Four Great Highways Llangollen: Active Travel Scheme Engineering Design
Introduction:
This new project aims to promote and enhance the four great highways of Llangollen, by providing landscape and engineering improvements works to enhance access, biodiversity and visibility to and interpretation of the Llangollen Canal and World Heritage Site, the former Ruabon to Barmouth railway line, the River Dee and Thomas Telford’s Historic Route: the London to Holyhead Road (A5). The aim is to improve the resident and visitor experience and encourage people to spend more time here.
We scoped and successfully delivered a desk study and ground investigation which ensured the rapid design and build programme could be achieved. A focused investigation strategy was developed in association with the designers and meetings throughout the works ensured emerging conditions were understood and technical requirements were met.
Details:
The Four Great Highways active travel route aims to provide landscape and engineering improvements to enhance access, biodiversity and visibility to the UNESCO world heritage designated canal, the historic railway and the River Dee. This included stepped and ramped access on sloped terrain where existing sensitive retaining and earthworks structures were present and preserving existing stability was key. Our investigation was separated into two main areas: The Wharf (adjacent to Llangollen canal) and Lower Dee Mill Park.
Access was limited and maximising the use of available desk-based information was vital to reduce uncertainty and improve understand of the investigation findings. Review of historical mapping and photography was important in developing an initial understanding of an evolving and variable conceptual model influenced by anthropogenic activity and historic fluvial/glaciofluvial processes.
The initial Conceptual Ground Model was then tested and proven through focused investigation, using a range of mechanical (windowless and window sampling) and hand techniques to maximise coverage, particularly at structure locations, and to limit disruption to the site and public.
Proving the variable Elwy Formation bedrock profile across the Lower Dee Mill Park site was critical to inform the foundation design for the proposed access structures. Acquisition of confidential historic BGS boreholes and outcrop mapping along the River Dee supplemented this and allowed hybrid solutions to be proposed consisting of both piling and traditional pad foundations.
The development and structural design in both areas needed to consider potential stability implications on existing highways and other infrastructure (e.g. canal and retaining structures). Advice in this regard was provided, and supplementary investigation/site inspection recommended during the construction phase to reduce any outstanding uncertainty where it remained.