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Soil Salinity Sensors

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Video 1: WP4 Dewpoint Potentiameter
When we talk about the contamination of soils, particularly 2 components that is the chlorideand sulphide become very, very important to be detected in the soils why any guess whypeople should be talking about chloride and sulphide contents. The reasons are very simple,because if you have soils which are having lot of chloride and sulphide what will happenas a geotechnical engineer where I will face problems "Professor - student conversationstarts" foundation could get corroded, very good, excellent. "Professor - student conversationends"So, the moment you do anything related with the concrete or cement and if the soils arevery aggressive having a lot of chloride and sulphide content or sulphate content, we aregoing to attack the foundations and there are several cases of foundation failure, whichhave induced because of these two culprits. So, what is normally done is that you takethe soils and dissolve them in water. Normally we maintain 2:1 liquid solid ratio that means,you take the weight of the soil and 2 times the weight of the soil is in the liquid formis added.And this is stirred for several hours on a hot plate and so that all the chemical speciesget leached out. And once the leaching process is over you measure filter the supernatantor the liquid and then the chloride and sulphide concentrations there are ion exchange kitswhich are available in the market. These kits are similar to the ones which are used forchecking your urine samples. So, this chemistry is same either its biochemistry or chemistryof soil normally call as soil chemistry.This is a sort of a titration and which you are doing and knowing the number of dropsof the ion exchange resin which you put in the solution you can compute what is the concentrationof the contaminants. So, change in the color of the solution is a good identification ofwhat type of chemicals and what is their concentration which is available in the system. This iswhere actually soil salinity sensors are also being used, they have been found to be veryuseful.And you know, there are lot of soil which has saline in the neck in the all over theworld and not one country and because of the salinity, you cannot even use them for agriculturepurpose, alright. So, suppose if I am creating underground facility storage of hydrocarbons,and the first question is what is the level of contamination in the soil because verysoon you will realize that if soils are heavily contaminated, even if you create a very thickconcrete wall as a containment or for the underground tanks.The chances are that all these chemical species will diffuse into the precious material whichyou are storing underground including hydrocarbons and the water because even concrete is porous.So, through pores the diffusion process may occur and the entire utility items which youhave stored might get contaminated any sort of a concentration gradient ultimately wouldcause the diffusive contaminant transport. The way you are dealing with the advectivetransport delta h upon l is i and the hydraulic gradient occurs and the seepage takes placethe same way when concentration gradient develops, these chemical species will migrate and becomea part of the hydrocarbons and the water you are storing.We will discuss in detail so, soil salinity in an indication contamination level of thesoils. The need of the hour is all the saline soils or heavily you know, sea water in throughthat impregnated soils can they be converted can they be rejuvenated will have a big problem,I hope and the entire country is facing the coastal area as well as the locations likeyou know Kutch area deserts, where you will realize a lot of impregnation of the saltwater is taking place in the geomaterials or soil and then this becomes a big questionthat how to construct the infrastructure, how to lay the foundations of the infrastructure.This is the technique which was developed by one of my PhD scholars during his PhD researchDr. Sredeep you have meet him all of you he is a professor at IIT, Guwahati. He did awonderful work to develop a lot of new innovative things in this context and what we have doneis we have used a new equipment not now 2003,4 when he was working with other first timein the country somebody had used this WP4 equipment, we what we call it as a dew pointpotentiometer dew point I think you understand what is dew point this is the moisture orthe water vapors getting condensed on a surface.So, that is the dew point and then potentiometer is an equipment which would measure what isthe volume of the fumes or the vapors which are getting condensed on its surface. WP4is the name of the equipment which is supplied by the company WP4 is normally used for developingsuction versus moisture content characteristics of the