Field axial loading tests of screw micropiles in sand

A screw micropile consists of a smooth shaft at the top, threaded shaft at the middle, and tapered shaft at the bottom. Research is required to investigate the axial behavior and capacities of this pile type in sand. A field load test program was performed on six types of full-scale screw micropiles installed at a sandpit site using torque. Selected piles were instrumented with axial strain gauges. A geotechnical investigation, including cone penetration tests (CPTs) and standard penetration tests, was undertaken. In total, 41 piles, including eight instrumented piles, were tested. The ultimate capacities and the unit shaft resistance distributions were determined. The shaft resistance was then compared with the tip resistance of CPT. The coefficient of lateral earth pressure and combined shaft resistance factor was determined for each pile segment, and then an effective stress method based on the combined shaft resistance factor was used to estimate the capacity of test piles. A theoretical torque model was developed using CPT sleeve friction; the model was verified by comparing the estimated torque to the measured torque of test piles. In the end, empirical torque factors were developed.