When Novartis Institute for Biomedical Research decided to build a research campus on a 4-acre parcel of land, located across the Massachusetts Avenue from its present building, they retained GZA to assist with the construction that required an excavation up to 50-feet deep in soft soils in close proximity to existing buildings and roads.
To excavate to the required maximum depth of 45+ feet below street grade, the Contractor faced several 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 soft clay deposits which presented lateral deformation and basal heave concerns, the close proximity to adjacent buildings and city streets; and the varying structural systems and slab elevations. The GZA design team provided the Contractor with innovative and cost-effective design solutions for the support of excavation (SOE) system, dewatering, monitoring, and support of the micro tunneling operation during construction.
A deep soil mix wall was used for the support at the deep portion of excavation, and steel sheet piling was used in the shallow part. The total area of excavation was 3.2 acres, or 138,500 square feet. Two to four levels of internal lateral bracing were used for lateral support. Due to the site location, the use of tiebacks was limited; and internal bracing system of steel pipe braces and wales was used to support most of the excavation with single brace lengths reaching an unprecedented 240 feet. Total length of bracing members reached 1.6 miles.
Finite element modelling was developed and employed to analyze the SOE performance during staged excavation. Unique finite element models were developed for each design case, to account for soil non-linearity, SOE stiffness, and varying depth of excavation. With lateral bracing design loads up to 1,200 kips, GZA designed and installed a comprehensive instrumentation monitoring system to provide the real time data of the loads in the lateral bracing and deflections of the excavation support system throughout excavation and subgrade construction. The fully automated monitoring system operated “24/7” from August 2012 until May 2013 and the data collected was displayed on a dedicated, customized and secure project specific website where all project participants (Owner, CM, GC, Engineers) could access the readings “on-demand” using any wireless device. The monitoring data demonstrated that the actual measured stresses and deformations were comparable to the stresses and deformations, predicted by FEM.
Due to the constraints presented by the proposed building structural system, the bracing could not be installed at the same elevation for the entire excavation. Continuous load paths have to be provided between wales and braces, requiring creative solutions for the connections.
In order to install a utility tunnel, under Massachusetts Avenue, connecting the existing Novartis Headquarters Building with the new construction a 76-inch diameter tunnel was constructed by micro tunneling at a 7-degree upward incline from within excavation. This unique installation method required an inventive design for the launch pad, including design of an internally reinforced concrete reaction beam inside the basement mat and additional tieback system to resist 780-kip jacking force.
Additional services were provided by GZA, including design of the dewatering system, existing utilities check for crane loading; new slab check for loads during brace removal; and additional SOE analysis for loads after partial backfill.
GZA’s understanding of the local area, combined with our experience working on projects in close proximity to other buildings and our state-of-the-art techniques of finite element modeling, instrumentation monitoring system and microtunneling enabled Novartis to complete the new construction in a timely and safe manner.