Three-dimensional finite element analysis of a complex excavation on the MIT campus

Abstract

Large excavation projects in urban areas are complex geotechnical problems when it is necessary not only to ensure the stability of the excavation and support system, but also to minimize impacts on adjacent buildings and structures. These problems require more comprehensive analyses that represent the four-dimensional (space-time) processes associated with performance of the excavation support system. The goal of this research is to evaluate the use of three-dimensional finite element analysis in Plaxis 3D 2011 to simulate ground deformations, pore pressures, and diaphragm wall deflections for the very well instrumented excavation for the basement of the Stata Center on the MIT campus. The model predictions are compared with the field measurements that include vertical inclinometers, settlement points, magnet extensometers, and piezometers. The Ray and Maria Stata Center building at MIT was designed with a basement for underground parking requiring a 42 ft deep excavation. The excavation was supported by a perimeter diaphragm wall that formed part of the permanent structure and extended 45 ft into a deep layer of underlying Boston Blue clay. The diaphragm wall was braced by a combination of prestressed tieback anchors, preloaded raker and corner bracing support elements. The control of ground movements was a critical aspect of the subsurface design due to the close proximity of the excavation to the historical MIT Alumni swimming pool building. This study has shown that the three-dimensional finite element analysis can be effectively used for such a complex excavation project and is capable to achieve reasonably consistent predictions of wall deflections, ground movements, and pore pressures for tieback, cornerbraced, and raker supported diaphragm walls despite of simplifications in the base case model. The simulation has also captured the three-dimensional effects causing the induced ground deformations to be smaller near the corner areas. Further numerical analyses are now needed to assess the importance of soil constitutive behavior on the observed field performance of the support system for the Stata Center basement.