Slope Process and Slope Stability Flashcards
What was Canada’s worst natural disaster?
- Frank Slide
- 1903
- 82 million tonnes of limestone
- Killed approx. 70 people in mining town of Frank
Objectives of slope processes
- Conceptualize slopes as systems where downslope forces move earth surface materials
- Review common features (Slides, flows, falls, spreads, creep) and classification schemes (morphological and rheological
- Expand with case study examples, discuss hazards and risks
- Discuss important morphometrics and indicators used to interpret activity
How do landscape materials get from mountain tops to valley floors?
- slope processes and mass wasting
Why are submarine failures of importance for terrestrial habitats?
- underwater displacement can cause tsunami’s
- infrastructure on deltas and other marine envrs
Frequency-magnitude relations
- Moderately sized transport events do the most geo work in the landscape as a consequence of the frequency of moderate sized events
What are 2 big factors for geomorphic work/potential damage?
- How bit is it
- How often does it happen
- i.e. Frequency-magnitude relationship
What affects the angle of internal friction for granular materials?
- Surface roughness
- Packing
- Grain shape
When is the angle of internal friction higher?
- Closer packing
- Grains of different sizes
- Angular grains
When is the angle of internal friction lower?
- Open packing
- Uniform particle sizes
= fewer points of contact for friction
Cohesion
- How well things stick together
- Rootlets, electro-static bonds in clays, cementing agents (salt oxides)
Internal Friction
- Planar friction angle
- Mechanical (bulk) resistance of grain-grain contact
- f (grain size, shape, sorting, compaction)
- Controls Stress in unconsolidated deposits, Failure when > than angle of internal friction
What are 2 measures that control rheological responses?
- Angle of repose
- Angle of sliding friction
Angle of repose
- Angle of rest of dry sediment (25 - 40 degrees)
- Static, stationary, friction
Angle of sliding friction
- Angle at which sediment fails
- up to 10 degrees > angle of repose
- Dynamic friction threshold
What is the primary driving force in the landscape?
- Gravity
On a slope, what is the Force of gravity (Fg) divided into?
2 vectors:
- Downslope component
- Normal component
What is the frictional force proportional to?
- frictional is proportional to normal force
- friction decreases as slope increases,
- Down slope gravitational component increase when slope increases and normal force decreases
What is the main thing slope failure is dependent on?
Slope!
What is the driving force?
- Shear stress
- Derived from soil bulk density, gravitational acceleration, and soil depth
What is the specific weight of the soil?
- soil bulk density x gravitational acceleration
What is the resistance to shear stress expressed by?
the Mohr-Coulomb eqn.
- Describes the ability of material to resist sliding
What does Soil strength depend on?
- Soil cohesion
- Normal force
- Pore water pressure
- Coefficient of friction
Angle of internal friction
- phi
- angle where shear failure occurs
- can be estimated in the field (driving a probe into the ground)
What is the normal force?
- imposed by the weight of the solids and water above a particular point in the soil and resists downhill movement
- Force per unit area
- Frictional resistance on the sliding plane
What does high pore water pressure do to the normal force?
- reduces normal force and the frictional strength of soil
- Forces the particles apart and reduces the friction
What is the difference between Phi and Theta?
- Phi is the angle where shear failure occurs
- Theta is the slope angle
- They are not the same thing
How is the Soil (Shear) Strength, S, calculated?
S = soil cohesion plus [(normal force per unit area - pore water pressure) x tan of the angle of internal friction]
What is Cohesion caused by?
- Chemical bonding and electrostatic attraction between particles of soil, not simply compressive forces (squeeze sand together and nothing happens)
- roots or inter-particle bonds
What holds soils with lots of organic matter together?
- roots can physically hold soils together
What happens in silts and clays that generally don’t chemically bond to each other?
- electrostatic forces due to the effects of capillary water between oil particles can provide a bond
- Clays which are charged are cohesive and stick together
How is pore water pressure calculated?
- It is the product of slope normal component of the bulk water density, gravity, and soil thickness, and cos of slope angle
- High pore water pressure decreases soil strength
- Measured in units of pressure, Newtons per square meter
Can pore water pressure be mitigated?
