 Today I will be discussing about the soil water. In short, what water does in the soil mass, what are the different types of waters which are present in the soils, how to differentiate them, what is their role, what is capillarity, what is bedow zone and how to answer the questions which society is asking in particular why infrastructure gets damaged, why buildings are collapsing, why roads are not functioning the way they should have. So, all these questions come or they become quite relevant when you see the perspective from the soil water and what water does in the soil system. When I am back I had talked about the different types of water which are present in soils and this is where I have given you some hint that there is something known as environmental water, there is something known as free water, there is something known as hygroscopic moisture, there is something known as adsorbed soft water, bound water, sometimes we also call it as hydrated water. Now, this is a topic of great interest to the researchers who are working in this subject and particularly those who are more interested in understanding how the environment causes contamination of soils and that is the reason nowadays this is being studied quite much in details and lot of emphasis is on understanding what moisture does. So, it so happens that most of the properties of the soil of the soil are dependent upon the moisture content and this moisture content is nothing but the soil water. Those of you who are interested in advanced research in this area should refer to soil water characteristic curve. Now, this is the most latest thing in the geotechnical engineering, but for undergraduates it is not a good idea to start discussing this right away in the class. So, when I say most of the properties of soil be it shear strength, be it compressibility, be it consolidation or be it permeability. Now, permeability has different roles to play in different subjects. I think I have been emphasizing upon this starting from petroleum to the motion of any fluid in the geomaterials to the water which is of interest to geotechnical engineers and from here we say that if soils are cohesive or non-cohesive or granular all their characteristics are bound to change because the water holding capacity is going to be different and soil water characteristic curve is nothing but a quantification of what is the water holding capacity of the soils. So, if I talk about the soil moisture, soil water is a general term and soil moisture is a particular term. Soil moisture is valid mostly for the fine-grained materials not for the coarse-grained materials. The first category of the soil moisture is the one which is adhesion characteristics. So, depending the mechanism of adhesion of moisture on the fine-grained materials is adhesion. Sometimes, we also call this as adsorbed. So, those of you who might get a chance to work in, you know, research laboratories, they are the terminology used is adsorbed or absorbed. Mostly, this is valid for the collides and for a quick understanding, collides is the particle size which is less than 1 micron, all right. Normally, we talk about the clay fraction up to 2 microns, collides are at the 1. So, it so happens that if I take a grain of a fine-grained material, this is the soil grain. The moment this comes in contact with water, there will be a adsorbed layer formation, all right. This is also known as adsorbed water. Sometimes people use this as bound water. Sometimes they also use the word hydrated water. In short, most of the adsorption is because of the surface characteristics of the grains. The surface are hyperactive, if you remember. Their surface area is going to be extremely high and hence their affinity is towards anything moisture or contaminant which comes in contact with it to adhere it on to itself. Now, in between the two particles or the grains of the soil, we will have free water. So, all this is free water, sometimes we also call this as gravity water, sometimes we also call this as a pore water, all right. So, it is the first time I am using the term pore water. Most of the characteristics of the soils would directly depend upon the pore water or in other words, the pore water which is the water which present in the pores of the soil mass would be controlling all of them and henceforth our focus would be to understand how the pore to pressure characteristics control all the fundamental properties of the soils. Adsorbed water also contains sometimes the cohesive water solidified water, all right. So, the combination of the two is adsorbed water which is a fundamental characteristics of the coloids. When you get a chance to work in this sub clay particle ranges, as I was talking about in petroleum industry, normally these type of deposits are quite frequently encountered where you are dealing with the sub fraction of the soil particles which fall in the category of coloids less than 1 micron. Now, if you compare the bound water and the free water with the free water, what you will observe is that the boiling point, what is your guess, bound water boiling point will be more or less than the free water. It is all bound by the particle and particle is having lot of electromagnetic forces. So, this is going to be higher than the free water, all right. So if I say that these are the characteristics, so if I write like this is okay, that means the bound water boiling point is higher than the free water. One of the ways to understand the difference between bound and free water would be if I take a sponge, if I soak it in water, if I take it out of the water column and just keep it on the surface, all right. So whatever gets drained out because of the