Video 1: Parameters Influencing Electrical Conductivity
A bit of theory of you know why these parameters influence the electrical conductivity of geomaterialsbecause the we are all aware that the conduction of the current is going to be through thefluids which are present in the pores provided the soil grains are bad conductors of current.So, most of the time the silica being a good dielectric material, it will not allow passageof current easily unless the particles of the sands are coated by cations or by someconducting layer of let us say bacterial skim.So, the moment this happens, the conductivity could be more on the surface rather than throughthe pores. So, these concepts are being utilized nowadays in designing different types of filters.So, these are the parameters you know which interplay or influence the electrical conductivity.The one is the pore continuity whether the pores are continuous or they are not. So,I will discuss this in the next lecture, where we will define the pore continuity.If pores are conduct are continuous the flow of ions will be continuous and the conductivityis going to be much more next is the water content the more the water content which ispresent in the soils. The soils will be more conducting and if I put more contaminantsin the soils, particularly the contaminants which have ionic species then also conductivitywill be more so, dry soil conductivities less than the moist soil.I think I discussed the mechanisms and the details. Another interesting thing is thatthe when the soils have high clay content on it, and a small waterfilled pores thatare quite continuous and these type of systems would conduct electricity better than thesandy soils. We compare the hydraulic conductivity of two soils, this means if the soil is havingcontaminants. Yes, answer is Yes. Because, imagine within the sample if I am missingthe electrical properties at two different points.When I can see how the contaminant front is migrating I can always establish that uncontaminatedsoil conductivities are going to be less as compared to the contaminated conductivity.But, the contaminants will be ionic then it will show. Ya, see simple presence of wateritself increases the conductivity. When you have more it conducting content in the water,the conductivity will increase further so, this concept can be utilized to detect thestate of materials.It is all about the conduction of the electrons or the current. Salinity level. So, when themore salts are present in the pores or when more electrolytes are present in the poresthe electrical conductivity is going to be higher. Similarly cation exchange capacityfor the soils which have very high cation exchange capacity their conductivity is goingto be more nut the soils which have more organic content in them conductivity is not goingto be higher then comes the effect of temperature.As temperature decreases towards the freezing point of water, the electrical conductivitydecreases this concept we are utilizing to capture the phase transition which occursin the geomaterials. Now, phase transition could be freezing or this could be boilingor this could be you know, precipitation of chemicals which are contaminating the soilsand because of the pH change and environmental conditions change they might precipitate inthe pores.So I am sure this must be given an idea that electrical properties appear to be a sortof a hidden I, for the technologist would like to see what is happening inside the system.Now as the water freezes its conductivity is going to decrease, because ionic movementdecreases. So if you want to differentiate between the 3 to 4 to 5 phases of the soilmass, I think this is what you were asking some time back, you know, you start with a3 phase system, where you have soil particles or the grains, minerals, then water and theair.If I freeze the sample, what is going to happen some part of the water which is present inthe post might get frozen till some part remains as the fluid water. So I have created fourstates. So this is a phase transition, which is actually now if I want to see how the freezingfront is migrating in the geomaterial. Again, I can use the concept of electrical conductivity.So the dielectric contrast or the in impedance contrast or the resistivity contrast withinthe sample is going to tell me what is happening.Where in the soil sample and this concept is used for locating the reservoirs of hydratesin the nature. So, what we call them as a resistivity plots or resistivity logs so ifyou do the resistivity log from top to bottom, you can identify the places where the hydratesare formed.A little bit on the frequency of the current because this governs the whole process andthe mechanism. Though I have talked about low frequency domain and the high frequencydomain, I am sure in your 10+2 physics you must have studied that how the capacitance,inductance and the resistance depends upon the frequency of the current is it not. So,this concept can be utilized to see whether a material is conducting or an insulator.So for insulators, the dielectric constant is going to be higher as compared to the conductors.Is this okay?. Because their charge storage capacity is high. Similarly, the soils whichhave very high water content would exhibit very high dielectric constant. So supposeif I am doing resistivity mapping and if I get a place or a geomaterial strata wherethe dielectric constants are extremely high, one of the interpretation is that these arethe natural aquifers.So, when I do satellite imagery, I get the same signals. For the places where the moremoisture content is present, I will be getting higher dielectric constant as compared tothe places where the less moisture is present and hence the less water is present in thesoils. Water is a dipole polar material alright?. So, they get resonated, when you use highfrequencies and hence the conductivity increases and hence the dielectric constant decreases.So, this concept is used most of the time for analyzing the results of the geomaterial.Most of time what we do is we try to develop the models all the materials which are electricalmodels by using the concepts of capacitance, inductance and resistance and we transformthe soil mass and the geomaterials to a electrical circuit and this is how the further analysisis done and this would answer your question that how will you differentiate between thesize which are contaminated or not.So, as we discussed earlier now, that DCs and ACs have their own strengths and limitations.AC is supposed to be a good current or quite useful current as far as the microstructuremapping of the size of the geomaterial will be concerned, but