- Yes
- ex. Drains to capture moisture and reduce pressure
Factor of Safety
- Describes the stability of a slope
- Ratio between forces resisting and driving movement
- Fs = 1 forces balanced, threshold for instability
- Fs > 1 (Strength > stress) = Stable
- Fs < 1 (S
Factor of Safety for a dry soil with no cohesion
- dry = no water pressure
- no cohesion = no clay
- The resisting force is the soil strength from the coulomb eqn and the driving force is shear stress
- Soil Density includes only the weight of soil particles which occupy 60 percent of the total volume (1590kg/m^3)
Factor of Safety for a dry soil with cohesion
- dry = no water pressure
- cohesion = clay
- Soil density remains the same (1600kg/m^3)
Factor of Safety for a wet soil with cohesion
- wet = water pressure
- cohesion = clay
- Soil density includes weight of the soil particles and the water in the pore spaces (2050kg/m^3)
What happens to the Fs if slope increases
- denominator (stress) becomes large
- Fs decreases
What happens to Fs if a wet soil loses moisture?
- numerator (strength) becomes large
- FS increases
What happens to Fs if a slope is logged?
- cohesion decreases
- numerator (strength) becomes smaller
- Fs decreases
What happens to Fs if a soil thickness is increased?
- denominator (stress) gets larger proportionally to numerator (strength)
- Fs decreases
- But balances out with cohesion present
External Factors that Increase Shear Stress
- Removal of support (erosion, human activity i.e. road cuts etc)
- Addition of mass (Natural i.e. talus/rain, or Human i.e. fills, buildings etc.)
- Earthquakes
- Regional tilting
- Removal of underlying support (undercutting, solution etc or human i.e. mining)
- Lateral pressure (Natural swelling, expansion, water addition)
Internal factors that decrease Shear Strength
- Weathering and physicochemical reactions (lowers cohesion):
- Disintegration (lowers cohesion)
- Hydration (lowers cohesion)
- Base exchange
- Solution
- Drying
- Pore water (Buoyancy, Capillary tension)
- Structural Changes (Remolding, Fracturing)
What do internal factors that control slope failure tend to do?
Decrease Shear Strength
What do External factors that control slope failure tend to do?
Increase Shear Stress
How do slopes achieve stability?
Through failure
Dalrymple 1968
- 9 slope elements described by geomorphology and dominant transport processes and pathways
- Provided a simple way to describe and map slopes to show down slope variation
- Reality rarely has all 9 components
- Stable profile while unstable shows steps and irregular features
9 Slope elements
- Interfluve
- Seepage slope
- Convex creep slope
- Fall face
- Transportational midslope
- Colluvial footslope
- Alluvial toeslope
- Channel wall
- Channel bed
Interfluve
Pedogenetic processes associated with vertical subsurface soil water movement
- Modal slope angle 0 - 1 degrees
Seepage Slope
Mechanical and chemical elevation by lateral subsurface water movement
- Modal slope angle 2 - 4 degrees
Convex Creep Slope
Soil cree, terracette formation
Fall Face
- Fall, slide, chemical and physical weathering
- minimum angle 45 degrees but normally over 65
Transportational midslope
- Transportation of material by mass movement (flow, slide, slump, creep), terracotta formation, surface and subsurface water action
- Frequently occurring at 26 to 35 degree slope angles
Colluvial footslope
- Redeposition of material by mass movement and some surface wash
- Fan formation
- Transportation of material, creep, subsurface water action
Alluvial toeslope
- Alluvial depostion
- Processes resulting from subsurface water movement
- 0 to 4 degrees
- Movement in a downvalley direction
Channel wall
- Corrasion, slumping, fall
- Movement in a downvalley direction
Channel Bed
- Transportation of material downvalley by surface water action
- periodic aggradation and corrasion
Dalrymple, where is the transition from pedogenic to colluvial processes?
- Between Seepage slope and Convex creep slope
Dalrymple, where is the transition from colluvial to alluvial processes?
- Between Colluvial footstep and Alluvia toeslope
Dalrymple, where is the transition from alluvial to fluvial processes?
- At the Channel wall between the alluvial toeslope and channel bed
How does Dalrymple’s model compare with reality?
- All 9 units rarely occur on one slope or in the same order
- But general shape of convex morphing to concave profile is fairly common
- Sometimes repeated units reflect compositional changes
Slopes reflect imbalanced forces acting on their mass controlled by:
- Geology (type, weathering, joints/fractures, orientation)
- Sediment properties (thickness, shape, sorting, rheology, Mc)
- Topography (steepness, aspect, pre-existing features, vegetation, land-use)
What is the ideal stable profile?
Convex-straight-concave
What do slopes out of equilibrium show?
- Steps or irregular features
- Steeps bring out of stability and it will try to return to convex-straight-concave profile
How do slopes naturally achieve balance with forces acting on their mass?
