UNIT-V Slope Stability - Land Slides.ppt
- 1. Landslides, Slope Stability and Earth Retaining Structures Er.D.Mythili, Asst Prof, Civil Engg. Excel Engg College
- 3. Content • Landslides • Slope Stability • Methods to determine factor of safety • Retaining Structures • Subsidence • Major Disaster in India
- 4. A DISASTER • the set of failures that overwhelm the capability of a community to respond without external help when three continuums: 1) people, 2) community (i.e., a set of habitats, livelihoods, and social constructs), and 3) complex events (e.g., landslides, ...) intersect at a point in space and time.
- 5. LANDSLIDES Represent permanent deformation caused by the downward and outward, down-slope movements of large volumes of soil and/or rock under the influence of the force of gravity.
- 6. PHYSICS OF LAND SLIDE • Landslides occur naturally on slopes. • Landslides can be triggered and/or exacerbated by: 1) water (from precipitation during a tropical storm, hurricane, or typhoon), 2) vibrations (from ground shaking during an earthquake).
- 7. SITING AND BUILDING ON UNSTABLE SLOPES LANDSLIDES SOIL AND ROCK SUCEPTIBLE TO FALLS SOIL AND ROCK SUCEPTIBLE TO TOPPLES SOIL AND ROCK SUCEPTIBLE TO SPREADS SOIL AND ROCK SUSCEPTIBLE TO FLOWS PRECIPITATION THAT TRIGGERS SLOPE FAILURE SHAKING GROUND SHAKING THAT TRIGGERS SLOPE FAILURE CAUSES OF DAMAGE CASE HISTORIES
- 8. LANDSLIDES • Types and Processes (Canada Natural Resources 2002)
- 10. Slopes
- 11. • 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 • Dynamic evolving feature, depending upon topography, rock types, climate, vegetation, water, and time • Materials constantly moving down the slope at varied rates
- 12. LANDSLIDES Flow: continuous movement of soil masses where shear surfaces are short lived. (Turner and Schuster 1996) Spread: is the sudden movement of water bearing rock masses
- 13. LANDSLIDES Slide: is the down slope displacement of soil or rock masses. It includes: rotational, translational, and debris slide
- 14. Slope Stability • Slope Stability Analysis (Abramson et al. 2002) – understand the development and shape of natural slopes – determine the short-term and long term stability conditions – evaluate the possibility of failure of natural or engineering slides – analyze and understand failure mechanisms – enable the retrofit of failed slopes – understand the effect of seismic loading on slope and embankments
- 15. Slope Stability Effect of Water on Soils • Dry sand grains will form a pile. The slope angle is determined by the angle of repose (i.e., the steepest angle at which a pile of unconsolidated grains remains stable - controlled by the frictional contact between the grains. It usually lies between about 30 and 37 degrees. (Nelson 2004) Dry sand Angle of repose Grain-to-grain frictional contact
- 16. Slope Stability Effect of Water on Soils • Slightly wet soil materials exhibit a very high angle of repose because surface tension between the water and the grains tends to hold the grains in place. (Nelson 2004) Wet sand Angle of repose Surface tension thin film
- 17. Slope Stability Effect of Water on Soils • When the material becomes saturated, the strength may reduced to a very small values and it may tends to flow… water (between the grains) eliminates grain to grain frictional contact. (Nelson 2004) Angle of repose Fully saturated sand Water surrounds the grain and prevent grain-to-grain contact
- 19. Common types of slope failure
- 20. • The quantitative determination of the stability of slopes is necessary in a number of engineering activities, such as: (a) the design of earth dams and embankments, (b) the analysis of stability of natural slopes, (c) analysis of the stability of excavated slopes, (d) analysis of deep seated failure of foundations and retaining walls.
