 We have been talking about geomaterial characterization and today I will be discussing about the electrical characterization. You will find it very interesting to note that during our 10 plus 2 physics when we were studying the electrical properties of materials, we never realized that where these concepts can be utilized particularly in the realm of civil engineering, geotechnical engineering and now in the realm of environmental geomechanics. The basics are same and these basics have been extended to solve very complicated looking problems which environmental geotechnologies faces in today's world. And this is what you are going to find out from today's discussion. So under the realm of electrical characterization, I will be talking about the importance then what are the electrical properties, electrical resistivity and electric constant of the material followed by influence of various parameters on the electrical properties of geomaterials. This will be followed by the method of measurement of electrical properties, the generalized relationships which we have developed and we have proposed and which are published in the literature which are helping people to obtain the electrical properties which can be utilized for understanding various micro mechanisms in particular which are happening in the geomaterials. Now this will be followed by the relationship between the thermal and electrical resistivities. This is an interesting concept where we can show that how thermal regime migration in the geomaterials can be related with the electrical regime or the potential regime which develops in the system. Now this will be followed by the laboratory and field investigations which have been conducted state of the art on different types of measurement techniques, then a little bit on what is the omiconduction in geomaterials and then I will be introducing the concept of electrical impedance. And this will be followed by again the determination of the properties of geomaterials by using the impedance which is a very state of the art and contemporary subject which some of my students have used as I discussed earlier also to exhibit how the unsaturated soils can be characterized, contaminant transport in unsaturated soils can be established. And nowadays we are utilizing the concept of electrical properties in defining the multi-phase of the geomaterials particularly gas hydrates. So gas hydrates is a subject where we are realizing that the electrical properties can be utilized as a signature of the material to demonstrate how the mechanism occurs. And I will also cover a bit on how the AC migrates in the geomaterials that is alternating current and the basic models which were developed by my students to understand the flow of current which is you know identical to the flow of water, the flow of thermal flux and the flow of you know electromagnetic flux also. So this concept becomes very, very interesting. And the last topic which I will be discussing during the next lecture would be the magnetic characterization of geomaterials. So to begin with what are the importance of electrical properties of geomaterials in contemporary geotechnical engineering this becoming essential for predicting and determining the most fundamental parameter related to the geomaterial which is the water content and saturation. See gone are the days when people used to take out samples from the field and they used to test them and then they used to analyze the results. Because first of all all of us are aware of the limitations associated with the sampling from the field and bringing these samples to the laboratory where we disturb the samples particularly its moisture state. So in most of the problems nowadays people are interested in measuring the in-situ water content and in-situ saturation all right because as I have discussed in the previous lectures most of the mechanisms depend upon the in-situ water content which is the volumetric water content and the saturation of the geomaterial. A good example of this would be whenever we talk about the coupled phenomena where we talk about let us say moisture migration out of the sample because of thermal gradients so this becomes a coupled phenomena. I would be very eager to understand how the saturation is changing within the sample of the geomaterial over a period of time and this is how we determine the unsaturated hydraulic conductivity of the soils. Another good example is that how the contaminant front is migrating in the geomaterials which is a function of saturation and in-situ volumetric moisture content of the geomaterials. Degree of compaction, so nowadays this is the age of electronics and people would like to measure or establish what is the degree of compaction of the soil mass, core cutter technique and sand replacement method and balloon method and you know all those methods have really been shelved off now and looking at the speed and the pace of infrastructure development it becomes very difficult to adopt the old techniques or the conventional techniques of finding out degree of compaction. So people are interested in finding out the in-situ densities so that the degree of compaction can be established by using sensors. So one more thing I think you will be realizing from this discussion is that in today's electronics era when the electronics is at its peak you know everybody would like to sense the parameters associated with the geomaterials by using sensors and this is where again the electrical properties of geomaterials become very very important. So most of the sensing techniques which I will be talking about would require electrical properties of geomaterials. The third one is porosity from the discussion which we had until now I hope you have realized that the porosity is the parameter which requires a very very dedicated efforts very intricate parameter to obtain and I will be discussing about this separately also how to determine the porosity of the geomaterials by giving you complete details of how the porous structure modeling is done in the subsequent lectures. So one of the techniques of finding out the porosity was to resort to molecular diffusion. So when we were talking about the contaminant transport through diffusion if you remember there I had introduced the concept of you know the equation where the diffusion coefficient is a function of porosity and free diffusion coefficient can be known and hence the porosity can be obtained. This concept is being utilized in most of the projects particularly which are of strategic importance or where you know the porosities are absolutely tending to 0 but you still want to establish that. A good example would be design of nuclear domes. What type of material should be utilized at what compaction states of the concrete they should be placed or compacted so that nothing diffuses out through the domes of the atomic reactors. Similarly, in case of soils