Mass MovementsGE 4150 – Natural Hazardshttp://www.ngi.no/vr2005/eng/big_image/kom650x4.jpgMass MovementsMass Movements• Large volumes of material move downslope under the pull of gravity. • The mass must overcome:–Friction-resistance to motion of a body that –Friction-resistance to motion of a body that keeps it from moving over another body, and– Inertia- tendency of a body to remain at rest until external force is appliedCreepCreep• Slowest moving and most widespread• Downward movement of soil and bedrockof soil and bedrock• Why does soil move:– Swelling– Expands (clays)– Heating from the sunExternal ProcessesExternal Processes• Addition of high mass material on a slope (sediment deposition, buildings)• Steeping the slope (due to construction or faulting)faulting)• Removing support from lower portions of the slope (stream erosion)Internal CausesInternal Causes• Inherently weak materials such as clays• Water weakens materials• Decrease in cohesion• Geological Structures (previous mass movements)Weak MaterialsWeak Materials• Clay crystals are very small• Form in sheets•Strength is lessened •Strength is lessened by:– Water absorbed in the exterior of clays– Water absorbed between the sheetsQuick ClaysQuick Clays• Occur in Scandinavia, Eastern Canada, and the Northeast U.S.• Begin as fine rock flour scoured off the landscape by glaciers and deposited in nearby landscape by glaciers and deposited in nearby seas• Glaciers retreat, isostatic rebound lifts sediment• Freshwater passes through the clays and can collapse by a jarring event, such as vibrations from construction equipmentWaterWater• Adds weight, making the mass heavier• Clays absorb water• Piping- water flowing underground erodes material, therefore making the ground less stablestable• Dissolve content, weakening the mass• Pore water pressure- pressure builds due to trapped water beneath the surface• Water tables- as they rise the ground becomes unstableVaoint, Italy 1963Vaoint, Italy 1963• Dam was built in 1960, the reservoir impounded 150 million m3of water• Dangers at the site–Rocks folded into a syncline–Rocks folded into a syncline– Fractures split the rocks apart– Slide surfaces were present in the rock from old slides– Some rock layers contained layers of clays– Limestone was present creating caverns– Water rising created elevation pore-water pressuresVaoint, Italy 1963Vaoint, Italy 1963• In addition the region had heavy rain raising the water table • The rain added large weight on the slopes• On October 9 the south wall of the reservoir failed and killed 3,000 people in 7 minutes. The failed and killed 3,000 people in 7 minutes. The slide was 1.1 miles long and 1 mile wide and had a volume of 240 million m3. The valley was filled with debris up to 500 feet• A wall of water entered the village over 230 feet highCohesionCohesion• Rocks expand and create fractures increasing the porosity•Porosity-void spaces in the material•Porosity-void spaces in the material• Reduces strength of the mass causing the mass to moveGeologic StructuresGeologic Structures• Ancient slide surfaces- create slick surfaces•Rock Orientation•Rock Orientation• Weaknesses in the rocksCommon TriggersCommon Triggers• Heavy rains• Earthquakes• Thawing of frozen ground• ConstructionTypes of Mass MovementTypes of Mass Movement• Falls- falls downward from a topographic high• Sliding- mass slips down and out•Flowing-mass behaves like a fluid flowing •Flowing-mass behaves like a fluid flowing down and out• Subsidence- downward collapse of the surfaceNatural Disasters 6thEdition by Patrick L. AbbottNatural Disasters 6thEdition by Patrick L. AbbottFallsFalls• Occur when elevated rock masses separate along joints or weaknesses• Yosemite 1996- 162,000 ton of mass gives way in two separate pieces. Each slide way in two separate pieces. Each slide down 540 ft and are launched into the air for a 1640 ft drop reaching a speed of 270 mph before hitting the valley floorYosemiteYosemite• The mass shattered into a cloud of rock, rolled across the valley and part of the way up the opposite wall before flowing down the canyon• The falling mass pushed air ahead of it which knocked down over 1000 trees and covered 50 acres with a thin blanket of rock• One person was killed by the air blastSlidesSlides• Landslides slide atop a basal surface which may be planar or curvedFlowsFlows• Behave like fluids• Water content varies between dry and wet• Velocities range from creep (very slow) to over 200 mphover 200 mph• Within the moving masses, internal movements dominate and slip surfaces are absent or short livedLa Conchita, CaliforninaLa Conchita, Californina• 1995- 1sta slump occurred moving slow enough for people to move, but destroyed nine houses. 2nda few days later a debris flow destroyed 5 more homes.• 2005- 15% of the 95 slide mass remobilized into a highly fluid debris flow traveling 22 mph. The flow overran a retaining wall and moved into La Conchita moving 10 mph, destroying 13 houses, damaging 23 others, and killing 10 people. The flow occurred after 15 days of heavy rainsLa Conchita Flow 1995La Conchita Flow 2005http://www.youtube.com/watch?v=W4KWxglDL3oSubsidenceSubsidence• Ground moves downward• Surface either sags gently or drops catastrophically • Occurs from the slow compaction of loose water-saturated sediments or the rapid collapse of saturated sediments or the rapid collapse of overlying earth into caves• Causes: Delta compaction (Louisiana), Groundwater withdrawal (Mexico City), Oil withdrawal (Houston), Long-term subsidence (Venice)SinkholesSinkholes• Occur in Southeastern U.S.• Limestone is composed of organic material (shells, skeletons etc) and is formed in shallow warm ocean waterswarm ocean waters• As water runs through limestone the organic material is dissolved and caverns are formed• During drought, water may be removed leaving caverns underground which cannot support the mass above, therefore creating sinkholesSnow AvalanchesSnow Avalanches• Also move as creep, fall, slide, and flow• Speeds up to 230 mph recorded in Japan• Mass movements initiate when snowfall builds to 0.5-1.5 m, but may not fail until 2-5 m.•Two types:•Two types:– Loose powder avalanches- low amount of cohesion, fails at a point source, runs downhill, and spreads out laterally– Slab