Field study on axial behavior of instrumented post-grouted steel pipe micropiles in tropical lateritic soil

This paper presents a field study on the axial behavior of new configuration of steel pipe micropiles in tropical lateritic soil. The micropiles were installed following a two-step grouting process with high injection pressures at a new experimental site located at the University of Campinas (Unicamp). The main innovation of the micropiles relies on the use of high-strength steel pipes as a drilling tool, casing protection, structural component and injection device. Full-scale pile loading tests were performed under compression and tension loads to investigate the axial behavior of the micropiles. The micropiles were instrumented along the shaft with electrical extensometers, and they were monitored during the progress of the loading tests. The load–movement responses, elastic moduli, axial load transfer mechanism, and unit bond stress relating to the micropiles and their interactions with the tropical soil were measured and evaluated. The bearing capacity of the post-grouted micropiles was considerably improved by the proposed post-grouting technique. The ultimate load capacity of the grouted steel pipe micropile under a compression load is approximately the same under a tension load. The average ultimate bond strength was 99 kPa and 100 kPa for the micropile under compression and tension, respectively. The distribution of unit bond stress along the shaft of the micropile under compressive and tensile load presented a post-peak softening behavior. The average ultimate grout-to-ground strength at the soil–micropile interface was 2.4 and 1.7 times greater than other nongrouted and non-injected piles installed in the same soil conditions at Unicamp.