The Behavior of a Deep Retained Excavation in Soft San Francisco Bay Mud

This paper presents portions of a five-part study done to further investigate the behavior of a deep braced excavation and improve upon existing techniques for estimating the magnitude of the transitory forces in such structures. This year-long project, constructed in San Francisco Bay Mud, was completely instrumented and monitored throughout the construction process. This initial part of the research provided the measured geotechnical and structural data with which to compute and compare load predictions made at various stages of construction. The second part of the study consisted of the development of a three-dimensional finite element structural model to allow for the application and study of various earth pressure theories proposed in the literature as well as a new earth pressure hypothesis developed and presented for this project. Comparisons were calculated and tabulated in the third and fourth parts of the study which included the evaluation of the relative performance of each model in predicting structural loads. Conclusions and recommendations were presented within the fifth part. With respect to the five earth pressure models selected for comparison, those with triangular shapes performed slightly better in the prediction of structural loads than did those with trapezoidal configurations. For the new model proposed and developed, the use of the Tributary Area Method along with earth pressures from K_o effective stress plus best-estimate pore pressures without reshaping of the newly suggested earth pressure diagram is recommended. Bending stresses within specific strut members were found to be surprisingly high with measured values averaging approximately 85% of P/A stresses. Augmentation of the predicted compressive stresses by 150% is recommended until further studies are completed. The finite-element structural model developed was a valuable research tool but incredibly labor intensive. Continued use of the Tributary Area Method is recommended for routine design of braced excavations.