geo-engineering in north wales a study of the a55 between chester and anglesey
TRANSCRIPT
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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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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
Penmaenbach Tunnels
Geological Context
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Penmaenbach Tunnels
Possible 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 Tunnels
Chosen 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 Tunnels
Problems 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
Rockfall protection measures installed prior to construction to protect workers and safeguard road
Construction 1
- Rockfall
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Penmaenbach Tunnels
Construction 2 - Rock face stabilisation
West portal vertical face supported by permanent rock anchors during and after blasting
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Penmaenbach Tunnels
Construction 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 Tunnels
Conclusion
Extreme environment Many engineering challenges
Geology Existing infrastructure
Wide range of solutions Used in combination to achieve best
result