7.2

• Ground subsidence


  The relative lowering of the earth's surface usually with respect to the mean level
• Causes of ground subsidence


  • Dissolution and collapse of limestone
  • Excessive groundwater withdrawal
  • Mining
  • Oil and gas extraction
  • Earthquakes
  • Changes of season
• Carbonate dissolution and collapse


  1. Acidic rainwater seeps into the ground and dissolves limestone

  2. Voids are created in the subsurface

  3. Ground surface sinks slowly or collapses rapidly, forming a sinkhole
• Carbonic acid (H2CO3)


  Formed by the mixing of carbon dioxide and water vapor in the atmosphere, dissolves limestone (CaCO3)
• Fluid pressure in permeable sedimentary rocks


  Supports the overlying rock layers and ground surface
• Withdrawal of fluids (e.g. by deep well pumps)


  Fluid pressure cannot be maintained, resulting in compaction of sedimentary rock layers and sinking of the ground surface
• Subsidence in CAMANAVA area of Metro Manila is manifested by perennial flooding
• Extraction of oil and natural gas is also a main cause of subsidence in the Philippines
• Droughts


  Ground gets dry, plants don't have enough water, hard for animals to find food and water
• Ground subsidence can also occur during liquefaction, which takes place in areas with fine-grained sediments or in areas that were reclaimed
• Underground mining method


  1. Can intentionally cause the collapse of the surface

  2. Subsidence can also occur unexpectedly in abandoned old portions of mine
• Change of season


  Clays in the soil expand during wet season, shrink during dry season
• Effects of ground subsidence


  • Slow rate, may not have terrifying effects like other natural disasters, but can cause serious, widespread damage to property
  • Sudden ground collapse events can be catastrophic but are usually isolated and very limited in magnitude
• Mitigating subsidence-related hazards


  1. Map out subsidence-prone areas

  2. Avoid development in the area or adapt by modifying or strengthening structures