The challenge
In 2012, Ofgem announced plans for a £500million fund to assist GB Transmission Owners in mitigating the visual impact of existing electricity infrastructure on nationally designated landscapes. Project VISTA (Visual Impact of Scottish Transmission Asset) is to play a key part in enhancing these designated landscapes within the north of Scotland transmission network.
As part of Project VISTA, Principal Contractor Morgan Sindall Infrastructure is engaged on a contract for SSEN Transmission to remove two overhead power lines from Killin to Inverarnan and St Fillans substations. The overhead power lines will be replaced with new underground cables buried in uPVC ducting, installed by open cut trenching technique and by horizontal directional drilling (HDD) under the River Dochart. Following energisation a total of 31 redundant steel towers will be dismantled and removed.
Given the rural nature of the project and the resulting site constraints, part of these works is being facilitated through the installation of our 45-metre-long temporary bridge.
Our solution
Constructed using the modular Mabey Universal bridging kit, the bridge has been designed and built to divert heavy construction traffic away from the village of Killin in the Loch Lomond and The Trossachs National Park, avoiding a narrow historic bridge which attracts thousands of tourists to the area.
We were initially approached by Morgan Sindall Infrastructure Scotland and its consultant, Baker Hicks, two years ago at the early planning stages of the work, when it was seeking competitive bids for the supply of a heavy duty access bridge to carry construction traffic up to a 55 tonne weight limit - including cable drum trailers and tractors - crossing the River Dochart.
As part of the liaison process involving the National Park as well as SSEN Transmission, we were able to illustrate the proposed installation process using time-lapse video footage from previous projects.
With abutments already built by Morgan Sindall to the designs of Baker Hicks – a Morgan Sindall group company - using a combination of reinforced concrete and Legato blocks, engineers took just three weeks to assemble the modular Mabey Universal Bridge sections and launch them using a 3600 excavator already on site. The completed 45 metre span bridge features an extra wide 4500mm clear road width and is one of the largest such structures deployed in the UK over recent years.
Our temporary bridge is expected to be in place for 62 weeks. The current work in progress at Killin is the third phase of VISTA projects Morgan Sindall Infrastructure has undertaken, with 15km of transmission lines already moved underground in the Cairngorms and three 1500m long stretches addressed near Sloy.
