 Bit of theory of you know why these parameters influence the electrical conductivity of geomaterials because the we are all aware that the conduction of the current is going to be through the fluids 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 passage of current easily clear unless the particles of the sands are coated by cations or by some conducting layer of let us say bacterial skim. So the moment this happens the conductivity could be more on the surface rather than through the 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 continuous the flow of ions will be continuous and the conductivity is going to be much more. Next is the water content the more the water content which is present in the soils the soils will be more conducting and if I put more contaminants in the soils particularly the contaminants which have ionic species then also conductivity will be more. So dry soil conductivity is less than the moist soil I think I discussed the mechanisms and the details. Another interesting thing is that the when the soils have high clay content alright and small water filled pores that are quite continuous and these type of systems would conduct electricity better than the sandy soils. We compare the hydraulic conductivity of two soils this means if the soils is having contaminants. Yes answer is yes because imagine within the sample if I am measuring the electrical properties at two different points when I can see how the contaminant front is migrating I can always establish that uncontaminated soil conductivities are going to be less as compared to the contaminated conductivity. The contaminants will be ionic then it will show higher. Yes see simple presence of water itself increases the conductivity when you have more ionic content in the water the conductivity will increase further. So this concept can be utilized to detect the state of materials it is all about the conduction of the electrons or the current salinity level. So when the more salts are present in the pores or when more electrolytes are present in the pores the electrical conductivity is going to be higher. Similarly cation exchange capacity for the soils which have very high cation exchange capacity their conductivity is going to be more but the soils which have more organic content in them their conductivity is not going to be higher. Then comes the effect of temperature as temperature decreases towards the freezing point of water the electrical conductivity decreases. This concept we are utilizing to capture the phase transition which occurs in the geomaterials. Now phase transition could be freezing or this could be boiling or this could be you know precipitation of chemicals which are contaminate which are contaminating the soils and because of the pH change and environmental condition change they might precipitate in the pores. So I am sure this must be given an idea that electrical properties appear to be a sort of a hidden eye for the technologist who would like to see what is happening inside the system. Now as the water freezes its conductivity is going to decrease because the ionic movement decreases alright. So if you want to differentiate between the 3 to 4 to 5 phases of the soil mass I think this is what you are asking sometime back. You know you start with the three phase system where you have soil particles or the grains minerals then water and the air. Now if I freeze the sample what is going to happen some part of the water which is present in the pores might get frozen still some part remains as the fluid water. So I have created 4 states. So this is the phase transition which is occurring. Now if I want to see how the freezing front is migrating in the geomaterials again I can use the concept of electrical conductivity. So the electric contrast or the impedance contrast or the resistivity contrast within the sample is going to tell me what is happening where in the soil sample and this concept is used for locating the reservoirs of hydrates in the nature. So what we call them as the resistivity plots or resistivity logs. So if you do the resistivity log from top to bottom you can identify the places where the hydrates are formed. A little bit on the frequency of the current because this governs the whole process and the mechanism though I have talked about low frequency domain and the high frequency domain. So make sure in your 10 plus 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 which have very high water content would exhibit very high dielectric constant. So suppose if I am doing resistivity mapping and if I get a place or a geomaterial strata where the dielectric constants are extremely high one of the interpretation is that these are the natural occupers. So when I do satellite imagery I get the same signals for the places where the more moisture content is present I will be getting higher dielectric constant as compared to the places where the less moisture is present and hence the less water is present in the soils. Water is a dipole polar material alright. So they get resonated when you use high frequencies 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 geomaterials. Most of the time what we do is we try to develop the models of the geomaterials which are electrical models by using the concepts of capacitance, inductance and resistance and we transform the soil mass and the geomaterials to a electrical circuit. And this is how the further analysis is done and this would answer your question that how would you differentiate between the soils which are contaminated or not. So as we discussed until 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 microstructure mapping of the soils or the geomaterials will be concerned. But I am sure you will realize that when you go beyond 100 hertz it requires special equipments and it requires special paraphernalia also. So the entire process of measurement of electrical properties changes when you switch over from low to high frequencies. So all this gets added up and then it creates background noise. So unless your experiments are