- 21. Slope Types • Infinite – – soil is homogeneous – Failure surface is plane and parallel to ground – Boundaries not known – Ex- Himalaya
- 22. • Finite – – soil is non homogeneous – Failure surface is curved with arc of rotation – Boundaries known – Example- Ooty
- 23. Slope Stability • Safety Factor: = Resisting/Driving Forces If SF >1, then safe or stable slope If SF <1, then unsafe or unstable slope • Driving and resisting force variables: – Slip surface – “plane of weakness” – Type of Earth materials – Slope angle and topography – Climate, vegetation, and water – Shaking
- 24. Factor of safety of Infinite slope • The factor of safety is commonly thought of as the ratio of the maximum load or stress that a soil can sustain to the actual load or stress that is applied. • Factor of safety F, with respect to strength
- 25. where ζff is the maximum shear stress that the soil can sustain at the value of normal stress of σn is the actual shear stress applied to the soil
- 26. Definition of Factor of safety
- 27. FINITE SLOPES Method of Slices In the method, the sliding mass above the failure surface is divided into a number of slices. The forces acting on each slice are obtained by considering the mechanical (force and moment) equilibrium for the slices. Each slice is considered on its own and interactions between slices are neglected because the resultant forces are parallel to the base of each slice.
- 28. • Factor of safety for (Ø=0 Soil) fully saturated clay soil = Cu R2 θ / W x • Factor of safety for C – Ø soils = C’ L + tan θ ΣW cos α / Σ W sin α
- 29. Modified Bishop’s method • The Modified Bishop’s method is slightly different from the ordinary method of slices in that normal interaction forces between adjacent slices are assumed to be collinear and the resultant inter slice shear force is zero.
- 30. • Assumptions:- • circular arc, radius R, centre O • The soil mass above a trial failure surface is divided into slices by vertical planes. Each slice is taken as having a straight line base. • The Factor of Safety of each slice is assumed to be the same, implying mutual support between the slices, ie. there must be forces acting between the slices.
- 33. Taylors charts
- 37. • As a whole, retaining structures are not particularly effective methods of remedy. • They are very difficult to construct on an already moving slide. • One use of them, though, is to ensure complete stability of an existing (old) landslide, which may in the future be reactivated. • The wall provides additional resistance which is only mobilized by further deformation of the slope. • The force then acts along the line of action into the soil or rock beneath the slope
- 38. SOIL ANCHORING
- 39. •Often these soil anchors are stressed, and the force they exert on the slope needs to be considered along with the other forces. •The axial load on the anchor increases the effective stresses at depth, therefore increasing the strength of the slope. •A vector component of the force may also act to help stabilize the slope against destabilizing forces.
- 40. DRAINAGE METHODS Drainage is best used as a short-term stabilizing method, due to the fact that, in the long-term, the drains need much maintenance and repair, which is often difficult to perform, and expensive.
- 42. Surface Drainage on a slope on the road into Runswick Bay, a coastal village in Yorkshire, England
- 43. LOADING THE TOE
- 44. • This method is most effective for deep-seated instability. • The berm can either be constructed from material which is removed from the crest of the slope (involving regarding of the slope), or from that which is brought to the site from elsewhere.
- 45. GRASSING-OVER THE SLOPE • By covering over a slope with either sand or grass, we immediately reduce the amount of water which can infiltrate it. • This method is often used in conjunction with more effective and long-term methods. It's benefits in being inexpensive and simple, whilst still performing a stabilizing function, make it worth serious consideration.
- 46. GEOTEXTILES
- 47. • Geotextiles are man-made (usually plastic based) soil reinforcement materials. • In the area of slope stabilization, geogrids are used eg. In an embankment fill to reduce the amount of movement possible, keeping the fill in place. • Very often the geogrid is used as an anchor, providing a reaction against the disturbing moment. • They are often also used to repair small slides in engineering earthworks. They perform this function well.
