CONSULTING engineers for the prestigious footbridge over the Grand Union Canal devised an innovation solution combining Leca® Lightweight Expanded Clay 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 unstable soils surrounding the canal plus the far bank of the canal being directly inaccessible to heavy plant via a sensitive residential area.
Drawing on previous experience of Leca® Lightweight Expanded Clay Aggregate 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 Leca® Lightweight Expanded Clay Aggregate not only as lightweight embankment fill but also as the stabilising backfill material for use with the Tensar reinforced soil wall system. It was the first structural application in the UK to use this combination of materials.
Leca® Lightweight Expanded Clay Aggregate’s weight – typically 75 % lighter than traditional crushed aggregates – met the requirement for lightweight bulk fill behind the bridge abutments and wing walls. It also 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 from the west bank to the point of use, resolving the access problem, minimising the need for trucking and spreading plant on the eastern bank and contributing to a reduction in contract costs as a result.
While the bridge abutments are piled, the use of the lightweight fill reinforced soil structure allowed the wingwalls to be founded directly upon the existing ground without ground improvement or further piling.
Simon Griffiths, Pell Frischmann’s Project Engineer commented,: “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 Leca® Lightweight Expanded Clay Aggregate and the Tensar Wall System in the UK, a solution that should have many applications in locations with poor non-load bearing soil structures as it was at this site. Besides easing access difficulties, the Tensar/Leca® solution was ideal as it required minimal foundations thereby reducing the use of hard construction techniques. Testing by Tensar and maxit demonstrated the validity of using Leca® LWA in place of conventional fill materials.”
Some 3,500m³ of Leca® Lightweight Expanded Clay Aggregate, 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 wing walls the material was placed in layers, combined with the geogrids to meet the specification for the Tensar Wall System. Minimal compaction was required.