Chapter 15 Mass Wasting
Mass wasting happens because tectonic processes have created uplift. Erosion, driven by gravity, is the inevitable response to that uplift, and various types of erosion, including mass wasting, have created slopes in the uplifted regions. Slope stability is ultimately determined by two factors: the angle of the slope and the strength of the materials on it.
In Figure 15.2 a block of rock situated on a rock slope is being pulled toward Earth’s centre (vertically down) by gravity. We can split the vertical gravitational force into two components relative to the slope: one pushing the block down the slope (the shear force), and the other pushing into the slope (the normal force). The shear force, which wants to push the block down the slope, has to overcome the strength of the connection between the block and the slope, which may be quite weak if the block has split away from the main body of rock, or may be very strong if the block is still a part of the rock. This is the shear strength, and in Figure 15.2a, it greater than the shear force, so the block should not move. In Figure 15.2b the slope is steeper and the shear force is approximately equal to the shear strength. The block may or may not move under these circumstances. In Figure 15.2c, the slope is steeper still, so the shear force is considerably greater than the shear strength, and the block will very likely move.
As already noted, slopes are created by uplift followed by erosion. In areas with relatively recent uplift (such as most of British Columbia and the western part of Alberta), slopes tend to be quite steep. This is especially true where glaciation has taken place because glaciers in mountainous terrain create steep-sided valleys. In areas without recent uplift (such as central Canada), slopes are less steep because hundreds of millions of years of erosion (including mass wasting) has made them that way. However, as we’ll see, some mass wasting can happen even on relatively gentle slopes.
The strength of the materials on slopes can vary widely. Solid rocks tend to be strong, but there is a very wide range of rock strength. If we consider just the strength of the rocks, and ignore issues like fracturing and layering, then most crystalline rocks — like granite, basalt, or gneiss — are very strong, while some metamorphic rocks — like schist — are moderately strong. Sedimentary rocks have variable strength. Dolostone and some limestone are strong, most sandstone and conglomerate are moderately strong, and some sandstone and all mudstones are quite weak.
Fractures, metamorphic foliation, or bedding can significantly reduce the strength of a body of rock, and in the context of mass wasting, this is most critical if the planes of weakness are parallel to the slope and least critical if they are perpendicular to the slope. This is illustrated in Figure 15.3. At locations A and B the bedding is nearly perpendicular to the slope and the situation is relatively stable. At location D the bedding is nearly parallel to the slope and the situation is quite unstable. At location C the bedding is nearly horizontal and the stability is intermediate between the other two extremes.
Internal variations in the composition and structure of rocks can significantly affect their strength. Schist, for example, may have layers that are rich in sheet silicates (mica or chlorite) and these will tend to be weaker than other layers. Some minerals tend to be more susceptible to weathering than others, and the weathered products are commonly quite weak (e.g., the clay formed from feldspar). The side of Johnson Peak that failed in 1965 (Hope Slide) is made up of chlorite schist (metamorphosed sea-floor basalt) that has feldspar-bearing sills within it (they are evident within the inset area of Figure 15.1). The foliation and the sills are parallel to the steep slope. The schist is relatively weak to begin with, and the feldspar in the sills, which has been altered to clay, makes it even weaker.
Unconsolidated sediments are generally weaker than sedimentary rocks because they are not cemented and, in most cases, have not been significantly compressed by overlying materials. This binding property of sediment is sometimes referred to as cohesion. Sand and silt tend to be particularly weak, clay is generally a little stronger, and sand mixed with clay can be stronger still. The deposits that make up the cliffs at Point Grey in Vancouver include sand, silt, and clay overlain by sand. As shown in Figure 15.4 (left) the finer deposits are relatively strong (they maintain a steep slope), while the overlying sand is relatively weak, and has a shallower slope that has recently failed. Glacial till — typically a mixture of clay, silt, sand, gravel, and larger clasts — forms and is compressed beneath tens to thousands of metres of glacial ice so it can be as strong as some sedimentary rock (Figure 15.4, right).
Apart from the type of material on a slope, the amount of water that the material contains is the most important factor controlling its strength. This is especially true for unconsolidated materials, like those shown in Figure 15.4, but it also applies to bodies of rock. Granular sediments, like the sand at Point Grey, have lots of spaces between the grains. Those spaces may be completely dry (filled only with air); or moist (often meaning that some spaces are water filled, some grains have a film of water around them, and small amounts of water are present where grains are touching each other); or completely saturated (Figure 15.5). Unconsolidated sediments tend to be strongest when
they are moist because the small amounts of water at the grain boundaries hold the grains together with surface tension. Dry sediments are held together only by the friction between grains, and if they are well sorted or well rounded, or both, that cohesion is weak. Saturated sediments tend to be the weakest of all because the large amount of water actually pushes the grains apart, reducing the mount friction between grains. This is especially true if the water is under pressure.