- 48. Slope formation using Geogrid
- 49. • This can be done in three ways:- – Regrading the slope to a flatter angle, – Reducing the overall slope height, keeping its profile unchanged, – Removing some material from the crest and placing it at the toe. REGRADING THE SLOPE
- 50. • Acheiving a flatter slope angle: can be done by either cutting material away from the slope, or adding (filling) material. This method is most effective where there are predominant shallow forms of instability. See the case study on Walton's Wood for a practical example.
- 51. • Reducing the height of the Slope: In a man- made (earthworks) slope, this method is not very practical as the height of the slope usually is not able to be changed due to design. For a natural slope though, this may well be considered. • However, the reduction in instability achieved using this method is much lower than that achieved by say loading the toe, unless the failure is deep-seated. It does not help much in the case of shallow failures.
- 52. WHICH METHOD IS BEST? METHOD NOTES 1 Regarding the Slope (includes Loading the Toe) Has an immediate effect Unlikely to become uneffective with time. 2Drainage Should be used if regrading is impractical Has immediate effect in high- permeability soils. Takes more time to take effect in fine-grained soils. Surface drainage is very common.
- 53. METHOD NOTES 3 Incorporating Structures into the Slope (eg. nailing, geotextiles, retaining structures). These structures can be active (eg. Stressed nails/anchors), or passive (eg. walls or sheet piling.) Passive schemes only take effect on further movement of the slope, which of course may not be desirable.
- 54. Subsidence • Subsurface ground failure • Natural or human- induced • Slow settling or rapid collapse • Causes: – Withdrawal of fluids (water, oil and gas, steam) – Removal of solid materials (dissolution, mining)
- 55. • 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
- 56. • 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 underground coal mining Removal of Solid Materials
- 57. Major Disaster in India Sl.No Name of Event Year State & Area Fatalities 1. Sikkim Earthquake 2011 North Eastern India with epicenter near Nepal Border and Sikkim Most recent disaster 2. Cloudburst 2010 Leh, Ladakh in J&K 3. Drought 2009 252 Districts in 10 States ----- 4. Floods 2009 Andhra Pradesh, Karnataka, Orissa, Kerala, Delhi, Maharashtra 300 people died 5. Kosi Floods 2008 North Bihar 527 deaths, 19,323 livestock perished, 2,23,000 houses damaged, 3.3 million persons affected 6. Cyclone Nisha 2008 Tamil Nadu 204 deaths
- 58. 7. Maharashtra Floods 2005 Maharashtra State 1094 deaths 167 injured 54 missing 8. Kashmir 2005 Mostly Pakistan, Partially Kashmir 1400 deaths in Kashmir (86,000 deaths in total) 9. Tsunami 2004 Coastline of Tamil Nadu, Kerala, Andhra Pradesh, Pondicherry and Andaman and Nicobar Islands of India 10,749 deaths 5,640 persons missing 2.79 million people affected 11,827 hectares of crops damaged 300,000 fisher folk lost their livelihood 10. Gujarat Earthquake 2001 Rapar, Bhuj, Bhachau, Anjar, Ahmedabad and Surat in Gujarat State 13,805 deaths 6.3 million people affected 11. Orissa Super Cyclone 1999 Orissa Over 10,000 deaths 12. Cyclone 1996 Andhra Pradesh 1,000 people died, 5,80,000 housed destroyed, Rs. 20.26 billion estimated damage
- 59. 13. Latur Earthquake 1993 Latur, Marathwada region of Maharashtra 7,928 people died 30,000 injured 14. Cyclone 1990 Andhra Pradesh 967 people died, 435,000 acres of land affected 15. Cyclone 1990 Andhra Pradesh 967 people died, 435,000 acres of land affected 16. Drought 1987 15 States 300 million people affected 17. Cyclone 1977 Andhra Pradesh 10,000 deaths hundreds of thousands homeless 40,000 cattle deaths 18. Drought 1972 Large part of the country 200 million people affected
- 60. KEDARNATH (Note: Temple in Foreground) URL: landslides.usgs.gov
- 64. Thank you