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Dr. Arindam DeyGeotechnical Engineering Division
Department of Civil Engineering
IIT Guwahati
Brainstorming Session
Quantification of Seismic Hazard
and Mitigation of induced effects in NER
Slope Stability
Slope stability is an age-old issue of soil and rock mechanics
Many things have been learnt
Still many things to learn
Natural and Artificially Engineered slopes
Slope instability is triggered when balance is disturbed
Natural causes
Extreme natural conditions (precipitation, earthquake) causing inequilibrium
Anthropogenic causes
Inadequate understanding of slope mechanisms and reckless constructions
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Natural Slopes
Natural Slopes
Slopes having inherent stability bounded
by the vegetative covers
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Engineered Slopes
Slopes artificially restrained or
protected from failure
Embankment and Fills
Highway and Railway embankments
Landfills
Earth Dams and Levees
Cut slopes
Landfill Cap and Liner system
Nailed or MSE Retention systems
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Slope Instabilities and Landslides
Movement of mass of rock, debris or earth down a slope
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Cruden (1991)
Classification of Landslides: Types of Failure
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Varnes, 1978; Cruden and Varnes, 1996
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Classification of Landslides: Velocity of Failure
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Cruden and Varnes, 1996
Landslides: CAUSE and TRIGGER
Landslide trigger
The single event that finally
initiates the landslide.
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Causes of landslide
Factors that make the slope
vulnerable to failure
Factors that predisposes the
slope to become unstable
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Landslide Triggers
Rainfall
Heavy or Prolonged rainfall
For rainfall occurring over a short time interval
Usually necessary to have very high rainfall intensities
For a long duration rainfall event
The intensity of rainfall may only be moderate.
Seismicity
Stress induced due to seismic shaking
Generation of pore water pressure
Toe-cutting (in many instances)
Inhabitation
Transport route development
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Typical Examples of Seismic Slope Instability
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Varunavat Parvat
Landslide
Uttarkashi,
Uttarakhand
24 September 2003
Typical Examples of Seismic Slope Instability
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Some Landslides
in North-East
2012
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Typical Examples of Seismic Slope Instability
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Rockfall at
Guwahati-
Shillong Road
Typical Examples of Seismic Slope Instability
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Seismic Slope
instability in
Saiphum,
Mizoram
2013
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Typical Examples of Seismic Slope Instability
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Seismic slope
failure due to
faulty excavation
technique in
North Guwahati
due to Steep
Excavation
2015
Components of Slope Instability Studies
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Infinite Slopes
Infinite Slope - Extend over long distances and great heights
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Ramche Landslide, Nepal, 2012
An example of progressive failure of slope
Infinite Slope and Analysis
Infinite Slopes - Extend over long distances and great heights
Translational Shallow Slip Analysis for Infinite slopes
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Finite Slope and Analyses
Finite slope – Local scale slopes bounded by surfaces in finite
measurable dimensions
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Finite Slope and Analyses
Slope Stability Analysis
Rotational slips – No rigid base stratum
Compound slips – Presence of rigid base stratum
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Finite Slope and Analyses
Slope Stability Analysis
Various types of failure surfaces
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Methods and Techniques of Slope Stability Analyses
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PLAXIS
GeoStudio
FLAC
GTS Midas
Talren
Geo5
Rocscience
Oasys
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Finite Slope Stability Analyses
Conventional Finite Slope Stability Analysis
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Finite Slope Stability Analyses
Conventional Finite Slope Stability Analysis
Limit Equilibrium based Method of Slices
Define the Factor of Safety of a slope
FoS = Strength / Stress developed
State of stability
FoS > 1 Stable
FoS < 1 Failed
FoS 1 Incipient failure
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Pseudostatic Slope Stability Analysis
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Issues in seismic slope stability analysis
• Factor of safety against failure
• Varies with acceleration coefficient
Pseudo-static Slope Stability Analysis
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Issues in seismic slope stability analysis
• Location of the critical slip surface
• Static and pseudo-static failure surfaces are not the same
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Hill-Slope Stability: Hydraulic and Seismic Effects
Effect of hydraulic and pseudo-static conditions
on the stability of hill slope
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Bedrock
β
Hill Slope
h
H
Static
Dry
Static with
water tablePseudo-Static Dry
Pseudo-Static
with water table
Hill-Slope Stability: Toe Cutting
Toe-cutting (A typical slope i=300, φ=200)
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Dry
Dry
Pseudo-static
Partially saturated
Pseudo-static
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Pseudo-static / Pseudo-dynamic Slope Stability Analysis
Pseudo-static analysis does not consider amplification of waves
More like a rigid block analysis of the active soil mass
Pseudo-dynamic analysis incorporates amplification
FoS governed by nature and magnitude of pre-defined amplification
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Seismic Slope Stability Analysis
Equivalent linear and Nonlinear dynamic analysis
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Bedrock
β
Hill Slope
h
H
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Landslide Analysis on a Local Scale
Slope Stability Limit Equilibrium Analysis
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Landslide Analysis
Continuum Analysis
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Rock Slope Stability: An intricate mechanism
Intricate presence of joints
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Rock Slope Stability: An intricate mechanism
Types of failure and analyses
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Rock Slope Stability: Pseudo-static analysis
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Rock Slope Stability: Time-history analysis
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Shear strain developed after the application
of dynamic load
Displacement contour of the slope near the
termination of seismic shaking
Sliding of the slope along the joint set J1 which was
predicted by kinematic and pseudo static analysis.
