Numerical Modelling of Axial Behaviour of Single and Grouped Hollow Bar Micropiles in Cohesionless Soils

Hollow bar micropiles are increasingly used to support a variety of civil structures due to their several practical advantages such as fast installation, small installation equipment requirements and efficient load transfer mechanisms. The main objective of this paper is to investigate the performance of hollow bar micropiles utilizing nonlinear three-dimensional finite element analysis. The cavity expansion theory was used to simulate the installation effect and radial stress changes due to the micropiles installation. Displacement- controlled cavity expansion analysis was conducted and a surface prescribed displacement was applied to a cylindrical cavity to replicate the increase in radial stresses during the micropile installation. The results obtained from the cavity expansion analysis were compared with those obtained from the more common method of simulating pile installations by increasing the lateral earth pressure coefficient (K₀) values to those back calculated from pile load tests. Finite element models were verified by simulating the field loading experiments of full scale single and grouped micropiles. The verified finite element models were then employed to investigate the effect of micropile installation in cohesionless soils and the group effect. The numerical model results confirmed that the micropile group efficiency is equal to unity for micropiles in a 2 × 2 and 3 × 3 arrangement.