Pre-Drilling for Deep Foundations: Estimating, Modeling and Obtaining Skin Friction for Tension Piles

Due to the ever-increasing size of commercial vessels, waterfront structures at major seaports are being designed and upgraded to withstand considerable mooring and berthing loads that often control design for lateral loads. A prime example of this case is the expansions of Berth 20 and Berth 22 at Blount Island Marine Terminal (BIMT), located in Jacksonville, Florida and owned by Jacksonville Port Authority (JAXPORT). To keep pace with the increase in both vessel size and vessel traffic, JAXPORT expanded the mooring and berthing capability of Berth 22 by adding two dolphins and subsequently expanded the existing pile supported wharf at Berth 20 by approximately 1000 linear feet (LF).

The expansion of both structures required new precast-prestressed concrete piles as deep foundations to provide adequate bearing and tension capacity. Due to the presence of a thick layer of weathered limestone, bearing capacity is easily achieved. However, dense limestone corresponds with high blow counts and the potential for refusal during conventional pile installation. Although refusal may indicate high bearing, the pile must be deep enough to achieve the sufficient skin friction to resist high tension demands. In these instances, pre-forming or pre-drilling offers a solution to ensure piles reach minimum tip elevations required to obtain both skin friction and lateral stability. Pre-drilling precluded the need for H-pile stingers, which are also common practice in the region to improve drivability and tension capacity but add significant time and cost to the production pile fabrication.

However, the designer must apply appropriate assumptions when modeling pile axial capacity to account for pre-drilled or pre-formed holes. Because of limited industry standard guidelines on how to account for pre-drilling, the engineer must apply a mix of fundamental principles, experience, and judgement to adjust the effective horizontal stress along the shaft of the displacement pile. Engineers applied two different methods for design of Berth 22 and Berth 20 respectively: 1) adjust geotechnical parameters conservatively to produce lower values of predicted skin friction and 2) reduce the lateral earth pressure coefficient (Kh) by the ratio of the area of pile vs. the area of the pre-drilled hole to determine a more accurate reduction in skin friction. Both methods assume that the pre-drill diameter will be less than the nominal size of the pile, and capacity for each will be verified by dynamic load testing via Pile Driving Analyzer (PDA) during the test pile program, prior to ordering production piles. Further, the projects introduced noteworthy practical construction considerations such as equipment availability, added installation costs, pile batter angles and configurations, test pile programs, and potential construction interferences with port operations, all of which greatly affect the pre-drilling procedures and the ultimately resulting skin friction values.