geomaterials. Now, this is a pointfrom there the initiation of unsaturated soils starts. Fine.So, those of you who are interested in learning how to characterize the soils for their unsaturatedstate you have to go through the enormous amount of work which our research group hasdone starting from Sneha Kurian this to you know, Kannan Iyer, I think Sneha Jayant andHanumantha Rao, Ravi Ranjan Rakesh, Sredeep and so many people who have used his equipmentand Vikas Thakur. So, these are the guys you should read their papers and try to see howthese equipments have been utilized and how the guidelines have been developed for establishingthe basic characteristic curve for the soils, which is known as soil water characteristiccurve.So, when we talk about the soil suction, there are two components, which everybody talksabout. One is the matrix suction, which is because of the matrix of the soils sometimeswe call this the matrix also. And the second one is the osmotic suction, which is becauseof the presence of the salts in the soil. And nowadays I am working on a third componentof the soil suction any guess what that would be? So, if you analyze this closely, you willfind that the soil matrix represent the physical state of the material, sort of compactiongamma D is very important.And what about this osmotic suction, this represents the chemically state of the material.So what is remaining? Very good. So we have started now talking about the bio suctionand to of M-Tech students they work in this context Asha and Meenu if you see the paperthey have tried to work on and they are established that these type of suctions are very importantand they cannot be ignored in the geomaterials. So, for the time being, I will restrict mydiscussion on metrix suction and the osmotic suction because the first time you are gettinginitiated into these type of discussions.So, you can always build upon once you have the basics. So, soil water characteristicis mostly utilized by the guys who are into the field of agriculture, or water resources.So, if you talk to your colleagues who are doing modeling of reservoirs, irrigation modeling,irrigation scheduling, you know, soil, nutritional analysis and all they would be very eagerto know as SWCC This is also known as soil water retention curve as the SWRC but beinga geotechnical engineer we like to use the term as SWCC rather than as SWRC, but theconnotation is same either soil water retention or soil water characteristic curve.So this essentially is a relationship between the moisture content which could be eithergravimetric or this could be volumetric also, it depends upon what technics you are using.So, if you are taking out a sample from the field, which is undisturbed, you can bringit cut in small pieces and use the WP4 or pressure membrane extractor to obtain thesuction. So psy corresponds to the total suction, and W corresponds to the moisture content.This could be theta also theta is the volumetric measurements. So I can plot a relationshipbetween moisture content and suction.So the way you read this graph is less of the moisture content, what happens to thesuction value, extremely high. And the more the moisture content, the soil, the suctionvalues less. That is true because saturated value will not show you negative suction ornegative pressures. So this characteristic which you are seeing the red color is a sortof a fundamental behavior of the soil. And if you are very good to learn this subjectplease refer to the papers by Vikas Thakur and Sredeep.We are we have talked intensively about measurement of ultra-high suction in the soil and interpretationof SWCC and how to develop SWCC and how to use this SWCC followed by the work of Dr.Hanumantha Rao who has used SWCC to derive unsaturated hydraulic conductivity of thesoils. So many times we have this question what is the state of the development of thissubject? I hope now we are getting the answer. Remember sometime back you are asking thesethings.It is not the first time this has been discussed. This has been under our radar since severalyears and we have done lot of work and I can proudly say that we are the pioneers in workingin these areas nobody thought of these issues in geotechnical engineering as back as 2002,2003, in fact early 2000, so as far as the interpretation is concerned, what I have shownis that the initial portion of this curve was almost flat and then as shown a pointover here as a air entry value. Now, air entry value is the value of the suctionbeyond which the water only, sorry at this point only the air will enter the pores notbefore this. So, the way the cycle valve works, you know, you have the valves there and thenyou pumping air it cannot come out. It is a sort of a reverse process. We are suckingout the water from the soil, but we are making sure that the air does not enter to the soilswhat are the meaning of this. I still assume soil is to be