 We have all studied this one dimensional consolidation process, a typical relationship is between void ratio and sigma prime, do you remember this correct? We define this tip as something what is this pre-consolidation pressure, so this is the pre-consolidation pressure. Remember do the mistake of delinking the chapters or the concepts which you have studied in geomechanics from your CE323 to last chapter of this course because everything is interrelated. It is only 50% of the material we could understand in the first course and now rest of the 50% of the response we are trying to understand in this course. Now this is the major player and this controls everything. Sometime back I was talking about that the more circles may shift on the left hand side or they may lift may get shifted on the right hand side also. That means in short the pore to pressures could be either positive or they could be negative clear, so the material is same. Now if you look at this curve carefully you will realize that on this curve if I consider 2 points 1 and 2 the stress is same but what is changing the mechanism is this part okay. The stress is same but what has changed we started loading from here we brought it up to this point and then we started unloading and this thing we defined as NC behavior normally consolidated and this is over consolidated. That means this type of soil is going or the soil response is going to exhibit maximum settlements because this has never been exposed to the pre-consolidation pressure in its past. Having come up to this point clear, we are unloading the samples. Sample has already got this thing in its memory that has already undergone this much of the high pressure and now you are releasing the pressure from it. So it is behaving in a swelling manner it is the swelling part is a compression part clear. So the pore to pressures here are going to be positive and what about this pore to pressures these are going to be negative. So we have introduced another classification scheme for the soils today which is much more you know liked by engineers and technologists and that is based on their pore pressures. So ultimately what I want I want to talk to you in such a manner that your pore to pressure does not increase your blood pressure then only we will be fine clear. At the same time I do not want to make you so dull that you die out of low blood pressure both possibilities are there is it not low and high and this is just emotions. I may say something you may collapse I may say something you become so furious that you may collapse both sides are problematic. So now we have got a better way of controlling the material we know that NC material is going to show positive pressures pore to pore to pressures why too much water is present in them these material by virtue of positive pore to pressures will always show C equal to 0 cohesion equal to 0 you remember the logic which I had given you sometime back take the sand particles fill them in a bottle make system saturated and shake it you see a lot of noise and that noise is because of the friction clear that means NC material is going to give you mostly the friction component life has become simple to equal to C plus sigma 10 5 which I wanted to use there I have eliminated one term very quickly got it however the OC materials are the tricky systems why they do not sit silent the moment you have loaded a system up to this point and you do unloading the chances are the failures will occur because of the negative pore to pressures and by virtue of this these type of materials will show you see not equal to 0 and 5 equal to 0 so I have been using this term very very wrongly the materials material is same what has created the distinction between the response of the material is pre-consultant pressure you got this point the material is same the same soil sample which is exhibiting the positive pore to pressures and negative pore to pressures and hence what is going to happen this gets reflected over here either the more circles are going to move on the left hand side or on the right hand side depends upon the portion that is the reason we wanted to measure the pore to pressures here it becomes like the sample is in ICU and we are doing the best possible monitoring of all sorts of pressures and the body parameters you know being displayed on the monitor for the doctors to decide that is the reason why I have introduced this term over here the pore to pressure measurement computationally I can obtain like this so this philosophy now I am going to extend to the shear strength theory is this part clear if I say e versus sigma prime has given me pre-consultant pressure if I write a situation sigma c is greater than or let us say sigma 3 is greater than sigma c prime what is going to happen meaning thereby I have caused this limit of stressing the sample I am working in a higher stress range if you complete this curve what is going to happen this is how it is going to traverse and it will cut at this point and this point becomes the pre-consultant pressure and this system behave as if nothing has happened to it so this becomes NC material so remember as long as the confining stresses are more than the pre-consultant pressures the pore to pressures are always going to be positive got it it is like balloon the balloon is filled with water or air and if I keep on pressing it what is going to happen the more and more pressure is going to build up which is going to be positive in nature so that means under this circumstances the pore to pressures are going to be positive however the reverse is going to be true when I am working in a stress range which is going to be less than sigma 3 prime so if I reverse this this is going to be negative pore water pressure is this concept clear so if this concept is clear to you I will extend this to shear strength envelopes so if I plot now tau versus sigma prime now if I project this information on a tau sigma prime plane what is going to happen is I am going to get something of this type I hope you can easily guess that is this point of discontinuity is going to be sigma C prime the soils are so intelligent you know they behave in this manner now if I extend this portion what you will find it that this is your friction angle and hence I can say that anything which is beyond sigma C prime in terms of sigma stresses this is going to be NC response and less than this is going to be OC response in real life what we do is if you do perform a test at very very small confining stresses of let us say 25 kPa 50 kPa you are going to get this type of arm what we do is we average it out and we plot it like this so this is the term which we ascribe to C for OC material average value of A to B so this is the average of AB and at this point we have pre-consortion pressure and then this continues as the NC material what is the practical applicability of this situation if I rotate this curve by 90 degree what is going to happen this becomes your undrained shear strength SU which is equal to 2 times Cu because what is Cu Cu will be equal to sigma 1 minus sigma 3 divided by 2 correct the SU term is sigma 1 minus sigma 3 this is undrained shear strength this is undrained cohesion sigma prime term is a function of Z that is the depth of the deposit this curve remains same because tau is nothing but SU sigma prime is nothing but a function of Z and this is what I am talking