Field axial cyclic loading tests of screw micropiles in cohesionless soil

Abstract

Screw micropile is a new type of steel pipe pile that consists of a smooth, threaded segment and a tapered segment. It is installed using the torque method. To examine the axial cyclic behavior of the piles, six one-way compressive and load-controlled axial cyclic tests were performed at a sandpit site. The axial cyclic load, with 15 cycles and an equivalent peak vertical acceleration ranging from 0.4 g to 0.6 g, was meant to replicate the vertical motion of an earthquake load. In-situ geotechnical investigation and lab testing were undertaken to determine the soil stratigraphy and properties. Three of the piles were instrumented with strain gauge to measure the unit shaft distribution. The pile-head cumulative displacement, stiffness and equivalent damping ratios were obtained. The effect of initial factor of safety on the cyclic behavior was examined. The re-distribution of unit shaft resistance of individual segments was obtained. Segment equivalent damping ratio and stiffness of the unit shaft resistance hysteresis were determined. The contribution of each pile segment to global pile behavior was delineated.