SLIDE 1
EAST CLIFF SLOPES
ENGINEERING ISSUES
SLIDE 2 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.
SLIDE 3
MAJOR LANDSLIDES WITHIN THE PROPOSED TOWN & VILLAGE GREEN AREA
SLIDE 4
Queens Avenue 12th March 1950
SLIDE 5
Miramar landslide January 1953
SLIDE 6 London Clay Oldhaven Sandstone sub-artesian water pressure slip surface lower slope profile slope 1 in 7 upper slope profile 1 in 3 cut fill
HOW WERE THE CLIFFS STABILISED ?
SLIDE 7 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
SLIDE 8
Re-grading and drainage works The Downs, circa 1974 RE-GRADING AND DRAINAGE WORKS
SLIDE 9
HERRINGBONE SHALLOW DRAINAGE SYSTEMS
SLIDE 10
DEEP DRAINAGE SYSTEMS
Vertical drainage (sandwick drains)
SLIDE 11
DEEP DRAINAGE SYSTEMS
Deep shafts and spider drains
SLIDE 12 RETAINING WALLS
Retaining wall at Cliff Cottage and Reculver Hatch
SLIDE 13 LARGE GRAVITY GRAVEL RETAINING WALLS
Located below Reculver Drive
SLIDE 14
LARGE GRAVITY GRAVEL RETAINING WALLS
SLIDE 15
TOE WEIGHTING Toe weighting provides sufficient weight to resist rotation of a landslide:
SLIDE 16 TOE WEIGHTING
Toe weighting for Miramar toe slide 1-3 tonne granite rock over 3 groyne bays
SLIDE 17 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.
SLIDE 18 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.
SLIDE 19 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
SLIDE 20 REGRADING & PLANNING RESTRICTIONS – COASTAL PROTECTION ZONE
line of possible future re-grading 15m buffer zone
X
Extent of Coastal Protection Zone
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
SLIDE 21 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.
SLIDE 22 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.
