Barton-On-Sea Instability

Barton-on-Sea Instability

Peter Ferguson NFDC Introduction

•Geological setting •Engineering works •Ground movement & instability •Public safety & engagement •Management & maintenance •Monitoring •Ground investigations •Future schemes •Funding

Location

Geology

• Geology of Hampshire & Isle of Wight

After Woodward 1904 Ian West 2000

Geology

• Stratigraphy

Plateau Gravel Bed L Pleistocene (fluvial deposits) Bed K Bed J Eocene Strata (Barton Sand Formation) Bed I Geology

• Internationally important exposure • English Nature • Maintain exposures • SSSI rating • Unique exposure of Barton Beds

Geology

Translational Slides Engineering Works • 1930 – First Defences • Timber groynes & cliff drainage • 1939 to 1945 – Disrepair • 1950 Works become ineffective • Erosion rate 1-2m year • 1964 – 1968 Major works undertaken • Vertical piled revetment • Re-profiled slope • Sheet pile cut off wall • 1970’s • 5 rock groynes • 1990’s • Timber revetment replaced with rock revetment

Engineering Works

Ground Movement & Instability

N • Landslide data

Naish

Barton-on-Sea

Continuing mass movement

200m Ground Movement & Instability

Cliff House Hotel Naish Farm

Marine Drive W

Barton Court Marine Drive E Ground Movement & Instability

•Cliff House Hotel •Deep -seated failure • Occurred early 2001 • Displaced rock revetment at toe

Cliff House Hotel

Hosseyni et al. (In Press)

Barton & Garvey (2011)

Translational failure with mudslides Shear on base of D and possibly base of F2 Cliff House Hotel

Cliff House Hotel

1st Nov 2012 29th Nov 2012

nd th 2 Jan 2013 14 Feb 2013 Cliff House Hotel

•Terrestrial laser-scan

– Undertaken by CCO • Sept 2012 (baseline) • Nov 2012 (post failure) • March 2013 (stable ground) • July 2013 (works)

Laser-scan data

Marine Drive

•Translational failure with mudslides • Shear on base of F2

Marine Drive

Monitoring

Marine Drive

Shear Stable area surface

Active area

Marine Drive 28/01/13

Marine Drive 30/01/13

Marine Drive 28/01/13

Marine Drive 30/01/13

Marine Drive 02/01/14

Marine Drive 30/01/13

Marine Drive • Marker stones in mobile unit 22/01/13 Day 0 • Set in line with score mark

Score mark in stable unit Marine Drive 24/01/13 Day 2

Score mark in stable unit Marine Drive 28/01/13 Day 6

520mm Marine Drive 30/01/13 Day 8

1410mm

Marine Drive 28/01/13 Day 0

Marine Drive 30/01/13 Day 2

815mm

Marine Drive

Marine Drive From undercliff

looking looking west east start

(Approximate end of visible crack)

Crack continues Into verge start Marine Drive

Public Safety & Engagement

• Emergency Level 1 – Cliff top cracking – Access closure / tape off areas – Regular inspections – Car park closure • Emergency Level 2 – Large scale ground movement not affecting property (but possible to affect) – Warn residents affected. – Escalate up • Emergency Level 3 – Large scale ground movement likely to affect property – Relocate residents etc. of property which might be affected – Inform Emergency services – Environmental health Emergency Level 4 – A serious threat to life / largely unforeseen / no warning – Civil emergency

Public Safety & Engagement

• Did Kent earthquake rupture seafront 150 miles along the coast? • THIS massive cliff crack threatening 50 beach huts was feared yesterday to have been triggered by the Kent earthquake 150 MILES away. • The 1,000ft rupture, 6ft deep and 6in wide, left a rocky outcrop hanging precariously over a beach and could spark a landslide weighing hundreds of tonnes. • In its path are fragile beach huts worth hundreds of thousands of pounds. • Police evacuated and cordoned off the danger zone at Barton on Sea, Hants, as structural engineers, surveyors and fire crews made safety checks. • The Marine and Coastguard Agency was alerted to the crack two hours after Saturday's quake left more than 70 homes in Folkestone in dangerous condition and caused at least £10million damage. • Barton is virtually due west of the epicentre of the quake, under the Channel 7.5 miles south of Dover. • The MCA said: "We don't know if the rupture is linked to shock waves but it's too coincidental to ignore. If the crack gets any bigger it could lead to a landslide." • Hut owner Val Branston, 57, said: "It'd be a disaster if the huts are swamped. They cost a bomb. We can only hope the council gets rid of the risk with a controlled explosion. We pay hundreds of pounds in premiums for our little plots by the beach, so the least we expect is safety." • Another hut owner Graham Young, 53, said: "Rains are forecast so it's only a matter of time until the cliff boulder gets heavy and the whole lot falls. • "It's frightening to think how the huts could be devalued. Ours is worth about £11,000 but there are places down the coast that go for £180,000. With risks like this, the price could plummet." • Builder John Dixon, 66, who walks the cliffs daily, said: "It's only a matter of time before the whole thing gives way. The edge has been looking dicey for quite a while. • "Add a bit of wind and rain and who knows what might happen?"

