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

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The paper presents an optimized approach to designing and constructing 161 drilled-shaft foundations for a 20-mile, 230/69 kV transmission line in central Arizona. To meet an expedited construction schedule while managing subsurface uncertainty, the project team combined targeted investigation, statistical design, and enhanced field inspection.<br /><br />A geological desktop study and site reconnaissance divided the route into four geologic zones and identified hazards and likely soil transitions. The team used 30 new and existing exploration sites, selected to represent these conditions and prioritize critical structures, rather than drilling at every pole. Statistical analysis of corrected SPT data and laboratory results established characteristic soil properties and 22 idealized profiles for foundation design.<br /><br />During construction, inspectors logged soil conditions and foundation geometry, checked materials and installation, and shared findings with the design team. This enabled real-time reassessment and adjustment when actual conditions differed from assumptions. Detailed spreadsheet-based records also supported quality control and verification of as-built capacity.<br /><br />The predicted soil profiles matched field conditions at 84.5% of locations. Eleven foundations were analyzed further because of potential capacity concerns; four initially appeared inadequate at design dimensions, but as-built geometry provided sufficient performance. Total concrete use was 5.8% above neat-line volume, within the 10% budget allowance, and construction finished within the required 23 weeks.<br /><br />Cost comparisons showed the optimized approach was comparable to investigating every foundation location (Alternative A), while avoiding an estimated two-month design delay. The conventional limited-investigation approach (Alternative B) provided less information and was estimated to cost about $507,000 more overall. The authors conclude that strategically located borings, statistical characterization, and enhanced inspection can balance cost, schedule, and reliability. They recommend more frequent sampling in geologically variable areas and investigation of anomalies identified during reconnaissance.

Keywords

drilled-shaft foundations

transmission line

geotechnical investigation

statistical design

subsurface uncertainty

SPT data

foundation inspection

geologic zones

Arizona

quality control