Field Axial Testing of Screw Micropiles in Sand and SPT-based Estimation Method

ABSTRACT: A screw micropile consists of a smooth shaft segment, one or two threaded shaft segments and one or two threaded-tapered shaft segments. Field test research is required to investigate the axial capacity and shaft resistance distribution of the pile in sand. Axial compression and tension tests were performed on screw micropiles installed at a sand site near Edmonton, Alberta. The diameters of the test piles were 76 mm, 89 mm and 114 mm and the length was 3 m. Selected piles were instrumented with strain gauges to measure load distributions along the shaft. Three axial compression load tests and three axial tension load tests were conducted for each pile type. From the load-displacement curves, the ultimate capacities of the test screw micropiles were estimated at an axial load corresponding to an axial displacement equal to 10% of shaft diameter. From strain gauge readings, axial load and shaft resistance distributions were determined. A comprehensive geotechnical site investigation was carried out to develop the soil profile and the soil parameters. Statistical relationships were developed between shaft resistance distributions and standard penetration test N value distribution using linear regression analysis. Regression coefficients were obtained for smooth, threaded, and threaded-tapered segment separately. The coefficients were used to estimate the ultimate capacities of all other screw micropiles.