gravity is free water, all right. This is not bound by any particle, there is no electromagnetic force acting on this. So this is just free to follow the gravity. However, the pore water is the one which still remains in the pores because of the capillary action and this we are going to discuss in details today. The freezing point, now what is your understanding, what will happen to the freezing point? Freezing point follows the reverse trend, all right. So the bound water freezing point is going to be less than the bound water, sorry bound water, the freezing point of the free water because this water is free, follows the gravity and hence it would get frozen first as compared to the bound water. Now if we talk about the viscosity, these concepts you require in R and D. So you cannot say that these concepts should not be discussed, these are very, very important concepts. So viscosity of the bound water is going to be more than the free water. Most of the industrial processes where you form different types of, you know, products, they all depend upon this type of series. Then surface tension, so surface tension also of the bound water is greater than free water. Those of you might get a chance to work in the field of agricultural sciences, lot of research is going on and lot of focus of interest is nowadays agriculture because of, you know, food assurance. The population has increased, everybody is talking about productivity of the soil. And by our human intervention what we have done, we have destroyed the productivity of the soil. So they call it as a food security, check it on net how much work is being done internationally. Now I hope you can realize when I talk about the food security for a country, for a population, these concepts will become very important because if you have most of the time the free water which is of not much of use and unless you design the systems, the chances are that this water might be available to the plants freely, but at the same time if the soil happens to be coarse material it may drain out. So you do irrigation and water is a commodity, you do not have much water. Question is can bound water be utilized for growing plants, hydrophonics? You go to 5 star hotels, what do you see there? In glass, crushed glass they are producing plants, is it not? So these are the plantations which are soil less. That means they may dose some chemicals, some activators, some raisins, some zeolites which have a very high tendency to retain moisture and this moisture slowly diffuses through the root zone. So this is a big subject, is this okay? I am not going into the details of this. So it is the free water which is ultimately going to guide all these properties. So free water and the pore water is going to which is the same thing is going to guide most of the properties of the soils. The second classification is remember this is the first type of water which we talk about, adsorbed. The second one is absorbs, absorbed water. There is a difference between the terminology which I am using, this is the adhesion or adsorbed and this is absorbed, any idea what this would be? So you take the rasugulla, take it out of the pan in which you are making it or you have soaked it in the syrup, squeeze it, those who are diabetic what do they do? They will squeeze it, squeeze out all the syrup and then they eat it. So the free syrup which is equivalent to the free water has been displaced. But still the rasugulla feels sweet, why? Because some amount of sugar has got absorbed into the pores, alright? So this water is the same like that. The wilting point has something to do with the absorbed moisture content which we, with the terminology which we use normally in agriculture engineering. Absorbed moisture content can be divided into two parts, hygroscopic and capillary. Remember sometime back when we were talking about the moisture content, determination of the soils, I was using these terminologies in a city like Bombay where the humidity is extremely high. If I dry a soil in the oven, particularly fine grained material and if I bring it back to the room temperature, what is going to happen? Because of the surface activity, a layer of hygroscopic moisture is going to surround the particles. So hygroscopic moisture content is equivalent to the moisture which normally spoils your table salt, correct? So this is because of the humidity, because of the hygroscopic behavior of the clay mineral or the material which makes it proactive to absorb moisture, correct? Another difference is adsorption is mostly a surface phenomena. Absorption could include surface as well as intermolecular and that is why I gave you logic. So without the syrup, Rasagulla is still sweet, is still sweet, correct? So absorb moisture will have two components, adsorbed and sorbed. So option becomes important when you are talking about contaminants in soil which is not the scope of the discussion in this class. Hygroscopic is the environmental moisture. Hygrow is, type out the word hygrow, what is the meaning of hygrow in your mobile? Hygrose is water, is it not? So what is hygrow? When you say anhydrous compound, hydrous compound, you understand the difference between hydrous and anhydrous? So what is hygrow, relating to moisture, simple moisture on the surface. So if you consider a small particle and because of the humidity in the environment, this particle uptakes some moisture that becomes hygroscopic. There is a difference, adsorbed water will have some sort of a energy of bending, clear? This is something which is not going to have any energy of bending, it is just free water. You take the soil, put it beneath the, a lamp of 40 watt, 20 watt, it will simply evaporate. So