(MODULE 7.3) GEOLOGICAL HAZARDS - MASS WASTING Flashcards
movement in which bedrock, rock debris, or soil moves downslope in bulk, or as a mass, due to gravity.
mass wasting
a general term for mass wasting
landslides
cause of mass wasting:
the driving force for mass wasting is ___.
gravity
the effect of gravity consists of two component forces:
___ is perpendicular to the slope and holds the block in place.
___ is parallel to the slope and indicates the block’s ability to move.
- normal force
- shear force
explain the role of slope in mass wasting through this figure.
at low slopes, the shear resistance would be stronger than the shear force, meaning it isn’t likely to cause a landslide.
at steep slopes, the shear resistance would be weaker than the shear force, causing it to be unstable and cause a landslide more easily.
___ counteracts the shear force.
friction
___ is the force that would be needed to move the block.
shear resistance
if the shear resistance its larger than the shear force, the block will ___.
not move
the resistance to movement or deformation of that soil is its ___.
shear strength.
the larger the normal force, the ___ the shear strength is.
greater.
if the shear strength is ___ than the shear force, the soil will not move or deform
greater
on the other hand, if shear strength is ___ than shear force, the soil will flow or slide.
less
mass wasting occurs when gravitational forces ___ frictional or shear strength of material.
exceed
shear strength of soil is controlled by the following factors:
1. the ___ of the material,
2. ___ between particles,
3. ___ of water, and
4. the ___ of plant roots
- cohesiveness
- friction
- pore pressure
- anchoring
cohesiveness of the material:
cohesiveness is the force which holds materials together and exists in one type of soils; when it comes to cohesiveness, ___ can be cohesive.
fine soils (silt and clay)
how does water affect the cohesiveness of clay?
water can saturate clay and turn it to a cohesive paste which acts as a binder. this allows the clay particles to rearrange and turn into a cohesive material.
too much water would separate the particles too much and make it less cohesive.
friction between particles: ___ relies more on friction for its shear strength. whereas clay is fine and weak, ___ is more sturdy.
coarse soil.
soil has pore spaces which have air or water on it. how does water enhance the shear strength of the soil?
putting water on the soil can cause it to carry more load. the more load on the soil, the higher the pore pressure of water would be.
as the amount of water in soil ___, the rate of movement tends to increase.
increase
___ have high ratios of water to earth. if one continues to gain water, it eventually becomes a ___.
- mudflows
- muddy stream
triggering factors:
a sudden event may trigger mass wasting of a hillside that is unstable. what factors may trigger this mass wasting?
1.
2.
3.
- earthquakes
- heavy rainfall
- construction work
how do earthquakes cause landslides?
earthquakes release pore pressure and cause landslides.
construction work can cause landslides through either one of two things: ___, or ___.
the extra weight of machinery or buildings, or when bulldozing for highway construction.
how does bulldozing for highway constructions cause landslides?
in order to create a highway, a part of a volume of soil has to be cut, causing a very steep slope which is prone to landslides.
since this part of the volume of soil serves as a support, removing it would cause landslides. additional load on the volume of soil with a steep slope could also trigger landslides.
vertical distance between valley floor and mountain summit
relief
how does the orientation of planes of weakness (bedding planes) in bedrock contribute to the chance of mass wasting? also, define bedding planes.
bedding planes are planes of weakness at the end of old material and beginning of new material (sedimentary rocks).
this weak material (fine-grained, clay sheets) wouldn’t be removed; if the bedding planes are parallel to the slope, that would lead to easier mass wasting.
on its own, it wouldn’t be a problem; however, if water infiltrates the bedding planes through rain, it would flow down the planes and uproot the surface soil on top.
The classifications of mass wasting is based on the following:
1.
2.
3.
- rate of movement (slow or fast)
- type of material (debris, earth, mud)
3, type of movement (flow, fall)
course-grained fragments predominant in the soil
debris
predominantly fine-grained (sand, silt, clay)
earth
clay or silt saturated with water
mud
this type of movement occurs when the descending mass is moving downslope as a viscous fluid.
flow
this type of movement occurs when material freefalls or bounces down a cliff.
fall
this type of movement occurs when the descending mass remains relatively intact, moving along one or more well-defined surfaces.
slide
in this type of slide, the descending mass moves along a plane parallel to the slope of the surface.
translational slide
in this type of slide, the descending mass moves along a curved surface, wherein the upper part moves downward while the lower part moves outward.
rotational slide
the types of mass wasting are as follows:
1.
2.
3.
4.
5.
