Boston Central Artery Tunnel Jacking (1991–2001)

Introduction

The Central Artery Tunnels (CA/T) in Boston, Massachusetts, constructed between 1991 and 2007, is a landmark project in the history of urban infrastructure redevelopment. It has rejuvenated the city which had become increasingly choked with traffic, establishing an enhanced environment and sustainable future for its citizens. A vital need was to minimise the disruption during construction - which, for this project, has been likened to prolonged urban open-heart surgery (Wheeler, 1997; Soudain, 1999). Known locally as the ‘Big Dig’, it was hailed as the single largest civil engineering project in American history (The Economist, 2003).

One of the largest sections, Contract 9A, required constructing three interstate highway tunnels beneath an operating railway (Figures 7.1,

7.2 and 7.13). The tunnel jacking, which provided the key solution to solving this huge construction challenge, was by far the largest, most complex application of its kind in the world. It needed a quantum leap in scale, with each jacked tunnel being well over ten times the size of any attempted in the US before and involved a wide range of innovation (Powderham, 2004, 2009). The construction delivered a low maintenance, robust product while bringing important environmental benefits and contributed to over US$300 million in construction savings (Angelo, 1996). From initial concept to construction in 2001, it took over ten years of sustained effort and teamwork. Contract 9A was officially opened on 17 January 2003. This case history provides an overview of the tunnel jacking within the wider project and describes the range of innovations and the key role of the observational method (OM).

/ Contract 9A of the Boston Central Artery project takes both interstate highways, 1-90 and 1-93, intersection underground

Figure 7. / Contract 9A of the Boston Central Artery project takes both interstate highways, 1-90 and 1-93, intersection underground.

Looking north before construction along the 1-93 towards the 1-90 intersection. Photo credit www.bigdig.com

Figure 7.2 Looking north before construction along the 1-93 towards the 1-90 intersection. Photo credit www.bigdig.com.

Key Aspects of Design and Construction

The Project

In its entirety, the CA/T project was the largest ever US Federal Works engineering undertaking and took over twenty years to plan, design and construct. It involved the replacement of an ageing steel elevated highway structure, built in the 1950s, which carried the Interstate 93 Highway through downtown Boston, with a new highway underground. Much of these tunnels were built directly beneath the existing elevated structure. In addition, a new highway, also mainly in tunnel, was constructed from the original termination of Interstate 90 (1-90) in downtown Boston, to Logan International Airport in the east in Boston Harbour. The new CA/T now takes much of the city’s interstate highway network underground thus bringing substantial relief from heavy traffic congestion and pollution. Overall, the project entailed construction of 7.5 miles of new highway with approximately half running underground (Figure 7.1). The final cost of the project in 2012 was estimated at $24.3 billion, with generally 70% of the funding from the Federal Government (through the Federal Highways Administration), and 30% from the Massachusetts State

Government. The Massachusetts Turnpike Authority (MTA) became the eventual owner and operator of the new highway system.

Geology

The ground conditions beneath the railway presented an exceptionally challenging array of mixed face conditions for tunnelling. This included contaminated fill and buried structures overlying compressible strata, which became major obstructions during construction (Figures 7.3-7.5 and 7.16). The geological profile in Figure 7.16 shows the alignment of 1-90 Eastbound (1-90 EB) tunnel, which was longest of the three jacked tunnels, and the confliction with the obstructions.

The upper part of the site’s geology consists of 6.1-7.6 m of miscellaneous fill material. This is primarily granular and contains numerous large obstructions including boulders and segments of concrete as well as an abandoned, low-level reinforced concrete track-way, old masonry foundations, reinforced concrete and hundreds of timber piles - in short, the various artefacts of two centuries of waterfront development (Figures 7.3,

7.4 and 7.17).

Groundwater levels at the site are typically 1.8-3.0 m below existing grade. Underlying the fill are extensive, variable deposits of organic materials in the range of 3.0-4.6 m thick, which consist largely of organic silt with fine sand and some peat. Below these are local lenses of relatively dense sand and inorganic silt of alluvial origin, generally less than

1.5 m thick. The thickest soil deposit at the tunnel alignments is the

Construction of buried trackway base in I890’s

Figure 7.3 Construction of buried trackway base in I890’s.

Figure 7.4 Timber piles showing at low tide.

Vertical alignment of jacked tunnel through buried trackway and sea wall

Figure 7.5 Vertical alignment of jacked tunnel through buried trackway and sea wall.

Boston Blue Clay, consisting of clay and silt. The deposit is stronger and less compressible at the top, over a thickness of approximately 4-5 m, where it has been over-consolidated by desiccation. The lower section of the marine clay is normally or lightly consolidated and considerably softer.

 
Source
< Prev   CONTENTS   Source   Next >