۱- Introduction

۲- Centrifuge test

۳- Test results and discussion

۴- Summary and conclusions

References

Abstract

A new type of micropile, the waveform micropile, has been developed to provide improved load-bearing capacity compared with that of a conventional micropile. The waveform micropile has a wave-shaped grout with a partially enlarged shear key formed by the jet grouting method on the cylindrical shaft of the micropile. Previous research has determined that the waveform micropile can be installed faster than the conventional micropile and that the bearing capacity increases as the wave-shaped grout provides additional shaft resistance between the ground and the grout. In this study, a series of centrifuge model tests were conducted on the waveform micropile model with various wave-shaped grouts to analyze the relationship between the arrangement of the shear key and the load-bearing mechanism of the waveform micropile. The load–settlement relationship and the load-transfer mechanism were analyzed based on the test results of six test micropiles, including three waveform micropiles with a single shear key at various depths, one waveform micropile with a multiple shear key along the pile depth, and two micropiles with only a cylindrical shape. The test results showed that the ultimate bearing capacity of the waveform micropile was over two times greater than that of the conventional micropile. The rate of increase in the bearing capacities of each waveform micropile differed with the shape of the shear key. Furthermore, the characteristics of the load-sharing ratio due to the shaft resistance and end bearing varied depending on the shape of the waveform micropiles.

Introduction

A micropile is a drilled and grouted foundation system that provides relatively high bearing capacity with a small diameter ranging from 100 to 300 mm. The micropile can be installed using fewer pieces of construction equipment than other existing foundation types. For this reason, since its introduction in Italy in the 1950s, the micropile has been recognized as an effective foundation solution for areas of limited access and low headroom. The construction process of the micropile involves drilling the pile shaft to the required depth, installing a temporary casing, and then placing the steel rebar and grout. The high-strength steel rebar transfers the load to the surrounding ground through the grout body to develop frictional resistance. Therefore, the micropile supports the load via the shaft resistance created between the pile shaft and the ground while the load carrying capacity resulting from tip resistance is generally neglected due to small diameter of the micropile [15]. Federal Highway Administration (FHWA) micropile design guidelines specify the bond strength (abond) considering the frictional resistance between the grout and the ground for micropile design [10]. The abond ranges are classified into four types (A–D) according to the ground conditions and grouting types.