almost always occcur with litttle or no w
warning and
d are over in a very s hort time, leaving
behind a legacy off death and destruction
d
(@ Table 11-1).
1
Mass wasting (also called mass movement) is defined as the downslope movement of material
under the direct influence of gravity. Most types of mass wasting are aided by weathering
and usually involve surficial material. The material moves at rates ranging from almost
imperceptible, as in the case of creep, to extremely fast as in a rock fall or slide. While water
can play an important role, the relentless pull of gravity is the major force behind mass
wasting.
Mass wasting is an important geologic process that can occur at any time and almost any
place. Though most people associate mass wasting with steep and unstable slopes, it can also
occur on near-level land, given the right geologic conditions. Furthermore, while the rapid
types of mass wasting, such as avalanches and mudflows, typically get the most publicity, the
slow, imperceptible types, such as creep, usually do the greatest amount of property damage.
_ FACTORS INFLUENCING Mass WASTING
When the gravitational force acting on a slope exceeds its resisting force, slope failure (mass
wasting) occurs. The resisting forces helping to maintain slope stability include the slope
material's strength and cohesion, the amount of internal friction between grains, and any
external support of the slope (~ Figure 11-2). These factors collectively define a slope's shear
strength.
Opposing a slope's shear strength is the force of gravity.
Gravity operates vertically but has a component acting parallel to the slope, thereby causing
instability (Figure 11-2). The greater a slope's angle, the greater the component of force
acting parallel to the slope, and the greater the chance for mass wasting. The steepest angle
that a slope can maintain without collapsing is its angle of repose. At this angle, the shear
strength of the slope's material exactly counterbalances the force of gravity. For
unconsolidated material, the angle of repose normally ranges from 25° to 40°. Slopes steeper
than 40° usually consist of unweathered rock.
All slopes are in a state of dynamic equilibrium, which means that they are constantly
adjusting to new conditions. While we tend to view mass wasting as a disruptive and
usually destructive event, it is one of the ways that a slope adjusts to new conditions.
Whenever a building or road is constructed on a hillside, the equilibrium of that slope is
affected. The slope must then adjust, perhaps by mass wasting, to this new set of
conditions.
Many factors can cause mass wasting: a change in slope gradient, weakening of material by
weathering, increased water content, changes in the vegetation cover, and overloading.
Although most of these are interrelated, we will examine them separately for ease of
discussion, but will also show how they individually and collectively affect a slope's
equilibrium.
Slope Gradient
Slope ggradient is probably
p
thee major cauuse of mass wasting. Generally
G
sppeaking, thee steeper
the sloppe, the lesss stable it is. Thereforre, steep slo
opes are mo
ore likely tto experience mass
wastingg than gentle ones.
mber of proocesses can oversteepeen a slope. One
O of the most comm
mon is undeercutting
A num
by streaam or wavee action (~ Figure
F
11-33). This rem
moves the slo
ope's base, increases th
he slope
angle, aand therebyy increases the
t gravitatiional force acting paraallel to the sslope. Wavee action,
especiaally during storms,
s
often
n results in mass moveements along the shoress of oceans or large
lakes.
Excavvations for road cuts and hillsidde building sites are another
a
maj
ajor cause of
o slope
failure (~ Figure 11-4).
1
Grading the sloppe too steep
ply, or cuttiing into its side, increases the
stress inn the rock or
o soil until it is no lonnger strong enough
e
to remain at the
he steeper an
ngle and
mass m
movement ennsues. Such
h action is aanalogous to
t undercuttting by streeams or waaves and
has' thee same resuult, thus exp
plaining whhy so many mountain roads
r
are pllagued by frequent
f
mass m
movements.
Weathering and Climate
Mass wasting is more likely to occur in loose or poorly consolidated slope material than in
bedrock. As soon as solid rock is exposed at the Earth's surface, weathering begins to
disintegrate and decompose it, reducing its shear strength and increasing its susceptibility to
mass wasting. The deeper the weathering zone extends, the greater the likelihood of some
type of mass movement.
