(Revised Fall 2009) Name Lab 3. Landslides and Mass Wasting INTRODUCTION Mass wasting is a general term for a variety of processes by which earth materials move downslope as a result of gravity. The actual process of downslope motion is referred to as mass movement. Mass movements, such as landslides, avalanches, and debris flows, occur when the amount of force of gravity acting in a downslope direction is greater than the strength of the rock (or soil or snow) material of the hillside. Slope movements are one of the most costly geologic hazards in the United States, causing 25 to 50 deaths and $1 billion to $2 billion in economic losses each year. However, the damage can be, and has been, reduced through the cooperative efforts of earth scientists, engineers, and public officials. THE ROLES OF GRAVITY AND WATER IN MASS MOVEMENT If mass movement is caused by gravity, which affects the entire Earth, why is it more likely to occur on some hillslopes than on others? Why do some mass movements occur suddenly and unexpectedly, perhaps after several days of rain? The answer to both of these questions has to do with resistance to the forces of gravity. For a mass movement to occur, the force of gravity must be greater than any resistance of the earth material. Because the force of gravity itself does not change substantially at Earth s surface, the amount of resistance, or strength, of the material relative to the force of gravity must be what varies. It can vary from one place to another, which depends largely on the slope of the surface on which the debris rests, and it can vary over time at a give location, which depends largely on the amount of moisture in the mass. Consider a flat surface, on which debris remains motionless. The force of gravity pulls the loose debris directly downward, holding it in place. However, on a sloping surface, the force of gravity can be separated into two components: (1) a slope perpendicular component that pulls the debris in a direction perpendicular to the slope and helps to hold it in place; and (2) a slope parallel component that acts to move the debris along and down the slope. The slope perpendicular component of gravity provides frictional resistance against mass movement, while the slope parallel component provides a driving force that favors mass movement. The slope parallel component on a hillside is greater on steeper slopes than on lower slopes. Every hillside has a critical angle of slope stability. Hillsides with slope angles lower than the critical angle are stable, but steeper slopes are unstable. This critical slope angle is an example of a threshold. When a slope system is at this critical angle, a minor change that adds weight or steepens the slope can be the proverbial straw that broke the camel s back. The critical angle above which failure occurs is called the angle of repose, and ranges from about 30 to 35 for loose, dry material such as sand and gravel. Therefore, determining the slope angle is an important first step in identifying potentially unstable and dangerous slope conditions. Slope steepness is not the only control on the strength of a material because some mass remains stable on steep slopes for long periods of time and then fails suddenly, usually after a prolonged, heavy rain. Water is the second factor in the strength of a material. If water enters the pore space between particles in weathered material, the material s resistance to movement changes. A certain amount of water can provide increased resistance to movement because it holds particles together by surface tension, a force caused by the attraction of water molecules to each other. This effect can be seen in the cohesion of moist sand used to build a sand castle. If, however, water completely fills up the pore spaces surrounding the particles, the pressure of the water in the pore spaces pushes the grains apart, causing them to move freely. In this case, water lowers the resistance to mass movement. The sand castle will collapse if too much water is added. 1
TYPES OF MASS MOVEMENT The basis of classification of mass movements is: (1) the type of earth material involved, such as rock, debris (coarse soil), or earth (fine grained soil); and (2) the type of movement, such as heave, slide, or flow. The heave mechanism is caused by alternating expansion and contraction of debris from freezing and thawing or wetting and drying and raises and lowers material in a direction perpendicular to the hillslope. Because of the slope and gravity, some material makes its way downhill as it is dropped back to the soil surface. Slide occurs when cohesive blocks of material move, or fail, along a well defined place. Thus, the term landslide has a very specific meaning. Flow occurs when debris moves like a fluid and, in contrast to sliding, no clear plane of failure exists within or below the moving mass. All types of mass movements, except for rockfalls, can be attributed to one of these three mechanisms, but the type of debris, the amount of water, and the speed at which the debris moves can all be quite variable. As a result, many different types of mass movements have been identified, such as rockslides, debris slumps, debris slides, debris flows, and mudflows (Figure 1). Soil creep is a very slow process that involves varying amounts of water and is the mass movement most closely associated with heave. Evidence for soil creep includes the downward curvature of trees and gravestone, and the tilt of power poles. 2