being done in a shielded environment clear, when you speak that frequency gets added up or there could be an interference with the frequency which is in the background and that might influence the results. So most of the time we use the frequencies in the megahertz or the gigahertz say curtsy modern day electronics, the cost of these gadgets is not very high but then very high is also a very relative term. So a modern day impedance analyzer would not cost you less than 50 lakhs. So the whole research is based on the gadgets which are quite expensive but they are useful in creating the micro mechanisms. There are efforts which have been done to capture the orientation, porosity, shape and size of the particle which I discussed in the past also. I will come to the laboratory and field investigations. Simple experiments can be done in laboratory by adopting two electrode configuration or four electrode configuration and the applications are sometimes we use surface network analyzer also. If you have done a course in electronics and you undergraduate you must have used this SNS surface network analyzers. If you want to find 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 this is where we use the impedance analyzer also which I was talking about or LCR meter. So LCR meter and impedance analyzer these are the devices which are used to find out the impedance of the sample at very high frequency. LCR meters basically give you the L component, C component and R component. These are inductance, capacitance and resistance of the soils up to 40 megahertz range. We use the concept of wave propagation when we deal with the megahertz regions of the frequencies which we use and we talk about the interference of the waves. However, when we deal with the low frequency which is less than a gigahertz range we use the equivalent element method and come to this. So this is a simple two electrode method take the sample of the geomaterial sandwich it within the electrodes and the electrodes are connected to the power supply. So this power supply passes current to the sample and you can measure the voltage across the sample and this becomes a two electrode method. The biggest problem with this technique is if you are using DC current heating up of the sample might occur and you must be realizing that when I keep the sample between two electrodes and apply DC what will happen whatever the polarity of the electrodes is the sample gets oppositely polarized. So there is a reverse electric field which gets generated in the sample I hope you must have done it in your 10 plus 2 simple electronic circuits alright. This concept can be utilized to analyze the samples and to find out their properties. I did lot of consulting work by using samples from different you know industries. This could be the sample of an ore, this could be the sample of a tile, this could be the sample of a 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 sample gets oppositely charged. So if you want to come out of this situation what you should be doing you can go for a four electrode method. How do you do profiling of the soils for obtaining the Westergaard's method exactly. So this is a geophysical method. So what we do is we apply the power, we apply the current to the sample and we measure voltage within the sample. So this becomes four electrode, one electrode, two electrode, three and the fourth electrode. These type of measurements are more precise as compared to the two electrode method because even if the polarization occurs at the interface of the sample I can utilize the properties of sample which is not connected with the electrodes directly. Simple devices but they reveal a lot. Based on this some people have done consolidation test by inserting electrodes into the odometer cell. And what they have done is they have placed let us say eight electrodes, these are point electrodes, small pins which have been inserted into the sample holders or the odometer cells of certain diameter made up of stainless steel. So this becomes an electrode and what they have done is they have measured the properties of the soil across four electrodes. So one and four electrode is the current carrying electrode and the voltage is measured in between. So this procedure is repeated by changing the electrode combination. So one, two, three, four first time, two, three, four, five next time, three, four, five, six next time and so on. And whatever average properties are obtained we can be, we can take the average where this has been used. So conventional odometer results have been compared with how the void ratio changes and how these changes influence the dielectric constant of the material. So dielectric constant as a function of log sigma prime is going to be very useful if I am let us say monitoring how a building is settling down in the real life. So suppose if I place the sensors below the foundations and as the consolidation occurs I can draw a relationship between E versus log sigma prime which is equivalent to dielectric constant versus log of sigma prime. 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 the foundations because that is related to the density of the geometry and the void ratios. So these type of tests have been done at low frequencies by several people. My MTech students long, long back particularly there is a group of students you know Rohini and Sneha Kurian and Reshma they work on these type of setups which were created by them. We took a small box we placed electrodes from inside across the two sides of this box and then we made a small container which is a sort of a capacitor and then we use this system for characterizing the geomaterials for measuring the electrical properties. The beauty of this system is that you can also decrease the spacing between the two electrodes, plate electrodes and you can generalize the laws which are being proposed. So you can show how much is the dependency of the electrical properties on the distance between the two electrodes because if you remember the equation of a parallel plate capacitor. So