Lesson 5: Mass Wasting Flashcards
It is the downslope movement of rock and regolith near the Earth’s surface
mass wasting
what force is mainly responsible for mass wasting
gravity
why is mass wasting an integral part of erosional process?
because it moves material from higher elevations to lower elevations
T of F: Mass wasting processes are occurring sporadically on all slopes; some act very slowly, others occur very suddenly, often with disastrous results.
FALSE: Mass wasting processes are occurring CONTINUOUSLY on all slopes; some act very slowly, others occur very suddenly, often with disastrous results.
Any perceptible downslope movement of rock or regolith is often referred to in general terms as a ________
landslide
during the Cherry Hills Landslide,
-how many people died?
-when did it happen?
-what type of landslide?
50 people died
on August 1-3, 1999
complex slide: started as slump, developed into debris flow near base
how many percent of the philippines is classified as highly susceptible to landslides?
80%
during the Guinsaugon Landslide in Leyte (St. Bernard)
-how many people died?
-when did it happen?
-what type of landslide?
1126 people died
on February 17, 2006
Complex side: rockslide-debris avalanche
what was thought to be the cause of the guisaugon landslide?
it rained continuously for 2 weeks and also the slope was known to be a splay of the philippine fault
during the Brgy Tinaan, Naga City, Cebu landslide
-what was the cause?
-when did it happen?
-what type of landslide?
- recent earthquakes caused groundwater to divert here & there was a fault 500m away
-Sept 18, 2018 - sinkhole collapse+ planar block slide = debris flow
Knowledge about the relationships between _________ and __________ can lead to better planning that can reduce vulnerability to such hazards
Knowledge about the relationships between LOCAL GEOLOGY and MASS WASTING PROCESSES can lead to better planning that can reduce vulnerability to such hazards
The material that is (practically) still in place and
adjacent to the highest parts of the main scarp
crown
A steep surface on the undisturbed ground at the
upper edge of the landslide
main scarp
It is the visible part of the surface of rupture.
main scarp
what causes the formation of the main scarp?
caused by movement of the displaced material away from the undisturbed ground
The highest point of contact between the displaced material and the main scarp
top
The upper parts of the landslide along the contact between the displaced material and the main scarp
head
A steep surface on the displaced material of the landslide produced by differential movements within the displaced material
minor scarp
The part of the displaced material of the landslide that overlies the surface of rupture between the main scarp and the toe of the surface of rupture
main body
The portion of the landslide that has moved beyond the toe of the surface of rupture and overlies the original ground surface.
foot
The point of the toe farthest from the top of the landslide
tip
The lower, usually curved margin of the displaced material of a landslide, it is the most distant from the main scarp.
toe
The surface that forms (or has formed) the lower boundary of the displaced material below the original ground surface
surface of rupture
The intersection (usually buried) between the lower part of the surface of rupture of a landslide and the original ground surface
toe of surface of rupture
The part of the original ground surface overlain by the foot of the landslide.
surface of separation
Material displaced from its original position on the slope by movement in the landslide
displaced material
what does the displaced material form?
both the depleted mass and the accumulation.
The area of the landslide within which the displaced material lies below the original ground surface.
zone of depletion
The area of the landslide within which the displaced material lies above the original ground surface.
zone of accumulation
The volume bounded by the main scarp, the depleted mass and the original ground surface.
depletion
The volume of the displaced material that overlies the rupture surface but underlies the original ground surface.
depleted mass
The volume of displaced material that lies above the original ground surface.
accumulation
The undisplaced material adjacent to the sides of the rupture surface.
flank
what is preferable in describing the flanks?
Compass directions, but if left and right are used, they refer to the flanks as viewed from the crown.
The surface of the slope that existed before the landslide took place.
original ground surface
8 types of states and distributions of mass movement activity
- active
- suspended
- reactivated
- inactive
- dormant
- abandoned
- stabilized
- relict
usually present if active
scarps and tension cracks
6 ways mass movements can be classified according to
- Type of movement
- Type of material involved
- Relative proportions of material and water
- Velocity of movement
- Thickness
- Type of triggering factor
reactivation is common for what types of mass movement
rockslides and slumping
how to tell if relict activity
if there are evidence of old landslides: significant weathering, look into morphology
the 5 different kinds of movement for mass movement
flow, slide, slump, creep, hello fall??
