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

Abstract

Underpinning in an existing foundation with piles is commonly used to improve their load-bearing capacity to allow vertical extension works. This study evaluated the effect of the piles’ stiffness on the load-sharing behavior of existing and underpinning piles during the vertical extension of existing buildings. A series of 1 g model tests, all with a scale factor of 1/20, was used to compare the underpinning performance of micropiles of varying stiffness. The performance was evaluated in terms of total settlement reduction and load sharing capacity. The results showed that the underpinning performance improved with the increase of underpinning pile stiffness. The ratio of carried loads of the underpinning pile to the existing pile increased linearly with its increasing stiffness (as expressed as a ratio relative to the stiffness of the existing pile).

Seismic Behavior of Micropiles and Micropiled Structures Used for Increasing Resilience: A Literature Review

“…Inertial interaction [40] Influence of MPs stiffness on underpinning applications Scaled experimental method (model test)…”

“…Influence on the inertial interaction by affecting the stiffness and on the kinematic interaction by modifying the soil MP shape was mainly studied in terms of the influence on the inertial interaction by increasing the MP stiffness or modifying the inclination angle, especially for type A, B, and WF MPs-this was studied both experimentally and numerically [6,7,11,[13][14][15][16]37,40]; The potential influence on the kinematic interaction, however, especially with high pressure grouting MPs, is not yet sufficiently studied; Additionally, the confinement effect due to MP inclination and installation method needs to be further investigated MP-to-structure connection…”

“…Not only the ability of the numerical model to accurately simulate the failure mechanism is interesting, but also the ability to model the confinement effect, which has a large influence on the seismic performance of MPs and the superstructure, since the size, depth, and location of soil which is contributing to the failure mechanism can affect the soil stiffness, which in turn can affect the seismic performance [38]. More examples of experimental methods can be found in the works of Elaziz et al [39], Wang et al [40], and Abbas et al [41]. Elaziz et al [39] performed field tests on full scale MPs of type E (self-drilling); the aim of the study was to evaluate the bond strength of grouting/ground; the outcome of the study showed that the value usually taken from FHWA [1] is less than the actual value of the bond strength that can be developed.…”

“…Wang et al [40] studied the load sharing between the existing piles and newly installed MPs through a model test; two types of MPs were studied, a waveform MP (WF), which is a new type of MP, and a conventional MP (type A). The outcome showed that the waveform MPs are always contributing more to the load sharing as a percentage vs. the conventional MPs used in the experiment; according to the authors this is due to the fact that the waveform MPs have a higher stiffness than the conventional MPs used.…”

“…As stated previously, MC is not the only CM used for MP or MPed structure analysis, but it is the one most used with sufficient calibration and validation. Other examples of More examples of experimental methods can be found in the works of Elaziz et al [39], Wang et al [40], and Abbas et al [41]. Elaziz et al [39] performed field tests on full scale MPs of type E (self-drilling); the aim of the study was to evaluate the bond strength of grouting/ground; the outcome of the study showed that the value usually taken from FHWA [1] is less than the actual value of the bond strength that can be developed.…”