capacitance is proportional to area and inversely proportional to the distance between the two plates. Maybe this is one of the answers to your question I can calibrate the setup by using some known materials first and then you can use those results for geomaterials characterization. This is how the calibration is done we can find out the resistivity of the geomaterials by filling up this box with some known fluid mostly sodium chloride, potassium chloride is used and A upon L becomes the cell coefficient. So whatever voltage you are applying whatever current you are measuring if area of cross section and the distance between the two 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 of the soil mass. This is how we started our studies we went ahead with another type of box where we embedded three electrodes on each face of 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 for the geomaterials because this point electrode and the one which is hiding behind would form a pair of electrode clear. So across the vertical plane I can get three values of the resistance and the capacitance and the inductance. So 3, 3, 6, 3, 9 I will be getting 9 values of the parameters and then I can average them. The setups look very simple but there was a time when we took enough time and efforts to synthesize them and the credit goes to my Masters students and they created all these facilities. In this case because this is a point electrode you cannot obtain the area of cross section of the electrode. So we can still go ahead by drawing some relationship as coefficient A which is the shape factor for the electrodes. So this type of mathematical manipulations can be done to obtain the results. My student Reshma created this probe which is now Dr. Reshma. This is an electrical LSD probe very interesting way of measuring the electrical properties of the geomaterials in their compacted form. What we have done is we have taken a cylindrical rod and on this cylindrical rod we have placed four electrodes. So this is one ring electrode, another ring electrode, 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 avonite ring in between. So this is the outer electrode, this is outer electrode which is connected to the current and this is the inner electrode and this is the inner electrode we are measuring voltage across this. So truly speaking this is the four electrode method which I showed you which resembles like what I discussed sometime back. This is the one you are applying current to the outer electrodes and measuring the voltage across the inner electrodes. So once this type of a probe is ready this probe can be inserted into the soil mass remember we had designed the thermal probes these are similar to the ones the concept remains same and you can find out what is the electrical resistivity of the materials. For easy insertion into the soil which is compacted the bottom portion of the probe has been tapered based on these simple experiments we obtain the relationship between saturation and resistivity of the soils. I hope this was the first effort which was done by researchers in geotechnical engineering fraternity to relate saturation and density, sorry resistivity and nowadays we find that these type of studies have become so useful when we have gone for advance material characterization. This is a relationship which deals with the electrical resistivity as a function of thermal resistivity and CR is some coefficient. So this coefficient can be defined as a function of saturation and A and B are the material properties. So I am sure you will realize that these simple experiments can also be utilized for characterizing the geomaterials provided their electrical properties are known. So this also proves the point that electrical resistivity can be utilized to differentiate the states of the material and to characterize them and subsequently we related A, B and C with the fine contents and fine contents is the one which differentiates between the course and the fine materials. Some field investigations which are becoming very popular in these days where the GPR is the leading example around penetrating radars, TDRs are being used for finding out the subsurface profiles of the material and the capacitance sensors which are used for finding out the degree of contamination of the geomaterials. There are portable dielectric probes also which are available in the market and all these techniques are being utilized for monitoring the environmental impact on the geotechnical structures particularly landfills would be a good example. So there are few landfills where we have adopted these techniques for profiling the state of material which is getting decomposed in the landfills. One of my PhD scholars Dr. Agnes she has worked in this area and you can go through her papers where the profiling of the landfills and when the landfill should be you know bio-mind this question has been answered by using the TDR probes and this type of concept was given by Dr. Patil one of my other PhD scholars who has used TDR probes and the capacitance probes 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. There are 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 there was a international symposium and workshop on TDR applications in geotechnical engineering. And what is shown over here is if you want to monitor the movement of the slope and if you want to design the early warning systems the TDR probes can be utilized. So what they do is they drill the bore logs and or the bore holes and in these bore holes they place the TDR cables and once the movement of the slope starts the cable gets stretched and because of the stretching of the cable the resistances change or the electrical signals change. These signals are recorded on the oxaloscopes and then you can make sure that whether the slope is stable or unstable. Now this concept can be utilized for monitoring the settlements of the building also. So wherever the monitoring is being done in today's professional activities these sensors are becoming quite useful particularly TDR and all.