Geo-Engineering In North Wales

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Geo-Engineering In North Wales

• A Study of the A55 Between Chester and Anglesey

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Introduction

• A55 main route into North Wales and Ireland
• Length approx. 145km
• Initially single lane highway following the
  railway along the coastline
• Expansion/bypass schemes created to ease
  congestion
• Schemes constructed between 1969 2001

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Schemes of Note

• Rhuallt Hill
• Slope stabilisation due to landslip
• Conwy
• First UK immersed tube tunnel
• Llan Glyn
• Settlement due to sink holes and abandoned mines
• Penmaenbach

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A55 Route Map
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Penmaenbach Tunnels
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Penmaenbach TunnelsGeological Context

• Rock Name and Type
• Rhyolite (Acidic Igneous Intrusion)
• Strength
• 250MPa (Strong to very strong)
• Orientation
• Intrusion of Rhyolite into existing extrusive
  Rhyolite flows
• Condition
• Strong rock with good engineering properties
• Loose scree deposits on slopes above

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Penmaenbach TunnelsPossible Solutions

• Build road elsewhere
• Where?
• Build around headland
• Large cost, impractical
• Build cutting through headland
• Large associated cost
• Environmental issues
• Tunnel
• Very hard rock

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Penmaenbach TunnelsChosen Solution - Tunnel

• 660m long, 12m diameter tunnel to accommodate
  west-bound traffic
• East-bound traffic uses existing headland tunnel
  and old road
• Tunnel driven by drilling and blasting through
  the Rhyolite volcanic rock

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Penmaenbach TunnelsProblems Encountered

• Rockfall danger
• during and post construction
• Slope instability
• anchorage required to stabilise vertical rock
  slopes during blasting
• Permeability
• water flow aided by joints
• water flow through tunnel lining
• Varied rock types
• Eastern end alluvial deposits slowed construction

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Penmaenbach Tunnels
Construction 1 - Rockfall

• Rockfall protection measures installed prior to
  construction to protect workers and safeguard road

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Penmaenbach TunnelsConstruction 2 - Rock face
stabilisation
West portal vertical face supported by permanent
rock anchors during and after blasting
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Penmaenbach TunnelsConstruction 3 - Tunnel lining

• Tunnel lined with unreinforced permanent concrete
  lining, 0.25m thick
• Minimal structural support
• Prevent seepage
• East and West portals supported by steel rims

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Penmaenbach TunnelsConclusion

• Extreme environment
• Many engineering challenges
• Geology
• Existing infrastructure
• Wide range of solutions
• Used in combination to achieve best result