- By adjusting slope angle, through failure
Classification of movements: Type of movement
- Fall
- Topple
- Slide (rotational, translational)
- Lateral Spreads
- Flows
- Complex
Classification of movements: Type of Material, Bedrock
- Rockfall
- Rock Topple
- Rock slump
- Rock block slide
- Rock slide
- Rock spread
- Rock flow (deep creep)
Classification of movements: Type of Material, Unconsolidated Coarse
- Debris fall
- Debris topple
- Debris slump
- Debris block slide
- Debris spread
- Debris flow (soil creep)
Classification of movements: Type of Material, Unconsolidated Fine
- Earth fall
- Earth topple
- Earth slump
- Earth block slide
- Earthslide
- Earth spread
- Earthflow (soil creep)
What are mass movement categories based on?
- Type of movement
- Further subdivisions based on material type (bedrock vs. unconsolidated sediments)
- Mechanism of failure
- Sedimentary and rheological properties
- Moisture content
- Speed of movement
Ternary diagram of mass movement classification
- Heave (up, down due to alternate freeze-thaw of soils, type of creep)
- Flow (wetter)
- Slide (dryer)
Parts of a landslide, top to bottom
- Crown (should have crown cracks)
- Main Scarp
- Head
- Minor Scarp
Foot: - Transverse cracks
- Transverse ridges
- Radial cracks
- Toe
Surfaces: - Surface of rupture
- Main Body
- Toe of surface of rupture
- Surface of separation
Rheological classification of mass movements
- Velocity vs. sediment conc.
- Fluid (liquid) to solid (plastic) behaviour
- Fast (inertial) vs. slow (viscous or friction)
- Streamflow (top left) to creep (bottom right)
What is the Sedimentological classification for movements?
- Based on sed facies which depend on sed support mechanisms, flow viscosity, water content, turbulence, material strength
- Ternary plot with viscous to non-viscous, Turbulent to no turbulence, Cohesive plastic to cohesionless flow
Name some evidence to distinguish mass wasting deposits from similar sed types (glacial till, fluvial)
Roundedness:
- Landslide angular, flacial moderate rounded, fluvial rounded
Internal Forms:
- Landslide transverse ridges, glacial sinuous moraines, fluvial lacks ridges
Sorting:
- Landslide very poor, glacial very poor, fluvial variable and generally good
What are the 5 main morphological classifications?
- Falls
- Topples
- Slides (translational, rotational)
- Spreads (Lateral)
- Flows (debris, earth, etc.)
- Combinations are common
- Consist of bedrock, sed, or both
Rockfalls
- Free or bounding downslope movement of loose rock material under influence of gravity
- Begin with detachment of rock from a steep slope along a surface on which little or no shear displacement takes place
- Materials bounce and roll once they impact lower gradient slopes
- Primary process leading to development of talus/scree
- Fall at 90 degrees, bounce at 70, roll less than 45 degrees
Why do rockfalls and talus coarsen downslope?
- Coarsens downslope b/c larger particles have more momentum and move further and crush upper sediments finer on their way down
What are main triggers of Rockfalls?
- Freeze-thaw, Earthquakes, Extreme precipitation
- Common on highway undercuts
What are used to mitigate rockfalls on highways?
- Fences
- Work for individual rocks but not substantial falls
- Also support walls and bolts
Talus (scree) cones
- Specific type of colluvium
- Product of gravity driven mass movements that form at the bottom of rockfall dominated slopes
- Shape dictated by angle of repose of debris composing them
- Mostly angular, irregular rock fragments
- Distal coarsening from fall sorting
- Downslope trajectories vary with surface properties (roughness) and rock properties
What happens to equidimensional boulders vs. oddly shaped boulders?
- Roll and bounce vs. wedge or break on impact
What does the degree of distal coarsening depend on?
- Clast size, shape, lithology (hardness)
- Surface roughness (frictional resistance, depends on size of clasts and surface irregularities like veg)
What is a positive feedback of talus cones?
- Large boulders get trapped in large holes
- and in turn create more surface roughness to trap more boulders
Why does Talus generally fine with depth?
- Fines infiltrate into matrix
- Large rocks have more momentum and roll further downhill
- Upper sediments crushed by rocks rolling over them
Topples
- Forward rotation outward from slope
- Axis of rotation below centre of gravity of displaced mass
- Generally in rocks with steeply dipping discontinuities
- Rotational stability of a particular block depends mostly on its aspect ratio (height to thickness) and angle
Block Toppling
- Common form of toppling
- Brittle failure flexural toppling occurs by plastic bending of weak rocks such as phyllite
Which topples more easily, slender or cubic blocks?
