Date of Award
Open Access Thesis
Civil and Environmental Engineering
Sarah L. Gassman
Charles E. Pierce
As drilled shafts have become a more popular foundation type in the Charleston, South Carolina area, there has been an ongoing goal of optimizing drilled shaft design while maintaining the structural integrity of the foundation. In the Charleston area, the primary bearing stratum for deep foundations is the Cooper Marl, a calcareous Oligocene formation. Research performed from 2002 to 2004 on load test data from drilled shafts constructed in the Cooper Marl and soil properties from three test sites for the Cooper River Bridge explored the relationship between the measured skin resistance and geotechnical properties. In the 15 years since the load tests for the Cooper River Bridge were performed, additional load tests have been performed throughout the Charleston area. Evaluation of this load test data, the Cooper River Bridge load test data, and earlier load test data allows better understanding of drilled shaft skin resistance in the Cooper Marl as well as the ability to use in-situ geotechnical properties to better predict axial capacity when a load test is not performed. Drilled shafts founded in the Cooper Marl are designed primarily for using skin resistance and LRFD design methodologies and load factors.
Using data from 27 drilled shaft load tests at 15 test sites in the Cooper Marl, the relationships between load test measured unit skin resistance and undrained shear strength, overburden pressure, and SPT N-values were evaluated. The distribution of unit skin resistance with elevation was also studied across the Cooper Marl. To derive a design unit skin resistance for use when a load test is not feasible, a statistical method vi evaluating the 97.5% confidence interval and the historical load test method were used. Finally, an empirical method was used to verify the LFRD resistance factor currently required for design in the Cooper Marl.
Based on the performed analyses, there is not a correlation between unit skin resistance and SPT N-values. Across the Cooper Marl, the unit skin resistance distribution was found to be constant with depth up to -80 ft-MSL. When evaluating the relationship between undrained shear strength and unit skin resistance, the α-value was found to be 0.85, which is approximately 60% larger than the α values for clay presented in the literature. Based on the load test data, a design unit skin resistance of 3.2 ksf is supported using the historical load test method and a unit skin resistance of 2.88 ksf is supported using the 97.5% confidence interval method for typical sites. Additionally, the current resistance factor for LRFD design of 0.45 is data supported. Finally, although the Cooper Marl is treated as a homogeneous formation, there are known geologic discontinuities that should be accounted for during design.
Gieser, W. J.(2015). Drilled Shaft Skin Resistance Design in the Cooper Marl. (Master's thesis). Retrieved from https://scholarcommons.sc.edu/etd/3667