- creep
- flow
- rockfall
- rockslide
- avalanche
this type of mass wasting exhibits very slow, downslope movement of soil.
creep (or soil creep)
since the shear forces are only slightly grater than the shear strengths in creep, the rate of movement is ___.
very slow and time-dependent, only moving for less than a centimeter per year and detected through observations taken over months or years.
the rates of movement in creep tend to be higher on ___ slopes and ___ soil moisture.
steep slopes, higher soil moisture
the best indicator of creep is a ___ because it is sure to be a natural occurrence in comparison to other indicators of creep.
curved tree trunk
tilted fences are considered indicators of soil creep when ___.
the tilted fences are not individual but rather a group.
this type of mass wasting occurs when motion takes place within a moving mass of unconsolidated or weakly-consolidated material.
flow
recall:
consolidated material means ___, whereas unconsolidated material means ___. weakly-consolidated material means.
- the material is compacted to the point where water has been squeezed out
- the material has not been compacted to the point where water has been squeezed out
- the material has been compacted but there is still some presence of water.
the main factors for flow is that the soil should be ___ and ___.
loose and saturated with water.
the common varieties of flow are
1.
2.
3.
4.
- earthflow
- debris flow
- mudflow
- debris avalanche
this variety of flow occurs when earth moves downslope as a viscous fluid; the process can be slow or rapid depending on the slope.
earthflow
earthflows usually occur on ___.
hillsides that have a thick cover of soil in which finer grains are more dominant, often after heavy rains have saturated the soil.
determine the parts of an earthflow from top to bottom.
- scarp
- hummocky surface
- flowing soil
explain the earthflow that occurs here.
the flowing mass remains covered by a blanket of vegetation, with a scarp (steep cut) developing where the moving debris has pulled away from the stationary upper slope.
this type of earthflow occurs when rotational sliding (slumping) takes place above the earthflow.
slump earthflow
how does slump earthflow occur?
soil remains in a relatively coherent block or blocks that rotate downward and outward, forcing the soil below to flow.
a landslide may be entirely an earthflow. a ___ lobe forms at the toe or front of the earthflow where soil has accumulated.
hummocky
earthflows are triggered through
1.
2.
- presence of too much water
- undercutting at the base of a slope during construction projects
this variety of flow involves soil in which coarse material (gravel, boulders) is predominant.
it can behave much like an earthflow or mudflow if it is saturated with water.
debris flow
this variety of flow occurs when a flowing mixture of fine-grained material and water (sudden influx) moves down a slope.
mudflow
mudflow differs from a debris flow in that ___ material is predominant.
fine-grained (sand, silt, clay)
mudflow is most likely to occur in places where soil is ___.
not protected by a vegetative cover.
___ is the fastest variety of debris flow. it is a rapidly moving, turbulent mass of debris, air, and water.
debris avalanche
this type of mass wasting occurs when a block of bedrock breaks off and falls freely or bounces down a cliff.
rockfall
rockfall
commonly, an apron of fallen rock fragments called ___ accumulates at the base of the cliff.
talus
this type of wasting occurs when there is a rapid sliding of a mass of bedrock along a bedding plane.
rockslide
much like rockfalls, rockslides can be caused by ___.
undercutting at the base of the slope from erosion/construction.
this type of mass wasting occurs when there is a very rapidly moving, turbulent mass of broken-up bedrock.
rock avalanche
the only difference between a rock avalanche and a debris avalanche is that a rock avalanche begins its journey as ___.
bedrock
how do you identify which method of preventing mass wasting to use?
1.
2.
3.
- type of soil
- slope
- cost
types of mass wasting prevention
1.
2.
3.
4.
5.
6.
7.
8.
- unloading
- benching
- installing proper drainage
- grouted riprap
- gabion walls
- soil nailing
- rockfall barriers and shelters
- vetiver grass
___ is the procedure of reducing the driving forces within a slide mass.
unloading
how is unloading undertaken?
unloading is done by removing materials from the upper part of the slope, removing loose and unstable materials, flattening of the slope, or benching.
___ is the transformation of one steep slope into a series of lower slopes with horizontal surfaces in between.
It’s much like creating terraces.
benching
___ minimizes the effects of water saturation and pore pressure on a slope by draining the soil through installed pipes containing perforated holes.
installing proper drainage
in cases of nearly vertical slopes, slope faces are protected using ___.
grouted riprap
what are gabion walls? what do they do? also, why is a geosynthetic filter cloth placed behind the wall during construction?
gabion walls use strong cobble-sized rocks placed within wire mesh baskets stacked and tied together in an interlocking pattern.
due to it being just a cage of cobblestones, water can seep through, and thus a geosynthetic filter cloth is placed behind the wall so as to prevent water from seeping through.
___ is a method of reinforcement that utilizes passive inclusions mobilized when movement occurs.
explain what this method is used for.
soil nailing; it is usually done to retain excavations and stabilize slopes by creating an onsite reinforced soil mass
___ have been used to protect major roads and pipelines in mountainous areas. they are designed to withstand the impact, as well as stop or divert falling bounders.
rockfall barriers and shelters
___ is often used for steep slope stabilization and rehabilitation of degraded and disturbed lands.
vetiver grass
