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Stirring stuff One of the largest soil mixing contracts in the US is being carried out to stabilise soft clay as part of the South Bay Interchange of the Boston Central Artery project.

USA FOCUS

Some 460,000m3 of Boston Blue Clay next to the Ford Point Channel will be treated.

East of the jacked tunnels of the South Bay Interchange, huge amounts of remediation and reclamation is needed to allow construction of Ramp D, the cut and cover tunnel link that will carry the I-90 on to the immersed tube tunnels of the Fort Point Channel crossing.

'The unusual thing about this contract is that very little is final construction,' says Joseph Baka of project management team Bechtel/Parsons Brinckerhoff.

This is because the ground, a 'bowl' of Boston Blue Clay next to the Fort Point Channel, presents a serious hazard to construction of the link. The marine clay is very soft and very deep, up to 37m, topped off with soft organic soil and up to 8m of fill. Some 460,000m3 of soil will be treated on the £125M contract.

Soil mixing was chosen as the most cost-effective way of stabilising the waterlogged ground and to reclaim some of the channel beyond the existing harbour wall. This will allow excavation of the cut and cover tunnels while protecting adjacent structures including the US Postal Service building.

Working for the joint venture main contractor Cashman and PKA, SMW Seiko - the Californian-based subsidiary of Japanese firm Seiko Kogyo - is carrying out a combination of shallow and deep soil mixing, using its in-house designed equipment.

SMW Seiko is using two large crane based rigs, each fitted with three 1.5m diameter augers that mix the wet cement grout/fly ash with soil to form overlapping columns to depths of up to 38m to key into the underlying stiffer glacial till. Designed to resist lateral loads, the columns act as shear walls transferring the loads to the till which resists movement through horizontal friction and should prevent damage of adjacent buildings during cut and cover construction. Columns also provide support for the final tunnel structures.

A third rig with a single 1m diameter auger is carrying out shallow soil mixing to a maximum 24m depth to stabilise the ground before excavation begins. Geotechnical contractor Trevi/ Icos will also be involved in this 'construction expedient' to jet grout the ground if any obstructions re encountered

during excavations.

Reclamation of part of the Fort Point channel is also required to accommodate the tunnels. Baka explains that this is carried out by forming 'cells' filled with soft sediment that is then strengthened by both shallow and deep mixing. Work is carried out on land and on specially built barges in the channel.

Final strength requirements vary between 2N/mm2 and 7N/mm2, Baka says. A combination of wet bulk sampling during mixing and testing of core samples taken after mixing has had time to work is used to check this specification is met. Checks will also be carried out from a 37m deep steel observation shaft and 12m deep trial pits across the site.

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