Highways Agency

The original junction consisted of a two lane underpass beneath the motorway, with roundabouts on each side, servicing the M1 slip roads, Rothersthorpe motorway service area and the Swan Valley development. As part of the original planning consent for the development, Swan Valley made a commitment to increase the capacity of the junction by providing a second underpass and remodelling the roundabouts.

Timescale

The project was completed in Spring 2003.

This was the first contract to be let by the Highways Agency to construct a motorway underpass using a 'box jacking' technique.

The finished scheme provides a dual carriageway between the two roundabouts on the A43, either side of the M1 at junction 15A, thus reducing delays on the A43 between Northampton and Towcester.

Construction

In order to minimise disruption to the 112,000 vehicles which use the motorway each day, the new underpass was tunnelled under the M1 using a 'box jacking' technique. A concrete box was constructed adjacent to the motorway and pushed through the embankment as follows.

Box Jacking Process

• Excavate jacking pit and construct jacking base
• Build box weighing 3,500 tonnes, measuring 8.5m high, 14m wide and 45m long
• Add tunnelling shield
• Add top anti-drag system ropes and anchorage
• Add bottom anti-drag system ropes and anchorage
• Add jacking equipment
• Jack box into place (100mm per hour, 24 hours per day, 7 days a week)

During the jacking process the surface of the motorway was monitored. The motorway surface was raised overnight in advance of the works to cater for calculated minor settlement.

The whole jacking process took 4 weeks and the motorway was kept open throughout.

Graphic showing the Box Jacking Process

See the FOI/EIR requests related to the road M1