extremely hard material which was tested in the small scale testing with a wide range of limestone strengths; the full scale tests could not practically test the same range of strengths. To this end, the large scale tests targeted what was thought to be the most likely scenario (N = 60). Small scale tests showed a higher reduction in side shear relative to the unconfined compression strength for stronger parent limestone (Figure 5.2). This is thought to be a byproduct of larger voids / higher roughness in the weaker material that promotes better bond even when debris from outside the casing is present. The evaluation of temporary casing used in rock socketed drilled shafts in simulated limestone showed construction procedures can lead to different side shear and hence an adjusted resistance factors could be considered. Based on the results of this study, the present FDOT specification requiring extending the socket depth by 50% of the unplanned additional embedment depth in Florida limestone formations is reasonable.
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