A field study on the load sharing behavior of a micropiled raft underpinned by a waveform micropile

Chengcan Wang,Jin-Tae Han,Seokjung Kim

doi:10.1139/cgj-2020-0547

Abstract

A waveform micropile (WMP) uses the jet-grouting method to generate shear keys along the pile shaft for improving shaft resistance and cost efficiency. In this study, field loading tests were performed to characterize the load sharing behavior upon inclusion of a WMP in a group of four micropiles. First, single-pile compressive loading tests were conducted on three WMPs and five type A micropiles (MP). Subsequently, a group-pile loading test was performed on a piled raft comprising 2 × 2 MPs and a central WMP. The load–settlements, axial stiffnesses, and load transfer mechanisms of individual MPs were analyzed during the tests, including the short- and long-term effects of the axial stiffnesses of the MPs on the load sharing ratio of the micropiled raft. The single-pile loading test results revealed that the shear keys along the WMPs caused its bearing capacities and axial stiffnesses to be 1.5 times and 2–5 times higher than those of MPs, respectively. In the micropiled-raft loading test, the load sharing ratios of the MPs increased with their axial stiffnesses, and the highest load sharing capacity was exhibited by the WMP, which constituted 30% of the total load and 2–3 times that of MPs. Moreover, the influence of raft on the load-sharing capacity should be considered as well.

Highlights

Owing to the rapid population growth and limited land in urban cities, the vertical extension of existing buildings is one of the possible alternatives to improve and increase the utilization of such buildings; the Government of South Korea has published guidelines stating that old existing apartment buildings taller than 14 and 15 floors can be extended vertically by adding and 3 floors, respectively (MOLIT 2013)
The load–settlement curve for waveform micropile (WMP) exhibit a plunging failure at 1670 kN corresponding to the settlement of 23 mm, which was less than 10% of the WMP diameter (300 mm), thereby indicating that most of the applied load was transferred through the WMP shaft
A field study was performed to evaluate the effect of the WMP stiffness on the underpinning performance in terms of the load sharing capacity

Summary

Introduction

Owing to the rapid population growth and limited land in urban cities, the vertical extension of existing buildings is one of the possible alternatives to improve and increase the utilization of such buildings; the Government of South Korea has published guidelines stating that old existing apartment buildings taller than 14 and 15 floors can be extended vertically by adding and 3 floors, respectively (MOLIT 2013). Kamash and Han (2016) conducted a parametric study to examine the micropile–soil–plate interaction and the load transfer mechanisms of soils and micropiles based on several factors They reported that the initial pressure ratio for underpinning and the micropile length pose a more significant impact on the load sharing of micropile than its elastic modulus. For achieving the first objective, three WMPs and five Type A MPs were subjected to compressive load for evaluating the axial performances in terms of bearing capacity, load transfer behavior, and axial stiffness. A loading test was performed on the micropiled-raft to evaluate the influences of the axial stiffnesses of the MPs on the short- and long-term load sharing characteristics of the.

Experimental setup

Findings

Discussion

Conclusions