Recall from Chapter 6 that some rocks are more susceptible to weathering than others and
that climate plays an important role in the rate and type of weathering. In the tropics, where
temperatures are high and considerable rainfall occurs, the effects of weathering extend to
depths of several tens of meters, and mass movements most commonly occur in the deep
weathering zone. In arid and semiarid regions, the weathering zone is usually considerably
shallower. Nevertheless, intense, localized cloudbursts can drop large quantities of water on
an area in a short time. With little vegetation to absorb this water, runoff is rapid and
frequently results in mudflows.
Water Content
The amount of water in rock or soil influences slope stability. Large quantities of water from
melting snow or heavy storms greatly increase the likelihood of slope failure. The additional
weight that water adds to a slope can be enough to cause mass movement. Furthermore, water
percolating through a slope's material helps to decrease friction between grains, contributing
to a loss of cohesion. For example, slopes composed of dry clay are usually quite stable, but
when wetted, they quickly lose cohesiveness and internal friction and become an unstable
slurry. This occurs because clay, which can hold large quantities of water, consists of platy
particles that easily slide over each other when wet. For this reason, clay beds are frequently
the slippery layer along which overlying rock units slide downslope (see Perspective 11-1).
Vegetation
Vegetation affects slope stability in several ways. By absorbing the water from a rainstorm,
vegetation decreases water saturation of a slope's material, and the resultant loss of shear
strength frequently leads to mass wasting. On the other hand, the vegetation's root system
helps to stabilize a slope by binding soil particles together and holding the soil to bedrock.
The removal of vegetation by either natural or human activity is a major cause of many
mass movements. Summer brush and forest fires in southern California frequently leave the
hillsides bare of vegetation. Fall rainstorms saturate the ground causing mudslides that do
tremendous damage and cost millions of dollars to clean up (~ Figure 11-5).
Overloading
Overloading is almost always the result of human activity and typically results from
dumping, filling, or piling up of material. Under natural conditions, a material's load is
carried by its grain-to-grain contacts, with the friction between the grains maintaining a slope.
The additional weight created by overloading increases the water pressure within the
material, which in turn decreases its shear strength, thereby weakening the slope material. If
enough material is added, the slope will eventually fail, sometimes with tragic consequences.
Geolog
gy and Slop
pe Stabilitty
The relaationship beetween topo
ography andd the geology of an area is imporrtant in deteermining
slope sttability. If the
t rocks underlying
u
a slope dip in the same direction as the slop
pe, mass
wastingg is more likkely to occu
ur than if thee rocks are horizontal or
o dip in thee opposite direction
d
(~ Figuure 11-6). When
W
the roccks dip in thhe same direection as thee slope, watter
major criteriaa (Table 11--2): (1) ratee of movement (rapid or
o slow); (2)) type of mo
ovement
three m
(primarrily falling, sliding, orr flowing); and (3) ty
ype of matterial involvved (rock, soil, or
debris)..
Rapidd mass movvements inv
volve a visibble movement of mateerial. Such m
movements usually
occur qquite suddenly, and the materiial moves very quickly downsslope. Rapiid mass
movem
ments are pootentially dangerous
d
aand frequen
ntly result in loss off life and property
p
damagee. Most rapiid mass mo
ovements occcur on relaatively steep
p slopes andd can involv
ve rock,
soil. or debris.
Slow mass moveements advaance at an im
mperceptible rate and are usuallyy only detectable by
the effeects of theirr movemen
nt such as tiilted trees and
a power poles or crracked foun
ndations.
Althouggh rapid maass movemeents are moore dramatic, slow mass movemeents are resp
ponsible
for the ddownslope transport off a much greeater volum
me of weatheered materiaal.
Falls
Rockfalls are a com
mmon typee of extremeely rapid maass movement in whicch rocks of any size
fall throough the airr (~ Figure 11-7). Rockkfalls occur along steep
p canyons, ccliffs, and road cuts
and buiild up accum
mulations of
o loose roc ks and rock
k fragmentss at their baase called ta
alus (see
Figure 6-5).
Rockffalls result from failu
ure along jjoints or bedding
b
plaanes in thee bedrock and are
commoonly triggereed by naturaal or humann undercuttiing of slopes or by eartthquakes. Rockfalls
R
range inn size from small rockss falling froom a cliff to
o massive faalls involvinng millions of cubic
meters oof debris thhat destroy buildings,
b
buury towns, and block highways.
h
Rockffalls are a particularly
p
common hhazard in mountainous
m
areas wherre roads haave been
built byy blasting and
a grading
g through ssteep hillsides of bedrock (~ Figgure 11-8).. Slopes
particullarly prone to rockfallss are sometiimes covereed with wirre mesh in aan effort to prevent
dislodged rocks froom falling to
t the road below. Ano
other tactic is to put upp wire mesh
h fences
along thhe base of thhe slope to catch or sloow down bo
ouncing or rolling rockss.
