 Hello, this is Dr. Mahesh Kalanshati, Associate Professor, Department of Civil Engineering, Volcanist of Technology, Solaq. In this session, we will discuss about the evaluation of soil strength parameters by triaxial shear test. The learning outcome will be at the end of this session, students will be able to perform the triaxial shear test to evaluate soil strength parameters that is cohesion and soil internal friction angle. Now, the determination of strength parameters basically we have two approaches. One is a laboratory test and another is a field test. So, in the laboratory test, these are the some important tests which are performed such as a direct box shear test and triaxial shear test. Whereas, on the field, we can perform these four tests, wind shear test, pressure meter, static cone penetrometer and standard penetration test. So, in this session, now we will focus on the triaxial shear test. Now, in the laboratory, first of all, we need to prepare a representative soil sample that we call it as a remolded soil sample. So, this is how the soil sample is remolded and the cylindrical specimen is prepared. So, initially there is no stress and then we will apply the stress to the soil sample to simulate a field condition and then we make the soil to fail. Now, based on our approach, there are two methods. So, if I apply the additional vertical stress and cause the failure, then that concept is used in the triaxial shear test. And if I apply the horizontal force and if I make the soil to fail, that particular approach is used in the direct shear test. So, today, now let us focus on the triaxial shear test. So, this is what is a triaxial mold or triaxial cell within which the soil sample is placed. So, first of all, we need to prepare this particular setup where at the center, you can see the soil sample, cylindrical soil sample and then we, you can see certain arrangement here. The two tubes, you can see one is at top, one is at bottom. These two tubes are used to measure the pore pressure. So, these particular pipes are to be connected to a pore pressure measurement unit. Then this, in this cell, the fluid is filled, usually the water is used. And then we have one more arrangement here to apply the pressure. So, there is a loading unit and through this loading unit, we apply the pressure to this water and this particular pressure, we call it as a confining pressure, which is present all along the soil sample. So, basically in this experiment, we are supposed to apply the confining pressure first and then we have to apply a vertical additional stress that we call it as a deviatoric stress. And we go on increasing this vertical stress till the failure takes place. So, this is what is the basic concept in the triaxial shear test. Now, the soil sample preparation, you can see. Now, this is a typical setup of the triaxial test. This is a loading frame and where you can see at the center, the triaxial mold. Such kind of sampling tubes you may use which are brought from the laboratories and from the sampling tubes, the samples are extracted using the extruder. Then it is placed in this particular mold because we need a cylindrical specimen. Then the trimming is done and you see this sample is placed here in the mold. So, this is how the sample is to be placed. And then a rubber membrane is used here just to avoid the reaction of water with the soil. And then this assembly is prepared. So, you can see this mold which is partially filled with water. And then this complete portion is to be filled with water and you can see some arrangement here. So, these all arrangements are to develop the pressure and to measure the pore pressure which is developed. So, this is a typical setup where you can see this mold already placed here and the soil sample is here at the center and then we start the machine. So, first of all, before we apply the vertical pressure, we need to apply the horizontal pressure that is confining pressure. So, that confining pressure is applied with the help of the pressure unit that is of known magnitude and then we apply, we start the motor and we start applying the vertical force. So, whatever vertical stress or vertical force is applied that can be measured in this pruning ring. So, we keep on continuing this load till the failure takes place. So, this concept is explained here. So, the first stage is all round pressure as we have seen in the previous slide. So, soil sample subjected to all round pressure and then we apply the deviatoric stress, additional vertical stress it is called as deviatoric stress. And then at one particular value the soil sample fails. So, we have a set of sigma 3 and sigma 1. So, this horizontal pressure is a sigma 3 everywhere. So, here everywhere the pressure is sigma 3 and here on the right hand side the sigma 3 is already present and in a vertical direction some increase in this stress has taken place. So, that sigma 3 plus delta sigma gives me sigma 1 that is called major principle stress. So, the major principle stress is acting in a vertical direction, minor principle stress is acting in a horizontal direction. So, using this sigma 1 and sigma 3 we can draw the