Geotechnical capacity of hollow-bar micropiles in cohesive soils

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.