Group behaviour of hollow-bar micropiles in cohesive soils

The use of hollow-core bars in micropiles has greatly increased over the past 10 years. In this study, the group behaviour of hollow-bar micropiles is investigated. The research methodology encompasses a full-scale field study on single hollow-bar micropiles and pairs of micropiles installed in cohesive soils and numerical investigations using three-dimensional (3D) finite element analysis. Four hollow-bar micropiles were installed in stiff silty clay deposit using large drilling carbide bits. Four axial monotonic tests on pairs of hollow-bar micropiles were conducted as part of the field study. The field test results were utilized to calibrate-verify a 3D finite element model (FEM). A parametric study was performed using the verified FEM to characterize capacity and performance of hollow-bar micropile groups. Group efficiency factors and interaction factors were established to evaluate the capacity and response of hollow-bar micropile groups under vertical loading. The results from the field tests and numerical investigations revealed that for hollow-bar micropiles, group efficiency factor, G(E), can be taken equal to 1. A method for calculating the group settlement utilizing the interaction factors approach is proposed and a family of interaction factor diagrams is established to evaluate the group settlement.