 Skelling shrinkage and cracking characteristics of soils. In your opinion what should have been the subtopics for this subject? Skelling shrinkage and cracking characteristics of soils. Mechanism of, do you find any sequential effect or correlation between swelling, shrinkage and cracking? You must have heard about swelling, good. That is right. So which one is more important out of the three? if I ask you a question or how would you grade them let us say swelling, shrinkage and cracking they are not independent so there should be something which links these three things together well this is the parameter if I ask you a question the mechanism sorry mechanism cracking is due to very good excellent and cracking could be because of swelling also so if you put it like this that swelling shrinkage and cracking they are interlinked and what is the most important parameter which would govern these three phenomena truly speaking not the moisture content moisture content itself will depend upon other parameters or the most prominent parameter would be sorry well not so prominent mineralogy temperature if I ask you a question that correlate these three mechanisms with some strength parameter so what would be your answer what type of strength is going to control swelling, shrinking or and cracking characteristics compressive strength or tensile strength compressive strength tensile strength well the correct answer is tensile strength governs most of the properties of geomaterials but very unfortunate we do not talk about tensile strength of geomaterials at all and that is the limitation of classical geomechanics that most of the theories are dealing with compressive strength of the material we never talk about tensile strength of the material so before we graduate to understand the tensile strength properties of geomaterials or soils you have to understand what are the components of tensile strength or in other words how tensile strength gets mobilized what are the parameters which influence tensile strength and what are the mechanisms which are responsible for generation or mobilization of tensile strength so coming back to the issue when we talk about swelling shrinking and cracking characteristics as you most of you are right and you said they are all interlinked and ultimately they control the tensile strength properties of the soil mass so in today's lecture and tomorrow's lectures I would like to give you the fundamentals of these three mechanisms and ultimately this discussion will lead to a discussion very elaborate discussion on tensile strength properties of soils clear determination of tensile strength is not a easy task most of the time you do triaxial test and from there you can get tensile strength and so on so with this in view what I am trying to do is I am trying to give you a good idea about these three mechanisms first is swelling characteristics you will be talking about intrinsic expansive behavior of soils followed by potential expansive behavior of soils what is the difference between intrinsic and potential expansive behavior of soils any idea okay then you have to wait for some time and you will have to wait till I discuss these things in details another subtopic is swelling a concern what is the meaning of word concern here problem problematic so there are two sides for any coin alright and then let us see swelling is a problem but swelling could be a boon also any guess how it could be we mostly talk about swelling as a problem problematic soils we never give importance to swelling properties of soil in a positive way can it happen do you think that this characteristic of a material is a boon for several engineers can be it's a good realization that's right good let's see so I'll be discussing in details both pros and cons of swelling that is swelling a concern or a problem to our profession and I will try to highlight that problems are less and benefits are much more so you should have more respect towards the soils which are swelling type of soils I will try to do this swelling and suction what is the correlation between these two mechanisms and I think this should give you an idea that why everybody has been talking about swelling as a problem because we have not gone in depth of these mechanisms but the moment you correlate the fundamental behavior of the soil as swelling to its suction you will realize that lot of things can be you know brought to a level where you can utilize them in day to day life very easily so this is where we discuss mechanism of swelling then the question is what are the methodologies for identification of materials or the determination of swelling soils I am sorry this should be methodologies for identification or determination of swelling soils it should not be off so this is where most of time we use classical concepts classical methodologies is it not anything can exist beyond this that is what we will be talking in new methodologies new methodologies are the methodologies which are coming up new methodologies followed by determination of swelling potential of soils and a novel technique to determine swelling characteristics of soils so I am sure that this will give you a very new perspective as far as swelling properties of soils are concerned and of course having completed this I will discuss shrinkage characteristics of soils and then once these two properties are done we will be talking about cracking characteristics and then ultimately I intend to correlate all these three to understand how would you determine the tensile strength of geomaterials I hope you will appreciate that this is more of a philosophy where people are trying to work on the models to interlink you know some modules in such a way so that you can get an answer to a big question what else you find in this subject more let us say provoking thought provoking when we talk about swelling what is this phenomena how would you define this phenomena in the context of what we have been discussing in the entire course can you use few keywords to say that yes whatever we have been discussing these three subtopics happen to be a good example of our perennial discussion can you use few keywords basically these three topics are nothing but interaction problems what type of interaction soil water interaction could be contaminated could not be contaminated soils of what type where the minerals are hyperactive clear so truly speaking these three mechanisms talk about the best possible interaction which a soil can have with nature or environment which could be either man made which could be even natural alright so with this intention I thought of including this discussion in this course so let us talk about swelling characteristics of soils basically whenever volume changes either because of intake or taking up of water by the soil we call it as a swelling characteristic this is mostly exhibited by clays and fine sills and such soils are termed as expansive soils not expensive soils only if you change the word later a to e it becomes a different term altogether so what happens here they shrink as moisture content decreases and they will swell as moisture content increases so what is the meaning of this type of mechanism that this is a system is very live it understands how it has to behave and this is a all those you know keywords like sentiments emotions memory of the material comes into the picture