ERTH Final (copy)

Lectures 11-20

Define Mass Movement

Massive failures of slope masses including
rock, debris, soils, and snow/ice under the
influence of gravity

What other disasters are sometimes associated with mass movements?

Earthquakes, floods, thunderstorms,
rainstorms.

T/F: Sometimes, mass movement can be catastrophic, destroying infrastructure and properties.

True

Describe the mass movement in Carabella, Venezuela - Dec. 15th, 1999

Heavy rain set off debris flow carrying boulders that killed thousands of people.

What are four characteristics of mass movements?

- They are gravity-caused disasters
- They cause large volumes of materials to move downslope under the pull of gravity
- They do so catastrophically
- They happen throughout the world

T/F: Everyone and everything is affected by the force of gravity

True

Describe Gravity

Force of attraction between masses m1 and m2, separated by distance r
(G m1 m2 / r2)

What is the term used to describe a variety of processes for the downslope movement of earth materials under the direct influence of gravity?

Mass Movement

Mass movement occurs whenever the ____ force is greater than the ____ force.

Driving, resisting

Which force is the component of gravity parallel to the slope?

Driving force

Which force is proportional to inertia and friction, and counteracts the driving force?

Resisting force

Describe some external conditions that will decrease slope stability

1. Adding mass high on a slope
2. Steepening of the slope
3. Removing support at the base of the slope
4. Removal of lateral support

AM, Steep, Base, Lateral

How does the external role of water affect slope stability?

- Rain erodes material, which can lead to unstable slope.
- Undercutting due to wave erosion
- Rotational slide from erosion

Describe internal conditions that decrease slope stability

1. Presence of weak material
2. Fractures in rock
3. Internal roles of water

Describe the qualities of weak material

- deforms plastically under stress and loses strength
- Crumbly, poorly-consolidated material
- Lubricants (clay)

Describe what occurs to weak material when water and temperature increase.

When weak materials come into contact with moisture they expand, and then collapse when they lose that moisture - causing cracks, change in mass, etc.

What is friction?

Force that resists motion between two surfaces in contact

What are some controlling factors of friction?

- Surface roughness on the surfaces
- Presence of lubricants

Describe the friction coefficients that coincide with:
1. Brick on moist clay
2. Brick on dry clay
3. Granite on granite
AND, describe which is most stable

1. μ0.3
2. μ0.5
3. μ0.6

Granite on granite is most stable

Rock fractures naturally in _____ planes of _______

parallel planes of weakness

Why do rocks fracture naturally in parallel planes of weakness?

Due to pressure, temperature and tectonic events

An unstable condition is created when the fractures are oriented in the direction __________ the slope

perpendicular to

Describe the different scales in which fractures in rock exist

- Regional (like a canyon)
- Local (like a rock outcrop)
- Hand Sample (piece of rock)
- Microscopic (self explanatory)

What causes the internal roles of water to decrease stability?

- Adding weight to porous earth materials
- Dissolving cement binding minerals together
- Erosion in the subsurface
- High pore pressure
- Congelifraction

Erosion in the subsurface causes what?

A network of caves

When there is ____ pore pressure there is ____ binding force, whereas when there is ____ pore pressure there is _____ binding force

Low, Strong
High, Weak

Why is there weak binding force when there is high pore pressure?

The grains are essentially floating in water rather than remaining compact (liquefaction)

What is congelifraction?

Disintegration of a rock due to several cycles of water freezing and melting

What happens during congelifraction?

9% volume increase, tension generated

What are the conditions that are favourable to congelifraction?

- Moist environment
- Temperature fluctuating around freezing point
- Fractures in rocks

What two elements account for around 75% of the earth's crust?

Oxygen (45%) and Silicon (27%)

______ are the building blocks of rocks

Minerals

Biochemistry studies structures of carbon, whereas geochemistry studies structures of _______ and ______

silicon and oxugen

Describe the basic building block of geochemistry

The silicon-oxygen tetrahedron molecule;
- Four oxygen ions surrounding silicon ion
- Tetrahedra combine to form rings, chains, sheets and 3D structures.

What are silicates?

Group of minerals containing Si and O combined with other elements

Describe clay

general term which describes a variety of complex stacked thin sheet silicates.

Because clay is _____ charged, it is looking for material to attract in order to become stable, and wants to bond with _____

Negatively, water.

The sheet surface is ____ charged, and attracts the _____ side of water molecules (hydrogen side)

negative, positive

What are swelling clays?

• Group of clays which tend to swell when exposed to water
– Addition of water molecules between sheets causes expansion

Swelling clays result in a _______

lack of strength (clay becomes an excellent lubricant)

Mass movements involving clay occur often in ___ conditions

wet (rainfalls, spring melt)

Sensitive clays can change ____________ almost instantaneously

Internal structure

Presence of ____ is a key factor in sensitive clays

salt

Describe flocculated structure of clay

Stiff and strong
Salt binds the clay sheets and silt together

Describe dispersed structure of clay

Soft and weak
Behaves almost like a fluid

How do igneous rocks form?

cooling and solidification from a molten liquid called magma

How do sedimentary rocks form?

– By the erosion and compaction of rock fragments or
– By precipitation

How do metamorphic rocks form?

Metamorphic rocks form by the alteration of existing rocks by heat and/or pressure

In what type of rock is mass movement most common in?

Sedimentary rock

Sedimentary rock conditions that make it favorable to mass movement

- Rich in clay
- Deposition planes are planes of weakness
- Some sedimentary rocks dissolve in water forming
a network of caves

Movement may occur along the ___________, ________ or both.

