Understanding Mass Wasting: Hazards and Mitigation Techniques

43 Exercise 5 Mass Wasting James S. Reichard Georgia Southern University Student Name: Kevin Hoffman In this lab, you will: examine various mass-wasting hazards and some of the techniques that can be used to help minimize the loss of life and property damage. Background Reading and Needed Supplies Before doing this exercise, you should read Chapter 7, Mass Wasting and Related Hazards in the textbook. Concerning supplies, you will need a calculator, ruler, and colored pencils. Part II – Basic Principles of Mass Wasting In this section, we will explore some of the mechanisms involved in the downslope movement of earth materials. 1) Figure 5.1 illustrates how earth materials on a sloping surface are subjected to some portion or component of gravity (g s ) that acts in a direction parallel to the slope. For material on a slope to remain stationary, frictional forces must be greater than or equal to g s . a) Describe what happens to the component of gravity in the slope direction (g s ) as the slope becomes steeper or gentler. The friction of the rock has a more gravitate pull to it when it becomes steeper. b) If you have two identical slopes, but earth materials are stable on one and not the other, what can you infer about the frictional forces on the two slopes? Based on the given information, it can be inferred that the frictional forces on the slope where earth materials are stable are greater than the frictional forces on the slope where earth materials are not stable. Figure 5.1 Ex 5 – Mass Wasting

44 2) As indicated in Figure 5.2, the weight of water in saturated materials creates an outward- acting fluid pressure in pore spaces and fractures. This so-called pore pressure becomes greater whenever the overlying column of water increases. a) Based on the forces shown in the diagrams, explain what happens to the frictional forces when pore pressure increases between sediment grains and planar surfaces. When the pore pressure increases between sediment grains and planar surfaces, the frictional forces decrease. b) Explain how a heavy or prolonged rainfall could trigger a mass-wasting event. Heavy or prolonged rainfall can trigger a mass-wasting event due to the increased saturation of the soil. As the rainwater infiltrates the ground, it fills the pore spaces between soil particles, reducing the friction and cohesion that hold the soil together. This weakened state makes the soil more susceptible to movement. Additionally, the added weight of the water increases the stress on the slope, further destabilizing it. The combination of reduced friction, decreased cohesion, and increased stress can lead to slope failure, resulting in a mass-wasting event such as a landslide or mudflow. It is important to note that the event’s severity can be influenced by various factors, including the slope angle, soil type, and vegetation cover. Therefore, it is Ex 5 – Mass Wasting

45 crucial to monitor and assess the conditions during heavy or prolonged rainfall to mitigate the risks associated with mass-wasting events. Figure 5.2. Microscopic views of (A) the pore space between sediment grains, and (B) a planar void space, such as a fracture, fault, or bedding or foliation plane. A) B) 3) Mass wasting is also commonly triggered by the vibrational waves associated with earthquakes. Explain how earthquakes can disrupt the frictional and gravitational forces within a slope, causing mass wasting. Earthquakes could disrupt the frictional and gravitational forces within a slope, leading to mass wasting. When an earthquake occurs, the ground shakes and vibrates, causing the particles within the slope to lose their stability. This loss of stability weakens the frictional forces between the particles, making it easier for them to slide and move downhill. Additionally, the gravitational forces acting on the slope can also be affected by an earthquake. The shaking motion can cause the slope to become unstable, leading to a decrease in the gravitational forces holding the particles in place. As a result, the particles can become dislodged and start to move downslope, resulting in mass wasting. Overall, earthquakes have the potential to disrupt both the frictional and gravitational forces within a slope, contributing to mass-wasting events. Ex 5 – Mass Wasting

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46 Part II – Recurring Movement at La Conchita, California In this section, we will take a closer look at the 1995 and 2005 mass wasting events at La Conchita, California. Here the steep coastal terrain combined with poorly consolidated sedimentary material causes the slopes to be inherently unstable. For additional information on the mass wasting at La Conchita, see Case Study 7.1 in the textbook and the following U.S. Geological Survey website:  http://pubs.usgs.gov/of/2005/1067/508of051067.html  http://landslides.usgs.gov/learning/photos/california__u.s._ 4) The blue and yellow ovals on the topographic map in Figure 5.3 show the location of the 1995 and 2005 mass wasting events, respectively. a) Notice the dirt road that traverses the hillside above the town. Using a SW-NE cross- sectional sketch of the land surface, show and explain how building this road would have made the slope less stable. The road looked like it was built on top of a hill or mountain. This road would have been less stable due to it being on a surface that can ease on a slope. b) Describe two ways in which heavy rains would cause the slope to become less stable. The two ways in which heavy rains would cause the slope to become less stable is due to the road collapsing and a landslide occurring due to the rainwater infiltration seeping through the surface and flowing along the slope generating water pressure, which causes the weakening of the rock mass strength, and the cracks in the slope fracture zone can further expand and extend to the top of the slope. The other way is that groundwater or aquifer below the surface generates porewater pressure. Ex 5 – Mass Wasting

47 Figure 5.3 – Topographic map showing the town of La Conchita and the general location of the movement in 1995 (blue) and 2005 (yellow). The contour interval is 20 ft . (From U.S. Geological Survey Open-File Report 2005-1067) 5) Take a blue-colored pencil or marker and carefully outline the area on the photo in Figure 5.4 where the movement took place in 1995. 6) The USGS has classified the 1995 movement at La Conchita as a complex slump . List the physical characteristics you see in the photo that support the interpretation that this is a slump. - Distinct curved scarps visible along the slope - Presence of multiple rotational slumps - Evidence of soil and debris movement downslope - Formation of tension cracks at the top of the slump - Distorted and tilted trees and vegetation - Accumulation of displaced material at the base of the slope - Lack of distinct bedding planes in the displaced material - Absence of clear-cut faulting or folding in the displaced material Ex 5 – Mass Wasting