you agree under a given circumstances so you put a certain load on the soil mass it knows how much it has to exhibit you release the load it knows how it has to exhibit its response and so on so swelling is caused due to presence of swelling clay minerals most of the minerals present in the clay mass will not sell except for if you have one more night in soils and the circumstances would be exposure to water so these minerals become very active because of their exposure to water so this exposure is nothing but the interaction and this will result in damage to structures which are founded on it over it in it with it and so on so you keep on changing these words and you have a different type of problem which come across you know in your profession so these are the basic swelling characteristics most of you are correct when you were defining the swelling properties of soil I was asking you what is meant by intrinsic expansive behavior any guess what it would be what is meant by intrinsic response when you use the word hydraulic conductivity do you find this term coming over somewhere what is this term what is meant by what is the difference between hydraulic conductivity and intrinsic permeability of the soil mass correct very good that is right so when you define hydraulic conductivity it is a mapping of two things one is fundamental property of the porous media which is capital K if I define a small K as hydraulic conductivity multiplied by pore solution parameters like this causative intensity yeah so you are quite close to answer this question that intrinsic expansive behavior is the property by virtue of which soil holds considerable amount of adsorbed and double layer water the fundamental behavior of the clay mineral alright what are the parameters which are going to govern this type of behavior you are talking about surface area cation exchange capacity and so on specific gravity structure of the rain and so on and this can be determined by the mineralogy of the soil each mineral will have its fundamental response or a behavior so this is how it becomes intrinsic response of the material of course when you change the pore fluid chemistry this response may change so this is basically determined by the mineralogy of the soil and pore fluid chemistry now this is very important the degree of aggregation of the particles what is meant by degree of aggregation clot formation so you add a little bit of water because of the charges which are present in the fine-gain material the tendency of these particles is to form a sort of an agglomeration or aggregation or the clod alright so more the clod formation it shows that the system is hyperactive more trivial the pore fluid chemistry the system is hyperactive the more trivial the mineralogy is the system is hyperactive and of course the particle size and shape are the fundamental variables for the materials why it is so because both on the shape and the particle size the surface area will depend strongly so find out the particle size more the surface area more the surface area more the degree of aggregation more the surface area is the fundamental property of a particular mineral of the soil more surface area the interaction is going to be much more with the pore fluid and so on what are the indicators of intrinsic expansiveness any guess what are the indicators of this mechanism sorry first is liquid limit so indicators are nothing but the traits so high liquid limit high plastic limit high indices and the indication is that the soil is going to be showing more intrinsic expansive behavior and clay fraction alright so more clay fraction more liquid limit more liquid limit more clay fraction more swelling properties more intrinsic properties of the material and so on and of course you must be knowing this relationship where C s is your swelling index equal to change in the void ratio divided by log of sigma prime starting from a known sigma prime value what about the potential expansive behavior now let me ask you a question why should be bisect or dissect in for that matter the expansive behavior of the soil why should be study intrinsic behavior separately and potential expansive behavior separately is the question clear very honest response alright I was trying to make you think that why do you require this type of you know philosophy to be generated yes please what is this potential okay think it like this individually we have a separate response and a behavior okay but when we contact with someone else what happens to our response what happens to our behavior how do we react how do we interact so something which is intrinsic is your behavior or nature clear so let us now see with this background what is meant by potential expansive behavior of the material so can you refine it further now properties like when it reacts with an external agent like what okay so meet the response of the soil when external load is coming on the soil and intrinsic means the internal behavior of the soil particles on the swelling but that is changing because of external see think think of a situation that you have soil grains which are separated and which are placed quite at a distance are you going to exhibit any swelling or shrinkage okay that means soil must be intelligent enough to understand that with what type of contaminants going to interact you are right that means the first thing is the material should identify a circumstances and it should be much more intelligent to understand how it is to react is this part okay so what is your guess about this okay let us let us go ahead with this what comes to my mind I have tried to put it here this is basically what you are saying is correct defined as the defined as the pressure exerted against an unyielding support due to swelling that means the system is understanding how it has to behave so potential expansive behavior is defined as the pressure exerted against an unyielding support due to swelling material wants to react clear it has a tendency but there is a fixed support fixed it is a constant volume so this is where intrinsic response is not very important this is where the potential behavior is coming to picture what is the potential of the material so that is why we always talk about swelling potential of the soil given a situation given circumstances given a chance how system is going to exhibit and what amount of pressure on a system which is rigid fixed retaining wall so you use the backfill material as this type of soils whatever pressure is going to get exerted on the retaining walls in the long run which is much more higher than the active or passive pressures which may get mobilized subsequently alright so this happens to be the potential response of the material that means the material is capable of or it has that potential inside so that is what I said two grains sitting quite at a distance are never going to show anything but when they come close in contact with each other this is where the mechanisms develop and this is where the response can be observed now potential expansive behavior can be determined by both intrinsic expansiveness and the void ratio because it is nothing but the matrix response of the system so for a soil of low void ratio if external agent is removed soil asserts potential expansiveness is this part clear now now if you use sodium monotronite which exhibits high intrinsic expansiveness