the 4 different kinds of material for mass movement
rock, debris, earth, mud
the 2 different kinds of thickness for mass movement
shallow or deep-seated
the 4 different kinds of triggers for mass movement
rain-induced, earthquake-induced, human-induced, volcanic-induced
mixture of rock, earth, plants and mud
debris
loose sediment, weathered rock fragments
earth
mixture of water and finer sized sediments
mud
for deep-seated landslides, do mitigation efforts work?
usually doesnt, and the only solution is relocation
comprise a detachment of soil or rock from a steep slope and the more or less free and extremely rapid descent of the material.
fall
Following the definition of fall, the movement is largely through _____, alternated with the _______ or ______ on the slope
Following this definition, the movement is largely through THE AIR, alternated with the BOUNCING OR ROLLING on the slope
what triggers falls?
triggered spontaneously by:
- lateral pressures: roots, crystallization pressures, ice wedging
- loss of underlying support: erosion on cliffs
what is preceded by falling?
falling will be preceded by SMALL SLIDING or TOPPLING MOVEMENTS that separate the material subject to fall from the undisturbed rock
scree vs talus
scree: coarse
talus: fine
constitutes a failure in the support conditions of bedrock
topple
is a forward rotation out of the slope of a mass of soil or rock about a point below the center of gravity of the displaced mass.
topple
identically to fall, associated with very steep slopes
topple
what can topples lead to?
Topples may lead to the sliding of the displaced mass
T or F: toppling is mostly occurring in combination with slides.
FALSE: toppling is mostly occurring in combination with FALL.
defined as a moving slab of soil or bedrock that moves downslope as a whole, Remaining in contact with the underlying material
slide
Slides can be sub-differentiated in two essentially different groups, what are they?
- rotational slides
- translational slides
slides that have a more or less circular failure surface
rotational slides
show in general a steep, almost vertical headscarp (crown), with the slided mass in front of it. The movement makes that the slide mass (body) is backtilting towards the headscarp.
rotational slides
what can occur in combination with tensional cracks for rotational slides?
small steps on the body
slides where the sliding mass moves along a preexisting failure plane
translational slides
4 examples of pre-existing failure planes fit for translational slides
bedding, schistosity, joint, discontinuity between slope debris and the underlying rock
2 differences between translational and rotational slides?
- the total displacement along the failure plane of translational slides is generally longer
= smaller depth/length ratio for translational slides (1/10) vs rotational (1/3) - Translational slides are also frequently showing a considerable larger width than a rotational slide
3 types of translational slides
- soil slips
- rocks slides
- debris slides
shallow translational movement in soil material
soil slips
usual thickness of soil slips
1-2 m
small scale translational slides
soil slips
2 kinds of soil slips
sheet slide and slab slide
soil slips in cohesionless and dry soils
sheet slide
soil slips in soil material saturated with water
slab slide
3 types of rock slides
planar slides, stepped slides, wedge failures
rock slide when the movement takes place along one and the same failure plane
planar slide
rock slide when the movement occurs along a number of parallel failure planes
stepped slides
rock slides when failure is controlled by two planes which define a wedge shaped block, which is loosened from the rock slope.
wedge failure
translational slides with movements occurring on steeper slopes, where the slope debris slides off along the discontinuity with the underlying weathered rock.
debris slides
the range of steep slopes of debris slides
25-35 degrees
involve great internal deformation
flow movements
what forces dominate over what for flow movements?
inter-granular movements dominate over shear surface movements
for flows: _______________ is not uniform, but the material near the surface runs at ______ velocity rate than the _________. The most common subdivision of flows is based on ________.
THE INTERNAL VERTICAL VELOCITY DISTRIBUTION is not uniform, but the material near the surface runs at A HIGHER velocity rate than the UNDERLYING MATERIAL. The most common subdivision of flows is based on THE TYPE OF MATERIAL
is a type of slow movement, in which the rate of internal deformation is limited.
earth flow
type of flow that includes solifluction
earth flow
The general form of faster earthflows is that _________, leaving a ________ bounded by a ________. The saturated material flows _______ to form a ________
The general form of faster earthflows is that MATERIAL SLUMPS AWAY FROM THE TOP, leaving a STEPLIKE TERRACE bounded by a CURVED, WALL-LIKE SCARP. The saturated material flows SLUGGISHLY to form a BULDGING TOE
is a type of a fast moving flow. The material is (over)saturated with water
mudflow
A mudflow mostly originates from ___________ in ________, dragging soil particles with it. The _______ can build up so high that and even stones can be taken along.
