NATURAL disasters like earthquakes, tsunamis, landslides, wildfires, cyclones, droughts, floods etc. continuously pose constant threats to our young, 10,000-years-old, civilization. It is estimated that these hazards have increased recently, in the decade from 1976 to 1985, close to one billion people were affected by disasters and this number had more than doubled in the decade from 1996 to 2005, in which nearly two and a half billion people were affected. In the last decade about 3 billion people were affected and it killed around 750,000 people and cost around US$ 600 billion.
To cope with the natural disasters it is required to understand the science behind these events and plan a comprehensive program to educate the masses. It is important to put more emphasis on pre-disaster planning, rather than on post-disaster reaction. There are ample evidences to demonstrate that lack of scientific awareness has proved fatal in most of the natural disasters we have witnessed so far in the past. For example, the earthquakes in New Orleans and Port au Prince, which had long been recognised as a catastrophe waiting to happen, but somehow even that awareness did not produce the desired effect! Similarly, there are several places in the world, for example South East Asia, where knowledge about earthquakes, tsunamis, landslides, cyclones, floods etc. is still in its infancy. This has caused colossal loss to life and property. For example in Muzaferabad and Aceh (Indonesia) the tragic examples, in which basic scientific ignorance and the inability to translate the acquired knowledge into timely planned action clearly shows the challenges earth sciences face today.
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 are one of the main causes, which trigger landslides and volcanic eruptions also contribute to these movements. One of recent examples of an earthquake induced landslides occurred in Muzaffarabad Pakistan, when a 7.6 magnitude earthquake struck this region in 2005. It is estimated that more than 87,300 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 hazard poses a consistent threat to people in Jammu and Kashmir, and it usually intensifies during rainy seasons. The recent landslide devastation that killing 16 people in Laden village of central Kashmirs Budgam district reminds us the importance of understanding the science of these hazards (see below) so that a proper scientific approach and technology is used to counter any future threat. This will also guide us in understanding why some areas in Jammu and Kashmir are more prone to such hazards and what can be done to avoid or minimize such hazards.
Gravity, the driving force for slope failure
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 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 (see Figure). For example the figure (c) shows these two components acting on a slope, one is parallel to its surface and the other one is perpendicular. The slope parallel component (Gs) will always try to make anything resting on its surface inherently unstable and cause a shear stress parallel to the slope, which pulls the object in the downward direction. However, the perpendicular component of gravity (Gp) 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. Water, makes slopes slippery and therefore, reduces friction, which facilitate the sliding, that is one of the reasons why there is more sliding in wet seasons. Cohesion is the force that keeps material (e.g. a rock) intact. When you pick a rock (e.g. granite) 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 the slope. 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 measured through the Safety Factor, which is the ratio of the resisting force (Shear Strength) to thedriving 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.
—-Author is Senior Lecturer, Department of Applied Geology, School of Engineering and Science, Curtin University, Sarawak Malaysia. Feedback : [email protected]
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