GNR 639 GNR 639 : Natural Disaster And Management Lesson 5 Landslide 5.01 Prof. R. Nagarajan, CSRE , IIT Bombay
GNR 639 GNR 639 : Natural Disaster And Management Landslide • What is it? • Causes • What happens • Where does it occur? • Indicators? • Monitoring • Prevention • Vulnerability • Preparedness? 5.02 Prof. R. Nagarajan, CSRE , IIT Bombay
GNR 639 GNR 639 : Natural Disaster And Management The cohesion of materials on mountain slope is disturbed by exogenous or endogenous factors. Movement debris from these slope inflict damages. Mass wasting is a natural process that continuously shapes the landscape that occurs without human involvement. Any movement of a mass of rock, soil or both down a slope by gravity, is called “ landslide ” . Depending on the thickness of the material removed from the in-situ surface, they are classified as • superficial (0-3m) occur in areas that have slopes with high permeable soils on top of low permeable bottom soils. • shallow (3-10m), intermediate (10-30m), deep-seated (30-100m), • very deep seated (100-300mm) the sliding surface is located below the maximum rooting depth of trees (> 10meters). It involves deep regolith, weathered rock, and/or bedrock and includes large slope failure associated with translational, rotational, or complex movement. • and extremely deep seated (>300m). 5.03 Prof. R. Nagarajan, CSRE , IIT Bombay
GNR 639 GNR 639 : Natural Disaster And Management Common causes of landslides in soil are: 1) Falls of the parent material or residual soil from above, due to natural weathering processes; 2) Increased moisture content and consequent softening of the soil, or a rise in the water table. They are due to excessive tree clearance, poorly executed soak-away drainage or septic systems, or heavy rainfall; 3) Excavation without adequate support, increased surface load from fill placement, or inadequately designed shallow foundations; 4) Natural erosion at the toe of the slope due to scour by a river or the sea 5) Re-activation of an ancient landslide. 5.04 Prof. R. Nagarajan, CSRE , IIT Bombay
GNR 639 GNR 639 : Natural Disaster And Management Causes include factors that increase the effects of down-slope forces and contribute to low or reduced strength. Natural causes • Increase in pore water pressure by saturation of slope material from either intense or prolonged rainfall and seepage • vibrations caused by earthquakes • undercutting of cliffs and banks by waves or river erosion • volcanic eruptions. Human causes • removal of vegetation • interference or modification of natural drainage • leaking pipes such as water and sewer reticulation • modification of slopes by construction of roads, railways, buildings, etc • overloading slopes • mining and quarrying activities • vibrations from heavy traffic, blasting, etc • excavation or displacement of rocks. 5.05 Prof. R. Nagarajan, CSRE , IIT Bombay
GNR 639 GNR 639 : Natural Disaster And Management Landslides are reported from landscapes are: • commonly characterised by cliffs, steep slopes of colluvium deposits or gentle slopes of unstable geology • slopes subjected to periods of prolonged or intense rainfall. • locations which have previous evidence of landslide activity • slopes made up of low strength, sensitive, collapsible weathered or disjointed material with internal and external weaknesses. 5.06 Prof. R. Nagarajan, CSRE , IIT Bombay
GNR 639 GNR 639 : Natural Disaster And Management Source courtesy: lampre-project.eu 5.07 Prof. R. Nagarajan, CSRE , IIT Bombay
GNR 639 GNR 639 : Natural Disaster And Management Potential landslide sites • Saturated ground or seeps in areas which are not typically wet • new cracks and scarps or unusual bulges in the ground, roads or pavements • movement of ancillary structures such as decks and patios in relation to a house • Sticking doors and windows • tilting or cracking of concrete floors and foundations • broken water lines and other underground utilities • leaning telephone poles, trees, retaining walls or fences • offset fence lines • sunken or displaced road surfaces or kerbs • a rapid increase in creek water levels, possibly accompanied by greater turbidity. 5.08 Prof. R. Nagarajan, CSRE , IIT Bombay
