Chapter 10 Slope Processes, Landslides and Subsidence

• • • • • • • • • Environmental  Geology   Chapter  10  SLOPE  PROCESSES,  LANDSLIDES,  and  SUBSIDENCE     Landslides  –  General  term  used  to  describe  all  mass  wasting  events   Mass  wasting  is  the  downhill  movement  of  rocks,  soil  +/-­‐  water,  under  the   direct  influence  of  gravity.    Mass  wasting  is  the  step  that  generally  follows   weathering  to  erode  a  landscape.   Landslide  and  other  ground  failures  posting  substantial  damage  and  loss  of   life   In  U.S.,  average  25  deaths;  damage  more  than  $1  billion   Landslide  and  subsidence:  naturally  occurred  and  affected  by  human   activities     Forces  at  Work   • The  controlling  force  of  mass  wasting  is  gravity.       • Addition  of  water,  oversteepening  (beyond  the  angle  of  repose)  of  slopes,   seismic  shaking  (earthquakes),  addition  of  weight  to  a  slope,  and  removal  of   vegetation  can  increase  the  likelihood  of  and  even  trigger  mass  wasting     Slope  Processes-­‐Angle  of  Repose:    The  angle  of  repose  is  the  steepest   angle  that  a  pile  of  sediments  or  soil  can  maintain  without  collapsing.    This  is   generally  between  25  and  40  degrees.    The  larger  and  more  angular  the   particles,  the  higher  the  angle  of  repose  will  be.    Loose,  dry  sand  (as  in  dunes,   etc.)  typically  has  an  angle  of  repose  of  33  to  34  degrees.     Slope  Processes-­‐Slopes:  The  most  common  landforms:  Dynamic  evolving   feature,  depending  upon  topography,  rock  types,  climate,  vegetation,  water   and  geologic  time     Types  of  Landslides-­‐Slope  gradient,  type  of  slope  materials,  amount  of   water  present,  rate  of  movement  are  factors.    The  rate  of  movement  may  be   imperceptible  creep  to  thundering  avalanches.    Types  includes  creep,  sliding,   slumping,  falling,  flowage  or  flow,  and  complex  movement  (sliding  and   flowage).    There  are  many  different  processes  lumped  under  the  term  “mass   wasting”.    These  processes  are  categorized  and  described  on  the  basis  of  (1)   the  type  of  material  involved  (debris,  mud,  earth,  or  rock);  (2)  the  type  of   motion  (fall,  slide,  flow);  and  (3)  the  rate  of  movement  (rapid  or  slow).     • Slow  Mass  Wasting  -­‐  The  slowest  forms  of  mass  wasting  include  creep,   the  gradual  downhill  movement  of  soil  and  fractured  rock  layers;  and   solifluction,  a  type  of  soil  flow  that  is  common  in  areas  with  permafrost   (permanently  frozen  ground)  under  a  thin  soil  layer  that  thaws  (but   remains  saturated)  in  the  warmer  months.     • Rapid  (Fast)  Mass  Wasting  -­‐  Slumps,  Rockslides,  Debris  flows,  Earth   flows     • • • • • • • • • • • Avalanches  -­‐  The  fastest  versions  of  these  processes  are  termed  rock   and  debris  avalanches   Slump,  (the  downward  sliding  of  a  mass  of  rock  or  soil  moving  as   a  unit  along  a  curved  surface)   Rockslides  (blocks  of  bedrock  breaking  loose  and  sliding   downslope)   Earth  flow  (an  unconfined  flow  of  saturated,  clay-­‐rich  soil  that   often  occurs  in  humid  areas  after  heavy  rains  or  snowmelt)   Debris  flow  (rapid  flow  of  soil  and  rocks  mixed  with  water)   Avalanches  -­‐  The  fastest  versions  of  these  processes  are  termed   rock  and  debris  avalanches   Complex  Slides  –    Can  start  out  as  one  type  and  change  to  another   as  the  slid  progresses  (e.g.  La  Conchita,  CA  –  1995  and  2005)     Slope  Stability   • Safety  Factor   SF  =  Resisting  Forces/Driving  Forces   If  SF  >  1,  Then  safe  or  stable  slope   If  SF  <  1,  Then  unsafe  or  unstable  slope  (MOST  DANGEROUS)     Driving  and  resisting  forces  determined  by  the  interrelationships  of  the   following  variables:     Ø Type  of  Earth  materials   Ø Slope  angle  and  topography   Ø Climate     Ø Vegetation  and  water   Ø Time     Resisting  Forces  (e.g.  Shear  Strength)  /  Driving  Forces  (e.g.  weight  of  rock)      FACTORS  THAT  INCREASE  CHANCES  OF  MASS  WASTING   • Steep  slopes   • Excessive  groundwater  pumping   • Adding  weight  (e.g.  building  a  house  or  using  a  bulldozer)   • Adding  water  –  irrigation  or  rainfall   • High  relief   • Lack  of  vegetation  (natural  or  removed  by  humans)   • Planes  of  weakness  parallel  to  slopes     Human  Land  Use  and  Landslide-­‐  Factors  that  can  increase  landslide   potential  due  to  human  land  use:   • Urbanization,  irrigation   • Timber  harvesting  in  weak,  relatively  unstable  areas   • Artificial  fillings  of  loose  materials   • Modification  of  landscape   • Dam  construction     • • • • • Vaiont  Dam  –  Worlds  Worst  Dam  Disaster  -­‐  ~  2600  deaths   • On  October  9,  1963  in  Italy  –  Landslide  displaced  water  –  overtopped  the   dam  –  