Civils and stations

Stabilising the Metropolitan Line

A close look at Cementation Foundation Skanska’s work in London

Work to stabilise an ageing earth structure for Metronet between Northwick Park and Preston Road Underground Stations has now been completed.

The contract to reinforce a six-metres high southbound embankment, which links the two bustling Metropolitan Line stops, was awarded to Cementation Foundations Skanska (CFS) in August 2005. The contract required Skanska to design and construct stabilisation measures to the 6m high embankment, which was installed in 1880, over a length of 1,200m.

The embankment has experienced considerable development over the past 126 years; the line which roofs it was soon widened to six tracks, and, in 1935, to compensate for space constraints, the upper slopes were over-steepened to allow room for the railway to develop. Maintenance was conducted regularly thereafter, and, in 2005, with settlement of the embankment continuing, and the lineside services being laterally displaced, an intensive monitoring regime and a temporary speed restriction were imposed on the line during January 2005.

Undermining of the lineside services had resulted in the cable posts leaning discernibly away from the track. Under the terms of the contract, the line had to remain open throughout the construction of the works, which were designed for a life of 120 years with minimal maintenance. CFS designed the works in partnership with their designers, Mott MacDonald. The design addressed both the shoulder stability and deep seated failure mechanisms throughout the length of the embankment.

Early contractor involvement enabled the project team to secure a full road closure of Windermere Avenue to provide plant access to the embankments. The site was otherwise landlocked, and securing the road closure was essential to enable the works to be completed in a fast track manner to secure the lifting of the temporary speed restriction on time. Several external stakeholders were involved by the project team with the negotiations, including the London Borough of Brent, the Police, London Transport Buses and the Local Residents Association. The road closure was granted, but only for a six-month period.

Before access could be made onto the embankments, a number of new cable posts were strategically installed along the lineside services to provide temporary support. Access platforms were then constructed along the entire length of the embankments to allow CFS’s specialist plant access to the embankments.

From the earthworks platform temporary cuts were made into the crest of the embankment. The cuts were supported using a combination of anchored and cantilever retaining systems. The running rails of the Metropolitan Line were monitored during engineering hours on a weekly basis throughout the project, and the temporary works were monitored on a daily basis, to ensure that the stability of the track was not compromised.

The earthworks platform also enabled vertical and inclined bored piles to be constructed with LU-approved plant. Each pile was constructed through the embankment fill material and into the underlying London Clay, pinning potential slip planes. A total of 4,500m of 308mm diameter vertical piles and 1,900m of inclined piles were constructed. Pile cages were installed in small sections to enable all of the piling works to be carried out during normal traffic hours.

The crest wall was constructed using CFS’s CemRailBeam system. The precast units were placed with 13T excavators whilst the piling works continued. The toe wall was located within a very narrow corridor of embankment and required placement with specialist plant from a narrow walkway approximately 1.5m wide.

The CemRailBeam is a unique reinforced capping beam system, designed by CFS to overcome safety, quality and production difficulties on traditional embankment stabilisation works. It involves factory-made pre-cast shuttering units placed over the piles, combined with reinforcement and concrete to form a continuous trackside retaining wall. The capping beam constructed in this manner significantly reduced the temporary works requirements, and in particular the depth of temporary excavations when compared to traditional cast in situ methodology. The capping beam was then fitted with a two-bar handrail.

On completion of the capping beam works, engineering fill material was placed behind the capping beam to support the crest of the railway and provide safe walking routes for railway personnel. Down slope of the capping beam, the slope was locally regraded to suit the existing site geometry in order to improve the factor of safety for down slope stability. Where necessary material was removed from the slope and granular material imported to create a smooth slope profile.

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

The earthworks sections involved a net import/export of up to 400m3 per day. Delivery vehicles were strategically scheduled throughout the works to deliver materials and remove muck away in a ‘just in time’ process. This minimised the number of deliveries through the residential area and was a necessity as only limited storage facilities were available on Windermere Avenue.

Gordon Sims, CFS’s rail operations manager, recently said of the project: “The works have now been completed. We have worked 40,000 man hours and have had no reportable accidents. The experienced project team successfully completed the project ahead of programme, which enabled the early lifting of the temporary speed restriction and the reopening of the road closure four weeks ahead of the contract programme.”

During the works, the project team found time to improve the conditions under the railway bridge sited within the road closure. This included improved lighting underneath the railway bridge, providing access to Thames Water to update the drainage systems, repainting of the railway bridge structure, and placing pigeon guano netting to the underside of the bridge to provide an improved environment for the local residents.

Metronet’s earth structures manager, Simon Welsh, expressed that he was “delighted with both the quality of workmanship and the rate of implementation. Lifting of the speed restriction early has been a great success for the project team and has been greatly appreciated by London Underground.”

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