soil%erosion%&%mass%wasting%1 all about soil · soil%erosion%&%mass%wasting%6!!! subsetof...

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SOIL EROSION & MASS WASTING 1 All About Soil: Soil Formation: According to the Natural Resource Conservation Service, “soil refers to the loose surface of the earth as distinguished from rock”. Soil is formed when weathering continuously breaks down rocks into pieces. These pieces are broken down into even smaller pieces to form soil. Soil Profile: Soil is made up of many layers called horizons, such as the humus, topsoil, subsoil, weathered rock fragments and bedrock. Leaching (drainage of minerals/chemicals away from soil, ash, or similar material by the action of percolating liquid) can occur from the humus to the bedrock. Factors Affecting Soil Profile (Layers), Soil Formation and eventually the Soil Composition: Soil Composition: Soil is composed of organic matter, mineral and nutrients. An average soil sample is 45% mineral, 25% water, 25% air and 5% organic matter (material that was once living or was formed by the activity of living organisms) Soil texture comes from different size mineral particles such as sand, silt and clay Pieces of weathered rock and organic material, or humus (Decaying organic material; becomes mixed with the top layers of rock particles, and supplies some of the needed nutrients to plants; also creates crumbly soil which allows adequate water absorption and drainage.), are the two main ingredients of soil. Factor: Explanation: Time The longer a rock is exposed to the forces of weathering, the more it is broken down. Mature soil is formed if all three layers have had time to develop. Climate In areas with heavy rainfall and warm temperatures, weathering takes place more rapidly. Heavy rainfall may wash much of the topsoil away. But since organisms are more plentiful these areas, the soil is quickly replaced. They speed up the chemical and mechanical weathering of rocks. Type of rock Rocks that do not break down easily do not form soil rapidly. In some climates it takes along time for granite to break down. So soil formation from granite is slow. Sandstone can break easily and forms soil quickly. Surface features of the region The surface features of the region also determine the speed at which soil is formed. On very steep slopes, rainwater running off the land erodes the soil and exposes rock to weathering. The presence of vegetation also speeds up biological weathering of rock.

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Page 1: SOIL%EROSION%&%MASS%WASTING%1 All About Soil · SOIL%EROSION%&%MASS%WASTING%6!!! Subsetof slides:% Slumps% H Alsoknownas!rotational!slidesastheyhavea!rotational%movement! H

SOIL  EROSION  &  MASS  WASTING   1    

All About Soil:  Soil  Formation:    According  to  the  Natural  Resource  Conservation  Service,  “soil  refers  to  the  loose  surface  of  the  earth  as  distinguished  from  rock”.  Soil  is  formed  when  weathering  continuously  breaks  down  rocks  into  pieces.  These  pieces  are  broken  down  into  even  smaller  pieces  to  form  soil.    Soil  Profile:    Soil  is  made  up  of  many  layers  called  horizons,  such  as  the  humus,  topsoil,  subsoil,  weathered  rock  fragments  and  bedrock.  Leaching  (drainage  of  minerals/chemicals  away  from  soil,  ash,  or  similar  material  by  the  action  of  percolating  liquid)  can  occur  from  the  humus  to  the  bedrock.      

   Factors  Affecting  Soil  Profile  (Layers),  Soil  Formation  and  eventually  the  Soil  Composition:  

 Soil  Composition:  

• Soil  is  composed  of  organic  matter,  mineral  and  nutrients.  • An  average  soil  sample  is  45%  mineral,  25%  water,  25%  air  and  5%  organic  matter  (material  that  was  once  

living  or  was  formed  by  the  activity  of  living  organisms)  • Soil  texture  comes  from  different  size  mineral  particles  such  as  sand,  silt  and  clay  • Pieces  of  weathered  rock  and  organic  material,  or  humus  (Decaying  organic  material;  becomes  mixed  with  

the  top  layers  of  rock  particles,  and  supplies  some  of  the  needed  nutrients  to  plants;  also  creates  crumbly  soil  which  allows  adequate  water  absorption  and  drainage.),  are  the  two  main  ingredients  of  soil.  

