Micropiles are being utilized to both enhance the foundation ultimate capacity as well as reduce foundation deflection. Load-displacement relationships may be used in the design of micropiles that conform to deflection and load-capacity criteria. Consequently, we develop a mathematical model for micropile load-displacement behavior. The model explicitly considers soil-micropile interaction. Motivated by field measurements, we assume the micropile to be partially bonded, consisting of a top debond zone that, typically, extends up to the casing depths and a bottom bond zone. Thus, the micropile is assumed to transmit its load to surrounding soil through the micropile-soil interface in the bond zone only. Furthermore, to ensure model simplicity and accessibility to designers, we assume the micropile-soil interface to be elastic-perfectly plastic. Closed form expressions of micropile deformation as a function of applied load are derived. The derived expressions are used to study the effect of model parameters on micropile yield behavior. Scaling factors are identified that are found to significantly influence the micropile load-displacement behavior. Micropile strain distribution and the consequent load transfer behavior predicted by the model are discussed. The model predictions are compared with the field measured load-displacement curves.
