United States Department of Transportation
i
LRFD Procedure for Piles with Pilot Hole in Rock
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2023-04-01
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[PDF-3.83 MB]
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Edition:Final; Start September 2019 – April 2023
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Abstract:A pile driven into the ground gets its bearing capacity from skin friction along the pile surface as well as from end resistance at the toe. The load transfer mechanism determines how much load is carried by the shaft and by the toe. In Georgia, when a hard/dense layer exists in the pile length or the vibration/noise during the driving causes secondary issues, a pilot hole is often adopted as a pile-driving assistance method to aid driving displacement piles through, especially if a competent hard rock layer exists in a reasonable depth. The use of a pilot hole reduces construction time and uncertainties related to driving through the problematic layers. However, it also reduces the side resistance within the drilled zone due to the disturbance and size of the hole. This process also complicates the prediction of long-term pile capacity. An objective of this study was to identify and document the current guidelines available and adopted by different states, and investigate the relationship between the load capacity of piles installed in rock and their design parameters with respect to the pilot hole, rock conditions, and installation method. Another objective was to identify a reliable design procedure that incorporates proper LRFD resistance factors, and a field verification method for quality assurance of rock. A compilation of best practice methods was necessary, which includes a literature review, a survey with state highway agencies, field tests, a review of past projects and testing data, and making final conclusions. Through these efforts, it is found that most states using a pilot hole in rock, the hole size is larger that the pile and the skin friction is ignored for the nominal resistance. In addition, geotechnical aspects control the design in soft rock, whereas structural aspects control the design in hard rock. Some states do not run a field test but use refusal/end of driving criteria for piles driven into rock. Nevertheless, PDA can be applied to the piles with a pilot hole on rock to check the internal stress to avoid the damage during striking. Moreover, it can verify the structural capacity of the pile if not the geotechnical capacity due to the higher bearing capacity on rock.
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