Slides
A slidee involves movement
m
of
o material along one or more su
urfaces of ffailure. The type of
materiaal may be soil,
s
rock, or a combiination of the two, an
nd it may bbreak apartt during
movem
ment or remaain intact. A slide's ratee of movem
ment can varry from extrremely slow
w to very
rapid (T
Table 11-2)..
Two types of sllides are geenerally reccognized: (1) slumps or rotationaal slides. in
n which
movem
ment occurs along a currved surfacee; and (2) rock
r
or blocck glides, w
which move along a
more-orr-less planaar surface.
A slu
ump involvees the down
nward moveement of material
m
alon
ng a curvedd surface off rupture
and is ccharacterizeed by the backward
b
rootation of th
he slump block
b
(~ Figgure 11-9). Slumps
occur m
most commonly in uncconsolidatedd or weakly
y consolidatted materiaal and rangee in size
from sm
mall individdual sets, su
uch as occuur along streeam banks, to massivee, multiple sets
s that
affect laarge areas and
a cause co
onsiderable damage.
Slumpps can be caused
c
by a variety off factors, bu
ut the most common iss erosion allong the
base off a slope, whhich removees support ffor the overrlying materrial. This loocal steepening may
be caussed naturallyy by stream
m erosion al ong its banks (Figure 11-9) or byy wave actio
on at the
base off a coastal cliff:
c
Slope over steepeening can also be caused by humaan activity, such as
the connstruction of
o highwayss and housinng developments. Slum
mps are par
articularly prevalent
p
along hhighway cuuts where they are ggenerally th
he most frequent typpe of slopee failure
observeed.
While m
many slumpps are merelly a nuisancce, large-scale slumps involving ppopulated arreas and
highwayys can causse extensivee damage. S
Such is the case in coastal southerrn Californiia where
slumpinng and slidinng have beeen a constannt problem. Many areaas along the coast are underlain
u
by poorrly to weakkly consolid
dated silts, sands, grav
vels, and cllay layers, some of wh
hich are
weatherred ash fallls. In additiion, southerrn Californ
nia is tecton
nically activve so that many
m
of
these ddeposits aree cut by faaults and jooint', which
h allow thee infrequennt rainsto percolate
p
downward rapidly,, wetting an
nd lubricatinng the clay layers.
l
Southhern Califorrnia lies in a semiaridd climate an
nd is dry most
m of the yyear. When
n it does
rain, typpically betw
ween Novem
mber and M
March, largee amounts of
o rain can ffall in a sho
ort time.
Thus, the ground quickly becomes saturated, leading to landslides along steep canyon walls
as well as along coastal cliffs (~ Figure 11-10). Most of the slope failures along the southern
California coast are the result of slumping.
A rock or block glide occurs when rocks move downslope along a more-or-less planar
surface. Most rock glides occur because the local slopes and rock layers dip in the same
direction (~ Figure 11-11), although they can also occur along fractures parallel to a slope.
Rock glides are common occurrences along the southern California coast. At Point Fermin,
seaward-dipping rocks with interbedded slippery clay layers are undercut by waves causing
numerous glides (~ Figure 11-12).
Flows
Mass movements in which material flows as a viscous fluid or displays plastic movement are
termed .flows. Their rate of movement ranges from extremely slow to extremely rapid (Table
11-2). In many cases, mass movements begin as falls, slumps, or slides and change into flows
further downslope.
Of the major mass movement types, mudflows are the most fluid and move most rapidly (at
speeds up to 80 km per hour). They consist of at least 50% silt- and clay-sized material
combined with a significant amount of water (up to 30%). Mudflows are common in arid and
semiarid environments where they are triggered by heavy rainstorms that quickly saturate the
regolith, turning it into a raging flow of mud that engulfs everything in its path. Mudflows
can also occur in mountain regions (~ Figure 11-13) and in areas covered by volcanic ash
where they can be particularly destructive (see Chapter 5). Because mudflows are so fluid,
they generally follow preexisting channels until the slope decreases or the channel widens, at
which point they fan out.
