Slope Stability, Triggering Events, Mass Wasting Hazards
The main force responsible for mass wasting is gravity. Gravity is the
force that acts everywhere on the Earth's surface, pulling everything in a
direction toward the center of the Earth. On a flat surface the force of
gravity acts downward. So long as the material remains on the flat surface
it will not move under the force of gravity.
On a slope, the force of gravity can be resolved into two components: a
component acting perpendicular to the slope and a component acting
tangential to the slope.
The perpendicular component of gravity,
, helps to hold the object in place on the slope. The tangential
component of gravity, gt, causes a shear stress parallel to the slope
that pulls the object in the down-slope direction.
On a steeper slope, the shear stress or tangential component of
gravity, gt, increases, and the perpendicular component of gravity,
The forces resisting movement down the slope are grouped under the
term shear strength which includes frictional resistance and
cohesion among the particles that make up the object.
When the sheer stress becomes greater than the combination of forces
holding the object on the slope, the object will move down-slope.
Thus, down-slope movement is favored by steeper slope angles which
increase the shear stress, and anything that reduces the shear
strength, such as lowering the cohesion among the particles or
lowering the frictional resistance. This is often expressed as
the safety factor, Fs, the ratio of shear strength to shear
Fs = Shear Strength/Shear Stress
If the safety factor becomes less than 1.0, slope failure is expected.
The Role of Water
Although water is not always directly involved as the transporting medium
in mass-wasting processes, it does play an important role. Think about
building a sand castle on the beach. If the sand is totally dry, it is
impossible to build a pile of sand with a steep face like a castle wall.
If the sand is somewhat wet, however, one can build a vertical wall. If
the sand is too wet, then it flows like a fluid and cannot remain in
position as a wall.
Dry unconsolidated grains will form a pile with a slope angle determined
by the angle of repose. The angle of repose is the steepest angle at which
a pile of unconsolidated grains remains stable, and is controlled by the
frictional contact between the grains. In general, for dry materials the
angle of repose increases with increasing grain size, but usually lies
between about 30 and 37 degrees.
Slightly wet unconsolidated materials exhibit a very high angle of repose
because surface tension between the water and the solid grains tends to
hold the grains in place.
When the material becomes saturated with water, the angle of repose is
reduced to very small values and the material tends to flow like a fluid.
This is because the water gets between the grains and eliminates grain to
grain frictional contact.
Another aspect of water that affects slope stability is fluid
pressure. In some cases fluid pressure can build in such a way that
water can support the weight of the overlying rock mass. When this
occurs, friction is reduced, and thus the shear strength holding the
material on the slope is also reduced, resulting in slope failure.
Troublesome Earth Materials
- As we have already discussed, liquefaction occurs when loose
sediment becomes oversaturated with water and individual grains loose
grain to grain contact with one another as water gets between them.
This can occur as a result of ground shaking, as we discussed during our
exploration of earthquakes, or can occur as water is added as a result of
heavy rainfall or melting of ice or snow. It can also occur
gradually by slow infiltration of water into loose sediments and soils.
The amount of water necessary to transform the sediment or soil from a
solid mass into a liquid mass varies with the type of material. Clay
bearing sediments in general require more water because water is first
absorbed onto the clay minerals, making them even more solid-like,
then further water is needed to lift the individual grains away from each
Expansive and Hydrocompacting Soils
- These are soils that contain a high proportion of a type of clay
mineral called smectites or montmorillinites. Such clay minerals expand
when they become wet as water enters the crystal structure and increases
the volume of the mineral. When such clays dry out, the loss of
water causes the volume to decrease and the clays to shrink or compact
(This process is referred to as hydrocompaction).
Another material that shows similar swelling and compaction as a result of
addition or removal of water is peat. Peat is organic-rich material
accumulated in the bottoms of swamps as decaying vegetable matter.
- In some soils the clay minerals are arranged in random fashion,
with much pore space between the individual grains. This is often
referred to as a "house of cards" structure. Often the grains are
held in this position by salts precipitated in the pore space that "glue"
the particles together. As water infiltrates into the pore spaces, it can
both be absorbed onto the clay minerals, as discussed above, and can
dissolve away the salts holding the "house of cards" together. Compaction
of the soil or shaking of the soil can thus cause a rapid change in the
structure of the material. The clay minerals will then line up with
one another and the open space will be reduced. But this may cause a
loss in shear strength of the soil and result in slippage down slope or
liquefaction. This is referred to as remolding. Clays that
are subject to remolding are called quick clays.
