mass movement

• Incompetent rocks are mechanically weak and unable pf supporting steep slopes. Prone to rotational slumping and mudflows when undermined with erosion or saturated with water. 
• Incompetent rocks have a high proportion of clay minerals, have a low shear strength (when wet) and fail along curved surfaces and create a toe of debris. 
• Competent rocks are mechanically strong and are capable of supporting steep slopes - failing produces landslides. 
• Competent rocks fail along bedding planes, joints or fractures. Rock moves together downslope before breaking up and becoming a landslide. 
• Mass of rocks on slopes ae held by friction. Increasing mass and slope too much or decreasing friction results in a downhill movement as gravity is large enough to overcome friction. 
• Increasing mass - adding buildings or adding water. 
• Decreasing friction - adding water to increase hydrostatic pressure or reduce clay strength. 
• Alter slope - erosion by waves or rivers, road excavation or tunnels. 
• Stability:

  Rock type influences the types of weathering processes that occur. Physical weathering creates joints, hydrolysis will make clay minerals - weakening granite. 

   
• Clay is prone to slumping. Water is easily absorbed onto surfaces of clay minerals - reduces cohesion. Alternating drying and wetting weakens clay so the m
• The number of bedding planes, laminations, joints, pore spaces, cleavages and faults controls porosity, permeability and the amount of water that can enter a rock. 
• Water content:

  Rainwater percolated through permeable rock, the higher the column of water above, the higher the hydrostatic pressure. The pressure forces the grains apart and weakens the rock. 

   
• Water content:

  As water content increases the stability decreases mass movement is more likely after periods of wet weather. Water reduces friction of slip surfaces. 

   
• The critical angle for loose material is 35 degrees. Slopes of clay are unstable over 10 degrees. As slope angle increases the force of gravity overcomes frictional force. 
• Vegetation helps to stabilise slopes. Roots bind weathered surface debris together and anchor into the bedrock. 
• Vibrations from heavy traffic or earthquakes reduce friction between rocks and particles. 
• Slope failure:

  Increased pore water pressure due to periods of prolonged rainfall and water table rising. 
• Slope failure:

  Seismic events trigger slope failures. 
• Slope failure:

  Removal of toes support due to erosion undercutting of riverbeds or cliffs. 

   
• Slope failure:

  Deforestation is the clearance of vegetation from slopes. 

   
• Slope failure:

  Increased mass at the top of slope. 

   
• Slope monitoring:

  Aerial photographs and satellite imagery can be used to identify changes over time. 

   
• Slope monitoring:

  Electronic distance measuring that uses a laser beam to measure distance between fixed points. 

   
• Slope monitoring:

  Tiltmeters and inclinometers measure any changes in slope angle. 

   
• Slope monitoring:

  Creep meters and strainmeters bridge the gap across faults joints and cleavages. Measures the changes in weakness stress. 

   
• Slope monitoring:

  Boreholes can be used to measure water levels and pore water pressure. Extensometers can be installed to measure movement of unconsolidated material at depth (borehole distortion). 

   
• Slope monitoring:

  Micro seismics monitor slopes form small earthquakes using geophones. 
• Incompetent rocks are mechanically weak and unable pf supporting steep slopes. Prone to rotational slumping and mudflows when undermined with erosion or saturated with water. 
• Incompetent rocks have a high proportion of clay minerals, have a low shear strength (when wet) and fail along curved surfaces and create a toe of debris. 
• Competent rocks are mechanically strong and are capable of supporting steep slopes - failing produces landslides. 
• Competent rocks fail along bedding planes, joints or fractures. Rock moves together downslope before breaking up and becoming a landslide. 
• Mass of rocks on slopes ae held by friction. Increasing mass and slope too much or decreasing friction results in a downhill movement as gravity is large enough to overcome friction. 
• Increasing mass - adding buildings or adding water. 
• Decreasing friction - adding water to increase hydrostatic pressure or reduce clay strength.