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Prof. Buddhima Indraratna, Professor of Civil Engineering, University of Wollongong, Australia

It is with interest that I read your article on "Exploitation and Earthslip Posers", about landslides in Sri Lanka. I have also received a large number of digital images of these earthslips from my fellow colleagues in Sri Lanka. In response to your editorial. I have the following information to offer, which will be of future benefit to the people of affected areas and to the relevant authorities in Sri Lanka, responsible for planning and implementation of preventive and remedial actions in connection with catastrophic landslides.

There have been numerous and dedicated efforts to study the landslide problem in Sri Lanka through expertise available at the National Building Research Organisation (NBRO), and I have been invited by the NBRO as a geotechnical engineering consultant under the auspices of UNDP to assist in related training programs.

A substantial amount of landslide hazard zone mapping has been produced at NBRO with the aid of a very focused UNDP programme under the leadership of Dr. Raj Bhandari. During the period 1995-1998, I visited NBRO several times, and I was actively involved in the training of a number of young, NBRO engineers and scientists, apart from contributing to the following efforts directly focused on landslides mitigation.

(a) Identification of the risks and mechanisms of earthslides as well as the nature of vulnerable soils under high internal water pressures, supported by detailed laboratory and field tests:

(b) Conducting field visits and examining the landslide prone areas in several districts, and recommending appropriate ground improvement methods for implementation to reduce the risk of landslides. Some of these schemes including the benefits of sub-surface drainage are elaborated in a handbook on "Ground Improvement with Special Reference to Landslides" which I have produced with the assistance of a few NBRO scientists.

(c) Through Rupavahini (eg. Ayubown Programme) and MTV, comprehensive landslides awareness programmes were launched, and I deliberately discussed the pertinent details of earth instability (causes, symptoms, identification, prediction and consequences) in Sinhala language for the benefit of the masses, including translations to Tamil.

I also recommended to NBRO that these programmes on video format be obtained from the TV stations and after further editing to be distributed to relevant government authorities for strengthening the existing landslides awareness programme already initiated by NBRO.

(d) Using landslides hazard maps, elucidating the need for exercising caution for various regions that are too risky for major development, unless pre-construction soil improvement methods are first put in place in order to reduce the soil instability potential.

These hazard zonation maps available at NBRO are imperative to be used by developers, state organisations and local councils before any form of deforestation and soil excavations can be approved.

Being in Australia, it is difficult for me to assess, how much of the above knowledge gained by NBRO through UNDP projects and other collaborative ventures have actually been put to practice in Sri Lanka.

It is not just deforestation and soil erosion that lead to the catastrophic landslides in Sri Lanka, although it is indeed true that denudation of vegetation reduces the safety of the soil slopes and increases the risk of earthslips. Landslides prone areas in Sri Lanka receive a phenomenal amount of rainfall within a short period of time, often followed by a drought.

The risks of slips are further exacerbated in the hilly terrains containing mainly soft and weak residual soils as well as historical landslide debris (already at a threshold potential threat). Given these circumstances, the main reason for the rapid instability of soils in Sri Lanka is the lack of adequate drainage, both surface and sub-surface.

While the surface drains installed along optimum ground contours can divert the surface water quickly to nearby culverts or streams (thereby preventing excessive infiltration of water to the soil), sub-surface vertical and horizontal drains must be installed in the landslides prone ground to dissipate the build up of excessive internal water pressures in the soil.

The inevitable increase of internal soil water pressures in the absence of sufficient sub-surface drainage rapidly decrease the effective soil strength to zero (i.e. soil then improvising the characteristics of a slurry or suspension) with the blatant consequence of initiating rapid lateral soil movement downhill. Of course, in the presence or a healthy and established vegetation cover (forests and native grasses) the process of building up the internal soil water pressures and subsequent earth movement may be delayed (roots obviously help to dissipate some water pressure by osmotic suction, apart from providing the soil some form of reinforcement like steel in concrete).

Knowing the torrential rainfall events that some districts of Sri Lanka encounter, even the existence of a thick forest cover will not be able to mitigate the risk of landslides to an acceptable level. Therefore, the civil engineers and authorities working in responsible organisations must appreciate and adopt the modern concepts of sub-surface drainage and launch a comprehensive ground improvement scheme to install both vertical and lateral drains.

The most economical and effective practice of sub-surface drainage includes the utilization of prefabricated drains manufactured by numerous companies in Southeast Asia. In most countries including in Asia, various types of geosynthetic (polymeric/vinyl/plastic) drains now replace the historical and conventional versions of natural sand drains and gravel piles, mainly because of the higher speed of installation of synthetic drains, as well as reducing the need for environmental degradation associated with sand mining and quarrying of rock. The field equipment required for installation of these synthetic drains are not sophisticated, and the existing drilling and pile driving equipment in Sri Lanka can be modified easily to install such drains. Some private engineering contractors may already have purchased or developed field equipment capable of installing these geosynthetic drains.

In Sri Lanka, there may be enough qualified civil engineers, geologists, and environmental scientists to plan and design effective technological schemes to reduce the risk of earthslips, and I also believe there may be some state organisations who feel responsible that they must have done more to reduce the risk of such hazards. They must all recognize that landslides in Sri Lanka are no longer just a natural disaster; the fact that the incidence of landslides has clearly increased in accordance with construction intensity proves otherwise. I propose that their action plans in the future would include the following for urgent consideration.

(i) the Institution of engineers, Sri Lanka and National Building Research Organisation in consultation with the relevant state organisations should now play a vital role to directly influence the land developers and earthworks contractors to follow stringent guidelines and specifications tailor-made for landslides mitigation through clear legislative procedures and administrative protocols.

(ii) state organisations and Local Councils in particular must continue to be proactive in educating the local populations by launching landslide awareness programs, distribution of updated information leaflets, regular TV and radio broadcasts, and simplistic hazard awareness programs integrated into the local school system in vulnerable areas.

The residents should be fully aware of the risks that they take, and must be trained to observe the tell-tale symptoms of imminent landslides (sudden vertical cracking of soil, movement and damage to surface drains, sudden tilting of tall trees etc.) in order to evacuate on time.

(iii) Sri Lankan universities must make advanced landslide analysis and preventive measures mandatory in both civil engineering and geology degrees for example in the final year soil mechanics and foundation engineering subject.

In countries such as Hong Kong and Australia, landslides risks have been reduced significantly through implementation of sound government policies and appropriate ground improvement technology, that land areas once considered as 'very high risk' are now being developed for vast housing schemes and industrial plants with no adverse consequences. There is no reason why Sri Lanka cannot follow suit.

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