1. Geo-Engineering In North Wales A Study of the A55 Between Chester and Anglesey

2. 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

3. 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

4. A55 Route Map

5. Penmaenbach Tunnels

6. 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

7. 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

8. 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

9. 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

10. Penmaenbach Tunnels Rockfall protection measures installed prior to construction to protect workers and safeguard road Construction 1 - Rockfall

11. Penmaenbach Tunnels Construction 2 - Rock face stabilisation West portal vertical face supported by permanent rock anchors during and after blasting

12. 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

13. Penmaenbach Tunnels Conclusion Extreme environment Many engineering challenges Geology Existing infrastructure Wide range of solutions Used in combination to achieve best result