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A new geogris/aggregate combination is resurrecting a Victorian design over the Grand Union Canal

For 200 years, the Grand Union Canal has gently wound its way through the area now occupied by the relative upstart new town of Milton Keynes. Canal boats first travelled the route from London to Birmingham in 1805, well before its arrival in the centre of southern England in the 1960s.

The town's presence has prompted many improvements to the waterway over recent years. One of the latest modifications will be the addition of a pedestrian footbridge over the canal off Stoke Road in the Water Eaton area of Milton Keynes. This will give residents direct access to a park on the other side, previously reached by a more remote crossing.

Access, as well as unstable soils surrounding the canal, also presented difficulties for contractor Jackson Civil Engineering. The solution, proposed by engineer Pell Frischmann for client English Partnerships, is the first structural application in the UK to combine the soil reinforcement of Tensar geogrids in the Tensar wall system (TWS) and Maxit lightweight aggregate (LWA).

Conventional aggregates and soils have been used with the TWS in the past, however, these proved to be unsuitable for the Milton Keynes bridge. Being only a quarter of the weight, the Maxit LWA eliminated the need for piled foundations beneath the abutment wing walls.

'This solution should have many applications in the future at locations with poor, non-load bearing soil structures like this site. Besides easing access diffiulties, it was ideal, requiring minimal foundations thereby reducing hard construction techniques, ' he says.

Apart from meeting the need for lightweight construction, site workers could pneumatically transfer (blow) the LWA across the canal to the bridge abutment on the far side for placement in layers with the geogrids. This minimised the need for trucking and spreading plant and reduced contract costs.

Jackson set the bridge footings on a concrete cap, with access slopes and steps on either bank constructed over the geogrid stabilised aggregate fill. Abutment retaining walls either side of the bridge, connect to the geogrid layers.

The 4.7m headroom footbridge has an 22m elliptical arch, constructed as it would have been in the Victorian era more than 120 years ago. The contractor is building it with bricks throughout, rather than the more popular concrete construction with brick face.

Because a flat profile was required, stainless steel reinforcement was incorporated into the three dimensionally bonded brickwork.

The project, which began in April 2005, is scheduled to be completed early in 2006.

Twin techniques

The TWS solution which has British Board of AgrÚment (BBA) certification comprises a dry laid wall of Tensar concrete blocks on a minimal concrete footing. The 'll behind the wall is completed with imported granular material - here Maxit LWA - stabilised by layers of Tensar uni-axial geogrid. The geogrid is connected to the blocks with a polymer key, to provide an integrated load bearing structure.

Maxit LWA is comprised of lightweight expanded clay pellets typically 10mm to 20mm across. Tensar says the material is up to 85% lighter than conventional aggregate that results in an average weight after compaction of 300 kg/m 3.It is 're and chemical resistant, allows free drainage and provides thermal insulation. It also has environmental credentials, requiring only 20% of the transport loads of conventional aggregates, which reduces the environmental impact on a site.

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