Factor:   Explanation:  Time    

-­‐ The  longer  a  rock  is  exposed  to  the  forces  of  weathering,  the  more  it  is  broken  down.      -­‐ Mature  soil  is  formed  if  all  three  layers  have  had  time  to  develop.  

Climate   -­‐ In  areas  with  heavy  rainfall  and  warm  temperatures,  weathering  takes  place  more  rapidly.    -­‐ Heavy  rainfall  may  wash  much  of  the  topsoil  away.    -­‐ But  since  organisms  are  more  plentiful  these  areas,  the  soil  is  quickly  replaced.    -­‐ They  speed  up  the  chemical  and  mechanical  weathering  of  rocks.      

Type  of  rock    

-­‐ Rocks  that  do  not  break  down  easily  do  not  form  soil  rapidly.      -­‐ In  some  climates  it  takes  along  time  for  granite  to  break  down.    So  soil  formation  from  

granite  is  slow.    -­‐ Sandstone  can  break  easily  and  forms  soil  quickly.  

Surface  features  of  the  region    

-­‐ The  surface  features  of  the  region  also  determine  the  speed  at  which  soil  is  formed.      -­‐ On  very  steep  slopes,  rainwater  running  off  the  land  erodes  the  soil  and  exposes  rock  to  

weathering.  -­‐ The  presence  of  vegetation  also  speeds  up  biological  weathering  of  rock.    

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SOIL  EROSION  &  MASS  WASTING   2    

• Rock  particles  form  more  than  80%  of  soil.    • The  composition  of  soil  varies  from  place  to  place.    The  type  of  rock  broken  down  by  weathering  determines  

the  kinds  of  minerals  in  the  soil.    The  type  of  weathering  also  affects  the  composition  of  soil.    Mechanical  weathering  produces  soil  with  a  composition  similar  to  the  rock  being  weathered.    Chemical  weathering  produces  soil  with  a  different  composition.  

•    Properties  of  Soil:    Property:   Description  &  Trend:  Texture   - Determined  by  the  size  of  mineral  particles  within  the  soil.    

- Too  many  large  particles  leads  to  extreme  leaching,  too  many  small  particles  lead  to  poor  drainage.  

 Colour   - Generally  speaking,  the  darker  a  soil  is,  the  more  nutrient  rich  it  is.  The  darker  color  often  indicates  

an  increase  in  decomposed  organic  matter  known  as  humus.  - Gray  soils  often  indicate  poor  drainage,  while  red  soils  can  indicate  very  poor  soils.  - These  general  rules  about  soil  colors  can  however  be  misleading.  Under  certain  conditions,  a  very  

poor  soil  can  appear  as  dark  black,  while  a  rich  healthy  soil  can  appear  as  red.  pH  (Acidity/  Alkalinity)  

- If  a  soil  has  too  much  acid  in  it,  the  nutrients  in  the  soil  will  be  dissolved  too  quickly,  and  leached  away  as  the  water  drains.    

- If  a  soil  is  too  alkaline,  or  in  other  words,  if  there  is  not  enough  acid,  than  nutrients  will  not  dissolve  quickly  enough.  

- Thus,  a  neutral  soil,  which  is  neither  too  acidic,  nor  too  alkaline  is  the  preferred  type  of  soil  for  plant  life  to  thrive.  

Structure   - Dirt  clods  (small  clump  of  soil  that  has  formed  a  rock-­‐like  clump)  are  called  peds  by  geologists.    - Many  soils  form  peds,  as  soil  fragments  bond  together.  These  peds  effect  how  easily  water  is  able  

to  move  through  the  soil,  as  well  as  plant  roots,  and  other  organisms.  (Related  to  particle  size)  - Peds  tend  to  form  more  often  in  wet  locations,  and  less  frequently  in  drier  locations.  Sandy  

locations  such  as  deserts  do  not  form  any  peds  at  all.    Types  of  Soil:  Type:   Description:  Residual  Soil:      

Sometimes  soil  remains  on  top  of  its  parent  rock,  or  the  rock  from  which  it  was  formed.  This  is  called  residual  soil.  Residual  soil  has  a  composition  similar  to  that  of  the  parent  rock  it  covers.      