this is not a bound water, this is a bound water, remember, correct? So when you say bound water, lot of energies associated with the binding of molecules are prevalent. See the third type of the water is free attic water. You must have come across this terminology in your irrigation engineering courses, is it not? Where you are designing dams and when you talk about the storage capacity of the dam and so on. Have you come across this sort or not, free attic? What is free attic? Check on the net, P-H-R-E-A-T-I-C free attic, which follows gravity, atmospheric, alright? So suppose if I give you an example that there is a hill and then this landscape becomes like this. See let us say ocean or a lake or it could be a river, alright? When the rainfall occurs, what will happen? Some portion of the rainfall will become runoff. So this is the rainfall and some portion of the rain would get percolated, alright? So this is the percolation. Now hypothetically if I draw a plane like this towards the land, this is the land side, this is the water body side. This zone is known as a Bedou's zone, you remember we have talked about this, unsaturated zone. However, this line is known as a free attic line. I hope now you can understand. What is the meaning of free attic world? Some of you could locate it, yes, and exactly. So what I have done, intentionally I have matched this line with the free water surface. So this piece, the gravity water, whatever gets runoff, whatever gets percolated, ultimately this will come and meet the free attic line. This is the gravitational water. Atmospheric conditions prevail over here. So the pressure all along this surface are going to be, pressure is going to be one atmosphere, always, fine? Now we will see how we are going to utilize this concept. So about the free attic water, yes please. What is? The dominant force. This is only a surface phenomena, thin layer of water which comes only because of the surface activity. Surface activity could be your cation exchange capacity or the surface area. So you have lot of parking places which are available on a fine particle because surface is very high. Water molecules just come and sit over there, there is no bonding of any type. So the moment you heat it, the moment you put it in a wind, everything will get blown off. Exhaust water also means? Exhaust water has energy because this is having the energy of what is known as the binding or the bounding. So that is why we use the word bound water. So this water is bounded to the soil particle because of some energy. It could be wind of all forces, is this fine? Yes, pressure at the free attic line is going to be always one atmosphere, natural STP conditions, standard temperature pressure condition. So the, I am going to talk about free attic surface. So basically free attic surface is the gravitational water, whatever gets percolated ultimately settles down over here because the level of the water has to be maintained, fine? Number 2, on the free attic line, the pressure conditions are atmospheric. This is how you define it because this is, see what is the pressure at this point? Atmospheric pressure. So it has to get balanced with this. So imagine you have in the offshore environment water and ultimately it has to come and meet the ground which becomes a ground water table. So the ground water table also has the pressure conditions same as the atmospheric conditions, is this okay? Now concentrate on what I am going to write. The free attic water is subjected to gravity, capillary action does not play an important role here. That means by virtue whatever you check on the net everything is going to be saturated. So this is the widow zone, this becomes your saturated zone. One of the ways to define the free attic line is, this is the boundary between saturated soil mass and the unsaturated soil mass, clear? And today after sometime we are going to discuss what is the state of stress in the widow zone and in the saturated zone. So then your funda will become very clear. Here the state of stress is going to be tensile and beneath the water table everything is going to be compressive. Is this part clear? Let us begin this, I think then we will catch it off. Number two, the free attic water will saturate the pores of the soil completely saturated situation. Beneath this point anywhere in the soil mass the pore water pressure will be more than atmospheric pressure. Have you understood this? So suppose if I consider a point over here, at point P the pore water pressure is going to be atmospheric pressure plus the pressure which is existing because of this much column of the soil and water. Another property of the free attic line is that it tends to flow laterally, why? After few days when we will be talking about the seepage in the soil mass, we will realize that the movement of the ground water is always in the horizontal direction because permeabilities are more as compared to the up and down. And this is a blessing in disguise, nature has created a system like this where the water table will not change in the vertical direction much but it will keep, it moves only in the lateral direction. So one of the properties of free attic line is that the water moves laterally, is this okay? So I will write a condition here, the free attic water is the gravitational water and the pressure at this point is equal to 180 M, 1 atmosphere. As a geotechnical engineer, most of my interest should be in this as per the undergraduate or the beginning of the subject or the initiation of the subject is concerned. But later on I think you will realize that in country, our own country, there are not many places where you have enough water and the country is