Mass Wasting - Chapter 9 Physical Geology (17th Ed.) PDF

Mass Wasting Reading Assignment: Chapter 9 th Physical Geology (17 Ed.)  Mass Wasting Mass Wasting -Definition » Movement in which bedrock, rock debris, or soil moves down slope in bulk , or as a mass, because of the pull of grav...

Mass Wasting Reading Assignment: Chapter 9 th Physical Geology (17 Ed.)  Mass Wasting Mass Wasting -Definition » Movement in which bedrock, rock debris, or soil moves down slope in bulk , or as a mass, because of the pull of gravity. » Includes small, almost imperceptible, movement called creep to landslides.  Mass Wasting Importance » Cost life and property » According to USGS mass wasting killed more people in last 3 months of 1985 than all other geologic hazards in the last 20 yrs. » Cost an average of $1.5 B in U.S. and about 25 lives per year » Cost more than triple the combined cost of floods, earthquakes, hurricanes, and tornadoes. » A little knowledge of geology, appropriate preventive action could avert destruction  Mass Wasting Controlling Factors that influence rate of movement of mass wasting » 1. Slope » 2. Relief (valley floor and mountain summit vertical distance) » 3. Water or Ice » 4. Abundance of loose material » 5. Vegetation » 6. Earthquakes  Mass Wasting Others: Orientation of plane of weakness - bedding planes, foliation planes etc. Climatic factors: » Precipitation (heavy vs intermittent drizzles) » Affect vegetation type » Freezing and thawing  Mass Wasting Gravity » Major driving force in mass wasting » Two forces involved » 1. Normal force - perpendicular to slope (ability to hold rock in place ) » 2. Shear force - parallel to slope (ability to move rock)  Mass Wasting » The steeper the slope and heavier the rock, the greater the shear force. » Shear Strength – Resistance to movement and deformation is controlled by the cohesiveness of the material and is related to the normal force. » If shear strength is greater than shear force, debris will not move or be deformed.  Mass Wasting Water: Critical factor in mass wasting. When debris is saturated with water it becomes heavier and is more likely to flow down Reduction in shear strength due to increased pore pressure forces grains apart.  Mass Wasting Triggers Triggers are also a critical factor in mass wasting. Seismic (earthquake) activity. Heavy Rainfall. Construction – added weight Lack of vegetation – no roots to hold rock/soil in place. Landslide triggered by an earthquake in New Zealand, 2016  Mass Wasting Classification of mass wasting: - Based on »1. Rate of movement »2. Type of Material »3. Nature of Movement  Mass Wasting 1. Rate of Movement: »< 1 cm/year - > 100 km/hour »Slow to fast 2.Type of Material: - Based on the bedrock or debris »Debris-Unconsolidated material at Earth’s surface e.g. soil, or rock fragments, earth, or mud  Mass Wasting 3. Type of Movement: »1. Flow –Descending downslope as a viscous fluid »2. Fall- Mass free-falls or bounces down a cliff »3. Slide Descending mass remains relatively intact  Mass Wasting »Slide: 2 kinds - A. Translational slide; Mass moves along plane parallel to slope. - B. Rotational Slide (slump) : Involves curve surface. Upper part moves downward while lower part moves outward  Mass Wasting Types of Mass Wasting 1. Creep » Very slow downward movement of soil or unconsolidated debris » Shear force only slightly greater than shear strength » Rate, less than 1 cm per yr » Factors: water and daily cycles of freezing and thawing » Plant roots help anchor the soil, especially in slopes  Mass Wasting Slump » Material moves en masse along a curved surface. » Involves rotational slide. » Usually does not travel fast nor very far  Mass Wasting 2. Flow: » Earthflow » Solifluction » Mudflow » Debris avalanche » Lahars  Mass Wasting Debris flow » Mass wasting in which motion is taking place throughout the moving mass. » Relatively rapid » Involves flow of soil and regolith containing large amounts of water and coarse material (gravel, boulders).  Mass Wasting Earthflow » Debris moves downward as viscous fluid » Process slow or rapid » Occurs usually on a hillside with thick debris cover after heavy rains saturate soil. » Humans can trigger earthflow by adding too much water to soil from septic tanks, sprinklers, undercutting slopes, etc. A slump-earthflow  Mass Wasting Solifluction and Permafrost » Type of earthflow usually associated with colder climate. » Flow of water saturated debris over an impermeable material » Impermeable material beneath saturated soil can be bedrock or permafrost (ground frozen for several years).  Mass Wasting Mud Flows » Flowing mixture of debris (fine-grained) and water usually through a channel. » Most of solid particles in slurry are normally clay or silt. » Occurs usually after heavy rains. » Likely to occur in areas without vegetative cover.  Mass Wasting Debris avalanche » Fastest variety of debris flow, turbulent mass of debris, air, and water Lahars are volcanic mudflows and debris 3. Rockfall and Rockslides » Rockfall occurs when bedrock breaks off and falls freely or bounces down a cliff, talus forms at the base  Mass Wasting 4. Slides » Rockslide: Rapid slipping of a mass of bedrock along an inclined surface of weakness, such as a bedding plane, a major fracture in a rock, or a foliation plane. Usually breaks up into rubble. » Rock Avalanche: a very rapidly moving turbulent mass of broken-up bedrock.  Mass Wasting Debris Slides and Debris Falls » Debris Slide is a coherent form of debris moving along a well-defined surface. » Debris fall refers to a free-fall mass of debris.  Mass Wasting Human impacts on Mass Wasting - Construction generally makes a slope susceptible to mass wasting due to several changes caused to the landscape. - For example, undercutting of a slope removes natural support, removal of vegetation increases erosion and amount of water in debris i.e. increases weight, building on upper part also increases weight.  Mass Wasting Preventing Mass Wasting A. Preventative measures against mass wasting in debris include : - 1. Building of retaining walls where cut has been made to a slope. - 2. Putting in drainpipes to take off water from debris. - 3. Avoid oversteepening of slopes by using by cutting back using a series of terraces.  Mass Wasting B. Preventing Mass Wasting from Rockfalls and Rockslides on Highways. - Planes planes of weakness parallel to the slope of hill can lead to rockslides when road cuts are made. - These can be prevented by doing a detailed geologic study before road is built by choosing the least dangerous route or by removing a rock that may slide.