ENV 101 Ch07 lecture ppt_a

7-1
Environmental
Geology
James Reichard
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

7-2
Chapter 7
Mass Wasting and Related Hazards
© AP Photo/Nick Ut

7-3
Mass Wasting
Okanagan Valley, British Columbia, Canada
British Columbia Ministry of Forests and Range/Kevin Turner.
Jump to long description

7-4
Slope Stability & Triggering
Mechanisms
• Nature of material
• Over steepened slopes
• Water content
• Climate and vegetation
• Earthquakes and volcanic
activity
USGS

7-5
Gravity

7-6
Slope Destabilization (1)
(top): Courtesy of Myron McKee/River of Life Farm; (bottom): Reproduced with the permission of Natural Resources Canada 2012 (Photo 2002-585 by Rejean Couture.)

7-7
Slope Destabilization (2)
(all): Montana Dept of Transportation

7-8
Influence of Geology
© Jim Reichard

7-9
Water Content

7-10
Types of Mass Wasting Hazards

7-11
Types of Mass Wasting
• Falls
• Slides
• Slump
• Flow
• Snow Avalanche
• Submarine Mass Wasting
© Rick Guthrie

7-12
Rock Falls and Rock slides
© Jim Reichard

7-13
Snow Avalanche
A. 2008, San Juan Mountains, Colorado
B. 2001, Logan Mountains, Utah
a: © Mark Rikkers; b: © Toby Weed

7-14
Slumps
Jump to long
description

7-15
Submarine Slump
NOAA

7-16
Creep

7-17
Mudflow and Debris Flows
Photo courtesy of Clatskanie, Oregon, PUD

7-18
Sinkholes

7-19
Subsidence
• Collapse
• General Subsidence

7-20
Reducing the Risks of Mass
Wasting (1)
Recognizing and avoiding the hazard
Engineering controls
• Retaining walls
a: California Department of Transportation; b: © Jim Reichard

7-21
Wasting (2)
• Rock bolts
(inset): © Doug Sherman/Geofile

7-22
Wasting (3)
Engineering
controls
• Controlling
water
(inset): © King’s Material, Inc.

7-23
Wasting (4)
• Terracing
© Jim Reichard

7-24
Wasting (5)
• Covering steep slopes
(a-b): Profile Products
Jump to long
description

7-25
Wasting (6)
• Reducing slope materials

7-26
Wasting (7)
Engineering
controls
• Protective
structures

7-27
Wasting (8)
Lahaina, Maui, Hawaii
© Doug Sherman/Geofile
Wolf Creek Pass, Colorado
© Jim Reichard

7-28
Appendix of Image Long
Descriptions

7-29
Mass Wasting Long Description
Photo showing how gravity caused part of a hillside to slide downslope in a process known as mass wasting.
Note how the rock and sediment slid into the stream valley, forming a dam and creating a temporary lake.
Running water will then slowly transport the material downstream. Together with water, wind, and ice, mass
wasting plays an important role in shaping the landscape.
Jump back to slide containing original image

7-30
Slope Stability & Triggering Mechanisms Long Description
Aerial photo showing the source area for a rock and snow avalanche that destroyed much of the Peruvian
city of Yungay in 1970—parts of Ranrahirca were also destroyed. Note how the cities were built at the
mouth of a canyon that leads up to the source area on Mount Huascarán.

7-31
Gravity Long Description
On horizontal (zero slope) terrain (A), the full weight of the rock due to gravity (g) is directed downward. On
a hillside (B), part of the gravitational force (gs) acts parallel to the slope, thereby directing some of the
rock’s weight downslope; the remaining component (gp) acts perpendicular to the slope. On a steeper slope
(C), more of the rock’s weight is directed downslope, which requires greater friction to hold it in place.

7-32
Slope Destabilization (1) Long Description
Both natural processes and human activity can destabilize a slope by increasing its steepness. (A) As a
stream channel naturally migrates and cuts into the outside bank, it creates highly unstable overhang, which
inevitably leads to mass wasting. (B) Construction of highways and buildings in hilly terrain requires that
material be removed in order to create a level surface. This removal of material causes the slope to become
overstepened and susceptible to mass wasting.

7-33
Slope Destabilization (2) Long Description
Photo shows the Beartooth Highway traversing steep terrain northeast of Yellowstone National Park.
Construction of the highway required excavating material from the hillside, which made the slopes even
more prone to mass wasting—note the scars left by repeated movements of rock and sediment. This photo
was taken immediately after a series of rockslides and mudslides had cut or blocked the road in 13 separate
locations, requiring extensive and costly repairs.

7-34
Influence of Geology Long Description
a) Differences in strength and resistance to weathering of sedimentary layers control the steepness of
slopes in the Grand Canyon. Steep cliffs develop in resistant sandstones and limestones, whereas
broad, gentle slopes form in the much weaker shales.
b) Planar surfaces such as bedding planes, faults, fractures, and foliation planes represent weaknesses
within rocks that can greatly reduce slope stability. Particularly dangerous situations occur when these
surfaces are inclined in the same direction as the slope, creating the potential for blocks of material to
slide downslope.

7-35
Water Content Long Description
Rain or melting snow will infiltrate and eventually cause intergranular and planar voids in the subsurface to
become saturated. The weight of the water causes the fluid or pore pressure within the saturated voids to
increase (A), which reduces the friction between the solids. Downslope movement occurs when the frictional
forces become less than the gravitational force in the slope direction. Note the enlarged view (B) shows the
irregular nature of most planar surfaces.

7-36
Types of Mass Wasting Hazards Long Description
Mass wasting can be categorized based on the type of material involved and the way it moves downslope.
Blank boxes in the chart indicate those combinations of materials and movement that take place rarely, or
not at all.