Transported  Soil:      

Some  soil  is  removed  from  the  parent  rock  by  water,  wind,  glaciers  and  waves.  Soil  that  is  moved  away  from  its  place  of  origin  is  called  transported  soil.  Transported  soil  can  be  very  different  in  composition  from  the  rock  it  covers.      

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SOIL  EROSION  &  MASS  WASTING   3      Soil  Erosion:    Erosion  is  the  process  of  detachment  and  transport  of  soil  particles  by  erosive  agents  (Ellison,  1944),  and  is  a  natural  geologic  process.    It  is  a  global  problem  -­‐  1/3  of  the  world’s  arable  land  has  been  lost  since  1950,  mostly  in  Asia,  Africa  and  South  America  at  a  rate  of  13-­‐18  t/a/yr  (tons  per  year  I  think,  I  don’t  know  what  the  a  stands  for).  In  the  USA,  30%  of  the  farmland  has  been  abandoned  through  erosion,  salinization  and  water-­‐logging.  90%  of  USA’s  cropland  is  also  losing  soil  faster  than  it  is  replaced,  at  a  rate  of  >1  t/a/yr.      Types  of  Soil  Erosion:  Type   What  it  does:  

4  Stages/Types:  

 1st  Stage:  Raindrop  erosion  -­‐ Begins  with  raindrops  striking  bare  soil,  hence  dislodges  particles  -­‐ Intense  rains  seals  off  the  surface  -­‐ When  the  rainfall  exceeds  the  maximum  infiltration  capacity,  water  is  stored  in  small  

depressions  on  the  surface  of  the  soil.  (Give  a  dimpled  appearance  to  the  surface)  -­‐ Once  the  depressions  are  filled,  surface  runoff  begins,  bringing  soil  particles  with  them.    

2nd  Stage:  Sheet  erosion    -­‐ Removal  of  a  layer  of  exposed  soil  by  the  action  of  raindrop  splash  and  runoff.    -­‐ The  water  moves  in  broad  sheets  over  the  land  and  is  not  confined  in  concentrated  flow  paths.    -­‐ Amount  of  erosion  increases  with  slope  and  distance.    

3rd  Stage:  Rill  and  gully  erosion    -­‐ Occurs  after  runoff  flows  concentrate  into  rivulets,  cutting  into  the  soil  surface.    -­‐ Amount  of  erosion  increases  with  slope  and  distance.    -­‐ Rills  =>  Narrow  and  shallow  incisions  into  topsoil  layers,  will  evolve  into  larger  fluvial  features  -­‐ Gullies  =>  Gullies  are  larger  than  rills  and  cannot  be  fixed  by  tillage  (agricultural  preparation  of  

the  soil  by  mechanical  agitation  of  various  types,  such  as  digging,  stirring,  and  overturning.).  Gully  erosion  is  an  advanced  stage  of  rill  erosion,  just  as  rills  are  often  the  result  of  sheet  erosion.    

Water  Erosion  

4th  Stage:  Streambank  and  bed  erosion    Occurs  with  an  increase  in  the  volume  and  velocity  of  runoff.  Water  is  a  great  erosional  agent:    

-­‐ Corrasion/Abrasion:  grinding  against  surface  by  materials  -­‐ Attrition:  colliding  together  to  become  smaller  rocks  -­‐ Solution:  Dissolve  in  carbonic  acid.  (Eg.  Limestone  –  calcium  carbonate  dissolves)  -­‐ Hydraulic  Action:  water  hitting  against  the  rocks,  loosening.    

 Wind  Erosion   -­‐ Wind  erosion  may  occur  on  any  soil  whose  surface  is  dry  and  unprotected  by  vegetation.    

-­‐ Saltation  (the  movement  of  hard  particles  such  as  sand  over  an  uneven  surface  in  a  turbulent  flow  of  air  or  water)  detaches  particles,  and  smaller  particles  are  suspended  while  larger  particles  creep.  

