Lateral-Load Tests on Drilled Pier Foundations for Solar Plant Heliostats

The first commercial-sized solar generating station in the United States, the 10-MW Solar Pilot Plant near Daggett, Calif., required 1818 sun-tracking mirrors (heliostats). To ensure accurate focusing of the mirrors on the central receiving tower, stringent wind load rotation criteria were specified for the top of the heliostat foundation. Soils at the site are medium dense to very dense sands and gravelly sands. Because little data were available to predict lateral deflections of drilled piers under low-level cyclic loads, a full-scale load test program was undertaken to obtain the necessary soil-resistance pier-deflection parameters. Piers with a diameter of 0.91 m (3 ft) and length of 5.5 m (18 ft) were tested under cyclic loads of up to 23.0 kN (5.16 kips) applied at a height of 5.43 m (17.8 ft). Test results indicated lateral movements of less than 0.61 mm (0.024 in.) under maximum loading. A design based on site-specific soil-pier parameters resulted in about a 40% reduction in pier length and significant cost savings when compared to the original design based on existing methods.