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 completed by our geotechnical engineers.
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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 a supplementary site investigation was recommended during the construction phase to reduce any outstanding uncertainty where it remained.
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