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To continue the pursuit of providing the most economical solutions for their customers, American Electric Power (AEP) sought to improve their current latticed steel tower (LST) grillage foundation design methodology. To accomplish this, AEP executed a detailed review of their current grillage foundation design methodologies and supplemented this work with a full-scale uplift load test program consisting of 18 independent load tests. The load test program was performed to verify existing geotechnical design equations and provide a basis for updating AEP’s design approach for grillage foundations. Pyramidal steel grillage foundations with two different base dimensions were tested. Installation of each test grillage used construction methods that attempted to replicate those utilized by AEP’s foundation contractors. The load tests were conducted at two different locations on an existing AEP substation site located in Galax, Virginia. The locations were selected to provide testing in an “all-soil” profile and a “partial soil/rock” profile within the planned excavation depth of the grillages used in testing. Subsurface exploration was performed at each test location to facilitate the development of geotechnical material design properties for use in the grillage uplift capacity analysis. This exploration included geotechnical borings, standard penetration tests (SPT), laboratory testing, K0 stepped blade tests (SBT), and pre-bored pressuremeter tests (PMT). Flat-plate dilatometer testing was performed at select locations within the test areas to provide information on the in situ horizonal earth pressure after foundation installation (pre-load testing) and following completion of load testing. This paper presents an overview of uplift geotechnical design methodologies for pyramid grillage foundations, a summary of the load test procedures and interpreted results, design philosophy improvements, and overall conclusions from the test program.