3Qs: For Big Dig, light at the end of the tunnel?

Boston’s Big Dig — the most costly highway project in United States his­tory — has been plagued with prob­lems since ground was broken in 1991. Last week, inspec­tors dis­cov­ered a sink­hole beneath the sur­face of the I-​​90 con­nector tunnel, pos­sibly caused by a ground-​​freezing process used during con­struc­tion. We asked Thomas Sheahan, a pro­fessor of civil and envi­ron­mental engi­neering at North­eastern Uni­ver­sity, to explain why the ground-​​freezing process was used and whether it caused the sink­hole to form.