Case history of a cost effective design approach for laterally loaded drilled shafts [for overhead power line supports]

A net savings of approximately one million dollars was obtained in the design and construction of 109 drilled shafts for phases I and II of the Pinnacle Peak-Papago Buttes 230 kV transmission line for the Salt River Project, Phoenix, Arizona. The savings were made possible by applying a rational design approach in conjunction with the Electric Power Research Institute´s computer program MFAD (Moment Foundation Analysis and Design). The so-called four-spring subgrade modulus model in the MFAD program computes the shaft overturning moment capacity considering all acting forces. The model considers both the stiffness of the different soil layers within the depth of the shaft and the stiffness of the shaft for response computations. A comparison between designs based on various methods indicates that MFAD can save up to 10 ft of foundation depth