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Retaining walls: Logistics solution

Early supply chain involvement has been crucial to the successful design and construction of retaining walls for a rail embankment at one of the UK’s largest distribution centres.

The increasing sophistication of the UK retail sector is evident in the scale of distribution centres built around the country in the last 20 years. One of the biggest of these is “DIRFT” – the Daventry International Rail Freight Terminal – off junction 18 of the M1 motorway in Northamptonshire.

Retaining walls

The design for the 800m long embankment involves a range of different retaining walls solutions

The site’s owner, Prologis, has been developing DIRFT since the 1990s, with the latest stage – DIRFT II – being a 54ha extension to the original 120ha site. The first stage of DIRFT II is already complete, with Tesco having recently moved into a 78,000m2 distribution centre and associated rail link. Next up is a similar 100,000m2 facility for Sainsbury’s, which is being built on the opposite side of the A428 to the existing development. A key element of this project is to extend the existing freight line that runs through the development, linking it to the Northampton loop of the West Coast Main Line.

The line is being taken over the A428 and then through the area being developed for Sainsbury’s to give the supermarket giant a railhead alongside massive new warehouses. This will enable the firm to brings goods in by rail on containers and unload them at its own intermodal rail facility.

The land on which the new development is taking shape is fairly flat, sloping only 2m from one end to the other, so it is perhaps surprising to find a new 9m high embankment being built along the site’s western boundary, ready to support the railway line and railhead.

The level was dictated by two constraints: the need to get over the A428 at sufficient height to allow large vehicles to pass underneath; and longer term plans for DIRFT, which include building a third phase to the north and east – on the other side of another main road, the A5. As a result, the new 800m long embankment will carry four lines of track, two of which will stop at the Sainsbury’s depot, and two that will continue through the site and over the A5 if and when the next phase of the development goes ahead.

“With space constraints making it
impossible to fit in a free standing
embankment, Volker Fitzpatrick
started talking to retaining
structures specialist Phi Group
about retaining wall options”

The embankment serves a number of purposes. It not only carries the rail lines, but also supports the “intermodal rail facility” – a 400m long, 30m wide concrete slab where goods will be unloaded from the trains. And it acts as a landscaping bund, providing an acoustic and visual barrier between the distribution centre and a proposed housing development to the west of the site. With space constraints making it impossible to fit in a free standing embankment with battered slopes, main contractor Volker Fitzpatrick started talking to retaining structures specialist Phi Group about retaining wall options.

“At the early stages they were looking at a vertical contiguous piled wall, which is not cheap,” says Phi managing director Julian Fletcher. “By fine tuning we worked out that they didn’t need to do that – we could put in walls that lean back and still achieve the overall size they needed for the slab.”

Volker Fitzpatrick had successfully built the first stage of DIRFT II – the Tesco’s distribution centre – for Prologis, so when the developer started to get interest for the second phase, it turned again to the contractor to come up with a budget price for the infrastructure work. This led to a period of detailed design, during which the contractor brought in specialist contractors like Phi – a division of ground engineering specialist Keller – to discuss options and to find ways to bring costs down.

Fletcher says the retaining wall design was developed over a period of six to nine months, during which it became clear that reinforced earth would be the most cost effective solution.

The resulting design for the 800m long embankment involves a range of different retaining walls solutions. The east side slopes up at an angle of 70° to a height of 9m. At the top of this is a flat section that supports the intermodal rail facility and four rail tracks. There is then another 5m high, 70° slope, which will eventually be topped by a 3.5m high acoustic fence. The embankment then slopes down at a gradient of 1:3 for a length of 21m, before steepening at the bottom to a gradient of 1:1.

Retaining walls

The 1:3 section has been designed as free standing, but Phi has designed retaining walls for the rest of the slopes, using its Textomur system for both the 9m high rail embankment and the 5m high additional slope, and a geotextile wraparound for a 425m length of the 1:1 slope.

Textomur consists of horizontal layers of geogrid or geotextile laid at regular intervals within compacted fill material. The facing to the slopes is formed using a steel mesh “cage” backed by another geosynthetic layer that, on this site, is filled with crushed concrete to give a maintenance-free finish.

As part of its value engineering process, Volker Fitzpatrick had made the decision to keep imported material to a minimum, and re-use all material it gained on site – including everything accumulated by demolishing an existing transport depot and yard. The contractor wanted to create the embankment using site-won materials, as well as clay stockpiled from the Tesco development.

Retaining walls

Retaining walls for the rest of the slopes use Phi’s Textomur system

But not all the material is ideal for using as fill for an embankment. Although the underlying ground is what Volker Fitzpatrick senior project manager Deon Scholtz describes as “really good consolidated Lias Clay”, this is overlain by 2m of weathered alluvium with pockets of sands and gravels. This material is marginal, at best, and very difficult to work with when it is wet.

One of the advantages of Phi’s Textomur system is that it can be used with a variety of different materials, with the design of the geogrid and the thickness of the layers varying according to the strength of the fill material. However, the soft alluvium that overlies the DIRFT II site really is marginal, and the only way it can be re-used for the embankment construction to treat it by adding both lime and ground granulated blast furnace slag (GGBS) to get an acceptable moisture content.

Earthworks contractor Blackwells is doing the main muckshift and placing the embankment core, using the stockpiled clay and the modified alluvium. Phi then builds the walls using the same material.

Between the face of the wall and the core is a 1m wide section of Class 6I granular material, which helps to drain the clay and give the structure stability. Again, this material has mainly been reclaimed from the demolition of the old transport depot.

Retaining walls

Volker Fitzpatrick – which is responsible for delivering around £130M of work on the site, including the demolition, infrastructure and warehouse been used for the abutments of the new railway bridge. The warehouse buildings have pad foundations and ground bearing slabs, with the fill beneath the high bay area of the warehouse surcharged to prevent differential settlement. The embankment is not being surcharged; instead, band drains were installed over its entire footprint at 1.5m centres, together with a 300mm drainage blanket, and both the embankment and retaining walls are being constructed in 300mm high lifts, with each layer being compacted until it achieves 95% of the required compaction level.

In addition to the Textomur walls on the rail side of the embankment, Phi has also installed a 5.4m high geogrid “wraparound” reinforced earth retaining wall at a 1:1 slope at the base of the front slope of the embankment. The geogrid contains topsoil, which will be seeded as part of the site’s landscaping.

Phi’s £2.3M contract – the largest single contract in the company’s history – also includes constructing concrete panel reinforced earth walls for the new rail bridge abutments, and a 4.2m high, 235m long Andacrib concrete crib retaining wall supporting a bund between the main warehouse buildings and car park.

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