Phase I and Phase II Ground Investigations

To facilitate the recent advancement in renewable energy generation across Scotland and increase load capacity, constructing a 275kV overhead line is proposed to replace the existing 132kV route from Inveraray to Crossaig substation.

CGL was appointed by Balfour Beatty Power Transmission and Distribution (BBPTD) to undertake an intrusive ground investigation at each of the 271 transmission tower locations by combining percussive and rotary drilling techniques. Groundwater monitoring wells were installed at selected tower locations. The position of borehole locations required extensive enabling works to achieve access to all proposed transmission tower locations, accomplished by CGL’s close working relationship with the partnering subcontractor and liaison with several SSEN subcontractors.

CGL co-ordinated the ground investigation phases to support BBPTD in designing transmission towers and the associated foundations. The intrusive ground investigation spanned three years across four distinct phases, totalling over 300 boreholes, including drilling for proposed quarries along the route. Given the number of transmission tower locations required, CGL deployed and managed up to two rigs concurrently throughout the project.

The proposed OHL route spans approximately 70km across heavily forested land or areas of rugged open hillside. Steep slope gradients and high density of trees led to challenging access for rigs and ancillary equipment and difficult working conditions, often involving steep slopes, remote areas, peat bogs, navigating SSEN forestry subcontractor ‘brash’ mats and inclement weather.

In order to combat the challenging access conditions, low ground bearing tracked rigs and plant were mobilised with assistance from 8T excavators to ensure access routes were navigated safely by the site team. CGL’s experience, innovation and strong relationship with the drilling team have developed the capability to access the most extreme locations – exemplified by access conditions on this project.

CGL managed this project throughout the investigation phases, meeting required timescales with each stage and delivered under budget to BBPTD. The extensive network of Engineers within CGL is highlighted by the cross-office working utilised to deliver this challenging project to a high quality, on-time and under budget.