Micropiles are used in various applications, including low-capacity micropile networks, underpinning, and seismic retrofitting of existing foundations and high-capacity foundations for new structures. Hollow-bar micropiles have an added advantage, as they provide fast installation with a high degree of ground improvement. The current Federal Highway Administration (FHWA) design guidelines designate hollow-bar micropiles as type B, even though the FHWA construction technique is different than the technique used for typical type B, which results in an overly conservative design. In addition, the current practice for construction of hollow-bar micropiles is limited to a drilling bit / hollow-bar diameter ratio of 2.5 or less. In this paper, full-scale load tests were conducted to evaluate the suitability of FHWA design guidelines to hollow-bar micropiles installed in cohesive soil and to evaluate the performance of hollow-bar micropiles constructed with a drilling bit / hollow-bar diameter ratio of 3. Eight micropiles were constructed using 76 mm (3 in.) hollow bars (76 mm outside diameter and 48 mm inside diameter) with the air–water flushing technique and advanced to a depth of 5.75 m: six micropiles were installed using a 228 mm (9 in.) drill bit and two micropiles were installed using a 178 mm (7 in.) drill bit. All micropiles were instrumented with vibrating wire strain gauges to measure the axial strain at three stations along the micropile shaft. The load tests included four axial monotonic and four cyclic axial loading tests. The results are presented and discussed in terms of load–displacement curves and load transfer mechanism. The load test results showed that the grout–ground bond strength values proposed by the FHWA (in 2005) for type B micropiles grossly underestimate the bond strength for calculating the ultimate capacity. In addition, the toe resistance can be significant for micropiles resting on sand due to the increased toe diameter. No tangent stiffness degradation was observed in the micropile capacity after applying 15 load cycles.
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.
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