Mullaperiyar Dam -Seismic Risks
Within a span of 2 days, two tremors of mild intensity on 5/11/10 (M2.0) and 6/11/2010 (M2.9) occurred in the 50KM range of Mullaperiyar Dam suddenly caused great anxiety in the minds of people, earthquake scientists and dam engineers. Even though, these earthquakes are minor in nature, the above earthquakes are seen as indication of the increased seismicity of the region surrounding Mullaperiyar Dam, which was once thought completely dormant. These warning signals by nature must taken very seriously, even though the present tremors are not able to create any significant damage to the dam due to its low intensity and large distance from the Mullaperiyar Dam.
The above tremors are coincident with the cosmetic beautification done by Tamil Nadu authorities to the grandma Mullaperiyar Dam to conceal its chronic ailments from the naked eyes. But the ailments of Mullaperiyar Dam are inherent and deep rooted. This dam was constructed during 1886-1895, when the dam engineering was at infancy. The dam is a composite structure with rubble masonry in lime surkhi mortar as facing on upstream and downstream and a lime surkhi concrete inner core. Later in 1980s, a concrete backing of 10 M width was added to the downstream of the old dam.
Structural Weakeness of Mullaperiyar Dam
The Mullaperiyar Dam, aged 116 years, was built of lime surkhi concrete, which occupies almost two-third of its volume. The above concrete is a very low strength concrete. The ‘History of Periyar Project’ by A. T. Mackenzie (1898) illustrate the tests conducted on determining the strength of this concrete “...the block generally beginning to disintegrate with a pressure of from 40 to 50 tons on the square foot...” ie, it is evident that its strength is almost equal to M5 concrete (This designation the letter M refers to the mix and the number to the specified 28 day cube strength of mix in N/mm2), which is having one third of the strength of the roofing concrete (M15) we are using for our homes. This must be compared with the M30 concrete we used for Idukki Arch Dam, which is almost having six times the strength of the lime surkhi concrete used in Mullaperiyar dam. The strength of this concrete has further reduced due to the continuous leaching of lime reported from its inception. Even though two attempts of grouting were attempted in 1930s and 1960s, it was not able to completely fill the cavities formed due to the leaching of lime.
History of Earthquake Design of Dams
While constructing Mullaperiyar dam, the dam engineers were not aware of the how to design the dams against the effect of seismic forces. The first seismic forces were taken care in a preliminary manner in the design of the dam by engineers only in the 1920s . The classic paper by H. M. Westergaard entitled "Water Pressure on Dams During Earthquakes" was published in 1933. In the period 1930-1970, design practice usually considered earthquake effects by simply incorporating in the stability or stress analysis for a dam a static lateral force intended to represent the inertia force induced by the earthquake. Most often this force was expressed as the product of a lateral force coefficient and the force of gravity. The coefficient generally varied, depending on the seismicity of the area in which the dam was located and the judgment of the engineer involved, between values of about 0.05 and 0.15. This method of approach was termed the pseudostatic analysis method, in recognition of the fact that the static lateral forces were only intended to represent the effects of the actual dynamic earthquake forces, which effects could well be substantially different from those of the static forces used in the analysis procedure.
In the 1960s and early 1970s a number of events occurred that caused engineers to reevaluate the adequacy of this approach:
· The observation was made that the pseudostatic method of analysis would not adequately predict the slope failures that occurred at many places in Alaska in the 1964 Alaska earthquake (M 8.3).
· Major cracking occurred in the Koyna Dam, in India, a 340-foot-high concrete gravity dam, subjected to a near-field M 6.4 earthquake in 1967.
· Major cracking developed near the top of the Hsingfengkiang Dam, a 345-foot-high concrete buttress dam in China, as a result of a near-field M 6.1 earthquake in 1962.
· A near failure occurred at the Lower Van Norman (San Fernando) Dam, and significant sliding occurred in the Upper Van Norman (San Fernando) Dam as a result of the San Fernando earthquake of 1971. In spite of the fact that both of these dams were judged to be adequately safe on the basis of pseudostatic analyses made before the earthquake, their performance east doubt on the validity of this approach.
· Accelograph records showed peak accelerations during earthquake shaking greater than 0.3g.
These events, and others, led to an increasing concern that the pseudostatic method of analysis could not always predict the safety of dams against earthquake shaking.
