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

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- 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 Take home
- 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) Take home
- 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 Take home
- 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 Take home
- 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 Take home
- 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) Take home 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% Take home
- 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 Take home
- 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% Take home
- 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 Take home
- 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 Take home
- 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 Take home
- 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 Take home
- 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 Take home
- 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 Take home
- 19. Overview Unconditionally unstable Failure criterion Models Unconditionally stable Setup Shape Stable Rainfall Intensity Time Unstable Volume Shalstab Take home
- 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 Take home
- 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 Take home
- 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 Take home

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