By Shanika Somatilake –

Shanika Somatilake

The National Building Research Organisation (NBRO) has stated that approximately 30% of Sri Lanka’s land area lies within landslide susceptible terrain. This classification reflects geomorphological and hydrological susceptibility rather than imminent failure. However, for hill country districts such as Badulla, Kandy, Nuwara Eliya, Matale and Kegalle, this assessment has direct and unavoidable implications for future construction policy.

These districts occupy steep terrain formed on deeply weathered residual soils over fractured bedrock and are subject to intense orographic rainfall. Under such conditions, slope stability is governed primarily by rainfall driven pore water pressure, with material strength defining the background susceptibility envelope. Fields investigations repeatedly show that uncontrolled roof water, roadside drains and slope surface runoff are the dominant triggers of shallow failures during extreme rainfall events. Structural failure is therefore most often a hydrological outcome rather than a pure geotechnical one.

Land availability in the hill country is constrained not by national land area but by the limited extent of terrain that can be safely serviced with drainage, access and slope stabilisation. Continued horizontal expansion up slopes increases the total disturbed area, fragments surface and subsurface drainage pathways and raises the probability of progressive slope failure at the settlement and catchment scale.

Each individual housing plot introduces additional cuts, fills and uncontrollable discharge points that cumulatively degrade slope stability beyond what any single intervention can correct.

From a risk engineering perspective, dispersed hillside development constitutes a high consequence, low controllability system. Geotechnical investigation, slope reinforcement and drainage control at the scale of individual houses are economically and institutionally infeasible. As a result, construction proceeds without adequate subsurface data, without rainfall design criteria aligned to extreme event statistics and without enforceable maintenance provisions.

Engineered classification directly addresses these constraints by reducing the number of slope interventions while increasing the level of control applied to each intervention. Concentrating development into fewer geotechnically verified sites allows for comprehensive subsurface investigation including borehole logging, groundwater assessment and slope stability modelling under extreme rainfall conditions. Shared retaining structures, slope drainage systems and controlled stormwater discharge networks can then be designed and maintained as integrated system rather than isolated fixes.

Densification, in the form of apartments, can enable drainage to be treated as a primary structural system rather than a secondary service. In hill country terrain, the capacity to rapidly intercept, convey and safely discharge rainfall determines slope performance during high intensity events. Centralised infrastructure makes inspection, maintenance and instrumentation feasible in a way that dispersed settlement patterns do not. Countries such as Colombia, Brazil and Vietnam have implemented urban planning strategies that involve building densification on safer land, often as a measure to mitigate the risk of landslides in hazard-prone settlements.

This approach requires explicit exclusion of certain terrain from future development. Slopes characterised by recurrent failures, concave morphology, active scars and saturated colluvial deposits cannot be reliably stabilized under non-stationary rainfall regimes. Retiring such land from habitation is therefore a rational engineering decision rather than a planning failure.

Relocation becomes unavoidable under these constraints, but it should be treated as an engineering and fiscal problem rather than a humanitarian one. Planned, phased relocation into denser, stabilised zones reduces long-term disaster expenditure and avoids repeated reconstruction of assets in locations with known failure histories. The cost of relocation is finite. The cost of recurring slope failure is cumulative.

In technical terms, Sri Lanka’s hill country has transitioned from a conditionally stable system to a dynamically sensitive one. Rainfall statistics can no longer be assumed stationary and design assumptions based on historical monsoon behaviour are no longer conservative. Under these conditions, unmanaged spatial expansion increases system fragility, while engineered densification reduces the number of degrees of freedom available for failure.

The policy choice isn’t between development and restriction. It’s between controlled concentration and uncontrolled exposure. From a physical systems perspective, engineered densification is no longer optional. It’s the only configuration compatible with the terrain, the climate and the limits of engineering control.