Influence of shape, antecedent conditions and rainfall intensity on shallow landslide activation

1. Influence of shape, antecedent
conditions and rainfall intensity on
shallow landslide activation
Ilaria Pretto, Cristiano Lanni
University of Trento
Eurac – Bolzano – 10/02/2010

2. Overview
TYPE OF ZONING:
Failure
criterion
susceptibility and
hazard maps
Models
Setup
TYPE OF LANDSLIDE:
shallow landslide
Shape
Rainfall
Intensity TRIGGERING FACTOR:
induced rainfall
Time
Volume
LEVEL OF ZONING:
Shalstab
advanced
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3. Overview
Failure
Infinite slope model
criterion
Models
The failure criterion, according to Bishop (1959) is:
Setup
Shape SUCTION
Rainfall
account the contribution of negative pore-water pressure on soil shear strength
Intensity
Time
The safety factor SF is defined as:
Volume
Shalstab
if < 0 (positive pore-water pressure)
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4. Overview
GEOtop hydrological model [Rigon et al. (2006)]
Failure
criterion 3-dimensional form of Richards’ equation
Models
Setup
Shape
follow the evolution of soil-pore pressure during the rainfall event
Rainfall
output maps of GEOtop hydrological model
Intensity
Time
each discertized soil layer
pore water pressure for each discertized soil layer
Volume every time step
Shalstab coupling the infinite slope stability model with the phisical based hydrological
model is possible to investigate the evolution of the hillslope SAFETY FACTOR
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5. Overview
Mualem (1976) parametic formula for hydraulic conducibility
Failure
criterion
Models
Setup
Shape
Rainfall
Mualem-Van Genuchten (1978) parametric formula for the retention curve
Intensity
Time
Volume
Where:
Se is the effective saturation
Shalstab ϑ is the saturation degree
ϑr is the residual water content
Ψ is the piezometric load
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6. Overview
Failure
The role of the ANTECEDENT CONDITION
criterion
Objectives
soil wetness degree
The role of the HILLSLOPE SHAPE
Setup
plan shape
profile curvature
Shape
Rainfall
The role of the RAINFALL INTENSITY
Intensity low
Time
medium
high
Volume
Comparison between hydrological models: SHALSTAB and GEOtop
Shalstab
simplified model vs complex one
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7. Overview
LANDSLIDE CARACHTERISTCS:
Failure
shallow landslisde, unpredictable with instruments
criterion
Models
SOIL COMPOSITION AND CHARACTERISTICS
Setup
silty-sand soil, internal friction angle 38°,
Shape saturated hydraulic conductivity Ks=10-4 m/s
soil depth 2m, area 900m2, average slope 35°
Rainfall
Intensity
impermeable bedrock
Time
ANTECEDENT CONDITIONS AND RAINFALL
Volume
3 different rainfall intensity: low 6mm/h (1.5% Ks)
medium 18mm/h (5% Ks)
Shalstab high 36mm/h (10% Ks)
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2 different soil-moisture antecedent conditions: dry (saturation degree = 25%)
wet (saturation degree = 75%)

8. Overview
TOPOGRAPHIC SHAPES
Failure
3 plan shapes: parallel, convergent, divergent
criterion
3 profile curvatures: straight, convex, concave
Models
Setup
Shape
Rainfall
Intensity
Time
Volume
Shalstab
Take home Evans formulation

