Landslide, also called Landslip, is defined as the movement down slope of a mass of rock, debris, earth, or soil (soil being a mixture of earth and debris). Landslides occur when gravitational and other types of shear stresses within a slope exceed the shear strength(resistance to shearing) of the materials that form the slope.
This geological phenomenon which includes a wide range of ground movements, such as rock falls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments. Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability.
In Sri Lanka Landslide Research and Risk Management Division of the National Building Research Organisation (NBRO) has implemented a landslide hazard zonation mapping programme within the 10 landslide prone districts of Kalutara, Galle, Hambantota, Nuwara Eliya, Matale, Kandy, Kegalle, Ratnapura, Matara and Badulla. The maps which display the distribution of the severity of landslide hazard potential in a given area, were intended to be used with associated guidelines as a decision making tool for development of central highlands of the country. It is also used for identification of elements at landslide risk and can be utilized in relocation, rehabilitation, allocation of relief funds and insurance purposes also
Of the 65,000 sq km of land extent of Sri Lanka, an area of nearly 20,000 sq km encompassing 10 districts is prone to landslides. Investigations carried out by National Building Research Organisation indicate that unplanned land use, inappropriate construction methods and wanton human intervention have lead to an increase in landslide susceptibility
Landslides in Sri Lanka
The central highlands of Sri Lanka often experience landslides during the rainy seasons. Watawala landslide in 1992, Hela Uda landslide in 1993, Naketiya landslide in 1997, Mulhalkele in 1986 and Elapatha, Abepura landslide in 2003 can be named as most devastating landslides that occurred in the recent history claiming many lives, damaging the property and having a significant impact on the country’s economy. Out of those, the Hela Uda landslide reactivated twice in 2003 and 2006, fortunately no deaths were recorded during those two reactivations.
Apart from the damage to life and property, several infrastructural as well as economically important facilities have also been affected by landslides, especially water distributor pipes, hydro electricity generating centres, and communication systems. At times, social interests such as educational and health services are also severely disrupted. Moreover, frequent land sliding has threatened the destruction to the environment including the flora and fauna of the areas concerned. Such damage caused to the environment, at times is irreversible and therefore cannot be estimated and perhaps will never be known.
Landslides can be divided into different types depending largely on the manner and speed in which material moves down a slope. The categories and subcategories of the landslides are listed below
- Slides – consist of blocks of material moving on well-defined shear plane. This is divided into two;
- Rotational slides are that move along a concave surface.
- Translational slides are that move parallel to the ground surface.
- Rock Falls are the sudden release of rocks or soils dropping freely through the air with little contact with other surfaces until impact.
- Topples are similar to rock falls except that the initial movement involves a forward rotation of the mass.
- Lateral spreads occur when liquefaction in underlying materials causes surface rocks or soils to move down gentle slopes.
- Creep is the almost imperceptible movement of material down a slope.
Causes of Landslides
Landslides occur when the stability of the slope changes from a stable to an unstable condition. A change in the stability of a slope can be caused by a number of factors including both human and natural causes.
Landslides mainly occur when there is a increase in shear stress and/or decrease in material strength
Increase in shear stress is due to increase of load, increase of lateral pressure, removal of underlying support and regional tilting (due to geological movements).
Decrease in material strength is due to weathering, change in state of consistency and changes in structure
The factors by humans causing landslides is listed below;
- Loosening of the soil caused by vibrations caused by vehicular movement and from heavy machinery
- Destabilization of slopes by deforestation and by construction of buildings
- Blasting for construction and mining
- Earthworks which alters the shape of a slope, or which imposes new loads on an existing slope
The factors caused by action of nature is listed below;
- Earthquakes adding loads to barely stable slope
- Volcanic eruptions
- Liquidification of soil by rain and floods
- Groundwater pressure acting to destabilize the slope
- Loss or absence of vertical vegetative structure, soil nutrients, and soil structure due to natural disasters such as wild fires
- Erosion of the toe of a slope by rivers or ocean waves
The process of lessening the effects of landslides is called Landslide mitigation. Landslide mitigation is done by constructing various man made projects on slopes which are vulnerable to landslides.
There are many landslide mitigations methods and techniques which can be used to reduce and prevent landslides
One of the quickest and easiest ways to prevent a landslide on a slope is to vegetate it. This landslide prevention method works best on slopes that are not too steep or if the movement hasn’t already begun. The roots of the plants anchor tightly to the soil increasing the stability of the soil hence reducing the chances of a landslide
Geometrical modification of the slope is another method in which the slope angle is changed or unstable land is removed. This method drastically reduces the chances of a landslide but the technique is very costly and time consuming.
Drainage system development is the process by which the rain water that falls in the slope area is systematically moved along slope through drains made through stable parts of the slope. This method reduces the moisture content of the slope hence making the slope more stable. This is a very cheap and effective method of landslide prevention.
Reinforcement techniques such as soil nailing, soil anchoring and geogrids can be used to increase the strength of the materials present in the slope.
Construction of retaining walls such as Gabion walls is also a very effective way of preventing landslides since this radically increases the slope stability.
Peradeniya landslide is located at Peradeniya town, by the Colombo-Kandy main road. This landslide occurred in November 2006 destroying a few boutiques in the town and interrupting road traffic for weeks. Peradeniya is the gateway to the historical city of Kandy hence the traffic hugely affected many passengers
The landslide was triggered by a unusually heavy rainfall of 80mm along with strong winds on that day.
The slope was weaken in the past due a combined action of both humans and nature which led to this disaster. There was a brief removal of soil from the base of the slope by human activities which considerably reduced the slope stability. There was also a underground water way which was going through the hill which increased the soil moisture content further reducing the slope stability.
Prevention of Future landslides in Peradeniya
The main cause of the landslide was unstable slope. Mitigation measures for mitigating this landslide include trimming and benching, rock blasting, soil nailing, rock bolting, and construction of surface drainage, and turfing
Construction of concrete retaining walls near the bottom of the slope also decreases the chances of future landslides.
Further educating residents of the area of landslide symptoms such as leaning of trees and other straight structures, unusual dying of vegetation and rock slips so that they can alert the local authorities and public of possible landslide can significantly reduce deaths and property damages from a landslide.
The local authorities should also conduct regular inspections and evaluation of unstable slopes so that further strengthening of slope can be done to prevent future landslides
The reference of the above report in given below;