Figure 1. Different types of mass movements Debris Slump Debris Flow Debris Slide CAUSES AND PREVENTION OF MASS MOVEMENTS 3 Rockslide
CAUSES Factors Related to Increased Forces Slope gradient (steeper slopes are more prone to mass movement) Factors Related to Decreased Resisting Forces (Reduced Strength of Mass) Lateral support removed by erosion or construction Moisture content (increased to the point of the slope material behaving more like a liquid than a solid) Vegetation (roots of vegetation provide much strength to materials on hillslopes; vegetation absorbs water and minimizes the accumulation of water in the soil) Nature of geologic materials (i.e. highly weathered; or rock layers inclined parallel to slope) PREVENTIVE MEASURES Reduce slope (e.g. constructing benches and retaining walls along road cuts and coastal cliffs) Reinforce base of slope with retaining wall or by grouting Seal surface cracks to prevent infiltration; drain surface water from potential mass movement material with ditches; install subsurface drainage system Replant slopes immediately after removal of vegetation during logging or development; protect slopes with cover or mulch while seedlings become established Construct pilings through mass; avoid building on slopes with rocks layers parallel to the slope THE GROS VENTRE SLIDE, WYOMING The Gros Ventre (pronounce grow vaunt) slide in Wyoming is considered the largest historical rockslide in the United States. It occurred during roughly 3 minutes on the afternoon of June 23, 1925. Ranchers who lived in this valley believed they heard water running beneath the ground years prior to the year of the slide. The mountainside was about 1.5 mi long and contained dense forest over much of its length. It is estimated that 40 million cubic meters of weathered sandstone, limestone, sand, silt, and clay rich, water saturated debris moved during the slide, much of it crossing the Gros Ventre River, and some of it even continuing up the opposite side of the valley. The toe of the slide created a 250 ft high dam, which impounded a lake of 11,000 acres. Two years later, in May 1927, rising waters from the previous winter s snow melt burst the dam and caused a fatal flood, killing 6 people in the town of Kelly, about 4 mi downstream. The Gros Ventre slide was one in which the slope of the land was parallel to the beds of rocks under it. The slide remobilized deposits of a smaller slide that had occurred at some time in the past. It left a newly exposed, less steep hillside. 4
5 Figure 2. Photo of Gros Ventre Slide, Wyoming (2007, MMH).
Exercise 1 Name Lab Instructor Section Purpose In this exercise, you will investigate the nature of critical angles of stability by conducting experiments to relate slope angles to the movement of different masses. You will also measure and review calculations of slope angles, consider the broad distribution of landslide prone regions across the United States, and calculate the rate of motion for a famous mass movement. (Note: This exercise is modified from the UT Geography 132 Laboratory Manual developed by Dr. Carol Harden, Dr. Sally Horn, and Dr. Henri Grissino Mayer.) Critical Slope Angles for Mass Movements Slope angle (α) is measured as degrees from an imaginary horizontal reference line. In this part of the exercise, you will conduct experiments to determine the critical (minimum) slope angle for initiating the movement of a particle on one planar surface. The slope will be a clipboard and the slope materials will be referred to as Mass A and Mass B. You will be experimenting with a static condition; in other words, the mass will be at rest on the slope when the experiment begins and the critical angle will be that at which movement begins. Because it is difficult to know the exact angle at which movement begins, you will do the experiment 10 times and average the results. As you do these experiments, remember that the experimental setup is a simplified model that represents much larger and more powerful landsliding processes in the natural environment. Instructions 1. Use the clipboard as an inclined plane and align the protractor with its center at the bottom of the board. Place Mass A flat on the board about 3 cm below the clip. 2. Slowly and steadily lift up the clip edge of the clipboard. Stop tilting the board when the mass begins to move. 6