having acute crisis of water. That means the widow zone becomes more important to study as compared to the saturated zone but I am not going to discuss this. So when you talk about the mechanics of the widow zone, this fundamental behavior of the soil becomes very important. But no, just hold on for few minutes, as I said in the widow zone the pressure is going to be because of the capillarity, so I am just going to come on that. So by virtue of the basic difference is anything which is the water table would be either dry. If it is a granular material, water cannot stand there because of gravity water goes out or if it is a fine-grained material, you remember we have discussed this case earlier also. If this is a fine-grained material, capillary action would be there, we will be discussing about this. And hence some portion of the widow zone would be either variably saturated or partially saturated or unsaturated, clear? So these are three cases which we have created out of this. So this dynamics becomes more important to understand for the present day technologists as compared to this, yes or then now you are complicating the whole game. But good, I am happy that you have asked this question. So now the sea water intrusion will start, the density driven flow will take place. So you have to now sit in my environmental geomechanics course. But it is a good question. So why this decant peninsular is having problem of drinking water? Because salt water diffuses into the freshwater, alright? And this becomes a diffusive contaminant transport, okay? So coming back to your point, anything which is below the phreatic line is going to be positively stressed. Anything above the phreatic line is going to be negatively stressed and we will see why it happens. Any other question? Yes please. Use the mic. How we theoretically compare the freezing point of two materials? I mean you can do calorimetry, remember in your 10 plus 2 calorimetry which you have studied, you were talking about the heat of reaction and if you really want to find out, Vinny can you answer some question? She is an expert in freezing of soils and freezing of the pore solutions. You can also add to this question. So are you particularly asking about the bound and free water, how can you measure the freezing point? Yes. So very in very simple thing which you can observe during the experiment is like you remember that whenever phase change is happening, freezing, boiling the temperature remains constant. So for a longer duration of time during freezing, for free water it might be zero but for bound water it will be like minus 2, minus 4. You will see the temperature plateau in that region. Thermodynamics of the system. Yeah. Use the word thermodynamics. They can follow. They have done a course in thermodynamics. Yeah. And as I said. How much energy you have to extract out to make the system freeze is what you are asking. Yeah. And you can use calorimetry also if you want to be very specific and quantify the phenomena. Actually these are the on the basis of practical knowledge, how we can, is there something so that we can get the difference by any theoretical knowledge or a guess? Can you please repeat your question? Basically what you are saying that it is after the practical experiment. Is there something to guess the freezing point of 2 substance? Not the freezing, guess the freezing point but to compare the freezing point of 2 substance. What salt does in water when you freeze it? What sodium chloride does to the freezing point? Excellent. So, I think now you can connect. Compare the freezing point of 2 substances. Am I right? Is that your question? And how to do this in calorimetry? That is your another question. Without doing experiments. Without, ok. Do you know what it means by freezing? That first of all we have gone through, right. What happens when freezing is happening to the state of the material? It is energy will increase or it is energy will decrease? You are taking out the, you are extracting the energy out of the system. Correct. So, this is the only way you can answer this question. This is what he was also asking that what is the difference between the bound water and the free water and the hygroscopic water? It is only the level of energy which is bounding a layer of water molecules onto the particles. Clear? So, when we said viscosity is high, under what circumstances viscosity becomes very high? High pressures, low temperatures. One of the examples. Clear? So, you can create similar types of situations and then you can justify these answers. Ok? Freezing point decreasing because of addition of salts, because of changes of rheology and so on. It is an interesting question what you are asking, but then it has very elaborate concept. You have anything else? Just to complete my response. So, we will look at like we will see how easy it is to extract the energy from the. Any system? That will govern the freezing point. So, if there is some mechanism which will oppose the removal of energy. No, no, no energy of crystallization you should talk about the. Crystallization. Yeah. So, basically freezing is nothing but crystallization, one form of crystallization. Correct? Read more about it. Yes. You should be having a mic here. Is there a hello? Sir, why is there a difference in surface tension between the bound and the free water? Ok. I will answer your question 5 minutes. Alright. So, one of the ways to characterize, let us go back to the basic capillary model to answer your question and what you have been asking some of you. Characterization of Vedo's zone. So, by definition first of all this is a zone in which capillarity is predominant. You remember the basic concept