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Samenvatting mass movements: land degradatie
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Overview of mass movements met de classification, safety factor, causes, slope stability assessment and slope model.
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MASS MOVEMENTS= mass movements include all processes that involve a downward transfer of slope materials under
the influence of gravity without the primary assistance of a transport agent. The happen due to a
trigger (excessive rainfall, snow melt and earthquakes). Causes: unstable slope, nature of material or
water amount. Many landslides are caused by an extraordinary chain-of-events. Characteristics
include movement, material, velocity, moisture content etc.
Mass movement cause direct and indirect damage. Direct damage includes loss of lives, structural
loss, destruction in part/entirety of a construction and the loss of serviceability (loss of a function).
Indirect damage is not directly caused by landslide and concern the loss of infrastructure, production
etc.
On site effects are those that occur in the source or deposition area of a landslide and offsite effects
occur below the landslide and refers to damage along the track of a landslide.
Risk=Hazard × Exposed Elem en ts ×Vulnerability
Risk =( Magnitude × Frequency ) × Exposed elements ×Vulnerability
Acceptable risk = elements at risk and the possible losses taken to be constant and the actual risk
only implied so that only the landslide hazard is assessed.
Classification of landslides:
- Material
- Initiation mechanisms
- Type of movement
- Morphology linked or specific conditions
Crudes and Varnes (p. 6+7 plaatjes uitleg): on the basis of morphology
- Material
o Bedrock
o Engineering soils: debris (coarse) and earth (fine)
- Type of movement
o Falls
o Topples
o Slides: translational and rotational (slumps)
o Lateral spreads
o Flows: creep can be grouped in this
o Complex
- Velocity
Siddle and Dhakal: triggering climate conditions, it also includes the timing of the landslide (p. 8). It is
useful to consider when assessing the impacts of land use and climate change.
- Shallow rapid landslides (debris slides, avalanches, and flows)
- Rapid, deep slides and flows (large debris slides and flows)
- Slower, deep-seated landslides (slumps, earth flows, lateral spreads)
- Slow flows and deformations (soil creep and solifluction)
, - Surficial mass wasting (dry ravel)
Failure happens if the applied loads (shear stress τ) exceed the maximum available shearing
resistance (τf), which will occur as a mass movement. When the shear stress is smaller than the
resistance, it is in equilibrium.
Safety factor = capacity/demand = F = τf/ τ. It expresses the ratio of shearing resistance that has not
yet been expended.
F > 1: Stable slope (1.3 is taken if we do not know)
F = 1: failure
F < 1: should have failed already, resistance is underestimated or imposed loads over estimated. \\
Causes of mass movements:
- Susceptibility: how likely a slope is to fail.
- Preparatory factors: processes that bring a location closer to the point of failure
- Triggering factors: bring the slope to failure
Internal: change in water regime, weathering, erosion, and progressive failure
External: loss of support, increased surcharge.
Factors influencing mass movements:
- Geology and paedology: lithology (kind of rock etc) can have a great impact. Discontinuities
cause more instability and form pathways for weathering. Volcanic activity can also lead to
weathering, furthermore they can cause slopes to become steeper (which can also occur due
to tectonic uplift, river incision and undercutting). Ground shaking by seismicity and
volcanicity also constitute potential triggers for mass movement.
- Topography: slope steepness and slope shapes play an important role. Slope steepness
causes the stability of the slope (can be positive or negative). Slope shape plays a role in the
accumulation of material.
- Hydrology: fast responses for shallow soils when the apparent cohesion due to matric
suction is lost, formation of perched groundwater table, rise of regional groundwater, fast
response of pore pressure.
- Vegetation and landuse: evapotranspiration and roots cause stability. (mechanics p.17, types
of roots p. 18-19)
Slope stability assessment
- Shearing resistance: capacity of the soil to withstand failure. material, ε = ∆L/ L0. The friction
component is proportional to the normal stress: σ = N/A = Wcosα/A. The shear stress is
expressed as: τ = Fs/A = T/A = Wsinα/A. The maximal available shear strength: τf = c + σ
tan(ϕ).
- Terzaghi’s principle of effective stress: The quantifiable changes in stresses exerted at the
particles in contact and the consequent deformation are a direct result of the effective stress
(total stress reduced with the pressure exerted on the pore water). This is stated on
assumptions: only total normal stress is considered; soil is saturated and soil particles and
pre fluid are incompressible. The frictional resistance between particles depends on the
normal forces between particles. A small bit of water causes cohesion while much water rips
the particles apart (saturated) (fig 22). σ’= σ – u τf = c’ + σ’tan(ϕ) = c’ + (σ – u) tan(ϕ)
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