-­‐ Sandy  and  silty  soils  are  the  most  susceptible  -­‐ This  process  causes  soil  accumulation  in  ditches  and  fence  rows.  

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SOIL  EROSION  &  MASS  WASTING   4    

-­‐ It  cannot  be  divided  into  such  distinct  types.  Surface  texture  is  the  best  key  to  wind  erosion  hazard  potential.    

Gravitational  Erosion  

-­‐ Gravity  is  the  principal  force  acting  to  move  surface  materials  such  as  soil  and  rock.  

Tillage  Translocation  

-­‐ Net  downhill  movement  by  tillage  (basically,  LAND)  results  in  the  smoothing  of  surface.  -­‐ Combined  with  water  erosion,  it  increases  relief  intensity  and  soil  variability.  -­‐ Boundaries  stop  the  movement  

 Mitigation  Measures:  Measure   Description   Picture:  Contour  Farming    

-­‐ Tilling  at  right  angles  to  the  slope  of  the  land.    -­‐ Rows  formed  slows  water  run-­‐off  during  rainstorms,  

acting  as  a  small  dam,  to  prevent  soil  erosion  and  allows  the  water  time  to  settle  into  the  soil.  

   

 Strip  Farming    

-­‐ Alternating  strips  of  closely  sown  crops  to  slow  water  flow.    

 

 Terracing   -­‐ Level  areas  constructed  at  right  angles  to  the  slope  to  

retain  water  -­‐  good  for  very  steep  land.      

 Windbreaks     -­‐ Planting  of  trees  or  other  plants  that  protect  bare  soil  

from  full  force  of  the  wind.    -­‐ Planting  sufficiently  dense  rows  or  stands  of  trees  at  

the  windward  exposure  of  an  agricultural  field  subject  to  wind  erosion.  Evergreen  species  are  preferred  to  achieve  year-­‐round  protection;  however,  as  long  as  foilage  is  present  in  the  seasons  of  bare  soil  surfaces,  the  effect  of  deciduous  trees  may  also  be  adequate.  

 

             

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SOIL  EROSION  &  MASS  WASTING   5    

Mass Wasting: What  is  Mass  Wasting?  

-­‐ Also  known  as  slope  movement  or  mass  movement  -­‐ Geomorphic  process  by  which  soil,  regolith  (loose  uncemented  mixture  of  soil  and  rock  particles  that  covers  

the  Earth's  surface),  and  rock  move  downslope  under  the  force  of  gravity  -­‐ Not  accompanied  by  a  moving  agent  such  a  river,  glacier  or  ocean  wave.    -­‐ Comprehensive  term  for  any  type  of  downslope  movement  of  Earth  materials  (rock  or  soil)  as  a  coherent  

mass  which  is  distinct  from  erosional  processes  and  follows  weathering  -­‐ Includes  landslides,  earthflows,  mudflows,  rockfalls,  debris  and  snow  avalanches,  and  subsidence  -­‐ Force  of  gravity  dominates,  and  is  often  initiated  by  heavy  rainfall  or  earthquake  

 Classification  of  Mass  Movement:  Important  variables  in  classifying  downslope  movement  of  Earth  materials  are:  1.  Types  of  mass  movements  (slide,  slump,  fall,  flow,  subsidence,  or  complex)  2.  Slope  material  3.  Amount  of  water  4.  Rate  of  movement    5.  Sudden  failure  or  flow  Speed  Diagram:  DRAW  YOURSELF!!            Types  of  Mass  Movement:  Type   Description  Slide   - Rock  or  debris  slides  down  pre-­‐existing  surface  e.g.  bedding  plane  

- Downslope  movement  of  coherent  block  of  Earth  material    - A  sheet  of  material  that  slips  over  a  failure  plane  ending  anywhere  from  a  meter  to  a  kilometer  

down  slope,  and  maintains  its  shape  and  cohesion  until  it  impacts  the  bottom  of  a  slope  - Slides  produce  concave  scars  (a  steep  high  cliff  or  rock  outcrop)  while  slumps  tend  to  produce  a  

scarp  (a  very  steep  bank  or  slope)  or  cliff  exposure.  Trees  are  broken  and  bent  and  the  slide  can  bury  the  soil  down  slope.    