Debris flows are composed of larger-sized particles than those in mudflows and do not
contain as much water, consequently, they are usually more viscous than mudflows, typically
do not move as rapidly, and rarely are confined to preexisting channels. Debris flows can be
just as damaging, though, because they can transport large objects.
Earthflows move more slowly than either mudflows or debris flows. An earthflow slumps
from the upper part of a hillside, leaving a scarp, and flows slowly downslope as a thick,
viscous, tongue-shaped mass of wet regolith (~ Figure 11-14). Like mudflows and debris
flows, earthflows can be of any size and are frequently destructive. They occur most
commonly in humid climates on grassy soil covered slopes following heavy rains.
Some clay spontaneously liquefies and flow like water when they are disturbed. Such
quick clays have caused serious damage and loss of lives in Sweden, Norway, eastern
Canada, and Alaska (Table 11-1). Quick clays are composed of silt and clay particles made
by the grinding action of glaciers. Geologists think these fine sediments were originally
deposited in a marine environment where their pore space was filled with salt water. The ions
in the salt water helped establish strong bonds between the clay particles, thus stabilizing and
strengthening the clay. When the clays were subsequently uplifted above sea level, the salt
water was flushed out by fresh groundwater, reducing the effectiveness of the ionic bonds
between the clay particles and thereby reducing the overall strength and cohesiveness of the
clay. Consequently, when the clay is disturbed by a sudden shock or shaking, it essentially
turns to a liquid and flows.
An example of the damage that can be done by quick clays occurred in the Turnagain
Heights area of Anchorage, Alaska, in 1964 (~ Figure 11-15). Underlying most of the
Anchorage area is the Bootlegger Cove Clay, a massive clay unit of poor permeability.
Because the Bootlegger Cove Clay forms a barrier preventing groundwater from flowing
through the adjacent glacial deposits to the sea, considerable hydraulic pressure builds up
behind the clay. Some of this water has flushed out the salt water in the clay and also has
Saturated the lenses of sand and silt associated with the clay beds. When the 8.S-magnitude
Good Friday earthquake struck on March 27, 1964, the shaking turned parts of the
Bootlegger Cove Clay into a quick clay and precipitated a series of massive slides in the
coastal bluffs that destroyed most of the homes in the Turnagain Heights subdivision (Figure
11-15b).
Solifluction is the slow downslope movement of water saturated surface sediment.
Solifluction can occur in any climate where the ground becomes saturated with water, but is
most common in areas of permafrost. Permafrost is ground that remains permanently frozen. It
covers nearly 20 % of the world's land surface (~ Figure 11-16a). During the warmer season
when the upper portion of the permafrost thaws, water and surface sediment form a soggy
mass that flows by solifluction and produces a characteristic lobate topography (Figure ll16b).
Construction of the Alaska pipeline from the oil fields in Prudhoe Bay to the ice-free port
of Valdez raised numerous concerns over the effect it might have on the permafrost and the
potential for solifluction. Some thought that oil flowing through the pipeline would be warm
enough to melt the permafrost, causing the pipeline to sink further into the ground and
possibly rupture. After numerous studies were conducted, scientists concluded that the
pipeline, completed in 1977, could safely be buried for more than half of its 1,280 km length;
where melting of the permafrost might cause structural problems to the pipe, it was insulated
and installed above ground.
Creep is the slowest type of flow. It is also the most widespread and significant mass
wasting process in terms of the total amount of material moved downslope and the monetary
damage caused "annually. Creep involves extremely slow downhill movement of sailor rock.
Although it can occur anywhere and in any climate, it is most effective and significant as a
geologic agent in humid regions. In fact, it is the most common form of mass wasting in the
southeastern United States and the southern Appalachian Mountains.
Because the rate of movement is essentially imperceptible, we are frequently unaware of
creep's existence until we notice its effects: tilted trees and power poles, broken streets and
sidewalks, cracked retaining walls or foundations (~ Figure 11-17). Creep usually involves
the whole hillside and probably occurs, to some extent, on any weathered or soil-covered,
sloping surface.
