Drilled shaft resistance based on diameter, torque and crowd (drilling resistance vs. Rock strength) phase II : final report.
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2016-06-01
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Edition:Final report
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Abstract:The research focused on evaluation of Florida limestone’s unconfined compressive strength, qu, through
drilling parameters – crowd, torque, penetration rate, rotational speed, and bit diameter – in both the
laboratory and field for assessing a shaft’s side shear. Synthetic homogeneous limestone blocks of
various strengths were cast in the laboratory and allowed to cure for 14 days; The blocks were then
drilled with rock augers of two different diameters at multiple penetration rates and drilling rotational
speeds. Both crowd and torque were measured in real time. Estimation of rock strength, qu from drilling
relationships developed by Karasawa (2002a) and Teale (1965) were evaluated. It was found that
Teale’s specific energy approach could estimate rock strength, qu using the multiple diameter drill bits
expected in field drilled shaft construction.
In the field, shaft installation was monitored (crowd, torque, penetration rate, etc.) at three sites (Little
River, Overland, and Kanapaha) from which rock strength, qu was estimated along with shaft side shear.
For side shear, FDOT’s design equation in limestone was used with split tension strength initially
estimated from qt/qu ratio established by field cores; subsequently, Johnston’s qu vs. qt/qu relationship
was used. The estimated unit skin friction in multiple rock formations and segments was compared by
instrumented segment during Osterberg, Statnamic, and top-down static load testing. The mean bias
(measured/predicted) unit side shear in limestone was 1.00, and the coefficient of variation, CV, was
0.07. The results suggest that estimating rock strength and shaft side shear using drilling parameters is
viable, especially considering the currently available rig monitoring equipment (Jean Lutz, Bauer, etc.).
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