 So, we have discussed a lot of things about slope stability and I wanted to show you some peculiar scenarios where the failure of the slope has occurred. Look at this, I mean if you just type on net you will get so much of information about the type of failures. This was a highway project, there is a highway multi-lane passing through the foothills and how it has been created what we were discussing over here, there was a hillock and certain portion has been chopped off to create a highway infrastructure. There is another highway which you can see over here at the top and then this much soil mass has caved in, this is a beautiful example of landslide, alright. Now if you analyze this picture further you will see that the material has flown out just like a slurry, it looks like but it is not slurry, there could be enough water. So, water happens to be the driving force for taking the debris from this point up to covering almost the entire highway during the rains, yes it could be because of also liquefaction. Another thing you must have noticed that this soil mass or the rock mass or the debris have come out and then there is a volumetric deformation, depression. So, there is a detachment of the entire material along this lip surface. If you look into the 3D you can realize that there is a scar formation and the whole mass has moved down, cleaning up operation is going on, the question is this much portion has caved in, this much portion has failed whether the rest of the slope is stable or not and how to rehabilitate the failed slope, then these are the techniques which a geotechnical engineer would be practicing. Now if you want to see these slides, these are some good pictures, you know these are deep seated, I will show you some more pictures so that you can understand how the failures are taking place. This is a beautiful example of there is a hillock on this side, left hand side on which you have a pavement or the road and there is a river which is flowing from the toe of the hillock. So, this situation can be easily studied by this type of idealization. In this case for creating infrastructure we remove the soil mass or the rock mass by blasting by cutting, yes. Now here what is happening in this case, the river which is flowing is responsible for cutting the toe of the hillock because of soil washing erosion, yes. So the mechanism of cutting is different, in the first case it was human activity, in the second case this is more of the natural process flooding takes place in the river and that water eats up the toe of the hillock. By all means what is happening is when this much of the soil mass, rock mass gets removed the failure takes place, these are the local failures because of the rains as we started discussing today's discussion, it may so happen that the water may percolate and the entire thing may get washed out. So there are different types of slides which are shown over here, this is a subject which attracts a blog also by the way and you will find that there is a blog on these landslides, 15th March 2021, very recent one, alright. So, whenever you get time you please visit this blog also, it is a very hot topic, people are trying to come out with the solutions but I hope you will realize the natural forces and the nature is much more, you know, powerful as compared to our knowledge of understanding the ambiguities associated with the natural structures, alright, heterogeneities of the natural structures and then why do they fail and why do they become susceptible, normally these type of situations occur because of the human interventions, it is very right thing you pointed out. Suppose if I consider a situation where the hillock is this, so this was the case when you had the cutting of the material, cutting at the toe. So cutting at the toe, it destabilizes, another situation, suppose rather than cutting I start building up something over here, somebody wants to build a villa or a multi-storey building, alright, now what is going to happen, you are loading the slope, so when you are loading the slope the state of stress inside the system becomes so critical that the material might fail, so this becomes another mechanism of pleasure, okay. So cutting which is equivalent to unloading, remember the stress parts, unloading is delta of sigma less than 0, loading is the situation where you have delta of sigma greater than 0, positive, both are detrimental, alright, the another situation could be because of the seepage, the seepage, yeah on the contrary suppose rather than loading it, this is the loading process which makes system unstable, if I do excavation on the top let us say what is going to happen, delta sigma v is becoming negative vertical loads, so what is going to happen to delta sigma h they also get equilibrated under active conditions, so that means if I would have excavated it let us say from the top, the loading on the slope or the hillock decreases, stability increases, so thumb rule says if you want to stabilize a hillock you should never cut it at the base, do excavation and remove the material from the top, now this is what is known as removal or unloading, removal could be of rehabilitated structures also, yes there is a colony, I mean you ask people to evacuate place, so this is the removal, alright, excavation, excavation should be again controlled, you know, you cannot use blasting to remove the material from the top, now having done this much a theory part of you know what causes instability in the slopes, we will have to bring in the mechanistic aspects, so the mechanistic aspects are like this, what is the role of forces which are acting on the system, alright, so what are the forces which are acting on the system, so we divide them in two categories, internal and external forces, what are the internal forces, pore water pressure, yes leaching of salts, yes it is possible why not, that means the loss in shear strength is because of internal seepage and what seepage does, it basically increases the pore water pressure, you are right, so the movement pore water pressure increases, the effective