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Environmental
Geology |
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MODULE 4 |
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| Reading assignment: Chapter 6, p. 131-160;
Keller. See Module Preview for initial information and a Powerpoint Presentation. |
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Introduction - Heavy rains saturate the Portland, Oregon area. Slick, debris-filled mud slides down steep slopes blocking the scenic Columbia River Gorge Highway. - Warm rains rapidly melts snowpack. Snowmelt-induced debris flow threatens lives and damages property. - A major earthquake hits the West Coast. It triggers numerous landslides in a 200-mile radius. Each of these events represents some type of slope failure. Slope failure, also referred to as mass wasting, is the downslope movement of rock debris and soil in response to gravitational stresses. Three major types of mass wasting are classified by the type of downslope movement. The types of movement, falls, slides, and flows, will be covered in this module. In addition, another type of ground failure, subsidence, will be covered. Take a Virtual Field Trip to the Slumgullion landslide. |
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Links to Good Landslide and Slope Failure sites: http://ilrg.gndci.pg.cnr.it/ International Landslide Research Group -- lots of general information Geologic Hazards--Landslides (USGS) information on landslides. http://walrus.wr.usgs.gov/elnino/landslides-sfbay/photos.html contains a computer-simulated landslide http://www.nps.gov/hafo/landslid.htm Landslides at Hagerman Fossil Beds N.M., Idaho Yosemite landslide News and Information--GEOLOGIC HAZARDS TEAM (... Oahu landslide News and Information--GEOLOGIC HAZARDS TEAM (...
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| SLOPES
Material is constantly moving downslope in
response to gravity. Movement can be Or, movement can be devastatingly rapid, apparent within minutes. Whether or not slope movement occurs depends on slope steepness and slope stability.
SLOPE PROFILE Some slopes are gently rounded, while others are extremely steep. Profiles of naturally-eroded slopes are primarily dependent on climate and rock type. Resistant rock and rock from semi-arid regions have similar profiles because in both cases chemical weathering is slow. Erosion may be slow or fast depending on the amount of material weathered (loosened) or freely available (uncovered) for transport. Typically, the crest of the slope is slightly convex to angular, the cliff (freeface) is nearly vertical, and a debris slope is present. The debris slope has an angle of repose of 30° -35° , which is the maximum angle at which loose material is stable. Resistant Rock -- Semi-arid Region
Non-resistant rock and rock in semi-humid regions also have similar profiles. Climatic conditions and softer rock types result in material that weather rapidly and erode easily. The crest of the slope is convex, while the base of the slope is concave. This type of slope contains a thick soil cover. Non-resistant Rock -- Semi-humid Region
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Slope Stability (a lesson in physics) When is a slope not stable? Slope stability is based on the interplay between two types of forces, driving forces and resisting forces. Driving forces promote downslope movement of material, whereas resisting forces deter movement. So, when driving forces overcome resisting forces, the slope is unstable and results in mass wasting. DRIVING FORCES The basic concept of these two types of forces is quite simple. You experience the interplay between driving forces and resisting forces each time you drive down a steep slope. The driving forces are gravity and the acceleration of the vehicle (if you step on the accelerator), the resisting force is the brake on the vehicle. The main driving force in most land movements is gravity. The main resisting force is the material's shear strength. Does gravity act alone? NO!! Slope angle, climate, slope material, and water contribute to the effect of gravity. Mass movement occurs much more frequently on steep slopes than on shallow slopes. Water plays a key role in producing slope failure. In the form of rivers and wave action, water erodes the base of slopes, removing support, which increases driving forces. Water can also increase the driving force by loading, i.e., adding to the total mass that is subjected to the force of gravity. Chemical weathering (interaction of water with surface rock and soil) slowly weakens slope material (primarily rock), reducing its shear strength, therefore reducing resisting forces. An increase in water also contributes to driving forces that result in slope failure. The weight (load) on the slope increases when water fills previously empty pore spaces and fractures. The shear strength of the slope material is decreased by increasing the pore water pressure (pressure that develops in pore spaces due to the increased amount of water). RESISTING FORCES Resisting forces act oppositely of driving forces. The resistance to downslope movement is dependent on the shear strength of the slope material. And shear strength is a function of cohesion (ability of particles to attract and hold each other together) and internal friction (friction between grains within a material).Water contributes to resisting forces when sediment pores are partially filled with water. The thin film of water acts as a binder, making the particles cohesive (remember surface tension of water? Module 2-part 2). The ratio of resisting forces to driving forces is the safety factor (SF):
NOTE: Usually a safety factor of ~10 is used in design to accommodate slight variances in materials and construction practices. Factors of Slope Stability Slope stability is therefore a function of material, strength of rock or soil, slope angle, climate, vegetation, and time. Each of these factors may play a significant role in controlling driving or resisting forces. Study in your textbook how each factor controls the type of failure and the likelihood that failure will occur. How does slope
angle affect both driving and resisting forces?
