Slop stability

Factors That Influence Slope Stability

GRAVITY The main force responsible for mass wasting is gravity. Gravity is the force that acts everywhere on the Earths surface, pulling everything in a direction toward the center of the Earth. On a flat surface the force of gravity acts downward. So long as the material remains on the flat surface it will not move under the force of gravity. On a slope, the force of gravity can be resolved into two components: a component acting perpendicular to the slope and component acting tangential to the slope.

The Role Of Water

Although water is not always directly involved as the transporting medium in mass wasting processes, it does play an important role. Dry unconsolidated grains will form a pile with a slope angle determined by the angle of repose. The angle of repose is the steepest angle at which a pile of unconsolidated grains remains stable, and is controlled by the frictional contact between the grains. In general for dry materials the angle of repose increases with increasing grain size, but usually lies between about 30° and 37° Slightly wet unconsolidated materials exhibit a very high angle of repose because surface tension between the water and the solid grains tends to hold the grains in place. When the material becomes saturated with water, the angle of repose is reduced to very small values and the material tends to flow like a fluid. This is because the water gets between the grains and eliminates grain-to-grain frictional contact. Another aspect of water that affects slope stability is fluid pressure. In some cases fluid pressure can build in such a way that water can support the weight of the overlying rock mass. When this occurs, friction is reduced, and thus the shear strength holding the material on the slope is also reduced, resulting in slope failure.

Troublesome Earth Materials

  • Liquefaction-Liquefaction occurs when loose sediment becomes oversaturated with water and individual grains loose grain to grain contact with one another as water gets between them.

  • Expansive and Hydro-compacting Soils

  • These are soils that contain a high proportion of a type of clay mineral called smectites or montmorillinites. Such clay minerals expand when they become wet as water enters the crystal structure and increases the volume of the mineral. When such clays dry out, the loss of water causes the volume to decrease and the clays to shrink or compact (This process is referred to as hydro compaction).

  • Sensitive Soils-In some soils the clay minerals are arranged in random fashion, with much pore space between the individual grains. This is often referred to as a “house of cards” structure. Often the grains are held in this position by salts precipitated in the pore space that “glue” the particles together. But this may cause a loss in shear strength of the soil and result in slippage down slope or liquefaction. This is referred to as remolding. Clays that are subject to remolding are called quick clays. Some clays, called thixotropic clays, when left undisturbed can strengthen. But when disturbed they loose their shear strength.

Triggering Events

A mass-wasting event can occur any time a slope becomes unstable. Sometimes, as in the case of creep or solifluction. The slope is unstable all of the time and the process is continuous. But other times, triggering events can occur that cause 8 sudden instability to occur.

  • Shocks-A sudden shock, such as an earthquake may trigger slope instability. Minor shocks like heavy trucks rambling down the road, trees blowing in the wind, or human made explosions can also trigger mass-wasting events.

  • Slope Modification-Modification of a slope either by humans or by natural causes can result in changing the slope angle so that it is no longer at the angle of repose. A mass wasting event can then restore the slope to its angle of repose.

  • Undercutting-Streams eroding their banks or surf action along a coast can undercut a slope making it unstable.

  • Changes in. Hydrologic Characteristic & Heavy rains can saturate regolith reducing grain to grain contact and reducing the angle of repose, thus triggering a mass-wasting event. Heavy rains can also saturate rock and increase its weight. Changes in the groundwater system can increase or decrease fluid pressure in rock and also trigger masswasting events.

  • Volcanic Eruptions-Produce shocks like explosions and earthquakes. They can also cause snow to melt or empty crater lakes, rapidly releasing large amounts of water that can be mixed with regolith to reduce grain to grain contact and result in debris flows. Mudflows. And landslides.

Assessing And Mitigating Masswasting Hazards

As we have seen mass-wasting events can be extremely hazardous and result in extensive loss of life and property. But, in most cases areas that are prone to such hazards can be recognized with some geologic knowledge. Slopes can be stabilized or avoided, and warning systems can be put in place that can minimize such hazards.

  • Because there is usually evidence in the form of distinctive deposits and geologic structures left by recent mass wasting events, it is possible, if resources are available, to construct maps of all areas prone to possible mass-wasting hazards. Planners can use such hazards maps to make decisions about land use policies in such areas or, as will be discussed below, steps can be taken to stabilize slopes to attempt to prevent a disaster. Short-term prediction of mass-wasting events is somewhat more problematical. For earthquake triggered events, the same problems that are inherent in earthquake prediction are present. Slope destabilization and undercutting triggered events require the constant attention of those undertaking or observing the slopes, many of whom are not educated in the problems inherent in such processes. Mass-wasting hazards from volcanic eruptions can be predicted with the same degree of certainty that volcanic eruptions can be predicted, but again, the threat has to be realized and warnings need to be heeded. Hydrologic conditions such as heavy precipitation can be forecast with some certainty, and warnings can be issued to areas that might be susceptible to mass-wasting processes caused by such conditions. Still, it is difficult to know exactly which hill slope of the millions that exist will be vulnerable to an event triggered by heavy rainfall.

  • Prevention and Mitigation: All slopes are susceptible to mass-wasting hazards if a triggering event occurs. Thus, all slopes should be assessed for potential mass wasting hazards. Mass-wasting events can sometimes be avoided by employing engineering techniques to make the s lope more stable. Among them are:

  • Steep slopes can be covered or sprayed with concrete to prevent rock falls.

  • Retaining walls could be built to stabilize a slope.

  • Drainage pipes could be inserted into the slope to more easily allow water to get out and avoid increases in fluid pressure, the po88ibility of liquefaction, or increased weight due to the addition of water.

  • Oversteepened slopes could be graded to reduce the slope to the natural angle of repose.

  • In mountain valleys subject to mudflows, plans could be made to rapidly lower levels of water in human-made reservoirs to catch and trap the mudflows. Some slopes, however, cannot be stabilized. In these cases, humans should avoid these areas or use them for purposes that will not increase susceptibility of lives or property to mass-wasting hazards.