also you must have noticed that I can cut short the advection phenomena but diffusion predominates. Now unless we understand the porous structure and how the pores are interconnected it becomes very difficult to do the modeling of the geomaterials. So porosity is a parameter which requires a very special treatment and I will try to do that. The next one is hydraulic conductivity conventional test for determining hydraulic conductivity have several limitations all of you are aware of it. So the question is if I really want to find out the in situ hydraulic conductivity of geomaterials what type of systems I should be utilizing. What type of mechanisms which I would like to study and how whether electrical properties can be utilized or not this is what is becoming quite useful in contemporary discussion. There are some efforts which have been made to obtain the liquefaction potential of the soil mass also. That means people have tried to determine the in situ density and the pore-water pressures which build up in the soil mass by using electrical sensors. Detecting and locating geomembrane failures the basal liners for the landfills when they get punctured because of their placement and the compaction the biggest issue is that there is no way to obtain whether or to establish whether the geomembranes have failed or they have got punctured while placing them in the form of the GCLs or CCLs. This is where the electrical properties could be utilized where the electrical sensors can be placed beneath the landfills and the compacted clay liners or geodextile clay liners. So when the contaminants migrate through the geomaterials the resistivity drops and this can be measured by using the electrical signals and analyzing them. To estimate corrosive effects of the soil on the buried steel structure. This is also a very contemporary subject. Most industries want experts who can establish the state of the structure which is buried inside the soil mass and lot of money is being spent by the industries to establish such type of you know state of the buried structures including the piles. So those of you who might become an expert tomorrow in the retrofitting of structures buried structures you will be using a lot electrical properties of geomaterials. To investigate the effects of soil freezing on buried structure freezing and thawing phenomena can also be captured by using the electrical properties and of course if you want to find out the salinity of the agricultural soils for agricultural activities the electrical properties would be very useful. There are sensors which can we call them as resistivity sensors or soil salinity sensors. They can be embedded into the ground and you can data along the entire network of the sensors to see whether the soils are more saline less saline whether they are losing the nutrition and how to replenish the nutrition by doing you know fertilizing. Another set of importance is that these are the concepts which have been used in understanding the electrical properties of the geomaterials. The basic hypothesis is that for any material the dielectric constant or the dielectric permittivity defines its properties. In other words for every material there will be a unique dielectric constant. The reason is that most of the geomaterials are made up of oxides of silica, alumina, iron, sodium, potassium, calcium and so on. So if you have done the chemical analysis of the material you can always link it with the dielectric constant of the geomaterial and the beauty is that this dielectric constant could be either for the dry soil partially dry soil or fully saturated soil. So if I know the water and dielectric constant of the water when the water goes and sits into the voids of the geomaterials this becomes a water geomaterial system and I can measure the dielectric constant of water geomaterial system and I can show that how the dielectric properties are changing over a period of time. Many sensing techniques are developed nowadays which use this concept of measurement of dielectric property or dielectric permittivity and depending upon the dielectric permittivity you can express the dielectric permittivity as a function of volumetric moisture content. So once these two are known I can manipulate the other properties of the geomaterials. Good examples of you know these type of techniques are capacitance probes and frequency domain probes, FD probes we call them or FD sensors we call them. There is another set of sensors which is known as TDR probes time domain refractometry probe. So both these probes are being utilized both in the laboratory as well as in the field to measure the electrical properties like dielectric permittivity of the material which is linked with the volumetric moisture content and volumetric moisture content is linked with the saturation and the density of the material. These are the techniques which are non-invasive and non-destructive alright. So that is the most advantage of resorting to electrical measurements that you are not changing the structure of the soil at all, you are not destroying the sample, you are not invading it and that is the biggest advantage of resorting to electrical properties of geomaterials for characterization. Any questions? Sir, how accurate are the results with respect to that oven drying from this? If I am applying voltage across two electrodes, what is the inaccuracy which I am going to impose to the system? So you must be realizing now see the whole society is graduating from conventional to electronic devices. The real answer to your question is these measurements are quite precise. So this is one of the concepts is that water happens to be a very you know dielectric material. So its dielectric permittivity is 81 and for air it is 1. So what it indicates is when you are dealing with the dry soils, the permittivity is going to be extremely less. But when you are dealing with the wet or saturated soils, the permittivity are going to be extremely high because of the presence of water okay. Now you can do the entire manipulation within a scale of 1 to 81 and then you can see how the whole system is behaving. So the electrical properties are electrical resistivity in 10 plus 2 physics I am sure all of you have studied and the dielectric constant k. So there are two broad methods of determination of electrical properties. One is we call as low frequency residue methods where the frequency of current is less than 100 hertz. However, there is a second category which is known as high frequency electric methods where the frequencies are up to the order of mega to gigahertz. So suppose if I ask you a question what is the advantage of pushing in let us say AC at a very high frequency into a material, what will happen? Why do you require very high frequencies of the current to be pumped into the sample? What have we high frequencies would do? We cancer would be when the AC frequency is