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 © Crown copyright and database rights 2014 Ordnance Survey 100026220 Geology Geology Geology 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 Geology 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 Cliff House Hotel Cliff House Hotel Cliff House Hotel Cliff House Hotel 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 Laser-scan data Marine Drive •Translational failure with mudslides • Shear on base of F2 Marine Drive Marine Drive Marine Drive Marine Drive Marine Drive Marine Drive Marine Drive Marine Drive Marine Drive 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 30/01/13 Marine Drive 30/01/13 Marine Drive 02/01/14 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 Marine Drive Marine Drive Marine Drive 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 Public Safety & Engagement 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 Public Safety & Engagement Management & Maintenance 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 Monitoring 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 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