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by Kapila Dahanayake, Senior Professor of Geology, University of Peradeniya and Director, Postgraduate Institute of Science, Sri Lanka.

Landslides refer to downward movement of soil and rock debris from a high elevation to a low elevation under the action of gravity along a hill slope. Landslides like earthquakes are geological phenomena.

Landslides can occur due to glacial activity, earthquakes, volcanism or even vibrations from nuclear explosions or movement of heavy vehicles. In Sri Lanka, landslides trigger due to high rainfall. During periods of intense rainfall of around 75mm or more within a few hours, landslides (mass movements) occur commonly in certain geologically weak hill slopes of the Central Highlands of Sri Lanka.

Some of these landslides are newly formed ones due mostly to unplanned construction of roads across hill slopes. The others are developed on unstable hill slopes due to reactivation of ancient landslides which had previously remained stabilized for a long period of time. The reactivation of landslides occurs due to disturbances caused by man or by nature or both. If one considers the recent spate of landslides in the hill slopes of different districts of Sri Lanka, it has been noted that most of them are reactivated ones mostly due to intervention of man for construction of houses.

Many networks of roads constructed across such landslides also have disturbed and caused the destabilization of otherwise dormant and stabilized ancient landslides. Such ancient landslides had earlier regained a stable position (of equilibrium) with time due to overgrown vegetation and natural sedimentary consolidation processes before being disturbed subsequently.

During the last three decades landslides have occurred with increasing frequency in the Central, Highlands of Sri Lanka perhaps due to increasing population and people migrating upslope for the construction of their houses on unstable lands. The landslides occur in areas of high relief characterized by ridge and valley topography and underlain by Precambrian meta-sedimentary rock successions.

These are triggered by continuous rainfall characterized by intense rains occurring in short periods of time. The observation on rainfall data of slide affected regions suggest that intense precipitation (200 mm in 12 hours at Watawala in 1992 and 170 mm in 24 hours at Helauda in Ratnapura in 1993 and more recently at Ratnapura in 2003 - 345 mm in about 12 hours) is responsible for these mass movements.

It has been noted that landslides have occurred on days of intense rainfall subsequent to periods of continuous downpour. Most of the killer landslides are reactivated ancient landslides with their axial length sometimes exceeding 1.5 km.

These have occurred with increasing frequency during the last three decades causing damage to life and property. Several hundreds of people have lost their lives and most catastrophes could have been averted if the people were aware of the signals of reactivation of impending landslides and the geological phenomena guiding them.

Majority of killer landslides of the Central Highlands occur on the steep slopes of ridges underlain by weak rocks (weak because slippery clay is formed due to reaction of the mineral feldspar with rain water).

These rocks which are banded in nature also have widespread fractures along which the landslides will commence their movement. After a landslide, the affected area resembles an area struck by an earthquake with extensive openings (cracks) between fracture planes spreading for hundreds of meters in the adjacent areas.

With the onset of next period of rains, water percolates along the opened up cracks and could prepare the terrain for another episode of perhaps a minor landslide movement reactivating the earlier slide. Depending on the local geological/hydrogeological conditions it is possible that the cracks will be sealed by sand/clay during rains. In such instances, the probability of landslides will be much reduced. The writer has observed this phenomenon at the Patulpana landslide (Ratnapura, 1982), Bambarakanda Landslides (Bulathkohupitiya, Kegalle) landslide.

The minimum damage to property and no loss of lives in a recent massive landslide about 2 km long and 400 m wide that occurred in a village of Naketiya in the Badulla District highlights the importance and impact of awareness programmes in educating people.

As recounted by the Grama Niladhari of the area, the programme on landslides conducted by the Science Education Unit of the University of Peradeniya, a few months before the landslide, for the school children of the affected area had helped to create an awareness of the causes that gave rise to this catastrophic landslide. As a result no lives were lost and the damage to property was minimized. Awareness Campaigns are cheap but could save many lives.

Mechanisms of landslides

During the process of movement, the landslide extends uphill by gradual headward caving of the scarp. The release of overlying debris downhill due to landsliding will help to release the stresses in the underlying rocks that are already characterized by one or two systems of fractures (joints). The release triggers wide openings in the joints and promotes rapid downward movement of disjointed blocks.

The effect of separation along the fracture planes could be transmitted on either side of a slide along existing fractures for considerable distances with widespread subsidence and further fracturing of the land surface.

This phenomenon which acts like a chain reaction may spread to tens and hundreds of meters from either flank of a slide. The cracks so developed will be manifested on floors and walls of dwellings and their orientation significantly reflects the fracture patterns of the underlying rocks. The cracks will also prepare the terrain for the future landslide since during continuous rains, water will disappear into such fracture systems causing "underground erosion" within the slopes and resulting in eventual downward movement.

The subsidence of terrain and development of cracks are evident on roadways and railways of the central highlands of Sri Lanka (e.g. Nawalapitiya - Gampola road near the rail track and Mahaweli river, at Cholankanda on Nawalapitiya - Dolosbage road; Galaha - Rupuress road, Watalwala rail track, Balangoda - Haputale road) consequent to occurrence of landslides and subsequent slow movements.

