Unstable Land Chapter 8

1. Unstable LandChapter 8

2. You will learn • How driving and resisting forces control slope stability • What factors initiate slope failure, including weather, earthquakes, and slope steepening • The characteristics of the different types of mass movements, including falls, slides, flows, and creep • What land subsidence is and how people’s activities can cause it • How people attempt to protect property against unstable land and help prevent slope failure • How science and engineering help guide land-use decisions and help mitigate the impact of unstable land

3. Slope Stability Basics Resisting forces oppose gravity and work to maintain slope stability. Slopes become unstable when driving forces >> resisting forces. The driving force—gravity Horizontal surface—gravity’s entire force is directed ┴ surface. Inclined surface—gravity’s force has two components: One component, Gp, acts perpendicular to the slope One component, Gs, acts parallel to the slope If the resistance to gravity’s downslope pull is insufficient, bedrock becomes unstable

4. Gravitational Forces on Slopes Figure 8-2 Gravitational Forces (a) On slopes, the force of gravity (Gt) can be divided into 2 components. One component, Gp, acts perpendicular to the slope, pulling the object againstthe slope surface. The other, Gs, acts parallel to the slope, pulling the object down alongthe slope surface. (b) As the slope steepens, Gp diminishes and Gs increases.

5. Resisting Gravity • The existence of slopes is proof that forces do resist gravity • Some rocks—internal strength (interlocking crystals) resists gravity • Broken or weathered—gravity makes loose fragments fall • Forces resisting gravity in loose materials are friction and cohesion • Friction is resistance to movement • Cohesion = force of attractions between grains of material • Varies with material and amount of moisture present • Safety Factor—assesses slope stability • The ratio of the resisting forces to the driving force • Ratio is slightly >1, the slope is close to being unstable • Ratio is significantly >1 the slope is stable • Depends upon material strength, density, volume, slope steepness

6. Friction on Slopes Figure 8-4 Friction Is a Resisting Force Frictional forces between blocks of solid material on potential failure surfaces oppose the force of gravity.

7. Slope Materials and Steepness • Solid materials—high internal strength = form stable slopes • Contacts—boundaries between different rock types = weak zones • Tilted contacts—weak zones for layers to break free • Faults and fractures—weak zones • Slopes with loose or unconsolidated materials—very weak

8. Slope Materials and Steepness (cont.) “Angle of Repose” = maximum slope; depends on size, angularity of grains Slope steepness Most important single factor in determining slope stability Slope gets steeper—the slope-parallel driving force of gravity increases On a shallow incline, friction may prevent failure If downslope pull of gravity << frictional resistance. Steeper—downslope component of gravity >> friction

9. Slope Failure and Lithology Figure 8-5 Slope Failure Discontinuities in rocks that are inclined in the direction of the slope are weak zones that can fail more easily under the force of gravity.

10. Water Content and Vegetation • Water content • Increasing water content of slope materials affects their stability: • Increases weight—increases downslope component of gravity • Ex) Thistle, UT (1983)—heavy rains led to landslide; direct and indirect costs exceeded record $400 million • Decreases internal resisting forces by filling pores • Increasing pore pressure • Decreasing internal strength of material • Vegetation • Plant roots strengthen unconsolidated slope materials • Plants work as pumps to remove water from the slope materials • Decrease the driving force • Increase the resisting forces

11. Types of Unstable Land • Slope failures • How quickly the material moves down the slope • The type of earth material involved • The type of movement • Falls • Fastest mass movements, characterized by the tumbling, rolling, or free fall of materials down a steep slope or cliff • Fractured blocks of rock, or talus, collect at the base • Slides • Material moves downslope along a sloping surface • Rate of movement varies from very slow to very rapid • Common on oversteepened and denuded slopes • Translational slide—slide surface is 2D planar surface

12. Mass Movement Classification Figure 8-9 Classification of Mass Movements Relationships between the type of movement, the type of material, and the rate of movement are used to classify mass movements.

13. Translational Slide Figure 8-12 Translational Slide The largest landslide in U.S. history, the 1925 Gros Ventre slide in Wyoming (a), was a translational slide. The slope failed along a weak and planar sedimentary layer—the Amsden Shale (b).

14. Rotational Slides Figure 8-13 Rotational Slides (a) In a rotational slide, or slump, the downslope movement is along curved surfaces at the base, and often between separate blocks of the slide material as well. (b) This landslide in Japan has the characteristic curved scarps and hummocky surface of a rotational slump.

15. Flows and Creep • Flows • Materials move more like a liquid • Debris flows involve coarse material mixed with water • Speeds of 16 meters per second (36 mph) or greater • Debrisavalanche—highly mobile • Fast flow of debris, water, and air • Mudflows—wet slurries of soil and other fine material • Quick clay soil • Creep • Very slow (1 mm/yr) • Driven by cycles of freezing and thawing • Solifluction = soil creep of upper layers in permafrost regions

16. Creepy Slopes Figure 8-16 Creep Creep on a slope is commonly caused by expansion of surface materials (blue arrows) during freezes (or wetting if expandable clays are present). When the surface material thaws (or dries), gravity pulls the material slightly downslope (red arrows). Figure 8-17 Evidence of Creep Curved tree trunks are evidence of creep in underlying surface materials.

