The science of soil mechanics and foundation engineering is relatively new. It began in the 1930s with the work performed and published by Karl Terzaghi. Since that time, well-known civil engineers, such as Arthur Casagrande, Ralph Peck and many others, have made great strides in the field. Initially, the development and verification of theoretically derived foundation design models were based, at best, on scale-model tests performed in the laboratory. Because of scaling issues with laboratory foundation-model tests, and the recent implementation of reliability-based design (RBD) approaches, the need to conduct full-scale foundation load tests has become apparent over the past 20 to 30 years.
Since early in the 1980s, the Electric Power Research Institute (EPRI) has been very active in sponsoring research for the design of drilled-shaft and direct-embedded foundations for transmission line tubular-steel single-pole, steel lattice tower and tubular-steel H-frame structures. These efforts have included collecting, analyzing and summarizing the results of past full-scale foundation load tests performed by electric utilities throughout the United States and other interested parties, as well as the EPRI-sponsored full-scale foundation load tests.
Full-Scale Foundation Load Tests
Past full-scale foundation load test efforts have been concentrated on drilled-shaft, steel-grillage and direct-embedded pole-type foundations. The mode of applied foundation test loads has been associated with tubular-steel single-pole and steel lattice tower foundation loading conditions; that is, moment, horizontal shear and compression loads for single and direct-embedded tubular-steel poles, and horizontal shear and uplift or compression loads for steel lattice towers. The availability of this full-scale foundation load test database made the development of EPRI's foundation design programs — MFAD, HFAD, CUFAD and TFAD — possible. In addition, this database provided the information needed to calibrate the EPRI foundation design models against the results of full-scale foundation load tests and to develop the resistance factors needed to implement the RBD approach with confidence. The results of calibrating any foundation design model can be summarized in a graph that plots predicted nominal capacity versus test capacity for each full-scale foundation load test.
Future Full-Scale Foundation Load Tests
Over the past several years, new foundation types have been developed to support transmission line structures, namely micropile foundations and helical pile foundations.
As a part of the increased use of these two foundation types, many site-specific full-scale micropile and helical pile proof tests have been performed. Most of these tests were performed for uplift or compression loads on a single micropile or helical pile on a project-by-project basis. However, only a few full-scale load tests have been performed on micro and helical pile “groups.” In addition, micropile and helical pile foundations are currently being designed using an allowable stress design (ASD) approach. The design approach leads to significant variations in reliability because of the uncertainty in establishing realistic factors of safety.
Thus, a need exists to assemble currently available full-scale load test data for micropile and helical pile foundations. Additional full-scale tests should be conducted as needed. This database can then be used to calibrate all applicable design models that are currently being used in practice.
Models Require Validation
Because of the increase in the use of these two foundation types, many site-specific proof tests have been conducted to date. However, there has been no effort to assemble the available load test data into a coherent body of data or to conduct additional tests and use the database to establish resistance factors for use in a RBD approach.
Since most current methods for designing micropile and helical pile foundations have not been calibrated, current foundation designs are based on the ASD approach, which requires the designer to use judgment to establish a factor of safety.
As a result of using the ASD approach, current micropile and helical pile foundation designs are likely over-designed and do not have a uniform level of reliability. The solution to this situation is to assemble the currently available full-scale load test data for both the micropile and helical pile foundations, conduct additional full-scale load tests, as needed, and use the data to calibrate applicable foundation design methods against the results of the full-scale foundation load tests.
In this way, a resistance factor for each design approach can be determined. This effort will result in optimized foundation designs as well as a uniform level of reliability.
Anthony M. DiGioia Jr. (firstname.lastname@example.org) is president of DiGioia, Gray & Associates, LLC. He has been a principal investigator on EPRI foundation research projects and is a leader in the development and implementation of the relaibility-based design approach.