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Video 1: Importance of Electrical Properties of Geomaterials
We have been talking about geomaterial characterization, and today I will be discussing about the electricalcharacterization.You will find it very interesting to note that during our 10+2 physics when we werestudying the electrical properties of materials, we never realized that where these conceptscan 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 havebeen extended to solve very complicated looking problems which a environmental geotechnologistfaces in today’s world.And this is what you are going to find out from today is discussion.So, under the realm of electrical characterization, I will be talking about the importance, thenwhat are the electrical properties, electrical resistivity and dielectric constant of thematerial followed by influence of various parameters on the electrical properties ofgeomaterials.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 arepublished in the literature, which are helping people to obtain the electrical propertieswhich can be utilized for understanding various micromechanisms in particular which are happeningin the geomaterials.Now, this will be followed by the relationship between the thermal and electrical resistivities.This is a interesting concept where we can show that how thermal regime migration inthe geomaterials can be related with the electrical regime or the potential regime which developsin the system.Now, this will be followed by the laboratory and field investigations which have been conducteda state of the art on different types of measurement techniques, then a little bit on what is theohmic conduction in geomaterials.and then I will be introducing the concept of electrical impedance and this will be followedby again the determination of the properties of geomaterials by using the impedance, whichis a very state of the art and contemporary subject, which some of my students have usedas 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-phaseof the geomaterials, particularly gas hydrates.So, guys, I refers to the subject where we are realizing that the electrical propertiescan be utilized as a signature of the material to demonstrate how the mechanism occurs, andI will also cover a bit on how the AC migrates in the geomaterials that is alternating currentand the basic models which are developed by my students to understand the flow of currentwhich is you know, identical to the flow of water, the flow of thermal flux and the flowof you know electromagnetic flux also.So, this concepts becomes very very interesting.And the last topic which I will be discussing during the next lecture would be the magneticcharacterization of geomaterials.So, to begin with, what are the importance of electrical properties of materials in contemporarygeotechnical engineering, this becoming essential for predicting and determining the most fundamentalparameter 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 usedto test them and then they used to analyze the results.Because first of all of us are aware of the limitations associated with the sampling fromthe field and bringing the samples to the laboratory, where we disturb the samples particularlyits moisture state.So, in most of the problems, nowadays, people are interested in measuring the in-situ watercontent and in-situ saturation, alright?.Because as I have discussed in the previous lectures, most of the mechanisms depend uponthe 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 wetalk 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 ofthe 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 whichis 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 establishwhat is the degree of compaction of the soil mass.Core cutter technique and sand replacement method and balloon method and all those methodshave really been shelved off now.And looking at the speed and the pace of infrastructure development, it is becomes very difficultto adopt the old techniques or the conventional techniques of finding out degree of compassion.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’selectronics era, when the electronics is at its peak.you know, everybody would like to sense the parameters associated with the geomaterialsby using sensors and this is again the electrical property the geomaterials become very veryimportant.So, most of the sensing techniques which I will be talking about, would require electricalproperties of geomaterials.The third one is porosity, from the discussion, which we had until now, I hope you have realizedthat the porosity is the parameter which requires a very very dedicated efforts very intricateparameter to obtain.And I will be discussing about this separately also how to determine the porosity of thegeomaterials by giving you complete details of how the porous structure modeling is donein the subsequent lectures.So, one of the techniques of finding or the porosity was to resort to molecular diffusion.So, when you were talking about the contaminant transport to diffusion, if you remember thereI had introduced the concept of you know the equation where the diffusion coefficient isa function of porosity and free diffusion coefficient can be known and hence the porositycan be obtained this concept is being utilized in most of the