- slender
Slides
- Movement of soil, sed, or rock mass along a failure plane with relatively thin zones of intense shear
- Determined by geology/stratigraphy (material type, permeability, shale, or clay layers, top of permafrost etc.)
What are the 2 main categories of slides?
- Translational
- Rotational
Translational Slide
- Planar rupture and slip face, no steps
- Surface roughly parallel to the ground surface and often shallow
- Shallower and longer than rotational slide
Rotational Slide (slump)
- Rupture along a concave (curved up) surface
- Rotation lowers the head and raises the toe
- Step-like features, back-tilted trees towards scarps, sag ponds on scarps
- Stratigraphy maintained usually
What is the surface morphology of a rotational slide characterized by?
- Steep scarp
- Flat upper surface
- Stair-step effect created by multiple events
Velocity profile of slides
- linear with depth where all depths are moving at the same speed
- Pure slides have little internal deformation
Rotational Slides (Slumps)
- Slower failure of massive blocks (usually sediment)
- Curved (rotational) failure planes often stepped (retrogressive)
Rotational Slides (Slumps), Causes/triggers
- Moisture effects (Precipitation)
- Under cutting
- Over steepening, undercutting
- Loading
- Logging
- ## Vibration (EQ’s)
Overstepping And Slumping
- Slopes naturally achieve balance with forces acting on their mass by adjusting slope angle
La Conchita, Ca
- Rotational landslide
- Rain instigated, pore pressure increase over rainy winter
- Geomorph indicated historical landslides evident, why put a town there?
- Happened again 10 years later, killed people, rain instigated again, Why no drain pipes?
La Conchita, Ca
- Evidence of instability?
- Many old slides
- Extensive gullying on marine terraces above town
- Large ancient scarp above modern failure indicated very large past events
Bank Slump Failure
- half way between translational and rotational
Lateral Spread
- Extension of a cohesive mass overlying deformable material
- Fractured cohesive material often subsides into the softer flow
- Trigger sets off deformable layer to move and upper firm layer ‘goes along for the ride’
- May occur on gentle slopes
- Spreads result from liquefaction, after EQ’s, and when sandy units overlie deformable clays
- Common in areas of quick clay
Quick clay landslide characteristics
- Landslides retrogressive (slide motion itself triggers more failures and often dams rivers- Range from 10m^3 to 10^6m
- arcuate or bottleneck shaped
- Rotated blocks of overburden common
- Overlying sands often shallow water facies
- Underlying clays deeper water seds
- Triggered by small disturbances (excavation, river erosion, small EQ’s)
Quick clay
- Salty, randomly stacked clay particles
- When salt leaches out and then vibration occurs the structure will liquify and flow
Debris Flows
- Viscous movement of soil and/or weathered bedrock
- Internal shear deformation (velocity variance along the flow profile of the viscous fluid)
- Includes mudflows, debris flows, earth flows, Lahars (volcanic), and some rock/debris avalanches
- Range in size form a few m^3 of sand down a dune face to collapse of several km^3 from volcano
- Common in BC
Characteristics of debris flows
- A form of rapid mass movement in which loose soil, rock, organic matter (logs), air, and water mobilize as a slurry and flow downslope
- Between water and sediment flow
- Rapid, depends on rheology of flow materials
- Occur in variety of climatic and physiographic zones
- Removal of trees increases infiltration capacity of water, need drainage to mitigate
What are debris flows typically initiated by?
- rapid addition of water by extended rainfall, localized areas of intense rainfall, ponding on surface upstream flow, snowmelt or rain on snow
What are the most damaging mass movements in BC year after year?
Debris Flows
Phases of Debris Flows?
- Phase 1: Snout is composed of coarse material brought to the front by kinematic sorting (shaking)
- Phase 2: Less viscous, main part of flow composed of sediments that are finer than the snout
- Phase 3: Highest on slope, usually finest material
Debris flow deposits
- Often ungraded or normally graded (coarsest seds on the bottom
- Stacked sequences of normally graded flows give frequency and recurrence interval
Anatomy of debris flow channel?
- Bare channel sides in upper reaches b/c of intense scouring produced when flow descends through a gully
- Sometimes only base is bare and sides are still mantled w/ unconsolidated materials
- Lower down w/ reduced slope gradient lateral ridges of coarse material levees b/c of friction along sides
- Lateral ridges channel remainder of debris, increasing flow depth and velocity
- Front is coarse grained snout
What are the implications of pulsating habits of debris flows?
- Affect the sedimentology of the deposit and the ability to measure frequency of events
Debris flow Prerequisites?
