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Load and Resistance Factor Design (LRFD) Resistance Factors for Tip Grouted Drilled Shafts
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Load and Resistance Factor Design (LRFD) Resistance Factors for Tip Grouted Drilled Shafts
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    Final Report, 01/17-08/19
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  • Abstract:
    Pressure grouting beneath the tip of drilled shafts, also known as postgrouting, has been used for more than fifty years throughout the world and has been shown to be an effective means to enhance both the usable and ultimate end bearing resistance. In short, postgrouting is a form of compaction grouting beneath the shaft tip (performed after concrete has cured) that can improve the soil strength and increase the axial shaft stiffness. Until 2006, there was no published design methodology, and hence, the anticipated performance was speculated to be a function of injected grout volume, shaft uplift, and/or the achieved grout pressure. Research leading up to a 2006 design method, funded by the Florida Department of Transportation (FDOT), found the grout pressure applied to the soils beneath the shaft tip to be the key factor most closely linked to the resulting end bearing. However, the research did not recommend safety factors or LRFD resistance factors for use in design. In fact, 13 years after the new design method and after hundreds of projects employing its use domestically and worldwide, there were no published resistance factors for postgrouted end bearing resistance of drilled shafts. The objective of this study was to establish LRFD resistance factors for postgrouted end bearing scenarios. As with all resistance factor calibration studies, the measured load test response was compared to the predicted capacity. The predicted capacity method was restricted to the 2006 design method and the FDOT method, adapted from the 2006 method. The pressure applied at the time of grouting was scrutinized for all 31 shafts and three values of grout pressure were identified: the highest field recorded pressure, which could have been the by-product of a blocked grout line; the office-calculated design pressure based on boring log information; and the truly applied pressure (termed effective pressure), which was verified by reviewing the simultaneous performance/trends of increasing grout volume, pressure, and shaft uplift. Bias values (the measured to predicted capacity ratios) were determined for each pressure level of each shaft and at all displacements under which the shaft was load tested. Resistance factors were found to be higher for effective pressure bias values and lowest for office-calculated design pressure. Further, both the 2006 and FDOT design methods resulted in the same resistance factor (0.65) for toe displacements up to 1% of the shaft diameter, D. The findings further recommend adoption of strict field quality control measures to support the use of the computed resistance factor.
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