“Pilot Study to Explore Similitude between Small and Large Diameter Drilled Shaft load Tests”

Project Description

Load tests such as standard top-down or bi-directional (i.e. O-cell) tests can help designers understand load-settlement behavior and estimate nominal resistance of drilled shafts. Implementation of these types of tests are usually justified for projects in which the substantial cost of the foundation can be offset by performing such load tests. Offset in cost is usually accounted for by a reduction in shaft length or diameter, leading to reduced costs in materials and construction.

Although load testing provides a clear advantage towards design and construction of drilled shafts, there are limitations in its current practice. Performing load tests on large-diameter drilled shafts with high capacities can present challenges when considering the loads required to fully mobilize the capacity of such foundations. Feasibility of these tests, at times, can be impractical because the associated initial cost is not guaranteed to outweigh the final savings. Finally, it is also common practice to specifically select load magnitudes that do not approach failure thus allowing for use of the test shaft into production. This practice oftentimes restricts our knowledge of the true capacity of the drilled shaft at failure.

This project has a specific focus on developing a cost-effective and innovative solution for performing load tests on smaller diameter drilled shafts and extrapolating the results to gain knowledge on the response of large diameter shafts. Our research will explore an innovative interpretation method through developing similitude relationships between small and large diameter drilled shaft load tests that allows tests to be performed on drilled shafts with reduced diameter (test shafts) and reasonably predict the capacity of the large-diameter shafts (prototype shafts).

Research will focus on three main tasks in developing similitude properties between small and large diameter drilled shaft load tests. The first task will be to collect all pertinent and available data of load tests performed in similar subsurface soil/rock conditions for drilled shafts that are comparable in length and differ in diameter. The second task will focus on interpretation of the collected load test data, establishing unit side resistance for different soil layers. These interpretations will be used to explore similitude relationships between load tests on test shafts and prototype shafts through an empirical relationship between side resistance and diameter ratio.

Project Team:
Ramin Motamed (PI), Joseph Toth (Graduate Student)

Project Advisory Panel:
Raymond Fassett (Condon-Johnson & Associates), Peggy Hagerty Duffy (Hagerty Engineering, ADSC), Kirk Ellison (Arup), Reda Mikhail (Golder Associates)