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Bump in the night signalled huge failure

HEAVY RAIN over Easter is believed to have triggered a sudden massive railway embankment failure on the Highland Line near Culloden, 15km south east of Inverness in north east Scotland.

Six failures occurred over a 300m length on the 15m to 25m high 30degreesto 35degrees embankment on the night of 27 April. Railtrack was alerted when a driver reported a 'bump'.

Railtrack engineers were at the remote spot within 30 minutes and closed the track immediately. Dawn revealed five translational failures and a large circular slip that in one section undermined nearly 20 sleepers. Tension cracks between the slips meant in effect the entire 300m section had failed.

Railtrack called in Glasgow-based consultant Donaldson Associates which had recently assessed the stability of cuttings and embankments on the Highland line using stereo oblique aerial photography (GE April). This classified the Culloden embankment to be 'at risk'.

The trigger was the exceptional rainfall on Easter Monday and the following Wednesday, according to Donaldson Associates director Andy Sloan. Drainage was unable to cope. The location is fed by a huge water catchment area and at the time of collapse water was running along the track into the failure area from a large cutting further up. Embankment materials in the failed section vary from silty sand to a 'stoney' fill.

Railtrack's initial assessment was that repairs could be undertaken in 21 days, but that this was subject to reassessment as the design and construction progressed. From Donaldson's preliminary evaluation it was clear buttressing the slope with a rock fill blanket and upgrading the drainage was the most appropriate solution.

Design was finalised within two days, allowing rapid implementation of the scheme.

First Engineering and the construction team have been working 24 hours a day installing 25,000m 3blocks topped by 240-50mm screened stone to create the new 32degreesto 34degrees strengthened embankment.

Work has progressed well, aided by unseasonally good weather in the Highlands since Easter. As GE went to press Railtrack was confident the line would reopen by mid-May.

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