The Ocean has been proposed as a potential resource for increasing the energy supply from renewable sources. Depending on how the energy is generated, several technologies have been proposed that tap into ocean energy. Examples of these technologies include Ocean Thermal Energy Conversion (OTEC) systems, wave, tidal, and current or Marine HydroKinetic (MHK) energy system. The deployment of ocean-based renewable energy generation devices, including those utilizing wind, waves, and tidal and ocean currents, requires the installation of cost-effective anchoring foundations. Micropiles provide a viable option for cost-effective subsea anchoring foundations. However, due to the lack of an acceptable approach for the subsea formation of micropiles in sandy soils, no cases that we are aware of have been presented in the literature.
The research work reported in this paper seeks to investigate the influences of construction parameters on the overall axial capacity of hollow-bar micropiles in sandy soils were quantified based on results from full-scale field testing. Eight micropiles were constructed at a site in the Outer banks of North Carolina in two phases. The field testing program included installation of micropiles to a depth of 25 ft. (7.62 m) while varying the drilling or drill-bit insertion rate (IR) and the grout flow rate (QR). In addition, the installation methods included micropiles that were continuously drilled and grouted with neat grout water-cement ratio (w/c) of 0.4, and others that were first drilled and grouted continuously with thinner grout (w/c - 0.7) and then flushed from bottom to top with thicker grout (w/c-0.4). Eight micropiles installation approaches were designated as Fast/Fast/ (w/c-0.4), Fast/Slow (w/c-0.4), Slow/Fast (w/c-0.4), Slow/Slow (w/c-0.4), Fast/Fast (w/c-0.7/0.4), Fast/Slow (w/c-0.7/0.4), Slow/Fast (w/c-0.7/0.4) and Slow/Slow (w/c-0.7/0.4). Four miocropiles were instrumented with temperature compensated vibrating wire strain gauges to measure the axial strain at three stations along the micropile shaft. The results of pullout field load testing are presented in terms of load-displacement curves and load transfer mechanism. The load-testing results showed an appreciable high ultimate pullout capacity by using Slow drilling and Slow grout pumping rate (w/c-0.4) as compared to the other three approaches. The result showed that an additional 50% pullout capacity could be achieved by using Slow/Slow construction approach with w/c-0.4 grout, as compared to commonly practiced (Fast/Fast) installation approach.