48 - Geological features consistent with a slump, such as hummocky terrain and internal deformation. 7) Take a red-colored pencil or marker and carefully trace the road as it comes down the hillside. Note that in the 1995 photo, the road can still be found in the middle of the slump. Figure 5.4 – Oblique aerial view of La Conchita after the 1995 event (Courtesy U.S. Geological Survey). Ex 5 – Mass Wasting

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50 Figure 5.5 – Oblique aerial view of La Conchita after the 2005 event (Courtesy U.S. Geological Survey, Mark Reid ) Ex 5 – Mass Wasting 8) The freshly exposed material visible on the photo in Figure 5.5 (previous page) represents the area where renewed movement occurred in 2005. Use a yellow-colored pencil or marker and carefully outline the recent area of movement on the photo. 9) The USGS classified the 2005 event as a debris flow . As before, list the physical characteristics you see in the photo that indicate this is a debris flow. - A large amount of loose sediment and debris visible in the photo - Chaotic and unorganized appearance of the flow - Steep slopes and rugged terrain surrounding the flow - Absence of well-defined channels or banks Ex 5 – Mass Wasting

51 - Presence of boulders and large rocks within the flow - Lack of vegetation or vegetation stripped away by the flow - Evidence of erosion and scouring along the flow path 10) Based on what you see in the two sets of photos, which event-- 1995 or 2005--was more deeply seated and which was shallower? Explain how you can tell. Based on the visual evidence from the two sets of photos, it is clear that the event in 1995 was more deeply seated compared to the event in 2005. This can be determined by examining the depth and extent of the visible impact in the photographs. The 1995 event shows a greater level of excavation and displacement of the surrounding area, indicating a more substantial and profound disturbance. In contrast, the 2005 event appears to have a shallower impact, with less visible excavation and displacement. Overall, the evidence suggests that the event in 1995 was more deeply seated, while the event in 2005 was relatively shallower. 11) From a hazard perspective, which event--the 1995 slump or 2005 debris flow--would have posed the greatest threat to people living at the base of the slope? Explain why. As far as hazards are concerned, the 2005 debris flow would have posed a greater threat to people living at the base of the slope. The reasons are as follows:  In 1995, the landslide was characterized by a deep, coherent slump--an earth flow that deformed plastically and moved slowly enough for people to escape. The 2005 landslide was a shallower remobilization of the same material into a rapid, highly fluid debris flow. Ex 5 – Mass Wasting

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52 In 2005, the rapid movement of the landslide would have given people less time to evacuate, increasing the risk of death and property damage.  As a result of the 2005 landslide, 10 people were killed and dozens of houses were destroyed or damaged. On the other hand, the 1995 landslide buried nine homes but did not cause a loss of life.  Conditions that triggered both landslides: Heavy rainfall initiated both slides. It is possible, however, that the 2005 event was intensified and more significant because it occurred at the end of 15 days of near-record rainfall levels. Therefore, considering these factors, the 2005 debris flow event posed a greater threat to people living at the base of the slope. 12) In both the 1995 and 2005 events, increased pore pressure associated with heavy rains is believed to have served as the triggering mechanism. From Figure 5.6 notice the March 4, 1995, slide occurred approximately one month after the period of heavy rainfall ended. In contrast, on January 10, 2005, the event took place near the end of a period of particularly heavy rainfall. a) Based on the timing of the slides relative to the rainfall pattern, which event-- 1995 or 2005--do you think the water would have infiltrated deeper into the subsurface? Based on the timing of the slides relative to the rainfall pattern, it is likely that the water would have infiltrated deeper into the subsurface in 1995 compared to 2005. b) Explain how this could account for the difference in the volume of earth material that you mapped in question #10. Based on the visual evidence from the two sets of photos, it is evident that there is a difference in the volume of earth material between the 1995 and 2005 events. This difference Ex 5 – Mass Wasting

53 can be attributed to various factors, such as erosion, deposition, and natural processes over time. The 1995 event likely had less earth material due to erosion and transportation, whereas the 2005 event may have experienced deposition and accumulation of earth material. These observations suggest that the volume of earth material can vary over time based on the specific event and the processes that occur. Figure 5.6 – Precipitation record in La Conchita area from October 1994 to March 1995. Figure 5.7 – Precipitation record in La Conchita area from October 2004 to January 2005. Ex 5 – Mass Wasting

54 13) List and describe 3 steps that could be taken that would help minimize the chance of mass wasting and loss of life at La Conchita. The three steps that could be taken that would help minimize the chance of mass wasting and loss of life at La Conchita are to Conduct a comprehensive engineering and geologic investigation of the landslide area, implement physical measures to reduce the danger of a further slide incident and Monitor the slide area and maintain drainage facilities or other structures to alleviate further effects of the slide. Conduct a comprehensive engineering and geologic investigation of the landslide area : This involves understanding the geology and structure of the area, identifying potential weak zones, and assessing the risk of future landslides. Ex 5 – Mass Wasting

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55 Implement physical measures to reduce the danger of a further slide incident: This could include the use of mechanical means such as rock bolts, drainage holes, and physical barriers like retaining walls. For instance, rock bolts drilled a few meters into a rock face can secure loose pieces of material that could pose a hazard 4 . Drainage holes can help to drain water out of a slope, reducing the risk of landslides caused by water saturation 2 . Monitor the slide area and maintain drainage facilities or other structures to alleviate further slide effects: Continuous monitoring by geological and geotechnical engineers is crucial to detect any signs of potential landslides. Maintenance of drainage facilities is also important to prevent water accumulation which can trigger landslides. Ex 5 – Mass Wasting

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