Exercise 15.1 Sand and Water
If you’ve ever been to the beach, you’ll already know that sand behaves differently when it’s dry than it does when it’s wet, but it’s worth taking a systematic look at the differences in its behaviour. Find about half a cup of clean, dry sand (or get some wet sand and dry it out), and then pour it from your hand onto a piece of paper. You should be able to make a cone-shaped pile that has a slope of around 30°. If you pour more sand on the pile, it will get bigger, but the slope should remain the same. Now add some water to the sand so that it is moist. An easy way to do this is to make it completely wet and then let the water drain away for a minute. You should be able to form this moist sand into a steep pile (with slopes of around 80°). Finally, put the same sand into a cup and fill the cup with water so the sand is just covered. Swirl it around so that the sand remains in suspension, and then quickly tip it out onto a flat surface (best to do this outside). It should spread out over a wide area, forming a pile with a slope of only a few degrees. [SE]
Water will also reduce the strength of solid rock, especially if it has fractures, bedding planes, or clay-bearing zones. This effect is even more significant when the water is under pressure, which is why you’ll often see holes drilled into rocks on road cuts to relieve this pressure. One of the hypotheses advanced to explain the 1965 Hope Slide is that the very cold conditions that winter caused small springs in the lower part of the slope to freeze over, preventing water from flowing out. It is possible that water pressure gradually built up within the slope, weakening the rock mass to the extent that the shear strength was no longer greater than the shear force.
Water also has a particular effect on clay-bearing materials. All clay minerals will absorb a little bit of water, and this reduces their strength. The smectite clays (such as the bentonite used in cat litter) can absorb a lot of water, and that water pushes the sheets apart at a molecular level and makes the mineral swell. Smectite that has expanded in this way has almost no strength; it is extremely slippery.
And finally, water can significantly increase the mass of the material on a slope, which increases the gravitational force pushing it down. A body of sediment that has 25% porosity and is saturated with water weighs approximately 13% more than it does when it is completely dry, so the gravitational shear force is also 13% higher. In the situation shown in Figure 15.2b, a 13% increase in the shear force could easily be enough to tip the balance between shear force and shear strength.
In the previous section, we talked about the shear force and the shear strength of materials on slopes, and about factors that can reduce the shear strength. Shear force is primarily related to slope angle, and this does not change quickly. But shear strength can change quickly for a variety of reasons, and events that lead to a rapid reduction in shear strength are considered to be triggers for mass wasting.
An increase in water content is the most common mass-wasting trigger. This can result from rapid melting of snow or ice, heavy rain, or some type of event that changes the pattern of water flow on the surface. Rapid melting can be caused by a dramatic increase in temperature (e.g., in spring or early summer) or by a volcanic eruption. Heavy rains are typically related to storms. Changes in water flow patterns can be caused by earthquakes, previous slope failures that dam up streams, or human structures that interfere with runoff (e.g., buildings, roads, or parking lots). An example of this is the deadly 2005 debris flow in North Vancouver (Figure 15.6). The 2005 failure took place in an area that had failed previously, and a report written in 1980 recommended that the municipal authorities and residents take steps to address surface and slope drainage issues. Little was done to improve the situation.
In some cases, a decrease in water content can lead to failure. This is most common with clean sand deposits (e.g., the upper layer in Figure 15.4 (left)), which lose strength when there is no more water around the grains.
Freezing and thawing can also trigger some forms of mass wasting. More specifically, the thawing can release a block of rock that was attached to a slope by a film of ice.
One other process that can weaken a body of rock or sediment is shaking. The most obvious source of shaking is an earthquake, but shaking from highway traffic, construction, or mining will also do the job. Several deadly mass-wasting events (including snow avalanches) were trigged by the M7.8 earthquake in Nepal in April 2015.
FAQs
What are three factors that contribute to slope stability? ›
Slope stability is a result of three factors: the angle of the slope, the friction between the slope and the soil, and the weight of the soil and rocks on the slope. 2. Creep is a type of slope movement that occurs when the soil and rocks on a slope slowly move up or down together.
What is slope stability in geology? ›Slope stability is the process of calculating and assessing how much stress a particular slope can manage before failing. Examples of common slopes include roads for commercial use, dams, excavated slopes, and soft rock trails in reservoirs, forests, and parks.
What is the most important factor controlling the stability of a slope apart from slope angle and type of material on the slope )? ›Apart from the type of material on a slope, the amount of water that the material contains is the most important factor controlling its strength. This is especially true for unconsolidated materials (e.g., Figure 15.4), but it also applies to bodies of rock.
