A field study on the behavior of a foundation underpinned by micropiles

Abstract

Micropiles have been used for many applications, including underpinning existing foundations to support structures subjected to additional loads, such as stories added to buildings. A field study was conducted to simulate a footing initially constructed on a natural soil to support a structure and later subjected to additional loads. This study investigated load transfer mechanisms during the connection of micropiles to the concrete plate under the initial load and during the additional loading. Earth pressure cells and rebar strain gauges were installed underneath the loading plate and in the micropiles, respectively, to monitor variation of stresses during the loading and connection processes. The behavior of micropiles in the underpinned foundation under the additional loading was analyzed and compared with that in the single-micropile loading tests presented in a companion paper. Theoretical solutions were adopted for examining the results obtained from this field study, including the overall stiffness of the underpinned foundation, the tip resistance, and the ratio of the load carried by the plate (raft). Guidelines are proposed for designing underpinned foundations using micropiles subjected to additional loads.Key words: underpinning, micropile, load transfer, skin friction, tip resistance, piled raft.