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Suction Measurement/Control Techniques - Basic

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Video 1
Hello everyone, we have seen several laboratory techniques and field techniques for the estimation of water content. Another important state variable for establishing soil-water characteristic curve which is a fundamental constitutive relationship for soils, for partly saturated soils is matrix suction or total suction. Let us see the different techniques that are available, either to estimate or control the suction of the soil. First, the basic technique is the hanging column method or negative column method. A pictorial or illustration of the hanging column is given here; where soil sample is placed in a container. You have a porous stone that is attached here, and this is soil. So, now the soil, this whole setup is connected with a flexible pipe, and a water level is maintained up to here. When you know the equilibrium to take place, at a steady-state you would see that the soil is completely saturated. If you raise this level, then water willstart flowing. So, now what if you bring this column down? For example, if the column is brought down to this level, so, the water level is here. The new water level is here or itcould be further brought down to this water level is here. So, what will happen to thesoil? Soil, because of the capillary action, soil tries to keep the water within the soil pores, depending on the air entry suction of the soil, water may remain in contact with the soil and, you may have completely filled tube. This U tube will be completely filled with water and that is a state you may have. So, now if you observe the pressure in the water, that will be negative pressure. Because, if this is a mids, midpoint point, or mid-section of the soil, there is a negative head off minus h meters or millimeters is maintained in this particular column. Under this negative head, the soil is completely saturated or some water content may exist. So, this is on a soil-water characteristic of the plot. If I plot the water content, either gravimetric or volumetric water content, this is the negative head, or if I multiply with gamma w, this is pressure. I can represent in terms of pressure. When the negative head is 0, the water content is fully saturated. This is a w s maybe, this is fully saturated water content. So, when the negative head started increasing, the water content may decrease, but still, the soil may be saturated. Because, as I explained earlier, the meniscus curvature might change, but still water will be completely filled within the soil pores and there is no air. So, it is completely saturated; however, the water content may slightly decrease. As you increase the head further, you would suddenly see that, there is a discontinuity between soil and the water and water is up to only here. That is the point where the air enters into the soil system. When air enters, there is a discontinuity between soil and this watercolor. So, this is the simple way of explaining the hanging column technique, where the, how the water content changes with negative head. A very good soil-water characteristic curve could be estimated using the hanging column technique in this procedure also In this particular procedure or method, we have a soil column. This is soil and, which isconnected to a water reservoir, and the water level is here. So, the water level in the soil is also here. It is only to understand this particular thing as of now. But, there are volumetric water content sensors, are placed at different levels. So, these are EC5 sensors; these are EC5 sensors that are placed at different levels, which can directly measure the volumetric water content. And these sensors are connected to the data logger. So, if you have such a system in place, now, when you maintain the water content within the soil to this particular level, you would see that there is some water, water content, that exist above this level also, because of the capillary action. For example, at this particular height above this water level, there may be water content theta 1 and sorry. Here at this level, the water content is theta 1. Here, the water content is theta 2. Here,water content is theta 3. Here, the water content is theta 4. Here, it is theta 5. Here, it is theta 6. Here, it is theta 7. So, you have around, 7 sensors placed at different depth levels. In this situation, now, if Iwant to plot the soil-water characteristic curve, on the x-axis I plot the head and the yaxis theta. This head is negative because we are measuring above the water level or water head is negative, above the water level. As we have seen the capillary, if I draw the capillary here, which is immersed in a water reservoir; this