A120 - Stansted
Engineers for a major trunk road project specified Leca® lightweight aggregate to meet construction challenges and speed up progress.
By using the versatile lightweight expanded clay granules, Balfour Beatty was able to reduce the amount of piling needed for bridges and to substantially cut the loading on three large diameter drains carrying surface water from Stansted Airport.
Working on behalf of Essex County Council to designs by WS Atkins Consultants Limited, Balfour Beatty was able to reduce earth pressure on the bridge and a nine metre embankment by some 75%, when compared with traditional fill materials (Class 1 or 6N).
Leca® was used to backfill abutments of a bridge carrying the new 15-mile A120 trunk road from Stansted to Braintree with around 1,000m3 completed in just two days, using only one small excavator to spread and compact the fill.
The reduced loading achieved by using Leca® allowed CFA piling to be used instead of bored piles due to the shorter length required. This had the added advantage of eliminating the use of a support fluid which would have been necessary with bored piles. Further abutment backfilling using maxit Leca® for a second bridge will take place later in the contract.
In the second stage, some 8,000m3 of Leca® is being used to form an embankment over three existing large drainpipes - two 1,650mm and one 1,800mm diameter - which are the main surface water outfalls from the airport. Conventional fill would have placed undue loading on the pipes with a likelihood of them collapsing.
In addition to solving the loading issues, Balfour Beatty also found Leca® quicker to place and compact - with deliveries reduced to just one fifth of the number required when using conventional aggregate fill - allowing faster progress and a shorter completion schedule.
Balfour Beatty's Design Manager, Martin Graham, explains: "The main reason for using Leca® for the structural backfill of the bridge abutment and the embankment was to reduce loading. The speed of use and the reduced environmental impact, because we needed fewer loads, were welcome bonuses."
Leca® is approved for both general fill and structural backfill applications by the Highway Agency. Leca® is totally natural and is made from pure clay which is expanded to form small, lightweight granules with a honeycomb core to produce a density approximately 25% that of traditional aggregate.