17. COMPLEX MASS MOVEMENTS • Subsidence • Gradual settling or sudden collapse of level/gently sloping land • Regional subsidence • Caused by the compaction of underlying porous material • Regional subsidence from withdrawal of fluids • Karst-related subsidence • Sinkholes (smaller, circular depressions)—develop when the land surface collapses into underground caverns and other cavities. • Karst landscapes—characterized by sinkholes, caves, and streams that disappear below ground or suddenly appear as springs. • Mining-related subsidence • Mining activity can create underground openings near the surface. Collapse or subsidence above these openings can damage roads, buildings, and buried utilities.

18. Formation of Karstlands Figure 8-21 Formation of Karst Interactions of the hydrosphere and geosphere produce the characteristic surface and underground features of karst. Dissolution starts along fractures and enlarges them into channels and openings, producing a complex network of interconnected fissures, channels, and caves in the underground bedrock.

19. Karst Distribution Figure 8-22 Karst Terrain (a) Mass movements can occur on level land where bedrock has been dissolved and overlying material gradually or suddenly collapses into underground openings. This produces undulating land surfaces dimpled with sinkholes and surface depressions characteristic of karst. (b) Soluble bedrock is widespread in the United States.

20. Causes of Land Failure • Natural Causes • Weather • Earthquakes • Wildfires • Slope steepening • People’s activities • People and slope failure • People and subsidence

21. Landslides—California Coast Landslides in La Conchita, CA, in 1995 and 2005.

22. People and Slope Failure People can significantly influence a slope’s stability: • Constructed facilities, like houses, add weight to the slope, increasing driving forces. • Excavating terraces and roads may locally steepen slopes and interrupt normal drainage patterns. Precipitation may either lead to erosion or saturate the soil and increase the driving force. • Septic systems, lawn sprinklers, and leaking water pipes can increase the amount of water that infiltrates the hillside, thereby increasing the driving force and reducing the resisting forces. • Clearing trees, brush, and other vegetation with well-developed deep roots decreases the slope’s resisting forces.

23. Excessive Pumping and Subsidence FIGURE 8-29 Groundwater Pumping and Subsidence Subsidence caused by groundwater pumping is widespread in the United States. Some of the areas most affected are shown on this map.

24. Living with Unstable Land • Assessing slope hazards • Aerial and field surveys • Undeveloped areas—warning signs: • Hummocky features • Bare scars on the landscape • Sharp banks (scarps) or gashes in the surface • Tilted trees with curved trunks. • In developed areas—warning signs: • Cracked sidewalks, fences, roads, foundations • Landslide hazard maps • Landslide susceptibility map—shows relationships among bedrock geology, previous landslides, and slope steepness • Acting on hazard information

25. Geologic Maps and Landslides Figure 8-33 Assessing Landslide Hazards in Kansas

26. Living with Unstable Land (cont.) • Engineering stronger slopes • Decreasing slopes increases stability—decreasing the driving force • Supporting slopes increases their resisting forces • Buttressing • Concrete retaining walls, steel plates bolted together, gabions • Wall of steel or concrete piles across the base of the slope • Surface pins and anchor bolts, soil nails • Nets of strong wire mesh can be laid across the slide area • Check dams—partially block drainages • Decrease flow velocity • Capture heavier material (typically boulders) • Real-time electronic landslide monitoring

27. Dewatering Slopes Figure 8-34 Dewatering Slopes Decreasing the water content of landslide-prone materials can increase resisting forces and help prevent mass movements.

28. Soil Nailing for Slope Stabilization Figure 8-37 Using Soil Nails to Stabilize Slopes Installing soil nails—commonly, steel rods—increases the internal strength and resisting forces of surface materials. This technique is often used to help stabilize steep to vertical walls of surface excavations

29. Living with Subsidence • Regional subsidence • Groundwater withdrawal—from overpumping • Regulate withdrawals • Encourage water conservation • Switch to surface water supplies • Slow the subsidence by: • Using imported surface water • Reservoirs and retention ponds (storm water runoff) • Injection wells to actively recharge aquifers

30. Living with Subsidence (cont.) • Karst-related subsidence • Identify areas especially vulnerable to ground collapse • Steer development away from hazardous areas • Storm water management • sInkholes in a city drain surface water into underground streams • Retention ponds • Concrete boxes with grates in sinkhole basins to prevent clogging • Vertical drywells that drain into the bedrock • Pumping concrete grout into the bedrock may seal cavities • Mining-related subsidence • Surface Mining Control and Reclamation Act • Maps showing underground workings and their depths below the surface can guide decisions about surface land use

31. Infrastructure on Karst Figure 8-41 Controlling Surface Runoff Bowling Green, KY uses drywells to direct storm water into selected underground openings of the karst terrain the city is developed on. Figure 8-42 Collapse of State Trooper Cave’s Roof The sinkhole that developed on Dishman Lane in Bowling Green, Kentucky, cost $1 million to repair.

32. SUMMARY • Slope stability ≈ interaction of driving force (gravity) vs. resisting forces such as friction and cohesion • Changes in slope steepness, material character, water content, and vegetation cover changes can lead to unstable land • When land fails, it can move downslope, subside, or collapse • There are several types of mass movement: falls, slides, flows • Gradual mass movements: creep and subsidence • Karst is a special type of unstable land due to dissolution of rock • Withdrawal of fluids, especially water from the subsurface, leads to regional subsidence

33. SUMMARY (cont.) • Both natural processes and people’s activities initiate surface land movements • Living more safely on unstable land first requires delineating and assessing hazardous areas such as by mapping, and engineering approaches can help stabilize the land • Real-time monitoring and warning systems can help in areas prone to land failure • How people use their understanding of unstable land—where it is and what causes it—is the biggest factor in mitigating these hazards • Land-use decisions are a critical part of living more safely with unstable land