is water; you would see thatthere is a water level, that exists up to a certain level. If you draw the pressure diagram, with depth, you would see that there is a negative pressure in the water, in the column above this level, and, here it is 0, and this is positive. This is positive. This is negative.Similarly, above this, you have negative pressure. So, here the head is some value. Correspondingly, the water content may be very high. Head is 0; the water current is theta s. And, similarly, you have theta 1, theta 2, theta 3. Likewise, you can plot a soilwater characteristic curve. In this particular case, you will have theta, not significantlychanging up to certain depth, but the soil is fully saturated and beyond that, you have water content decreasing drastically up to that point or that head is called air entry value or unity suction head. So, that is somewhere here. Beyond that, you have a partly saturated; you may have a fully saturated system up to a certain depth. That is, h A E V; air entry value head. This is an actual setup, that is available at IIT, Guwahati, geo-tech lab. So, this is the soil column. And there is a reservoir, a water reservoir. And, several sensors are placed at different depths. They are connected to the data loggers. These 2 white color boxes are data loggers. They are EM50, which are supplied by decagon. So, here you get several data points. Similarly,now if I raise this water level to again certain depth. Now, this sensor realizes, that the water content at this particular depth and at this particular depth, both are theta s only. Now, this theta 3 would change, and theta 4 would change. Now, you need to estimate,what is the head from this particular point onwards. You get new points in between again. This way, you can establish the entire soil-water characteristic curve for coarsegrained soils, like sands. Because, generally, in coarse-grained soil, because the diameter, the pore diameter is smaller. So, the air entry value is smaller. Also, the totalcapillary effect, that is considered, or that is available within the soil mass will be less. You can conduct such tests, in the laboratory. In coarse-grained soils, this is possible. Otherwise, if you have fine-grained soil when you compact; as we have seen that the capillary rise expresses to very high values of height. So, therefore, it is not possible to conduct the test by considering such columns. So, this hanging column method is a wonderful technique for establishing the soil-water characteristic curve accurately, for coarse-grained soils.So, here whatever is described here, is a wetting technique where the path is from the right-hand side to the left-hand side because soil initially is a dry strait. As the head is decreased by raising the water level, the water content increased. So, this follows awetting path. One can conduct the drying technique also; drying, one can establish the drying path also, by lowering the water level from the initial depth. For example, the water level is slowly raised up to here, and maybe, it reaches the maximum value. Now, the soil is completely saturated and, you obtain this particular point. Now, when you lower down this value, this particular value to here somewhere, you can measure the water contents at different points. These points represent the drying path. So, as the drying path lies above the wetting path, you would get, the hysteresis can beestablished using this particular technique. So, the hysteresis loop can be established using a single technique that is hanging column technique, on the same soil sample, without considering duplicate samples. However, you cannot have a very lengthy column, like, beyond 1 meter, 2 meters. It is not possible to conduct the test, and not possible to have so many numbers of water content measuring sensors, at different depths. This particular test is limited to coarsesands. Coarse sands and maybe for fine sands generally, this is limited for coarse-grained soils. 
Video 2
A very common method, that is used in unsaturated soil mechanics, and also in soil science is Tensiometer; for measuring the suction value. This is a popular technique, for measuring the suction in both science and engineering disciplines. And also, this particular technique could be used in the laboratory, as well as in the field. The earlier technique which we discussed, that is the hanging column method, cannot be used in the field; which is purely a laboratory technique. When you alter the density of the soil, you can alter, because, it is a reconstituted sample. So, then you get different soil-water characteristic curves. But, it is not possible to conduct this particular hanging column technique in the field. But, however, this tensiometer is very advantageous, because, this can be carried to the