- Abundant water (extended or intense precip, ponding, rapid snowmelt, rain on snow)
- Abundant fine seds (volcanics, glacial seds)
- Slopes steeper than 15 degrees
Geomorphic evidence of debris flow deposits
- Lobate margins, convex surface
- Coarse clasts at snout and sides
- Surfaces studded with boulders
- Flow levees
- Boulders, logs in lower part
- No gravel imbrication
- Consolidated sed packed into nooks and crannies
- Bare channels at top with increasing debris thickness downslope
- Muddy coatings on boulders, logs etc.
HCF
Hyperconsolidated Flow
Largest slide in Canadian History?
- Mount Meager 2010
- Lahar
- 13km runout, 270m runup
- Dammed lake burst
- Debris flow 65km downstream
- Seismic equivalent of M2.6
- Lilloet river morphology changed due to sed load increase
Earth Flows
- Mass movements of relatively dry, fine-grained materials
- More sediment rich and slower than most debris flows
- Typically exhibit both sliding and flowing
- May be long-lived features, with movements occurring intermittently (decadal)
- High viscosity flows, dry, slow
Lemieux landslide
- Geomorphology success story
- Small 1971 landslide initiated study of glaciomarine clays
- Indicated town was in danger zone so it was abandoned
- 1993 rapid earth flow consumed farmland adjacent to town, no lives lost
- Don’t need large slopes for extensive failure
Creep
- Upward heave with downslope (plastic) displacement
- Flow is not rapid, very slow
- Most widespread and poorly understood process
- Associated with ‘heave’ (periodic expansion and contraction of a soil or sediment that is usually linked to clay swelling and dewatering or freeze/thaw)
- Frost heave special consideration in Canada
- Look for bending trees
Why is frost heave important and how does it work?
- Process that contributes to creep
- Frost wedging acts perpendicular to the ground surface resulting in individual particles moving outwards from slope,
- Melt causes gravity to bring material vertically downwards, not back where it came from on a slope
- Net movement is downslope
How can creep be significant?
- Compress and wrinkle cas pipelines
Solifluction
- Creep in saturated beds
- Slow gradual downslope movement of saturated sed lobes
- Common in permafrost envrs
- Permafrost impermeable to water so soil above saturates and moves slopes as shallow as 0.5 degrees
Oso, Wa
- Large late winter rainfall
- No EQ
- Logging in upper watershed
- River undercutting
- Glacial seds (marine clays)
- Saturated above impermeable layer, very permeable sands above, increased pore pressure
- Rotational slide that morphed into moist debris flow
- Very dangerous for rescue workers, kept sinking on top
- Dammed river, caused major flooding
- Geomorphic evidence for past slides evident in lidar
Mitigation examples
- Geomorphic slope hazard maps
- Geomorphic Monitoring programs
- GPS surveys of monuments on colluvial aprons (but expensive)
- Corner reflector interferometric synthetic aperature radar (Crinsar) for repeat measurements form space of coherent targets established on colluvial aprons (detect mm scale movement)
Geomorphic slope hazard maps
- Map past failures, evidence of current or incipient slope movement, slope materials prone to failure, slope angles etc.
- Develop setback-distance maps
- eg slope plane map of NEBC
Slope plan map NEBC
- Hazard map based on local relief and proximity to rivers
- Hazard parameters based on sed characteristics and empirical data on historical failures
- Slope planes > 7 from toe to surface
- Zones created btwn valley side where slope plan represents an area where slope movement could occur
- Distribution of previous failures mapped from geomorphic data
Geomorphic monitoring programs
- What are typical velocities
- When is most likely to occur
- Can climate variables correlate to events
- eg Colluvial apron monitoring program NEBC
Colluvial Aprons
- Area accumulations of bedrock and/or sediments below cliffs or escarpments, convex or concave upper surface
- silt and clay-rich matrix, high water table (pore water pressure)
- Common, found on valley sides, river valleys, and upland escarpments
- Associated w/ continuous, low-mag, down-slope movement
Mitigation for pipelines in danger zones
- Put pipe on slip plates so ground can slide under pipeline
Synthetic Aperature Radar (SAR)
- RadarSat-1
- C-band wavelength (5.6 cm)
- Orbit every 24 days
- Non-intrusicve, non-destructive
- Precise, reliable, cost-effective
- Permits pro-active attention to instability
- Enables retroactive analysis back to 1992
- Reflectors on cliff top bedrock to provide datum, representative colluvial aprons and pipeline corridors
What are the forms of mass wasting most characterized by?
- Materials
- Moisture
- Speed of failure
Strength and Stress most controlled by?
- Moisture/hydrology
- Sediment properties
- Slope
- Land use
