An Optimized Approach For Managing Overhead Transmission Line Foundation Design and Construction Risk-PPT Presentation

PPT Presentation

Pdf Summary

The presentation describes an optimized approach to managing geotechnical risk for a 20-mile, double-circuit 230/69 kV transmission line near Phoenix. The project required 161 reinforced-concrete drilled-shaft foundations, generally supporting laterally loaded steel monopoles, and had an expedited 23-week construction schedule.<br /><br />The approach combined reliability-based design, targeted subsurface exploration, and enhanced construction inspection. Before drilling, the team reviewed existing investigations and geologic maps, conducted field reconnaissance, assessed geologic and other hazards, and mapped four geologic zones. Borehole locations were selected to represent changing soil conditions, with priority given to dead-end and heavy-angle structures, geologic transitions, and at least one exploration per mile. Existing nearby data were also incorporated. The resulting program used one boring per approximately 5.4 structures and represented the anticipated deposits across the alignment.<br /><br />Soil data were evaluated statistically and adjusted using laboratory results. Lower-bound characteristic blow counts were established for similar soils, producing nine geotechnical layers and 22 idealized soil profiles for design.<br /><br />A detailed inspection program tracked excavation, tooling, reinforcement and anchor bolts, concrete placement, and final foundation geometry. Qualified inspectors documented work with standardized forms and photographs, providing rapid feedback. If field conditions differed from design assumptions, profiles could be updated and foundation embedment reanalyzed.<br /><br />The optimized approach was compared with two alternatives: boring at every structure, and using predetermined test sites without detailed reconnaissance. Testing every site improved profile accuracy and reduced installed concrete volumes by about 2.5%, but required 131 additional borings. The optimized program had comparable cost and completed exploration more quickly, while providing stronger quality control and adaptability during construction. The predetermined-site alternative provided less reliable subsurface information and was estimated to increase concrete volumes by 12%, resulting in higher overall cost.

Keywords

geotechnical risk

transmission line

drilled-shaft foundations

reliability-based design

subsurface exploration

soil profiling

construction inspection

laboratory testing

foundation geometry

Phoenix project