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Slope stabilisation

Foundations : Skanska

Five years of development have perfected a new technique for stabilising rail embankments.

Dan Simpson reports from the Piccadilly Line.

Poorly compacted Victorian embankments on overland sections of London Underground's network have exceeded their design life and are undergoing a long awaited overhaul.

Foundation contractor Cementation Foundations Skanska has developed a production line method of stabilising old clay embankments and cuttings, and enhancing the quality of the track bed. The system, known as the CemRailBeam, has been developed and evolved over the past five years, and is said to be quicker and cheaper than other methods.

In its current form, CemRailBeam comprises vertical and raking piles beneath a small wall structure that retains the track bed. The vertical piles stabilise the embankment by intersecting possible slip surfaces and the raking piles act as anchors to support the vertical piles.

Cementation's £1.46M contract to stabilise a 460m section of the Piccadilly and District lines between Turnham Green and Chiswick Park demonstrates CemRailBeam's benefits.

The original embankments were constructed by trial and error and 90% of them are more than 70 years old.

'London clay was excavated and simply tipped to form the embankments, ' says Cementation project manager Iain Barr.

'The clay has taken a long time to become unstable but now, 70 years, on it is failing. This leads to track closures, speed restrictions and excessive maintenance costs.'

LU first commissioned a topographical survey and site investigation to determine the condition of its embankments and cuttings. These were mapped and classified as poor, marginal, serviceable and good according to their condition.

Howard Humphreys carried out an earth structures investigation, Soil Mechanics carried out the ground investigation and Ove Arup produced the interpretative report.

Cementation is working for client Infraco Sub-Surface Limited (Infraco SSL) - part of LU - set up to manage the sub-surface sections of the District, Metropolitan, Circle, Hammersmith & City and East London lines. Joywheel is the partnering contractor for the CemRailBeam and Mott MacDonald is consultant.

Stabilisation of the embankments on the current section began with the installation of piles from a temporary earth platform built onto the side of the embankment and covered with a concrete blinding layer cast up to the required pile cut off level and providing a clean platform. The platform was later incorporated into the permanent works as the base of the CemRailBeam.

Vertical piles, 310mm in diameter, were then installed through the concrete blinding layer down to 2m to 3m below the base of the embankment at 1,200mm intervals. Raking piles were installed every 3600mm at an angle of 45degrees and reinforced with a T32 Dywidag bar forming an anchored bored pile wall.

To minimise the danger of the piling rig falling on the tracks and to allow the installation of piles at the top of embankments, the majority of piles are installed using a compact TD 308 low headroom rig.

A capping beam cast between two parallel precast concrete shuttering units ties in the tops of the piles. The factory-made shuttering units, supplied by Solway Precast, are assembled in 3m long sections and joined on site with a half check joint to form a beam over the piles.

Lengths vary between 12m and 24m, separated by a construction joint.

Each unit is made up of two parallel reinforced concrete slabs separated 600mm apart by reinforcing bars. The front slab of the shuttering unit is 125mm thick and between 1300mm and 1500mm high, and the rear slab is 100mm thick and 600mm high.

Once the shuttering units are in place, four extra layers of reinforcement are placed between the concrete slabs and a beam is formed by pouring concrete up to the level of the rear shuttering units. Handrail fixing bolts are cast into the top of the front face, so reducing installation time.

The continuous trackside retaining wall formed supports 6N/6P fill which is placed and compacted in layers behind the wall to provide a stable track bed. The embankment is then regraded and topsoil and vegetation is added.

'Using precast concrete shuttering means that there is no need to use wooden shuttering and therefore saves a lot of time.

The whole system reduces the amount of temporary works and also provides a factory finish to the capping beam, ' says Barr.

'By constantly developing and improving the CemRailBeam, we have been able to increase production from 40m to 50m a week to in excess of 350m a week. There has been a 65% reduction in costs and construction time of the CemRailBeam over the past few years.'

Movement in the embankments also causes movement in the cable stays at the trackside which can lead to signalling problems. By stabilising the top of the embankment, the system supports the cable runs. It also provides extra room at the side of the track for new cables to be laid, while the old ones are still in use, allowing a changeover to take place at night.

'Because the work is carried out under a design and build contract, there has been a lot of feedback between ourselves, the designer and the client allowing us to constantly improve the system, ' says Barr.

Design of the system has been aided by applied research funded by LU and numerical and physical modelling.

'The upgrading is part of the PPP plan, all cuttings and embankments must be brought up to a new standard within the next 5-10 years, ' says Infraco SSL project manager Jez Haskins. 'So far, all the embankments classified as poor have now been brought up to standard, ' he adds.

Embankment remedial work also calls for care in choosing vegetation to provide a finish for the stabilised slopes. 'A lot of research has gone into choosing suitable vegetation for our embankments, ' says Haskins.

'We had to choose plants which maintain an even moisture over the embankment and minimise seasonal shrinkage and swelling.

These tend to be plants such as small shrubs.'

Barr adds: 'We have construction of the CemRailBeam down to a production line process with a dedicated rail team going from one job to the next. There is about ten years of work left, so we must keep developing the system and moving forward.'

Work at Turnham Green is due to finish in early March. Cementation is also using the CemRailBeam on three further jobs.

Work started on a £2M contract in the summer to stabilise 700m of Piccadilly Line embankments between Boston Manor and Osterley Stations. Approximately 230m of embankment is being stabilised near Hanger Lane in a contract worth £685,000 and 1185m of cutting is being stabilised in a £1.74M contract around the area of West Acton station.

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