Maximum deformation of the slope occurs in the
vertical direction after 5.9 s
Post seismic displacement more than 50 mm
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Topographic Amplification
Amplification due to heterogeneity,
soil stratification, bedding planes etc.
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Topographic Amplification in Slopes
Slope face acts as reflective boundary
Wave directivity
Wave generation
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Rayleigh waves
Rayleigh waves
P reflected
SV reflected
SV incoming waves
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Buildings on Slopes: Foundation Interaction
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Regional Scale: Spatial Variability
Landslide Analysis in regional scale
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Variability of Rainfall across Himalayas
Substantial spatial and temporal variation
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Anders et al. (2006)
Bhatt and Nakamura (2005)
Variability of Himalayan Soil Profiles
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Singh (2013)
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Variability in Himalayan Bedding Planes and Faulting
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Kothyari et al. (2010)
Meghalaya
Spatial Variability of Soil Properties
Salient variable parameters
Shear strength parameters
Permeability characteristics
Geological and geomorphological variability
Rainfall distribution
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Mangan, Sikkim
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Probabilistic Analysis of Landslides (Regional Scale)
Probabilistic framework of analysis
Defines a margin of safety and a
probability of failure instead of a specific
safety factor
Soil parameters are defined as random
variables with a probability distribution
of occurrence (single or joint probability)
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Probabilistic Analysis of Landslides
A typical example of parameter distribution
Simulation of spatial variability of soil shear strength parameters (c,φ)
Isotropic correlation – Formation of parameter pockets
Anisotropic correlation – Formation of stratified layers
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θx = 1.0
θy = 1.0
θx = 5.0
θy = 5.0
θx = 10.0
θy = 10.0
θx = 10.0
θy = 2.0
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Landslide Monitoring and Instrumentation
Mass Movement monitoring
Electronic Distance Measurement (EDM)
Inclinometers, Extensometers, and Strain Meters
Ground tiltmeters
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Landslide Monitoring and Instrumentation
Mass Movement monitoring
Ariel photographs and Advanced surveying techniques
using GPS and Satellite images
Time domain reflectometry
Use of Optical fiber sensors
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Ariel/Geodetic Surveys
LIDAR Technique
Velocity of soil movements
Type of movements – Rotational or
Translational
Extent of damage
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Hydro-Geological Surveys
Identification of hydrological issues
Ground water table
Suction capacity and Unsaturated zones
Perched water table
Infiltration
Surface runoff
Precipitation
Evapotranspiration
Seepage
Springs
Piping
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Landslide Monitoring and Instrumentation
Ground water monitoring
Piezometers and In-situ Tensiometers
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Hydro-Geological Surveys
Determination of GWT and pore-water pressure
Hydraulic Piezometers (Stand Pipe, Casagrande)
Pneumatic Piezometers
Electric Piezometers (vibrating wire)
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Landslide Monitoring and Instrumentation
Rainfall event monitoring
Strategically located Gain Gauges
Seismic event monitoring
Accelerographs
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Slope Stabilization
Myriads of non-unique ways to stabilize a slope
Primary objective
Reduce the driving forces
Excavation of material from appropriate part of unstable ground
Drainage of water to reduce the hydrostatic pressures acting on unstable zone
Increase the resisting forces
Drainage that increases the shear strength of the ground
Elimination of weak surfaces or other potential failure zones
Building of retaining structures or other supports
Provision of in-situ reinforcement in the ground
Chemical treatment to increase the shear strength of the ground
or, Both
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Slope Stabilization Techniques
Unloading – To reduce the driving forces in a slide mass
Excavation and Filling techniques
Removal of the weight from upper part of the slope
Removal of all of the potentially unstable materials
