The improvement of Junction 15A on the M1
motorway was a requirement of the planning consent for the Swan
Valley Industrial area. The existing junction consists of a two
lane underpass beneath the motorway, with roundabouts on each
side servicing the M1 slip roads. The capacity of the junction
was to be increased by providing a second underpass and
remodelling the roundabouts.
In order to minimise disruption to the 112,000 vehicles which daily use the motorway the new underpass was tunnelled under the M1 using the ‘Jacked Box Tunnel’ technique rather than by conventional 'cut-and-cover' methods that would have involved diversions, lane closures and contra-flow systems.The under-bridge was to be installed at a minimum of 1.5m below the motorway surface.
The ground conditions consisted of stiff clay and glacial deposits with clay fill and pulverised fuel ash in the motorway embankments. The water table was below the box.
A three level concrete cellular shield with steel cutting edges was used. Face excavation in the upper cells was carried out by miners and in the middle and lower cells by machine excavators. The motorway was monitored for movement throughout the construction and installation periods.
The patented ‘anti-drag system’
(ADS) was used at the top and bottom of the box and anchored to
the jacking base. Due to geometric constraints the top ADS was
anchored at each side to the sheet pile side walls of the jacking
pit, and at the centre by a series of tendons that were passed
around pulleys mounted on the back of the box and then fixed to
anchorages at the rear of the jacking base. Jacks were used at
the tendon anchorages to compensate for stretch in the tendons as
the load in the tendons progressively increased to around 350t.
The tunnel was installed to within 30mm of level and 60mm of line and required a maximum jacking thrust of 4,500t.The jacking operation took three weeks following headwall entry and was successfully completed in December 2002. Surface movements were well within specified tolerances and motorway traffic continued uninterrupted throughout.
The first application of the technology beneath a live motorway proved to be highly successful.
Client: Highways Agency
System design: John Ropkins Ltd
Contractor: Edmund Nuttall Ltd
Scope of work: Design and construct
Contract period: 17 months
Completion date: April 2003
© John Ropkins Ltd