The deep seated cracks developing in the upper reaches of ancient dormant slides at the slightest disturbance to such a slide could trigger catastrophic slides to reestablish equilibrium during continuous rains blocking highways/road tracks or causing damage to life and property. The Watawala landslide is a case in point. In the case of such deep seated slides, even the overlying vegetation will not be able to prevent infiltration along such deep cracks and thus even areas with good forest canopy could be destabilized due to internal erosion or underground percolation of water.

Remedial measures

Most of the landslides indicate some common mechanisms that cause them.

Understanding of such mechanisms is needed for preventive and remedial measures.

Recognition and investigation of unstable areas and the design of preventive treatment should be considered as an essential phase of the preliminary planning and design of a project to reduce potential hazards. Although many a landslide reported in Sri Lanka, had been influenced by man's activity, it should be borne in mind that landslides can occur without any human disturbance.

There are instances of residential developments on old quiescent landslides that have become active again as a result. In highway construction, proposed routes may cross old inactive landslides as evidenced by many instances of subsidence on some key highways of Sri lankan highlands. Therefore, it is important that evidence of such ancient slides is recognised in any development project - be it residential or road construction.

Correct interpretation of geological, geophysical, boring and test data is cardinal for landslide prevention or correction. Familiarity with local conditions and a background of experience in landslide work will assist in exercising sound judgement. Soils derived from rocks are characteristically non-uniform.

The stability of an embankment or excavations influenced by many complex factors-geological, structural and sub-surface water conditions as well climate or man's interference. Therefore, analysis of landslides and design of control treatment cannot be standardized or routine. The prevention of all types of landslides may be accomplished by one or more of the following geotechnical methods.

(a) Reduction of activating forces - draining and removal of surface as well as underground water in the shortest possible time period.

(b) Increasing the forces resisting movement by the construction of suitable retaining structure on geologically stable foundations

(c) Avoidance of slide during construction or elimination of the slide to attain equilibrium

Although most catastrophic landslides of Sri Lanka may not be prevented and if preventable can be very costly. Therefore, people living in landslide-prone terrains can in most cases avoid their ill effects if there is an awareness about the causes, signals and dangers of impending landslides. Since landslides usually occur in continuously wet periods, the dry periods that occur during most time of a given year are relatively safe for living. Construction of dwellings on unstable rock foundations or on ancient landslides should be discouraged. Such constructions are widespread in many districts of the Central Highlands.

Preventive and remedial measures are absolutely necessary in view of the impact on the life and economy of the people concerned resulting from such large landslides. The measures should necessarily include in addition to geotechnical methods, a programme of educating the people of landslide prone areas about signals of impending landslides and precautions to be taken to minimize the effects of a landslide.

Coming to terms

Avoid excavations/road construction on ancient/dormant landslides, which may be currently inactive having attained equilibrium due to abutment against a riverbank or hillock or due to overgrown vegetation. The dormant landslides are characterised by disrupted and irregular drainage patterns and hummocky topography and could be easily identified with little experience even on air photographs.

The fracture systems underlying rock presently covered with soil and/or vegetation should not be exposed to running water, particularly in hilly terrains with histories of landslides. The wanton destruction of forests and setting fire by hooligans to patana grasses on hill slopes of the Central Highlands of Sri Lanka should be stopped perhaps by an awareness program or by stringent legislation.

The denudation of such regions will expose the underlying joints to water percolation that could eventually cause block slides. The construction on hill slopes should only be allowed after proper geological and engineering investigations.

Proper drainage should be maintained particularly in densely populated areas with histories of landslides. There should be regular monitoring of drainage channels so as not to clog them with plastics and polythene or other materials. Overflowing drains on unstable populated terrains could trigger landslides and destruction life and property. Minor slides could be prevented by construction of suitable retaining structures on stable bedrock against unstable slopes.

Large trees with extensive root systems should not be grown on slopes/road cuttings or embankments bordering roads or home gardens. The root system could pierce through the fractures of underlying rocks destabilizing and causing the displacement of rock and soil debris. Such occurrences are common mostly in town areas. Such slopes would remain stable when overgrown with grass. Efforts should be made to stabilise and convert massive landslides into paddy fields by way of construction of suitable drainage systems to remove water systematically as already practised by people living in ancient villages.

People living on hill slopes particularly with histories of landsliding should be on the alert during periods of continuous rains characterized by intense rainfall for signals of impending landslides such as: slanting trees or lamp posts, development of cracks on walls of dwelling houses, sudden sputing of springs on the floors of houses, ground subsidence, disappearance of surface waters along cracks developed on the surface and reappearance downslope as muddy water, sudden withering and death of tall trees such as coconut, arecanut trees.

This is due to displacement of their root systems due to slow downslope movement and sudden slow rolling of big rock boulders downslope.

It has been noted that up-country paddy farmers have over the years transformed massive ancient dormant landslides into contour-type paddy fields with an effective system of drainage. The clay layer prepared for sowing paddy effectively prevents/hinders downward percolation of water. This age-old practice of villagers seem to have controlled many landslides and furthermore, they have transformed them into safe and economically viable ventures.

It should be noted here that in the control of landslides, as highlighted above, a mechanism for quick removal of rain water is cardinal and such water should not be allowed to percolate into the soil.

The water should be removed and diverted to a nearby stream/river by a suitable system of canals or by using electrical pumping in the body region of the landslide or by using horizontal pipes at its foot region. Planting of grass cover to deflect water and prevent percolation can also be practised.

Water absorbing trees also should be planted to stabilize landslides.

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