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Chapter 7

Chapter 7. Mass wasting and Subsidence . Objectives. Basic processes, types of flow Driving and resisting forces (safety factor) and how it is related to slope stability Slope processes and the influence of slope angle, topography, vegetation, time etc Influence of human use

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Chapter 7

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  1. Chapter 7 Mass wasting and Subsidence

  2. Objectives • Basic processes, types of flow • Driving and resisting forces (safety factor) and how it is related to slope stability • Slope processes and the influence of slope angle, topography, vegetation, time etc • Influence of human use • Methods of identification, prevention., warning etc • Processes related to land subsidence

  3. Introduction • Landslides and related phenomena cause damage and loss of life • General term: • Mass wasting (gravity driven) • Downslope movement of rock or soil as more or less coherent mass under the action of gravity • Other terms: earth flows, landslides, mudflows, rockfalls, debris flows, creep • Also to include: subsidence

  4. Hazard Fact Sheet (USGS) • The term landslide includes a wide range of ground movement, such as rock falls, deep failure of slopes, and shallow debris flows. Although gravity acting on an over steepened slope is the primary reason for a landslide, there are other contributing factors: • erosion by rivers, glaciers, or ocean waves create oversteepened slopes • rock and soil slopes are weakened through saturation by snowmelt or heavy rains • earthquakes create stresses that make weak slopes fail • earthquakes of magnitude 4.0 and greater have been known to trigger landslides • volcanic eruptions produce loose ash deposits, heavy rain, and debris flows • excess weight from accumulation of rain or snow, stockpiling of rock or ore, from waste piles, or from man-made structures may stress weak slopes to failure and other structures • Slope material that become saturated with water may develop a debris flow or mud flow. The resulting slurry of rock and mud may pick up trees, houses, and cars, thus blocking bridges and tributaries causing flooding along its path.

  5. Bases for classification • Type of material involved • How the material moves • The moisture content • How fast the material moves

  6. Types of flow (remolded) Creep Mm per year Debris flows High velocities (in the ocean,  100 km/h)

  7. Landslides that move as a unit • Slumps • Rotational slides • Glide blocks • Coherent masses • Falls • talus

  8. Types of landslides • Example on rock slide • Norwegian valley

  9. Video clip

  10. Slope types • Hard rock/talus • Typical for Norwegian valleys/fjords Soft rock/sediments

  11. Causes of debris flows • Rain induced • Antecedent rainfall, saturated soil • Access rain • Depend on various parameters such as vegetation • Earthquake • Man made effects

  12. Forceson the slopeMechanics of slides • Stability of a slope • Relationship between • Driving forces • Resisting forces • Slope stability is evaluated by computing a safety factor –SF-, defined as • The ratio of resisting forces to driving forces • SF > 1, stable, SF< 1 unstable

  13. Cy = C sin C  Cx = C cos mg sin 30o mg cos 30o W= mg

  14. h=d sin  d = h/sin 

  15. Role of Gravity and Slope Angle • Gravitational force acts to hold objects in place by pulling on them in a direction perpendicular to the surface. • The tangential component of gravity acts down a slope: it causes objects to move downhill.

  16. Role of Gravity and Slope Angle • Shear stress is the downslope component of the total stress involved. • Steepening a slope by erosion, jonting it by earthquake, or shaking it by blasting, can cause an increase in shear stress. • Normal stress is the perpendicular component.

  17. Figure B 13.1

  18. Figure B 13.1

  19. Effective stress Friction angle cohesion

  20. Effective Stress • Total stress consists of two parts: • One portion of stress is carried by water – equal intensity in all direction • The other portion of stress is carried by the soil solids at their points of contact • Total stress =effective stress ’+pore water pressure u • s = s’ + u • Effective stress – the sum of the vertical components of the forces developed at the points of contact of the solid particles per unit cross-section area of the soil mass. • ’ =  - u applicable to fully saturated soil Principle of effective stress (Terzaghi, 1925 &1936)

  21. Infinite slope analyses • http://www.tagasoft.com/Calculators/slope/eqn_is1_html#input

  22. Driving and resisting forces depend on: • Type of materials • Slope and and topography • Climate • Vegetation • Water • Time

  23. Role of material • Two basic patterns of movement • Rotational • Translational

  24. Rotational slides • Occur along curved slip surfaces • Movements follows a curve • Comment in slope soils

  25. Translational slides • They are planar • Occur along inclined slip planes with a slope • E.g bedding plane • Weak clay layer • Soil slips • Very shallow slides in soil above