field, and test can be conducted, and directly it can be measured.Suction can be measured. Now, the let us understand the principle behind this tensiometer. So, this tensiometer has, a sensing probe; and this is connected to the water-filled body tube. So, here, the body contains the water fully, and de-aired water, and there should not be any vapor or air component in this. And, de-air water should be filled completely in this, water, this body; this tube. Then, it is connected to a sensing probe. This is a sensing probe. And, I willdiscuss what exactly the sensing probe contains. And, this water-filled body tube is connected to a gauge; pressure gauge. So, essentially this has a service cap that can be opened up to fill the water, de-aired water; and which can be closed during the measurement. Now, this sensing probe, when it is in contact with water which is at negative pressure, the negative pore water, the negative pressure is transmitted through this tube, and negative pressure is transmitted to the water, which is contained in this body tube; waterfilled body tube. So, that can be measured using a pressure gauge. So, what does it contain? The sensing tube contains a high-air, high-air -entry ceramic disk. High-air-entry ceramic cup, which has uniform pores so, these are all pores. It is a pore diameter of or pore radius of R, and this is a surface. This is surface, and these are pores. Now, uniform capillaries that contain in your sensing probe. So, as you fill this whole tube; so, this is initially saturated completely. The sensing probe should be completely saturated. So, the capillaries are completely filled with water. That is the initial condition. This is the tensiometer, T8 tensiometer, supplied byDecagon, which is available, at out IIT Guwahati lab; geo-tech lab. Now, this tensiometer, when it is in contact with the soil when the soil is fully saturated, the negative pore water pressure within the soil mass is 0. Soil pore water maybe at, atmospheric pressure so, then, water in the water-filled bodies, also at the atmospheric pressure. Then, there is no issue. But, as, when the soil contains when the soil is partly saturated. So, then, there is a negative pore water pressure that exists within the soil. So, when that soil, comes in contact with the high-air-entry disk, for example, the soil is placed here. This is soil; contains several grains, and within this, there is water. When this is in contact with this, with these capillaries, air cannot enter into this, provided, the air entry suction of this pore space is higher than the air pressure that exists within the soil mass. Earlier using (Refer Time: 15:58) equation, we have estimated, what is the air entryvalue so, which is simply, 2T s by r. If you know the radius of the high-air-entry disk, so, you can estimate, what could be the, u a minus u w, that could be sustained. So, this is the pressure across the interface that can be sustained. So, assume that this pressure that can be sustained here is 100 kPa. So, then, when the soil contains negative pore water pressure, say up to 100 kilo Pascal, so, air cannot enter. But, water within the soil pores can establish good contact with the water in the high-air-entry disk, which is again connected to the water-filled body. So, therefore, negative pore water pressure is transmitted through the water in the high-airentry disk pores and is communicated to the gauge. So, the gauge reads, that, this muchof suction is existing. Because, there is negative water pressure, so, when it is transmitted, the water is sucked from that water-filled tube. So, that much pressure that can actually detect, and it can be read on the gauge pressure. So, when it is in contact with the soil here, the pore water, which exists in the soil is in good contact with the high-air-entry disk. And, the water in the high-air-entry disk is incontact with the water through this and water in this tube. So, therefore, this negative pore water pressure in this particular, negative pore water pressure, that is u a minus u w,positive value indicate, that there is a negative pore water pressure in the soil. So, therefore there is a suction or suction pressure that is exerted on the water which is available here. That pressure can be recorded on the pressure gauge. This way, you can establish the soil-water characteristic curve, for example, you take a soil column, and in which the tensiometer is placed. So, this is the tensiometer and this is the soil. So, when the soil gets dried with time, directly, the reading of the suction can be monitored or measured. If you place atensiometer along with the EC5, or the volumetric water content sensors, both water content and suction using tensiometer, both can be measured simultaneously at different equilibrium water contents