projects, particularly whichare of strategic importance or were, you know, the porosities are absolutely attending to0 but you still want to establish them.A good example would be design of nuclear dorms.What type of materials should be utilized at what compaction states of the concretethey should be placed or compacted so that nothing diffuses out to the dorms of the atomicreactors?.Similarly, in case of soils also you must have noticed that I can cut short the advectionphenomena, but diffusion predominates.Now, unless you understand the porous structure and how the pores are interconnected, it becomesvery difficult to do the modeling of the geomaterials.So, porosity is a parameter which requires a very special treatment and I will try todo that the next one is hydraulic conductivity, conventional test for determining hydraulicconductivity have several limitations all of you are aware of it.So, the question is, if I really want to find out the inch to hydraulic conductivity ofgeomaterials, what type of systems I should be utilizing, what type of mechanisms whichI 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 thesoil mass also.That means people have tried to determine the in-situ density and the porewater pressureswhich build up in the soil mass by using electrical sensors.Detecting and locating geomembrane failures, the basal liners for the landfills when theyget punctured, because of the placement and the compaction.The biggest issue is that there is no way to obtain whether or to establish whetherthe geomembranes have failed or they have got punctured by placing them in the formof the GCLs or CCLs.This is where the electrical properties could be utilized, where the electrical sensorscan be placed beneath the landfills and the compact clay liners or geotextile clay liners.So, when the contaminants migrate to the geomaterial, the resistivity drops and this can be measuredby using the electrical signals and analyzing them to estimate corrosive effects of thesoil and the buried steel structure.This is also a very contemporary subject most of the industry is wants experts who can establishthe state of the structure which is buried inside the soil mass.And lot of money is being spent by the industries to establish this type of you know state ofthe 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 the geomaterials.To investigate the effects of soil freezing on buried structures, freezing and thawingphenomena 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 agriculturalactivities, the electrical properties would be very useful.There are sensors which can be we call them as resistivity sensors or soil salinity sensors.They can be embedded into the ground.And you can data log the entire network of the sensors to see whether that the soilsare more saline less saline, whether they are losing the nutrition and how to replenishthe nutrition by doing, you know, fertilizing.Another set of importance is that these are the concepts which have been used in understandingthe electrical properties of the geomaterials.The basic hypothesis is that for any material, the dielectric constant or the dielectricpermittivity defines its properties.In other words, for every material there will be a unique dielectric constant.And the reason is that most of the geomaterials are made up of oxides of silica element ion,sodium, potassium, calcium and so on.So, if you have done the chemical analysis of the material, you can always link it withthe dielectric constant of the geomaterial.And the beauty is that this dielectric constant could be either for the dry soil, partiallydry soil or fully saturated soil.So, if I know the water and dielectric constant of the water, when the water goes and sitsinto the voids of the geomaterials, this becomes a water geomaterial system and I can measurethe 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 dielectricproperty or dielectric permittivity.And depending upon the dielectric permittivity you can express the dielectric permittivityas a function of volumetric measurement.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 props and frequency domainprobes, 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 reflectometry probe.So both these probes are being utilized.Both in the laboratory as well as in the field to measure the electrical properties likedielectric 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 thematerial.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 notchanging the structure of the soil at all.You are not destroying the sample, you are not invasing it.And that is the biggest advantage of resorting to electrical properties of geomaterials forcharacterization.Any questions?How accurate are the results with respect to that over drying, from this.If I am applying voltage across two electrodes, what is the inaccuracy which I am going toimpose to the system so you must be realizing no see the whole society is graduating fromconventional 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, dielectricmaterial.So, it is dielectric permittivity the 81 and for air, it is one.So, what it indicates is when you are dealing with the dry soils, the permittivity is goingto be extremely less, but when you are dealing with the wet or saturated soils, the permittivityare 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.
