In 2012, the Novartis Institute for Biomedical Research embarked on a $600 million expansion of its Cambridge, Massachusetts Campus with the addition of a 550,000 square foot complex which includes multiple levels of underground space.
To excavate to the maximum depth in excess of 45 feet, the Contractor faced several logistical challenges presented by the congested, urban location. These challenges included the sheer size of the excavation, which occupied an entire city block, the presence of very soft clay deposits, the close proximity to adjacent buildings and varying structural systems and slab elevations.
A Support of Excavation (SOE) system was designed which consists of a combination of deep soil mix wall and steel sheet piling to support a total excavation area of about 3.2 acres (138,500 sq ft). Since the use of tiebacks was limited due to the site location, the SOE was mostly supported by internal bracing; with single strut lengths reaching 240 feet.
Finite element models were developed to account for soil non-linearity, SOE stiffness, and varying depths of excavation. With strut loads up to 1,200 kips, an instrumentation system provided real time data acquisition of the loads in the bracing and deflections of the SOE. The monitoring data were comparable to the stresses and deformations predicted by finite element model.