I am sure you will realizethat when you go beyond hundred hertz, it requires special equipment is and it requiresspecial paraphernalia also. So, the entire process of measurement of electrical propertieschanges when you switch over from low to high frequencies.So, all these gets added up and then it creates background noise. So, unless your experimentsare being done in a shielded environment, clear?. When you speak, that frequency getsadded up or there could be an interference with the frequency which is in the backgroundand that might influence the results. So, most of the time you use the frequencies inthe megahertz or the gigahertz range, curtsy modern day electronics, the cost of thesegadgets is not very high.But and very high is also a very relative term. So, a modern day impedance analyzerwill not cost you less than 50 lakhs. So, the whole research is based on the gadgetswhich are quite expensive, but they are useful in creating the micro mechanisms. There areefforts which have been done to capture the orientation porosity, shape and size of theparticle which I discussed in the past also.I will come to the laboratory and field investigations. Simple experiments can be done in laboratoryby adopting 2 electrodes configuration of 4 electrodes. And the applications are sometimeswe use surface network analyzer Also, if you have done a course in electronics in yourundergraduate, you must have used this SNAs, surface network analyzers, if you want tofind the fault in a cable system or where there is a short circuit which has occurred.You can use the network analyzers and you can find out the fault in the system and thisis where we use the impedance analyzer also which I was talking about or a LCR meter.So, LCR meter and impedance analyzer these are the devices which are used to find outthe impedance of the sample at very high frequency. LCR meters basically give you the L componentsC component and R component. These are inductance, capacitance and resistance of the soils upto 40 megahertz range.We use the concept of wave propagation when we deal with the megahertz regions of thefrequencies that you use and we talk about the interference of the waves, however whenwe deal with the low frequencies which is less than a gigahertz range, we use the equivalentelement method. I will come to this.
Video 2: Measurement of Soil Resistivity
So, this is a simple to electrode method, take the sample of the geomaterial, sandwichit within the electrodes and the electrodes are connected to the power supply. So, thispower supply passes current to the sample and you can measure the voltage across thesample and this becomes two electrode method. The biggest problem with this technique isif you are using DC current heating up of the sample might occur.And you must be realizing that when I keep the sample between 2 electrodes and applyDC, what will happen whatever the polarity of the electrodes is the sample gets oppositelypolarized. So, there is a reverse electric fuel that gets generated in the sample. Ihope you must have done it in your 10 + 2 simple electronic circuits. Alright?. Thisconcept can be utilized to analyze the samples and to find out their properties, I did alot of consulting work by using samples from different you know, industries, this couldbe the sample of an ore, this could be the sample of a tile, this could be the sampleof the granite rock, this could be samples of the concrete, soil mass and whatever. So,the biggest problem as I said is when you apply voltage across the electrodes, the samplegets oppositely charged. So, if you want to come out of this situation, what you shouldbe doing, you can go for a four-electrode method. How do you do profiling of the soilsfor obtaining ya wester Gard’s method. Exactly. So, this is the geophysical method, so, whatwe do is we apply the power supply, we apply the current to the sample and we measure voltagewithin the sample. So, this becomes four electrode 1 electrode 2 electrode 3 and the fourth electrodethese type of measurements are more precise as compared to the two electrode method. Becauseeven if the polarization occurs at the interface of the sample, I can utilize the propertiesof sample which is not connected with electrodes directly. Simple devices, but they reveala lot.Based on this some people have done consolidation test by inserting electrodes into the odometercell. And what they have done is they have placed let us say eight electrodes. Theseare point electrodes, small pins which have been inserted into the sample holders or theodometer cells of certain diameter made up of stainless steel. So, this becomes an electrodeand what they have done is they have measured the properties of the soil across four electrode.So, 1 and 4 electrode is the current carrying electrode and the voltage is measured in between.So, this procedure is repeated by changing the electrode combination so, 1234, firsttime, w345 Next time 3456 next time and so on, and whatever properties are obtained wecan be, we can take the average where this has been used. So, conditional oedometer resultshave been compared with how the void ratio changes and how these changes influence thedielectric constant of the material.So, dielectric constant as a function of log sigma prime is going to be very useful ifI am let us say monitoring, how a building is settling down in the real life. So, supposeif I place the sensors below the foundations and as the consolidation occurs, I can dryrelationship between e versus log sigma prime, which is equivalent to dielectric constantversus log of sigma and dielectric constant can be monitored easily.And this is what is going to tell me how much settlements are going to take place in thefoundations because that is related to the density of the geomaterial and void ratio.So, these type of tests have been done at low frequencies by several people.My M.Tech. Students’ long long back, particularly the group of students, you know, Rohini andSneha Kurian and Reshma, they worked on these type of setups which were created by them.We took a small box, we placed electrodes from inside across the 2 sides of this box.And then we made a small container which is a sort of a capacitor. And then we use thissystem for characterizing the geomaterials for measuring the electrical properties.The beauty of this system is that you can also decrease the spacing between the twoelectrodes read letters, and you can generalize the laws which are being proposed. So, youcan show how much is the dependency of the electrical properties on the distance betweenthe 2 electrodes, because if you remember the equation of a parallel plate capacitor,so