saturated.So, as long as the soils remain saturated, the moisture content is very high, but theirsuction is extremely low. So, this entry value can be a sort of inflection point, you knowhow to opt in the inflection point, you extend this straight line portion and then use extendedthis straight line portion wherever they cut this becomes the air entry value. And airentry value is a unique signature of the soil, which has something to do with the type ofthe soil the gamma D.The particle sizes and of course, the method of measurement and the environmental conditions.So, nowadays the philosophy is like this that if I know that as SWCC I need not do any otherexperiment, long back I had talked about the speculative modeling application of artificialintelligence and geomechanics. I had talked about different types of software's whichare used for speculative modeling. So, if I know one of the parameters or if I knowthe as SWCC curve, I can speculate all other properties.So, air entry value it would be of great help to people like us who are into designing thebarrier systems designing the disposal repositories for the nuclear waste, why? Because soil isunder continuous threat of elevated temperatures and the moisture migration is taking placefrom the soil. So, I would like to study you know, because of this thermal influence, howthe moisture gets changed in the sample and how the properties are changing over a periodof time.So, these are the applications and the implications of this study. The second term which you areobserving here is Wr, which would be very useful for the guys who are into agriculturalsciences, because Wr corresponds to the residual moisture content. So, residual moisture contentcorresponds to the wilting point. Correct. So, people like you who can devise the automaticsprinkling systems if you have SWCC and if this SWCC in the softwares, what you can dois you can link this whole thing with an actuator and that actuator will actuate the irrigationsystem.So, I can do precise irrigation, the moment my set drops below Wr the suction values aregoing to be extremely high I can measure the suction in-situ by using different type ofsensors. And I can put an algorithm in such a manner that the moment moisture contentand the suction drops below a certain value, I get a trigger and that trigger induces theirrigation. This could be a technique for disposal of the toxic waste also, where whatwould you like to do? If because of the inherent heat of the of the disposed waste?The tendency of the geometry is to crack. What I would like to do, I might like to haveto do you know, artificial saturation also keep the system saturated, are you realizingthis. So, this is what the state of affairs is, here you have to do complete engineeringof the systems which are extremely life, I hope you are getting an idea about the applicationswould be you were asking something back. Air entry value is the point beyond which themoisture will drop, but air will not enter into it this also known as the bubbling point.So, this is the point beyond which the moisture loss is taking place, but you are stoppingair to enter and that is used for different applications you are not allowing system tobecome unsaturated, but of course, what is running out. So, this is a sort of equilibriumwhich is existing at the interface of the contractile surface. I was thinking like so,at the shrinkage limit, what happens is the air enters, no slightly below shrinkage limitno never at shrinkage limit also your sample is saturated, clear and you take all at mostcare that the air should never enter.Remember, if you have done haste and if you have fried your sample it is not a correctestimation of shrinking limit sample should never crack number one if sample has crackedso, after taking out that you should really use a lens to see whether the sample has crackedthe error has entered into it or not. If air has entered discard the sample should neverbe used. So, by concept, the shrinkage limit is the one below which is still the soil isfully saturated or at this point the soil remains saturated and air has not enteredinto it that means the cracks have not formed agreed.So let us not complicate the things use this air entry value sir. I told you air entryvalue is the one if you look at this graph, you know the moisture remains constant. So,this is the point where the tendency of the air is just to enter into the system. Andbecause of this what is happening is the moment you increase they will be a decrease in themoisture content and decrease in the moisture content is still holding from air to enterinto the system. Clear. And then after that what is going to happen is if you apply pressureon the soil sample lot of water will come out. So, that is why we call this as the initiationof entry of the air into the soil sample. Type on Google air entry value and bubblingpoint of the membranes and then see the videos.