Some clays, called thixotropic clays, when left undisturbed can
strengthen, but when disturbed they loose their shear strength.
A mass-wasting event can occur any time a slope becomes unstable.
Sometimes, as in the case of creep or solifluction, the slope is unstable
all of the time and the process is continuous. But other times, triggering
events can occur that cause a sudden instability to occur.
- A sudden shock, such as an earthquake may trigger a slope
instability. Minor shocks like heavy trucks rambling down the road, trees
blowing in the wind, or human made explosions can also trigger
Turnagain Heights Alaska, 1964
During the Good Friday earthquake on March 27, 1964, a suburb of
Anchorage, Alaska, known as Turnagain Heights broke into a series of slump
blocks that slid toward the ocean. This area was built on sands and
gravels overlying a marine clay.
The upper clay layers were relatively stiff, but the lower layers
consisted of a sensitive clay, as discussed above. The slide moved
about 610 m toward the ocean, breaking up into a series of blocks. It
began at the sea cliffs on the ocean after about 1.5 minutes of shaking
caused by the earthquake, when the lower clay layer became
liquefied. As the slide moved into the ocean, clays were extruded
from the toe of the slide. The blocks rotating near the front of the
slide, eventually sealed off the sensitive clay layer preventing
further extrusion. This led to pull-apart basins being formed near
the rear of the slide and the oozing upward of the sensitive clays into
the space created by the extension.
75 homes on the top of the slide were destroyed by the movement of the
mass of material toward the ocean.
Nevados de Huascarán, Peru, 1962 and 1970.
Nevados de Huascarán is a high peak in the Peruvian Andes Mountains.
The peak consists of granite with nearly vertical joints (fractures)
covered by glacial ice. On January 10, 1962 a huge slab of rock and
glacial ice suddenly fell, with no apparent triggering mechanism.
This initiated a debris flow that moved rapidly into the valley below and
killed 4,000 people in the town of Ranrahirca, but stopped when it reached
the hill called Cerro de Aira, and did not reach the larger population
center of Yungay.
On May 31, 1970 a magnitude 7.7 earthquake occurred on the
subduction zone 135 km away from the Nevados de Huascarán.
Shaking in the area lasted for 45 seconds, and during this shaking another
large block of the Nevados de Huascarán between 5,500 and 6,400 meters
elevation fell from the peak.
This time it became a debris avalanche sliding across the snow covered
glacier and moving down slope at velocities up to 335 km/hr. The
avalanche then hit a small hill composed of glacially deposited sediment
and was launched into the air as an airborne debris avalanche. From
this airborne debris, blocks the size of large houses fell on real houses
for another 4 km. The mass then recombined in the vicinity of Cerro
de Aira and continued flowing as a debris flow, burying the town of Yungay
and its 18,000 residents.
The debris flow reached the valley of the Rio Santa and climbed up the
valley walls killing another 600 people on the opposite side of the
river. Since then, the valley has been repopulated, and currently
large cracks are seen on the remains of the glacier that still covers the
upper slopes of Nevados de Huascarán.
- Modification of a slope either by humans or by natural causes can
result in changing the slope angle so that it is no longer at the angle of
repose. A mass-wasting event can then restore the slope to its angle of
Undercutting - streams eroding their banks or surf action along a coast
can undercut a slope making it unstable.
Example: Elm Switzerland, 1881
In 1870s there was a large demand for slate to make blackboards throughout
Europe. To meet this demand, miners near Elm, Switzerland began
digging a slate quarry at the base of a steep cliff. Slate is a
metamorphic rock with an excellent planar foliation that breaks smoothly
along the foliation planes. By 1876 a "v" shaped fissure
formed above the cliff, about 360 meters above the quarry. By
September 1881, the quarry had been excavated to where it was 180 m long
and 60 m into the hill below the cliff, and the "v" shaped fissure had
opened to 30 m wide. Falling rocks were frequent in the quarry and
their were almost continuous loud noises heard coming from the overhang
above the quarry. Realizing that the slope had become unstable, the
miners stopped working, thinking that the rock mass above the quarry would
probably fall down.
