Week 9

 

Mass

Wasting

Introduction

Mass wasting is the downslope movement of materials under the influence of gravity. Mass wasting can occur at rates of only a few inches per year and cause little damage and no loss of life, or it can occur at over 100 miles per hour and result in great loss of life and property damage. Each year mass wasting causes nearly $2 billion in damage and 30-50 deaths. Mass wasting can often be unpredictable. A mass wasting event in China in 1920 resulted in over 220,000. deaths. Mass wasting events may also be linked to other geological events that trigger the failure of an unstable slope. In 1985 in Nevado del Ruiz, Columbia a volcanic eruption resulted in the melting of a glacier near the top of the mountain The meltwater from the glacier caused a mudflow that flowed down the mountain at almost 20 MPH and killed over 20,000 people and destroyed the town of Armero.

A slump landslide in Teranganu, Malaysia.

Factors that Effect Slope Stability

Although the root cause of mass wasting is the force of gravity, there are a number of other factors that increase the risk of slope failure. When one or more of these risk factors is high, mass wasting is likely to occur.

Slope Angle. The angle at which material slopes is the major determining how much of the force of gravity is directed downslope. If a block of rock weighing 10 pounds is placed on a flat surface, gravity acts vertically and perpendicular to the flat surface, as shown in below (left), and the full force of gravity is directed downward onto the surface. If the slope is rotated, some of the force of gravity is directed, or resolved, perpendicular to the sloped surface, called normal force, and part is resolved parallel to the surface, called shear force. As the angle of the sloped surface increases, the force of gravity remains the same however the amount of that force resolved as shear force increases and the amount resolved as normal force decreases as shown below (center). At some point the ratio of shear/normal force, called the coefficient of sliding friction, reaches a critical level and the block begins to slide down the slope below (right). Every material and slope type has an inherent angle at which the material becomes unstable, called the angle of repose. Most unconsolidated materials, such as soil or sediment, have angles ofbetween 30 and 40 degrees. The angle of repose for solid rock materials depends on the smoothness of the sloped surface and the nature of the rock material, and can vary from 20 – 45 degrees.

Other Factors that Affect Slope Stability

Pore Water. Pore water is the water held within the void spaces, or pores, in the rock or sediment. Pore fluid has two distinct effects on mass wasting risk. Pore water has a tendency to liquefy and disaggregate unconsolidated materials, such as sediment or soil. Pore water tends to destabilize rock layers on sloped surfaces. When pore water is under pressure it reduces the normal force holding rock layer stable on the sloped surface without reducing the shear force that causes the downward motion of the rock.

Material. The type of material within a sloped terrain is another important risk factor.Unconsolidated materials, such as soil and sediment, tend to be more prone to slope failure than rocks. Sand-rich sediment tends to be the least stable because there is more void space and the packing of individual grains is not as close as sediment with clay sand and silt. Layered rocks are more stable than sediment or soil but are less stable than massive igneous rocks. Layered rocks tend to be more friable and can be fragmented and broken away from the bedrock. Massive rocks tend to be less prone to fracturing and fragmentation except when they are highly fractured.

Orientation. The orientation of rock layers has a significant effect on the stability of the slopes containing layered rock units. When rock layers dip in the same direction as the slope, failure of the slope is most likely. Horizontal layering is a somewhat more stable configuration. The most stable configuration is when the rock layering dips into the slope, in the opposite direction of the slope direction.

Vegetation. Vegetation has an anchoring effect on slopes. The roots of the vegetation form a physical anchor for the soil and sediment along the slope. Vegetation also reduces the amount of water in the pore spaces for nutrition. When human development occurs vegetation is often stripped from slopes, increasing the risk of slope failure. The full effects of human development on slope stability will be discussed in detail later in this chapter.

Classification of Mass Wasting

Mass wasting events are classified according to the type of movement, the type of material involved in the movement, and the speed at which the failed material moves. The velocity of mass wasting varies from a few centimeters per year to more than 50 km per hour. The types of material involved in mass wasting include unconsolidated materials such as soil, sediment and rock debris, or consolidated materials such as rock layers or large boulders. The types of movement include flows, slides, and falls. Flows involve the movement of rock fragments, soil, or sediment in a fluid fashion. Slides occur when consolidated layers of rock, soil or sediment slide down slope in discrete blocks. Falls are the rapid movement of materials vertically.

Types of Mass Wasting

Rock Falls. Rock faces or vertical outcrops are prone to rock falls. Rock falls occur when rock material breaks way from an outcrop and falls rapidly to the ground. Rock falls occur when weathering or frost heave weakens the rock in an outcrop along joints or fractures. This is the most rapid form of mass movement and is particularly a problem in road cuts in mountainous areas. Areas prone to rock fall often display accumulation of small rock fragments called talus slopes.

Rock Avalanche. Rock avalanches occur when large masses of rock fragments and boulders move rapidly down a steep slope. This type of movement is a flow because the rock fragments and boulders are unconsolidated and display little cohesion. Rock avalanches are often confined to broad valleys in areas in which there is little or no vegetation or soil cover.

Rock Slides. Rock slides involve the down slope movement of relatively intact rock layers that have broken away from sloped rock outcrops. In most cases rock slides occur when the orientation of the bedding dipping in the same direction as that of the slope or horizontal. The failure plane is often parallel to a bedding or fracture plane, with an orientation similar to that of the slope orientation.

Slumps. Slumps, also known as rotational slides, involve the movement of relatively intact masses of rock or sediment downslope along a curved, concave upward, failure plane, as shown in Figure 9-7. The material involved in the slump rotates along the failure surface as it slides down the slope. In many cases small flows of unconsolidated sediment or soil move out in front of the slump. Slumps usually occur in rock units where there is some unconsolidated or weak rock layers.

Mudflows. Mudflows involve the downslope movement of soil or unconsolidated, clay-rich sediment in a fluid motion. Mudflows occur when the material within the sloped surface are saturated or nearly saturated with water. The slopes are stable when dry, but become unstable when saturated with water. Mudflows are often confined to valleys and can be several hundred feet thick. When the flows exit the valley they expand outward. Areas prone to mudflows often display an uneven, undulating surface due to localized movement of soil and sediment before the main flow.

Debris Flow. Debris flows are similar to mudflows except that they involve soil, sediment and a significant number of boulders. Debris flows occur when high slope angles, water saturation and unconsolidated soil and sediment, and poorly consolidated rock layers combine to create an unstable situation.

Creep. Creep is the slow downslope movement of soil, sediment or rock along more gentle slopes than give rise to the more rapid forms of mass wasting. The surface itself gives little indication that creep is occurring, however features above the surface of the slope provide evidence that slow downslope movement is occurring. Trees display bent after being tilted by the creep movement. Trees tend to grow vertically, towards the sun, and so when they are tilted by downslope movement, the trunks bend to grow vertically again. Gravestones and fences in areas of creep are also are tilted downslope. Rock layers near the surface are also tilted downslope.

Solifluction. Solifluction occurs in cold regions in which the water near the surface freezes and thaws repeatedly. Solifluction is a relatively slow form of mass wasting. When the water near the surface freezes the soil and rock is moved upward and in the downslope direction due to the expansion of the water. When the water thaws the rock and soil moves vertically back to the surface. Although the downslope motion may only be a fraction of an inch for each freeze and thaw cycle, the repeated cycles each day result in significant downslope movement over time.