Abstract:
In dense urban environments, projects often maximize development opportunities by planning deep excavations up to the property limits. While this practice optimizes the financial benefits of expensive downtown real estate, it also requires engineers to contend with the interaction of the planned project with surrounding environment – which often includes nearby utilities, buildings, or infrastructure. As such, excavation-induced movements are critical topic from early planning stages and subsequently through the duration of construction activities. Earlier stages of project planning require estimations of excavation-induced movements, while later stages focus on monitoring of adjacencies to ensure that excavation-induced movements stay within accepted criteria.
What adjacencies should be monitored? What structures may be deemed beyond the “ zone of influence?” The answers to these questions define the scope of analysis and instrumentation plans, thereby carrying significant cost implications. While engineers have typically assumed generic zones of influence such as 1 horizontal to 1 vertical (1H:1V) or 2 horizontal to 1 vertical (2H:1V), the zone of influence for large, deep excavations in softer soils can exceed three to four times the excavation depth due to associated global movements (basal heave).
This presentation will discuss the mechanisms which inform zones of influence for excavations and will provide recommendations for estimating the extents of these zones depending on site and soil conditions. These recommendations will be demonstrated using instrumentation data from several excavations in the Greater Boston area, which includes geologic conditions with shallow bedrock (subjecting smaller zones to excavation-induced movement) to deep deposits of Boston Blue Clay (which can affect a much larger zone).