Analysis of the shape effect on the axial performance of a waveform micropile by centrifuge model tests

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.

Experimental Investigation of Micropile Stiffness Affecting the Underpinning of an Existing Foundation

“…This finding indicates that shear keys along the pile increased its shaft resistance. This effect was more significant in the soil region with a higher confining pressure, which is in good agreement with the results of Jang and Han [36,37]. Interestingly, WMP2 provided a similar shaft resistance to CMP1 despite being only about 80% of its length.…”

“…The shaft resistance of WMP1 increased sharply by increasing the applied load and at the final loading level, the mobilized shaft resistance was about 2.5 times that of CMP1 and WMP2. The shaft resistance of WMP2 and CMP1 increased slowly with the increasing load, and both showed almost identical behavior, indicating that the shear keys along the pile's shaft significantly increased the shaft resistance, as explained by Jang and Han [37]. The shaft resistance of CMP2 also increased slowly with the applied load and the increase is the smallest measured here for these micropiles.…”

“…The piles' lengths, L, are related by the scale factor N. The effects of the piles' form and length, and consequently their axial stiffness, were evaluated by comparing two types of conventional micropile (CMP) and two types of waveform micropile (WMP) in foundation underpinning tests. The WMP (Figure 2a), with shear keys along the pile shaft, demonstrated a higher stiffness and load bearing capacity than the CMP [35][36][37]. Although the limitation of the soil stress effect in small-scale tests is difficult to overcome, to establish a reliable relation between the small-scale model and the prototype model requires adopting a suitable scale and satisfying the scaling law [38,39].…”

Inclusión de micropilotes como elementos recuperadores de carga en cimentaciones: revisión del estado del conocimiento

“…En Corea del Sur [39] se efectuaron pruebas de centrífuga en seis modelos a escala: con micropilotes convencionales, de inyección y cuatro especiales conocidos como de forma de onda, tres de ellos contaban con una llave de cortante que es un elemento estructural del mismo material del micropilote en forma de prisma hexagonal, construido en diferentes intersecciones con la finalidad de aumentar la capacidad de carga, tal y como se ve en la Figura 4. Fuente: elaboración propia a partir de [39].…”

“…Las cargas axiales de los micropilotes convencionales pueden mostrar mejoría dependiendo de la metodología de construcción utilizada con el uso de micropilotes en forma de onda [39], [46], los cuales han demostrado óptimos resultados en capacidad de carga; lo que comprueba la transferencia de carga compartida completa al fuste y a la punta, demostrando que las cargas compartidas se comportan de manera diferente dependiendo de la ubicación de las llaves. Cuando estas llaves están construidas en la parte superior o central la resistencia del eje aumenta alrededor de la llave, debido a que la presión de confinamiento es relativamente baja.…”