A mudflow mostly originates from OVERLAND FLOW in HEAVY RAINFALL, dragging soil particles with it. The KINETIC ENERGY can build up so high that and even stones can be taken along.
where do mudflows commonly occur?
along rivers: if riverbanks (consolidated/semiconsolidated sediment) + lots of precipitation = mudflow
T or F: lahar is technically mudflow
true
when can debris from falls and high-energy slides also behave as a flow (debris flow)
this happens especially if:
- air or water is entrained in the initial movement.
- the trapped fluid may develop high pore pressures
= which buoy up the individual debris particles. Alternatively:
- the numerous interparticle impacts may produce an
effect analogous to intermolecular motion in a real fluid
=thus enabling even dry, non-cohesive, debris to flow
the movement of debris from falls behaving like debris flows taking place in larger material is called?
block flow
an extension of a cohesive soil or rock mass combined with a general subsidence of the broken mass of cohesive material into softer underlying material.
lateral spread
T or F: for lateral spread, the surface of rupture is a surface of intense shear
FALSE: The surface of rupture is NOT a surface of intense shear
Spreads may result from ________ or _______ of the softer material
Spreads may result from LIQUEFACTION or FLOW (AND EXTRUSION) of the softer material
for lateral spreads, the horizontal movement is _________ over ________ and often also partly induced by ________ deformation of the underlying material
for lateral spreads, the horizontal movement is A SLIDING OF THE DETACHED BLOCKS OF COHESIVE MATERIAL over THE SOFTER and often also partly induced by A PLASTIC deformation of the underlying material
is the slow movement of soil or bedrock down slope.
creep
where does creep typically happen
It occurs where the stresses on the slope material are
too small to create a rapid failure, or as a result of thawing/freezing effects
can be classified those slope movements whereby at least two different types of landslides together are responsible for the occurrence or continuation of the slope movement in the area
complex
Some slopes are inherently _______ than others and are therefore more prone to mass wasting
LESS STABLE
As the slope increases, so too does the _______ component acting _______ to the slope, which, in turn, allows more of the rock’s weight to be directed in the ________ direction. When gravitational force becomes _______ than the _________, ___________ can occur.
As the slope increases, so too does the GRAVITATIONAL component acting PARALLEL to the slope, which, in turn, allows more of the rock’s weight to be directed in the DOWNSLOPE direction. When gravitational force becomes GREATER than the FRICTIONAL FORCES, DOWNSLOPE MOVEMENT can occur.
6 factors that may trigger mass wasting
- slope (steepness)
- nature of slope material: angle of repose, oversteepened slopes
- climate
- vegetation
- earthquakes
- volcanic activity
example of rocks that are inherently so strong and homogeneous that they are able to form stable cliffs
granitic rocks of Yosemite Valley, California.
explain how loose sediments are more prone to mass wasting than hard lithologies
frictional forces within loose or unconsolidated sediments are usually lower than solid rock, making sediment more prone to mass wasting and less able to form vertical slopes.
is the steepest angle at which a sloping surface of unconsolidated sediments is stable
angle of repose
typical range of angle of repose
typically less than 35 degrees
Large angular fragments generate greater _________, and therefore are capable of ________ compared to _________.
Large angular fragments generate greater FRICTIONAL FORCES, and therefore are capable of MAINTAINING STEEPER SLOPES compared to SMALL, WELL-ROUNDED SEDIMENTS.
Slippage can occur along ________, ______, ______ and _______ that are inclined in ________ direction as the slope
bedding planes, foliation planes, fractures and fault surfaces inclined in the SAME DIRECTION as the slope
structures facing the slope
daylight structures
On _______ hillsides the component of ______ operating the direction of slope is _______, which, in turn, _______ the potential for _________.
On STEEPER hillsides the component of GRAVITY operating the direction of slope is GREATER, which, in turn, INCREASES the potential for MASS WASTING.
example of how natural oversteepened slopes can cause mass wasting
An common example:
- the undercutting of stream banks
- due to the natural migration of stream channels
= results to destabilizing/overhanging of riverbanks.
= River valleys become wider over time
undercut stream banks
cutbanks
example of how human induced oversteepened slopes can cause mass wasting
flat surfaces are required for the construction of roads, buildings, and parking lots
- means that material must be excavated from hillsides
- in order to create a leveled surface
= weakens the slope significantly
= greatly increases the potential for mass wasting
The long-term average weather for a region is defined as
Climate
2 reasons why climate is an important factor in slope stability
- because it ultimately determines how and when precipitation falls.