On September 11, 1881 the 10 million m3 mass of rock above the quarry
suddenly fell. But, it did not stop when it hit the quarry
floor. Instead, it broke into pieces and rebounded into the
air. Residents in Untertal, on the opposite side of the valley from
the slide, saw the mass of rebounded rock coming at the them and ran
uphill. But the mass of rock continued up the walls of the valley and
buried them. The avalanche then turned and ran an additional 2,230 m as a
dry avalanche traveling at 180 km/hr burying the village of Elm. The
avalanche killed 115 people.
Changes in Hydrologic Characteristics
- heavy rains can saturate regolith reducing grain to grain contact
and reducing the angle of repose, thus triggering a mass-wasting event.
Heavy rains can also saturate rock and increase its weight. Changes
in the groundwater system can increase or decrease fluid pressure in rock
and also trigger mass-wasting events.
Example: Vaiont Reservoir, Italy, 1963
In 1960 a dam was built across the Vaiont Valley in northeastern Italy
near the border with Austria and Slovenia. The valley runs along the
bottom of a geologic structure called a syncline, wherein rocks have been
folded downward and dip into the valley from both sides (see cross section
The rocks are mostly limestones, but some are intricately interbedded with
sands and clays. These sand and clay layers form bedding planes that
parallel the syncline structure, dipping steeply into the valley from both
Fracture systems in the rocks run parallel to the bedding planes and
perpendicular to bedding planes. The latter fractures had formed as
a result of glacial erosion which had relieved pressure on the rocks that
had formed deeper in the Earth.
Some of the limestone units have caverns that have been dissolved in the
rock due to chemical weathering by groundwater. Furthermore, the dam
site was built near an old fault system.
During August and September, 1963, heavy rains drenched the area adding
weight to the rocks above the dam. On October 9, 1963 at 10:41 P.M. the
south wall of the valley failed and slid into the reservoir behind the
dam. The slide mass was 1.8 km long and 1.6 km wide with a volume of
240 million m3. As the slide moved into the reservoir it displaced
the water, forcing it 240 meters above the dam and into the village of
Casso on the northern side of the valley.
Subsequent waves swept up to 100 meters above the dam. Although the dam
did not fail, the water rushing over the dam swept into the villages of
Longorone and T. Vaiont, killing 2,000 people.
Waves also swept up the reservoir where they first bounced off the
northern shore, then back toward the Pineda Peninsula, and then back up
the valley slamming into San Martino and killing another 1000 people.
The debris slide had moved along the clay layers that parallel the bedding
planes in the northern wall of the valley. A combination of factors
was responsible for the slide. First filling of the reservoir had
increased fluid pressure in the pore spaces and fractures of the
rock. Second, the heavy rains had also increased fluid pressure and
also increased the weight of the rock above the slide surface. After
the slide event, parts of the reservoir were filled up to 250 m above the
former water level, and even though the dam did not fail, it became
totally useless. This event is often referred to as the world's
worst dam disaster.
Example: Portuguese Bend, California, 1956
Portuguese Bend lies on the Palos Verdes Peninsula just to the south of
Los Angeles, California, but still within Los Angeles County.
In this area the rocks have been folded into a synclinal structure with
rock layers dipping gently toward the Pacific Ocean.
Rocks near the surface consist of volcanic ash that has been altered by
chemical weathering to an expanding type clay called bentonite. Below
these altered ash layers are shales that are interbedded with other thin
volcanic ash layers that have been similarly altered to bentonite
clay. The area had the appearance of an earth flow, with a very
hummocky topography with many enclosed basins filled with lakes.
Prior to the 1950s the area had been used for farming. In the
1950s demand for ocean views led to the development of the area as an
upscale suburb. But, no sewer system was available, so wastes were
put into the ground via septic tanks. In 1956 the area began moving down
slope toward the ocean. Rates of movement were fastest several months
after the end of the winter rainy season and slowest during the summer dry
season. In the next three years the earthflow moved as much as 20
meters, but in the processes the expensive homes built on the flow became
uninhabitable. Movement was caused by a combination of wave erosion
along the coast removing some the mass resisting flow, added water due to
the disposal of wastes, watering of lawns, and rainfall causing the
bentonite clays to expand and weaken, and by the added weight of
development on top of the flow. Property owners looked desperately
for someone to sue, and eventually won a suit against the county of Los
Angeles who had added fill dirt to build a road into the development (note
that since the property owners could not sue themselves, nor could they
sue the clay layers responsible for the movement they found the only
agency with deep pockets that was available).