The Earthquake Committee of the International Commission on Large Dams (ICOLD, 1975) recommended that: “As regards low dams in remote areas, they may be designed by the conventional (pseudostatic) method for any type of dam. As regards high gravity or arch dams or embankment dams whose failure may cause loss-of-life or major damage, they should be designed by the conventional method at first and they should then be subjected to dynamic analyses in order to investigate the deficiencies which might be involved in the seismic design of the dam in case the conventional method is used.”
These requirements and recommendations led to a major review of design procedures by many design agencies, as a result of which increasing emphasis was given to the use of dynamic analysis methods.
Need for Site Specific Seismic Hazard Assessment and Dynamic Analysis
Mullaperiyar dam and the site lies in seismic zone-III as per the seismic zoning map of India. This area is close to a large number of major faults in this region, some of which are active and others having the potential to become active. Earthquakes of intensities around 5.0 in Richter scale were experienced in this region in the recent past (5.0 on Richter Scale on 12/12/2000 and 4.8 on 7/01/2001 in the Palai/Erattupetta region, concurrent multiple events called doublets).
This is a high hazard dam as per the hazard potential classification considering the human and economic loss as per the criteria fixed by the Central Water Commission (CWC). In this context, it is highly necessary to get the structural stability of this dam must be verified by proper estimation of the seismic parameters and carrying out detailed dynamic analysis. This becomes imperative considering the high hazard nature of this century old weak dam and consequent catastrophe, if it fails.
As per CWC guidelines itself, stability assessments are to be done utilising in situ properties of the structure and its foundation and pertinent geologic information. It also recommends dynamic analysis and higher seismic coefficient, if the structure is in the proximity of major faults.
CWC’s Certification in 1986
In spite of the above clear guidelines, the CWC in 1986 rushed to the conclusion in the ‘Memorandum of Rehabilitation of Mullaperiyar Dam’ that the Mullaperiyar dam is safe for an FRL of 152 ft after carrying out simple pseudo-statistical method of analysis meant only for preliminary design of dams. Neither the CWC had carried out dynamic analysis taking into consideration the present actual in situ properties of the structure, its foundation, geology and the site specific seismic parameters till date.
In this context, it is worth to note that IS: 1893, ‘The Criteria for Earthquake Resistant Design of Structures’ was revised in the year 1984. As per clause 126.96.36.199 the horizontal seismic co-efficient to be considered in the case of pseudo static analysis is 0.12g in Seismic Zone III for the preliminary analysis of dams. As per clause 7.3.1, this value must be multiplied by 1.5 times in the case of Dams. CWC, totally ignoring the revision of the Indian Standard, carried out the certification of Mullaperiyar Dam using a seismic co-efficient of 0.10g. Also the same Indian Standard clearly recommends site specific seismic hazard assessment study for dams and critical structures (nuclear power stations, major bridges, etc.) considering its magnitude of hazard. It is surprising that CWC completely ignored to carry out site specific seismic hazard assessment. In the above context, Government of Kerala has not concurred with the Memorandum of Rehabilitation and rejected the same.
GSI Recommendation in 1995
Geological Survey of India conducted a geological mapping of the Mullaperiyar Dam area in 1995 as part of the Dam Safety Assurance and Rehabilitation Project aided by the World Bank. Sri. V. Balachandran, Senior Geologist, Geological Survery of India in his report, remarks that “…Based on the available data, the seismic status of the Periyar and Suruliyar-Cumbum lineaments are inferred to be active. However it is desirable to assess the seismogenic capability of these lineaments by instrumentation and monitoring. It is also desirable to study the provision of RCC capping over the structure from the seismic consideration…” But no positive action in this account was taken by Government of Tamil Nadu.
Expert Committee of 2000 not tested, analysed Main Mullaperiyar Dam!
In this context, it may be noted that the Expert Committee of 2000 (EC of 2000) appointed by the Union Ministry of Water Resources, had never carried out any NDT (Non Destructive Tests) or taken or tested any core samples or made any analysis as far as the Main Mullaperiyar Dam is concerned. EC of 2000 analysed using pseudo static method and adopting a zonal seismic co-efficient of 0.12g while checking the Baby Dam. It is an irony that the EC of 2000 concentrated all their efforts to check the Baby Dam, a saddle dam of one third height of the Mullaperiyar Main Dam, which is fully available for upstream sealing. They have taken physical samples, conducted non-destructive tests and structural stability analysis of the Baby Dam and concluded that that the Mullaperiyar reservoir is safe for water levels upto 142 ft.
