The Vertical Bearing Mechanism of Hybrid Bored Pre-stressed Concrete Cased Piles

Finite element analysis (FEA) models for sophisticated hybrid bored pre-stressed concrete cased piles, where the unique grouted interface between the pile and strata is highlighted by cohesive elements, are established in this paper and their applicability is demonstrated by comparing the in situ experiments and FEA simulations. The underlying bearing mechanism is investigated from the aspects of energy dissipation due to the damage of the grouted interface, the axial force along the pile length, and the deformation evolution in the surrounding strata. Under the failure criteria controlled by both the stiffness of the pile/soil system and pile head allowable settlement, the ultimate bearing capacity increases with the interfacial shear strength until reaching an asymptotic value. By energy analysis, it is demonstrated that the buffering effect of the strata to redistribute the stresses in the system is another key factor for the bearing capacity and failure mode of the pile foundations. The decrease in modulus attenuation (from approximately 90% to 50% in the studied case) leads to a larger asymptotic bearing capacity (from approximately 2.17 × 104 to 2.88 × 104 kN in the studied case, respectively) if the ductile failure mode of the pile foundations is guaranteed. However, the minimum interfacial strength needed for the presence of the preferred ductile failure mode is also increased (from 0.6 to 2.0 MPa in the studied case), which indicates more rigorous conditions for the grouting of pile/soil gap. As improved grouting quality and less modulus attenuation are commonly contradictory to each other from the aspects of construction practice, it is suggested that, if the grouting quality cannot be guaranteed, optimizations should be carefully carried out to achieve the presence of the preferred ductile failure mode of the pile foundations.