Pressure Grouting Drilled Shaft Tips: Full-Scale Research Investigation for Silty and Shelly Sands

The tip (toe) capacity of drilled shafts in sands can often be many times greater than the side shear component; however, the tip capacity is often discounted from the total shaft capacity due to the relatively large displacements required to mobilize this end bearing component. Concerns of soil disturbance at the shaft tip (i.e. insitu stress relief) and cleanliness also discourage the use of end bearing as available capacity within any reasonable service load displacement criteria. Pressure grouting the shaft tip has been successfully employed throughout the world as a method of mitigating these conditions. However, there is an apparent lack of ration design procedures and construction guidelines for its use. In cooperation with the Florida Department of Transportation, the University of South Florida is researching the effects of post-grouting on shaft capacity. This paper describes two sites at which full scale shafts have been tested: (1) five shafts, including one control shaft, tipped in a shelly sand, and (2) three shafts, including one control shaft, tipped in a silty sand. The results of these tests show that significant tip capacity improvement can be realized through the use of pressure grouting drilled shaft tips.