The Dorset Visual Impact Provision replaced 22 pylons and 9km of overhead electricity lines with a new underground cable system to enhance the Dorset Area of Outstanding Natural Beauty.
This project was the first of several Visual Impact Provision schemes, which make use of a £465m provision by Ofgem to carry out work to reduce the impact of existing transmission lines in English and Welsh Areas of Outstanding Natural Beauty (AONBs) and National Parks.
The cables were laid in four 1.4m deep and 2.2m wide trenches, each containing three 250mm cable ducts and backfilled with Cement Bound Sand (CBS). As this was the first of the VIP schemes to be delivered, the team encountered several technical challenges that required innovative solutions and extensive pre-construction trials to overcome.
Part of the trench route ran through the Dorset Ridgeway, which drops 60m in level over a 1:1 gradient, making the area inoperable for conventional plant. The team used a Menzi Spider Excavator to construct this section of the haul road, which became known on site as the ‘Alpine Pass’, reducing the gradient to 16%, which then enabled conventional plant to be used to dig the trenches.
Electric cables buried underground generate heat. Consistent temperatures above the design temperature shorten cable life and can cause catastrophic failures. It was crucial, therefore, for the team to understand the thermal resistivity of the ground (i.e. how well it could dissipate heat) to ensure that the cables were lined and backfilled with the correct mix of CBS, before construction began.
Soil testing carried out in the preconstruction phase revealed that parts of the cable route had exceptionally high thermal resistivity, so to mitigate this, the team conducted extensive offsite trials of different mixes of CBS to ensure the optimal ratio of sand, cement and water was used to achieve the required resistivity levels and voids ratio.
Whilst trench digging is a conventional practice, doing so to highly prescribed measurements, over many kilometres of distance at a consistent rate of 100m per day represented a unique technical challenge.
Certainty over the depths and levels of the trenches was crucial, along with an accurate as-built record of the 9km route, so GPS machine controls were installed on all plant to ensure accuracy and to produce a BIM-style record of the completed trenches.
To minimise plant/person interface in the trenches, the team also designed and built two bespoke cable laying trailers. The trailers were fitted with automatic ‘conveyor-belt’ style offloading mechanisms to lay the ducts, alongside a bespoke levelling box and sledge mechanism which spread the CBS to the required depth depending on the thermal resistivity of the surrounding soil. The trailers removed the need for personnel to enter the trenches and were subjected to extensive off-site trails before being deployed on site.
