It is important to understand landslides to predict their locations and avoid damage and loss. This first part of the series is going to give a general overview of the science behind it
LANDSLIDES are frequent in Jammu and Kashmir region. Even as it is largely predictable to mark the locations of future landslide sites, the unfortunate loss of people and disruption due to landside is still a continuous affair. This clearly suggests that landslide hazards are a major problem that must be resolved. Therefore, the motivation of writing this article is intended to educate people about the science of landslides, and the subsequent articles will look at the problem from a scientific perspective with a focus on Jammu and Kashmir, and how to avoid the future disasters.
Landslide is the movement of a mass of rock or debris down a slope. The dimensions of a landslide may be very small or huge, and its movement can be sluggish or very swift. There are various reasons for such movements; for example precipitation (rainfall), topography, geology (rock and soil types) and human activities, can all trigger landslides. Anything affecting slope conditions can cause slope failure, potentially in an area, which is prone to landslides. This includes human induced slope failures, especially during construction, mining etc. Earthquakes and volcanic eruptions also contribute to these mass movements.
One of the recent examples of a large-scale earthquake induced landslide occurrence was mapped in Muzaffarabad in Pakistan, when a 7.6 magnitude earthquake struck this region in 2005. It is estimated that more than 86000 people lost their lives and several millions were rendered homeless. This has affected an area of more than 30,000 Kilometre square and triggered thousands of landslides.
The landslide risk has potentially increased because of the post-seismic landslides and avalanches. Such a risk remains high during wet seasons and particularly in places like Muzaffarabad, Balakot, Garhi and Dopatta. There are several rivers, which are dammed. This could also cause hazard, if not managed properly. Generally, during an earthquake such a situation can occur, if a fault crosses a river. Artificial lakes, dams, waterfalls can form during such a process. This happened during the Kashmir earthquake and was seen in many other places.
Any future town planning must take care of these risks, which are extremely important to save lives and property. This must be particularly done in places like Jammu and Kashmir where earthquake hazards are high, and conditions are right for mass sliding during earthquakes.
Gravity, the driving force
Slopes are generally unstable over a long period of time and therefore, they tend to stabilise by moving to a new and stable condition. This is achieved through the force of gravity, which is the driving force and will always act on the slopes to drag them down. There is always a tug of war between the driving force (gravity) and the resisting force, which is the strength of the rock-mass to upload against the gravity. A slope may fail, if the driving force exceeds the resisting one.
Gravity makes it possible for us to walk on the surface of Earth. When a surface is flat, it is quite easy to walk, because the gravity acts perpendicular to our feet. However, if the surface is inclined, it is hard to walk, because the force of gravity in this condition has two components. These two components act on a slope: one is parallel to its surface and the other one is perpendicular. The slope parallel component will always try to make anything resting on its surface inherently unstable and will cause a shear stress parallel to the slope, which pulls the object in the downward direction. However, the perpendicular component of gravity helps to hold the object in place on the slope.
Friction and Cohesion
The forces resisting movement down the slope are grouped under the term shear strength which includes frictional resistance and cohesion among the particles that make up the object. For example, when we want to push an object over a slope, we need some force to do that and that applied force must overcome the resistance due to friction (which is the area of the object in contact with the sloping surface).
In natural conditions, the applied force is from the gravity and it has to overcome the friction and the cohesion among the particles. If the surface is rough, the friction will be more because the particles of the object are tightly held to the surface. However, when a surface is slippery or polished, it is easy to slide down.
One of the reasons why there is more sliding in wet seasons is because water makes slopes slippery, reduces friction and facilitates the sliding. Cohesion is the force that keeps material (e.g. a rock) intact. When you pick a rock and look at it carefully, you can observe that it is made-up of a number of minerals of different colours. These are held together or interlocked by the forces of cohesion.
When the frictional and cohesive forces become smaller than the shear stress, the object on a slope slips down. Alternatively, when the cohesive forces, which hold rock particles or soil together are weaker than the shear strength, the rock will fall apart under the influence of gravity.
Slope stability can be assessed through the Safety Factor, which is the ratio of the resisting force (Shear Strength) to the driving force (Shear Stress).
Fs = Shear Strength/Shear Stress
If this ratio is slightly more than 1, the slope is close to being unstable, however, if it is significantly greater than 1, (1.5 or 2), then the slope is stable, because the shear strength (resisting and cohesive forces) is much greater than the driving force.
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