Video 2: Electrical Resistivity and Dielectric Constant
So the electrical properties are electrical resistivity in 10 + 2 physics I am sure allof you have started and the dielectric constant K.So, there are two broad methods of determination of electrical properties.One is we call as low frequency resistivity methods where the frequency of current isless than hundred 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 giga hertz.So, suppose if I ask you a question what is the advantage of pushing in let us say ACat 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 heavy high frequencies would do?.Quick answer would be when the AC frequency is more the passage of current becomes easyas compared to the current which is of low frequency.So, this concept has been utilized to analyze the results of the electrical signatures ofthe materials.Now, the advantages of these techniques over other methods is that these are non-destructive,and these are fast and easy most of 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.Now, as far as the accuracy of the results is concerned, you have to rely on the measurementsare 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. Suchit Gumasthe, who has tried to quantifythe degree of you know, anisotropy in the system in the soil mass by quantifying whatis the degree of flocculation 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 fabricof the geomaterials, your measurements are going to be quite precise.Now, there are two more things which we should remember that electrical properties of thegeomaterials are their response to the applied electric field.And I am sure you must have come across this function in your 10+2 physics that dielectricconstant is the ratio of epsilon s.epsilon s is the permittivity of soil divided by epsilon not epsilon notis the permittivityof 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 inherentporosity and its pore structure, so I am sure you have come across these concepts of whypermeability is more in the flocculated state of the geomaterial or the soils as comparedtheir dispersed state.That means the pore structure has to play with Important to the orientation the grainshave 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 goingto adhered re on the surface and they act as the conductor of correct All right.So, as compared to sense, the fine grained materials which have negatively charged grainswould be better conducting of correct.Of course, the temperature plays an important role in the previous lecture I was talkingabout when you pass current through the cables, heat gets generated and if soils are not ableto dissipate the temperature quickly, the heat within the soil mass increases, whichindirectly increases the temperature of the conductor or the cables.Now, once the temperature of the cables increases their resistance is going to increase andonce the resistance of the cable increases the ampacity drops or the amount of heat whichis getting emitted into the soil mass also increases alright.So, temperature is a phenomena which can be considered as a coupled phenomenon, it couldbe because of heating of the geomaterial, how the temperature of conductor is changingor if I pass current through the conductor, because of this conduction, how the temperaturegets generated in the system and as discussed sometime back the electrical properties wouldget influenced by the type of current which is used, I am sure from your 10 + 2 physicsyou must have learned that which current is non-heating type, which one is a heating currentyou 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 interms of its moisture content would get altered because of the heating of the sample and thatis the reason most of the time high frequency AC is used for these measurements and theresistance offered by the material when you use AC of high frequencies is known as impedance.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 ideato draw a parallax between the parameters which influence the liquefaction potentialof 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 geomaterial.And we know that liquefaction is associated with a coarse grained materials which arevery regular in shapes or a spherical in shape.So, we tried studying the grain shape and size effect by using the impedance and wewere successful.These studies are still ongoing porosities and relative densities which again are relatedto the liquefaction potential can be easily captured by electrical properties and variationin water table.So, what a table is variation in nothing but saturation as a function of depth, includingtheir capillary zone.Alright?.So, if you derive a relationship where the saturation is a function of dielectric constantof 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 inthis area of you know what really happens when shearing takes place and whether we cancapture the shearing response by using the electrical signatures of the material a lot.So, the philosophy is like this resistivity the function of void ratio and void ratiois 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 madeby researcher in this context.And please remember all this is being done in-situ, so, those of you who are aware ofdisadvantages of finding or liquefaction potential of size by conditional SPT would appreciatethat this technique would give you the liquefaction potential under in-situ state and you canmonitor when the systems are going to liquefy this can also be extended to the fact thatdelta of resistivity that is a change in the resistivity is going to be a function of changein the void ratio and change in the density of the geomaterials efforts can be made tofind out the e critical value, if you remember the critical void ratio when you shear thesample, and then you plot the dilatancy and compressibility of the soils.And then you define a band of 10 to 15% of the void ratio which falls under the categoryof e critical void ration where the system becomes constant.That can be used as the reference electrical signal for establishing the liquefaction potentialof the soils.
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