Osterberg Load Test on a Drilled Shaft in Gravels and Sands with Cobbles

An Osterberg cell (O-cell) load test was performed on a drilled shaft (typically referred to locally in Colorado as a caisson or drilled caisson) to evaluate the side and tip resistance, and validate the values used in design. The drilled shaft was constructed as part of the Colorado Department of Transportation (CDOT) project for the replacement of the Colorado State Highway 82 Bridge and adjacent pedestrian bridge located in Glenwood Springs, Colorado. Due to concerns of potential impact to nearby geothermal resources the shafts were not designed to penetrate into bedrock, and all of the resistance was developed in the overburden soils. The overburden soils consist primarily of gravels and sands with cobbles and boulders. Strain gauges attached to the rebar cage were used to monitor strain during the test and to evaluate load transfer. The results of the O-cell test are presented and compared to the values that were used in design. A back analysis was also performed to see what design parameter values produced the best fit when compared to the test results. The current AASHTO design methods agreed reasonably well with the O-cell results.