more the passage of current becomes easy as compared to the current which is of low frequency. So this concept has been utilized to analyze the results of the electrical signatures of the materials. Now the advantages of these techniques over other methods is that these are non-destructive and these are fast and easy. And most of the time these techniques can be utilized under in situ conditions. So I will just insert a probe or a sensor and then I can get the properties. As far as the accuracy of the results is concerned, you have to rely on the measurements or the best way would be you repeat the experiments and see how reliable the results are. Another interesting thing is when we talk about the electrical properties of the geomaterials, the microstructure of the soil or the geomaterial gets. So one of my PhD scholars did his PhD thesis Dr. Sochandumaste who has tried to quantify the degree of you know anisotropy in the system in the soil mass by quantifying what is the degree of localization and what is the degree of dispersion of the clays. So I hope you can realize that when you can go up to this minutest details of the fabric of the geomaterials, your measurements are going to be quite precise. Now there are two more things which you should remember that electrical properties of the geomaterials are their response to the applied electric field. And I am sure you must have come across this function in your 10 plus 2 physics that dielectric constant is the ratio of epsilon s, epsilon s is the permittivity of soil divided by epsilon nod, epsilon nod is the permittivity of the free space. So we utilize these concepts to decode the geomaterial, I will show you how. Now comes the parameters which are influencing the electrical properties of geomaterials, porosity and the pore structure is number one and because the passage of current through the geomaterial is because of its inherent porosity and its pore structure. So I am sure you have come across these concepts of why permeability is more in the flocculated state of the geomaterial or the soils as compared to a dispersed state. That means the pore structure has to play a very important role, the orientation of the grains have to play a very important role. Water content, the salinity level, the cation exchange capacity of the soil, why? The more the cation exchange capacity of the geomaterial, the cations are going to adhere on the surface and they act as the conductor of current alright. So as compared to sands, the fine-grained materials which have negatively charged grains would be better conductor of current. Of course the temperature plays an important role. In the previous lecture I was talking about when you pass current through the cables, heat gets generated and if soils are not able to dissipate the temperature quickly, the heat within the soil mass increases which indirectly increases the temperature of the conductor or the cables. Now once the temperature of the cables increases, their resistance is going to increase and once the resistance of the cable increases, the opacity drops or the amount of heat which is getting emitted into the soil mass also increases alright. So temperature is a phenomena which can be considered as a coupled phenomena. It could be because of heating of the geomaterial or the temperature of the conductor is changing or if I pass current through the conductor, because of this conduction, how the temperature gets generated in the system. And as discussed sometime back, the electrical properties would get influenced by the type of current which is used. I am sure from your 10 plus to physics you must have learnt that which current is non-heating type, which one is a heating current, yeah you are right. So DC is a heating current and AC is a non-heating current, why? So you know the answer. So suppose if you are using a DC current, the chances are that the soil properties in terms of its moisture content would get altered because of the heating of the sample. And that is the reason most of the time high frequency AC is used for these measurements and the resistance offered by the material when you use AC of high frequencies is known as impedance. So impedance is the resistance which is dependent upon the frequency of the current. In this context, because we are geotechnical engineers, I thought it will be a good idea to draw a balance between the parameters which influence the liquefaction potential of the soils and how the electrical properties can be utilized to map them. So grain shape and size can easily be mapped by using electrical properties of geomaterials. And we know that liquefaction is associated with the coarse-grained materials which are very regular in shapes or spherical in shape. So we tried studying the grain shape and size effect by using the impedance and we were successful. The studies are still ongoing. Porosities and relative densities which again are related to the liquefaction potential can be easily captured by electrical properties and variation in water table. So water table is variation in nothing but saturation as a function of depth including the capillary zone, alright? So if you derive a relationship where saturation is a function of dielectric constant of the material which is a function of depth, you can map the water table also. External forces like shearing effects. So some of my PhD scholars are now working in this area of what really happens when shearing takes place and whether we can capture the shearing response by using the electrical signatures of the material or not. So the philosophy is like this, resistivity is a function of void ratio and void ratios are a function of density. So if I can map these three parameters by conducting good electrical properties experiments I can even find out the liquefaction potential of soils. So on net if you search you will find that there are some efforts which have been made by researchers in this context. And please remember all this is being done in situ. So those of you who are aware of disadvantages of finding out liquefaction potential of soils by conventional SPT would appreciate that this technique would give you the liquefaction potential under in situ state and you can monitor when the systems are going to liquefy. This can also be extended to the fact that delta resistivity that is the change in the resistivity is going to be a function of change in the void ratio and change in the density of the geomaterials. Efforts can be made to find out the e-critical value. If you remember the critical void ratio when you shear the sample and when you plot the dilatancy versus compressibility of the soils and then you define a band of 10 to 15 percent of the void ratio which falls under the category of e-critical void ratio where the shear strength becomes constant that can be used as the reference electrical signal for establishing the liquefaction potential of the soils.