Experimental Study of Vertical and Lateral Load-Bearing Characteristics of Long Piles Anchored in Rocky Soil at Deeply Backfilled Sites

The foundation of nuclear power plants is special as large-scale earth filling is often required. The properties of the backfill soil differ significantly from naturally deposited soils with regard to deformation and bearing capacity. For pile foundations, a thick backfill layer near the top may change the bearing mode around the pile. In this paper, six cast-in-place rock-socketed piles were tested, with three vertical loading tests and three horizontal loading tests. The lengths of four piles are 35–40 m, while the other two piles reach 55 m. The results show that shorter piles with more parts in the backfill layer can endure a hoop-tightening effect that caused by dilatancy at the upper part of the pile, resulting in very little frictional resistance being provided by the lower soil and smaller vertical displacement of the whole pile. The typical mechanism of transition from static to dynamic friction between soil and piles that leads to shaft resistance is more apparent for longer piles, but inhomogeneous soil like the backfill layer will make the transition complex. When subjected to lateral loading, piles with better integrity show more pronounced elastic features, smaller maximum horizontal displacement, and less residual horizontal displacement. The selection of the proportional coefficient for determining piles’ horizontal bearing capacity should correspond to the specific load and displacement in backfill soil. The results and in-depth analysis of the piles’ bearing capacity in backfill soil will provide intuitive experience for the analysis of pile foundations, thus offering references for the design and construction in similar engineering.