Tuesday, January 22, 2008

Mitigation of the Landslide

Effects of Landslide
1. Loss of human life
2. Economic losses (both direct and indirect)
3. Environmental impacts

As can be seen from the picture, there was most probably a loss of human life, along with possible financial losses in terms of direct and indirect costs. The impact to the environment can be seen as minimal and will not be discussed at length.

As we are aware of the causes of landslides, as can be read from the below entry on the Causes of Landslides, what steps can be taken to reduce the hazardous impacts of landslides? We will now explore the possibilities of how to mitigate a landslide.

Management of Landslide Hazards
1. Hazard Mapping and Zoning

A landslide hazard map is constructed by conducting extensive engineering surveys of the characteristics of a chosen slope, together with evidence of past landslide activity (which can be found in the form of distinctive deposits and geologic structures). The finished maps indicate those areas which are at greatest risk from landslides and if used properly in the planning process, should serve to reduce the landslide hazard by keeping people away from the most vunerable slopes; or by taking steps to stabilize the slope in the attempt to prevent the a disaster.

Aerial photography has been used extensively to produce landslide inventory maps; but air photos are not readily available in all areas. Scientists increasingly rely on global satellite data to help assess the risk for potential landslides.

A possible reason why the landslide shown in the photo below had not been predicted and the people evacuated, could be that the town could not afford to hire professionals to examine the slope. Such services are expensive and require considerable technical expertise, and require planners to respond appropriately to their findings, which is not always the case when pressure for development is high. Urban sprawl onto marginal hillsides in some cities because of the short-term demands for housing and land outweigh the possible problems associated with a landslide, which, of course, might never happen.

2. Modifying the Event

Slope/Cliff Pinning
This could have been a possible method of preventing the landslide from occurring. The way this is done, is by pinning the cliff through the likely shear planes in the cliff. Piles, made up either of metal, plastic of wood are used to pin the sheared block which seems to be unstable and vunerable to potential mass movement. Toe revetments are often built at the foot of the cliff to provide toe support.

This method is relatively inexpensive and if the potential shear plane was effectively identified, the landslide could have been altogether avoided.

However, the downside to cliff pinning is that it can further disturb the cliff materials and at times could accelerate the process as the materials of the slope become less compacted with the intrusion of foreign materials. Also, identification of potential shear planes can prove to be a difficult task.

Slope Drainage
This is the cliff-face method of control which is most widely used in the UK, particularly for cliffs with high clay content. The way it works is that pore water pressures are reduced by drainage lines within the cliff face, field drains and gravel trenches. Overland flow is intercepted by shallow surface drains and vertical drainage removes water from the cliff face as well as the body of the cliff.

The landslide in the photo could have been avoided using this method, as the abundance of foilage on the slope and the surrounding area suggest moderate rainfall - which can be enough to trigger a landslide with a little help from other factors.

3. Modifying the Vulnerability

Public awareness and Education
Achieving widespread awareness of landslide hazards will enable communites and individuals to make informed decisions on where to live, purchase property or locate a business. Local decision-makers will know where to permit construction of residences, business and critical facilities to reduce or zero potential damage from landslide hazards.

If the community which was affected in the photo knew that being at the base of a slope which had a road cut into its side, and possibly had some of its essential toe support removed, would increase the possibility of a landslide, they would most definitely have moved out and away from the base of the slope.

Wednesday, January 16, 2008

Causes of the landslide

Physical factors
1. Steep slopes
Steep slopes are abundant because ongoing tectonic deformation pushes up rock that is too weak to support the steep slopes. Some of the uplifted material is geologically very young sediment that has not consolidated into more resistant rock. However, even older and harder rocks have been sheared and fractured by tectonic movement along the plate boundary, and thus the landslide occurred.

2. Precipitation
Excess water from precipitation that does infiltrate through to the water table drains along the path of least resistance and follows the force of gravity; the faults that exit through the cliff face provide this path of least resistance. These fault and their scarps can be seen in the photo. The rainfall that caused the landslide must be of sufficient duration and strength to raise the field capacity of the soil, the point where under gravity infiltration will equal percolation. At this point, the soil layer will become saturated with additional infiltration. With an abrupt increase in infiltration that exceeds percolation, the pore-pressure between soil particles decreases, and the upper layers of soil become saturated. As water replaces air between the soil granules, the resistance of soil layers to shear stress decreases and the mass of the saturated soil tumbling down the slope creates the landslide.

3. Type of rock
Geologic stability of subsurface soil layers and the slope of the hillside are important factors to consider. The area of the landslide may be underlain by weak, geologically young rock that is being uplifted and severely deformed by compressional forces generated along the tectonic plate boundary. The strata of weak clay and shale may have caused the landslide.

4. Earthquakes
As California is located near plate boundaries, it is often prone to earthquakes. However, in this case, it is unlikely that the cause of the landslide is an earthquake as the houses are still intact.

Human factors

1. The construction of roads
As seen from the photograph, a road has been constructed along the slope. This form of slope modification reduces the shear strength of the slope, hence making the slope more vulnerable to slope failure. The movement of cars along the road may cause vibrations while the mass of the cars may add weight to the slope, increasing the vulnerability of the slope to slope failure.

2. Building of houses
The houses built at the toe of the slope may add weight to the slope, thus increasing shear stress and causing the downward movement of soil particles.

3. Irrigation by people living at the toe of the slope
Irrigation has the tendency to raise the water table and potentially cause weakness in hill slopes. In the area around the toe of the slide water may emanate from beneath the cliffs.


Picture of the landslide at La Conchita, CA, 1995