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Freeze frame

An integral part of the tunnel jacking operations at the South Bay Interchange is the treatment of the ground beneath the mainline railway tracks serving Boston's South Station.

Ground freezing subcontractor Freezewall installed 1800 steel freeze pipes an average of 2m apart in and around the railway tracks. A calcium chloride brine was then used to freeze the ground to between -25degreesC and -30degreesC over three months, up to one month before jacking operations started.

Ground is frozen down to a maximum of 18m, just 1m above the base of the tunnel in places, so that the front face is frozen solid from the ground surface almost to the invert.

The freezing process gives the soil quite high strength, says Hugh Lacy from New York-based Mueser Rutledge Consulting Engineers, geotechnical adviser for this aspect of the job. 'It is comparable to soft rock.'

Because of the nature of the fill, the organic layers and the Boston Blue Clay, some heave was expected in the tracks. Up to 180mm of vertical movement was allowed by the railway operators, counteracted by removal or tamping down of track ballast. Lacy says that heave has been within the acceptable levels, with heat pipes installed across the site used to thaw the ground and reduce heave if necessary.

He adds that to aid jacking and to reduce lateral movement of the ground as the tunnel sections are pushed, heat tapes have been fitted on the outside walls to thaw a few centimetres of soil around the sections. As jacking proceeds and the freeze pipes are encountered, they will be simply dug out.

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