Probabilistic Subsurface Characterization for Transmission Line Foundation Design-Paper

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The paper presents a risk-based approach to geotechnical investigation and foundation design for a 191-mile transmission line project in Iowa, involving approximately 1,066 large drilled-shaft foundations.<br /><br />Because only a limited number of structure sites could be investigated, the team combined existing borings and cone penetration tests with supplemental field testing, geological desktop studies, surface observations, and nearby water-well records. Direct subsurface testing covered about 15% of structure sites. The geological information helped classify soils and bedrock by depositional setting and select representative locations for additional testing.<br /><br />Design parameters were developed by grouping similar subsurface materials and statistically analyzing field and laboratory data. The team used lower-bound values at a 5% lower exclusion limit to account for variability, while applying engineering judgment where data were sparse or results conflicted with geological understanding. These parameters informed idealized soil profiles and the design of laterally loaded reinforced-concrete shafts.<br /><br />During construction, inspectors logged excavations and compared actual conditions with design assumptions. Foundations were reassessed when weaker-than-assumed soils, unexpected bedrock elevations, or other significant differences were found. Only 22 foundations—about 2%—required modification, compared with an anticipated modification rate of roughly 5%.<br /><br />The authors conclude that probabilistic characterization, supported by geological reconnaissance and construction-stage verification, can provide reliable designs despite limited sampling. The approach also produced significant savings compared with conventional methods: the proposed standard investigation was estimated to cost 45% more than the geologic and geotechnical program used. Design and analysis added costs, but foundation modifications remained limited. Similar reliability-based approaches may also reduce foundation costs by avoiding overly conservative assumptions.

Keywords

geotechnical investigation

foundation design

transmission line

risk-based approach

drilled-shaft foundations

Iowa

subsurface testing

geological reconnaissance

probabilistic characterization

foundation modifications