GNR 639 GNR 639 : Natural Disaster And Management The observations prior to the reported landslide events 1) open cracks, or steps, along contours; 2) ground water seepage, or springs; 3) bulging in the lower part of the slope; 4) hummocky ground; 5) trees leaning down slope, or with exposed roots; 6) debris/fallen rocks at the foot of a cliff; 7) tilted power poles, or fences 8) cracked or distorted structures. 5.09 Prof. R. Nagarajan, CSRE , IIT Bombay
GNR 639 GNR 639 : Natural Disaster And Management • Rapid snow melting caused by sudden warming spells or by rain falling on snow can add to hillside soil. Rain-on-snow events commonly reduce the water content of the snowpack and add sufficient water to soils to be significant in triggering landslides. • The sudden lowering of the water level on a slope can trigger landslides in earth dams, along coastlines and on the banks of lakes, reservoirs, canals and river. • Increase in groundwater levels on hill slopes following periods of prolonged above- normal precipitation can build up pore pressure and reduce effective strength of saturated slope materials and can trigger landslides. • Landslides occur during or immediately after earthquakes as a result of two separate but interconnected processes - seismic shaking and pore-water pressure generation. Seismically generated landslides tend to be more widespread and sudden. • Cliff failures - Coastal cliffs and headlands exist are subjected to repeated cycles of wetting and drying. They are also accompanied by the expansive effect of salt crystal growth in gaps in the rocks. 5.10 Prof. R. Nagarajan, CSRE , IIT Bombay
GNR 639 GNR 639 : Natural Disaster And Management • Sand dune escarpment failures - Waves produced by large oceanic storms erode beaches and cut escarpments into dunes. After an escarpment has eroded, assets located on the land are damaged by subsequent slumping of the dune. • landslide dam , is a barrier lake across a river by debris flow, rock avalanche or volcano. A common failure scenario is overflowing with subsequent dam breach and erosion by the overflow stream. Landslide dams are responsible for two types of flooding: back flooding (upstream flooding) upon creation and downstream flooding upon failure. Compared with catastrophic down flooding, relative slow back flooding typically presents little life hazard, but property damage can be substantial. Preventive measure are 1) Stabilization of slope forming material; 2) Surface drainage control to prevent infiltration and pre pressure development; 3) Arrest of boulder movement – net, wall and Diversion of debris movement. Warning Devices such as trip wires, telephone alarm, sirens, evacuation plane etc are used. Warning devices of landslides would alert the effects of a materialized danger by slight modification. 5.11 Prof. R. Nagarajan, CSRE , IIT Bombay
GNR 639 GNR 639 : Natural Disaster And Management Type of movement Type of material Engineering soils Bed rock Predominantly coarse Predominantly fine Fall Rock fall Debris fall Earth fall Topple Rock topple Debris topple Earth topple Slide Rock slide Debris slide Earth slide Spread Rock spread Debris spread Earth spread Flow Rock flow Debris flow Earth flow Velocity Description Velocity Typical velocity Probable destruction class (mm/sec) > 5 X 10 3 7 Extremely > 5 m/sec Catastrophe of major violence, Rapid building destroyed by impact of displaced material 5x10 3 to 5x10 1 6 Very 5 m/sec to 3 Some lives lost; velocity too great to Rapid m/min permit all persons to escape 5x10 1 to 5x10 -1 5 Rapid 3 m/min to 1.8 Escape evacuation possible; structu m/hr possession and equipments destroyed 5x10 -1 to 5x10 -3 4 Moderate 1.8 m/hr to 13 Some temporary and insensitive m/month structures can be temporarily maintained 5x10 -3 to 5x10 -5 3 Slow 13 m/month to 1.6 Remedial construction can be m/year undertaken during movement; insensitive structures can be maintained with frequent maintenan work if total movement is not large during a particular acceleration phase 5x10 -5 to 5x10 -7 2 Very slow 1.6 m/year to 16 Some permanent structures mm/year undamaged by movement < 5x10 -7 1 Extremely < 16 mm /year Imperceptible without instruments, slow construction possible with precautio 5.12 Prof. R. Nagarajan, CSRE , IIT Bombay
GNR 639 GNR 639 : Natural Disaster And Management Source coutressy: usgs.gov 5.13 Prof. R. Nagarajan, CSRE , IIT Bombay
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