Through this article, I try to explain some engineering truths about Mullaperiyar Dam, as a follow-up to my previous article. This article was originally published in arkives.in, by the same author, read the blog for a better reader experience as tables in the medium are not properly visible.
License: This article is licensed under the Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) license.
Preface
I am repeating my claim that I am neither a dam safety expert nor someone who can vouch for the competency of people who explain the dam will break. Expert studies are available from IITs and other institutions who studied the matter. I am also not neither demanding the dam be decommissioned or to be retained, let the experts decide. Here all I am trying to do is educate society on the certain technicalities of the dam, which will help you identify the fearmongers and their personal goals. With the little Civil Engineering knowledge I have and the research I made, stating some facts about the dam, its engineering and technology. The intention is to help society think more scientifically(not pseudo-science) and increase the scientific temperament. Also, I am not getting into the integrity and logic of the lease agrement made between the two states.
If you have not read There is a Bend on Mullaperiyar Dam? Is Google Map True? the first part of this article please read it as well.
Dam Structure
Mullaperiyar Dam is a Gravity Dam which became operational in 1895, 129 years before (as of the article’s publish date). Does it mean the dam is old enough to decommission?
Let’s understand what is a dam. A dam is any structure which holds the water behind it. Usually, the dams are built near the areas with the least cross-sectional areas to optimise the construction of additional structures. The water behind it is called the reservoir. The dams may or may not have gates(openings) to control the outward flow of the water. For those who don’t have the gates on the dam structure, there will be an adjacent dam or some other mechanism capable of controlling the outward flow of the water. Ex: Idukki Arch Dam, Cheruthoni Dam, Kulamav Dam, and Kulamavu Saddle Dams together consist of the reservoir catchment area of 650 Sq km (527 Sq km for Idukki-Cheruthoni + 123 Sq km for Kulamavu). Here the gates or shutters are present only in the Cheruthoni Dam.
Gravity Dam
Gravity Dam is a dam structure built across a waterway to hold the water, utilizing its weight. This means a gravity dam stops the pressure exerted by the water from upstream by the weight of the structure. Gravity dam structures are hence bulky with huge self-weight. While designing a dam structure there is a set of forces considered such as
- Hydrostatic Force
- Vertical Load (Self-Weight)
- Base Pressure
- Seismic Forces
- Thermal Expansion and Contraction
- Wind Loads
Where everything is pretty self-explanatory. The dam foundation majorly has to withstand the load of the dam(self-weight). I am going to touch upon the one with the major impact which is Hydrostatic Force.
Hydrostatic Force
Hydrostatic force on a dam is the total force of the water pushing against the structure. The deeper you go stronger the force as the water column above increases. Hence pressure is at the bottom so as we can see the width of your dam base is huge and heavy. The thickness of the dam at the base and the angle of the upstream face are key design elements here and not the width of the water column.
The figure above explains the force exerted by the water on the gravity dam. This can be summarised as only the height of the water in the reservoir has impact on the force over the dam, while the volume of water in the reservoir doesn’t matter, be it 1m or 100km. Generally the self weight of these dams are multiplied with factor of safety (1.5–2 times), making it way higher than the pressure of water.
What is a Surkhi Mix?
“Mullaperiyar Dam is not made of Concrete it is Surkhi Mix and it is unsafe” — this is a common misconception people possess over the years. But the truth is that none of them exactly knows what the Surkhi Mix is! Surkhi is not a substitute for cement. Surkhi mix is a traditional construction material used in India made by mixing lime with burnt clay, brick dust, or powdered bricks. The mixture is used as a mortar or plaster, particularly in historical structures. Surkhi has been favoured for its durability, workability, and resistance to weathering, and it is known to improve the compressive strength of lime-based mortars.
Here is a comparison of the components of Surkhi mix and M30 grade concrete:
Here is a table giving the components used in ordinary Portland Cement and Surkhi Mix
In Surkhi mix, the proportions of these compounds can vary depending on the source of the materials used (e.g., brick dust, burnt clay).
From the above tables we can understand the usage of Surkhi Mix is not reason to say the structure is unsafe. Surkhi Mix has lime, a major component (~70%) used in the cement, the mortar components are almost same.
Still not convinced?
Here is a list of 10 oldest operational dams worldwide that were constructed using surkhi mix and have a height above 30 meters:
In Mullaperiyar Dam the Surkhi Lime Mortar is used as the binding material to hold the masonry. Even if the surkhi mortar erodes the dam will not break all of a sudden. As explained above the dam can hold the water with the weight of the masonry. This doesnot certify that the dam is safe and will not break even after the surkhi erodes. The water seepage happens through the masonry blocks and that dam losses lesser weight due to the uplift force by the seepage water, and fails. But this process requires a longer period to happen.