there is a small glass tube and which is kept in a water bath. So, this is the glass tube normally we call this as a capillary tube. That means the diameter is going to be extremely small, very, very small diameter alright. And this is a glass tube or a capillary tube. Where do you find capillary tubes in day to day real life situations? Not really. So, very good, but then keep this question in mind you should relate with this answer. The straw diameter is much more. So, a good example of capillary would be pores of the skin. You have gone to a different level altogether apart from this simple devices which you use in day to day life. Sorry. Sorry. Roots. Roots, yes. But that again is a capillary action in the soil. So, I am just asking about a simple device which you use in day to day life. Capillary action. Towel. Towel again falls in the same category. Thermometer. Thermometer is a beautiful example of a capillary tube alright. So, the moment temperature increases there is a rise in the temperature. Sorry, the moment temperature increase there is a rise in the height of the capillary mercury going into the capillary tube alright. So, this is a glass tube or a capillary tube of a very small diameter. So, this you have done in your 10 plus 2 physics. If you leave it like this, this is the water column. Free surface. Atmospheric pressure. Friatic surface. Agreed? Suppose A A, what is the dynamics of the system? The moment you put this glass tube and a capillary will get formed, sorry, menisci will get formed. This is the meniscus. Draw the free body diagram of the meniscus. This height H C is known as capillary rise. The free body diagram would be something like this. The glass tube and this is the meniscus. This is the surface tension. Your question was related to surface tension. This is the contact angle alpha between the water and the glass tube. Glasses of quartz and sand particles are also of quartz. Minerals are mostly quartz. So, this alpha can be obtained by help of most of the devices which are available nowadays. They call them as guinometer. And because of DVSYNC's lab, they have guinometer where you can measure the contact angle for the capillary action, alright. Most of the R and D is being done in this context. So, if the diameter of the tube is D, now can you draw the pressure distribution? I hope you have understood that why this, there is a rise in water column in the capillary tube. That means there is a negative pressure which is acting over here. It is a suction pressure as if the water has been sucked by the glass tube. So, if I consider a point here, let us say point number 1 and point number 2. What is the state of stress at point number 1 and point number 2? If this is the Z at point 1, this is gamma W into Z. Is this okay? This is positive pressure. Anything which is beneath the phreatic line, atmospheric line is positive. How would you find it out? If I take another small tube which is known as a piezometer. Have you ever seen a piezometer? You tubes you have seen. So, if you connect a piezometer over here, please understand this concept carefully because this is going to be very useful forever. So, this tube is known as a piezometer. Nowadays, I can put an electronic sensor which will measure the pressure. So, I can measure the pressure. Earlier days, this is how the pressures used to be measured. Go to any ICU where doctors measure the pressures of your body, clear? Body fluids. So, piezometer always gives positive pressures. Piezo is a sort of a pressure which is atmospherically induced. However, if you keep a tensiometer at this point, you know, normally tensiometers are oppositely designed. This is how the tensiometers would be. It is a tube like this. So, if I keep the tensiometer over here, there will be a drop in water column up to this level which is at sea. So, a tensiometer measures negative pressures. Tension is the word which is used for negative pressures. So, capillarity always gives you negative pressure. Somebody was asking this question, alright? Piezo pressures are always going to be positive. So, state of stress at point number one is positive. State of stress, this is suppose if I say normal stress, sigma. I will remove point. I will simply say sigma 1. So, sigma is the normal stress. Water cannot take shear stress. I hope you agree. At this point 2, the state of stress would be, there is a drop in water column. So, minus at sea multiplied by gamma w. Is this okay? What is at sea? Can you compute at sea? Hydrostatics concepts you can use. So, what is the force which is acting on the system? If I take components, this will become T cos alpha and T sin alpha will vanish. I hope you understand because T sin alpha is on the periphery. So, this is not going to contribute. Yes, please. Sorry. Sorry. P1. Normally, we define pressure as sigma. Normal stress. All right. So, because P1's are normally used at the point. So, this is the normal stress at point number one. And these are normal stress at the point number two. Clear? P1 we do not use. The shear stress we will be using as a tau. But because you are dealing with water, so shear stress is going to be zero. This is pressure, correct? Pressures and stresses are same at that point. Is it not? Stress is the sort of a pressure. So, this is your sigma 2 which is negative and this is your sigma 1 which is positive. So, T sin alpha gets cancelled all along the periphery of the system. So, if I have to draw equilibrium condition, can I write like this T cos alpha which is getting on which surface? The meniscus. And because of that, the entire thing is getting lifted up. So, T cos alpha into pi D, is this correct? This is the periphery, is this