Introduction Chapter 11 Mass wasting - The downslope movement of material resulting from the force of gravity. Mass Wasting Mass wasting results when the force of gravity acting on a slope exceeds the strength of that Mass wasting often results in the loss of life and property. Landslides are the most well-known examples of mass wasting. Introduction Deadly Landslides Shear strength - Forces that help maintain slope stability January 2011: Flooding and landslides in southeast Brazil kill more than 800 people and leave at least 14,000 homeless. Landslides kill about 25-50 Americans per year and cause more than $2 billion damage in the US annually. Forces The material s strength and cohesion Internal friction between grains Any external support of the slope Opposing a slope s shear strength is the force of gravity. Gravity Acts vertically Steeper slopes are generally less stable. Angle of repose is the steepest angle that a slope can maintain without collapsing; dry sand is ~30 o Whenever the gravitational force acting on a slope exceeds the slope s shear strength, slope failure and thus mass movement occur. 1
Slopes are in dynamic equilibrium. Slopes adjust to changing conditions. If a highway is built on a slope, the equilibrium of the slope is affected. Slope may undergo mass wasting to reestablish equilibrium. What causes mass wasting? Usually more than one factor is involved in the failure of a Factors affecting mass wasting include: Slope Angle Slope angle is probably the major cause of slope failure. Generally the steeper the slope, the less stable it is and the more susceptible it is to failure. What causes mass wasting? Usually more than one factor is involved in the failure of a Highway excavation disturbs the equilibrium of a slope and can result in mass wasting. What causes mass wasting? Usually more than one factor is involved in the failure of a Weathering and Climate Mass wasting is more likely to occur in loose ore poorly consolidated slope material than in bedrock. Water Content Large quantities of water from melting snow or heavy storms increase the likelihood of slope failure. What causes mass wasting? Usually more than one factor is involved in the failure of a What causes mass wasting? Usually more than one factor is involved in the failure of a Water content is an important factor in slope stability at Point Fermin, California. When fine-grained sediments become wet, they become slippery and slopes fail. Vegetation Vegetation can help absorb water from the rains, and its root network can help stabilize a Overloading Overloading, almost always from human activity such as dumping, filling, or piling up of material, can increase water pressure within the slope material, which decreases its shear strength and thus weakens the slope material. 2
Geology and Slope Stability Triggering Mechanisms Generally, rocks that are horizontal or dipping in the opposite direction of a hillside s slope are more stable than the rocks that dip in the same direction as the The most common triggering mechanisms are earthquakes and excessive amounts of water, although anything that disturbs the slope s equilibrium will result in mass wasting. Fig. 11.5, p. 278 Summary of Types of Mass Movement Mass movements are classified according to Classification of Types of Mass Movements 1. Rate of movement (rapid vs. slow) 2. Type of movement (Rockfalls, Slides or Flows) 3. Type of material (rock, soil, or debris). Rapid sudden, visible movement Slow recognized by their effects Table 11.1, p. 266 Rockfalls Falls Rockfalls, the free-fall of rocks, are a common type of mass movement. 3
Slump on the coast of California Slides Slumps and rock slides are the two types of slides. Slumps are rotational slides in which material moves along a curved surface, and usually involve poorly consolidated or unconsolidated materials. Slides Slumps and rock slides are the two types of slides. Laguna Beach, California Rock (or block) slides take place on a planar surface and usually involve solid pieces of rock. Turtle Mountain Rock Slide Alberta, Canada Mudflows, debris flows and earth flows are the three main types of flows. The different types of flows are recognized by their 1. Rates of movement (rapid or slow) 2. Water content 3. Material composition (rock, sediment, or soil). 4
Mudflows Mudflows consist largely of clay- and silt-sized particles, contain more than 30% water, and generally follow preexisting channels. Debris Flows Debris flows contain less water than mudflows and are composed of larger particles. They are more viscous than mud flows and move more slowly. Mudflows are most common in arid and semiarid climates. Earthflows Earthflows move downslope as thick, viscous masses of wet regolith that move more slowly than debris flows or mudflows. Quick-Clays Clays that spontaneously liquefy and flow when disturbed are known as quick-clays. Quick-Clay Slide Quebec, Canada Quick-clay activated by ground shaking Anchorage, Alaska 1964 Earthquake Solifluction The slow downslope movement of water-saturated surface sediment known as solifluction is most common in areas of permafrost. 5
Creep Most expensive and most difficult to stop The slowest and most widespread type of mass wasting is called creep. This involves the imperceptible downslope movement of soil and rock. Creep Most expensive and most difficult to stop Creep produces tilted trees and power poles, broken streets and sidewalks, cracked foundations, and bent rock layers. Complex movements Minimizing the Effects of Mass Wasting A thorough geologic investigation of an area is the most important way to reduce or eliminate hazards. Combinations of different types of mass movements, most involving sliding and flowing, are considered complex movements. Making slope stability maps Recognizing former landslides by observing scarps, open fissures, tilted objects. Collecting bedrock and soil samples. Combine with topographic/slope info Recognizing and Minimizing the Effects of Mass Movements How to eliminating or minimize the effects of mass wasting Regrading slopes Retaining walls, draining excess water, regrading slopes, and planting vegetation can all be used to help stabilize slopes. Draining Excess Water Cut-and-Fill Benching 6
Retaining walls Rock Bolts and Netting 7