  26. Other types of materials and flow • Shale slopes/ slopes on weak volcanic pyroclastic materials • Creep • Earthflows; mudflows • Downslope flow of saturated materials

  27. Role of slope and topography • Slope, relative importance, note • The difference in height h is the important one • More frequent slide on steep slope • Note in marine conditions, • Steep slope, minor slides, more frequent

  28. Slope angles and landslides The slope angle for each of the 145 recent landslides was determined from the U.S. Geological Survey topographic maps. The slope angle is the angle between the horizontal and the ground surface. Figure 13 relates the frequency of landslides to the slope angle. The average slope angle for landslides is 22.2 degrees with 75% of the landslides on slopes greater than 15 degrees. One landslide is on a slope of only 5.7 degrees. Fig. 13. The frequency of recent landslides for a given slope angle. The frequencies were determined by counting the number of landslides for 5-degree intervals of slope angle. The average slope angle for recent landslides is 22.2 degrees.

  29. Marine slides • 2 3 4 5 6 7 8 9 10 12 14 14 • slope

  30. The role of climate • Climate influences • the amount and timing of water • the form, rain or snow • Note different slides types in various climatic regions • Dry/cold/humid

  31. The role of vegetation • Veg. provides a cover • Cushions the impact of rain • Veg/root systems • An apparent cohesion • Veg adds weight to the slope • Note the impact of deforesting

  32. The role of water • Slopes becomes saturated • Infiltration of water deep into the slope, late response • Water can erode, decreasing stability • Quick clay

  33. Quick clay • Marine clay – flocculated (card house) • Subaerially exposed (isostatic uplift) • Fresh water drainage and leaching • Reduced content of e.g. K+ • Reduced stability and collapse of clay structures

  34. The role of time • Classical case • Vaiont dam • Slide into the dam

  35. Other factors • Timber harvesting • Urbanization • E.g. Rio de Janeiro • Combination of steep slopes and fractured rocks covered with thin soil • Slopes were logged • Urban development on slopes • Vegetation cover has been removed • Etc

  36. Identifying Potential Landslides • Aerial photographs • Information on past landslides • Soil properties etc • Risk/probability of occurrence

  37. Preventing landslides • Difficult to prevent! • Possible trigger mechanisms: • Loading the top of slopes • Cutting into sensitive slopes • Placing fills on slope • Changing water conditions on slope

  38. Landslide Hazard Zonation • Inventory maps • Land risk map • Control and Stabilization

  39. Techniques for landslides prevention • Provisions for surface and subsurface drainage • Removal of unusable slope materials • Construction of retaining walls • Supporting structures

  40. Drainage control Surface and subsurface drainage Note the importance of effective stress Grading, material from upper part of a slope is removed and placed near the base Increased resisting forces Cut into a series of benches

  41. Slope support • Retaining walls • Other suggestions?

  42. Warning methods • Electrical systems • Tilt meters • Geophones • In most cases, the cause of the slides is an increase in water pressure • note effective stress • Note snow avalanches

  43. Withdrawal of fluids-Subsidence • Withdrawal of fluids • Oil/gas • Ekkofisk oil field in the North Sea • Groundwater • Consequence • Subsidence • E.g. Ekkofisk oil field • Central California, pumping of ground water

  44. SinkholesSalt domes • Voids, large open spaces such as caves formed by chemical weathering within soluble rocks, limestone, dolomites etc • Serious subsidence associated with salt mining • Water is injected, salt dissolves and supersaturated water is pumped out

  45. Summary (1) • Slope failure involve • Flowage, slumping, sliding, falling of earth materials • Complex combination of sliding and flowage • Forces – interaction of several variables • Type of material, topography, slope angle, climate, vegetation, water and time

  46. Summary (II) • Common driving forces: • Weight of slope materials • Note safety factor: • Ratio of resting versus driving forces, • SF > 1, stable slope

  47. Summary (III) • The importance of water • Water can erode the base of slopes • Excess water increases the weight • Reducing effective stress • Reduction of resting forces • Importance of human impact • Change of groundwater, logging, change of vegetation etc.

  48. Summary (IV) • Minimizing landslides hazard • Mapping and monitoring • Prevention is difficult • Possible techniques • Drainage control, grading of slopes, construction of retaining walls • Subsidence • Due to withdrawal of fluids, water, oil and gas

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