9. Overview
INITIAL CONDITION
Failure Rainfall intensity = 18 mm/h
criterion
Rainfall time = 10 ore
Models
Setup Slope maps
Shape
Rainfall
Intensity
SAFETY FACTOR
Time
Volume
PLANARE CONVESSO CONCAVO
Shalstab Percentuale instabile = 50% Percentuale instabile = 28.57% Percentuale instabile = 85.73%
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10. Overview
• Straight profile curvature
Failure non saturo tanα< tan Φ’
criterion
Models tanα> tan Φ’
saturo
FS water table slope
Setup • Convex profile curvature
tanα< tan Φ’
Shape non saturo
tanα> tan Φ’
Rainfall saturo
Intensity FS water table slope
Time • Concave profile curvature
Volume tanα< tan Φ’
non saturo
tanα> tan Φ’
Shalstab saturo
water table slope
FS
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11. Overview
INITIAL CONDITION
Failure Rainfall intensity = 18 mm/h
criterion
Rainfall time = 10 ore
Models
Setup Slope maps
Shape
Rainfall
SAFETY FACTOR
Intensity
Time
Volume
PARALLELO CONVERGENTE DIVERGENTE
Shalstab Percentuale instabile = 50% Percentuale instabile = 72.5% Percentuale instabile = 34.9%
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12. Overview
• Parallel shape
Failure non saturo tanα< tan Φ’
criterion
Models tanα> tan Φ’
saturo
water table slope
FS
Setup
• Convergent shape
non saturo
Shape tanα< tan Φ’
saturo tanα> tan Φ’
Rainfall
Intensity
FS water table slope
Time
• Divergent shape
Volume
tanα< tan Φ’
non saturo
Shalstab tanα> tan Φ’
saturo
FS water table slope
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13. Overview
INITIAL CONDITION
Failure Total rainfall volume = 180 mm
criterion
Models
Setup
SAFETY FACTOR (convex-convergent hillslope)
Shape
Rainfall
Intensity
Time
Volume Rainfall intensity = 6mm/h Rainfall intensity = 18mm/h Rainfall intensity = 36mm/h
Percentuale instabile = 16.8% Percentuale instabile = 65.7% Percentuale instabile = 81.1%
Shalstab
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14. Overview
Time needed to achieve specific percentages of destabilized hillslope area for
Failure
a continuous rainfall simulation of 5 days event
criterion
SHAPE has no
Models
influence for small
percentage of
instable area
Setup
For high percentage,
Shape CONVERGENT
hillslope reaches
instability earlier
Rainfall
Intensity
Time Hillslope
propensity to
Volume landslide
Shalstab
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15. Overview
Rain-volume and Total-volume needed to achieve specific percentages of hillslope
Failure
area for a continuous rainfall simulation of 5 days event
criterion
Models
Setup
Shape
Rainfall
Intensity
Time
Volume
Shalstab
For DRY scenarios the RAINFALL
The TOTAL VOLUME of water to reach
intensity influences more the instability a specific percentage of unstable area is the
Take home process same for both WET and DRY scenarios

16. Overview
THRESHOLD BEHAVIOUR
Failure
criterion
Models
Setup
Shape
Rainfall
Intensity
Time
Volume
Shalstab
Long rainfall time is needed to achieve a small percentage of unstable area (5%) but only a short time is therefore
needed to quickly increase the destabilized area for both wet and dry cases
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17. Overview
DRY SCENARIOS: pore water pressure evolution pixels numeration
1) SUBSURFACE NORMAL FLOW
Failure
criterion
2) SUBSURFACE LATERAL FLOW: very
significant in cases of HIGH rainfall intensity
Models
Setup
low rainfall intensity high rainfall intensity
Shape pore water
pressure
Rainfall
Intensity Critical
pressure
Time head 
FAILURE!
Volume
Shalstab
pixel number pixel number
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18. Overview
doesn’t consider sliding processes caused by saturation from above. Instability
conditions can be reached only from below
Failure
criterion
consider sub-superficial flow in steady-state conditions
Models
doesn’t take into account the shear resistance in unsaturated zone
Setup
Shape
Rainfall
Intensity
Time
Volume
Shalstab
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19. Overview
Unconditionally unstable
Failure
criterion
Models
Unconditionally stable
Setup
Shape
Stable
Rainfall
Intensity
Time
Unstable
Volume
Shalstab
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20. An example on the usage of JGrass
consolle
Overview
Failure Instability
criterion
propension:
JGrass consolle
Models Safety Factor with
SHALSTAB
Setup formulation
Shape
Rainfall
Intensity
Time
3D visualization
Volume
Shalstab
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21. Overview
concave-parallel
Failure
criterion
Models
real Ip effective Ip GEOtop
Setup
concave-convergent
Shape
Rainfall GEOtop
Intensity real Ip effective Ip
concave-divergent
Time
Volume
real Ip effective Ip GEOtop
Shalstab
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22. Overview
Failure
• the WETNESS DEGREE plays a fundamental role on the instability: when 5% of
criterion hillslope area reaches instability (relevant soil wetness reached) failure propagation
velocity increases
Models • WET antecedent moisture condition induces faster development of LATERAL
SUBSURFACE FLOW
Setup
the DRYER the ANTECEDENT CONDITION is and the LOWER the
Shape
RAINFALL INTENSITY is, the more the HILLSLOPE SHAPE is relevant on
the triggering of shallow landslides
VERTICAL and LATERAL subsurface flow
Rainfall
Intensity
Time
• the controller is the TOTAL VOLUME
• the HIGHER the RAINFALL INTENSITY is, the HIGHER total volume is
needed
Volume • when RAINFALL INTENSITY is MEDIUM-HIGH, the triggering of shallow
landslide occurs BEFORE STEADY_STATE CONDITIONS are reached
Shalstab
Take home
Simplified models need simple parameters which are difficult to estimate

23. Overview
Failure
the contribution of unsaturated soil condition provide an additional shear strenght
criterion to the soil so that it may ensure the stability of steep slope (in areas where the
internal friction angle in smaller than the slope inclination)
Models
Setup
Shape
Rainfall
Intensity
Time
Volume
Shalstab
ilaria.pretto@gmail.com cristiano.lanni@gmail.com
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