within the soil mass; and one can establish the relationship between theta and psi. So, similarly, therefore, it can be used for laboratory, or it could be used in the field. So, that is the advantage of this particular technique. As have seen, the limitation of this particular technique is the air entry disk. Because, if the soil contains a negative pore water pressure of more than 100 kPa or minus 500 kPa, but the air entry value of the high-entry-disk is only 100 kPa, water enters through the high-air-entry disk, pores, and water enters into this body. Then, there is a negative pressure within the soil mass which is in contact with the high-air-entry disk. The negative pressure may be as high as 1000 kPa. So, in that particular case, if the high-air-entry, high-air-entry disk capacity is only 100 kPa, then the air enters into the high-air-entry disk. Because, the sustainable air-water interface or sustainable pressure at the air-water interface, is only up to 100 kPa. That is the capacity of the high-air-entry disk. So, therefore, the air enters into the high-air-entry disk, and air enters into your system. So, your pressure gauge will not read the values. So, therefore, the maximum value that can be measured by the, measured by the tensiometer, is dictated by the high-air-entry disk capacity. You have several high-airentry discs. The high-air-entry disk, which is ceramic disk; the capacity, HAE capacity, is expressedin bar. 1 bar is atmospheric pressure. So, if it is a half bar, that ceramic disk will have a pore diameter of in mm, approximately 6 into 10 power minus 3 millimeter, which is equivalent to the air entry value of in kilo Pascal, that can be estimated using equation as we have done earlier, u a minus u w is, 2T s by r, to into itsstandard temperature. You may use 72.75 milli Newton. If I represent in kilo Newton, then it is minus 6 kilo Newton per meter, divided by, the radius is 6 by 2. So, 3 times 10 power minus 3 mm. If it is represented in meters, then this is 10 power minus 6. So, 10 power minus 6, 10 power minus 6 gets canceled. So, this is equal to about, 48.5 kilo Pascal. So, the air entry of the particular disk is, it may vary around, 48 to 50, or 55 kPa. So, that is why it is called, half bar. A half bar is around 50 kPa. Similarly, if it is 1 bar, so, the 1 bar higher entry capacity, will have the pore diameter, approximately, 1.7 times 10power minus 3. Here, however, in the calculation, we did not use the contact angle. We are assuming the contact angle is 0. So, with that approximation, this comes to be around, 48.5 kilo Pascal. For 1 bar, this pore diameter comes out to be, 1.7 times, 10 power minus 3, and 2 bar, 1.1 into 10 power minus 3, and 3 bar point,7 into 10 power minus 3 millimeters so on and so forth. So, one can choose. And, you have up to, 5 bar and 15 bar as well. So, 15 bar will havethe pore diameter of,16 times 10 to the power minus 3 millimeters. So, this is very fine. So, one can choose, what ceramic disk we can use in the tensiometer. One can use 15 bar, or one can use 5 bar, one can use 1 bar. But this high-air-entry disk pore diameter isnot the only parameter that controls the maximum measurable suction in tensiometer. There is another important factor if we revise our knowledge of the phase diagram of pure water. So, this is the diagram which I got for you. So, here, with respect totemperature on the x-axis, and pressure on the y-axis if you plot so, this is the phase, this is the phase diagram. It represents several phases. This is the water phase; this is a vapor phase, and this is the water phase. We have seen that there is cavitation that takes place,when the absolute pressure decreases for the same temperature, at the same temperature. Pardon about the scaling here. This is not a linear scale. This is some scale we have used here. And, similarly, the y-axis some scale we have used so, here the same temperature. For example, at say 25 degree, at 25 degrees, when you are decreasing the absolute pressure of water. When it reaches the vapor pressure, this is the vaporization curve; this is the vaporization curve. If ittouches the vaporization curve or if it reaches the saturated vapor pressure then, the water turns to vapor so, therefore, how this is useful for tensiometer? So, in the tensiometer, we have seen that there is a water-filled tube, which has a cap. And, here, this is connected to a gauge, and here I have vent etcetera. So, this isconnected to, a flexible tube; through which you have a sensor that is connected, this a sensor. This is a high-air-entry disk, which is connected. Now, with this sensor, when it comes in contact with soil; the soil has negative pressure. So, water pressure is negative within the soil mass. So, u minus u w is positive; that means, pore water pressure