It may be noted that the EC of 2000 was neither taken any physical samples nor non-destructive tests nor any safety analysis of the Main Dam. Moreover, they have not done any re-analysis of the Main Dam using the revised zonal seismic co-efficient of 0.12g which they used for Baby Dam, instead they simply relied the CWC document of 1986 to conclude that Mullaperiyar Main Dam is safe. Moreover, they have not checked the structural stability of the Baby Dam against the flood conditions, which will clearly render the Baby Dam not safe for an FRL of 136ft.
Combined Study of CESS, Trivandrum and IISc, Bangalore in 2001
In 2001, Kerala had conducted a detailed seismic assessment of the area by the scientists of CESS and a finite element study by Prof. R.N. Iyengar of Indian Institute of Science, Bangalore for assessing the safety of the Mullaperiyar dam for seismic forces (‘Stability of Mullaperiyar Dam in the light of recent earthquakes’ – Final Report by the Expert Committee of Govt. Of Kerala- June 2001).
The CESS scientists have concluded noting the major tremors viz. Nedumkandam 1988 (M4.5), Wadakkancheri 1994 (M4.3) and Earttupetta 2000 (M5.0), 2001 (M4.8) ”...although these tremors have not serious damage, it should be noted that all these earthquakes have occurred during a short span of twelve years, quiet unprecedented in the earthquake history of the State..” They further observed that there is every chance of occurrence of an earthquake with a maximum magnitude of 6.5.
Considering the above observation, detailed dynamic studies of Prof. Iyengar, IISc had indicated that the Mullaperiyar dam is unsafe for water levels above 136ft. The seismic acceleration values adopted by Prof. Iyengar are 0.16g considering Uttarkashi Earthquake and 0.4738g considering the Koyna Earthquake, as possibility these of earthquakes happened in Pennisular India are possible in Mullaperiyar region with almost same geology. He had done detailed dynamic analysis using both scenarios and found that the dam is unsafe for both cases with water level above 136ft. Then, Prof. Iyengar had concluded that it is not prudent to raise the water level in the reservoir from the existing level of 136 feet under any circumstances.
However, the said studies of Prof. Iyengar were rejected by the CWC on the ground that Prof. Iyengar had adopted PGA based on values of Uttarkashi and Koyna Earthquakes. They rejected the seismic inputs used by him citing that those are not based on site specific assessment of seismicity accelogram relevant to tectonics of Mullaperiyar site.
Site Specific Seismic Hazard Assessment by Earthquake Engineering Department of IIT, Roorkee in 2008
Indian Standard specifies that instead of using the general seismic co-efficient applicable for zones, detailed site specific studies must be conducted for special structures like Dam, Nuclear Power Stations, Major Bridges, etc to determine the seismic co-efficient. But neither CWC nor Expert Committee has ever conducted such a study for Mullaperiyar Dam even though they questioned the logic of Prof. R.N. Iyengar’s adoption of seismic co-efficient. Instead they decided to adopt the zonal seismic co-efficient, which are used for general structures.
In the above context that, after detailed deliberations, Government of Kerala decided to entrust the Earthquake Engineering Department of IIT, Roorkee to study the site specific seismic parameters of the Mullaperiyar dam region and to carry out a detailed Structural Stability Analysis using Dynamic Method. The above Institution was selected because they are the premier institute in India for such studies and of high international repute.
It is worth to note that all site specific studies are to be conducted in a radius of 300KM zone of influence by the national and international practises. IIT, Roorkee team have identified 22 major faults in a radius of 300km around Mullaperiyar dam site. Out of this, the Tekkady-Kodaivannalur Fault is identified as the one which will create most devastating effect on the Mullaperiyar dam site, which is capable of producing an earthquake of 6.5 Magnitude in Ritcher scale with a close distance of 16km from dam site. The intensities of maximum possible earthquakes due to the 22 faults and their effect on Mullaperiyar dam site were studied in detail. IIT, Roorkee in their report on Seismic Hazard Assement recommended that the existing Mullaperiyar dam has to be checked for its safety under Maximum Considered Earthquake (MCE) condition since eventuality of its failure may result in huge economic and human loss. Finally Dr. D.K. Paul and Dr. M.L. Sharma of IIT, Roorkee have concluded that the (MCE) possible in Mullaperiyar region will produce a Peak Ground Acceleration (PGA) of 0.21g at the site.