Graduation Year


Document Type




Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department


Major Professor

Gray Mullins, Ph.D.

Committee Member

Manjriker Gunaratne, Ph.D.

Committee Member

Kingsley Reeves, Ph.D.

Committee Member

Michael Stokes, Ph.D.

Committee Member

Danny Winters, Ph.D.


Bias, Postgrouting, side shear, Tip Capacity, Tri-axis


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 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 parameter 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 post grouted end bearing resistance of drilled shafts.

Today, FDOT restricts the use of shaft end bearing in sands and as such no resistance factors are provided in FDOT design manuals. However, end bearing is permitted if postgrouting is employed and an adaptation of 2006 design method is provided. Even then, when post grouting is used there is no resistance factor for design computations and a load test is usually required from which the load test specific resistance factor is used. The objective of this study was to establish LRFD resistance factors for post grouted end bearing scenarios. To this end, a database of 36 test shafts was established into which the shaft diameter, length, boring logs, grouting logs and load test reports were compiled.

As with all resistance factor calibration/determination studies, the measured load test response was compared to the predicted capacity. The predicted capacity methods were restricted to the 2006 design method on which the FDOT method is founded. This design method is dependent on two factors: (1) the amount of end bearing displacement and (2) the grout pressure imparted to the end bearing strata. Where no grout pressure is applied, the end bearing responds as a convention ungrouted shaft. So, the pressure applied at the time of grouting was scrutined for all 36 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, 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 ratio) 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. The 2006 design method resulted in a resistance factor of (0.60) 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. A postgrouting resistance factor of 0.6 was adapted and referenced on page 173 of the 2022 Soils and Foundations handbook