Geohazards in Britain

• UK Earthquakes:

  • 1.30am 7th June, 1931 - M6.1 earthquake with epicentre at Dogger Bank in North Sea + remote location of epicentre (60miles offshore) limited damage but shaking felt throughout country
  • Colchester Quake, 22nd April 1884 - M4.6 on Richter Scale (focus shallow as 2 miles below ground); 1200 buildings damaged, £10,000 damages, chimneys collasped/walls cracked
  • Dover Earthquake, 1580 - M5.3-5.9 on Richter Scale; caused 2 deaths/buildings damaged, and sections of white cliffs fell
• Dudley Earthquake, 2002:

  • early hours of September 23rd, M5.0 earthquake struck with epicentre at junction of High Arcal Road/Himley Road (20seconds of tremors felt)
  • impact felt Black Country, South Staffordshire, Shropshire (+ North Yorkshire, London and Wiltshire); chimneys shaken loose, glass shattered and walls cracked (families fled into streets)
  • Dudley Castle damaged during quake, splitting the stonework; stones from 16th Century kitchen walls fell to ground
• Earthquake in UK:

  • occur in UK as crustal stresses within tectonic plates are relived by movement on pre-existing fault planes
  • driving forces; regional compression from motion of tectonic plates + isostatic rebound (uplift from melting of ice sheets)
  • 200-300 earthquakes detected in UK each years; most small/cause no damage + M4 roughly every 2yrs/M5 around 10-20yrs (largest possible is M6.5)
  • most in Western Britian; quakes absent from eastern Scotland/north-east England (North Sea more active)
• Tsunamis in Britain; evidence that significant tsunamis have affected British Coast in recent geologic past. Evidence; coarser sediment deposited inbetween younger/older layers of fine sediment indicating a tsunami.
• Storegga Landslide/Tsunami:

  • 6200BC, tsunami in North Sea affected coastlines of Norway-Scotland from sudden collaspe of 240km of continental shelf near Norway displacing 3000km$^3$ of sediment
  • largest submarine slide to have produced a tsunami (3rd largest submarine slide) + Shetland Islands, 25m wall water above normal sea level travelling to 80mph recorded
  • prominent sand layer in Holocene deposits along eastern Scottish coast (dated 7,000yrs ago) may have been deposited by tsunami
• Storegga Landslide/Tsunami:

  • in periods of intense climate change, events like Storegga became more common; in last 45,000yrs 70% of continental slope failures off western Norway occured in periods of intense glaciation (16,000-4,000yrs ago)
  • continental shelf destablised by increase in volume of water on shelf, earthquake or methane being degassed
• Meteotsunamis are caused by the weather; caused by changes in atmospheric pressure associated with storms in North Atlantic + more common (one in 2011 impacted southern britain).

  • tsunami like storegga would be devastating due to higher population in proximity to coast
• Subsidence from Mining:

  • parts of country underlain by coal affected by subsidence from longwall mining (deep mining phased out from 2015); shallow coal mining left unstable voids underground
  • longwall mining; excavation of seam followed by collaspe of roof + rock above seam heavily fractured/resulting subsidence spreads out affecting area 1.4x height from workings to surface
  • angle subsidence spreads out towards surface depends on strength of strata/depth of cover to coal seam + subsidence at surface less thickness of coal removed as fallen rock has voids after collaspe
• Subsidence from Mining:

  • some parts subside before others; results in horizontal extensions then compressions (horizontal displacement can be greater than vertical subsidence)
  • faults can extend duration of hazard; subsidence from longwall extraction done in weeks, but reactivation of faults can extend subsidence hazard
• Crown Holes/Sinkholes:

  • sudden deep holes opening up in roads/under houses with catastrophic consequence; more common as climate change means more prolonged/intense rainfall, eg. 2014, wet weather created sinkholes in Yorkshire and Southeast
  • crown holes result of human activity, eg. Yorkshire gpysum mining; heavy rains enlarges voids + cavity forms/holds more water (increases in size until roof collaspe)
  • eg. 662m deep Xiaozhai Tiankeng Sinkhole, Chongquing
• Crown Holes/Sinkholes:

  • in limestone country, continual collaspe of cavities creates 'Karst Topography' ; sinkholes result from natural dissolution of bed rock (areas of salt/gypsum/carbonates at risk)
  • joints/bedding planes enlarged allowing more water/dissolution forming cave systems; as rock dissolves, soil carried down leaving funnel-shaped depressions in ground (collect more rainwater)
  • if clay in superficial layers, sinkholes may hold water creating circular pond/lake - some caves suddenly collaspe/cavity breaks surface; sudden increase in surface water adds to load on roof
• Sinkholes - saucer-shaped hollows that are release of collaspe/removal of underlying layer of rocks that used to support layers of material at surface.
• Sinkhole Triggers:

  • heavy rain/flooding - initiate collaspe of stable cavities especially those developed within superficial (youngest) deposits
  • water leaks - leaking drainage pipes/burst water mains/irrigation can trigger sinkholes
  • building works - construction/development triggers (modifying surface drainage/altering loads without adequate support)
  • change in water levels - drought/groundwater abstration cause sinkholes as water level changes (removes buoyant support)
  • mining - lower water table/intercept clay-filled voids that then collaspe
• Solution Sinkholes - formed by chemical weathering of rock where water accumulates around fissure/joint in rock (underneath soil or on surface) + hollow form drained of water through fissure/joint - depression enlarges from removal of soluble rock (chalk/gypsum/limestone); sides gentle-vertical slopes + saucer-like/cone shape.
• Collaspe Sinkholes - gradual collaspe of cave passage occur and eventually causes subsidence at surface level; collaspe may gradually propogate up through overlying strata to cause surface subsidence.
• Suffosion Sinkholes - form where solution of rock creates a depression on bedrock surface under soil + unsupported soil subsides into cavity leaving depression in landscape; if cover material sandy, it will slump into fissure but if cohesive (clay) cavity grows large before collasping.
• Sinkholes in UK:

  • most are small and in upland/rural locations + in areas underlain by carboniferous limestone; Mendips, Peak District and northern Penines
  • most susceptible is Permian gypsum deposits in north-east England (+ around Ripon), salt deposits (eg. Cheshire), and chalk group notable in Dorset/Hampshire
• Shrink-Swell: the volume change that occurs as a result of changes in moisture content of clay-rich soils. Swelling pressures cause heave/lifting of structures whilst shrinkage causes settlement/subsidence.

  • most damaging geohazard in britain; cost £540million after 1991 drought + 2003/6 heatwaves led to peaks in subsidence claims
• Causes of Shrink-Swell:

  • fine,grained/clay rich soils aborbs large amounts of water after rain becoming sticky/heavy + become hard when dry resulting in shrinking/cracking of ground (shrink-swell behaviour)
  • shrink-swell determined by water content near surface, type of clay in soil, clay's tendency to change volume
  • natural seasonal occurance (change in rain/vegetation) + enhanced movement; planting/removal of trees, change to surface drainage, leaks, and long term subsidence
• Effects of Shrink-Swell:

  • Heave; damage to buildings when volume change of soil unevenly distributed beneath foundations + if difference in water content in ground beneath building, swelling pressures causes heave (walls lift; happens at corners/centre of buildings).
  • Subsidence; triggered by artificial disturbance (change in drainage/heavy rain/water abstraction) + potential damage to foundations, buildings or infrastructure + dry weather/high temperatures major in subsidence in clay soils.
• Areas affected by Shrink-Swell:

  • rock formations most susceptible mainly in south-east of Britain (many clay formations here that absorb/lose moisture) + clay rocks elsewhere older/hardened by deep burial
  • eg. around the Wash/Lancashire Plain, clays deeply buried under soils (not susceptible to shrink-swell)
  • superficial deposits (alluviam, peat and laminated clays) susceptible to soil subsidence/heave (eg. Cheshire Basin)
• Mitigating Shrink-Swell Hazards; deep foundations below flucuations in water table + continious/reinforced raft foundation + effective drainage system + changing properties of clays (treated with 'lime', Ca-Smectite 100% expansion by shrink-swell but Na-Smecite 1500% expansion).