Video 2: Working Principle and Case Studies
So, working of the WP4 said equipment which is commercially available, this appears tobe a sort of a chamber where you have a dryer in which you can mount the sample over hearin a small cup and there is a block chamber inside. So, if you look into it, this workson the principle of relative humidity. That means if I take a sample which is saturated,and if I leave it beneath the fan what will happen. The moisture will evaporate and thesemoisture in the form of the evaporation if the get condensed on a mirror.That mirror I can use some infrared technique to measure what is the volume of the moisturewhich has got condensed over there. So, this is the calibration which is done in the formof electronics to measure what is the relative humidity of the sample and this relative humiditycan be converted into the suction value directly this is this technique is also known as chilledmirror technology, chilled mirror technology read about it, which gives you the total suction.So, the point here to be noticed is that most of these WP4 type of instruments they workin the range of 0 to 80 MPa but of course the new ones in the market go up to 350 megapascals also of the suction value remember these are negative questions, but what weare proven is that these equipment cannot be utilized for the range of 0 to 1500 KPafor that the pressure membrane extractor works better. So, in short pressure membrane extractorand the WP4 result should be utilized together to get this SWCC more about this please readfrom the papers which have referred to particular the paper which have been written by becauseVikas Thakur. Alright?Now, the pretext what we take from here is that if I take the soils in their native state,which might be contaminated, and if I establish this SWCC and if I washed the soil sampleseveral times by using water or different chemicals, and if I keep on measuring theair suction values will be a difference. And that is what the contamination is now, youmay ask a question that, you know in real life we cannot do this. They will be millionsof tons of the millions of metric cube of the soil which might be contaminated.So, how would you draw a parallel between the laboratory exercise and the exercise?So, the answers could be like, you know, I am using this technique first of all to establishwhat is the level of contamination, whether the soils are contaminated or not. Secondis I can create a small prototype sort of a thing in the lab to show if this is thevolume of the material and this is the level of contamination, how much washing is requiredto get rid of all the chemicals which are adhering to it and then this becomes a thiscan be scaled up in the real life.So, these type of challenges are still a lying ahead of us, we have to overcome them laboratorystudies have been done, but when you have to take them to the field at a bigger scale,you have to be very judicious in applying them. Does this answer to your question thatwhat is the significance and how the practical applications of these techniques have beenmade. If you look at this, it is not a matter of time, what is more interesting is I haveto come up to the tail of the graph.So, for few soils, we have seen that this take months. All these experiments are basicallythe test your nerves patience, particularly when you are working on the organic materials,it is very difficult to lose out moisture from them, even after drying and if you aredrying them, the chances are that they may catch fire or they may distort the volumetricdeformations could be there so becomes very tricky to work on it, but I hope you can understandthat these are the questions which are being answered, which will become tomorrow's guidelinesand tomorrow's code of conduct.So, this is a case study which I wanted to share with you. Incidentally, you only talkedabout the case studies today. And then this is the case study which I am talking aboutmost of the coastal regions, particularly the ports are very much concerned about thequality of the soil and all this comes under the green initiative of government of India.So, most of the boards have to comply with the green ports status.I hope you understand what the meaning of the green ports that means first of all, thisis a zero waste discharge, nothing goes out of the port area in the gaseous form liquidform or the solid form. But if you do the audit of most of the ports will realize becausethe cargo handling and the soil is becoming contaminated, imagine chemicals which arebeing brought in urea particularly different type of assets, petroleum hydrocarbon itemsof day to day life and so on.So, they get spilled over when you stack them and this is becoming a big work for environmentalgeotechnologists to establish that the soil is there are uncontaminated in case they arecontaminated or to clean it up. There are many instances where the fire has taken placeat this place of storage is of these consignments which are known as containers, they have explodedalso and these terrible accidents happen in the factories. And once they meet this fate,you have to clean up the entire area. So, if you characterize the soils of the marinenature, which are in the coast of Bombay.These are the properties of the soils, you have specific gravity, particle size distribution,liquid limits classification is typical average size of a soil, and these chemical propertiescome from the SRF analysis X-ray fluorescence. And then if you look at the chloride and sulphitecontent, these