Geotechnical Investigations at a Remote Location for Major Transmission Project

CGL supported this transmission reinforcement project between Beauly substation and Corriemoilie substation near Mossford from 2012 to 2016. The project was to replace two single circuit, 132kV lines with one higher capacity, double circuit 132kV line to enable renewable energy providers to connect to the transmission network.

The overhead power line (OHL) passes through 30km of remote and rugged terrain near Inverness, presenting difficult access conditions at many tower locations. On behalf of Scottish & Southern Electricity Networks (SSE), the project client, Balfour Beatty Utility Solutions (BBUSL) engaged CGL to review 97 tower locations.

CGL initially provided a detailed desk study and a walkover report for three alternative routes, including a risk rating for each tower location and the works needed to enable construction. CGL provided recommendations for targeted ground investigations at appropriate tower locations. CGL carried out these intrusive investigations using wide track rigs and equipment in the most challenging sections to facilitate safe access and to avoid damage to the sensitive environment. Using the data captured an interpretative report was prepared with specific recommendations for tower foundations and temporary works design.

Difficult Access

BBUSL instructed CGL to undertake detailed geomorphological assessments of three slopes along remote mountainsides within the route options. Two experienced engineering geologists spent several days in the area assessing the ground form, slope angles, drainage and the underlying geology to reach appropriate conclusions on the potential risk of instability. At one point both the road and rail access north of the site were closed by a landslide indicating the instability of the local landscape.

Temporary Works Design

The engineering geologists recordings allowed identification of specific risks to be mitigated during construction, including rock fall protection, slope instability and design of earthworks for access tracks cutting through steep mountainous terrains. CGL provided specific advice on upgrading work required to the access roads to enable construction of the new towers.

Value Engineering

Upgrades principally used slope cuttings and gabion baskets to provide sufficient stability of the loose material in the excavated face. Acting on CGL’s advice, BBUSL made significant savings by reducing the number of gabion baskets compared with the original design concept. This reduction was made possible by CGL’s detailed understanding of the potential for slope failures and substitution of gabions by pinned netting in less risky locations.

Verification

On completion of the construction works CGL carried out a walkover to check the access roads were in a suitable condition before handing back to the Forestry Commission, the original landowner.