Landslides and Mass Movements

• Landslides are a destructive natural hazard in Canada, causing $100-200 million in damages annually and 1000 deaths since 1940.

Mass Movements

• These processes transport Earth material (rock, soil, sediment, ice) downslope due to gravity.
• They are a major source of sediment production.
• Occur when downslope forces (gravity) exceed resisting forces (friction).
• Failures can be rapid and catastrophic or slow and gradual.
• Over geologic time, all slopes are inherently unstable.

Triggers of Mass Movements

• Usually triggered by an event, but most failures have complex, multiple causes.
• Slopes weaken over time through numerous events and near-failures.
• Underlying causes bring slopes to the brink of failure.
• Immediate causes trigger the final collapse, releasing significant energy.
• Common triggers include earthquakes, volcanoes, heavy rains, thawing ground, and human activities.

Factors Influencing Slope Failures

• Slope angle/support: Increased by river or wave downcutting, making slopes more unstable over long periods.
• Steeper incline = more unstable. Support at the base helps stabilize the slope.
• Slope composition: Weathered/fractured rock, high clay concentration, or unconsolidated material = unstable.
• Vegetation: Presence of vegetation anchors unconsolidated material.
  • Roots stabilize soil, so deforestation or fires increase failure risk.
• Weight: Water, overlying sediment, or buildings increase instability.
• Angle of repose: The angle at which grains are stable, affected by grain size (bigger = more stable) and angularity (more angular = more stable).
• Water has varying amount based on saturation:
  • Small amounts: Holds unconsolidated material together due to water's dipole nature.
  • Large amounts: Reduces friction between grains and buoys up slope material.
• Pore-water pressure: Pressure on water within rock pores increases with sediment weight; over-pressurization lifts sediments, causing instability..

Internal Causes of Slope Failures

• Adverse geologic structures (composition) contribute to instability.
• Ancient slip surfaces (faults): Sliding creates smooth layers that easily reactivate, especially when wet.
  • Faults from tectonic activity create gaps; old faults can reactivate with slight pressure.
  • Large faults allow water infiltration, increasing instability.
• Exposed bedding (layer orientation in hillside): Layers at low angles to the hillside allow slippage, while steeper angles resist it.
  • Beds are prone to slipping, especially with soft, slippery layers like clay.
• Rock structures: Uncemented/uncompacted rocks, clay layers, soft rock on strong rock, and fractures weaken the rock.
• Exfoliation joints: Decrease cohesion.
  • Buried rocks compress, then expand upon uplift, causing fractures and increased porosity, which weakens rocks and increases water infiltration.
• Inherently weak materials: Clays form during chemical weathering; their small size and book-like shape create electrostatic bonds. Clay's composition is unstable and changes, altering strength and water content.
• Expansive and hydrocompacting sediments and soils: Some clays expand with water (negative charge attracts water), leading to collapse upon drying.

The role of water

• Water weakens earth materials internally by:
  • Weight: Water is heavier than air in pore spaces.
  • Absorption and adsorption: Decreasing material strength.
• Quick clays: Mobile deposits, fine rock flour scoured by glaciers, deposited in seas, and later exposed.
  • Weak solid (elongated and tabular shapes) held by salt collapses when freshwater dissolves salt, turning ground into liquid.
  • Common in eastern Canada where a large sea basin was left by subsidence.
• Dissolving (natural) cement (gypsum and clay): Water dissolves minerals holding rock together, creating water conduits.
• Piping: Water physically erodes away loose material, creating springs.
• Quicksand: Sand grains are supersaturated with pressurized water, pore-water pressure equals the weight of sands → no shear strength.
  • Water-pressurized sand flows downhill into a depression, becoming a high-viscosity liquid.
• Water table: Top of groundwater; gravity saturates subsurface rocks.
  • Infiltrates the ground by gravity.
  • Rivers are higher when the water table is, draining it in the dry season.
• Acid rain: Dissolves calcite in limestone, decreasing stability of the surface.

Classification of Mass Movements

• Mass movements: Erosion, transport, and accumulation of material on slopes due to gravity.
• Vary by material size, moisture, speed, and presence of a slip surface.
• Classified by movement type: downward, downward and outward.