Flattening of slopes
Benching of slopes
Application of a lightweight fill
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Slope Stabilization Techniques
Buttressing of slope
Offset or counter the driving forces of
a slope
Externally applied force system that
increases the resisting force
Various techniques of buttressing
Soil and Rock fill
Counterberms
Shear keys
Mechanically stabilized embankments
Pneusol (Tiresoil)
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Slope Stabilization Techniques Drainage
Surface Drainage
Concrete ditch drains or Geopipes
Catchment parameters
Area and shape of catchment zone
Rainfall intensity
Steepness and length of slope being drained
Condition of ground surface and nature of the subsurface soils
Nature and extent of vegetation
Redirection of surface runoff
Subsurface drainage
Drain blankets
Trenches
Cut-off drains
Horizontal drains
Relief drains
Drainage tunnels
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Slope Stabilization Techniques
Reinforcement
Soil Nailing
In-situ passive inclusions penetrating
the failure plane
Mobilize when movement occurs in
the soil
Stone columns
Mitigate or prevent landslides
Reticulated micropiles
Create a monolithic rigid block of
reinforced soil to a depth below the
critical surface
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Slope Stabilization Techniques
Reinforcement
Geosynthetic reinforced walls
Wrap around geosynthetic faced soil systems
Reinforced retaining walls
Concrete block walls
Gabion-faced retaining walls
Driven piles
Drilled shafts
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Slope Stabilization Techniques
Reinforcement
Tie-back walls
Anchored slopes
Vegetated slopes
Reinforcement by roots
Check on soil erosion
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Slope Stabilization Techniques
Several other stabilization techniques
Erosion control mats and blankets
Biotechnical stabilization
Surface slope protection
Shotcreting (With caution)
Chunam plaster
Masonry
Riprap
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Slope Protection/Stabilization Measures
Stabilization and Mitigation measures in a nutshell
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Some Novel Slope Protection/Stabilization Measures
Rockfall protection measures - DRAPERY
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Some Novel Slope Protection/Stabilization Measures
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Some Novel Slope Protection/Stabilization Measures
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Some Novel Slope Protection/Stabilization Measures
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Some Novel Slope Protection/Stabilization Measures
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Some Novel Slope Protection/Stabilization Measures
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Some Novel Slope Protection/Stabilization Measures
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Poor Hillside Practices
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Australian Geomechanics Society
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Good Hillside Practices
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Australian Geomechanics Society
Final Remarks
Seismic slope stability is influenced by several factors
Geometry and composition of the slope
Degree of seismicity and strong motion characteristics
Presence of joints, bedding planes, fractures, shear zones
Topographic amplification and wave directivity
Geohydrologic conditions
Spatial variability of soil properties and ambient conditions
Instrumentation and continuous/intermittent monitoring of vulnerable and
potentially vulnerable landslide sites are a must
Stabilization and mitigation measures are site-dependent
Many techniques are available, and it is required to choose the most viable one
which suits the requirement
Domain of slope stability and landslides is interdisciplinary
Geotechnologists, Geologists, Hydrologists, Climatologists, Seismologists,
Earth Science experts, Instrumentation and signal processing experts,
Transportation engineers to achieve a sustainable hillslope practice
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Acknowledgments
First of all to my Ph.D. and M.Tech. students, many of whose
slides I borrowed with/without permission
Rubi, Rana, Anangsha, Madhulatha, Chiranjib, Amalesh, Aswathi
To the industry collaborators with whom I am working on
many of the landslide related projects
Maccaferri, Genstru, Terre Armee, OST slope, GeoBrugg, NEPC and
others
To all those researchers who are working tirelessly to make
hillslope practices more sustainable. I have collated many of
their findings in the presentation.
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Thank You for Patient Hearing
