Optimal Subsurface Investigation Approach for Overhead Transmission Structures-Paper

Paper

Pdf Summary

The study compares three subsurface investigation methods—Standard Penetration Testing (SPT), Cone Penetration Testing (CPT), and Flat Dilatometer Testing (DMT)—for overhead transmission structure foundations, which must resist substantial lateral loads. All three methods were used at three sites in Maryland, with soundings extending at least 50 feet.<br /><br />Overall, the methods produced broadly similar soil profiles and elastic-modulus trends, but differed in detail. SPT provides physical samples for laboratory testing and is relatively simple, but its widely spaced measurements can miss thin soil layers, and samples may be disturbed. CPT records soil response continuously, making it the most sensitive method for detecting changing strata and the strongest option for locating groundwater. However, CPT-derived SPT-equivalent resistance values do not always match measured SPT results. DMT measures soil response to lateral membrane expansion and is therefore particularly useful for assessing lateral strength; it also provides overconsolidation information relevant to settlement. Its soil classifications sometimes differed from those of SPT and CPT, and it is less effective for identifying groundwater.<br /><br />CPT and DMT generally require smaller crews and automated data processing, while SPT remains useful in dense sand or bouldery conditions where probe-based methods may have limited penetration. CPT and DMT can also be used in marine settings where SPT is unsuitable.<br /><br />The authors recommend selecting methods according to project conditions rather than relying on one test universally. DMT is preferred for dead-end poles exposed to high lateral loads and where long-term settlement matters. CPT is recommended for lighter-angle poles with moderate lateral loads and for soft granular soils with shallow groundwater. SPT remains valuable for sample recovery, but should not be used alone; combining SPT with CPT is recommended for more reliable interpretation of soil properties and foundation design.

Keywords

Standard Penetration Testing

Cone Penetration Testing

Flat Dilatometer Testing

subsurface investigation

transmission structure foundations

lateral loads

soil profiling

groundwater detection

soil classification

foundation design