LWA Plays Part In Solution For Brunel-Style Bridge
CONSULTING engineers for a prestigious new footbridge over the Grand Union Canal devised an innovation solution combining maxit LWA lightweight aggregate and Tensar geogrids to solve soil stabilisation and access challenges.
Designed for client English Partnerships, with Jackson Civil Engineering as main contractors, the new 22-metre span elliptical brick arch bridge provides pedestrian access to homes in Milton Keynes.
The brick-built bridge, which echoes the style of Isambard Kingdom Brunel, presented the consulting engineers Pell Frischmann with major challenges due to ground conditions and the far bank of the canal being directly inaccessible to heavy plant via a sensitive residential area.
Drawing on previous experience of maxit LWA and combining its use with Tensar geogrids and wall blocks to create reinforced earth retaining walls on each side of the bridge, Pell Frischmann utilised maxit LWA not only as lightweight embankment fill but also as the stabilizing backfill material for use with the Tensar reinforced soil wall system.
maxit LWA's weight - typically 75 per cent lighter than traditional crushed aggregates - met the requirement for lightweight bulk fill behind the bridge abutments and wing walls and solved the challenge of 'delivering' material to the eastern bank of the canal by enabling some 2,000m³ of material to be pneumatically 'blown' for 39 metres across the canal to the point of use, resolving the access problem and contributing to a reduction in contract costs.
Comments Pell Frischmann's Project Engineer, Simon Griffiths: "For these spans few of these solid brick arch footbridges have been built since the heyday of Isambard Kingdom Brunel, so it is entirely in keeping with the Grand Union Canal. It is also a first combination of maxit LWA aggregate and the Tensar Wall System in the UK, a solution that should have many applications in locations with poor non-loadbearing soil structures as it was at this site. Apart from access difficulties, we wanted to avoid piling for the retaining walls, so the Tensar/maxit solution was ideal. Testing by Tensar and maxit demonstrated the validity of using LWA in place of conventional fill materials."
Some 3,500m³ of maxit LWA, in 10-20mm grade, was delivered in phases over a three-week period with material "blown" into position for the eastern side of the canal and the remaining 1,500 m³ tipped into position on the western bank. Behind the wingwalls the material was placed in layers, combined with the geogrids to meet the specification for the Tensar Wall System. Minimal compaction was required.
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