How do you build tunnels of interstate highway four lanes wide through poor soil beneath vital railroad tracks without shutting down the rail lines? This was the dilemma faced by the Slattery/ Interbeton/White/Perini Joint Venture, working on a portion of the Central Artery/Tunnel Project—often called the Big Dig—which will result in 7.5 miles (161 lane miles) of I-93 and I-90 buried under central Boston. The multibillion-dollar undertaking is reported to be the largest and most complex urban infrastructure project in American history. Tunneling under railroads usually involves relocating tracks—or taking all or a portion of the tracks out of service—and building staged cut-and-cover tunnels. This was not a viable option in Boston because of the many rail lines funneling more than 300 daily commuter and Amtrak trains into the city's South Station. These vital tracks bisect eastbound and westbound extensions of I-90 that are now being built and that will provide high-speed access to Logan Airport via the new Ted Williams Tunnel under Boston Harbor.
Of three other tunneling options—mining, excavating multiple drifts by micro-tunneling, and tunnel jacking—jacking is the least expensive. Jacking also reduces the volume of excavated soil and minimizes contact with subsurface obstructions. The 25 feet of soil under the rail beds contain hundreds of years of Boston history: long abandoned subsurface tracks, building foundations, and coarse fill. And because the groundwater level is only 6 to 10 feet below the present rail beds, the deeper soil is a soft, watery mixture of silt, sand, peat, and marine clay.