Location of Thesis Examination

Room 3102 Spencer Engineering Building


Doctor of Philosophy


Civil and Environmental Engineering


M. Hesham El Naggar

Delay of Publication



The use of hollow core bars in micropiles has greatly increased over the past ten years. Hollow core construction, also known as “self drilled”, is becoming a popular option because it allows a faster installation processes and ground improvement at the same time. Despite the growing demand for hollow bar micropiles, little work has been devoted to evaluating the nominal bond strength between the micropile grout and the surrounding soil, especially in clayey soils. Moreover, the performance of such micropiles under different kinds of loading is still largely unknown and needs to be investigated.

In this study, a research methodology encompassing two primary elements is adopted. The first element is a series of full scale field studies on hollow bar micropiles installed in cohesive soils, while the second is numerical investigations on hollow bar micropiles. To accomplish the study, four hollow core micropiles were installed using an air flushing technique employing large drilling carbide bits. Twenty-two load tests were conducted on the four hollow bar micropiles. The hollow bar micropiles were loaded in four consecutive phases, which included; five axial monotonic, five axial cyclic load tests on single micropiles, four axial monotonic tests on pairs of hollow bar micropiles, two monotonic and six cyclic lateral tests on single micropiles. The results of each set of tests were utilized to validate a numerical model. Parametric studies were conducted on the calibrated model to provide design guide lines for hollow bar micropiles under different loads.

An equation is proposed to estimate the axial capacity of hollow bar micropiles in cohesive soils depending on the installation method adopted. In addition, an equation for the stiffness degradation under axial cyclic loading is proposed. It reveals that the group efficiency factor for hollow bar micropiles should be taken equal to 1, despite the spacing to diameter ratio employed. Moreover, a family of interaction factor diagrams is established to estimate the settlement of hollow bar micropiles group. Finally, the study demonstrated that hollow bar micropiles can carry moderate lateral loads with proper reinforcement configurations and pile head fixity condition.