Experimental Study on Vertical and Lateral Bearing Characteristics of Rock-Socketed Long Piles at Deeply Backfilled Sites in Nuclear Power Engineering

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, parallel multi-pile tests were conducted to thoroughly investigate the vertical and horizontal bearing characteristics of long rock-socketed piles at backfill nuclear site. The results show that longer piles tend to have higher vertical bearing capacity, however, they do not necessarily exhibit smaller displacements when subjected to vertical load. When the applied vertical load increases, the effect of end resistance of piles becomes more pronounced, meanwhile, deeper soil layers exert a more significant impact on the axial forces within the pile body. Short piles with more part in backfill layer may endure a hoop tightening effect at the upper part of the pile, resulting in very little frictional resistance being provided by the lower soil. At the vertical load level preceding failure, the distribution of axial force and shaft resistance along the pile length will change. The typical mechanism of transition from static to dynamic friction between soil and piles that lead to shaft resistance is more apparent for longer piles but inhomogeneous soil like 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. The results and in-depth analysis of the piles’ bearing capacity will provide intuitive experience for the analysis of pile foundations, thus offer valuable references for the design and construction in similar engineering.