Axial monotonic and cyclic compression behaviour of hollow-bar micropiles

Abstract

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.