Steelgrid prevents concrete spalling onto A38 at Liskeard

22/12/2005

The A38 is a key holiday route to the West Country, carrying very high volumes of traffic, particularly during peak holiday periods.  In 1976, a new by-pass was built around Liskeard, in Cornwall, to alleviate congestion in the town centre.  During construction of the new dual carriageway, a deep rock cutting was excavated through Devonian slates, supported by a retaining wall structure formed utilising concrete kingposts secured by ground anchors.  It was important to preserve the existing steep-sided cutting to support an existing hospital building on the crest of the slope.

The king-posts were spaced at three metre intervals, with pre-cast concrete panels spanning the intervening gap between adjacent king-posts.  The space between each panel and the excavated rock face was back-filled with loosely compacted granular fill.

This solution, designed by Freeman Fox & Partners, has stood the test of time.  However, routine monitoring of the concrete revealed that although the anchored concrete posts were sound, some of the panels were beginning to show signs of localised weakness.  To ensure there was no risk of concrete spalling onto the highway, The Highways Agency requested its retained consulting engineer Parsons Brinkerhoff to design a cost effective solution that could be installed with minimal disruption to the traffic flow.

Parsons Brinkerhoff completed the preliminary design for an additional support system to the concrete panels of the retaining wall.  This involved encasing the wall within a strong rock-netting system.  This would be supported by rock nails drilled through each panel and fixed into the slate bedrock beyond.  To provide the design strength envisaged by the preliminary design, a flexible, yet structurally strong mesh with a wire strength of 1770 N/mm², was required.

In winning and undertaking the contract, with a value of just under £1/2 million, Exeter-based main contractor Dean & Dyball worked closely with their regular geotechnical sub-contractor, Saxton Drilling Ltd and geotechnical consulting engineers Applied Geotechnical Engineering (AGE).  A number of factors influenced their successful bid and method of undertaking the works, including invaluable first hand knowledge of the site, acquired by AGE Director Mike Turner who had been involved with the original ground anchoring works in 1976.

Commenting on the site history and the original construction, Mike Turner said, "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.  Given this requirement, a 'top down' form of construction was specified by the Engineer, so that the slope was excavated in three metre deep benches, and the cut face supported by the anchored 'soldier' king posts before the next bench was excavated.  These considerations drove the choice of the 'post and panel' retaining wall solution adopted by the original design engineers.

"The panels are not, therefore, directly supported by the original rock anchors.  The current scheme is designed to provide additonal support to these panels now deemed to be necessary."

Dean & Dyball was also unable to offer an innovate solution, proposing a product new to the UK market.  Sub-contractors Saxton and AGE both work regularly with Maccaferri, specialists in retaining structures, soil reinforcement and erosion protection and were invited by the company to pioneer a new high strength rockfall netting product it was about to introduce to the UK.  Developed by Maccaferri in Italy and already used in a number of applications across Europe and the USA, Steelgrid is a woven geocomposite steel mesh reinforced with 8mm high tensile steel cables.

Designed for use in applications which demand greater strength than that offered by traditional rockfall protection netting, Steelgrid can be used with soil nails in slope stabilisation applications.  A versatile product, it is also appropriate for projects where traditional rockfall netting requires reinforcement with cables, combining both elements in a single product, Steelgrid can save significant construction time on the rock face.

Manufactured from double twisted wire mesh, galvanised with Galfan, a Zn-5%A1-MM (mischmetal) alloy, high tensile steel cables are woven into the mesh during manufacture.  Available with the option of 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.  For applications requiring structural strength in two dimensions (bi-oriented), transverse cables are inserted through the twists within the mesh and secured around the edge cables with aluminum suaged connections.

Although standard products are available with longitudinal cable inserted at 1.5m spacing, the location of cables can be customised to meet individual specifications, avoiding any redundancy or over specifcation in materials.

For the A38 at Liskeard, Steelgrid was manufactured to the exact specification.  Dean & Dyball was able to demonstrate to the Highways Agency and Parsons Brinkerhoff that the solution was both innovative and more cost effective than standard high tensile steel mesh products.  Equally as important, Steelgrid meeting the required specification was available from Maccaferri on a very short lead-time, ensuring there were no delays during the construction period.

With a total of 2,200 square metres of Steelgrid to be installed and the challenge of ensuring the mesh was both flexible and structurally strong, AGE, Saxton and Maccaferri worked very closely to resolve any unexpected challenges on-site.  With the job well-defined, and with Mike Turner's inside knowledge of the site, there were no major problems, but there were a number of minor issues with fixings that are inevitable when using a new product for the first time in a new application.

"We were a little apprehensive", acknowledged Andrew Thomas, Managing Director at Saxton Drilling, "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 prevailing ground conditions, and Maccaferri was equally helpful with advice and technical support.  And all deliveries were on-time, taking a major headache out of the construction schedule."

Work commenced on site in September, and with the main A38 carriageway reduced to one lane, there was a very tight deadline.  The job was successfully completed and the road fully open by 16 December well in time for the Christmas holiday traffic.

For further information, contact:

Matt Showan, Sales & Marketing Manager, Maccaferri Ltd, 7400 The Quorum, Oxford Business Park North, Garsington Road, Oxford OX4 2JZ.

Tel: 01865 770555  Fax: 01865 774550

Steve Jones, Account Director, Elixir Marketing Communications Limited, 20 High Street, Theale, Reading, Berks RG7 5AN

Tel: 0118 930 3151  Fax: 0118 930 3252