A geosynthetic solution has been used not only to contain contamination, but also for its drainage properties on an access road built on a former landfill site. Gemma Goldfingle reports
The rundown area of Orford Park in Warrington is undergoing a facelift thanks to £30M of public funding.
Warrington Borough Council, the North West Regional Development Agency and Sport England are among the diverse range of investors in the regeneration project, which will transform a former landfill site into a community sports village.
Sport will be the catalyst for regeneration in the area, which is in the top 7% of most deprived areas in the UK.
A range of world class sports facilities will be built on the derelict land, including four full-size football pitches, a dance studio, a six lane swimming pool and a new training academy for local rugby team Warrington Wolves.
Tenders were submitted to construct the main sports facilities in March, and the project is due for completion in early 2012. But before this can happen an access road must be created to connect the new development to the wider community.
The road will link into the A49, one of Warrington’s key transport corridors, which itself connects to the M62 and M6 motorways.
“This should be a simple cut and fill operation,” says Craig Steele, site agent for BAM Nuttall, which is building the access road.
“However, this site has been used as a landfill for the past 30 years. There are lots of nasties lurking in the ground.”
Before work started consultant Aecom carried out 14 borehole tests and dug trial pits across the 20ha site, but Steel says they were inconclusive:
“It’s such a large site that it is pretty hit and miss as to what you find. There are hotspots where contaminant gases are found. These can be easily missed in ground investigation. It is safe to say we know there is some level of contamination.”
“This should be a simple cut and fill operation. However this site has been landfill for 30 years”
Rather than cap the poor material using clay, Aecom decided to use a geosynthetic membrane to contain the contamination.
“We are working less than 5m away from a block of flats. We’ve got to keep disturbance to a minimum,” says Steele. “Using a clay capping layer requires importing the material to site, which means constant lorry movements.
A lot of compaction would need to be carried out using a clay cap, which means noise, vibration and dust.”
Using a geosynthetic solution was therefore considered the most neighbour-friendly solution.
Aecom originally suggested using a geosynthetic membrane to prevent any gases escaping, and a sandwich of geogrids and fill levels to stabilise the existing ground.
But manufacturer Naue suggested integrating Secudrain, a geosynthetic material with inbuilt drainage.
Secudrain is a composite material consisting of a drainage core made of polypropylene monofilaments that are firmly attached to a non-woven geotextile.
It can be rolled out easily, and acts as an efficient drainage mechanism, especially in road construction.
Drainage was originally to be provided by 150mm of Environment Agency-specified stone.
But this would also require importing to site and excavating into the landfill before filling.
Naue’s solution proposed a simple roll-out of material.
The material acts as protection as well as drainage, so it saves the need for another geo-membrane to be used to contain contamination, along with offering an easy drainage solution.
As aggregates become more expensive Secudrain is becoming more commonly used,” says Naue managing director Chris Quirk.
Installing the geotextile solution on the Warrington job is simple, according to Steele.
The road requires both cut and fill along its 260m route.
Around 1m of cut is required where the access road connects with the A49, while at the furthest end of the road, deep into Orford Park, 1m of fill is added.
The fill is carried out before the ground improvement takes place.
Next a geotextile layer is laid.
This material, Secutex, is a fibre textile which, to the naked eye, looks like carpet underlay.
Like all the geotextiles used on site, this material comes in large rolls.
It acts as a cushion for the geomembrane that contains the contaminated land, and is simply rolled out across the stretch of road.
The membrane, a 2mm thick high density polyethylene (HDPE) material, is rolled out directly on top of the textile.
HDPE is one of the most inert plastics, so will not be permeated by any leachate.
The material comes in 9.4m wide rolls, and two types are being used in Orford Park, one to stretch across the 7m wide road and pavements, and another that will be cut to size and laid over the utility trench next to it.
The drainage textile is rolled directly on top of the membrane, and then 150mm of locally sourced recycled Type 1 stone is scattered on top.
This is compacted, and then a 30kN Secugrid is laid to strengthen the ground.
Two more layers of compacted stone are added, interspersed with 30kN geogrids to create the road, designed to take 18,000 axle loads.
Secugrid geotextile grid is manufactured from extruded and drawn bars, which are laid and welded together.
It has tensile properties, so can resist tensile force loading without deforming.
The grid interlocks with the fill material through its 32mm apertures, ideal for Type 1 stone, and absorbs the tensile forces.
“Ideally, for road construction you want a high-strength material with very little elongation.
This material experiences only 2% elongation at 12kN loading. This is important as, in road construction, more than 10% elongation will cause a failure,” says Quirk.
The road can be built on top of this platform immediately after it is laid.
The grids are already in tension, so no differential settlement is expected.
Progress has been steaming ahead since BAM Nuttall began building the access road.
When GE arrived on site in early April, the contractor had completed 25m of the 260m stretch in only four days.
“It is much quicker than capping,” says Steele. “It is much easier to install and less likely to go wrong on site.”
It has been calculated that this method has saved more than 60 lorries from entering the site.
BAM Nuttall is on course to meet its June deadline to complete the access road.