DFI Journal - The Journal of the Deep Foundations Institute
Volume 13, Issue 2, November 2019
DOI: 10.37308/DFIJnl.20190613.205
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Behaviour of Hollow Bar Micropiles Under Uplift and Lateral Loading
Article Type: Research Paper
El Hadi Drbe, O. F. & El Naggar, M. H.
Abstract
In many applications, micropiles are subjected to uplift and lateral loads due to wind and or earthquakes. Hollow bar micropiles (HBMP) have become popular because of their fast installation and small equipment required for their construction, which allow installation in difficult access and remote areas. As part of HBMP installation, grout is applied under pressure, which results in improving the soil in their vicinity. Most of HBMP applications are limited to drilling bit/hollow bar diameter (dbit/dbar) ratio of 2.5 or less. This study evaluates the uplift and lateral performances of HBMP constructed with two different bit/hollow bar diameter ratios. Load tests were conducted on full-scale instrumented HBMPs constructed with dbit/dbar = 3, and dbit/dbar = 2.35. Four HMPs were subject to uplift loading and eight were tested under lateral loads. In addition, a laboratory test program was conducted to evaluate the mechanical properties of grout reinforced using micro steel fibers for application in HBMP construction. The effect of reinforced grout on HBMP lateral performance was then evaluated numerically. The results showed that the performance of HBMP is sensitive to the construction technique and to the drill bit specifications. The grout/ground strength values proposed by FHWA (2005) for type B micropiles were found to underestimate the ultimate uplift capacity for hollow bar micropiles. As expected, the larger drilling bit resulted in enhanced lateral performance and increased the capacity due to the larger diameter. In addition, using fiber-reinforced grout can increase the micropile lateral capacity and enhance its ductility. Finally, fixing the micropile head into the pile cap can increase its lateral capacity significantly.
Keywords:
hollow bar micropiles, uplift capacity, skin friction, lateral capacity, cohesive soil