Mass Wasting

Mass Wasting (also Mass Movement): – is the down slope movement of earth materials under the influence of gravity. The detachment and movement of earth materials occurs if the stress imposed is greater than the strength of the material to hold it in place. – Mass movement is a naturally occurring process that contributes to the cycle of tectonic uplift, erosion, transportation, and deposition of sediments. They are responsible for the topography of mountain ranges and river canyons that has developed over geologic time. Types of mass wasting: A. Slide involves movement of coherent blocks of material along a well-defined surface 1.

Rockslide • Also called debris slides or “landslides”. Occurs when blocks of rock, or masses of unconsolidated material slide down a slope. These are among the most destructive of mass movements. May be triggered by rain or melting snow, or earthquakes. 2. Slump • Slumps involve a mass of soil or other material sliding along a curved, rotational surface. (Shaped like a spoon. ) Slumps are sometimes seen along interstate highways where the graded soil on the sides of the road is a little too steep. 3. Creep • A SLOW downhill movement of soil and regolith.

Creep results in tree trunks that are curved at the base, tilted utility poles, fence posts, and tombstones, and causes retaining walls to be broken or overturned. B. Fall involves free fall of material (no contact with any surface except to bounce) Rock fall -The free fall of detached pieces of material of any size; may fall directly downward or bounce and roll. May occur as result of freeze-thaw, or the loosening action of plant roots. Causes the formation of talus slopes. Signs along highways warn of rolling rock in mountainous areas where the road has been cut into the hillside.

C. Flow involves continuous movement of material as a viscous fluid 1. Debris flow or mudflow • Commonly occur in volcanic areas, where they are called lahars. Mudflows generally follow established drainage patterns (valleys). 2. Earthflow • Form in humid areas on hillsides following heavy rain or melting snow, in fine-grained materials (clay and silt). Also occurs at the toe of slumps. Rate of movement varies (less than 1 mm per day to several meters per day), but may be long-lived (days to years). Includes the liquefaction associated with earthquakes. 3. Solifluction also known as soil fluction or soil creep, where waterlogged sediment slowly moves downslope over impermeable material. It can occur in any climate where the ground is saturated by water, though it is most often found in periglacial environments where the ground is permanently frozen (permafrost) Forces involved in mass wasting are: 1. gravity, a vertical force that can be split into vectors parallel to (tangential) and perpendicular to a surface(normal) 2. friction on the surface or between grains 3. shear strength, a measure of material strength and cohesion Kinds of Material Moved 1. Bedrock. . Soil (Regolith) 3. Water Mass movements are caused by various conditions: • Volcanic activity many times causes huge mudflows when the icy cover of a volcano melts and mixes with the soil to form mud as the magma in the volcano stirs preceding an eruption. • During heavy rainfall or rapid snow melt, water rapidly accumulates in the ground, changing the earth into a flowing river of mud. • Earthquake shocks cause sections of mountains and hills to break off and slide down. • Human modification of the land or weathering and erosion help loosen large chunks of earth and start them sliding downhill. Vibrations from machinery, traffic, weight loading from accumulation of snow; stockpiling of rock or ore; from waste piles and from buildings and other structures. • Gravitational pull of the earth on soil, rocks, and mud is the force behind mass movements. Factors Affecting Mass Wasting Several factors that determine the extent of mass wasting are: 1. Slope stability. The steeper the slope, the less stable it is. If the angle on the slope is great than the angle of repose, the slope will fail, resulting in mass wasting. Angle of Repose -maximum angle at which unconsolidated material on slope is stable • Slope stability depends on the nature of material • Slope stability also depends on the driving and resisting forces that act on the slope • 2. The degree of chemical weathering. In regions where there is more chemical weathering, there is a greater chance for mass wasting. • 3. The water content. Water adds weight to the slope, increase pore spaces in between grains, makes it easier for material to slide down the slope. • . Vegetation. The amount of vegetation can help reduce the rate of mass wasting. The roots help absorb water and help keep the soil in place. Removal of vegetation will speed up mass wasting. • 5. Overloading. Adding too much weight on a slope can increase water pressure. • 6. Geological features. When bedding is dipping in the same direction of the slope, there is a greater chance for mass wasting. Joints in the rocks also allow more water to seep into the ground. The type of rocks and their composition also affects mass wasting. Rate of Movement Slow movements primarily affect unconsolidated material at depths less than 1 meter, where movement rates average from 1 mm/yr to 1 mm/day: • Moderate velocity movements have movement rates that average from 1 cm/day to 1 cm/s. Mass movements of this type include: • Rapid movements, which have movement rates ranging from meters/second to 100 km/hr, occur on steep slopes. Social and Economical Impact of Mass Movements Mass movements produce a variety of effects. 1. loss of life 2. floods, damming up bodies of water 3. destruction to habitable land 4. Damages to structures or property . loss of tax revenues on devalued properties 6. reduced real estate values in landslide prone areas 7. loss of productivity of agricultural lands affected by landslides 8. loss of industrial productivity because of interruption of transportation systems by landslides 9. damage to railroads, building structure and underground pipes Preventive Measures 1. Bridge 2. Slope reduction and weight reduction 3. Retention Structures – walls or ground covers 4. Fluid removal – bore holes, subsurface drainage 5. Vertical piles driven into slide 6. Rock bolts 7. Flood control channels