slides

• what are slides?

  • failures along a slip plane, where an entire mass of material moves downslope as a coherent mass
  • material in a slide usually remains intact until impact at ground causes it to break up
  • velocity tends to remain similar throughout the displaced mass
• speed of slides


  rate of slides highly variable 1-10m/sec
• causes of slides

  • triggered mainly by earthquakes or sudden rock failures rather than by heavy rains
  • also by oversteepening of the base of a slope could also cause it
• materials involved in a slide

  hard rocks and consolidated regolith
• location of slides


  along bedding planes/ lines of weaknesses and can occur in areas of moderate relief
• types of slides

  • translational slides
  • rotational slides
• difference between the 2 types of slides

  • shape of surface of failure along which the mass of rock moves
  • translational slides -> relatively uniform slope
  • rotational slides -> curving, concave surface
• what is a translational slide?

  • occurs when there is a mass movement over a surface separating 2 different types of materials
  • the displaced mass slides along a pre-existing surface such as a bedding plane or joint surface
• cause of translational slide

  • occurs when a coherent mass of rock/soil detaches itself from the slope
  • mass slides downslope along the planar surface of the bedrock
  • if the material is a mix of soil, rock and organic matter -> debris slide
• location of translational slide

  • occur late in the rainy season in the humid tropics following a buildup of antecedent rainfall -> saturation
  • more common in humid tropics
  • arid tropics following a bout a heavy convectional rainfall
• evidence of translational slide

  piles of talus are common at bottom of slope as these fragments break up
• types of translational slides

  • rockslides
  • -> rapid displacement of masses of rock along an inclined surface like bedding plane
  • -> travel downslope at rapid speeds due to presence of a layer of compressed air trapped between the slide and ground surface that reduced frictional resistance
  • form a talus slope made up of angular fragments
  • angle of repose (angle at which debris stable) lies between 30-37 degrees
• what is a rotational slide (slump)?

  • movement of rotational slide moves in a downward and outward motion and occurs along a concave slip plane
  • top of the displaced material usually tilted backwards
• material involved in rotational slides

  occur in near heterogeneous materials, usually fine grained, where such masses of material are well weathered
• cause of rotational slides

  • oversteepening of base of the slope due to natural (eg: wave erosion) or human causes (eg: road construction)
  • removal of lower part of slope removes physical support for upper part o slope causing formation of new fracture in sediment/rock
  • soon the slope will begin sliding downwards
• location of rotational slides

  • common in deeply weathered materials which is relatively uniform in compostion
  • common in humid tropics due to intense weathering + thick regolith
  • occur along coastal cliffs
• evidence of rotational slide

  • at the top-> material falls away leaving a scar
  • upper surface of each slump block remains relatively undisturbed
  • at the bottom-> soil accumulates as a sluggish mass that piles up in ridges or lobes to form a bulging toe