By Afroz Ahmad Shah
The famous Portuguese Bend landslide in the summer of 1956, when people noticed that the ground beneath them had begun to move, remind us of the potential of such forces to move the ground (Figure, a). It is estimated that a 105-hectare (260-acre) section of the slope was breaking free at a rate of about 2.5 centimetres per day, and also carrying Portuguese Bend with it. However, by 1961, the rate had slowed to about 1 centimetre per day, but by then it already had destroyed or damaged about 154 homes within the slide area.
Similarly, the Hyogoken-Nanbu earthquake of Kobe, Japan in 1995 triggered a total of about 674 landslides, which were mapped within an area of about 700 square kilometres. This quake killed about 5500 people and destroyed 200,000 houses and caused direct economic loss of about 100 billion US dollars. The observed landslides were mainly rock slides, rock falls, and rock/debris avalanches (for example Figure, b). Debris slides, complex slides, and slumps were also found.
These examples emphasise the importance to understand the cause of such mass movements.
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 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 (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 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.
Afroz Ahmad Shah is a research fellow at Earth Observatory of Singapore, Nanyang Technological University, Singapore.