7-37
Types of Mass Wasting Long Description
Photo showing the aftermath of the 2006 debris slide that killed over 1,100 people in the village of
Guinsaugon on Leyte Island, Philippines. A period of unusually heavy rains is believed to have played a
major role in triggering the deadly slide.

7-38
Rock Falls and Rock slides Long Description
Rockfalls generally result from repeated freezing and thawing of water within fractures in steep exposures of
solid rock. Expanding ice creates a wedging effect that eventually pushes a slab outward to the point where
it free-falls. Over time this process creates a deposit of broken rocks at the base of the cliff called a talus
pile. This material is then carried downstream by rivers and glaciers.
Rockslides (A) consist of blocks of solid rock sliding on top of a weakness plane such as bedding, foliation,
faults, and fractures. Earth and debris slides (B) are less coherent and tend to break up and move as a
jumbled mass. Common triggering events include streams undercutting their banks and infiltrating water
that increases the pore pressure along less permeable layers.

7-39
Snow Avalanche Long Description
Most snow avalanches occur in mountainous areas where weak layers form within the snowpack. Common
triggers are heavy snowfall events and human activity that add weight to the slope and overwhelm the
frictional forces along a weak snow layer. The avalanche in (A) began when a slab started sliding along a
weak layer. Closer view (B) of a detached slab—note the skier for scale.

7-40
Slumps Long Description
Slumps are complex events where material moves by sliding along a spoon-shaped surface near the top and
then flows toward the bottom or toe. Note the distinctive scars called scarps at the top and the jumbled
terrain near the toe.

7-41
Submarine Slump Long Description
A sonar survey of the ocean floor near St. Croix Island in the Caribbean Sea reveals a complex submarine
slump over 4.5 miles (7 km) across. Note the prominent scarp at the top of the slump.

7-42
Creep Long Description
Creep is the extremely slow movement of unconsolidated materials caused by repeated expansion and
contraction resulting from freeze/thaw and wet/dry cycles. The inset shows how expansion and contraction
cause particles to take a zigzagging path downslope. Because the motion decreases with depth, the
weathered rock layers appear to bend downslope. This movement can cause damage to a variety of human-
made structures.

7-43
Mudflow and Debris Flows Long Description
Mudflows and debris flows typically form in areas where there is an abundance of loose sediment on
relatively steep slopes. Water carries this material off the slopes, funneling it down steep channels where it
forms a fan-shaped deposit near the base of the slope. Debris flows can also occur on heavily vegetated
slopes when unusually heavy rains saturate loose material. Photo shows a 2007 debris flow that buried
several homes and closed a highway near Clatskanie, Oregon.

7-44
Sinkholes Long Description
Groundwater flowing through soluble limestone (A) will create large voids or caverns. As rivers cut
downward (B), they force the water table to lower, leaving void spaces drained of water and their roofs in a
weakened state. Infiltrating water will eventually cause the cavern roofs to weaken to the point where they
collapse, forming sinkholes.

7-45
Subsidence Long Description
a) Gradual subsidence can take place when large volumes of water or oil are removed from the subsurface.
Compaction occurs in clay-rich layers as water molecules between individual clay particles are slowly
removed. Heavy withdrawals of water and oil in the Houston area have resulted in as much as 10 feet
(3 m) of subsidence, creating a bowl-shaped depression that has led to serious flooding problems.
b) The amount of overburden pressure, or weight, that sediment grains must bear (A) is offset by the level
of pore (fluid) pressure within the sediment. Compaction and subsidence can occur (B) whenever there
is a reduction in fluid pressure, or when additional sediment is deposited, allowing more weight to bear
down on the grains.

7-46
Reducing the Risks of Mass Wasting (1) Long Description
Retaining walls are commonly used both above and below highways to strengthen oversteepened slopes.
The retaining wall in (A) helps stabilize an oversteepened slope created when part of the hillside was
removed, whereas the wall in (B) is supporting fill material that was placed on the slope to create a flat area
for the road.

7-47
Rock bolts are used to attach loose slabs of rock to more massive, solid bodies of rock, thereby reducing the
chance of rockfalls or small-scale rockslides along highways and in tunnels.

7-48
Drains can be used to prevent water from accumulating in porous earth materials, thereby minimizing weight
in the slope direction and the buildup of pore pressure. Berms and channels can also be used to divert
surface water away from unstable slopes.

7-49
Terraces were built into this road cut in order to keep rocks from falling onto the highway. By breaking the
slope into shorter segments, terracing allows rocks to come to rest on the terrace as opposed to tumbling
down the entire length of the slope.

7-50
Covering slopes with vegetation and synthetic materials increases friction within the slope, thereby
decreasing the potential for mass wasting. Photo(A) shows workers applying a hydroseed mixture over a
synthetic fabric, which had been draped over the sloped surfaces of a construction site. Photo (B) illustrates
how the slopes soon became covered with vegetation. Note how large rocks were used to stabilize the slope
in certain areas.

7-51
Removal of a slope altogether is sometimes the most practical and cost-effective means of reducing a mass
wasting hazard.

7-52
In areas where it is not feasible to prevent mass wasting, buildings can be protected by constructing
retaining walls (A) that will divert material, or by installing large barriers in valleys designed to trap debris
(B). Shelters (C) are used to allow material to safely move over transportation routes.

7-53
a) A heavy chain-link mesh was draped over a crumbling rock mass in Hawaii in order to keep small- to
medium-size rocks from falling onto the roadway.
b) This tunnel was constructed as a long-term and cost-effective solution for
protecting both the highway and motorists from earth materials moving downslope. Note that the old
roadway (to the left of the tunnel) is no longer open to traffic.

ENV 101 Ch07 lecture ppt_a

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ENV 101 Ch07 lecture ppt_a