Public Safety & Engagement

Public Safety & Engagement 2005

Public Safety & Engagement 2008

Public Safety & Engagement

Management & Maintenance

This map is reproduced from Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationery Office © Crown copyright. Unauthorised reproduction infringes Crown copyright and may lead to prosecution or civil proceedings. New Forest District Council licence no. 100026220 2004 Management & Maintenance

•Maintenance of rock strongpoints

Monitoring

Cliff Monitoring • Instrumentation – Peizometers – Inclinometers • Deployed by Geotechnical Company • Cliff Surveys – Regular Survey Points – Rain gauge • Managed by NFDC • Data collection Monitoring

Rainfall Data

Monitoring

Rainfall Data

Monitoring

Monitoring Movement E-W

Monitoring Movement N-S

Monitoring Vertical movement

Monitoring

Ground Investigations

• The aims of the study are to: • Enable a better understanding of the site to be gained • Gain better information on current landslide stability, cliff recession processes and groundwater behaviour of the site • Develop viable engineering and management options

Ground Investigations

Ground Investigations • Extensive database of past boreholes

Ground Investigations Fm Zone Description Z (m) Becton ‘J’ Stiff, dark grey (weathering grey-brown), shelly, sandy, silty clay 7.5 Sand ‘I’ V. dense, dark grey (weathering yellow-white), silty, shelly sand 8.0 ‘H2’ V. dense, blue-green (weathering orange-brown) silty, shelly sand 2.4

‘H ’ Stiff, blue-green (weathering orange-brown) sandy, silty, shelly clay 3.1 Chama 1 Sand ‘G’ V. stiff/indurated, grey-brown (weathering red-brown), sandy, silty, clay/limestone 0.3

Barton ‘F2’ Stiff, laminated, dark grey-brown, silty clay with sandy/shelly partings and 0.3m basal mudstone 4.5 Clay ‘F1’ Stiff, laminated, dark grey-brown, silty clay with sandy/shelly partings 9.0 ‘E’ Stiff, laminated, brown-grey, silty clay with sandy/shelly partings and 0.2m, grey-brown siltstone 2.0 at top ‘D’ V. stiff, laminated, grey-green, silty variably sandy, clay 7.0 ‘C’ V. stiff, laminated, grey-green, sandy clay with calcareous mudstone nodules at top and base 3.8 (<0.3m) and pale grey horizon near middle

‘B’ V. stiff, laminated, dark grey silty clay with sandy/shelly lenses 1.5

‘A 3’ V. stiff, green/brown-grey sandy clay with regular beds of shelly sand 2.5 • Well documented beds of sand and clay. • Can be recognised in boreholes if carefully logged. • Shelly/sandy partings often associated with development of shears

Ground Investigations

• Uncertainties • Ground models – particularly at Cliff House Hotel • Monitoring – replace lost equipment to ensure continuity of data • Triggers of movement – threshold rainfall / groundwater levels • Detailed stratigraphy of site – thickness, continuity, dip • Hydrogeology – pattern of springs, perched water tables, pre water pressure recovery in Barton Clay. Ground Investigations • G. I. Plan

Ground Investigations

• Geotechnical Engineering

Ground Investigations

Resistivity Survey – single electrode probe Resistivity Survey – line of connected electrode probes Seismic Survey – seismic source

Seismic Survey (line of geophones) Ground Penetrating Radar Survey Ground Investigations

BH15 - Open cored hole Open coring – separating drilling flush and recycling the water

Down-hole Geophysics Survey Down-hole Geophysics Survey Example of down-hole geophysics log Ground Investigations

BH12 – Excavating inspection pit by hand BH12 – Coring BH12 – Core sample

BH12 – installing piezometers BH12 – Rig set-up in graben behind steel sheet piling BH12 – Located within active area Ground Investigations

BH10 – Winching equipment down the slope BH10 – Assembling equipment on upper terrace area

BH4 – Cliff House Hotel BH10 – Winching arrangement Ground Investigations

Location 3: Top of mudstone Ground Investigations

Ground Investigations • 3D overview of the site

• Showing principle geological boundaries 5x vertical exaggeration. Showing position of boreholes, interpolated stratigraphy and gentle dip to the east.

5x vertical exaggeration. Showing broad correlation of strata outcrop elevations and landslide geomorphology Ground Investigtions • Association of strata, beds & geomorphology

Cliff House Hotel Sea Road 5x vertical exaggeration showing association of strata and landslide geomorphology Cliff face cut in Becton Sand and Plateau Gravels Base of Bed I (Becton Sand) Becton Sand and Chama Sand debris on bench formed by F2 shear

Base of Bed G (Chama Sand)

Landslide bench formed on F1 shear Base of Bed F2

Base of Bed F1 Ground Investigtions • 3D geological model – Bed F2 in Barton Clay

Plan view showing elevation and morphology of the base of the F2 Bed in the Barton Clay (thought to form a shear surface)

This bed crops out in the cliff for much of the site associated with a series of landslides, but is below 0m OD at the easternmost area. Ground Investigtions

• Monitoring – Inclinometers

Ground Investigations

• Monitoring – Inclinometers

Ground Investigtions

• Monitoring – Inclinometers

Ground Investigations

• Monitoring – Piezometers

Ground Investigtions

• Monitoring – Piezometers

Future Schemes

• G.I. to inform development of viable management and control options

– As detailed in Christchurch Bay Strategy Study & SMP • Siphon drains • Maintenance of existing defences • Cliff Stabilisation • New rock groynes / rock revetment • Beach recycling • Monitoring

Future Schemes • At present (from SMP/Strategy) – Preferred Option (100 yrs) • Siphon drain system (or similar groundwater control system) • Rockwork • Maintenance – £12m – £50M benefits – ~750 properties

• Partnership score: – 50 - 55% – Require ~£6m contributions

• Task & Finish – Members and Officers Group

Future Schemes

• G.I. to develop lower cost schemes – Increase partnership sore – Reduce contributions

Thank you

[email protected]