stress are going to decrease, instability is going to occur, internal, external all type of forces is deepening of the slope, somebody might come and deposit the material on the edges, why not, so suppose if you overload it from here, construct a building, this is supposed to be a building on the face of the hillock, it is in fashion, everybody wants to stay on the hillocks, hillside areas, yes, so you are loading the slope either at the top or on the sides, this is going to be detrimental on the slope surface, what else could be, earthquakes, rains, yes, earthquakes, vibrations, manmade, so in case of the internal forces we were talking about the seepage, leaching of salts is the main problem, when leaching occurs what happens, the bonding between the particles is going to get lost, so either the bonding becomes weak or it becomes you know, desoluted, cementing material, you remember in CE323 you were talking about the cementitious agents which bound the particles together, yes, that could be the situation, so bonds between the particles get broken because of the seepage, it is because of leaching, salt migration and so on, it is a difficult thing to say, now let us go back to the, because now the whole discussion which we have done so far has to be given a sort of a mechanistic overview, fine, so the sliding is, this is the hillock, this portion of the rock mass becomes unstable, it starts sliding down, this is the slip surface, it becomes linear translational motion, you had a surface and there is a fault, is this okay, so we talked about the translational motion, I am going to talk about the circular slide, alright or nonlinear translation, so because of this fault and suppose if this is the layer 1 of the strata, alright, so what may happen, there could be a geological activity or manmade activity, this side of the material I am loading, I am creating a dam over here, there are famous faults at several locations in the country where the infrastructure has been done, big dams have been done, you know, all these things have been under debate, please check it on the net, so because of that movement it may so happen that the failure might occur like this and this surface which was looking like this might become like this, so this is a circular slide, fine, so the whole description which we have done so far, now I am bringing it to a situation from where the mechanistic models can be developed, so when we talk about the mechanics, you know, these situations can easily be handled by considering 2 types of forces which cause instability, there also we have talked about 2 types of forces, internal and external, clear. So these 2 are the same thing that means from here I have to use the concept of 1 and 2 in such a manner that I can imbibe it on mechanics aspects of the slope stability, how will we do that, external forces are the ones which are causing destabilization, so let us talk about the internal forces which stabilize the system, number 2, the ones which are external, external forces destabilize the system, what are the stabilizing forces, internal forces, shear strength, yes, because shear strength is the one which gets affected because of seepage, leaching of salts, truly speaking this is nothing but C and pi which you have studied so far, the shear strength parameters, they could be drained, they could be undrained, they could be effective, they could be total depending upon the drainage conditions and the structure, life of the structure, correct, you are right. And the rate of shearing, quick failure, slow failure and all those things can be imbibed together over here, so stabilizing forces are the ones which are mostly shear strength of the soil or the rock mass, they prevent against failure, now the failure could be any one of them and when we are talking about the simple mechanics models, sliding, yes, these mechanics concepts are not valid for flow, fall and creed, what about the destabilizing forces, all these forces which are acting externally are the destabilizing forces, gravity, yes, number one, so gravity is the destabilizing force because the slide or the failure is taking place because of the gravity, weathering process, yes, but weathering is a mechanism, it is not a force, fine, yes, water, yes, you are right, so water when it enters into the system creates over the pressure enhancement and what it does, it is again internal because the water goes sits inside the cracks, applies the pressure on the system, so you may say that the water after tension cracks have developed acts as a external force which actuates failure, yes, so what should be done with the water, drain it out of the slopes, provide drainage systems, how to do that, you provide, you know, drainage, insert pipes, this is not the correct way to insert pipes, it should be the other way around, alright, so what we should be doing, so this is not a correct thing, what we should be doing, water drains freely in the direction of the gravity, yes, so what you should be doing is, you can drill weep holes and install them, so that if it is a freely draining material, the pore water pressures will not build up, the seepage which has gone because of the rains can be collected and channeled out, the simplest possible method would be, this is again a difficult method because when you install drains, when you are drilling and putting the weep holes, you are destabilizing the soil mass of the hillock, alright, so the best thing would be you do proper drainage control on the surface, so suppose if I look at the top view of this hillock, this is how the top view looks like, okay, so here I should provide proper drainage, channelized drainage, a network of drainage, correct, so this way the water will move only in the direction and then I can control it by collecting and then draining out of the slopes, so a proper network to channelize the rain water has to be provided to stabilize the slopes, these are all techniques for stabilization, alright, so now what I will do is, I will discuss maybe the analysis of the slope instability from the next lecture onwards.