Shaking table test study on dynamic behavior of micropiles in loose sand

Micropiles are small-diameter piles installed in soil. Due to wide application of micropiles in seismically active areas, investigation of their behavior during earthquake is of great significance. In this study, shaking table tests were conducted on a small-scale physical model of micropiles consisting 16 vertical micropiles embedded in loose sand fill and subjected to harmonic sine waves. Influence of amplitude of input excitations, presence of a single degree of freedom superstructure and variation of concentrated mass of this model superstructure on the dynamic response of micropiles system were examined. Response of physical model to the base excitation was investigated in terms of the horizontal acceleration at different locations of physical model and also bending moment generated along two instrumented micropiles during the experiment. Increase in the amplitude of input wave and cyclic nature of shaking resulted in densification of underlying soil and modification of soil-micropile stiffness, which accordingly reduced dynamic amplification and shifted fundamental frequency of vibration of micropile cap. However, presence of superstructure and increase in its concentrated mass resulted in a shift of fundamental frequency the other way. There was a maximum bending moment induced by vibrations in the mid-length of model micropiles which increased with augmentation of amplitude of input wave. Inertial effect of superstructure resulted in significant elevation of bending moment in the vicinity of micropile head, and also caused bending moment in the corner micropile to exceed bending moment in center micropile.