3. Determine the minimum angle of the inclination of the board at which the mass will slide all the way to the bottom. Measure this angle using the protractor. 4. Repeat steps 1 3 ten times with Mass A. Record your observations in this table: Mass A Trial 1 2 3 4 5 6 7 8 9 10 Angle Calculate the average of the 10 trials. The average critical angle for Mass A =. 5. Repeat steps 1 3 ten times using Mass B. Record your observations in this table and compute the average of the 10 angles. Mass B Trial 1 2 3 4 5 6 7 8 9 10 Angle The average critical angle for Mass B =. Questions 1. Which mass had the higher average critical angle? 2. Why where the critical angles different or not different? 3. What do you think would happen to the critical angles if water was added to the clipboard and why? Part II. A review of slope angles Any angle may be expressed as the ratio: rise/run where rise = difference in elevation between two points run = distance between those two points 7
Example: If the rise = 20 and the run = 50, the slope angle is 20 /50 = 0.40 This may also be expressed as a percent: a slope of 0.40 is a 40% slope. The rise/run ratio is the tangent of the slope angle, so we can convert the slope angle ratio to degrees by finding the arctangent (using a calculator or trigonometric table). In this case, arctan(0.40) = 21.8. Note that the percent slope is roughly twice the number of degrees. This is a useful rule of thumb. 1. You are driving on a mountain highway that crosses the Great Smoky Mountains and see a highway warning sign that says 6% downgrade, trucks use lower gear. Approximately what is a 6% slope when expressed in degrees? 2. In the diagram below, the rise and run are indicated: a) Express the slope angle as a ratio:. b) Express the slope angle as a percent:. c) Estimate the slope angle in degrees:. Part III. Go outside and measure the slope Go outside with your TA to the hill at the rear of the Geology Building. Break up into groups of 3 4 students so each group can measure the slope of different parts of the hill. The TA will provide each group with a measuring tape, some string and a line level. One student should stand at one spot (x). Have another student walk directly down slope from that spot for about 6 feet. How would you measure the slope of that distance using the figure above? (Hint: you have walked the hypotenuse of the triangle). This is shown below. In this case, the slope can be calculated as the arctangent of rise/run. Thus, slope = arctan(rise/run). 8
Questions 1. If the distance between x and y on the hill (see above) = 6 feet, then what is the slope? Make the needed measurements of rise and run. Show your calculations. 2. Starting at y, walk downhill another 6 feet and repeat the procedure. What is the slope of this segment? 3. Which segment (the 1 st or 2 nd that you did) would probably experience the most rapid mass wasting? Why? 4. Which segment (the 1 st or 2 nd that you did) would probably experience the most water runoff during a rainfall event? Why? 5. If you had a rise of 6 feet and a run of just 1 inch, what slope would that be? What kind of land form is that? Part IV. Landslides in the contiguous United States The map of landslide areas in the contiguous United States was produced in color and distributed by the U.S. Geological Survey (http://pubs.usgs.gov/pp/p1183/plate1.html). According to this map, some states have no landslides, while others have considerable landslide activity. Looking at the map, hypothesize the type of terrane and the type of environment that is most susceptible to landsliding. Use an atlas to confirm or redevelop your hypotheses. 1. I hypothesize that: 2. Why do you think the eastern part of Tennessee has greater susceptibility to landslides than the western portion? 9
3. What state has the greatest proportion of lands with a high incident of landsliding? Velocity of Famous Mass Movement Part IV. On May 31, 1979, a magnitude 7.7 earthquake triggered a huge debris avalanche on Mt. Huascaran (pronounced was car RON) in Peru. The moving mass descended from the mountain s summit (elevation 6654 m) to the town of Yungay (young GUY) (elevation ~2800 m) where it killed an estimated 20,000 people, many of whom had come to town from the surrounding villages to visit the marked. Above the town of Yungay, the moving mass split into two lobes, one of which spilled over a hill (C) and covered the town to nearly the top of the church steeple. Examine the map view and the cross section of this catastrophic debris avalanche. 1. From the map view, measure the length of the land surface from H, the top of the landslide source, to Yungay (measure to the center of the street grid). The distance is ~ km. 2. Timed observations indicate that the avalanche reached Yungay 3 minutes after its initiation. What was the average velocity (km/hr) of the debris avalanche? 10
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