Geo-Engineering In North Wales

A Study of the A55 Between Chester and Anglesey

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

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

A55 Route Map

Penmaenbach Tunnels

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

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

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

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

Penmaenbach Tunnels

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

Construction 1

- Rockfall

Penmaenbach Tunnels

Construction 2 - Rock face stabilisation

West portal vertical face supported by permanent rock anchors during and after blasting

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

Penmaenbach Tunnels

Conclusion

Extreme environment Many engineering challenges

Geology Existing infrastructure

Wide range of solutions Used in combination to achieve best

result