correct? Multiplied by T cos alpha. And this is what is balancing the weight of the water column in the capillary tube. So, this will be HC into gamma W and what is the volume of the water? Is this okay? So, area into volume, this is the volume multiplied by density is the force. Is this correct? Hope I have not made mistakes. So, the weight of the water which has gone into the capillary tube is pi D square by 4 into height of the column volume multiplied by gamma W. Equate this and what you are going to get? You will be getting HC equal to 4T cos alpha pi D gets cancelled out upon D. Is this okay? T is the force per unit length, surface tension. In most of the cases, when you are dealing with the glass and water, alpha is 0, not a very good assumption. So, nowadays people can find out the alpha value. As I said, you can substitute over here and if you are very interested in seeing how all this works, you please contact Ghanaraj and see in the laboratory mercury intrusion porosimeter which works on this concept of thin capillary tubes by which we find out the pore diameters present in the soil mass, concrete or coal sample or steel sample, anything, alright? So, those who are interested, you can show them. So, this gets simplified to 4T upon D into gamma W. Another simplified form of this would be, I will come to that later. Now, very cleverly what I can do? You are asking this question. Hello. HC into gamma W is also defined as UW, but this water is under tensile stresses. So, height of the column multiplied by gamma W would give me a sort of a pressure term. So, this is a pressure. This is okay now. We call this as the pore order pressure and the pore order pressure happens to be negative over here. There is one approximation which people have done. They say HC is equal to 0.3 upon D and this is in centimeters where D is also in centimeters. Just let me check. Yeah, it is correct, alright? This is a thumb rule equation which has been derived by people. Now concentrate for a minute. You will understand a lot of geotechnical engineering without much of efforts. Ready? Stop writing please. The interpretation is like this. D is the diameter of the pores in the soil mass, correct? Smaller the diameter, higher the capillary action, agreed? Number one, I can create this D by compacting the soil. So, you lose material. If I compact it, what I am doing? I am reducing the D size. So, the more and more you compact the soil, the capillary action increases. Remember in your index properties when we are talking about different types of gammas, there I told if the granular material is standing out of the water table, it could be dry. But suppose if it is a compacted fine-grained material, the capillaries are going to be extremely active, that is the capillary zone. So, what this indicates is under the hydrostatic equilibrium, a capillary will show you the height of water in it as Hc, which is guided by this. And it so happens that D is defined as E into D10. A very interesting equation. What is E? What is E? Very good, excellent. So, E is the wide ratio and what is D10? Particles finer than 10 percent of the mass. So, D10 is a standard material concept or the property which you can get from the particle size distribution curve. This is one of the applications of PSD, particle size distribution characteristics. If they are known, immediately you can find out what will be the D value substituted over here, you get the capillary action. Most of the disasters which are taking place in the country in terms of infrastructure development are because you are not selecting the material properly, number one. Number two, you are not compacting the material properly. And then number three, you are not designing the whole system properly. The entire thing comes from here. You do any type of consulting, this is the first rule of the game. Contractors do not compact. Now, this E into D10 is also known as effective pore size or effective diameter also, both. So, imagine if I take the soil compacted properly, E is decreasing, D10 remains same, D10 is the fundamental property of the material, E is the matrix, clear? So, heavily compacted systems are going to give you higher SC. What I have to do? I have to take care of capillary rise in most of the practical situations so that highways do not remain submerged. So, in the coastal areas where you are developing road networks, if you are defining this concept, most of the time the water table will be sucked up up to SC value. Is this part clear? If these are the two particles, I can assume this as D, correct? Because ultimately that the pores themselves are being assumed as the tube. So, you are right, absolutely right. So, this is D. When you compact them, what is going to happen? This D is going to get decreased, clear? They come closer, nice. So, truly speaking, this capillary tube is an assumption that you have two particles of the soils and then you are having water inside because of the capillary action and then this happens. So, if I draw the free body diagram of the particle, how does it look like? If this is the particle and if this is the particle and there is a thin layer of water on this, let us say capillary water. Have you come across this somewhere in mechanics? You must have solved this problem I am sure in engineering mechanics. Belt. Belt, correct. So, if I cut it, what is going to happen? This is the grain and there is a layer of water which is nothing but surface tension. So, what surface tension does? It is sort of a rubber band. Put a rubber band and do like this. What happens? The tendency of the rubber band is to bring this back to the original position because of the tension. Exactly same thing is