Plane of deposition, fracture

Conditions favourable to mass movement

- Rough topography (out West)
- Sediments moving down river valleys (mid)
- Sensitive clays (East)

What are the two causes that trigger mass movement?

Natural causes and human causes

Name natural causes of mass movement

- Earthquakes
- Volcanic activity (Lahars)
- Erosion
- Water (rain, spring melt, freezing/melting, groundwater fluctuations)

What are the five types of mass movements?

Falls
Slides - Translational and Rotational
Flows
Complex (mix of falls, slides and flows)
Subsides

How is mass movement classified?

1. Type of movement
2. Material involved
3. Speed of movement

Describe falls

- Rapid, free-fall mass movement
- bouncing, rolling, end-over-end motion
- fragments ranging from small grains to large blocks
- develops in material weakened by fractures

What contributes to the formation of talus slopes in mountainous areas?

Freezing/melting cycles

Describe slides

- Mass movement involving motion along a failure surface
- Material remains as a block
- Preslide coherence maintained
- Failure surface is well-defined

Slides move as a ________ mass

semisolid

Slide on top of ______ slip surface that may be ____ or ______

basal, planar, curved

T/F: "Landslide" is a generic term applied to almost any kind of slope failure

True

Slides are classified according to...

The nature of the failure surface

What type of sliding surface creates a Translational slide?

Planar

What type of sliding surface creates a Rotational slide?

Curved

What is a translational slide?

A slide in which earth material moves parallel to planar failure surfaces

In terms of translational slide, if the material is unconsolidated, it's called

Debris or earth slide
(debris is small, earth is tiny particles)

In terms of translational slide, if the material is bedrock, it's called a ___

rock slide

What is critical in the development of rock slides?

Presence of fractures

Describe lateral spreads

Movement of the earth material results from liquefaction of subjacent material

When are lateral spreads common?

When Tsunamis happen

What type of slides are lateral spreads?

Translational slides

Lateral spreads are related to distinct _______ conditions present in what environments?

Geological, northern

Where is it common to see lateral spreads?

- St. Lawrence river lowlands, Canada
- Scandinavia
- Alaska

Define liquefaction

The phenomenon in which the strength of soil is reduced by rapid and violent shaking, or loading. It does not have the strength to support a load.

What type of soils does liquefaction occur in? And how does it behave?

Saturated soils, behaves like a liquid

Describe how clay becomes present above sea level from glaciers

1. Marine clays are deposited in glacial regions. Salt is present due to the sea water leading to flocculated structure
2. Glaciers melt. The earth's surface is uplifted. Clays are above water level
3. Clays are leached by fresh water from rain and snowmelt (salt is removed)
4. Clays instantly change to dispersed structures (liquefaction)

Clays in a marine environment are ______, and clays above sea level are ______

strong, weak

What are triggering factors of lateral spreads?

Loading, earthquakes, vibrations from construction activities, heavy rain, spring melt, etc.

With lateral spreads, failure is ______. It often starts on a river bank and proceeds rapidly inland

retrogressive

With lateral spreads, material flows rapidly even on very _____

Gentle slopes

In lateral spreads, large masses of clay become completely ______ and flow _________

liquefied, as fast as a river

Describe the 1971 St. Jean-Vianney lateral spread-south nation river lateral spread

Retrogressive lateral spread in sensitive clays
- Started on the bank of a small creek and retrogressed in minutes
- occurred within the scar of another lateral spread
- 7.6 M3 displaced, created a large crater
- Triggered by engineering work on a small underground creek
- 31 deaths
- Damage to infrastructure (40 houses, village abandoned)

The 1971 St. Vianney lateral spread lead to investigation of slope stability and protection of ______, removal of ______ in _____

Seguinburg, Lemieux, 1991
*Lemieux lateral spread ended up occurring in 1993

Ottawa area was covered in _______ km of ice during the last _______ (23 000 to 10 000 years ago)

2-3, glaciation

When glaciers first melted around Ottawa area, marine invasion formed the _______, which deposited up to 100m of _______ and _______

Champlain sea, marine clay and silt

What type of clays are known for being sensitive and prone to lateral spreads?

Leda clays

What caused the Champlain sea to retreat and expose the clays to rain?

Isostatic rebound

What was the name of the small village on the banks of south nation river that was exporpriated and dismantled in 1991

Lemieux

How much volume of material was displaced during the Lemieux lateral spread of 1993?

2.4m3

T/F: there were no casualties from the Lemieux lateral spread

True

Mass movements is also known as ___________

Mass wasting

The cause of mass movements can be ______ or ______, whereas the trigger is usually _______

Human-made, Natural, Gravity

Describe a rotational slide

Slide in which failure occurs on a very steep slope, along a concave rupture surface

Why do rotational slides often occur?

Multiple blocks fail due to natural factors like wave erosion or human activity like road cuts.

What is the top and bottom of a rotational slide called?

Scarp and toe

Which type of mass movement involves continuous internal deformation of the moving material?

Flows

What is the main difference between flows and slides?

Slides have little deformation within the moving material, whereas the material with flows is thoroughly deformed during movement

What are the two main types of flows and what do they depend on?

They depend on velocity. Slow is creep, and fast is rock, debris or earth flow.

Describe creep

Creep: gradual, slow movement of earth and debris downhill
– Creep is assisted by the alternating seasons; expansion during wet conditions and contraction during dry conditions.