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Extensive soil nailing is providing temporary support for phased construction of a cut and cover section of the new Channel Tunnel Rail Link in Ashford, Kent.

As work on section two of the Channel Tunnel Rail Link gears up (see News this month), major sections of the high speed route's first phase are rapidly taking shape.

Main contractor Skanska Construction (UK) (formerly Kvaerner Construction) has been working for just over two years on the 15.2km section of the route that passes through the heart of Ashford.

Worth £150M, Contract 430 includes construction of the mainline and station approaches to Ashford International Station, as well as the line from Charing Heath to Sevington.

Skanska is building 32 bridges (including viaducts), a cut and cover tunnel at Westwell Leacon and a 1.7km long cut and cover tunnel with props and retained cuts.

Engineering design and project management of section one is being carried out by Rail Link Engineering (a consortium of Bechtel, Systra, Ove Arup & Partners and Halcrow).

At Ashford the route runs alongside existing track into the international station and must pass under Beaver Road Bridge, a main arterial route through the town. The alignment of the new line takes it through the north abutment of the six-lane bridge. This is being achieved by constructing a 13.6m wide,10m high concrete box.

The main problem for Skanska's piling arm, Cementation Foundations, was that no more than two lanes of the road could be closed at any one time. Also, the bridge was widened several years ago and the abutments contain elements of the new structure, as well as mass concrete temporary works used in the widening.

Because of these obstructions, the limited working area and the need to keep most of the bridge open, the box had to be built in three phases. Skanska chose soil nailing to provide temporary support for the excavation face and the bridge deck during each stage of construction. Most nails are removed during construction although some, supporting a pedestrian subway box running under the bridge, are permanent.

Rail Link Engineering senior field engineer Adam Chodorowski says soil nailing was chosen over a piled box because installation is relatively quick and cheap and, because the working area was restricted, small rigs had to be used. He says piling would have been expensive, would have caused much more disruption and was not really an option because of the road closure restrictions.

For the first phase of excavation, 11m long nails were installed at 1.2m horizontal centres in rows 1.5m apart on the near vertical excavation face. Geology consists of about 2m of fill overlying Hythe Beds, Atherfield Clay and Weald Clay at the base of the excavation.

The face was excavated in benches, with soil nailing in rows followed by a final 150mm thick layer of sprayed concrete reinforced with a single A252 square mesh fabric, connected to the nails by 225mm square by 15mm thick steel plates. Weep holes were also installed to keep water levels down, although groundwater is low and has not caused any problems.

Percussive drilling was needed for the top three rows to cope with the anticipated obstruction in the fill and 114mm diameter nails were used here. The rest of the nails were 168mm diameter, because the ground below was weaker and obstructions were not expected.

Each nail comprises cement grout reinforced with a 25mm diameter 500 Gewi bar installed at 12.5degrees below horizontal, to ensure it finished in the Atherfield Clay and did not go into the Weald Clay beneath.

A tracked Casagrande C6 drilling rig was used to install the nails, and proved capable of overcoming the significant numbers of obstructions in the fill, says Cementation's Alan Day. However, progress was slow and some nails had to be abandoned because part of the bridge buttress was encountered. The first phase finished at the beginning of last year.

'We learnt lessons from the first phase, ' Chodorowski says.'You have to be prepared to react to conditions.' The second phase went more smoothly as a result, and work was finished in only three weeks, just before Christmas. This phase involved 10m long soil nails installed the same way as before.

Day explains that longer working hours and careful sequencing of excavation, benching, soil nailing and concreting of the face all helped make things easier. Although the exposed face had a maximum two-day stand-up time, on long rows, shotcreting followed directly behind the nailing, he says.

Following installation, grout cubes, sprayed concrete tests and soil nail load tests are undertaken. The latter are carried out cyclically on expendable nails which were debonded for the first 2m from the face. Day says that empirical estimates of movements suggested that surface settlement of 15mm to 20mm could be expected during excavations and movements have been well within this range.

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