Mohr's circle. So, here at the bottom you see on the right hand side or sorry left hand side on x axis the normal stress is plotted and on y axis the shear stress is plotted. So, the first circle you can see the first Mohr's circle corresponding to first trial because sigma 1 and sigma 3 are known to us. Then we go for a second trial where we increase this confining pressure. We increase the confining pressure and for that we find what is the extra vertical stress required. So, in this case we get another set of sigma 1 and sigma 3 of course of higher magnitude. So, corresponding Mohr's circle will draw. So, the second Mohr's circle you can see the second Mohr's circle is corresponding to second trial. Then we go for a third trial we increase the confining pressure again and again we apply the stress. So, we find another value of sigma 1. So, a new setup of sigma 1 and sigma 3 is developed. So, that is used and again a number of Mohr's circles are plotted like this. So, usually the four trials are desired. So, therefore you can see the four Mohr's circles we get. So, once we get the Mohr's circle then the common tangent line need to be drawn here and this common tangent line is nothing but a failure envelope. The moment I get a failure envelope I can find out the intercept on a vertical axis that we call it as a cohesion and the inclination of this line with horizontal is called as a friction angle. So, on the right hand side also you can see a straight line developed that is nothing but the failure envelope. So, this is how the failure envelope is developed in the laboratory using triaxial shear test. So, let us have a review questions quickly go through this take a pause answer it and resume the video. The two questions I have posted the main advantage of triaxial compression test is four options and the second one is strength envelope of purely cohesion soil is four options take a pause and get the answer okay. So, these are the answers the first question was the main advantage of triaxial compression test is the failure takes place on weakest plane complete control over drainage condition and stress distribution on failure plane is uniform. So, these are all the advantages. So, therefore the answer will be all of this and the strength envelope of purely cohesion soil is horizontal because we know that phi is zero therefore the strength envelope becomes horizontal. So, these are some drainage conditions which are used in the triaxial test. In fact, this is a advantage of the triaxial test because we can conduct the experiment under different drainage conditions. The three major drainage conditions are used one is called unconsolidated undrained test that is UU second one is a consolidated drain test which is called CU and third one is consolidated drain test that is called CD. So, in UU test what happens that the cell pressure is applied without allowing drainage and then keeping all pressure constant deviatoric stress is increased to failure without drainage. So, drainage is not allowed throughout the process. In the second step consolidated undrained test here the drainage is allowed during the cell pressure application right. So, initially when the cell pressure is applied the drainage is allowed and then when the sharing takes place at that time the drainage is not allowed. So, it is called undrained so consolidated undrained test and third one is the most realistic method it is called consolidated drained test where the consolidation is allowed when the confining pressure is applied and at the same time during the sharing also the drainage is allowed. So, this is most realistic but it takes a lot of time it is UU test is relatively quick test and CD test is a time consuming one but it is more accurate. Then now this conditions you can see here now the drainage conditions we can understand from this slide in the first step the confining pressure is applied and then under all-round confining pressure is applied here then then during this process if the drainage is allowed or not. If it is allowed then it is consolidated if it is not allowed then it is unconsolidated. Then the second stage is the application of the day waitress stress here and making the soil to fail. Now, during this process during the sharing if you are allowing the drainage or not based on that also we have two options if the drainage wall is open it means if you are allowing a drainage it is called as a drained loading if you are not allowing it is called undrained loading. So, based on this we can have these particular tests say as I told in the first step if suppose the drainage is allowed then it is consolidated and during the sharing also if the drainage is not allowed then it is undrained. So, therefore it is called CU test consolidated undrained test blue line indicates. Then if it is consolidated and then during the sharing if the drainage is allowed then it is called CD test which is shown in red color and if suppose the drainage is not at all allowed during the first stage as well as second stage then it is unconsolidated undrained load. So, the green color indicates UU test references which are used. Thank you. Thank you very much.