Downward Movement

• Fall: Free-fall, dominantly vertical, as separate blocks.
• Subsidence: Collapse into a void, dominantly vertical, as separate blocks.

Downward and Outward Movement

• Slide: Movement on a basal slip surface (planar or curved), as a semisolid mass with some coherence.
• Flow: Movement as very viscous fluids with turbulence throughout.

Creep:

• Slowest, most widespread slope failure.
• Almost imperceptible downhill movement of soil and uppermost bedrock.
• Caused by swelling and shrinking of soil:
  • Freezing/expanding water in pores.
  • Absorption of water, expansion of clay minerals.
  • Heating by the sun increases volume.
• Soil expands perpendicular to the surface and shrinks straight down from gravity.
• Dry to moist conditions; extremely slow movement of rock/debris/earth.

Solifluction/Gelifluction

• Occurs in permafrost regions, where the top layer of soil (active layer) thaws and slowly flows over frozen subsoil.
• Moist conditions; extremely slow movement of rock/debris/earth.

Slump/Debris Flow

• Flow over the landscape as very viscous, turbulent fluids.
• Saturated conditions; extremely fast movement of debris/earth.

Earthflow

• Rock layers tilt seaward, containing bentonitic clay (from volcanic ash weathering, easily absorbs water), and ocean waves erode the toe.
• Unstable land is used for farming until residential development occurs (adding weight).
• Stays as coherent blocks.
• Associated with high water content in a layer.
• Dry to saturated conditions; extremely slow movement of debris or earth.

Slump (Rotational Slides)

• Block movement above a curved slip surface, rotating about an axis parallel to the slope.
• Head moves downward and rotates backward.
• Toe moves upward onto the landscape.
• Moves short distances until equilibrium is reached by the toe slide adding base support.
• Pore-water content and materials affect morphology (rocky = one failure surface, mixed = multiple failure surfaces).
• The first landslide may not reach equilibrium, leaving the land behind it unstable.
• Moist to saturated conditions; slow movement of rock/debris/earth.
  • Turnagain Heights, Anchorage, Alaska, 1964:
    • M9.2 earthquake triggered mass movements.
    • Glacially ground, clay-rich sediment.
    • Sliding began after 90 seconds of shaking liquefied deep clays.
    • Rotational slides trapped by deep clay deformed internally, moving the block above.

Slides (Translational Slides)

• Move on a planar slip surface (fault/joint/clay-rich layer).
• Move as long as there is a downward-inclined surface and driving mass.
• Can remain coherent, deform and disintegrate into debris slides, or cause underlying material to fail.
• Sandstone can slide as a coherent block on a main slip surface when clay expands from water infiltration.
• Dry to saturated conditions; slow to rapid movement of rock/debris/earth.
  • Turtle Mountain, NWT, 1903:
    • 30M m3 of dipping limestone slid 1000m down a daylighted bedding surface into the valley.
    • Beds dip more shallowly than the slope.
    • Limestone can be dissolved by acid rain, weathering daylighted beds.

Debris Avalanche/Debris Flow

• Falls down the face of a mountain (debris flow = channelized down valleys).
• Can start dry.
• Moist to saturated conditions; rapid to extremely rapid movement of debris/earth.

Sturzstroms

• Massive rock falls convert into highly fluid rapid debris flows that travel far.
• Large rock falls travel up to 25 times their vertical fall.
• Hypotheses:
  • Water provides lubrication, but some flows were dry.
  • Frictional melting fluidizes the mass, but some deposits contain ice and lichen.
  • Falling mass traps and rides on air, but some were in contact with ground and identical flow features on ocean floor, moon and mars.
• Fall → jump → surge: Mass began to disintegrate as it fell, hitting the floor and shooting out from the mountainside ledge.
• Varying levels of water; extremely rapid movement of rock/debris/earth.

Falls

• Elevated rock mass separates along a joint, bedding plane, or weakness and falls downward through the air until hitting the ground, bouncing and rolling.
• Solid mass displaces air quickly, creating an air blast that can knock down trees.
• Such a heavy mass falling for so long registers on the magnitude scale.
• Can roll and bounce or pulverize and be redeposited.
• Dry to moist conditions; extremely rapid movement (air resistance only) of rock/debris/earth.

