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Stars and stripes

Phi Group did not need badgering to complete work on a West Midlands slope retaining scheme. A deadline was set by a higher authority than even its client and paymaster.

When badgers start to get broody, Natural England, the countryside and land management agency, listens up. In some regions badgers have been culled as possible carriers of tuberculosis that can spread to cattle, but in Dudley, West Midlands, they get the red carpet treatment.

House and flat building for a new residential estate required cutting into a slope to maximise the development's footprint. But something had beaten client Taylor Wimpey to the site and taken up residence.

Phi Group regional director Julian Fletcher says it all comes down to timing when working around the urban badgers that had moved into the made ground embankment running along the edge of the site. "You can't disturb them between November and June when they are breeding," he says. "What Taylor Wimpey said to us in September was 'your price is good as long as you can finish part of it in October'."

Phi Group came up with a combination of techniques for the project but decided on its timber product Permacrib for two areas, including the part where the black and white burrowers had made a home.

This comprises 1m deep timber cribs built on top of one another and filled with graded granular infill. But speed was of the essence to beat the clock and get a retaining solution installed before the badgers turned their minds to more amorous thoughts.

"A good thing about it [Permacrib] is the procurement times that can be just two weeks," says Fletcher. "So we were able to start on the first week in October in the section where the badgers are roaming around and we finished this first phase two weeks later."

Fortunately, considering the time available, this first section of the timber crib retaining wall is relatively low, at just 3.5m high, and runs for about 100m. Had the badgers decided to live in the much higher embankment further along, things might have been impossible. But Phi Group was able to finish phase one before the cut-off point.

Phase two is a taller prospect and includes wall sections rising to 9m in places. Typically, for such heights, site workers would install the timber crib retaining wall at multiple depths – placing layers of cribs one in front of the other along the face. But although it often acts as a gravity wall leaning at up to 1:4, at this site it is a single depth.
"If we had had good ground, such as rock or chalk, we could have used a multi-depth wall," says Fletcher. "But here in Dudley there is a lot of made ground and you can't get away with that as there is a chance the material won't stand up as it's old [Ford] Cortinas, rubble, concrete and steel. That whole hillside has been built up over the years with rubbish."

But Fletcher explains that the problem is not that a multi-depth wall would not support the slope. It is that contractors could not cut the slope back to install it without suffering a potential collapse.

This meant it was necessary to install soil nails up to 9m into the face of the cut (typically 6m) as site workers re-profiled the slope from the top down. Since these Dywidag nails are already holding back the slope, a single-depth Permacrib is added, almost as a cosmetic touch. "It's hardly a gravity wall at all, as the nails are doing the work," says Fletcher.

Over 5000 linear metres of these nails were planned, but more ended up being used as the slope was cut back further than Phi Group had expected. This has seen extra long nails with an additional 3m being installed in places to account for the space between the 76˚ crib wall and the cut face of the slope. The nails are at 1.5m centres horizontally and 1.2m vertically, with the longest being 12m in the area where the slope was overcut.
Phi Group installed soil panels on a third area of the cut with a 60˚ face. Along with the phase two Permacrib work, this has created a 1200m2 wall.

This angle is not far off the shallower gradients where geotextiles could be used for a more gentle batter and, like the Permacrib, the panels were used on this project for aesthetic rather than functional reasons. Taylor Wimpey had said it did not want Permacrib all the way around the slope that forms the border on two sides of the new development.

The choice of a different face using soil panels was intended to offer a green vegetated finish. But this has changed following the client's decision to have the 150mm deep wire mesh panels filled with crushed rock for cost-saving reasons, as well as it being maintenance free compared with a heavily planted face. This said, both the Permacrib and soil panel sections will feature some Permabags, planted at intervals, from which creeping plants will grow.

The about £450,000 job is nearly complete and Fletcher says the firm tends to win such jobs because it offers a one-stop shop that includes nailing and facing. Phi Group marketing manager David Selwyn points out that one of the reasons Keller bought it was it was winning many jobs from under the nose of the much larger company.

But Fletcher says times are changing. "When Keller bought us about two years ago, we were the only firm that could offer this. Now others are copying us and offering more," he says.

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