Seismic Safety
“The Mullaperiyar Dam is in Seismic Zone III” — another statement by the fearmongers. Lets do a fact check on this. Yes, not only Mullaperiyar Dam but the whole state of Kerala is in the Seismic Zone-III. In India Seismic Zones are defined by the Bureau of Indian Standrads (BIS) and is mentioned in the Indian Standards Code IS-1893 (Part-1) : 2002 — June 2002,Criteria For Earthquake Resistant Design Of Structures. In its fifth revision it combined zone II and zone II into one and name as zone II. Hence theoretically India now has only 4 zone starting from zone II (least risk), zone III, zone IV and zone V (highest risk). The map below shows the zone wise division of the Indian subcontinent.
The studies shows there are no valid proofs or extensive studies done on the region for the Reservoir-Triggered Seismicity (RTS). Though Idukki and Sholayar reservoirs are the list as per a journal paper (referenced below), it says there are no extensive studies done on those.
The map below shows the past historic earthquakes(as per Geological Survey on India records) happened in the district, none of it is near the Mullaperiyar Dam.
As per my knowledge there are no equipments installed at the Mullaperiyar Dam to know the seismicity in the region, we have some equipemnts installed at the Idukki Dam. Having such equipments help us in identifying the slightest of the tremors very closely and studied, helping us to know the trends and actions can be taken before its been too late.
BIS Seismic zones in are defined considering the following factors
- Historical Earthquake Data
- Geological and Tectonic Features
- Soil Type
Analysing the factors we can understand these are the major reasons for an earthquake, hence lower the zone number lower the possibility of earthquake
Can Dams be Repaired?
“Repairing Dams! Have you ever heard of such things? If structures surpass their design age, they should be decomissioned. This was another statement which was passed by the pseudo experts to create the panic. The table below shows some famous strcutures around the world which are operational even after their design life span after proper retrofitting measures.
Retrofitting is the branch of engineering which deals with the strengthening of structures and making it operational. Structures which are not to be feasible to retrofit are decommissioned, and that is the more sustainable way.
Here is a table showing Indian Standards providing guidelines for retrofitting of structures.
Is Mullaperiyar Dam Strong?
I was not able to find the original design document or dam operation manual of the Mullaperiyar Dam, hence all the information here is based on secondary sources. We have seen what is a Gravity Dam and Surkhi Mix Conctere is in detail. We have also seen structures made safe and operational through retrofitting around the world.
Mullaperiyar Dam was repaired using cable anchors and RCC concrete cap as part of the retrofitting of the dam. It is also observed that reports on improper or incomplete methods of cable anchoring were raised by Kerala state. Kerala and Tamil Nadu appointed various institutions to study on the dam and prepare reports that are also one-sided majorly favouring the views of appointed parties.
The issue of Mullaperiyar Dam is a matter of importance and has to be dealt with utmost scientific way and not politically or emotionally. As said “Water for Tamil Nadu and Safety for Kerala”, the solution should be comprehensive to solve concerns of both sides. Hence a combined technical committee from both Kerala and Tamil Nadu should be appointed to study the present condition, and operation of the dam and publish the results from time to time. Only then someone can say about the safety of the dam. One thing is sure the Mullaperiyar dam is not as dangerous as it is spread on social media by the fearmongers. But it is a serious matter to be monitored and called for action.
Some References
There are a lot of technical documents and references listed below, for people who want a crisp fact check and explanation beyond the article at least watch the 2 YouTube videos added below.
- Surkhi Mix
1.1. IS 2250: Code of Practice for Preparation and Use of Masonry Mortars
1.2. Punjab PWD secification on surkhi
1.3. UltraTech Cement Article on Surkhi - Dam Safety Manual by Central Water Commision (CWC)
- Seismicity
3.1. Study on Reservoir Triggered Earthquakes
3.2. SDMA visit report 2011
3.3. An Old Study on Idukki Reservoir
3.3. Seismic Zones of India — Press Release - Idukki Dam Operational Manual
- New Mullaperiyar Dam
5.1. DPR
5.2. EIA for new Dam - Supreme Court verdict of Mullaperiyar
- On Ministry of Water Resources and River Development Website
- YouTube Videos (Malayalam)
8.1. Fact Check about Advocate’s fearmongering by Saji Markose and Sunitha Devadas
8.2. About Gravity Dam by Science 4 Mass