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| Types of Mass Wasting Study the following table before reading this section. Return to the table to review. |
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FALLS
SLIDES Slides, either in rock or
soil, will have rotational or translational movement. The behavior of the slide
depends mostly on the type of material and whether that material is:
FLOWS Flows are the downslope movement of unconsolidated material in which the material behaves like a viscous fluid. Flows can be very slow or can be exceedingly fast. Creep - a type of flow Have you ever noticed a group of trees on a slope where the base of each tree bows outward in the downslope direction? Have you ever driven through an older, well-established neighborhood where houses are located "above" the sidewalks and streets? Where retaining walls try to hold the sloping lawns in place? Where some of the retaining walls are failing, bulging out over the sidewalks?
Other types of flows are all quite similar. Generally the size of individual particles and the amount of water present is the distinguishing criteria. Frequently, the same event is referred to by more than one name. Confusing, huh?!! So, don't be surprised if you discover what one source called a mudslide another source referred to it as a debris flow! Generally, the following rules apply:
TASK 1: Visit the U.S. Geological Survey Geological Hazards site and peruse the various types of landslides and their effects on human lives. Select FOUR different events from the Landslide Images directory. Record their names and the general features of each one. Write a very short synopsis of the problem that was encountered during the failure and answer the question: How did it fail? TASK 2: Continue visiting the U.S. Geological Survey Geological Hazards site and find one example of each of the four types of flows outlined above. List them by name and/or place and date of the event, if known. The U.S.G.S. Landslide Image site
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The Human Impact of Landslides Landslides are natural occurring phenomena. Landslides, or slope failure, occur whether people are there or not! But, human land-use does have a major impact on slope processes. The combination of uncontrollable natural conditions (earthquakes, heavy rainstorms, etc.) and artificially altered landforms can result in disastrous slope failures, and does.
Harvesting timber can also have an impact. Removing slope-supporting material (trees and brush), as well as creating roads, affects the landscape. If the surficial (and sometimes subsurface) geology is unstable, mass wasting often occurs. BUT, if clear-cutting (harvesting all trees in an area) and road building occur on geologically stable land, timber harvesting has considerably less impact. Urbanization also has an effect on slope stability. Grading hillsides (cutting benches for building homes on) greatly increases landslide potential. Construction of homes on unstable slopes has similar effects. Changing the slope face, the additional weight (homes and fill material), plus the added water (homeowners' sprinkler systems and septic tanks) make a formerly stable slope unstable. Add a heavy rainy season and you have lots of landslides!
MITIGATION Minimizing landslide hazards requires three steps: 1) identification of landslide potential areas, 2) prevention of landslides, and 3) corrective measures when a landslide occurs. Identification Identification is accomplished by 1) studying aerial photographs to determine sites of previous landslides or slope failures, and 2) field investigations of potentially unstable slopes. Potential mass-wasting areas can be identified by steep slopes, bedding planes inclined toward valley floors, hummocky topography (irregular, lumpy-looking surface) covered by younger trees, water seeps, and areas where landslides have previously occurred. The information is then used to generate a hazard map depicting the various landslide-prone areas.
Prevention Controlling drainage and reducing the slope angle reduces landslide potential. Concrete interceptor drains can be constructed to contain runoff and prevent infiltration. Steep slopes can be graded into gentler slopes. A series of "stair-steps" can be created on very steep slopes. Engineering methods can be used to help prevent slope failure. Retaining walls, rock bolts, and "shotcrete" (coating of concrete-rock mixture on slope surface and crevices to prevent water entry) are used to inhibit slope failure. Wire cables and wire fences minimize the danger of rockfall. Correction Correction of some landslides is possible. This is accomplished by installing a drainage system, which reduces water pressure in the slope, thereby preventing further movement. TASK 3: Locate in your local area or on the web an example of how a slope is being stabilized (or needs to be stabilized). Write only a sentence or two that describes what you have found.
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SUBSIDENCE
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| TASK 4: What is
the major internal cause of slope failure? What three conditions are external causes
of slope failure? ==================================================================== ALL TASKS: Send your answers to hughscot@isu.edu as soon as you have completed them. Terms to Look Up and Know Pore water pressure Saturation Progressive wetting Rapid Drawdown Liquefaction Quick clay Seeping Avalanche Oversteepening by erosion Hummocky terrain |
