Side Resistance of Drilled Shaft Socketed into Wissahickon Mica Schist

Drilled shafts socketed into bedrock are a common foundation type for support of large and/or important structures in downtown Philadelphia. The bedrock beneath the city is the Wissahickon Formation, a mica schist bedrock encountered with various degrees of weathering. The interface between soil and rock is known to be transitional, varying significantly with depth, and is usually difficult to identify. Based on an extensive geotechnical investigation program, the properties of mica schist rock in the city of Philadelphia area are summarized. The side resistance values in the rock socket measured from O-cell test results for a recent project in the city of Philadelphia are presented. The side resistance values are then compared with measured rock properties, such as, the unconfined compressive strength, RQD, and elastic modulus.