Hollow-bar micropile construction, also known as self-drilled, is becoming a popular option because it allows faster installation processes and ground improvement at the same time. This paper presents a field study and numerical investigation on the behaviour of single hollow-bar micropiles embedded in a stiff silty clay deposit. Four hollow-bar micropiles were installed using an air-flushing technique employing large drilling carbide bits. Five axial tests were conducted on the four micropiles, comprising three compression and two tension monotonic axial tests. The results of the field tests are presented and analyzed in terms of load–displacement curves. A two-dimensional axisymmetric finite element model (FEM) was created and calibrated using the field test results. The calibrated FEM was utilized to select an appropriate failure criterion for hollow-bar micropiles depending on the load-transfer mechanism of the micropiles. In addition, the model was employed to carry out a parametric study to investigate the effect of the installation methodology, hollow-bar micropile geometry, and shear strength of the surrounding soils on the micropile capacity. Based on the outcomes of the parametric study, an equation is proposed to estimate the axial capacity 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, GE, 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.