are extremely high and the calculations capacity is also high. So, chloridecontent tells you that the soils are impregnated heavily with this marine seawater, and thenyou have to do something to create the facilities over there. So, what we did is, we take tookthis other initiation point for studies.And we started washing them to nullify the contamination level. So what we will observehere is that we have given number of washings to the soil sample, LS is the liquid to solidratio, process simple, you take certain amount of soil and add 2 times the water. So, thisbecomes L / S is equal 2 measure the chloride content and sulphite content and keep on increasingthe volume of the water to wash the soil sample or you can literally wash it also.So then L / S becomes 4 times, 6 times, 8 times, 10 times and then keep measuring thesethings and then you get something which is within the permissible limits. So, this isone of the techniques which we worked on. And what we have shown here is that if youmeasure the electrical conductivity of the supernatant, and number of washing, what willobserve is that as the number of washing increases, the conductivity of the pore solution decreases.So remember, we were talking about the pore solution and its application to establishwhether the soil itself contaminated or uncontaminated. So, in literal words what you have to do isyou have to bring down these salinity from this point to this point if you want to makethis soil usable for different applications. So, electrical conductor the data has to bemanipulated in terms of the time and money. So, these type of relationships have to bedeveloped.And then if you measure the SWCC of the contaminated and washed soils, so what you will realizeis that these are the peculiar SWCC curves for the contaminated soils and uncontaminatedsoils or the washed soils. And what will realize is for a given moisture content, there isso much suction different. Now this is the osmotic suction. So what I am trying to conveyhere is that soil water characteristic curve itself happens to be an interesting way toestablish the level of contamination of the soil."Professor - student conversation starts" This washing processes are done in large scale.So that is what I told you sometime back that before you launch something in the market,what do you do? The thing? Is that wash water, it is like you ever read our mind? So thatthis is going to be a secondary source of you are right. What is the other technologieswhat you have to do to keep on trying different options? You cannot say that I cannot do it.It is like we are just transferring the pollutants from one so that is right so from your ownmistakes, you learn, is it not? Somebody must have washed soils in the beginning.And then someone else must have asked this question that what you are going to do withthis slurry third person would I was how to dry it up this why fourth person would haveasked how would you reconstitute the samples from the slurry state to this state. Thisis how the growth develops when a lot of questions your questions are valid. Now, coming backto your point the question would be any not to do the washing is you know, a connotation,there could be another interesting way of getting rid of the contaminants by makingthis a stand mark is a is a benchmark.And saying if I do any treatment clear and if my results get superimposed on this graphon the left hand side or right hand side, I know that this is the hundred percent dryingor the cleaning process "Professor - student conversation ends" and where do I stand ifI adopt any other technique and quantify my efforts that what type of decontaminationof the soils I have achieved by other methods, now this is what exactly Ganaraj is doing.Suppose if I give you a heap of mountain like the mountain which you have IIT campus onwhich you climb and see the Mulund.This is the size of the mountains of the red mud, let us say industrial byproducts whichline here and there. Somebody asked me a question, can you neutralize this whole mud so thatI can use it for some purpose. Are you getting this point, this is where this type of studybecomes very useful. So what I will do is I will inject some gases and I will injectsome chemicals and I will inject water. And I will do all sorts of logistics to take outthe effluents so that they do not pollute the world, geoenvironment.And after doing all this, if I take out a sample and do this test, and superimpose itover here, I know that complete washing was this and I am somewhere here it shows that,I have the contaminated the soil, yes the answer is better than saying nothing can bedone and lower down the extent of contamination of the soils. Because these are the solutionswhich have to be given to the industry and the governance. So imagine if I remove allthis chloride and sulphide with pH equal to 13, I am sure you can walk on this materialwith your rubber shoes why?It is got forward if you slip over there and if you touch the material you are gone. Itis very dangerous to work on this type of deposits and go to Australia and there arethe soils which might be having some paid contents and acidity of the soil for thatit could be other 2.52. Again, you can access this places forget about taking of samplewith the help of a drill bit itself will get dissolved in no time for these other challengeswhich people are facing and trying to work on. So this must be giving you an idea aboutthe complexities associated so the people who asked the question where the solutions.