- Note  the  concave  scar  typically  produced  by  a  slide.  

 

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SOIL  EROSION  &  MASS  WASTING   6    

 Subset  of  slides:  Slumps  

-­‐ Also  known  as  rotational  slides  as  they  have  a  rotational  movement  -­‐ Involve  the  movement  of  relatively  intact  masses  of  rock  or  sediment  downslope  along  a  curved,  

concave  upward,  failure  plane.  -­‐ The  material  involved  in  the  slump  rotates  along  the  failure  surface  as  it  slides  down  the  slope.    -­‐ Occur  on  weaker  rocks  e.g.  clay  and  the  material  is  saturated.  -­‐ Usually  undercutting  has  weakened  the  base  or  the  pressure  on  the  rocks  is  intensified  by  human  

activity.  -­‐ In  many  cases  small  flows  of  unconsolidated  sediment  or  soil  move  out  in  front  of  the  slump.  Slumps  

usually  occur  in  rock  units  where  there  are  some  unconsolidated  or  weak  rock  layers.  

 

 Flow   -­‐ Liquidy  mass,  highly  fluid  and  lacks  cohesion,  saturated  with  water.    

-­‐ Moves  quickly  down  a  well-­‐defined  channel,  the  toe  (end)  of  the  flow  spreads  out  -­‐ Leaves  a  scar,  and  over  33%  of  the  material  is  fine-­‐grained  -­‐ Down  slope  movement  of  water-­‐saturated  soil,  regolith,  weak  shale,  or  weak  clay  layers.    -­‐ TYPES  OF  FLOWS  

o Earthflows  (the  contents  are  made  of  soil)  and  Mudflows  (clay-­‐sized  particles,  suspended  particles  and  silt)  

o Earthflows  have  a  fairly  slow  speed,  occurring  over  a  few  hours  or  so  slow  that  they  are  almost  imperceptible.    

o Earth  flows  are  accompanied  with  slumping,  but  unlike  slumping,  there  is  no  backward  rotation.    

o Earth  flows  differ  from  mudflows  in  that  they  (1)  tend  to  be  slower,  (2)  are  not  confined  to  

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channels,  (3)  are  more  common  in  humid  areas  than  dry,  and  (4)  have  a  lower  water  content.  

o A  mudflow  is  the  rapid  down  slope  movement  of  water-­‐saturated  soil,  regolith.  The  higher  water  content  creates  a  flow  rapid  enough  to  be  perceptible  to  the  eye.    

o Conditions  favorable  for  the  development  of  mudflows  are:  (1)  unconsolidated  surface  materials,  (2)  steep  slopes  abundant  but  intermittent  precipitation,  and  (3)  sparse  cover  of  vegetation.  Mudflows  tend  to  be  more  prevalent  in  dry  regions  where  vegetation  is  sparse  and  heavy  rains  may  form.  When  set  in  motion,  they  occupy  stream-­‐cut  channels  rushing  along  in  a  torrential  flow  of  mud.      

 

 Subset  of  Flow:  Creep  

-­‐ Is  the  slow  downslope  movement  of  soil,  sediment  or  rock  along  more  gentle  slopes  than  give  rise  to  the  more  rapid  forms  of  mass  wasting.    

-­‐ The  surface  itself  gives  little  indication  that  creep  is  occurring,  however  features  above  the  surface  of  the  slope  provide  evidence  that  slow  downslope  movement  is  occurring.    

-­‐ Trees  display  bent  after  being  tilted  by  the  creep  movement.  Trees  tend  to  grow  vertically,  towards  the  sun,  and  so  when  downslope  movement  tilts  them,  the  trunks  bend  to  grow  vertically  again.    

-­‐ Gravestones  and  fences  in  areas  of  creep  are  also  are  tilted  downslope.    -­‐ Rock  layers  near  the  surface  are  also  tilted  downslope.    

