Construction Method Effects on Axial Drilled Shaft Performance: Fid-Bau Portal

Construction Method Effects on Axial Drilled Shaft Performance

Camp, W. M. / Brown, Dan A. / Mayne, Paul W.

A large load test program was performed for the US Highway 17 Replacement Bridge over the Cooper River in Charleston, South Carolina, USA. A total of 12 drilled shafts (1.8-m and 2.4-m in diameter, 30-m or 46-m long) were constructed at three different test sites along the 11+ km (7+ mile) long alignment. The bearing stratum for all shafts was the Cooper Marl, a stiff calcareous marine clay. The load test program was structured to help answer many design-related questions, one of which is the consequences of using different drilled shaft construction methods. Shafts were constructed dry, with bentonite slurry, with polymer slurry or with plain freshwater. To evaluate bottom conditions prior to concrete placement, all shafts were inspected with a Mini Shaft Inspection Device (Mini-SID). Steel casing was driven through loose sands and/or soft clays into the Cooper Marl and was pulled after concrete placement on some shafts and left in place on others. All shafts were instrumented with vibrating wire and resistance type strain gages and axial testing consisted of single-level and double-level Osterberg Cell load testing and Statnamic load testing. For all shaft construction methods, average back-calculated unit side shear values within the bearing stratum were within 10% of each other and there were no discernible construction-method-related trends in the data. However, there was a marked reduction in unit side shear values between uncased portions of the shafts and the cased portions of the shafts. This was true even when the casing was pulled. Within the range of Mini-SID results that were observed on the project (10mm<40mm), there was no apparent correlation between unit end bearing resistance and bottom cleanliness.