Snow Avalanches

• Behave like earth mass movements: creep, fall, slide, flow.
• Small to large, barely moving to 370 km/h, few meters to kilometers in length.
• Small avalanches typically fail at one steep point in loose, powdery snow, triggering more snow movement downhill.
• Usually begin when snow reaches 0.5-1.5m deep.
• Snow depth can reach 2-5m before big avalanches if snow particles become rounded and packed.
• Stability is affected by snow morphology (humidity and temperature conditions for snowflake type).
  • Porosity and density will be affected by morphology of snowflakes.
• Loose-powder avalanches:
  • Low cohesion with up to 95% volume as pore space (mostly air).
• Slab avalanches:
  • Slabs of snow that break free from the base like translation slides and turn into flows.
  • Snow mass is composed of layers with different ice, snow characteristics that have different strength.
  • Numerous potential failure surfaces.
• Dry snow (less cohesion + dense) forms faster avalanches than wet snow.
• Avalanches may flow for many miles up and over ridges.
• Dry to saturated conditions; extremely fast movement of snow.

Subsidence

• Ground surface gently sags or catastrophically drops as voids in rocks close due to tectonic processes.
• Smaller-scale mass movement where the ground is caving in.
• Causes: Oil/natural gas extraction, mining, limestone dissolution, groundwater changes.
• Groundwater: Ground is constantly recharged, rivers lose water downstream as it's absorbed.
• The water table is the upper surface of saturation in the ground.
• An aquifer is fed by the water table.
• Effluent streams emanate from the water table.
• Sandstone is more porous than shale so aquifers are more permeable.
• Shale units that are impermeable act as confining layers.

Rock Dissolution (Piping)

• Caves usually occur in limestone (CaCO3 shells of marine organisms).
• Equilibrium equation exists for creating or dissolving limestone.
• Cave formation (karst landscape) takes 10,000 years to 1 million years, controlled by carbonic acid (CO2).
• Catastrophic subsidence: Groundwater levels drop, caverns empty, and the buoyant water support is removed, causing roofs to collapse and form sinkholes.

Mine Collapse

• Coal, salt, and metallic ore mines create voids that can collapse, causing surface subsidence.
• Mine water breakout: Mine voids can collect water that can burst onto the surface.

Fluid Withdrawal

• Removing water causes loose sand to pack tighter due to the overburden weight, causing sinking.
• Grain-to-grain interaction is reduced in water due to buoyancy.
• Cubic packing is lighter than rhombohedron packing.
• Dewatering removes buoyancy force, making sediment grains heavier and causing sediment compaction.
• Sedmiment and old faults that act as landslide surfaces can be renewed with movement.

Sediment Compaction

• Loose piles of water-saturated sand and mud at the end of rivers compact and sink.
• Mississippi River/New Orleans region: Sinking due to sediment compaction, dewatering, and isostatic adjustment; 45% of the city is below sea level and prone to flooding in hurricanes.
• Building on sediment adds weight, increasing pore-water pressure; water migrates to low-pressure areas, removing water and volume, grains pack tightly, sinking buildings.

Monitoring and Mitigating Mass Movement

• Measurements of different levels at varying risks to monitor changes over time.
• GPS: Satellite signals determine 3D locations and track real-time movement.
• Data logger: Centralized system controls sensors, gathers readings, and transmits data from multiple instruments.
• Piezometer: Detects water pressure fluctuations in boreholes. Rising pressure weakens slopes.
• Inclinometer: Monitors ground motion at various depths in boreholes.
• Tiltmeter: Mounted on the surface to detect subtle land tilts.
• Controlling rotational/translational landslides: Unloading the head, reinforcing the body, and supporting the toe.
• Drainage removes water from the slope.
• Rocks can also be held in place with wire mesh nets and fences can be used to to catch fallen rocks.
• Controlling earth/snow flows: Steering flows with walls and digging channels, removing rock, decreasing slope angle, using avalanche tunnels, avalanche fences, and booms to start avalanches on stable surfaces.