OVERVIEW OF UNDERGROUND CONSTRUCTION FOR BOSTON'S CENTRAL ARTERY AND TUNNEL PROJECT

Interstate Highway 93 slices through the heart of Boston, south to north, in the form of an obtrusive viaduct. Completed in the 1950s and designed to carry 75,000 vehicles per day, it was carrying 190,000 per day within 30 years. The solution to the resultant delays, traffic jams and pollution caused by idling engines was to replace the aging elevated Artery structure with a mostly underground expressway that totals nearly 4 miles in length, is up to 120 ft deep and 250 ft wide, and has the capability of carrying 245,000 vehicles per day. The Artery, coupled with extension of Interstate 90 eastward to Logan Airport (also mostly underground), is known as the "Central Artery/Tunnel" Project, generally shortened to "CA/T" and more informally referred to as the "Big Dig." Geology along the alignments is extremely variable and challenging, consisting of late Pre-Cambrian argillite country rock overlain by glacial till & outwash deposits, blue marine clay (Boston Blue Clay), marine sands & stream deposits, organic deposits, and miscellaneous fill interlaced with large boulders, old building stone & brick, thousands of old timber piles, and remnants of old wharves & sea walls. The geological situation is further complicated by a high tidally-influenced groundwater table. The CA/T is the largest infrastructure project in the nation today, and it showcases a number of cutting edge underground construction advancements, including many that are U.S. or world record holders. Among other things they include deep soil mixing, ground freezing, very large jacked tunnels, concrete immersed tube tunnels fabricated on site, mined stacked drift grouting galleries, a gigantic circular cellular cofferdam, and extensive use of deep slurry walls.

This presentation will provide an overview of the above as an introduction to more detailed follow-on presentations on individual aspects of the CA/T underground.