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Subset  of  Flow:  Soilfluction  

-­‐ Freeze  thaw  cycles  is  the  agent.  -­‐ Occurs  in  cold  regions  in  which  the  water  near  the  surface  freezes  and  thaws  repeatedly.    -­‐ Relatively  slow  form  of  mass  wasting.    -­‐ When  the  water  near  the  surface  freezes  the  soil  and  rock  is  moved  upward  and  in  the  downslope  

direction  due  to  the  expansion  of  the  water.  The  underlying  frozen  ground  acts  as  sliding  plane.  -­‐ When  the  surface  of  permafrost  melts  creating  mobile  water  saturated  layer,  soil  moves  vertically  

back  to  the  surface  -­‐ Although  the  downslope  motion  may  only  be  a  fraction  of  an  inch  for  each  freeze  and  thaw  cycle,  

the  repeated  cycles  each  day  result  in  significant  downslope  movement  over  time.  

   

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SOIL  EROSION  &  MASS  WASTING   9    Falls   - Occur  because  of    

o Excessive  vibration  o Freeze  thaw  action  o External  stresses  o Jointing  and  faulting  in  the  rock  mass  o Sudden  shock  e.g.  earthquake  o Slope  modification  

- Rock  faces  or  vertical  outcrops  are  prone  to  rock  falls,  it  usually  occurs  on  steep  slopes  >70  degrees  - Rock  falls  occur  when  weathering  or  frost  action  weakens  the  rock  in  an  outcrop  along  joints  or  

fractures,  and  the  rock  material  breaks  away,  falling  rapidly  to  the  ground  under  the  influence  of  gravity.  

- This  is  the  most  rapid  form  of  mass  movement  and  is  particularly  a  problem  in  road  cuts  in  mountainous  areas.    

- Areas  prone  to  rock  fall  often  display  accumulation  of  small  rock  fragments  called  talus  slopes,  if  the  fall  is  short,  it  produces  a  straight  scree,  if  it  is  long,  it  forms  a  concave  scree.    

- Topples:  Blocks  of  rock  pivot  and  fall  away  from  a  slope  

 

 Avalanches   - Rapid  movements  of  snow,  ice,  rock  or  earth  down  a  slope  

- Common  in  mountainous  areas,  occur  on  steep  slopes  >22  degrees  - Dry  avalanche  (newly  fallen  snow  falls  off  older  snow)  and  wet  avalanche  (partially  melted  snow  

moves),  often  triggered  by  skiing  - Debris  avalanches  moves  sediments  and  organic  material,  and  are  often  associated  with  saturated  

ground  conditions.  

-    

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SOIL  EROSION  &  MASS  WASTING   10    

-    Trigger  Factors  for  Mass  Wasting  Events:  Trigger  Factor   How  does  it  work:  Sudden  Shocks   A  sudden  shock  from  an  earthquake  may  release  so  much  energy  that  slope  failures  are  

triggered.    Slope  Modification   When  a  slope  is  modified  through  human  activity  such  as  road  cuts  into  regolith,  the  angle  of  

repose  for  these  artificial  slopes  may  be  exceeded,  and  hence  trigger  a  landslide.  Undercutting  by  streams  or  larger  bodies  of  water  (e.g.  streambank  erosion)  may  result  in  this  as  well.      

Exceptional  Precipitation  

This  causes  saturation  of  the  ground  and  hence  making  it  unstable.  It  decreases  the  shear  strength  of  an  oversteepened  slope,  causing  failure.    

Volcanic  Eruptions   Stratovolcanoes  have  inherently  unstable  piles  of  lava  flows,  rubble  and  pyroclastics  such  as  ash.  When  an  eruption  occurs,  slope  failure  is  common,  causing  pyroclastic  flows  and  landslides.  If  the  volcano  supports  glaciers  or  snowfields,  melting  of  snow  and  ice  can  cause  avalanches  and  various  flows  when  combined  with  unconsolidated  deposits  on  the  slopes.    