capacitance is proportional to area and inversely proportional to the distance betweenthe two plates.Maybe this is one of the answers to your question I can calibrate the setup by using some knownmaterials first and then you can use those results for geomaterial characterization.And this is how the calibration is done, we can find out the resistivity of the geomaterialsby filling up this box with some known fluid, mostly sodium chloride potassium chlorideis used and A upon L becomes the cell coefficient.So, whatever voltage you are applying whatever current you are measuring, if area of crosssection and the distance between the electrodes is known, you can find out the resistivity.So, I can place the soil mass in between and I can measure the electrical properties ofthe soil mass. This is how we started our studies.We went ahead with the another type of box where we embedded 3 electrodes on each faceof the box. And then we measured the electrical properties across the pair of electrodes.I hope you can realize that the efforts were on to find out the equivalent circuits forthe geomaterials because this point electrode and the one which is hiding behind would forma pair of electrodes. Clear?. So, across the vertical plane, I can get 3 values of theresistance and the capacitance and inductance.So 33639 I will be getting 9 values of the parameters and then I can average them. Thesesetups look very simple, but there was a time and we took enough time and efforts to synthesizethem and the credit goes to my master’s students and they created all these facilities.In this case, because this is the point electrode you cannot obtain the area of cross sectionof the electrode. So, we can still go ahead by drawing some relationship as you know coefficient‘a’ which is the shape factor for the electrodes. So, this type of mathematicalmanipulations can be done to often the results.My student Reshma created this probe which is now Dr. Reshma. This is a electrical resistivityprobe, very interesting way of measuring the electrical properties of the geomaterialsin their compacted form, what we have done is we have taken a cylindrical rod and onthis cylindrical rod, we have placed 4 electrodes. So, this is one ring electrode another ringelectrode and another ring electrode another ring electrode.So, four ring electrodes have been fitted and so, that they do not get short circuited,what we have done is we have put ebonite ring in between. So, this is the outer electrodethis is outer electrode, which is connected to the current and this is the inner electrodeand this is the inner electrode, we are measuring the voltage across this. So, truly speakingthis is a four electrode method which I showed you which resembles like what I discussedsometime back.This is the one, you are applying current to the outer electrodes and measuring thevoltage across the inner electrodes. So, once this type of probe is ready this probe canbe inserted into the soil mass, remember we had designed the thermal probes these aresimilar to the ones the concept remains same and you can find out what is electrical resistivityof the materials. For easy insertion into the soil which is compacted at the bottomportion of the probe has been tapered.Based on this simple experiments we obtained the relationship between saturation and resistivityof the soils. I hope this was the first effort which was done by researchers in geotechnicalengineering fraternity to relate saturation and density, sorry resistivity and nowadayswe find that these type of studies have become so, useful when we have gone for advancedmaterial characterization. This is a relationship which deals with the electrical resistivityas a function of thermal resistivity and CR is some coefficient. So, this coefficientcan be defined as a function of saturation.A and B are the material properties. So I am sure you will realize that these simpleexperiments can also be utilized for characterizing the geomaterials provided their electricalproperties are known. So, this also proves the point that electrical resistivity canbe utilized to differentiate the states of the material and to characterize them. Andsubsequently we related A B and C with the fine contents and fine contents is the onewith differentiate between the coarse and the fine grained materials.Some field investigations which are becoming very popular in these days, where the GPRis the leading example, from ground penetrating radars, TDRs are being used for finding outthe subsurface profiles of the material and the capacitance sensors which are used forfinding out the degree of contamination of the geomaterials. There are portable electricprobes also which are available in the market. And all these techniques are being utilizedfor monitoring the environmental impact on the geotechnical structures particularly landfillswould be a good example. So, there are few landfills where we have adopted these techniquesfor profiling the state of material which is getting decomposed in the landfills oneof my PhD scholars, Dr. Agnes, she has worked in this area and you can go through her paperswhere the profiling of the landfills and when the landfill should be, you know biomined.This question has been answered by using the TDR probes and this type of concept was givenby Dr. Patil, one of my another PhD scholars, who has used TDR probes and the capacitanceprobes to relate the degree of decomposition of the municipal solid waste in the landfills.There are some electrical conductivity probes also which can be used in the market. Thereare a lot of specialized conferences which are being done on application of TDRs.This is a beautiful example of why TDR is becoming so popular almost 18 years back therewas a International Symposium and workshop on TDR applications in geotechnical engineeringand what is shown over here is if you want to monitor the movement of the slope, andif you want to design the early warning systems, the TDR probes can be utilized. So, what theydo is they drill the bore logs and or the boreholes, and in these bore holes, they placethe TDR cables and once the movement of the slope starts the cable gets stretched andbecause of the stretching of the cable the resistance is change or the electrical signalschange. These signals are recorded on the oscilloscopes. And then you can make surethat whether the slope is stable or unstable. Now, this concept can be utilized for monitoringthe settlements of the building also. So, wherever the monitoring is being done in today’sprofessional activities, these sensors are becoming quite useful, particularly TDR andall
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