about the natural deposits what is happening there so if you look at the natural deposits what you will observe is the shear strength most of the times in marine environment particularly where you are doing most of the projects of national importance international importance this is how the stress shear strength decreases and then it picks up and it goes linear and this is how we can extrapolate this alright this is the zone in which what we call is the tricky zone this is approximately 1 meter 1 to 1.5 meter and this is what is known as desiccation zone most of the time in the marine environment particularly in the coastal area because of the very high humidity very high temperatures the top layer of the soil gets desiccated like cake when you bake a cake what happens the top layer in the bottom layer as stiff and inside you have quite soft material clear so something like this happening now imagine if by virtue of your mistake and at lesser less understanding of the material if you create an embankment on the top of this what is going to happen the strengths are going to be less over here and this embankments are going to collapse so that is the reason why OC behavior cannot be ignored and by investigations normally we fix the desiccation zone thickness here again we can apply this as a normalization and we can say that this is the average value we can assume SU variation in the desiccation zone to be constant followed by a linear increase in the shear strength of materials so one part which I have not discussed yet about the CU test is how to identify the material based on AF parameter so if I do continuous monitoring of the pore-water pressures all right and if I say that this is the A parameter at failure what I can do is I can plot AF versus percentage epsilon A and the way cardiologist see the cardiogram of a patient I can tell you how the material is behaving and how should I use this material for construction so until now you have understood that AF is going to be positive for NC materials this is correct because the pore-water pressures are going to be positive so if I plot AF as a function of strain and this is how I get it is a typical NC response however in case of OC materials what is going to happen there would be a response like this there will be slight positive pore-water pressure followed by rest of it would be a negative pore-water pressure so just by looking by doing an online test where I am instrumenting everything I can plot these type of graphs and I know how the soil sample is behaving and ultimately what we are going to do is we are going to use these parameters to classify the soils the last in the series is the UU test later on what we will do is we will link this AF with OCR over consolidation ratio and we will define this OCR as the sigma C prime over cell pressure so I hope now you can realize sigma C prime is the pore this is the pre-consolidation pressure divided by the sigma 3 which we are applying to the sample and then we will correlate this effects again and we will characterize the soil whether it is OC response or NC response now let me come back to UU test the third one now as I said this is another category of the response which we are trying to capture unconsolidated undrained typically offshore environments unconsolidated undrained soil samples are saturated there is no way they can get consolidated young materials they are all being deposited by the reverse into the ocean bed they will take millions of years to get consolidated clear because of sulphate no drainage condition all marine clay peculiar example of offshore construction piling everywhere in the world so no consolidation no drainage fine so what you are going to get is something known as total stress parameter undrained total stress we talked about effective we talked about CD and now we are talking about the total stress analysis clear so this is what is going to be UU test normally we define the parameter which we get as phi U undrained Cu as undrained and we make sure that this is equal to 0 remember another request is phi is never computed in decimal places 23.2 no phi in geomechanics is always a absolute number it cannot be 23.23 it cannot be 23.1 even because the degree of accuracy is not so much so this is the condition which we are creating where we are not allowing any friction to get mobilized even if it is coming out to be 2 to 3 degrees we always assume this to be 0 the reason is simple unconsolidated test over the pressures are building up is this fine I can measure it also by doing a UU prime test and then what I am doing is I am sharing the sample under undrained conditions to get the response of the material typical UU response would be any guess when phi is 0 what is going to happen it is a horizontal line clear so all your Mohr circles are going to be perfectly setting like this and I can give a reasonably average value as phi U equal to 0 and this becomes my Mohr Coulomb envelope okay in effective form if I have to write what is going to happen all these Mohr circles are going to get overlapped on one so this is how the effective Mohr circle will look like by subtracting the pore pressure you will be having a unique circle what should I do then should I allow phi or C or what NC material C is going to be 0 and only phi will get mobilized so this is your effective stress envelope I have understood this I can measure the pore pressure and I can compute the or I can compute the pore pressure and I can apply this correction from sigma 1 sigma 3 to get the effective stress envelope it is very important that the type of testing which we are doing and how we are going to interpret the results one more thing which I will just take another one minute once I have got the total stress and effective stress I will like to see what is going to happen if I test the sample in the UCS unconfined shear strength of the material so a UCS will always show you the negative pore water pressure always negative pore water pressure because there is no confinement so what we have done by creating situations of loading and drainage we have characterized the soil in the best possible engineering bay so the last remark would be as a designer you will be confused what I should be doing I have CT I have CU I have UU I have UC as what I am supposed to do plot all of them on a same scale clear and in general you will be getting the CU like this UU like this CU like this UU like this so this is the relative you know strength parameters which we will be getting from here so CU gives the upper bound UU gives the lower bound CU is in between if I can perform test CU or UU I can extrapolate interpolate in such a manner that I may be getting the range of parameters in which I am working it might be possible because normally a CD test would take you at least a month to conduct on clays continuously because you are consolidating the sample it will take you 2 weeks and when you are shearing at very very slow test slow rate it is going to take you another 2 weeks you might not be having so much time in the real life practice so CD test unless somebody specifies is not done and the quick test would be which I can do from the body of the ship is UU I will load on a plunger have in I will do the test I will get the UU parameters or I will retrieve the samples bring them to the laboratory conduct the test superimpose this condition gets C parameter