Time Running Out for Digana Locals?
By Faadhila Thassim
The recent tremors in the Digana area close to the periphery of the Victoria reservoir have heightened the sensitivity and concerns about the safety of the communities living in the villages close to and below the dam.
There are concerns that the tremors might be a sign of a large earthquake to come and that they are connected to the local dolomite marble mining operations, to which the Minister of Environment appointed a twelve-member Expert Committee to investigate the incidents.
A series of earth tremors were experienced by the residents in the Digana-Haragama area between 29 August and 5 December 2020.
Task carried out by the committee
The members of the committee collected information pertaining to the tremors from various sources and carried-out preliminary geological, geophysical, and seismological investigations. They made their own observations on marble mining in the area by aerial observations using drones and by making a few explosions in a selected quarry.
Instrumental records of the earth tremors available through seismic stations of the Geological Survey and Mines Bureau (GSMB) at Buddhangala, Hakmana, Mahakanadarawa and Pallekele and seismic sensors of the Mahaweli Authority installed at Kothmale, Randenigala and Victoria dam sites were examined.
The objective of appointing this expert committee was to scrutinise the current situation and investigate possible causes of a series of earth tremors that occurred around the Victoria reservoir, and make recommendations to ensure the dam’s safety and avoid possible damage to vulnerable communities.
Cause of tremors...
According to the committee report, the instrumental records revealed that the ground motions around the Digana area have characteristics of natural tremors and the instruments monitoring the dam have not indicated any unusual behaviour during or after the tremors.
Further the peak ground acceleration caused by the tremors was only one tenth of the acceleration to which the dam has been designed.
The report revealed that the test blasts carried-out at the dolomitic marble mining sites located in the vicinity of the reservoir area did not record any ground motion in the instruments installed on the dam or in the Pallekele station.
The direct and instant effect of mining activities on tremors is trivial, as per the regular practices of blasting procedures. The quarry blast tested at the site to observe such mechanism, has not indicated any significant violation.
On the other hand, several hundred kilograms of explosives have to be detonated to generate a magnitude 2.5-3 earth tremors. However, uncontrolled explosions may create the situation favourable for triggering natural earth tremors.
The report revealed that the blasting of quarrying and mining activities of dolomitic marble around the Victoria reservoir may however, have indirect influence on triggering the earth tremors particularly if the bands in the area are cavernous.
According to Mining Expert Committee Report on Investigation of Recent Earth Tremors in Digana Area, quarrying blasting has triggered earthquakes and earth tremors in other countries. The report on the Digana tremors however, noted that this aspect in the Digana area remains to be investigated in-depth with detailed seismological monitoring, geophysical and geodetic measurements.
The earth tremors in Digana area may also have been caused by ground motions associated with the uplift of the Central Highlands. However, long-term seismic monitoring has to be carried out to produce conclusive evidence.
The rate of such upward motions could be only a few millimetres per year and is possibly highly localised occurring in different parts of the island from time to time. Prof. Vitanage has shown that the benchmarks used for surveying the island have moved over the years during the said uplifting while there were also two such large tremors around Haputale in 1974.
The report also revealed that there has been no sudden water level change observed during this period.
After initial reports of the incident, Geological Survey and Mines Bureau has initiated a ground observation survey, to collect information about any possible impact of the tremors on the ground, buildings, dams and on human settlements.
Also, a drone survey was carried out by University of Peradeniya, in order to collect aerial views, especially to estimate the extent of quarry operations in the nearby area. There are many Dolomitic-Marble quarries around the Digana- Aluthwaththa-Ambakote mining industrial zone and some of them are located within the near buffer zone.
The area is highly fractured due to development of excess stress under the operation of controlled blasts during the allocated time period for conducting blasts .The issue has been brought to the notice from different Government agencies and especially from those living in the vicinity. However, when the economic importance is taken into consideration, operation of mining sites are vital as many quality dolomitic-marble rock belts have been exposed within the area, and those may be of the best quality in the country.
Based on the information processed by acquired data, the morphology over the long running mega quarries were obtained. This would be useful in delineating the quarry sites that make the reservoir vulnerable in several aspects, and make decisions on banning or imposing high priority regulations on the surface and deep operations of mines, evaluating the effect.
During the survey period, a network of individual blast monitoring equipment was deployed, in order to identify the effecting parameters and after a two-week monitoring period, there was no significant signal pattern observed. The network is still in operation. However, if any vibration is activated, due to a tremor, this could be a useful equipment to catch the surface disturbance.
The limited survey that has been conducted in the study area reveals that there have been deeply weathered cavity formations from the near-surface with tendency for deep extension. As the depth penetration ability of the survey system is limited, the deployed techniques in the field are insufficient to provide concrete conclusions on the deep cavity formations, a probable cause for earth tremors.
It is proposed to limit and continuously monitor or eliminate the mining and quarrying activities around the Victoria Dam and the reservoir area to prevent a national disaster.
Disaster Management Preparedness plan for Victoria Reservoir
A detailed disaster management plan has been developed, based on the requirements in terms of dam safety and assessments of downstream vulnerable possibilities. The Mahaweli Authority is the operational agent and focal point in this regard and the disaster management plan has been submitted to the Disaster Management Centre.
Mining Techniques and impacts to be used on surface mining
Blasting technique may be considered as the most economical method used for fragmenting rocks masses in construction, quarry and mining projects. Nonetheless, only 20-30 per cent of the used energy is served for rocks fragmenting and displacing, while the rest is wasted in the form of ground vibration, air blast, noise and fly-rocks.