is negative right? So, because u a is, if you take 0, as gauge pressure, then u w is negative. Otherwise, if you take absolute, this is 101.325 kilo Pascal. And then, u w maybe some value. So, then you have a positive value right? You have some suction. So, there is anegative pore water pressure. So, now, due to this negative pore water pressure which is transmitted through this tube, and the pressure is transmitted to this water-filled body, which is the red on the pressure gauge here. Now, imagine the water pressure within this tube is decreasing. Because, the soil is in contact with this, and the soil which is; the soil that is getting dried with time. So, therefore, the negative pore water pressure is increasing with time, which istransmitted through the tube, and the water pressure in this one is changing. So, if it is changing. So, if you consider the absolute water pressure, p w, within this tube which is decreasing with time. Initially, it was at atmospheric pressure, which is equal to atmospheric pressure, which is decreasing continuously. When it reaches, so, vapor pressure maybe at 3.1625 kPa, at standard temperature, this is the saturated vapor pressure. u v sat. And, depending on r h, you may have some u v, value slightly less than or significantly less than 3.1625 kPa, at standard temperature. So, when the pressure, water pressure reaches this particular value then vaporization occurs. So, this water turns to vapor. So, you will see a lot of vapor bubbles inside the tube. So, when the vapor bubbles form within this tube, then you cannot measure because water volume does not significantly change. There is a pressure that is exerted on the water, that is you are able to measure. But, when there is a vapor, are bubbles that are formed within the system, then the measurement unit will not work because there is a discontinuity that is, that has happened here. Because of the discontinuity of water. So, because of which it does not measure the negative pore water pressure. Actually, this vapor pressure that controls the maximum measurable suction, by the tensiometer, if the atmospheric pressure is, say, 101.325 kPa; if it is decreased continuously to the vapor pressure, so, vapor pressure value. This is atmosphere pressure; u a. This difference controls what is the maximum measurable range of yourtensiometer; suction measurable range of your tensiometer. So, for example, the atmospheric pressure is u a, and the vapor pressure is u v, the difference between these two quantities would dictate what is the measurable range of suction of your tensiometer. So, for example, at sea level, this u a value maybe, 101.325 kPa and this u v, if it is 100 percent r h, u v equals u v sat. At standard temperature, this value may be 3.16 kPa. This difference, whatever the value may be, 98 kilo Pascal, is a maximum measurable value by your tensiometer. Beyond that, vapors, vaporization takes place, and tensiometer does not work. Similarly, if you take the tensiometer to a hillside, maybe, 1500 to, 2000 meter above the mean sea level, then the atmospheric pressure drastically drops. Atmospheric pressuremay become, maybe 80 kPa. Your vapor pressure at that point, maybe, that can be estimated, depending on the temperature. One can estimate, maybe, it may vary in the range of 5 or something. Then, the measurable range will become 75 kPa only. So, as your measurement is done at elevated places, the measurable range of tensiometer will decrease. This is at elevated places. This is at mean sea level. This is at mean sea level. So, we have seen the negative column technique or hanging column technique,where, you can probably measure the maximum value of about, maximum suction range. You can establish maybe if you have a 1-meter high column, then, you can establish the soil-water characteristic curve up to 10 kilo Pascal suction. Beyond that, it is not possible. Then, you should have increased the height of the columns. But, it is not possible to have whatever the length of the column you want to have.Therefore, it has severe limitations in establishing the soil-water characteristic curve for other than coarse-grained soils. Coarse-grained soils, generally, the air entry value lies in the range of 1 kPa to maybe, 3 kPa at max, for fine sands. So, beyond that immediatelythe water content decreases as the suction increases. So, be within 10 kPa range, you can establish the entire soil-water characteristic curve. So, for red soils or other soils, where the water content significantly varies between a suction range of 0 to 150, 200 kPa, you cannot use the hanging column technique. But, you can use tensiometer up to the atmospheric pressure minus vapor pressure. In that particulars range, one can use a tensiometer to establish the soil-water characteristic curve.
