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Early expertise means Eurostar stays on track

Early involvement of geotechnical specialists ensured Eurostar trains kept running during embankment construction for the Croydon Tramlink project.

MAIN CONTRACTOR for the Croydon Tramlink, Amey/ McAlpine Joint Venture, commissioned reinforced soil slope and retaining wall specialist Comtec Bio Engineering to design and construct a ramp to provide disabled access from an adjoining main road to the Beckenham tram stop platform.

In this section the new tramline and tram stop are being installed beside an existing railway line supported by a 6m high embankment, consisting of made ground varying from sandy clay at the base through silty and clayey sand to sandy gravel at the top.

Work had to be incorporated within the railway land take, so reinforced soil design was used to steepen embankment side slopes, creating space for the new works.

'The real challenge was that existing Eurostar signalling cables clashed with the construction of the wall system,' says Comtec Bio Engineering managing director Jerry Fox.

'Access and working space for construction were extremely restricted, and good supervision was required to ensure that the reinforced walls were constructed without damaging the cables.'

The problem was overcome by the main contractor enclosing the cables in a split plastic duct strapped to steel cables which spanned between reinforced concrete mini piles installed at 8m centres.

Comtec then excavated the embankment and constructed the 4m wide reinforced soil 'block' of geogrid and fill material below the Eurostar cables and between the mini piles.

For the new embankment walls Comtec designed a scheme using Textomur on the lower side of the pathwa, and a Tensar block faced reinforced soil system on the upper side.

The 3m high Textomur wall incorporated Tensar 40RE reinforcing geogrids, and used site-won sandy gravel as the fill material. Tensar SR geogrids were used to reinforce upper 4.5m section. Again, fill material was site won, using railway ballast.

The walls, both 58m long, tie on to a Textomur embankment constructed earlier in 1998 to support a tram passing loop. The two new walls for the access ramp were constructed in five weeks.

Fox says: 'The Beckenham Tramstop project is a good example of the way in which main contractors can include specialist subcontractors at a very early stage in problem solving, to develop innovative solutions and to promote value engineering.'

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