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I'll take the high road

SLOPE ENGINEERING

Extensive remediation of steep slopes with a history of instability will ensure that an important road link in northern Scotland stays open. Ian Nettleton reports.

The A890 Stromeferry Bypass in north-west Scotland links South Strome to Attadale along Loch Carron's southern shore. Built between January 1968 and October 1970, the 8km single track road replaced a ferry at the western end of the loch.

Running along the landward side of the Kyle of Lochalsh to Inverness railway line, the road involved cutting into the glacially over-steepened sides of the fjordlike Loch Carron valley. The lower slopes consist of 60°-90° relict sea cliffs with occasional overhangs and 30°-45° superficial slopes rising up to 100m; the upper slopes are 22°-45° superficial deposits with occasional natural rock outcrops forming cliffs.

Blasting was used in places to form rock cuttings up to 65m high in the lower slopes. The cuttings are typically less than 2m from the road and often less than 0.5m.

Numerous major stream gullies cut through the hillside, some them extending a kilometre or more.

The geology is Pre-Cambrian metamorphic Moinian granulitic and pelitic schists and Lewisian gneisses. The rock strata have been affected by the Moine thrust which outcrops close to Stromeferry Pier and then runs parallel to the loch shore.

This thrust and associated large scale folding have lead to the inversion of strata. The younger Moine schists occupy the lower parts of the slopes with the older Lewisian gneiss inlier forming the upper parts.

The rock mass contains localised, complex and highly variable patterns of jointing from the large scale and parasitic folds.

Tight to isoclinal folds have sheared parallel to their hinge planes, creating shear zones.

These factors, as well as the excavation process, have caused ongoing instability.

While the road was being built a large landslide blocked the Kyle of Lochalsh to Inverness railway line for five months. As a result an 80m long concrete avalanche shelter was built to protect the railway and the new road.

Although the A890 only carries about 2000 vehicles a day, it is the only north-south link on the west coast north of Kyle of Lochalsh.

If the bypass is blocked, the alternative is a 233km long drive via Inverness or Beauly.

Since the road was finished there have been frequent minor rock falls and several larger ones of tens to hundreds of tonnes.

The Highland Council requires annual inspection of the area.

In 1996, TRL Scotland undertook a risk assessment of the slopes and a risk-based maintenance management strategy was developed. Two phases of remedial works have been undertaken to bring the slopes into a manageable condition. The second phase, completed in August 2002, was a partnering contract between client/resident engineer the Highland Council, designer and technical adviser Edge Consultants and contractor CAN Geotechnical.

The works were designed to minimise any adverse effects on a local geological site of special scientific interest and to take into account the future maintenance and management of the Stromeferry Bypass.

Failures that would not undermine larger overlying areas were dealt with by scaling or controlled removal. The control and containment element of the scheme involved remediation of areas of extensive ravelling failures using draped rock fall netting, locally reinforced with wire ropes and installing rock fall catch fences on slopes below cliffs and across gullies prone to rock falls.

Finally, the slopes were strengthened using bolting and dowelling of specific plane, wedge and toppling failures where removal was impracticable. A masonry-clad concrete toe buttress was built to provide support for large undercut wedge failures at the base of one rock slope.

Because of the single track design of the road, the height of the slopes and the restricted access to the slope crests, most of the work had to be carried out by rope access techniques. As road closures were generally limited to 15 to 20 minutes, these works had to be carefully programmed.

Proximity of the railway meant temporary protection measures had to be used to prevent rock material affecting the track.

With the completion of the remedial works, the slopes are now being managed by monthly and annual inspections, which form the final element of the maintenance management strategy being implemented by the Highland Council and Edge Consultants. On-going maintenance will be required along with remedial works for failures that develop due to deterioration of the rock slopes.

Heavy autumn and winter rain can lead to debris flows within the major gullies. The most significant of these was a 400m 3 debris flow that blocked the road and the railway during October 2001 (GE February 2002).

Remedial measures involved construction of 100m of slope crest cut-off drain, a collection sump and outflow pipe to the base of the slope, and erosion protection for the failure scar. Because of the scale of the works and the difficult access, a helicopter was used to airlift a 1t excavator and materials to the crest of the slope, 100m above the road level.

Ian Nettleton is principal geotechnical engineer for Edge Consultants.

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