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SEVENTIES SURVIVOR

SLOPE ENGINEERING

A high tensile steel wire mesh, new to the UK, has strengthened a 1970s retaining wall on the A38 in Cornwall.

A 1970s retaining wall on a key route to the West Country was repaired to a tight schedule to avoid disrupting Christmas holiday traffic.

The dual carriageway section of the A38 was built in 1976 to bypass Liskeard in Cornwall. During construction a deep rock cutting was excavated through Devonian slates, supported by a retaining wall formed using concrete kingposts secured by ground anchors.

The kingposts were placed at 3m intervals with precast concrete panels spanning the gaps. The annulus behind them was back'lled with loosely compacted granular fill.

Designed by Freeman Fox & Partners, this solution has stood the test of time. However, routine monitoring revealed that while the anchored concrete posts were sound, some of the panels were showing signs of localised weakness.

To ensure there was no risk of concrete spalling on to the carriageway, the Highways Agency asked its consulting engineer Parsons Brinckerhoff (PB) for a solution that could be installed with minimal disruption to traffic.

PB's answer was to encase the wall in a strong rock-netting system supported by rock nails drilled through each panel and fixed into the slate behind. A flexible yet structurally strong mesh with a wire strength of 1770N/mm 2 was needed.

Dean & Dyball won the contract, worth just under £12M. It worked with its regular subcontractor, Saxton Drilling, and consultant Applied Geotechnical Engineering (AGE).

AGE director Mike Turner says: 'The design and construction of the retaining wall was driven by the need to provide continuous support to the cutting as it was excavated, because the dip of cleavage meant there was a very real risk of the rock mass sliding into the cutting.' It was particularly important to prevent this as there is a hospital building on the slope crest of the steep-sided cutting.

'Given this, top-down construction was specified by the engineer.

The slope was excavated in 3m deep benches, and the cut face supported by the anchored soldier king posts before the next bench was excavated, ' Turner says.

'The panels are not, therefore, directly supported by the original rock anchors. The [current] scheme is designed to provide the additional support to these panels that is now deemed necessary.' Dean & Dyball proposed a product new to the UK market. Saxton Drilling and AGE both work regularly with geosynthetics manufacturer Maccaferri and the firm invited them to pioneer a new high strength rockfall netting product.

Developed in Italy and already used across Europe and the USA, Steelgrid is a woven geocomposite steel mesh reinforced with 8mm high tensile steel cables.

Designed for uses which demand greater strength than that of traditional rockfall protection netting, Steelgrid can be used with soil nails in slope stabilisation applications.

Maccaferri says it is also appropriate for projects where traditional rockfall netting requires reinforcement with cables.

Manufactured from double twisted wire mesh, galvanised with Galfan (a zinc and aluminium alloy), high tensile steel cables are woven into the mesh during manufacture.

Available with Mono (Steelgrid M) or bi-oriented (Steelgrid B) cable reinforcement, vertical cables are used in place of the conventional selvedge wires and are also inserted longitudinally in the woven mesh.

Where structural strength is required in two dimensions (bi-oriented), transverse cables are inserted through the twists within the mesh and secured around the edge cables with aluminium connections.

Standard products have the longitudinal cable inserted at 1.5m spacing, but this can be customised, avoiding redundancy or overspecification in materials, the firm says.

Inevitably when using a new product for a new application, there were a number of minor issues with fixings.

'We were a little apprehensive, ' says Saxton Drilling managing director Andrew Thomas, 'but there was nothing that couldn't be sorted out in site discussions. We had excellent support from AGE, particularly in adapting fixings to the ground conditions.' Work began in September, reducing the A38 to one lane. It reopened on 16 December, in good time for Christmas traffic.

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