What factors control slope stability? ›Slope stability is ultimately determined by two factors: the angle of the slope and the strength of the materials on it.
What are three 3 main factors that could contribute to the slope failure? ›Weathered geology: Weak, weathered bedrock, jointed rock, or bedrock that dips parallel to the slope can decrease stability. Vegetation removal: Droughts, wildfires and humans can remove vegetation from the slope, decreasing stability. Freeze/thaw cycles: Water in rock joints or in soils can decrease slope stability.
How do you determine the stability of a slope? ›If the forces that resist the movement are greater than those driving the movement, the slope is considered stable. A factor of safety (FS) is calculated by dividing the resistance by the driving forces. A factor of safety greater than 1.00 suggests that the slope is stable.
What are the different types of slope stability? ›Types of slope stability analyses include rotational slope failure, translational failure, irregular surfaces of sliding, and infinite slope failure.
What is slope stability examples? ›(a) Earth dams and embankments, (b) Excavated slopes, (c) Deep-seated failure of foundations and retaining walls.
What are the factors affecting slope? ›In general terms the factors include climate, lithology, topography and vegetation. At a more site-specificlevel, rainfall fre- quency, intensity and duration, slope length and inclination, soil properties, ground cover, and soil-disturbing activities are also important.
What are the 3 types of slope failure? ›- Rotational failure. When rotational failure occurs, the failed surface will begin to move outwards and downwards. ...
- Translational failure. ...
- Compound failure. ...
- Wedge failure.
How do you control slope failure? ›
To ensure slope stability in constructed soil slopes, one common method is to take soil core samples, determine the stratigraphic layout of the soils, and then cut benches into the weaker soil. Next, a more stable and uniform soil type can be placed overtop to promote slope stabilization.
What are the two opposing forces in controlling slope stability? ›Slope stability is controlled by 2 main factors: the driving and the resisting forces.
What measures should be considered to protect or stabilize slope from failing? ›Support stabilization: Structural supports aim to increase the stability of the slope. Those techniques include the implementation of pre-stressed anchors, rock bolts, piles, soil nailing, geosynthetic reinforcement, retaining walls, shotcrete, etc.
How does temperature affect slope stability? ›A certain correlation exists between temperature change and slope stability. That is, the higher the temperature is, the lower the slope safety factor. The effects of atmospheric temperature on slope stability are also influenced by many other factors, such as rainfall condition, soil type, and climatic conditions.
What are three factors that affect stability? ›Factors Affecting Stability
The moment the size of the base support will be increased the stability of the respective object will start to change. If the line of gravity will be central to the base of support, the stability of the object will increase. The object with a larger mass will have a greater stability.
The stability of the compounds depends on the energy of sublimation, the lattice energy and the solvation energy.
How many factors influence the slope stability of embankment? ›Five factors influence slope stability of an embankment: 1) Shear strength of the soil; 2) Unit weight; 3) Embankment height; 4) Slope steepness; and 5) Pore pressure within the soil. Failure generally occurs in two ways.
How does water affect slope stability? ›The Role of Water
Water can seep into the soil or rock and replace the air in the pore space or fractures. Since water is heavier than air, this increases the weight of the soil. Weight is force, and force is stress divided by area, so the stress increases and this can lead to slope instability.
The base failure occurs at a steeper slope, i.e., slope angle equal to 36.87°. Decreasing slope angle increases the factor of safety of slopes nearly linearly while decreasing slope height increases the factor of safety at different rates.
What are the methods of slope protection? ›Slope protection approaches discussed below include erosion control blankets and turf reinforcement mats, which can also be used for ditch protection, surface roughening, slope drains, gabion structures, and cellular mats.
How do you determine more stability? ›
- Make stability a top priority. Commit yourself to consistency. ...
- Establish a routine. Go to bed and wake up at the same time every day. ...
- Limit your alcohol. ...
- Live within your financial means. ...
- Don't overreact. ...
- Find stable friends. ...
- Get help making decisions. ...
- End a bad relationship.
Slopes come in 4 different types: negative, positive, zero, and undefined. as x increases. The slope of a line can also be interpreted as the “average rate of change”. It tells us how fast y is changing with respect to x.
What are the 5 modes of slope movement? ›The term "landslide" encompasses five modes of slope movement: falls, topples, slides, spreads, and flows. These are further subdivided by the type of geologic material (bedrock, debris, or earth).
What does the slope control? ›Slope control gradually reduces the amount of current flowing through the electrode as it approaches the weld puddle, which prevents the electrode from sticking to or “welding” to the puddle. Some welding power sources have a fixed slope, which is set by the manufacturer.
