Saltford Cutting

Sometimes it takes a failure to provide stability, which is true of the Saltford cutting near Bath.

As odd as this first seems, one of the most stable parts of this earthworks, and the area that needs least attention, is a depression resulting from previous slope movement. This scooped out section was the site of a failure that took place about 100 years ago, and having moved once it is not as prone to shifting as nearby parts.

Subcontractor Ritchies, part of Edmund Nuttall, is busy clambering over the downside slope using rope access and drilling rigs to stabilise the rest of the face. It has a history at the site, having also carried out the upside stabilisation about five years ago. The firm is using Tecco mesh, manufactured by Swiss company Geobrugg, which it represents in the UK. This is a high tensile active system stressed over the face. Underneath is Maccaferri's Enkamat that prevents erosion and helps vegetation take hold.

The strata on this side of the 483m long cutting is Fuller's Earth clays in the upper third of the embankment height, overlying a band of inferior Oolitic limestone. The limestone is visible in small vertical outcrops at about two thirds of the way up the cutting, which in turn overlays lower Lias clays.

Past slips have formed in the weaker, near surface superficial materials. Recent slip scars revealed typical failure depths of between 1.5m and 2.5m with the main mode of failure seeming to be mainly non-circular slab slides.

The plane of weakness or separation appears to be the interface between the superficial, saturated and often weakly cemented material that overlies the more competent underlying mudstone. This plane of weakness is often saturated, assisting in mobilising the failures.

To prevent further problems, the ground anchors going in will secure the face with lengths of 5.5m to 7.5m, depending on the depths of superficial weathered material at the top.

Ritchies general foreman Chris Motion describes the installation process: "The anchors go in with down the hole hammer rigs using an air mist flush to dampen the dust, and we add weep holes at the same time. Then we dig out scallops, which are depressions at the anchor heads that aid mesh tensioning.

"The first layer of Enkamat goes on the bottom third of the slope to cover the weep hole section then a second layer goes top to bottom with the Tecco mesh over the top. Then spiked plates are locked off at 30kN before we add boundary cables at the top, sides and bottom. These spread the load in case of failure across several anchors, rather than just the nearest two."

The £2M stabilisation project began in February and should finish next month.

Article courtesy of Ground Engineering (August 2007)

Further Info

For further information, contact Peter Bishop, Head of Public Relations at:
Edmund Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax:01276 66060
E-Mail: peter.bishop@edmund-nuttall.co.uk