1. Mass wasting and Subsidence
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

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

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

13. Forces on the slope Mechanics 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

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

15. h=d sin 
d = h/sin 

16. 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.

17. 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.

18. Figure B 13.1

19. Figure B 13.1

20. Effective stress
Friction angle
cohesion

21. 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)

22. Infinite slope analyses

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

24. Role of material Two basic patterns of movement Rotational
Translational

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

26. 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

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

29. 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

30. 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.

31. Marine slides
slope

32. 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

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

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

35. 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

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

37. 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

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

39. 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

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

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

42. 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

43. Slope support
Retaining walls
Other suggestions?

44. 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

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

46. Sinkholes Salt 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

47. 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

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

49. Summary (III) The importance of water Importance of human impact
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.

50. Summary (IV) Minimizing landslides hazard Possible techniques
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

51. Vaiont Dam, Italy

52. Vaiont Dam, 262m high, on the Vaiont River, a tributary of the Piave River, in Venetia, NE Italy in the south-eastern part of the Dolomite Region of the Italian Alps, near Belluno, about 100 km north of Venice.
The dam, one of the highest in the world, was completed in 1961
After heavy rains in 1963, landslides into the Vaiont reservoir caused the stored water to spill over the dam, sweeping away the village of Longarone and flooding nearby hamlets.
The Vaiont reservoir disaster is a classic example of the consequences of the failure of engineers and geologists to understand the nature of the problem that they were trying to deal with.
During the filling of the reservoir a block of approximately 270 million m3 detached from one wall and slid into the lake at velocities of up to 30 m sec-1 (approx. 110 km h-1). As a result a wave over topped the dam by 250 m and swept onto the valley below, with the loss of about 2500 lives. Remarkably the dam remained unbroken.

55. The slip surface of the slide was along weak bedding planes in the dipping limestone valley.
Inadequate geological surveying of the mountain surrounding the projected lake contributed greatly to the disaster.
The measures taken by the authorities (lowering the water level in the lake after prolonged rainfall), while intended to stem movement of the hillside, actually made the situation worse. 
As the valley filled with water after completion of the dam in 1960, an ancient landslide on its upper southern side, adjacent to the dam, began to move again, in episodes of slow creeping movement.
It was later established that this was due to groundwater, unable to escape into the floor of the now - flooded valley, saturating a layer of clay within the rocks beneath it.

56. Vaiont Dam
This accumulation of water high in the slope was most marked during periods of heavy rain, although the association of heavy rainfall and creep went unnoticed.
The presence of the impermeable clay layer in the bedrock was not recognized at the time and it was assumed that the movement was due to local saturation of the rocks below the level of water in the reservoir, at the toe of the creeping landslide, rather than accumulation of water pressure in the entire mountainside.
It was therefore proposed to regulate the movement of the landslide, and thus allow it to settle to a new equilibrium, by lowering the level of water in the lake when an episode of creep was in progress, until the toe of the landslide was no longer saturated and the creeping stopped.
The reservoir was then allowed to refill and the drainage cycle repeated whenever creeping movement occurred again.

57. Events that led to the disaster
The engineers concluded that if there were any shearing, it would take place such that it would cause a chair like deformation. This would then result in a braking reaction. After analysing the seismic information gathered, it was determined that the walls had a very high modulus of elasticity and that even though small slides were likely to occur, the outcome of these slides could be managed

58. Events that led to the disaster
Around Feb 1960, the dam was filled and only one month later a tiny detachment slide occurred.
Significant amounts of mass material continued to slide and eventually on Nov 4th, m3 of material slid down into the lake.
The team of engineers decided to solve the problem by varying the water level in the reservoir by constructing drainage and bypass tunnels.
From Oct 1961, to Sep 1963, the engineers who increased and decreased height of the water controlled the reservoir levels. This was done with the intent to control the landslides.
On Oct 9th 1963, a mass of earth and rock slid down the hill and blocked the gorge completely.
The material eventually travelled 140m up the opposite bank and finally terminated its movement after 45 seconds. At the time the reservoir contained 115 million m3 of water.
A wave of water was pushed up the opposite bank and destroyed the village of Casso, 260 m above lake level before over-topping the dam by up to 245 m. The water, estimated to have had a volume of about 30 million m3, then fell more than 500 m onto the villages of Longarone, Pirago, Villanova, Rivalta and Fae, totally decimating them. However the dam was not destroyed and is still standing today.

59. Vaiont dam
It is likely that increasing the level of the reservoir drove up pore pressures in the clay layers, reducing the effective normal strength and hence the shear resistance.
Resistance to movement was created by the chair-like form of the shear surface.
Dropping the level of the reservoir induced hydraulic pressures that increased the stresses as water in the jointed limestone tried to drain.
It has been estimated that the total thrust from this effect was million tonnes.
Failure occurred in a brittle manner, inducing catastrophic loss of strength. The speed of movement is probably the result of frictional heating of the pore water in the clay layers.