EAST CLIFF SLOPES ENGINEERING ISSUES BACKGROUND The majority of - PowerPoint PPT Presentation

EAST CLIFF SLOPES ENGINEERING ISSUES

BACKGROUND • The majority of coastal slopes were stabilised in major sea defence contracts 1968-1970 and 1974. • Prior to that time the cliff was constantly being eroded by the sea with large landslides occurring from time to time. • Large landslides at Miramar, Queens Avenue and Beacon Hill and Reculver Drive.

MAJOR LANDSLIDES WITHIN THE PROPOSED TOWN & VILLAGE GREEN AREA

Queens Avenue 12 th March 1950

Miramar landslide January 1953

HOW WERE THE CLIFFS STABILISED ? upper slope profile 1 in 3 original slope profile cut lower slope profile slip surface slope 1 in 7 fill London Clay sub-artesian water pressure Oldhaven Sandstone

HOW WERE THE CLIFFS STABILISED ? Every square metre of the slope has been modified and engineered in some way or other by: • Re-profiling the cliff to a flatter gradient i.e. cut and fill • Herringbone shallow drainage systems • Deep drainage systems • Retaining walls Reinforced concrete Mass gravel • Toe weighting

RE-GRADING AND DRAINAGE WORKS Re-grading and drainage works The Downs, circa 1974

HERRINGBONE SHALLOW DRAINAGE SYSTEMS

DEEP DRAINAGE SYSTEMS Vertical drainage (sandwick drains)

DEEP DRAINAGE SYSTEMS Deep shafts and spider drains

RETAINING WALLS Retaining wall at Cliff Cottage and Reculver Hatch

LARGE GRAVITY GRAVEL RETAINING WALLS Located below Reculver Drive

LARGE GRAVITY GRAVEL RETAINING WALLS

TOE WEIGHTING Toe weighting provides sufficient weight to resist rotation of a landslide:

TOE WEIGHTING Toe weighting for Miramar toe slide 1-3 tonne granite rock over 3 groyne bays

WHAT HAPPENS IF WE DO NOT MAINTAIN THE SLOPES? • Shallow land slides will occur in the steep upper 15m. • The deep seated land slides at Miramar, Beacon Hill and Queens Avenue will become re-activated. • Loss of land at the cliff top affecting property and infrastructure. • The sea wall will be breached by the land sliding.

CONSULTANTS VIEW OF THE FUTURE In 1971 Ove Arup and Partners undertook a review of the then recent works: • They reported that the slopes have useful life of 40 to 50 years (from 1971). • They recommended Planning restrictions for development near to the cliff top. As a result, the former Herne Bay Urban District Council applied restrictions, which Canterbury continue to do today.

WORKS WILL BE NEEDED IN THE FUTURE AND MAY CONSIST OF THE FOLLOWING: • Possible localised re-grading of the steep upper 15m of slope to a gradient of 1 in 4 where required • Ground anchors • Replacement of slope drainage

REGRADING & PLANNING RESTRICTIONS – COASTAL PROTECTION ZONE Extent of Coastal Protection Zone X 15m buffer zone line of possible future re-grading The landward extent of the Coastal Protection zone area is based upon re-grading to a safe gradient of 1 in 4 plus a 15m buffer zone

GROUND ANCHORS • Ground anchors literally anchor the slope back to good ground beyond any potential slip surfaces. • Likely to be used in situations where we cannot re-grade because of the proximity of property.

FINALLY OUR CONCERNS • Town & Village Green status will prevent us from carrying out any works at all in the future. This is a major concern as the Council are the landowner and neighbour to many cliff top properties. • We have a duty of care to take reasonable steps to prevent the effects of instability on our land affecting our neighbours. • How will all the paths, steps, currently enjoyed by walkers and necessary to access the beach be maintained if we are not allowed to carry out works? • Without maintenance these benign looking grassy slopes will revert to the muddy quagmire of former times as shown on the previous slides.