Prediction and Measurement of Unit Side Resistance in Franciscan Complex Bedrock - The Longest Drilled Shafts in San Francisco

摘要

The evaluation of the bedrock present in San Francisco, known as the Franciscan Complex, can be an integral part of the design of the drilled shafts because the rock socket accounts for a significant amount of the load capacity. Franciscan Complex comprises a mix, or "melange", of predominantly sedimentary sandstone and shale that has undergone extensive faulting and shearing related directly to plate tectonics. This faulting and shearing action results in a chaotic mixture of materials, many of which have been broken down and weakened by shearing. The Franciscan Complex is especially unique in its irregularity across San Francisco, even within the footprint of a single building site (such as the case with the Oceanwide Center project). This paper will present the challenges in predicting unit side resistance in the Franciscan Complex using: 1) conventional equations like those presented in AASHTO (2008) and FHWA (2010) based on RQD; 2) using alternative equations incorporating GSI instead of RQD; and 3) using the Hoek-Brown strength criterion in finite-element modeling. All of these methods will then be compared to the results of bi-directional load test data collected at the project site with a discussion about the best methods to estimate drilled shaft capacity in Franciscan Complex melange. In addition, a brief discussion of base resistance developed in the shafts is included herein.