STIFFER THAN expected ground conditions are hampering efforts to complete Britain's first box jacking operation beneath a live motorway before Christmas, engineers admitted last week.
A new £6M underpass is needed on the A43 between Towcester and Northampton where the dual carriageway narrows beneath junction 15A of the M1 west of Northampton.
'The stiff ground has slowed us down by about a week with rock intrusions the main culprit, although the ground itself is steadier and therefore safer than first feared, ' said Don Higgs, project manager for design and build contractor Nuttall.
The jacking operation is expected to last four weeks.
The 3,500t concrete box is currently halfway across the M1, as over 100,000 vehicles per day hurtle by overhead.
Work by Nuttall is progressing in 150mm bursts. This is the amount excavated by miners over about one and a half hours before the whole box is pushed forward by hydraulic rams.
'Cut and cover would have been far easier, and much cheaper to build. But delays caused by a diversion scheme on the M1 could have cost up to £30M, ' said project manager for client the Highways Agency, Roy Brunsden.
Measuring 8.5m tall, 14m wide and 45m long, the reinforced concrete box was first constructed in a specially excavated pit, which eventually formed the jacking base.
Then the three storey concrete tunnelling shield was fitted to the front to enable miners to work at the higher reaches of the tunnel face while excavators operate below.
'This shield is angled so the face is protected by a small gradient. Protecting the stability of the face is paramount, ' explains Higgs. 'To ensure that the face never settles or deteriorates, excavation is continual.'
An anti drag system designed by specialist box-push engineering consultant Ropkins was fitted top and bottom to protect ground around the box from being pulled out of place as it moves forwards.
It is a system of heavily greased 13mm thick steel ropes that remain stationary at the box-ground interface and unfurl beneath the front end of the box. This allows the box to be pushed without pulling the pulverised fly ash, glacial deposits and stiff clay ground with it.
Before the push began, the road construction was gradually built up by a maximum of 25mm across the area above the box push, to allow for possible settlement caused by advance excavation.
'This was to limit the predicted 50mm settlement. But as we have excavated, the ground has been much stiffer than expected and so we have found little settlement, ' said Higgs.