Lesson 2.2

• Geological Hazards


  Natural phenomena that cause major problems around the world
• Most geological hazards are related to natural conditions, although some may be due to human activities
• Landslide


  Ground movement on a sloping terrain

  
• Landslides do not happen on flat ground because of the angle on the ground, gravity induces the land to move downward

  
• Landslides are aggravated by rain because water is a natural agent for erosion
• Structures built on steep-slope mountains

  • Have a high vulnerability to landslide hazards especially during heavy rains
  • Areas with steep slope, dense population and denuded terrain are distinguished by a high susceptibility to rainfall-induced landslide hazards

  
• Without plants and trees whose roots can absorb water and hold the soil together, subsequent rain water can continue to loosen up the soil that anchors the buildings
• Types of Landslides


  • Soil Creep
  • Slumping
  • Debris Flow
  • Rock Fall

  
• Soil Creep


  Very slow downslope movement of particles that occurs in every slope covered with loose, weathered material
• Slumping


  Downward movement of rock debris, usually the consequence of removal of buttressing earth at the foot of a slope of unconsolidated material

  
• Debris Flow


  Happens when the slope becomes saturated with water, this then triggers a landslide of water-soaked mass of rock and soil that slides down the slope

  
• Rock Fall


  Sudden slides caused by heavy rain the rock on the slope loosens and then slides down the slope

  
• Causes of Landslides


  • Climate
  • Earthquake
  • Weathering
  • Erosion
  • Volcanic eruption
  • Forest fires
  • Gravity

  
• Climate


  Long-term climatic changes can significantly impact soil stability. A significant upsurge in precipitation or ground saturation would dramatically increase the level of ground water. When sloped areas are completely saturated with water, landslides can occur. If there is absence of mechanical root support, the soils start to run off.
• Earthquake


  Seismic activities have, for a long time, contributed to landslides across the globe. When earthquakes strike areas with steep slopes, on numerous occasion, the soil slips leading to landslides. Ashen debris flows instigated by earthquakes could also cause mass soil movement.

  
• Weathering


  It is the natural procedure of rock deterioration that leads to weak, landslide-susceptive materials. It is brought about by the chemical action of water, air, plants and bacteria. When the rocks are weak enough, they slip away causing landslides.

  
• Erosion


  Erosion caused by sporadic running water such as streams, rivers, wind, currents, ice and waves wipes out latent and lateral slope support enabling landslides to occur easily.

  
• Volcanic eruption


  If an eruption occurs in a wet condition, the soil will start to move downhill instigating a landslide. Volcanic gases partially dissolve in groundwater which turns into acidic hydrothermal systems that weakens rock by altering minerals to clay.
• Forest fires


  Since it takes time for vegetation to re-establish on steep slopes, the risk of landslides prevails or up to 20 years in dry climates. Wildfire removes the protective ability of vegetation which increases runoff, erosion, and debris flow because the water cannot penetrate through the soil.
• Gravity


  Steeper slopes coupled with gravitational force can trigger a massive landslide. Steeper slopes can also cause the debris to move farther from the foot of the area.

  
• Human Causes of Landslides

  • Mining
  • Clear cutting

  
• Mining


  Mining activities that utilize blasting techniques contribute mightily to landslides. Vibrations emanating from the blasts can weaken soils in other areas susceptible to landslides. Earthwork alters the shape of a slope, imposing new loads on an existing slope.
• Clear cutting


  Clear cutting is a technique of timber harvesting that eliminates all old trees from the area. This technique is dangerous since it decimates the existing mechanical root structure of the area. It changes the amount of water infiltrating the soil.
• Warning Signs of an Impending Landslide

  • Earlier landslide as indicator
  • Tension cracks
  • Things Moving
  • Water Doing Something Different

  
• Earlier landslide as indicator

  Inspecting an area of an old landslide for scarps and deposits is a clear indicator that a landslide will reactivate. This is also a sure sign that much of the region's underlying geology is fragile and vulnerable to landslides.

  
• Tension cracks


  These are caused by the stress and friction produced by geologic materials moving apart which forms steep lines of cracks in the terrain. Tension cracks above an existing landslide can hint at a future reactivation. Cracks that are found on flat terrain are caused by fault movement and not landslide indicators.

  
• Things Moving


  Deformation and movement of non-living objects not caused by human manipulation can also indicate a landslide. Slow landslides are categorized by movement of floor tiles, deformation of door frames which causes difficulty in closing and opening the door, and broken electric posts, gas, water and sewage pipes.

  
• Water Doing Something Different

  A debris flow is a very wet, very mobile landslide, where water is loaded with trees, mud, rock, and everything else caught in the current. Low water level precedes the arrival of the debris flow surge.

  
• Sinkhole

  A topographic depression created when groundwater dissolves the underlying limestone bedrock. Often known as "sink" or "doline".
• Sinkholes


  • Occur in areas where the soil foundation is made of soft minerals and rocks such as limestone, salt beds, or any acidic rocks
  • The depth of sinkholes ranges from a couple of meters to several mile deep
  • Water from the rainfall seeps underneath the soil through the cracks and fissures
  • As water passes through these cracks and fissures, it erodes the soil and forms a conduit system, these underground water systems increase in size as the soil is carried by the water through internal erosion
  • It develops suddenly (over an hour period) thus, causing catastrophic damage
• Types of Sinkholes


  • Cover Collapse
  • Cover Subsidence
  • Dissolution
  • Artificial
• Cover Collapse Sinkhole


  They occur where the covering sediments contain a significant amount of clay.
• Cover Subsidence Sinkhole


  It gradually grows where the sediment covers are permeable and contain sand.
• Dissolution Sinkhole


  It occurs in areas where calcareous is exposed on the ground or where thin layers of soil and permeable sand are also covered. Limestone or dolomite dissolution is most intense when the water first reaches the rock surface.
• Artificial Sinkhole


  Such types of sinks may be caused by various human activities, including groundwater pumping and building.
• Causes of Sinkholes


  • Dissolution of Sedimentary Rocks
  • Underground water pumping
• Dissolution of Sedimentary Rocks

  Sinkholes occur when sedimentary rocks are dissolved. When these rocks are dissolved, it can either form a void of water or air. Sinkholes can be in the form of holes or depressions on the ground.
• Underground water pumping


  Pumping water underneath the ground causes drainage to alter their flow which causes the soil to be eroded by water. Broken pipes underneath the ground can also cause internal erosion making a void underneath the ground. Altering drainage systems cause water to branch into other routes where there is soluble rock can cause erosion through the flow of water.
• Warning Signs of an Impending Sinkhole


  • Trees or fence posts that tilt or fall
  • New small ponds that appear after rain
  • Sudden drainage of a pond
  • Rapid appearance of a hole in the ground
  • Well water that is discolored or contaminated with debris
  • Cracking or buckling of home's concrete slab
  • Earthly odor in home after rain
  • Uneven floors, warping of hardwood, bulging or sagging sections
  • Dead patches of grass or plants
  • Separation between walls and ceiling or floors
• Signs of an impending geologic hazard may or may not always cause a landslide or a sinkhole but the presence of two or more of these signs may indicate that there is something wrong happening in the area which alerts people to be cautious and careful for possible dangers that it may cause.