The Loading Test on the Singe Pile with Pile Cap in Transparent Soil Model

Abstract Pile-cap-soil interaction is important to understand the load transfer mechanisms of pile foundations. However, studies on pile-cap-soil interactions are quite limited. This article reports a series of small-scale model tests in laboratory that was carried out on the single piles with and without pile caps. Transparent soils made of fused quartz and refractive index–matching fluids are employed to represent natural angular sand. Pile cap effects, diameter effects, and pile length effects are investigated. Elastic load limits are found to increase linearly with pile cap area, pile length, and pile diameter for the tested pile models. Soil deformation around the pile head is greatly influenced by the pile cap size, pile diameter, and pile length. Soil deformation at the pile head can be described by Terzaghi’s bearing capacity failure theory. The soil particles below the pile cap next to the pile body move downward, which might reduce the shaft friction or induce the negative shaft friction. All the loading tests show that the pile cap can increase the pile’s bearing capacity. For small-diameter model piles, soil deformation is centered below the pile base, no sliding surfaces are formed at the pile base when settlement S = 4 mm. For large-diameter piles, it is possible that a bearing capacity failure forms two sliding surfaces on the left and right of the pile base. This study indicates that the appropriate usage of the bearing resistance of soils below the pile cap can increase the bearing capacity of pile foundations. This study reveals the soil deformation below the pile cap, which benefits the utilization of the bearing resistance of pile caps. The soil deformation zone at the pile head and around the pile base is also investigated.