Numerical Analysis of Drilled Shaft O-Cell Testing in Mica Schist

Osterberg-cell or O-cell tests are an effective method for load testing drilled shaft foundations, and can identify the relative contributions of side resistance and end bearing. A series of O-cell tests was conducted for several production shafts socketed into mica schist in downtown Philadelphia. The bearing material is the Wissahickon Formation, which is a mica schist bedrock typically encountered with various degrees of weathering. This paper describes a finite element model developed to simulate the drilled shaft rock socket subject to axial loadings. The material parameters used in the analyses were calibrated based on the O-cell test data during the "bottom up" loading and supported by results from shaft instrumentation. The calibrated model is used to investigate the load distribution in the weathered rock socket and the contribution of end bearing, both when loaded from the bottom during the load test and under the service loading from the top.