and very high potential expansiveness now this is where we are talking about the matrix and even the osmotic type of a thing the type of pressure which may develop because of the double layer or the osmotic behavior between the two grains which are saturated with water so kaolinite exhibits low intrinsic expansiveness and potential expansiveness so these are two extremes this is part clear now let me talk about why swelling is a concern of course all of you are aware of that the swelling of a expansive soil poses a major problem with respect to serviceability performance of light weight structures on shallow footings is very well known to everybody what are the reasons for this this occurs due to moisture near ground fluctuates as a result of rainfall watering of gardens water logging and so on even fluctuation of water table seasonal changes in the rainfall which may cause swelling of the material and because of these type of mechanisms the structures which are founded may be under stresses or distress swelling as a boon you find this okay or no expensive soils are attracting great retention as a buffer and backfill material and that is why this property is a boon for repository sides of high level nuclear waste if you do not have any clay mineral where swelling is extraordinarily high you cannot design buffers for the disposal sites or the repositories so if you remember in my first or second or third lecture I was talking about selection of the ideal buffer material you compact the clay and as long as its intrinsic response it is not suitable for preservation of disposed waste or isolating the disposed waste but the moment potential behavior comes in the picture even after compaction what's happening the system is active and then what happens if for the chokes the pores which are present in it because of swelling so this is where swelling is required definitely so that all the pores which are present in the matrix of the soil they get clogged because of the material itself and based on this concept most of the time gcls or buffer materials are selected for designing the backfill material the function of the buffer material is to create an impermeable zone around containers or canisters and this is where the concept of self-sealing self-sealing minerals comes into the picture so if nature was not you know so helpful you would have always synthesized materials which have been good self-sealing self-sealing minerals but we are thankful to nature that we get some soils where the swelling potential itself is very high and these type of minerals are very useful for all sorts of you know waste handling and disposal and they are isolation from the environment a good example of this type of so a good example of this is a vermiculite type of a clay where the swelling is too high so buffer materials like bentonite and sand bentonite mixtures are used together why why they should be used together why do you use bentonite and sand bentonite mixtures you require excessive swelling by the way you think of a situation where all the pores gets filled up or clogged because of the swelling of the soil so this is what actually you require because by simple compaction you can create a structure which is totally impervious now sand is always mixed to bentonite because it is very difficult to compact clays all right so you cannot compact clays alone unless you mix a bit of sand into it so you will find in 21st century everybody is trying to characterize sand bentonite mixtures most of the studies which are being taken up in the present day scenario everybody is trying to understand the behavior of sand bentonite mixtures and these are all the they fall under the category of repository characterization schemes a good buffer material must exhibit swelling properties and hence development of impermeability in the backfill so I am sure with these concepts you will be now respecting swelling properties of soil much more is it not rather than being so much bothered about that if swelling is too much what is to be done because since 40 years back we have decided we have adopted different techniques of laying the foundations in expansive soils is it not but the present day requirements are required materials which are going to cater to the needs of the environmental protection you agree or no so why choice of bentonite swelling characteristics of compacted bentonite and sand bentonite mixtures need to be established estimation of the soil pressure resulting from swelling on the structures of the disposal sites has to be studied and you have to establish hydro thermo mechanical properties I have coined a new term today in this course hydro thermo mechanical properties what is your understanding about this term yes niraj very good excellent that is true so hydro thermo mechanical properties indicate to the fact that how mechanical properties of the soils are going to change when they come in contact with elevated temperatures what about hydro water interaction that is true particularly water retention so if you read this term hydro thermo mechanical response if you heat up a soil mass particularly in case of your waste repositories where the temperatures are very high how mechanical properties are going to change at elevated temperatures and at those temperatures what is the amount of moisture which still is retained in the soil mass if this moisture goes out what is going to happen the soil mass is going to crack that means you will see later on tensile strength is very much susceptible to drop in moisture content so the moment moisture content goes beyond a certain limit the tensile strength of the soil will be less and it cannot stop formation of cracks and once the cracks start in the system the whole engineering work gets defeated this is quite clear so whether you are constructing a basement whether you are constructing a blackfill whether you are constructing a repository if after placement of the blackfill if it cracks because of any reason the whole engineering has gone in waste so this term is becoming very important in the present scenario people are trying to study hydro thermo hydro thermo mechanical response of sand bentonite mixtures at elevated temperatures and later on I will tell you that the characterization schemes for this type of phenomena would be heavily dependent upon suction properties of the soil mass problem is that bentonite itself is a thexotropic material it has any strength or not so at the time of drilling a hole itself you are disturbing the soil how do you stop this disturbance the vibrations will be good enough to create thexotropic effect in bentonite your point is valid slurries will never expand first of all they will always shrink then you should always keep basics in your mind the whole idea of creating a slurry is it has no further chance to soil it will always shrink so shrinking will create more problems that means you will be having shrinkage cracks in your material rather than expansion cracks that's right so but these two mechanisms are totally different alright yeah but it's a good idea you can include here that you require a certain type of material to give temporary stability to the structures that's right my idea here was to show you that these minerals are becoming very important in certain industries we are now geotechnical engineering is finding lot of scope to deal with it's okay