However, a part of the explosive energy is always exhibited in the form of elastic waves during rock breakage by blasting. These waves travelling in all directions from the blasting site give rise to ground vibrations, which in excess may cause damage to the nearby structures. Continuation of rock blasting without endangering the safety of surrounding structures is of great concern to all.
Over the last two decades, Department of Earth Resources Engineering, University of Moratuwa has been associated with several case studies and research involving rock excavation at different quarry sites close to various engineered structures such as bridges, roads tunnels, buildings and other civil structures, etc.
The experiences helped to outline the general methodology of blasting to ensure safety of structures against blast vibrations. In this methodology of blasting, the safe vibration level that a structure could withstand without producing any damage is adopted, attenuation relations describing propagation characteristics of blast vibrations are developed, safe charge weight per delay for different distances is estimated, and the blasting pattern used for rock excavation is optimised based on field trials with vibration monitoring.
This methodology has been successfully used in a number of construction projects to ensure the safety of a wide spectrum of structures including engineered structures, residential and commercial buildings in urban areas, village houses, historical monuments, etc. The use of this generalised method not only helps to ensure the safety of structures against blast vibrations but also is equally effective in minimising other unwanted effects associated with blasting such as air blast, fly rock, over-breakage.
Shock waves generated due to continuous blasting activities for limestone and rock quarrying around the Victoria Dam and the reservoir area travel through the earth mass and increase pore water pressure. Increase in pore water pressure reduces friction and may trigger earth tremors due to relative motion between blocks of rock whenever natural shear stresses can overcome the friction along suitably oriented planes.
The detonation energy is immediately imparted into the rock and soil, causing fracture of the limestone rock and propagates large cavities and collapsing cavity walls and displacement of soil and this may cause land subsidence. Therefore, it is proposed to limit and continuously monitor or eliminate the mining and quarrying activities around the Victoria Dam and the reservoir area to prevent a national disaster.
Societal impact, vulnerability, risk and disaster preparedness plan and possible long-term plan for a resilient built environment.
Despite the fact that Sri Lanka is located far away from seismically active zones, possibility of damaging phenomena cannot be totally ignored and prediction of such incident is also unlikely.
While prevention of earthquake hazard is impossible, implementation of mitigation and preparedness measures with a view to reducing the damages are vital. In accordance with the Hyogo and Sendai frameworks, comprehensive disaster risk management plans have been formulated and are enacted within Sri Lanka.
These plans attempt to address pre-disaster, during-disaster and post-disaster risk reduction measures with a view to protect already existing communities and facilities, as well as future developments. Disaster risk reduction via development planning, formulation of disaster preparedness plans for response, establishment of multi-hazard early warning systems and dissemination mechanisms, establishment of emergency operations and response mechanisms, relief and temporary shelter management, relocation of affected persons, recovery, rehabilitation and reconstruction and training and public awareness activities are some of the major activities which have been carried out under the national disaster management plans.
However, in the present context, such plan for facing an earthquake hazard is not in focus and hence communities are highly vulnerable for such event. Rapid growth of human population coupled with expansion of human settlements to susceptible areas in the country has increased the exposure of humans to natural hazards.
Disorganised and non-engineered development activities in such areas increase the vulnerability, leading to greater overall risks. Hence, the country is in a necessity of having short-term, medium-term and long-term earthquake risk management plan.
The National Research Organisation is in a strong view to implement resilient built environment against seismicity in future planning purposes to reduce the damages as a long-term plan. Such resilient manuals have already been published by NBRO and possible to update with the requirement of the country as per the magnitude of earthquakes that may possibly hit any part of the island in future.
The report recommended the development of a comprehensive investigation and research programmes on all aspects of geological and seismological and mining impacts and risk reduction with proper mechanisms to implement preparedness plans in order to enhance the scientific understanding and minimise the societal unrest while taking the immediate actions in related to mining procedures.
1. Actions to be taken to safeguard the reservoir buffer zone while sustaining the mining industry by implementing a high prioritised monitoring and rehabilitation system:
- Restrict the mining and quarrying within the 200m limit from the reservoir full supply level.
- Cancel the mining and quarrying licences within 100m limit from the reservoir full supply level
2. Comprehensive investigation and research programmes:
- Deploying a dense short period seismic network to monitor natural anthropogenic micro seismic activities in addition to accelerometers already installed at dam sites.
- Seismic Micro Zonation Mapping with reference to dam safety and building safety.
- Detailed structural geological mapping and geophysical surveys with the instrumentation of micro-gravity, Magneto Telluric and other necessary technologies around the area.
- Establishing of Satellite-based GPS monitoring system particularly geodetic measurements, for studying crustal movements.
- Proper assessments on measuring of the impact due to limestone and other quarry blasting activities in and around the Victoria dam and reservoir area.
- Landslide mapping in all reservoir areas around the central highlands
- Analysis of the relationship between reservoir dynamics and local seismic activities (water level fluctuations, conditions on the banks, down streams)
-Conducting awareness programmes for relevant authorities and general public in vulnerable areas
- Water quality analysis and use of traces for Leakage detection
The Victoria Dam
Victoria dam was designed to bare a maximum peak ground acceleration of 0.1g and the maximum ground acceleration value generated at Victoria dam during the past earth tremors is less than one tenth of that permissible value. However, now the dam is 35 years old, hence it is important to evaluate the present structural conditions.
More than 400 instruments and devices have been installed to monitor the behaviour of the dam and its foundation. Since installation of each instrument is available. A crack in the block 9 has been observed since 2002 and being monitored at regular intervals. This dam has been fully renovated under the Dam Safety Project during 2015/2017 and the operational reliability of spillway gates is enhanced for real time operation to discharge the flood storms during rainy periods.