Stabilising Sterling
BAM Ritchies was contracted by Forth Electrical Services to carry out ground stabilisation works in Sterling, Scotland. During routine scaling and vegetation clearance works, Forth had noticed that the steep, rock slope behind one of its offices and car parks had become weakened, and was now in a critical condition.

BAM Ritchies suggested the use of 1,500m2 of high-tensile TECCO mesh in conjunction with 250 profile and spot rock bolts to provide an active support on two of the most friable faces.

The mesh was installed using rope access techniques and a skid-mounted Ripamonte drill rig fitted the rock bolts. Rock debris from the site was used to fill gabion baskets, which were installed at the base of the slope, helping to further stabilise the rock mass, save money on removal costs and reduce the environmental impact of the project. After further assessment, it was decided that extra barriers were needed to fully mitigate the risk of falling rock.

Although flexible rockfall barriers have been available for a number of years (with the Swiss BAFU guidelines denoting best practice), the publication of the European Organisation for Technical Approval (EOTA) guidelines, ETAG 027, in February 2008, has led to the design and development of barriers based on the European standard.

Following detailed rockfall simulations for several sections of the slope, BAM Ritchies chose a Geobrugg GBE-500-AR barrier for 500 kJ, which fully complies with ETAG 027 and carries CE markings. Three sections were needed, each 100m in length and 3m in height.

As the barrier had no up-slope restraining ropes, the layout had a minimal environmental impact on the surrounding wooded area. Corrosion protection on the barrier will also enable many years of maintenance-free service. For each foundation to the barrier posts (spaced 10m apart), two ground anchors of 12m and 8m in length were installed with 1,200 concrete capping blocks measuring 600x900mm.

A Boart Longyear Deltabase 100 tracked-drill rig with top-drive percussion equipment and water flush was used to install them. The base anchors were 35mm-diameter Dywidag Systems GEWI+ threaded bars, which were secured using OPC (ordinary Portland cement) grout. Lateral support anchors consisted of 28mm GEWI threaded bars with OPC grout.

The concrete bases were cast in-situ immediately after the installation of the base anchors. Once the concrete had achieved significant strength, the installation of the fence was straightforward.

Stuart Jackman, site agent for BAM Ritchies, said: "While it took some time to install the anchors owing to their depth in difficult ground conditions, the barrier installation was remarkably quick!"

Article courtesy of GeoDrilling International - October 2010