Ultimate Capacity Comparison of Test Results Upper portion (above O-cell): skin friction only Lower portion (below O-cell): skin friction + end bearing Predicted (kips) [MN] Measured (kips) [MN] Displacement at Failure Predicted (in) [mm] Measured (in) [mm] 9,300 [41.4] 9,371 [41.7] 0.40 [10] 0.31 [7.9] 9,300 [41.4] 9,402 [41.8] (no failure observed) 0.40 [10] 0.10 [2.6] (no failure observed) to sustain additional load. After a review of the test results and confirmation of design values, construction of the production shafts commenced. Up-close photo of the O-cell O-cell Test Results — The bi-directional load test was performed in general accordance with the standard Quick Load Test Method for Individual Pi les (ASTM D1143) using 15 equal loading increments of about 620 kips (2,758 kN) based on the rated capacity of the O-cell (9,300 kips or 41.4 MN). Each load increment was maintained for approx- imately 8 minutes, and a creep test was conducted for 30 minutes at the maximum load of 9,300 kips. It was intended that if failure of the side friction or end bearing was not observed, loading would continue above the rated capacity until reaching the O-cell limit (typically between 1.5 and 2 times the rated capacity of the device). Failure did occur, however, along the length of the shaft above the O-cell, where the geotechnical resistance along this length was controlled by side friction. For side friction along the upper portion, the predicted and measured values for load and deformation at failure were very similar. However, no failure in end bearing was observed. The measured values shown in the table correspond to the resistance and displacement along the lower portion at the instant of failure along the upper portion. Once the upper portion of the shaft failed, the lower portion was not able Conclusions The bi-directional load test results for the CSVT project illustrate how realistic design values are now compared to actual test values of ultimate side friction resistance and deformation in competent rock. Although design manuals and codes are allowing a significant contribution of the resistance from end bearing, it is very likely that the ultimate end bearing resistance of a drilled shaft in competent rock is controlled by the structural capacity of the shaft and not the geotech- nical capacity. Using bi-directional load test results, such as those described in this article, results from other static and dynamic load tests, and more than 1,600 load tests in the new version of the FHWA Deep Foundation Load Test Database (DFLTD v.2.), we are getting closer to predicting the true ultimate geotechnical capacity for drilled shafts. Acknowledgments We would like to express our appreciation to the Pennsylvania Department of Transportation, District 3-0, for taking a proactive approach and including the requirement for this load test program in the contract documents for the referenced project. The load test results validated the d e s i g n a n d p r o v i d e d a b e t t e r understanding of load-deformation behavior, which can be used for future projects in the area. We would also like to thank STV (project design prime firm), HNTB Corp. (bridge designer), Trumbull Corp. (general contractor) and Moretrench Cons t ruct ion Co. (dr i l led shaf t s subcontractor). A special thanks to Rochelle Dale, P.E., Yojiro Yoshida, P.E., and Melissa Lieberman from A.G.E.S. for their valuable contributions during the geotechnical design of the shafts and the associated testing. Vishal B. Patel, M.S.C.E., P.E., is a geotechnical project engineer at American Geotechnical and Environmental Services (A.G.E.S.) in Pittsburgh, Pa. He has more than 7 years of geotechnical experience in the design of bridge and building foundations, earth retaining structures, and soil/rock slide stabilization. Sebastian Lobo-Guerrero, Ph.D., P.E., is a geotechnical project engineer/AAP laboratory manager at A.G.E.S. in the Pittsburgh, Pa. headquarters. He has more than 16 years of experience in geotechnical engineering, specializing in the design of deep/shallow foundations, earth retaining structures, and landslide stabilization. He is a member of the DFI Tiebacks and Soil Nailing Technical Committee. James G. Ulinski, P.E., is a project manager at A.G.E.S. in Pittsburgh, Pa. He has more than 40 years of experience in all aspects of geotechnical engineering. DEEP FOUNDATIONS • SEPT/OCT 2017 • 85
