Contract 330 of the Channel Tunnel Rail Link has led to the largest use of geogrid on the project so far. Dan Simpson reports.
Land take restrictions on Contract 330 of the Channel Tunnel Rail Link have meant forming reinforced slopes of up to 700 for the steep cuttings required.
Contract 330 of the CTRL forms the wastern end of section 1 of the UK's first dedicated high-speed railway line.
Running from just north of the River Medway to just south of Ebbsfleet in North Kent, the £80M, 16km long Medway Valley section includes the Waterloo connection to Fawkham junction.This will allow Eurostar trains to travel to Waterloo International Station in central London while construction of section 2 of CTRL is under way.
Work on Contract 330 is being carried out by joint venture contractor Alfred McAlpine Construction and Amec Civil Engineering. Union Railways, a subsidiary of CTRL builder and operator London and Continental Railways, is acting as project client.Design and project management is being carried out by Rail Link Engineering (RLE) a joint venture between Ove Arup, Bechtel, Halcrow and Systra.
Geosynthetic firm Tensar International has designed and supplied 36,000m 2of geogrid, costing some £80,000, for two reinforced soil slopes on the section, where land take restrictions meant that steep cuttings were needed.
One is on the Waterloo connection and the other on the main CTRL route where it runs parallel with the A2 north of Cobham.
Tensar International area civil engineer Geoff Carter explains that as well as being designed to Highways Agency Standard 68/94 and BS8006, cutting slopes were also designed to parameters set by RLE.
The Waterloo connection is largely being built along the route of the abandoned Gravesend West Railway line.
Just before the link converges with the main CTRL line north of Southfleet, near the B262, trains will run in an existing cutting.
This cutting had to be widened and reprofiled to accommodate the high-speed trains. Because of land take restrictions, the slopes at Dale Road had to be steepened to 600 up to a height of between 3m and 5.1m. The slope from the top of the reinforced soil to the top of the cutting has been reinstated to its previous profile of 220.
Geology comprises a clay head deposit overlying a thick layer of sand and clay, both part of the Thanet Sand formation, which overlies Chalk at track level.
Tensar supplied 13000m 2of its uniaxial 40RE geogrid to reinforce the 600 slope at the base of the cutting. The existing slope was then benched back at 450 with step heights of 865mm, to match the vertical spacing of the geogrid layers.
A 1000mm thick layer of class 6N granular fill reinforced with geogrid provides the foundation for the reinforced soil structure.The base geogrids, which are between 6000mm and 9000mm long depending on the height of the embankment, provide the overall stability against slip circle failure. Above track level, geogrids of between 3000mm and 4500mm long were placed between 865mm thick layers of compacted granular fill to build the main structure. A category 1 embankment fill was then placed behind the geogrid structure and profiled to an angle of 220 above the geogrids.
The face of the slope comprises turf behind a steel mesh, attached by looping the geogrid back over the horizontal mesh bars and connecting it back to itself using Tensar's bodkin connector.
One of the main hazards on site is the legacy of ancient chalk mining. Four chambers of a dene hole, branching off a single shaft excavated through the Thanet sand formation, were found just below the level of the Chalk. Two of the chambers were dug out and the two remaining in the cutting were infilled with concrete.
Land take is also a major issue for the cutting north of Cobham where the CTRL runs close to the A2 carriageway, which is now being widened (see page 30 this month).
The sides of this cutting are being supported by a combination of a timber crib wall, supplied by Phi Group, and 22750m 2of geogrid reinforcement, again supplied by Tensar.The reinforcement is continued above the top of the cutting to form a bund next to the A2 at Scalers Hill to prevent vehicles from veering off the road and on to the railway line.
Geology here comprises Lambeth Group beds overlain by 8m of Harwich Formation Blackheath Beds. The Lambeth Group deposits are predominantly clayey, with some silty and siltstone bands.
At the eastern half of the Scalers Hill cutting there is also a 2m thick layer of London Clay overlying the Harwich Formation. A band of lignite at the base of the Harwich Formation was removed when it was encountered at formation level.
Two types of primary uniaxial geogrid reinforcement and a secondary biaxial reinforcement were used to provide support for the cutting from its base to the top of the bund next to the A2.To help support the lower part of the cutting a 4m high timber crib wall was used to form a slope of 760 in front of the 700 reinforced soil slope. The slope above the crib varies between 450 and 270.
The crib wall is founded on a concrete base. Behind it, the reinforced section comprises geogrids spaced at 475mm.Tensar 80RE geogrid was used in the lower 4m of the 14m deep cutting.
To prevent the loss of fines during construction, the slope is faced by a Tensar geotextile liner fixed behind a steel mesh panel.The mesh is attached to the slope using the same method as in the Waterloo connection cutting.
The space between the crib wall and the reinfored earth section is filled by class 6N granular fill.
Above the crib wall the spacing between the 40RE geogrids increases to 1m. To prevent small circular slips above the crib wall, 2m lengths of Tensar biaxial SS12 geogrid were placed half way between each primary geogrid layer.
The face of this slope is protected by an erosion mat, also supplied by Tensar. The bund on top of the cutting comprises 6N fill, also has a face angle of 450 and is protected by an erosion mat.
Contract 330 should be one of the first major contracts to be completed on section 1 of the CTRL, with work due to finish by June this year.