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.