Mechanisms of soil plug formation of open-ended jacked pipe pile in clay

The mechanisms of soil plug formation during the jacking of an open-ended pipe pile into clay is investigated by using Coupled Eulerian-Lagrangian (CEL) method. According to the evolution of soil velocity field, the formation of soil plug is divided into: the upwelling stage, the partially plugged stage and the fully plugged stage. A concave arch is formed below the pile tip when the stress bulb expands from the pile wall to the pile axis. The initial soil plug and the fully soil plug are formed when the vertical stress increment of the soil below the pile tip reaches 6 and 9-10 times undrained shear strength (S-u), respectively. The results show that soil plug is easier to be formed in the pile with smaller diameter. The pipe pile shows a fully plugged mode in the stiff soil overlaying soft soil, while pipe pile shows a partially plugged mode in the soft soil overlaying stiff soil. The soil plug is hard to be formed in inhomogeneous clay due to the higher soil strength of deeper soil. An effective method is proposed to estimate the formation of soil plug with the equation for vertical stress increment of the soil below the pile tip.