- determines the types of vegetation we see blanketing the various slopes
= influences the fraction of rain or snow that infiltrates into the subsurface
describe how vegetation can affect slopes
- fairly dense vegetation tends to stabilize slopes plant roots
- help bind together loose particles of rock and sediment. - during unusually large rainstorms or rapid snowmelts, dense vegetation will increase infiltration ( since it reduces the ability of surface water to move downslope)
- excessive infiltration adds significant weight to a slope
- reduces friction through higher pore pressures
which factor is heavier? geology vs climate
geology
2 ways seismic waves affect slopes
- as the seismic waves pass along the surface,
= the least stable slopes will tend to fail
BECAUSE the ground vibrations suddenly reduce the frictional forces within the slope materials - seismic waves may also cause surface materials to liquefy
= immediately destabilize a slope
=trigger a mass wasting event
how does rain/snow affect cause mass wasting
- Rain or melting snow will infiltrate
= cause subsurface void spaces to become saturated
= weight of the water in the saturated zone causes the fluid/pore pressure within the voids to increase
= reduces the friction between the solids
= Downslope movement occurs when the frictional forces
become less than the gravitational force in the slope direction
what are the 4 types of causes of mass wasting?
- geological
- morphological
- physical
- human
geological causes of mass wasting
Weak material
* Sensitive material
* Weathered material
* Sheared material
* Jointed or fissured material
* Adversely oriented mass discontinuity (bedding, schistosity,
etc.)
* Adversely oriented structural discontinuity (fault, unconformity etc.)
* Contrast in permeability
* Contrast in stiffness (stiff, dense material over plastic material)
morphological causes of mass wasting
*Tectonic or volcanic uplift
* Glacial rebound
* Erosion of slope toe
* Erosion of lateral margins
* Subterranean erosion (solution, piping)
* Deposition of load on the slope or its crest
* Vegetation removal
physical causes of mass wasting
*Intense rainfall/ Prolonged exceptional precipitation
* Rapid snow melt/ Thawing/ Freeze-and-thaw weathering
* Earthquake/ Volcanic eruption
human causes of mass wasting
*Excavation of the slope or its toe
* Deposition of load on the slope or its crest
* Drawdown (of reservoirs)
* Deforestation
* Irrigation
* Mining
* Artificial vibration
* Water leakage from utilities
how to identify landslides: 6 indications
- Vegetation cover: intact/disrupted
- Presence of cracks or scars
- Disturbance of the drainage pattern
- Excessive soil moisture or abrupt soils moisture changes.
- Terrain form: hummocky, niche-lobe sequence
- if there is growing or tilting of trees, and electrical posts
what are the 9 morphological diagnostics of mass wasting
- concave-convex
2.semicircular niches
3.back tilting of slope faces
4.hummocky relief - formation of cracks
6.steepening of slopes
7.vegetational clearances on steep scarps - irregular linear clearances along the slope
is associated with landslide niches and associated deposits.
The sequence of a concave and a convex part of the slope
is related to retrogressive sliding
A step-like morphology
are associated with the head part of a slide with the outcrop of the failure plane
semicircular niches
how are semicircular niches recognized on photographs?
as a light toned scarp, with small light curved lineament
indicates rotational movement of slide blocks and appears as oval or elongated depressions with imperfect drainage conditions
Back tilting of slope faces
Irregular slope morphology. Micro-relief associated with shallow movements or small retrogressive slide blocks
hummocky relief
what lithology is hummocky relief common?
areas underlain with limestone
how is hummocky relief recognized in aerial photos?
coarse surface texture of hummocky terrains stand contrast with smooth surroundings
an indication for recent mass wasting activity
The formation of new cracks
occurs with sliding and toppling movement
crack formations
how do crack formations appear?
They appear as lineaments more or less parallel to the existing scar
can indicate the presence of a landslide scar. This type of slope anomaly is clearly visible on aerial photographs
steepening of slopes
coincides with morphological steps. Headscarps and steps in a slide body often show this
Vegetational clearances on steep scarps
how do vegetational clearances on steep scarps show on photos?
On photographs this shows as light-toned elongated areas at the crown or on the body of the mass movement.
what causes the irregular linear clearances along the slopes?
- Flows and avalanches strip parts of the slope from vegetation; leaving the flowpath uncovered.
- Slip surface of translational slides as well are usually uncovered.
what do irregular linear clearances along the slopes look like?
The denudated areas are showing light tones, often with linear pattern in direction of movement
