Army Manoeuvres
Work is under way to stabilise a slope running alongside a road used by the MoD in the west of Scotland.

A Road in mid-Argyll used by the Ministry of Defence for critical services is undergoing repairs to its upslope to prevent debris falling on to the road. Rock cuttings, up to 14m in height, exist on the upslope side of the road, which is approxi mately 4km in length.

Over recent years several small slope failures have occurred on the cuttings adjacent to the road. The developer, Turner Estate Solutions, commissioned consultant URS to design a solution to improve the level of safety along the entire length of the road.

Investigation, including preliminary emergency remedial work, was undertaken between March 2007 and March 2008, by URS' geotechnical and soil and groundwater technical director Ermanno Rambelli.

Its aim was to fully assess the risk of further slope failures and to give an initial assessment of the extent of potential work required. "Due to the nature of the rock, the gradient of the hillside and the orientation of discontinuities within the rock mass, extensive sections of the road cutting were unstable and prone to failure," says Rambelli. "For ease of management we have broken down the 4km long road into 26 smaller sections and prioritised the remedial works on the basis of the likelihood of failure and associated risk."

As part of these initial works specialist geotechnical contractor BAM Ritchies undertook de-vegetation, investigation and preliminary emergency scaling work on a critical, high-priority slope.

In October 2008 URS completed the design of the remedial works on five sections of the road considered to have the highest priority for remediation. As part of these works, detailed geostructural analysis of the rock mass was carried out to assess the stability of the rock cuttings within the top priority sections.

The design solution, which, in principle, will be common to all the slopes to be stabilised, included de-vegetation, slope re-profiling, scaling of loose and/or fractured rock, rock bolting and the installation of permanent stabilisation systems.

The systems comprise rock anchors of length and diameter suitable to local conditions, active- tensioned facing mesh and, where required, erosion-control matting under the facing mesh for soil retention leading to vegetation development.

To provide value for money, the design revolves around galvanised GEWI bars provided by Dywidag and high-strength TECCO-facing mesh and TECMAT erosion-control matting supplied by Geobrugg.

In November 2008 BAM Ritchies commenced work on the full stabilisation works at the first of the highest priority sections. These works have been carried out under the supervision of URS' Geotechnical Department.

In this section, the works comprised the full de-vegetation of the rock face, followed by heavy scaling using excavators and rope access personnel. Some 3,500m3 of material has been removed and the slope re-profiled accordingly. Following the re-profiling of the slope an active remediation system has been installed to secure the rock face and contain further rock slips/falls.

Within this first section, 233 rock anchors have been installed using a combination of a drilling mast on a Merlo telehandler and rope-supported rigs. These anchors consist of 25mm GEWI and 30mm GEWI Plus that are galvanised for enhanced corrosion protection.

Grout socks
Owing to the fractured nature of the Schist rock many of the anchors have been grouted-in using grout socks to ensure that the grout is not lost and adequately secures the bolt in the ground. Approximately 5% of the rock anchors are pull-out tested to ensure that the design loadings are achieved by the rock anchors.

"The slope face has been found to be heavily fractured, even at depth, after the slope re-profiling," says BAM Ritchies' senior geotechnical engineer Stuart Jackman.

"The use of the grout socks is necessary to ensure no grout wastage and to bond the rock bolts into the rock. The rock bolt testing programme has confirmed the adequacies of this method."

For this section 2,730m2 TECCO mesh has been installed, with a 450m2 area of TECMAT installed for localised erosion control.

The mesh is secured to the rock anchors by the system "Spikeplates", that ensures proper transmission of the surface loads from the actively- tensioned mesh to the rock anchors. Boundary ropes are used around the periphery of the treatment area and pre-filled gabion baskets, provided by Maccaferri, placed along the road side as a barrier between the road and the stabilised slope.

This first phase of work has recently been completed. Work on the next four priority sections of the road commenced on 26 January. Despite very cold weather at the end of January and throughout early February the contractor is making good progress and this second phase of works is due for completion at Easter. The value of the first two phases of work is about £900,000.

URS is designing remediation solutions for the next eight sections of the road, with design of the remaining sections to follow. Remediation works at these additional sections will be awarded and started as the design is finalised and budgets approved.

Planning and preparation for further phases of work is under way and it is hoped that this work will commence in the new financial year for completion by April 2010. These further phases of work will more than double the spend to date.

Article courtesy of Ground Engineering - april 2009


Further Info
Peter Bishop - Head of Public Relations & Corporate Communication
BAM Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax: 01276 66060
peter.bishop@bamnuttall.co.uk