flooding  downstream   • Landslide  at  30  m/s  or  60  mph   • Generating  waves  of  water  up  to  300  ft  high     • Multiple  factors   Ø Weak  carbonate  rocks  and  clayey  layer   Ø Geologic  fractures,  sinkholes   Ø Steep  slope  surface  and  creep  due  to  the  increased  water  pressure  of  the   reservoir     Preventing  landslides   Ø Drainage  control:  Reducing  infiltration  and  surface  runoff   Ø Slope  grading:  Reducing  the  overall  slope   Ø Slope  supports:  Retaining  walls  or  deep  supporting  piles   Ø Avoid  landslide  hazards:  Warning  of  and  correcting  landslides     Warning  of  Impending  Landslides   • Monitoring  changes     Ø Human  surveillance  –  visual  inspections   Ø Instrumental  survey:  Tilt  meter  and  geophones   Ø Shallow  wells  –  signal  when  slopes  contain  too  much  water   • Landslide  warning  system     Ø Info  for  public  awareness  and  education   Ø Enough  time  for  public  evacuation   Ø Stop  or  reroute  traffic  flow   Ø Emergency  services     Snow  Avalanche   • Mountainous  region     • Rapid  downslope  movement  of  snow  and  ice   • Thousands  of  avalanches  in  the  western  U.S.   • More  deadly  if  large-­‐slab  avalanches   • Preventive  measures:     Ø Well-­‐designed  explosives   Ø Engineering  structures  to  retain,  divert,  or          retard  avalanches     Lahars  -­‐  The  eruptions  of  snow-­‐  and  ice-­‐capped  volcanoes  commonly  trigger   large  mass  wasting  events  involving  the  rapid  downslope  movement  of   water-­‐saturated  mud  and  rock  flows  termed  lahars.  These  are  often   significantly  larger  hazards  than  the  lava  and  ash  erupted  by  the  volcanoes   because  they  can  travel  so  rapidly  and  for  such  large  distances  (tens  of   kilometers  or  more).     • • • • • • Subsidence   • Subsurface  ground  failure   • Occurred  naturally  or  induced  by  human  activities   • Slow  settling  or  rapid  collapse     • Causes:  Withdrawal  of  fluids  or  removal  of  solid  materials     Withdrawal  of  Subsurface  Fluids   • In  exploration  and  extraction  processes   Ø Petroleum  and  natural  gas   Ø Groundwater  -­‐  sinkholes   Ø Geothermal  water  and  steam   • Louisiana  –  1980  -­‐  Oil  rig  drilled  through  salt  dome  into  a  mine.    A   shallow  lake  drained  –  No  lives  lost.  10  barges,  oil  rig  and  a  tugboat  lost     Karst  Topography  -­‐  In  wet  climates,  limestone  will  get  highly  dissolved  by   groundwater,  and  caves  and  karst  topography  (from  sinkholes  that  form  by   collapse  into  empty  caves).    The  danger  of  sinkholes  is  greatly  increased   when  man  drops  the  water  table  and  empties  out  these  cave  systems,   removing  the  buoyancy  force  that  helped  hold  up  the  cave  roofs.    Florida  is   an  area  where  this  geologic  hazard  is  significant  today.     • Sinkholes   Ø Dissolution  of  carbonate  rocks,  limestone,  and  dolomite   Ø Affecting  most  of  the  conterminous  states   Ø Natural  or  artificial  fluctuations  in  water  table  increasing  the  problem   Ø Triggering  other  problems:  Sinkholes  as  waste  dumping  sites       Removal  of  Solid  Materials  -­‐  Salt  and  coal  mining   Ø Salt  dissolution  and  pumping   Ø Active  coal  mines  and  abandoned  coal  mines   Ø Ground  failure  due  to  depleted  subsurface  pressure   Ø More  than  8000  km2  of  land  subsidence  due  to  underground  coal   mining     Ground  Subsidence  and  Earth  Fissures  -­‐  As  groundwater  is  withdrawn,   the  spaces  between  grains  collapse.    Excessive  pumping  of  groundwater  can   lead  to  ground  subsidence,  or  a  dropping  of  the  land  surface,  which  can   produce  earth  cracks  (fissures)  and  destroy  buildings  and  roads.    Arizona   has  experienced  this  in  the  past  due  to  excessive  pumping  for  irrigation  of   crops  like  cotton  in  the  Phoenix  and  Casa  Grande  areas.      In  the  San  Joaquin   Valley  of  California,  there  has  been  26  feet  of  subsidence.     Perception  of  the  Landslide  Hazard   • Landslide  hazard  maps  not  preventing  development   • • •     Common  perception:  “It  could  happen  on  other  hillsides,  but  never  on   this  one.”   Infrequency  and  unpredictability  of  large  slides  reducing  awareness  of   the  hazards   Often  people  taking  the  chances   •   What  Can  You  Do?   • Professional  geologic  evaluation  for    a  property  on  a  slope   • Avoid  building  at  the  mouth  of  a  canyon,  regardless  of  its  size   • Consult  local  agencies  for  historic  records   • Watch  signs  of  little  slides—Often  precursor  for  larger  ones   • Look  for  signs  of  structure  cracks  or  damages  prior  to  purchase   • Be  wary  of  pool  leaking,  tilt  of  trees  and  utility  poles   • Look  for  linear  cracks,  subsurface  water  movement   • Put  observations  into  perspective,  one  aspect  may  not  tell  the  whole  story