Bored Piles in Rock: Recent Design Lessons Learned

Over the last two decades, widespread implementation of bi-directional axial load testing has resulted in a wealth of data from full-scale load tests of rock-socketed bored piles. Accordingly, valuable insights are now available to help advance the state of practice with respect to both design and construction, making it practical to achieve satisfactory performance economically.  Demonstrated behavior indicates when good design, construction, and quality assurance/quality control (QA/QC) procedures are followed, the design of bored piles in rock is controlled by structural considerations, even in rock that has traditionally been considered to have very poor quality. Specifically, this paper will describe in detail the following lessons learned:  1) Rock socket side resistance and base resistance mobilize immediately and at compatible displacements.  2) What has traditionally been considered poor or very poor bedrock is often capable of providing axial resistance in excess of the structural concrete, and the concrete strength typically controls the axial design rather than geotechnical considerations.  3)  Construction controls the performance of bored piles and as an extension, design and construction of bored piles are inseparably linked. This relatively new body of knowledge, when appropriately applied, has allowed the optimization of bored pile design and construction, reducing required rock socket sizes. Such optimizations are beneficial not only for project cost and schedule, but also reduce the carbon footprint associated with the construction of bored pile foundations in rock.