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

Why is the ground-​​freezing method used for con­struc­tion pro­jectssuch as the Big Dig?

The sec­tion of the Big Dig where ground­freezing was used is unique. Because of Amtrak lines, con­struc­tion workerscouldn’t dig a large trench, as they did in some of the other parts of the project.Doing so would have inter­rupted rail ser­vice for a long period of time. The areais also rel­a­tively soft, which can be dif­fi­cult to tunnel through, espe­cial­ly­when the water table is so high. As a result, the method of ground freezing wasused to turn the soil into the equiv­a­lent of soft rock, which keeps the soil­stable and allows rel­a­tively con­ven­tional tun­neling methods to be used.

Did ground-​​freezing cause the sink­hole to form?

It is very unclear at this point. It seems,however, that an effect of the ground thawing — which con­tinues to occur a fewyears after project com­ple­tion — is that frozen water pre­vi­ously held in soil­pores is now seeping out and col­lecting under the tunnel. Think of a house ofcards (the soil par­ti­cles) that has its voids filled with water. When it isfrozen, the house of cards is very stable and the water is obvi­ously solid ice.When the water melts, how­ever, it escapes out of the voids and the cards may bemore prone to com­press, leading to set­tle­ment of the soil.

In ret­ro­spect, was ground-​​freezing the best approach to use for­this type of project?

It seems to have been a very effec­tive way tocom­plete this stage of the project. There appears to have been a pre­dic­tion­about this sink­hole and set­tle­ment, but not to this degree.