Lyme Regis Environmental Improvements Phase IV Presentation by Chris Hill CEng FICE

Dean & Dyball Civil Engineering Project Manager

Project Timeline

• Contract award – May 2012 • Start date - September 2012 • Site set up - through September and October 2012 • Detailed design by URS - September 2012 to March 2013 • Ecological works & site investigations - September 2012 to February 2013 • Main construction works - April 2013 to April 2014 • Landscaping - May to August 2014

Contract Details

• NEC3 Option C Design & Construct • Tender based on a well developed reference design • Won convincingly on quality • 116 week programme with planned Completion 29/04/14

Key Construction Stages • Ecological surveys & mitigation • Site clearance • Treatment of invasive plant species • Ground investigations • Installation of new instrumentation • Monitoring • Trench drains • Foreshore haul route • Seawall • Rock armour • Soil nails • Drainage • Dowel piles • Large diameter piling • Landscaping

Ecology & Environment • World Heritage Site • Site of Special Scientific Interest • Special Area of Conservation • Area of Outstanding Natural Beauty • Jurassic Coast designation • Rock ledges • An abundance of protected species including

dormice • Japanese Knotweed • Other controlled plant species • Asbestos • Disused tip • Old gas works

Ecological Surveys & Mitigation

Site Clearance

Trench Drains

Foreshore Haul Route • 5m wide and built from gabions and stone fill • Designed to protect the rock ledges, offer some protection from the tide and to allow

construction vehicles to travel along the foreshore

Seawall • Stops further erosion by preventing waves hitting

the coastal cliff • Recurve profile to deflect waves • Provides a pedestrian walkway, a splash wall,

seating and access for maintenance and emergency vehicles

• Reinforced ready mix concrete – in the order of 7000m3

• Built in the inter-tidal zone • Straight and curved 8m long panels, up to 6m high • Follows the profile of the existing wall • Demands an extremely high standard of

workmanship • To be backfilled with stone from the haul route

Seawall – 60 years ago

Seawall Foundation • The seawall foundation needs to be sufficiently deep to prevent the new wall being

undermined by tidal scour • This means founding the wall on a harder limestone band – not on the weaker mudstone • A lot of work has taken place to identify the level of these harder bands

Rock Armour • 400T of 3-6T rock armour from the Mendips • Designed to protect the eastern end of the new seawall

from further ‘outflanking’ • Placed in position by a large excavator with a grab • Delivered to site early to be used as temporary works

Soil Nails • Approx. 2500 steel bars drilled into the coastal cliff to pin the

unstable material near the surface to deeper more stable material

• Bars are grouted into position, steel mesh is placed on the surface of the slope and bars are tensioned with a cover plate and nut

• Nail heads will be recessed into the slope and covered over prior to landscaping

Soil Nail Installation Methods

Drainage • 20-25m long drilled drains installed just above the new

seawall to intercept ground water and allow it to escape before it destabilises the ground

• Gravity drainage to collect the run off from the trench drains and to replace the existing over-land pipes

• 8m long 300mm diameter dowel piles on East Cliff to enhance the stability of the cliff above the soil nails, protect the new seawall and to ensure that landslips cannot progress inland

• Dowel piles are filled with grout and reinforced with a steel tube

• Up to 25m long 750mm to 1200mm diameter piles below Charmouth Road Car Park and through the allotments to prevent landslips progressing inland but also positioned to allow the SSSI site to be preserved

• Larger piles filled with concrete and reinforced with a steel cage

Piling

Piling Installation Methods

Landscaping • The coastal slopes will be sprayed with a hydroseed

mix to prevent erosion and to quickly establish native grassland

• Planting areas on the flatter parts of the slope will have native scrub planting and deterrent species along property boundaries

• New footpath leading from Charmouth Road Car Park to the seawall

Artwork • Geological timeline

concept under discussion • Stone insets or curb

stones marking the timeline

• QR codes used to provide easy access to web sites

Thank You