Hybrid bored prestressed concrete cased piles: Equipment and construction procedures

To widen the applications of large-diameter prestressed high-strength concrete (PHC) piles to stiff strata, this paper introduces a novel hybrid piling technique, including the equipment and construction procedures. The spiral drilling operation provides space for simultaneously lowering the piles, and the detected real-time parameters characterizing the construction status can be also automatically transferred to additional exciting force or constraints acting on the piles by a sophisticated feedback loop. The application of the proposed technique is verified with two large-diameter test piles installed into stiff weathered granite. It is experimentally demonstrated that, taking advantage of the unique grouted gap between the piles and the bored hole, the pile-strata adhesive strength and therefore the shaft resistance of the pile are significantly enhanced. Comparison of the tested piles shows that the trade-off relation between the grout quality and the modulus reduction of the strata plays an important role in controlling the bearing performance of the piles because the shaft resistance does not always increase with the modulus of the surrounding strata if the strength of the grouted interface is relatively small. The findings can provide fundamental guidance for further optimization of the proposed technique from the aspects of both design and construction.