ABSTRACT
The Eryri Visual Impact Provision project in the Eryri National Park, North Wales UK, is promoted by infrastructure owner, National Grid. The aim of the project is to reduce the visual impact of electricity transmission infrastructure in a designated Area of Outstanding Natural Beauty. The existing overhead lines and pylons crossing the Dwyryd Estuary have a significant visual impact on the surrounding landscape which the Eryri VIP project aims to reduce by removing a section of the existing overhead lines and pylons and replacing them with electricity cables housed underground within a 3.3km long, 3.5m internal diameter tunnel.
Construction of the tunnel utilises two shafts, which form permanent access structures, topped by headhouses, which accommodate ventilation and other services.
The Llandecwyn reception shaft is circular in cross-section with a 12.5m internal diameter and is approximately 60m in depth. It is to be constructed using a combination of secant piles and sprayed concrete lining. Secant pile support is utilised within superficial deposits at the top of the shaft with sprayed concrete support provided in Mudstone of the Maentwrog Formation, which underlies the site from a depth of approximately 20m below ground level. Geotechnical investigations carried out within the mudstone found varying rock conditions, including weathered rock, intact rock and very fragmented rock associated with fault zones. Anticipated Geotechnical Strength Index values varied between 25 and 70.
Design of primary ground support provided several support classifications for the sprayed concrete lining to suit these varying rock conditions. For completeness, and to avoid potential delays during construction, designs also considered the possibility of rock conditions worse than those anticipated by the ground investigations.
Support design is sensitive to the convergence, or relaxation, of the rock mass during excavation. When replicating established methods used in previous projects to calculate convergence (i.e. the use of Finite Element analysis under plain strain conditions), it was found the weak rock led to computational challenges, so an alternative analytical approach was proposed and adopted. This paper explores the analytical method used to calculate the convergence of the excavation and therefore to determine loads applied to the support system.