Not only is creep difficult to recognize, it is difficult to control. Although engineers can
sometimes slow or stabilize creep, many times the only course of action is to simply avoid
the area if at all possible or, if the zone of creep is relatively thin, design structures that can
be anchored into the solid bedrock.
Complex movements
Recall that many mass movements are combinations of different movement types: When one
type is dominant, the movement call is classified as one of the movements described thus far,
if several types are more or less equally involved, it is called a complex movement,
The most common type of complex movement is the slide-flow in which there is sliding at
the head and then some type of flowage farther along its course. Most slide flow landslides
involve well-defined slumping at the head, followed by a debris flow or earthflow. Any
combination of different mass movement types can, however, be classified as a complex
movement.
A debris avalanche is a complex movement that often occurs in very steep mountain
ranges. Debris avalanches typically start out as rockfalls when large quantities of rock, ice,
and snow are dislodged from a mountainside, frequently as a result of an earthquake. The
material then slides or flows down the mountainside, picking up additional surface material
and increasing in speed. The 1970 Peru earthquake set in motion the debris avalanche that
destroyed the town of Yungay (see the Prologue).
RECO
OGNIZING
MOVE
EMENTS
S
AND
D
MINIM
MIZING
THE
EFFECTS
E
S
OF
MASS
The moost importannt factor in eliminating
e
or minimizzing the dam
maging effeccts of mass wasting
is a thoorough geoloogic investiigation of thhe region in
n question. In this wayy, former landslides
and areeas suscepttible to maass movem
ments can be
b identifieed and perrhaps avoid
ded. By
assessinng the risks of possiblee mass wastting before construction
n begins, stteps can be taken to
eliminaate or minim
mize the effeects of suchh events.
Identiifying areaas with a high
h
potenttial for slo
ope failure is importaant in any hazard
assessm
ment study; these stud
dies includee identifyin
ng former landslides aas well as sites of
potentiaal mass moovement. Sccarps, openn fissures, displaced
d
orr tilted objeects, a hum
mmocky
surface, and suddeen changes in vegetatioon are som
me of the feaatures indiccating formeer landslides oor an area susceptible to slope failure. The
T effects of weatherring, erosio
on, and
vegetation may, hoowever, obsscure the evvidence for previous
p
maass wasting..
Soil aand bedrockk samples are
a also studdied, both in
n the field and
a laboratoory, to asseess such
characteristics as compositio
on, susceptiibility to weathering,
w
cohesiveneess, and ab
bility to
transmiit fluids. Thhese studiess help geoloogists and engineers predict
p
sloppe stability under
u
a
variety of conditions.
y cannot be
b preventeed, geologists and
Althoough most large masss movemennts usually
engineeers can empploy various methods tto minimize the dangeer and dama
mage resultin
ng from
them. Because water plays such an important role in many landslides, one of the most
effective and inexpensive ways to reduce the potential for slope failure and to increase
existing slope stability is through surface and subsurface drainage of a hillside. Drainage
serves two purposes. It reduces the weight of the material likely to slide and increases the
shear strength of the slope material by lowering pore pressure.
Surface waters can be drained and diverted by ditches, gutters, or culverts designed to
direct water away from slopes. Drainpipes perforated along one surface and driven into a
hillside can help remove subsurface water (~ Figure 11-18). Finally, planting vegetation on
hillsides helps stabilize slope, by holding the soil together and reducing the amount of water
in the soil.
Another way to help stabilize a hillside is to reduce its slope. Recall that overloading or
over steepening by grading arc common causes of slope failure. By reducing the gradient of.
hillside, the potential for slope failure is decreased. Two methods are commonly employed to
reduce a slope's gradient. In the cut-end-fill! method, material is removed from the upper part of
the slope and used as fill at the base, thus providing a flat surface for construction and
reducing the slope (~ Figure 11-1'1a). The second method, which is called benching, involves
cutting a series of benches or steps into a hillside (Figure 11-19b). This process reduces the
overall average slope, and the benches serve as collecting sites for small landslides or
rockfalls that might occur. Benching is most commonly used on steep hillsides in conjunction
with a system of surface drains to divert runoff:
In some situations, retaining walls can be constructed to provide support for the base of the
slope (~ Figure 11-20). These are usually anchored well into bedrock, backfilled with crushed
rock, and provided with drain holes to prevent the buildup of water pressure in the hillside.
Answers
Addiitional Rea
adings