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CHAPTER 7 SLOPE PROCESSES, LANDSLIDES, and SUBSIDENCE Introduction Case History: La Conchita Landslide

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CHAPTER 7 SLOPE PROCESSES, LANDSLIDES, and SUBSIDENCE Introduction Case History: La Conchita Landslide 9/23/2013 Introduction • Landslide and other ground failures posting substantial damage and loss of life CHAPTER 7 • In U.S., average 25–50 deaths; damage more than $3.5 billion SLOPE PROCESSES, LANDSLIDES, AND SUBSIDENCE • For convenience, definition of landslide includes all forms of mass-wasting movements • Landslide and subsidence: occurs naturally and affected by human activities 1995 Slide Case History: La Conchita Landslide • La Conchita: small coastal community 80 km (50 mi) northwest of Los Angeles • Landslide occurred on January 10, 2005 • Triggered by heavy rainfall, reactivation along an older landslide surface (35,000 years ago, 6000 years ago, and 1995) • Debris flow up to 45 km/h (30 mph) • La Conchita should not be built on the landslide deposits and under the foot of the slope • Potential solution: relocate people and better land use regulation 2005 Slide http://landslides.usgs.gov/recent/events/laconchita/ 1 9/23/2013 Slope Processes (1) • Slopes: The most common landforms • Consists of cliff face (free face) and talus slope or upper convex slope, a straight slope, and a lower concave slope Types of Landslides (1) Slope Processes (2) • Slow or rapid failure of slope: Slope gradient, type of slope materials, amount of water present, rate • Dynamic evolving feature, depending of movement upon topography, rock types, climate, vegetation, water, and geologic time • Rate of movement: Imperceptible creep to thundering avalanches • Materials constantly moving down the • Types: Creep, sliding, slumping, falling, flowage slope at varied rates or flow, and complex movement (sliding and flowage) Types of Landslides (2) 2 9/23/2013 Soil Slip Debris Flow Rockfall, Yosemite Home Destroyed, 2 Lives Lost Stream Erosion Soil Slip on Steep Slope Roots too shallow! 3 9/23/2013 Beachfront Erosion Japanese Landslide Tennessee Landslide Why slip surfaces are curved Driving vs. Resisting Forces can change… 4 9/23/2013 Slope Stability Imagine the following situation… • Safety Factor SF = Resisting Forces/Driving Forces If SF >1, then safe or stable slope If SF <1, then unsafe or unstable slope • Driving and resisting forces determined by the interrelationships of the following variables: Existing slip surface Type of Earth materials Slope angle and topography Climate, vegetation, and water Time Geologists need to do some math… Geologists need to do some math… Using some trigonometry Vector diagram illustrating and information on the relationships between materials involved, the vectors D (the weight Safety Factor can be downslope, given by caluclated. WsinΘ2), N (the normal force or force perpendicular FS = Safety Factor to the slope), and W, the S = shear strength of overall weight of the the material, in material in the situation. this case a clay L = Length of slip plane T = Unit thickness W = weight of the overlying material SF > 1.25 is best. SF for curved slip surfaces can be estimated Human Land Use and Landslide For a curved slip surface, using the • Urbanization, irrigation rotational geometry and applying the idea of torque (force around a • Timber harvesting in weak, relatively unstable areas moment arm) instead of direct force. The safety factor is still the ratio of resisting to driving forces/torques. • Artificial fillings of loose materials Also, note the recalculation if fluid • Artificial modification of landscape pressure is from increased moisture content is included! • Dam construction 5 9/23/2013 Warning of Impending Landslides • Monitoring changes Human surveillance Instrumental survey: Tilt meter and geophones • Landslide warning system Info for public awareness and education Enough time for public evacuation Stop or reroute traffic flow Emergency services The Vaiont Dam Disaster The Vaiont Dam Disaster On October 9, 1963 2600 lives were lost due to a Monitoring for 3-years leading up to the disaster massive landslide and subsequent flood in northern showed variable rates of slip from 1-30 cm per Italy. 238 million cubic meters of material slid into week. the nearly full reservoir, moving at speeds of nearly 60 mph. The displaced water made a wave 90 Starting in September 1963, slip increased to 25 cm (~300 feet) meters high that swept over the dam and per day and finally up to 1 m per day just before the down the valley, drowning people and washing fateful night. away homes in the valley below the dam. The Vaiont Dam Disaster The Vaiont Dam Disaster 6 9/23/2013 The Vaiont Dam Disaster What caused the Vaiont Dam disaster? 1. Adverse geology – weak rocks with open fractures, sinkholes and weak clay layers inclined towards the reservoir. 2. Steep topography = strong driving force. 3. As the reservoir filled, water pressure increased reducing friction and weakening the clay layers even more. 4. Heavy rains in late September increased the weight of slope materials and runoff into the reservoir caused the water level to rise further in spite of attempts by engineers to lower it. Minimizing the Landslide Hazard (1) Landslide Hazard Map • Identifying potential landslides Photographic analysis Topographic map and detailed field check Historic data • Landslide hazard inventory map Grading code from the least stable to the most stable • Application of geologic and engineering knowledge before any hillside development Minimizing the Landslide Hazard (2) • Preventing landslides Drainage control: Reducing infiltration and surface runoff Slope grading: Reducing the overall slope Slope supports: Retaining walls or deep supporting piles • Avoid landslide hazards Landslide warning for critical evacuations Correcting landslides 7 9/23/2013 Subsidence • Form of subsurface ground failure • Occurred naturally or induced by human activities Retaining Walls • Slow settling or rapid collapse can Prevent Mass Movements • Causes: Withdrawal of fluids (water, oil and gas, steam) or removal of solid materials (dissolution, mining) 8 9/23/2013 Process of Subsidence Removal of Solid Materials (1) • Sinkholes Dissolution of carbonate rocks, limestone, and dolomite Affecting most of the conterminous states Natural or artificial fluctuations in water table increasing the problem Triggering other problems: Sinkholes as waste dumping sites Removal of Solid Materials (2) November 21, 1980 • Salt and coal mining Salt dissolution and pumping Active coal mines and abandoned coal mines Ground failure due to depleted subsurface pressure More than 8000 km2 of land subsidence due to Lake Peigneur – an unexpected underground coal mining subsidence incident. 9 9/23/2013 Perception of the Landslide Hazard What Can You Do? (1) • Landslide hazard maps not preventing development • Professional geologic evaluation for a property on a slope • Common perception: “It could happen on other hillsides, but never on this one.” • Avoid building at the mouth of a canyon, regardless of its size • Infrequency and unpredictability of large slides reducing awareness of the hazards • Consult local agencies for historical records • Often people taking chances and unknown risks • Watch signs of little slides—often precursor for larger ones What Can You Do? (2) • Look for signs of structure cracks or damage prior to purchase • Be wary of pool leaking, tilt of trees and utility poles • Look for linear cracks, subsurface water movement • Put observations into perspective, one aspect may not tell the whole story 10.
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