Innovative ground engineering solutions were celebrated at the 2010 Fleming awards last month. By Adrian Greeman
Ground breaking use of tyre bundles for lightweight road embankments took this year’s Fleming Award despite some strong competition.
The winner was up against detailed entries from a difficult analysis of cofferdam stability in a tidal zone and from a major London air-rights station scheme which used complex micropiling and pile reuse.
After hearing presentations from the three shortlisted projects, the British Geotechnical Association judges panel decided that the unusual and economical tyre bales solution to an awkward problem had mobilised excellent cooperation between client contractor and designer.
“The untried method carried risks, and it was a brave decision to go ahead, backed up with necessary testing and documentation which helped convince the client, by going through all the necessary technical steps.
Buy-in was secured from the Highways Agency and the county council which will manage the road,” said judges chairmanDinesh Patel, director at Arup Geotechnics.
The judges said the team had thought about the immediate project and the strong potential for future use of tyre bales “which indicated their desire to improve the industry generally”.
“The untried method carried risks, and it was a brave decision to go ahead”
The £3,000 prize was awarded to the winning team of Alex Kidd from the client Highways Agency, Stephen Beales from designer URS Scott Wilson and Russ McNeill from contractor Balfour Beatty Civil Engineering.
The project was a section of improvements to the A421 road in Marston Vale between Junction 13 of the M1 motorway and Bedford, a route which is part of a key east to west corridor linking the university towns of Oxford and Cambridge (GE October 2009). Around £200M is being spent to dual a 13km stretch of the road, greatly increasing capacity and relieving congestion.
A short length of the road passes the edge of the small Brogborough lake said Kidd, At this point the main carriageway and a realigned section of side road had to cross a former clay borrow pit, on embankments up to 7m high.
The excavated pit contained a number of clay slurry fans, the result of spoil deposition from a second adjacent pit.
These were made of soft material up to 21.5m deep.
For the main carriageway, a piled support with a geotextile load transfer platform was selected to cope with this.
The slip road was done with a surcharged lightweight embankment, with a six month maximum for loading.
In places wick drains were also used. In low embankment areas, lightweight fill was chosen for soil replacement.
For this purpose PFA would have been too dense and would have had to be placed in stages.
Commercially available fills such as Maxit expanded clay were considered too costly.
So the contractor proposed tyre bales during Early Contractor Involvement discussions. These were cheaper and recycled material that would otherwise be incinerated.
The bales are made by compressing used tyres into bundles 1.5m long and 0.8m high, secured with wire straps.
The bales have a fairly low density of 500kg/m3. But they needed to be carefully assessed for engineering properties as well as for durability, and fire risk.
The team was able to draw on information from suppliers and also from Mike Winter of TRL who has been championing their use.
An existing PAS 108 specification for consistent production of the bales was used.
In construction, the bales were used in layers to build up the embankment above a Maxit working platform used for drain installation.
More Maxit was used to infill gaps and the whole was enclosed with a geotextile, above which a 1m layer of conventional fill created space for services.
Balfour Beatty looked around the UK to find existing uses of tyre bales.
They had been used in walls and some other structures though never before in embankment.
“There was a feeling it would be complex element of the job” said McNeill, “especially on a soft ground area.” In the event he said, it turned out to be one of the easiest and fastest sections. “It is almost like laying bricks.”
Some 3,000 bales were laid up to six deep in just under a month, he said. A small excavator-mounted handling clamp was developed for the operation.
The bales were easy to move, handle and store and could be obtained from three or four suppliers in the country.
“Time saving was of great programme benefit which saved up half a million pounds in itself,” says McNeill.
The lessons learned have since allowed the use of the bales in other projects elsewhere in Balfour Beatty he says.