LRFD Resistance Factors for Base-Grouted Drilled Shafts in Cohesionless Soil

Abstract

Base grouting below a drilled shaft tipped in sand has been shown in many cases to dramatically improve both the initial stiffness and ultimate end-bearing resistance. After promising results from base-grouting case studies in the early 1960s, numerous grouting methods were developed, but end-bearing prediction methods remained largely proprietary for over 40 years, none of which were provided via design codes or specifications. No safety factors or resistance factors for any design method had been codified prior to this study. Instead, load test verifications routinely were performed, and the safety or resistance factor associated with the test method was applied. This paper assigned resistance factors for base-grouted end bearing from the results of 35 base-grouted shafts that were load-tested throughout the United States. Like other geotechnical capacity prediction methods (e.g., driven pile capacity), a higher resistance factor is established when increased field inspection is implemented, specifically when grout pressure verification demonstrates that the end-bearing soils have been effectively grouted to the specified design pressure. Safety factors are a necessary parameter in the design of any structural element. Currently most codes either mandate load and resistance factor design (LRFD) or provide the option to use LRFD, which separates load and resistance factors to better isolate uncertainties and to achieve an acceptable level of safety. To date, no resistance factors have been incorporated into standard specifications for base-grouted drilled shafts. Hence, a load test is required to provide a suitable degree of certainty. This paper provides resistance factors for end-bearing resistance for base-grouted shafts and recommends a minimum level of field inspection to ensure that the grouting process is performed effectively.