happening. So, same thing is happening over here. So, this is the T, this is the T surface tension. The more surface tension gets developed, particles have a tendency to come closer to each other. All right? There are a lot of ways to interpret this. Now, suppose what is going to happen now because of this? Suppose this is what is going to happen. The particles come absolutely close to each other. At this point of contact, there will be a normal stress. This is what is defined as effective stress. So, please do not talk about the pressure term. All right? Do not use the term pressure. We always talk in terms of the stresses. This is the inter granular contact stress which is acting at the contact of the two grains. And what is the root cause of this? Surface tension. Clear? So, if I were applying a normal stress sigma on this control volume, this sigma would be equal to sigma prime plus uw. This is the theory of effective stress. So, what we say here is the sigma prime value will be equal to sigma minus uw. Have you understood this? uw is the pore water pressure. Sigma is the pressure which we applied externally and what is getting delegated between the particles is sigma prime which is what is known as effective stress. This theory was given by Terzaghi. Now, suppose if I ask you to plot the variation of pressure along the length of this tube. So, along the length of the tube if I ask you to draw the pressure distribution. Clear? How this will look like? This is hydrostatic pressure. You have already drawn this in your engineering mechanics course. Is this okay? Where this is that? What about the upper one? Upper portion. What is the pressure at this point? We are going to ignore it. So, we will put it as neutral pressure 0. This is also known as neutral pressure. Coming back to your question, when you apply stress externally, this stress gets transmitted into the pores and pores are filled up with water. The distribution of the pressure in the water is all around and it is equal. So, the net pressure at that point is 0 under atmospheric conditions and hence we call this as neutral pressure. So, pore water pressure is also known as neutral pressure unless you increment the sigma value. How this sigma value will get incremented? I started constructing a building and today the height of the building is let us say 3 storey tomorrow it becomes 5, 7, 10, 20. What is happening every day? Every day sigma is getting increased, clear? So, what is happening because of sigma getting increased? The pore water pressure is also getting increased. As long as there is no way for water from the pores to escape, typical fine grained soils and then we will discuss quite in detail about consolidation. So, now come back to this question. So, what I was talking about is the pore water pressure is also known as neutral pressure. Is this okay? Have you heard this? Why? Because this under static condition this is the pressure in equilibrium in the pores unless you create some disturbance. Now, this is what is going to be the pressure diagram here. What will be this magnitude? This is negative and this is positive. Where else you have come across this type of a pressure diagram? Sorry. Very nice. Where? Beams. RCC. Correct? Now, what I am talking about here? I have created a beam situation in the soils itself. Initial 2, 3 lectures when we are talking about why it is so easy to drive a car on beaches which are wet and why cannot you drive a car on the beaches which are dry? Are you getting the answer? What has happened? What water does? It provides tensile strength to the material. Clear? So, under tension also you have so much of stresses which are, which can be negotiated by external stresses. So, the pressure which is getting transmitted because of the moving vehicle on the wet sands on the beach is going to be less than gamma WSC. Beaches and beach sands are notorious for sucking and retaining water in them. Find out the material. Come back to this. Find out the material. D10 is going to be extremely low. When D10 is going to be extremely low, D is going to be less. Find out the material. Capillary action is going to be maximum. Clear? So, what I have to do when I am designing infrastructure on the clay soils where the grains are very small, what I should be doing? I should be putting a layer of cutting off the seepage which keeps on moving up. That is the concept or design of infrastructure in marine clays. Are you following these concepts? They are very simple. I mean all throughout your life you will remember. You cannot forget them and nothing has to be mucked up. You agree? Yeah. What is on how the pressure can be linear because it is, it will be up to a height of further capability, but you down the graph is linear. Very good question and very difficult to answer. So, truly speaking it is not linear, but about the sake of convenience. We are assuming this to be linear. You are right that the pressure would not be linear. You are right, but this is an approximation. So, what you should be doing is you should insert different type of tensiometers all along the column of the soil which is in vedo zone. If you are so eager, come to our lab. We can show you the most electronic, most recent sensors which are normally used to measure the negative pressures, but do not get involved in all these things at this stage, but your point is correct. So, have you understood the concept of reinforcement of the soil because of the water present in it and that to under suction. So, suction helps you. Have you followed everything? Any question which you think would you like to ask? Rest of the things are absolutely simple. There is no complication if you have followed these concepts.