Video 3
We have another technique that is developed, taking inspiration from tensiometer. Thetechnique is called the axis translation technique. Actually, the name itself indicates theprinciple behind the technique. In this axis translation technique, you have a high-airentry disk, similar to the tensiometer. And in a closed chamber, thick vessels you shouldhave. In a closed chamber, where the one top is connected to the air pressure valve, and another one is connected to the water pressure valve at the bottom. So, here the axis is translated. As we have seen earlier in the tensiometer, as elevation increases atmospheric pressure decreases because, decrease in the atmospheric pressure causes the decrease inthe, or the decrease in the maximum measurable suction value by tensiometer. What if, you increase atmospheric pressure? So, that is the principle behind the axis translation technique. Air pressure is maintained at elevated values may be as high as, you can maintain very high values. Then, water pressure can be maintained at lowervalues; maybe at atmospheric pressure itself. So, then, if you see, your u a minus u w, u a is positive. Because, you are maintaining very high values; maybe 500 kPa, at air pressure you are maintaining. And, u w is maybe 0 or which is equal to atmospheric pressure. At, gauge values if you consider, a suction of 500 kPa could be applied, without causing any vaporization; without causing any cavitation. So, therefore, that is a principal, here it is used. When the water level is up to here, so, the air entry disk is completely saturated, and these are the pores that are formed. And, the curvature indicates the pressure drop across the air, air-water interface. On this air entry disk, if you play some soil like this. Now, soil pore water is in very good contact with air entry discs. So, therefore, the water in the soil pore, and water in the high air entry disk, both are in good contact. They form a good channel. Now, when you start increasing the air pressure, so, u w is constant; maintained constant, may be equal to 0. So, then, you are increasing u a, which is the positive value; thatmeans, this indicates the suction; matrix suction. So, you are actually maintaining a certain suction. So, then, slowly the water has a, u a minus u w is increasing, water starts bleeding from the soil because water leaves from the soil because you have increased the suction. As there is good contact between the high-air-entry disk and soil, the water leaves through this and water comes out. So, you can collect this particular water. If you collect the burette stand, you may collect the amount of water that is coming out. So, as and when ua minus u w is increased, the amount of water that is coming out is known. So, when u minus u w is increased, which is plotted on the x-axis; u minus u w is increased, the water content; it could be gravimetric or volumetric, or here you are measuring the volume of water which is coming out. So, it could be volumetric water content which, starts decreasing. For example, initially, it has certain water content and then you have placed inside. Then, you have increased certain u a value, and u w is 0. Then, a 5 kilo Pascal of air pressure isapplied then you have water content slightly decreased because some amount of water has come out. Then, at equilibrium, once the water stop coming out; that means, it has reached, soil reached equilibrium with u a minus u w, are atmospheric conditionssurrounding ambient conditions. Then, you increase further value and some more water comes out. So, this is how you can continue the experiment, just like applying a suction value, which is similar to your consolidation test. In consolidation, you have sigma applied and you have void ratio on the y-axis. So, this void ratio is similar to your theta. This is water content, and this is the void ratio. Both are state variables. Here, u a minus u w; that is, the negative suction or matrix suction. And sigma is a positive load that is applied to thesoil. At equilibrium, where 100 percent or 90 percent consolidation takes place, this sigma is equal to sigma dash, then the curve is similar. The curve is, this is similar to this. Similarly, here also, you are applying some load, matrix suction load. When you have increased, then water comes out. So, the water content drops. The same thing is happening.However, the difference is the consolidation is conducted in a fully saturated state. Here, at any given point, the soil is at the saturated condition. But, here, it is the unsaturated case. Beyond air entry, there is unsaturation in the soil. So, both are similar. But, here air pressure is applied. Instead of directly measuring, we are controlling the suction. So, this is not a measurement this is the controlling suction. We control the suction and measure its corresponding water content to establish the soil-water characteristic curve. So, there is, this is again a laboratory technique. This cannot be used in our field. So, to understandthe field dynamics you may have to get other representative soil samples and then place it in the chamber and conduct the test by increasing the air pressure. And, this way you can establish the entire soil-water characteristic curve. Here in this particular technique also, you can do both, drying and wetting.