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How to nail a steep problem

Geotextile protection is being used in Pembrokeshire to prevent oil spills from spreading. By Mark Lewis

Scheme:Milford Haven refinery
Client:Murco
Main contractor:Quantum GB
Terram materials contract value: £55,000
Materials delivered onsite: May 2009
Environmental bund completion: August 2009

The people of Milford Haven could be forgiven for fearing the effects of an oil spillage more than most. In 1996, a tanker, The Sea Empress, ran aground in the Irish Sea on its way to the rainy, Welsh town spilling 73,000 tonnes of the stuff into the water around the Pembrokeshire coast.

Birds died in their thousands and it was five years before the coastline was properly cleared up. Paranoia is necessarily hard-wired into the local psyche. But one man’s paranoia is another’s careful planning, and in commissioning Welsh contractor, Quantum to geo-synthetically reinforce the holding basin at its Milford Haven oil refinery, oil firm, Murco has taken what looks like sensible precautions against allowing toxic nasties to leech into the surrounding earth.

The holding basin, or lagoon, is designed to store oil or excess water which could be contaminated, until Murco can lift it back out and clean it up. It has installed a geosynthetic membrane, like a giant bin-liner on the banks of the lagoon to prevent anything leeching into the surrounding soil, but now has what amounts to a steep wall of black plastic, gently baking in the sun.

“There are two issues with that,” says Martin Lambley, business development manager at geosynthetics manufacturer, Terram. “Firstly it is black and shiny and ugly, and secondly it is very susceptible to degradation from ultra-violet light. So it will become weak and eventually fail.” The solution is to put a layer of top soil above it, wait for the grass to grow, and enjoy as it becomes part of the surrounding green landscape.

But persuading the soil to remain on top of the membrane would mean fixing the slope of the 12 m high banks at around 18 degrees. And, at that gradient, the brow of the hill would be nearly 40 m back from the foot. A more practical ratio is a 45 degree bank, and a 16 m slope. But this requires a much more radical solution.

“If you tried to put top soil on that slope, it would just role down,” says Lambley. Terram’s solution has been to supply a modified version of one of its geosynthetic landscape products, the Erocell 20/15. Supplied as a flat panel, it is expanded on site to create a honeycomb structure and then filled with earth and seeds to create a flat vegetative surface.

The geosynthetic fibre is a non-woven geotextile made out of small individual porous filaments. It was originally developed by the US army to provide fast access roads on difficult terrain. Laid flat on the sand and stuffed with earth, it makes a durable layer of makeshift road, tough enough to carry armoured vehicles. It was used in Kuwait during the first Gulf War in 1991. So packing it with topsoil to encourage vegetation which protects the geosythetic layer underneath, and blends in with the surrounding Welsh landscape seems comparatively simple.

But working in South Wales creates its own problems. The Arabian Gulf landscape is so unforgiving because rain there is less common than a Victoria Beckham smile. Not so in Pembrokeshire, where deluge after deluge has forced Murco to repeatedly employ the lagoon to store rainwater run-off, which might otherwise end up in local tributaries, until it could manage its discharge content. Each time, the job has stalled until the water could be drained, and the lagoon made dry enough until hopefully there is enough dry weather to get the cells laid, filled, and industrially “hydroseeded” with a combination of mulch, seeds and other fast growing agents.

“Whenever the dumping basin has been filled we have had to pull off the job,” says Hayden Thomas, Quantum contract supervisor. “We were supposed to be finished before last Christmas, but we had to pull off the job then, and were not able to get back on before May. And then because of all the rain we have had in the last few weeks it has been put back by another three weeks again. Now we are hoping to have it completed by the end of August.”

The membrane liner is now fixed in place with the top folded into a 2-3 m filled trench at the top of the bank and a comparable trench at the bottom, running the 600 m along the side of the lagoon. Ordinarily the Erocell structure would then be attached on top by concertinaing the honeycomb structure down the side of the slope, and fixing it to the ground with pins. In normal circumstances, each sheet of cells would be pinned with 3 m pins at the edge of each cell around the perimeter. Then additional pins would be used to fix the sheet at 1 m intervals horizontally and vertically across the rest of the structure.

In the case of the Milford Haven this process was not possible because, like a Viking wearing a helmet with the horns on the inside, puncturing the underlying geosynthetic membrane with pins would have defeated the purpose of protecting it in the first place. “There is a theoretical datum line towards the top of the slope,” says Mr Thomas. “Anything below that point we assume that crude oil can reach, if Murco dumps it into the basin.” Above the datum line, the honeycomb structure has been fitted conventionally. Below it, Terram had to find an alternative solution.

“What makes this interesting is how we avoid stapling it to the slope, because if you did that it would puncture the membrane and that would create a flow pathway,” says Lambley. “Instead we have used galvanised steel cables to allow the cells to hang. They are nailed on at the top and then the cells are opened out.

The sheets of cells hang down like marionettes, supported by the tendons, each 7 m apart, which are nailed on at the top of the slope, grouted and covered in earth. The Erocell panels then have suitable holes drilled through to allow the insertion of the galvanised steel cables (threaded, for ease, before the cells panels are opened.) And, in the absence of individual pins maintaining the correct shape of the individual cells, cell panels rest against regular 25 mm galvanised steel washers, secured along the cables. The overall effect is very much like Venetian blinds, affixed at the top of the window, with individual panels secured by washers along the strings which suspend them.

At the moment, the cells are stuffed with sandbags, waiting to be filled with the earth and seeds which will effectively complete the project, awaiting nature to take its course. Ironically, a touch of rain then, might just urge the grass along.