 Problems  with  Mass  Wasting:    - Caused  by  instability  of  a  slope  under  the  force  of  gravity  

o 2  Forces  of  gravity:  Tangential  (pulls  material  down  slope)  and  Perpendicular  (keeps  material  on  slope)    - When  tangential  force  >  perpendicular  force  and  friction,  material  falls.  This  is  the  same  as  tangential  force  

overcoming  shear  strength.    Material  is  mobilized  when  the  shear  stress  imposed  on  a  surface  exceeds  the  shear  strength.    

- Shear  strength  (=  perpendicular  force)  is  a  measure  if  the  resistance  of  earth  materials  to  be  moved.  The  interlocking  of  soil  particles  increases  the  ability  of  material  to  stay  in  place.  Plant  roots  also  help  bind  soil  particles  together.    

- Shear  stress  (=tangential  force)  is  a  function  of  the  force  exerted  by  the  weight  of  the  material  under  the  influence  of  gravity  acting  in  the  down  slope  direction.  The  slope  of  the  surface  determines  the  amount  of  stress  that  occurs  on  earth  materials.    

- Water  destabilizes  hill  slopes  by  creating  pressure  in  the  pore  spaces  of  earth  materials.  (Factor  =  Pore  Water)  Water  infiltrating  into  slope  materials  saturates  the  soil  particles  at  depth  by  filling  the  pore  spaces  between.  The  weight  of  water  lying  above  creates  water  pressure  that  drives  soil  particles  apart.  This  lessens  the  friction  between  them  and  enables  them  to  slip  past  one  another.    

- The  movement,  especially  in  the  case  of  slides  and  slumps,  is  along  a  failure  plane.  The  failure  plane  may  be  a  well-­‐defined  layer  of  clay  or  rock  upon  which  sets  the  destabilized  surface  material.    

- Slope  instability  is  when  the  angel  of  repose  is  reached,  where  material  cannot  lie  on  the  slope  without  falling.    - Humans  induce  mass  movement  when  subjecting  a  slope  to  a  load  that  exceeds  its  ability  to  resist  movement.  

(Exceed  the  angle  of  repose).  People  building  houses  on  scenic  hill  slopes  often  find  their  homes  threatened  by  a  landslide.  Undercutting  of  hillsides  during  road  construction  commonly  creates  unstable  slopes  making  them  prone  to  failure.  Removal  of  vegetation  can  too.  

 

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   Causes  &  Mitigation!    Cause   Measure  Steepened  slope   Oversteepened  slopes  could  be  graded  to  reduce  the  slope  to  the  natural  angle  of  

repose.    Added  weight   Drainage  pipes  could  be  inserted  into  the  slope  to  more  easily  allow  water  to  get  

out  and  avoid  increases  in  fluid  pressure,  the  possibility  of  liquefaction,  or  increased  weight  due  to  the  addition  of  water.      

Decreased  shear  strength   Increase  shear  strength  • Use  iron  rods  and  anchors  • If  the  slope  is  made  of  highly  fractured  rock,  rock  bolts  may  be  emplaced  

to  hold  the  slope  together  and  prevent  failure.    Saturation  of  soil   Prevent  Saturation:  

• Re-­‐compact  soils  • Prohibit  over-­‐irrigation  • Install  surface  drains  • Install  subsurface  drains  

Reworking  of  fill   Construct  retaining  wall  with  anchors    In  mountain  valleys  subject  to  mudflows,  plans  could  be  made  to  rapidly  lower  levels  of  water  in  human-­‐made  reservoirs  to  catch  and  trap  the  mudflows.  Steep  slopes  can  be  covered  or  sprayed  with  concrete  covered  with  a  wire  mesh  to  prevent  rock  falls.    

Devegetation   Re-­‐vegetate  soil  slopes    Sediment  flows:  Slurry  flows:  - Soilfluction  (waterlogged)  - Debris  flows  (Material  more  coarse)  - Mudflows  (High  water  content)  - NEED  WATER  Granular  flows:  - Creep  - Debris  avalanches  - Flowing  mass  supported  by  grain  to  grain  contact  or  collision