The tensile forces affecting pile foundations are usually transferred to the bedrock by rock anchors. If drilled piling pipe could transfer some of these tension forces, foundations could be lighter, execution works easier and the whole system could be more cost-efficient.
Several recent studies have examined the tensile resistance of piles, revealing three main factors affecting bond strength: the relation between the diameter of the drill hole and that of the pile, the roughness of the steel surface and the quality of the grouting. Furthermore, the stresses are not distributed uniformly along the length of the piling pipe, but are highest on the top of the pile and lowest at the tip of the pile. Hence, the bond strength cannot be increased by increasing the bond length. The results of the pull-out test proved that the bond strength of the pile is significantly increased by the grooved surface. However, over half of the steel piles could not be pulled out of the bedrock as loading had to be discontinued at the yield capacity of the steel piles for safety reasons. Thus, the actual tensile capacity of the piling pipe remained undetermined. Anyway the grooved pipe piles proved to have a great potential to be used as tensile force transferring structures.
