Eryri
As part of the Visual Impact Provision project in Eryri National Park, the removal of 10 electricity pylons and three kilometres of overhead powerlines required the construction of two shafts and a 3.4 km tunnel to carry new electricity cables under the Dwyryd Estuary. VolkerGround Engineering, contracted by HOCHTIEF UK Construction Ltd, was tasked to build a launch chamber and headwall to enable this construction.
The initial scope involved installing 2,794m2 of steel sheet piles and nine rotary bored concrete piles for a secant headwall. However, during the installation, the team encountered difficult drilling conditions caused by a combination of sand and high water pressure above the rock. A further site investigation revealed layers of sand over very hard mudstone, requiring immediate reassessment of the design and construction plan.
To address these challenges, the project scope was revised through close cooperation with HOCHTIEF and their design consultant. The scope was expanded to include additional rotary bored piles and the use of polymer drilling fluids, to stabilise piles and prevent contamination of the concrete from the sand.
The ABI Delmag RH34 drilling rig, equipped with a 1,180mm diameter segmental casing and 1,080mm tooling kit, was mobilised alongside the TM22 Leader Rig and Soilmec SR95 concrete piling rig, to manage the new requirements.
The launch chamber was redesigned and segmented to be treated as two distinct sections:
Box 1: Constructed using steel sheet pile.
Box 2: Built as a concrete secant wall.
Soft-Eye piles: Two soft-eye piles were added to the headwall, designed with GFRP-reinforced cages, to allow for the tunnel boring machine (TBM) to penetrate without clashing with reinforcement.
VolkerGround Engineering successfully adapted to the unexpected ground conditions, completing the revised construction scope while maintaining project timelines and environmental sensitivity. Key achievements included:
- Effective stabilisation of challenging ground conditions using polymer drilling fluids, proving their suitability in environmentally sensitive settings.
- Seamless integration of advanced equipment and innovative design adjustments, ensuring structural integrity.
- Completion of the launch chamber and headwall, enabling the continuation of the tunnel construction under the Dwyryd Estuary.
This case study highlights VolkerGround Engineering’s ability to manage complex projects and overcome unforeseen technical challenges, ensuring successful project delivery while safeguarding the environment.