- produce shocks like explosions and earthquakes. They can also cause snow
to melt or empty crater lakes, rapidly releasing large amounts of water
that can be mixed with regolith to reduce grain to grain contact and
result in debris flows, mudflows, and landslides.
Examples - We have previously discussed the mudflows and debris avalanche
produced by the 1980 eruption of Mount St. Helens, and the devastating
mudflows that killed 23,000 people in Armero that resulted from an
eruption of Nevado del Ruiz volcano in Columbia.
Assessing and Mitigating Mass-Wasting Hazards
As we have seen mass-wasting events can be extremely hazardous and result
in extensive loss of life and property. But, in most cases, areas
that are prone to such hazards can be recognized with some geologic
knowledge, slopes can be stabilized or avoided, and warning systems can be
put in place that can minimize such hazards.
Prediction and Hazard Assessment
If we look at the case histories of mass-wasting disasters discussed
above, in all cases looking at the event in hindsight shows us that
conditions were present that should have told us that a hazardous
condition existed prior to the event.
Exploration could have revealed the sensitive clays beneath Turnagain
Heights, located in known earthquake prone area.
The area beneath the slopes of Nevados de Huascarán was littered with
debris from prior landslide events, and even thought the first event in
1962 was not caused by an earthquake, it should have been known that the
area was susceptible to such a hazard. The 1962 event should have
provided fair warning to inhabitants of the area and the death and
destruction caused by the 1970 event should have been avoided.
Miners in Elm, Switzerland, certainly realized that undercutting of the
mountain could cause the mountain to fail, but did not consider the more
widespread effect of the avalanche.
In the Portuguese Bend area, planners should have realized that the slope
was an earthflow, fine for farming, but not a very desirable place to
construct houses of any sort.
In both of the volcanic mudflow cases, the hazards were known before the
event. In the Mount St. Helens case, hazards assessments were
available and plans were in effect to minimize further damage once the
event occurred. In the case of Armero, warnings were given, but
ignored. The town was built on mudflow deposits from prior mudflow
Because there is usually evidence in the form of distinctive deposits and
geologic structures left by recent mass wasting events, it is possible, if
resources are available, to construct maps of all areas prone to possible
mass-wasting hazards (see the example in figure II.1, page 63 of your
text). Planners can use such hazards maps to make decisions about
land use policies in such areas or, as will be discussed below, steps can
be taken to stabilize slopes to attempt to prevent a disaster.
Short-term prediction of mass-wasting events is somewhat more
problematical. For earthquake triggered events, the same problems
that are inherent in earthquake prediction are present.
Slope destabilization and undercutting triggered events require the
constant attention of those undertaking or observing the slopes, many of
whom are not educated in the problems inherent in such processes.
Mass-wasting hazards from volcanic eruptions can be predicted with the
same degree of certainty that volcanic eruptions can be predicted, but
again, the threat has to be realized and warnings need to be heeded.
Hydrologic conditions such as heavy precipitation can be forecast with
some certainty, and warnings can be issued to areas that might be
susceptible to mass-wasting processes caused by such conditions.
Still, it is difficult of know exactly which hill slope of the millions
that exist will be vulnerable to an event triggered by heavy rainfall.
Prevention and Mitigation
All slopes are susceptible to mass-wasting hazards if a triggering event
occurs. Thus, all slopes should be assessed for potential
mass-wasting hazards. Mass-wasting events can sometimes be avoided by
employing engineering techniques to make the slope more stable.
Among them are:
Steep slopes can be covered or sprayed with concrete to prevent rock
Retaining walls could be built to stabilize a slope
Drainage pipes could be inserted into the slope to more easily allow water
to get out and avoid increases in fluid pressure, the possibility of
liquefaction, or increased weight due to the addition of water
Oversteepened slopes could be graded to reduce the slope to the natural
angle of repose
In mountain valleys subject to mudflows, plans could be made to rapidly
lower levels of water in human-made reservoirs to catch and trap the
Some slopes, however, cannot be stabilized. In these cases, humans
should avoid these areas or use them for purposes that will not increase
susceptibility of lives or property to mass-wasting hazards