Before the huge Airbus A380 jets could become operational at Gatwick Airport, a vast bund has had to be built to shield the taxiway the planes will use from local residents. Gemma Goldfingle reports.
At 350m long and 16m high it is difficult to miss the latest addition to Gatwick airport. The newly constructed screening bund is part of a £43M investment in the airport and is designed to shield nearby villages from the new taxiway being built, and the giant A380 planes which will be housed there.
The six new aircraft stands at the north-western side of the airport stretch over 100,000m². The stands are designed as remote parking areas, two of which will hold Airbus A380s - the world’s largest commercial aeroplane. This will give aircraft operators a greater degree of flexibility on housing planes on site.
Hiding the jets
Noise will not be an issue as the nearest neighbours to Gatwick’s northern front are 2km away, but views could be altered. The existing visual bund on site is only 8m tall, which is not enough to conceal the tail fins of the huge aircraft that will reside in the expanded area.
“It was vital that we used the right materials. We spoke to local firms in the formative stages of this project so we knew exactly what was available”
“Gatwick Airport is committed to ensuring that any development carried out does not negatively impact the surrounding area,” says Gatwick Airport project leader Jeremy Rowton. “Our first task was to determine where the viewpoints were. This would help decide how high the bund had to be to be completely enclosed.”
A visual impact assessment was carried out around the north and north-west side of the airport. Beyond the confines of the bustling airport is the rather beautiful setting of rural Surrey. The nearest conurbation is the small parish of Charlwood, although public footpaths popular with ramblers circumnavigate the outskirts of the northern airport boundary.
Gatwick and its consultants worked closely with Charlwood Parish Council and Crawley Council to identify the viewpoints and came to an agreement that 16m was a suitable height. The consultation went beyond merely the height issue - the local community were passionate in ensuring that the bund itself did not become a blight on the landscape.
Broccoli Wood, a continuous belt of trees, surrounds the north-westerly side of the airport and the River Mole meanders through it. It was vital for the screening bund to have a varied profile and be well vegetated to integrate into the woodland setting.
With minimal land at the cramped Gatwick site the inside face of the new bund is 63 degrees, which is as steep as they could get it according to Rowton, allowing the slope to be far more gentle on the public-facing side. To engineer this 3:1 slope face, geogrid reinforcement was be needed.
Scott Wilson developed the scheme design and Maccaferri, designer of the Paragrid, carried out detailed design to determine the performance of the reinforcement on different material sources, both on site and external materials.
Paragrid is made from high tenacity polyester with a tough polyethylene protective sheath, which gives a high resistance against damage during construction. The grid is particularly suited for use with marginal fills as the apertures interact well with the soil.
“It was a logistical nightmare. Communication with the team was critical. We had regular meetings with the entire project team making sure that everyone was on the same page.”
The design team worked together to see how they could integrate various materials available into the 180,000t bund to avoid sending waste to landfill. On site materials, fills from other local sites and other recycled granular materials were tested. Initially, contractor Carillion planned to use all site-won material in line with the airport’s environmental strategy, but granular fill was needed in parts to strengthen the bund and the quality on site was not adequate.
“It was vital that we used the right materials. We spoke to local firms in the formative stages of this project so we knew exactly what was available,” says Rowton.
“There was a lot of clay on site,” adds Scott Wilson aviation associate Kieran Bradley. “We reused much of it, but we needed free-draining material otherwise the water pore pressure could cause collapse.”
The team considered using a geogrid with in-built drainage that would allow lower quality fill to be used, but decided it did not have a sufficient cost benefit for the project.
It was determined that within the reinforced zone, at the steep front face, fill had to have an internal friction of at least 28 degrees. Outside this area poorer quality materials could be used.
A local firm, PJ Brown, was used to source out recycled high-quality granular material and class 7C1 clay, which was also not available on site, from across the region.
Overall 22,000t of local clay was imported; 96,000t of recycled granular material under the WRAP protocol; and 60,000t of site-won clay was reused.
The bund required a 2m toe into existing ground because of the high ground water table. Where there was an existing bund it was cut off and reused as it was in good condition and made from good quality granular material, so the new bund could sit on top of it.
The Paragrid is installed horizontally in the works and then 600mm of structural backfill is placed and compacted upon it. To form what is an unnaturally steep slope the paragrid is wrapped back upon itself using a temporary framework to hold the grid at an appropriate angle whilst backfilling is taking place.
Two layers of grid was laid before stepping back the slope by 200mm. This created the 630 angle and also allows more rainfall to get into the face topsoil to help vegetation to grow - very important for long-term stability of the bund.
As a range of materials were used to create the bund, regular testing was carried out on every 1,000t of material used in an on-site laboratory to ensure it had the right grading. The Highways Agency grading system was used to identify soil types.
Placing the Paragrids
As well as a range of fill materials, varying strengths of Paragrids were also used across the bund. Grids of 200kN were placed at the base where stresses would be at their highest. This gradually reduced higher up the bund, reducing to 80kN at its peak.
Using a range of materials and paragrids of varying strength made logistics challenging on site. Subcontractor PML Geotech, a specialist in building bunds, would often have to construct a course where all materials used were different from the previous section.
“It was a major logistical nightmare,” says PML Geotech director Phil Lettieri. “Communication with the team was critical. We had regular meetings with the entire project team making sure that everyone was on the same page.”
Despite the treacherous winter weather the team managed to stay on programme, often designing a section just before its construction.
As the bund needed a vegetated finish to integrate with the rural landscape, compacted top soil was placed behind the front-facing grids. A biodegradable erosion protection blanket was used to stop the soil spilling out.
Hydroseeding was carried out in April to kick-start the growth of the vegetation. Now ramblers will think this grassy mound has long been part of the Sussex countryside.