A55 North Wales Coast Road

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A55 North Wales Coast Road

• Jemma Oakley Hannah Grimley

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Introduction

• 145km dual carriageway from Chester to Holyhead
• Diverse geological terrains-glacial features,
  abandoned mines igneous rock intrusions

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Location Map
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Holywell Bypass

• 9km dual carriageway, completed in 1986
• Two main geo-engineering features
• 1)Cutting through glacial deposits
• 2)Abandoned mines mineral veins

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Holywell BypassGlacial Geology

• 16m deep cutting through sand gravel layer
  which was sandwiched between clayey tills.
• Low permeability of the upper till layer
• High groundwater pressures
• Drainage to lower groundwater pressures

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Holywell BypassAbandoned mines

• 36 of route affected by mining of lead, zinc
  silver ceased in early 20th century
• Abandoned mine-collapse of unsupported mineshafts
  subsidence due to subsurface workings in
  mineral veins

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Distribution of mineral veins along A55 Holywell
Bypass
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Holywell Bypass Mining investigations

• Few records
• Specialist mining consultants
• Historical documents, old mining records maps
• CCTV mine access

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Holywell BypassMining Solutions

• 34 mineshafts capped at rockhead
• Mineworkings mineral veins spanned by
  reinforced concrete rafts

• Continuous annual monitoring

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Pen-y-Clip Tunnel
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Pen-y-Clip Tunnel

• Mountainous headland creating a natural obstacle
• Scheme completed 1994. 1.9km in total
• Tunnel 930m long
• Geologically Igneous intrusion of Upper
  Ordovician age (450 Ma)

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Pen-y-Clip Tunnel
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Pen-y-Clip Tunnel

• Construction
• - very heavily jointed
• - central section shorter than anticipated
• Joints excavated and supported
• Central zone driven and blasted

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Pen-y-Clip Tunnel

• Still concerns about rockfalls
• Rockfall countermeasures

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Conclusion

• 32 yr project
• Encountered many geological problems
• Constant monitoring ensures future safety
• Findings enhance future schemes

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References

• Geo-engineering along the A55 North Wales Coast
  Road. D.Nichol. Quarterly Journal of Engineering
  Geology and Hydrology.
• www.soil.co.uk/AppHTunn.htm
• http//www.nwales-traffic.co.uk/english/road/road_
  users/highways_pages/newtra/hwell_bypass.asp
• http//www.nwales-traffic.co.uk/english/road/road_
  users/highways_pages/newtra/penyclip2.asp