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Vertical and raking bored piles have stabilised a settling embankment on London's busy Metropolitan Line.

Stabilisation work has nearly finished on an ageing earth structure on a busy section of London Underground's Metropolitan Line.

A temporary speed restriction on the line between Northwick Park and Preston Road stations has been lifted early and a public road closed to enable the works is due to reopen four weeks ahead of schedule.

The remaining third of the project is now in the final earthworks stage.

The contract for stabilisation of a 1200m stretch of the 6m high southbound embankment was awarded to Cementation Foun tions Skanska in August 2005 by Metro et, which is responsible for maintenance and upgrading of two-thirds of London's underground network.

Ongoing settlement of the earth structure and lateral displacement of lineside services led to an intensive monitoring regime and a temporary speed restriction being imposed on the line in January 2005.

The embankment was built by the Metropolitan Railway in 1880.

It opened with two tracks (now the Metropolitan northbound fast and slow lines), and in 1914 was widened on the south side to accommodate two more tracks (now the Chiltern Line up and down lines).

In 1935 two more tracks (now the Metropolitan Line southbound slow and fast lines) were added to the northern side of the embankment, giving a total of six lines.

By then the surrounding farmland had given way to housing, which meant space at the embankment toe was limited. As a result, the upper slopes were over-steepened to allow room for the new lines.

Built on London Clay, the structure is made up principally of cohesive embankment fill, with ash and track ballast on top. Below the over-steepened shoulder is a shallow upper slope (between 15° and 18°) and a steeper lower one (about 25°).

The cohesive fill is generally present in two layers, one from the original embankment and an upper layer, placed during the final stage of widening.

Ash forms the shoulders of the embankments and is also found on the slopes, in layers between 1m and 2m thick. Ash was also found within the embankment, thought to be material from the original slope face.

Cementation designed the stabilisation scheme with its consultant Mott MacDonald. The design criteria required a 120-year life, with minimal maintenance, and an increase in the factor of safety to more than 1.3 for deep seated failures and to more than 1.15 for shallow ones.

Differential settlement had to be restricted to 1:500 along the track and 1:300 across it. A crest walkway was also needed because of limited clearance in the cess. And all this had to be built while the line remained open.

The design addressed both shoulder stability and deep-seated failure mechanisms along the embankment, using a combination of an anchored bored pile retaining structure running roughly parallel to the track for 450m, earthwork berms over a 450m stretch and a toe wall over a further 300m.

The retaining structure uses vertical and raking piles tied together with a capping beam. The vertical piles, at 900mm centres, pass through the slope failure plane and embed in the stable ground beneath.

Site workers installed inclined piles at 1.8m and 3.6m centres.

Cementation says early contractor involvement allowed the project team to secure full closure of a local road to provide access to the embankments.

The site was otherwise landlocked and the road closure was essential to enable works to be finished quickly and allow the temporary speed restriction to be lifted, the firm says.

Parties involved in negotiations included the London Borough of Brent, the Metropolitan Police, London Transport Buses and the local residents' association. The road closure was granted - but only for six months.

The fist step was to install a number of new cable posts along the lineside services to provide temporary support - undermining meant the old cable posts were leaning away from the track.

Access platforms were then built along the entire length of the embankment using a recycled Highways Agency Class 1A material. Temporary cuts were made into the crest of the embankment from the access platform, supported by a combination of anchored and cantilevered retaining systems.

The track was monitored weekly and the temporary works daily to ensure stability of the track was not compromised.

The earthworks platform also enabled the vertical and inclined bored piles to be installed. Each was drilled through the embankment fill and into the underlying London Clay, pinning any potential slip planes.

A total of 4500m of 308mm diameter vertical piles and 1900m of inclined piles were placed. Pile cages were installed in small sections to enable all the piling to be carried out during normal traffic hours.

The crest wall was built using Cementation's CemRailBeam system, with the precast units placed by 13t excavators while the piling works continued. The toe wall is within a very narrow corridor of embankment and required placement from a walkway only 1.5m wide.

Cementation says the CemRailBeam is a reinforced capping beam system designed to overcome safety, quality and production difficulties on traditional embankment stabilisation works.

It involves factory-made precast shuttering units placed over the piles, combined with reinforcement and concrete to form a continuous trackside retaining wall.

On this contract, the capping beam signifiantly reduced temporary works, particularly the depth of temporary excavations when compared with traditional cast insitu methodology, Cementation says. The capping beam was then fitted with a two bar handrail.

Once the capping beam was finished, engineering fill was placed behind it to support the crest of the railway and to provide safe walking routes for railway personnel.

Down slope of the capping beam, the slope was locally regraded to suit the site geometry and improve stability. Where necessary material was removed from the slope and granular material imported to create a smooth profile.

Over the bottom third of the slope, a modified Highways Agency Class 1A granular fill was benched a minimum of 800mm below the lower of the pre-existing and finished ground profiles. Where the regrade angle exceeded 1:2, a geotextile antierosion mat was installed, extending 1.5m down slope.

The final interface between the track and the stabilisation works involved placing additional ballast trackside of the lineside services.

The team finished two-thirds of the project ahead of target, which meant early lifting of the temporary speed restriction and the road closure.

When the earthworks are finished, the embankments will be revegetated with wildflowers, grasses and shrubs.

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