What are the factors that influence the position and slope of the IS curve? ›The slope of the IS curve also depends on the saving function whose slope is MPS. The higher the MPS, the steeper is the IS curve. For a given fall in the interest rate, the amount by which income would have to be increased to restore equilibrium in the product market is smaller (larger), the higher (lower) the MPS.
How does slope angle affect slope stability? ›Slope angle is one of the most important factors in creating instability of slopes. Based on morphology, each study area may have different slopes. The higher the angle of a slope, the greater the unstable force.
What is the failure of the stability of slopes? ›When the stability conditions are not met, the soil or the rock mass of the slope may experience downward movement which could be either slow or devastatingly rapid. This phenomenon is known as slope failure or landslide.
What are the four slope elements? ›Slope elements: crest, cliff (scarp slope, free face), talus (debris, scree slope) and pediment. Characteristics of the slope elements: crest, cliff, talus and pediment.
What are the most common types of slope processes? ›The slope processes include weathering, erosion, transport and deposition of the material. Weathering is the process by which material is prepared for transport. Weathering is the response of the materials within the lithosphere to conditions at or near its contact with the atmosphere, hydrosphere and biosphere.
How do you control erosion in slope protection? ›- Plant Grass and Shrubs. Grass and shrubs are very effective at stopping soil erosion. ...
- Use Erosion Control Blankets to Add Vegetation to Slopes. ...
- Build Terraces. ...
- Create Diversions to Help Drainage.
How does cohesion affect slope stability? ›
(3) The influence of Cohesion on the loose slope stability: Under the condition of slope type and other soil parameters, the increase of cohesion will lead to the decrease of slope stress, strain and displacement Make the slope safety factor increase.
What is the best slope protection? ›In order to prevent slope erosion, plant grass and other vegetation. Grasses are great for slope stabilization because of their roots. They also absorb rainwater and other precipitation, making water erosion less common. Erosion control blankets work to add vegetation to slopes.
What are at least 3 ways we can engineer a slope to prevent it from failing? ›Flattening the slope. Eliminating part of the soil/rock. Eliminating load from the top of the slope and therefore reducing the shear stresses on critical planes. Constructing pressure berms at the toe of the slope and thereby providing extra safety against toppling failure.
What are two ways in which climate affects slope development? ›Examples of climate change impacts on slopes include an infiltration increase causing loss of soil suction, a reduction in effective stress due to rising groundwater levels, a loss of root reinforcement due to changes in the type of vegetation or dying of vegetation, an increase in seepage forces due to frequent and ...
How does temperature affect the stability of the solution? ›When a drug is stored in temperatures that are too high or too low, the drug's chemical stability will likely be impacted. That means that the drug may degrade and form impurities. While these impurities may not be visually noticeable, this degradation can cause real problems when the drug is administered.
What are the three factors of stability? ›The three most critical factors affecting stability are the size of the base of support, the relation of the line of gravity to the base of support, and the height of the center of gravity.
What are the factors affecting slope development? ›The mechanics and rates of slope movement are controlled by many factors: slope gradients, overburden depth, structural rock properties, water content and soil pore water pressure, and certain engineering properties of overburden and weathered rock, such as cohesion and coefficient of friction.
What factors affect slope? ›In general terms the factors include climate, lithology, topography and vegetation. At a more site-specificlevel, rainfall fre- quency, intensity and duration, slope length and inclination, soil properties, ground cover, and soil-disturbing activities are also important.
What are the 4 factors that affect stability? ›Common factors that affect this stability include temperature, light, pH, oxidation and enzymatic degradation.
How many stability factors are there? ›Three stability factors are defined as follows, Stability factor S: The above equation can be considered as a standard equation for the derivation of stability factors of other biasing circuits.
What are the stability factors? ›
Stability factors are calculated as functions of the frequency or another stimulus parameter. They provide criteria for linear stability of two-ports such as amplifiers. A linear circuit is said to be unconditionally stable if no combination of passive source or load can cause the circuit to oscillate.
What are the 5 factors that affect slope stability? ›- Strength of soil and rock.
- Type of soil and stratification.
- Discontinuities and planes of weakness.
- Groundwater table and seepage through the slope.
- External loading.
- Geometry of the slope.
There are many factors that significantly affect soil structural stability like climate, organic matter content, adsorbed cations, tillage, type of vegetation, plant roots, soil organisms, manurial practices and crop rotation, alternate wetting and drying (Shreeja n.d.).
How does gravity affect slope stability? ›But if gravity is stronger, the slope will fail. The steeper the slope, the stronger the friction or rock strength must be to resist down slope motion. The steepest angle a slope can be before the ground will slide is about 35 degrees, called the angle of repose.
What is the minimum factor of safety for slope stability? ›For global stability of a slope, a minimum factor of safety of 1.3 is required for both the long-term drained condition and the short term undrained condition.