Case Study: The Use of Post Tensioned Piles for Wind Turbine Foundations-Paper

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The case study examines a post-tensioned pile-cap (PTPC) foundation supporting 3.15–3.4 MW wind turbines at a 63-turbine Texas wind farm. Each foundation used a 33-ft-diameter concrete cap supported by 24 tension piles. Four distinct soil profiles required tailored pile designs, with lengths generally ranging from 45 to 57 ft.<br /><br />A pre-production testing program evaluated Auger-Pressure-Grouted-Displacement (APGD) and Continuous Flight Auger (CFA) piles, with and without post-grouting. In the first phase, ungrouted piles often failed creep or load requirements, or showed excessive movement. Post-grouting was therefore introduced to improve shaft resistance and uplift capacity. In the second phase, most post-grouted piles met the specified creep and apparent-free-length criteria and sustained high test loads. APGD piles showed slightly greater tension capacity and somewhat smaller movements than CFA piles.<br /><br />Despite this performance advantage, APGD installation caused grout to migrate between nearby piles, creating uncertainty and slowing construction. The team selected post-grouted CFA piles for production because they reduced grout communication and provided more consistent, efficient installation. The change cut installation time for a 24-pile foundation from about three days to one day.<br /><br />The study demonstrates that post-grouting can make pile foundations suitable for the high uplift demands of large wind turbines, while emphasizing that construction reliability and productivity may outweigh modest differences in pile capacity.

Keywords

post-tensioned pile-cap

wind turbine foundation

Texas wind farm

post-grouted piles

APGD piles

CFA piles

pile testing

uplift capacity

shaft resistance

construction productivity