 This is Dr. Mahesh Kalyansheti, Associate Professor, Department of Civil Engineering, Vulture Institute of Technology, Soolabh. In this session, we will discuss about the evaluation of soil strength parameters by direct box shear test. The learning outcome of this session will be, at the end of this session, the students will be able to perform the direct box shear test to evaluate soil strength parameters that is cohesion and soil internal friction angle phi. Before we actually go for the field, the laboratory conditions, let us understand the field conditions. So, usually before the construction, this is what is the stress system present in the soil mass, wherein you can see in the slide the soil mass is subjected to a stress in the horizontal direction as well as a vertical direction. And the vertical stress at this moment is the overburden pressure due to depth of soil equal to Z. When the structure is rested on the soil mass or any construction takes place, then the load of the structure is transferred to the soil mass and that particular pressure is an additional pressure which is developed on the soil mass. And therefore, the total stress in a vertical direction becomes sigma v plus delta sigma. So, this is what is actually happening on the field and after continuation of the stress, the soil failure takes place. Now let us see the determination of strength parameters in the laboratory. So, there are basically two approaches to find the strength parameter. One approach is called as a laboratory test and another is a field test. So, in the laboratory, we have the two very important tests which are performed. One is a direct box shear test and another is a triaxial shear test. And in the field also, there are many tests which we can perform such as a vane shear test, pressure material test, static cone penetrometer test and standard penetration test. Now, in this session, we will focus on the direct box shear test. Now, before we understand the laboratory procedure, so let us understand the phenomena of the failure of the soil. Now, this is what is the represented to soil sample which is taken from the site and right now there is no stress. Then we apply the initial stress condition to the soil mass to simulate it, simulate to the field conditions and then we make the soil to fail. Now, there are two approaches by which we can make the soil to fail. The first approach is by applying the additional vertical stress delta sigma and that particular approach we call it as a triaxial test. The second approach where we apply the horizontal force and we make the soil to fail and that approach is used in the direct shear test. So, these are the two different process used to fail the soil right, so in the triaxial test additional vertical stress is developed in the vertical direction or it is applied in the vertical direction then in the box shear test horizontal pressure is applied. Before we proceed further, let us have these questions. I request you to answer these questions, two MCQs are posted, so read carefully and once you get the answer you can resume. The first question is which of the following is the laboratory test to determine strength parameter of soil, four options are given and the second question is in direct box shear test the soil specimen is failed by applying again four options are there. Take a pause and get the answer. Welcome back, now these are the answers, the first question which of the following is the laboratory test to determine strength parameters of soil, of course it is a box shear test just now we have seen in the previous slides that remaining all are the laboratory tests right therefore the sorry, remaining all are the fill test, only box shear is the laboratory test. The second question is in direct box shear test the soil specimen is failed by applying a horizontal shear force just now we discuss, basically in the box shear test we are making the soil to fail by applying the horizontal shear force. Let us have a look at the apparatus of the box shear test, so on the right hand side you can see apparatus which is used where we have a facility to keep the soil sample in the box and then we apply the normal stress to the soil sample by some cantilever arrangement. So some lever arrangement is made in this machine so that normal stress of known intensity is applied to the soil and then we start the machine and we apply the horizontal force. Whatever horizontal force is applied that is been read on the dial gauge so we have to continue the force till the failure takes place and when the failure takes place the indicator of the dial gauge starts moving in the reverse direction. So we can find the maximum shear force. So knowing the maximum shear force I can find the shear stress. So these are some accessories you can see this is a basically a box two parts of the box then there are certain plates which have got the impressions. Now first of all you have to arrange the two parts of the boxes like this then fill the soil in it and make sure that the soil is compacted properly here with the appropriate moisture content and then you have to place the loading pad. So first you have to level the surface and then you have to place the pressure plate here and this whole assembly now is to be kept in this particular box. So this is an enlarged view of that particular box arrangement. So here you see the box is actually placed on the rulers and then the normal stress is applied here P is a normal stress which is a known value and then you can see the two part of the box independent part bottom portion as well as top portion and within these two the soil mass is placed. So this all is a soil sample. And then we start applying the horizontal force. So this arrow indicates the application of the horizontal force. Now when I apply this force then this particular whole assembly will try to slide towards the right hand side however the top portion is not connected to the bottom portion therefore the soil will offer resistance to this particular movement and along the horizontal plane the resistance is developed. So as long as the resistance is there the soil will go on taking the load and that all load is been measured using the proving ring. So this proving ring gives us a force. Then at one particular force the failure takes place. When the failure takes place then you can see the indicator starts moving in a reverse direction. So we have to identify the maximum reading then those are the proving ring readings then these values are to be converted into force by knowing the calibration chart or loading factor we can find out how much is the shear force. So once I know the shear force I can find out the shear stress. So shear stress is nothing but the force divided by area which is resisting that particular force. So with the help of this particular animation let us see how actually the movement is taking place so the normal stress is present P and the shear force is now applied and as a result of this force you can see the movement takes place like this. So here the bottom portion will slide and you can see a separation line here along this particular line the resistance is developed and that resistance is measured on the proving ring. So that we call it as a shear stress. So in this experiment basically our intention is to find the normal stress and shear stress. So normal stress of known magnitude is applied and for that stress how much shear stress is developed that we are measuring in the laboratory. Then this will be one trial then we go for a second trial where we increase the normal stress and again we apply the shear force and we find the shear stress. So in this way we will develop a number of sets of normal stress and shear stress and knowing this normal stress and shear stress we have to draw the failure envelope. So as you can see here first of all the normal stress you have to find which is nothing but the normal force upon area of cross section of the sample and shear stress that is nothing but the shear resistance developed at the sliding surface divided by area of cross section of the sample. Now this we will understand with the help of the example. Let us consider the four trials are taken in the laboratory and the normal stress observed is 0.51, 1.52 and the shear stress which is measured in the laboratories 0.4, 0.7, 0.9 and 1.1. The stresses are expressed in kg per centimeter square. So we have to develop the strength envelope. Now you can see here in this particular graph on the x-axis the normal stress is plotted on y-axis the shear stress is plotted and then four points you have to obtain here. The first trial that is normal stress 0.5, shear stress 0.4 gives me a first point. Second trial gives me a second point, third trial gives me a third point and fourth trial gives me a fourth trial. So in this way all the line points are joined and then a straight line is developed joining all the points then the intercept of this line with the vertical is cohesion and the inclination of this line with the horizontal is friction angle. So this is how c and phi is developed using the box shear test. So these are the results from the graph cohesion and phi. These are the references which are used for the presentation. Thank you. Thank you very much.