Video 1: Cracking Characteristics of Fine-Grained Soils
We have been discussing about the geomaterial characterization and in particular thermalcharacterization.So, as discussed last time one of the real-life application of thermal characterization wouldbe the formation of heat induced cracking of the soils.And this is a big challenge for geotechnical engineers to overcome and design the foundationsand the structures so that they do not get exposed to the environment even after thethermal cracking occurs.So, in today’s discussion I will be elaborating about the mechanisms what thermal crackingis all about?What are the mechanisms of cracking of soils and how to quantify the cracking characteristicsand how to use this information in the practice of geotechnical engineering and of course,this will be followed by electrical and magnetic characterization, mostly the cracking is associatedwith the fine grained materials.Such as clays or the expensive soils where the minerology is very active or the clayminerals which are active in nature, when we say active minerals the tendency of theminerals is to react with water or the environment and in the process they shrink and swell.So, shrinkage is because of the expulsion of the moisture and when the drying takesplace.And swelling is the reverse process when the minerals come in contact with water they havea tendency to adhered the water onto their surface.In short, the swelling and shrinking causes the cracking to occur.That means there will be a limit of swelling and shrinking process before the crackingoccurs.So, cracking is something which is the ultimate effect of the swelling and shrinking process.These type of situations become very critical when we talk about the clays which are usedas a you know core of the dam, earther dams, sometimes landfill liners also and the landfillcovers, these could be the liners and covers for the radioactive waste repositories alsoas we have discussed embankment, slopes in the sporting world.Normally this concept is used quite a lot, where they apply the concepts of crackingof soils and how to migrate it, how to remediate it, particularly in case of cricket pitchesand tennis courts.In fact, my research in the cracking of fine grained soils and so I started with my associationwith the BCCI when we were doing this pitches for World Cup 2011 and then I found that thisidea became so intricate that I guided almost 2 PhDs on this.And we devised lot of you know philosophies and the facilities, which can be utilizedfor characterization of fine grained materials.It is a very interesting topic to study by geotechnical engineers as well as the peoplewho are associated with these type of projects.Now, another thing is that when you construct something on the soil mass which is proneto cracking, the safety of the structure becomes a very very important issue, most of the timein the realm of environmental geotechnology, where we talk about the permittivity of thecompacted soil mass.The big question would be after the soils crack, particularly the fine grained soils,they become more porous, more conducting, sometimes we also call this as a secondaryporosity of the material.So, after cracking the material becomes highly conducting and this could be alarming situationwhen you deal with the liners or the covers for the landfills.So, imagine a situation where the thermal gradients are coming from the landfill andthey are getting exposed or the clay liners are getting exposed to the thermal gradientswhich are coming from the landfill, where lot of chemical reactions are occurring.And if these liners crack, the chances are that the environmental water will migratethrough the covers which have been designed as well as the liners through which the leachateswill go into the environment.As far as the conventional geomechanics is concerned, the tracking is going to becauseyou know, loss of bearing of the strata.And sometimes if the tracking tendency is too much, then differential settlements alsomight prevail in the system.It so happens that the cracking of the soils is linked with the tensile strength of soils.And I am sure you will realize that in conventional geomechanics, we have not given much weightageto the tensile strength of geomaterials all the test which are performed are related tothe compression of the soils, and we rarely talk about the tensile strength of the soils,but it has been realized in the recent past that the tensile strength of the soils isthe one which controls their stability rather than the compressive strength.And hence, it becomes very important to study the mechanism of crack formation in the finegrained soils and which is attributed to the tensile strength mobilization.This also necessitates development of instrumentation and the hypothesis which can be utilized fordetermination of tensile strength of soils.I had another interesting encounter with the geomaterials which used to crack and thisis an example of a large structural fill which was created somewhere close to Bombay, inGujarat.Soils in Gujarat are quite, you know, active because of the mineralogy and this case happenedwhere the structural fill was created.And unfortunately, there was a flash flood and because of the flash floods several hectaresof the area which was structurally filled up with the soils cracked, and then this becamea very interesting case to study.So here what I have shown is, if you look at the pattern of the cracks, which get developedon the entire structural fill, the basic idea of compacting the soil to create a structuralpad gets defeated number one, you can always say that this was a poor choice as far asthe material is concerned to create a structural fill and Yes, that is true.So, the contractor need not to be paid for this type of a filling because he was supposedto be extra cautious while selecting a material as a structural fill material.Now in short ones these type of cracks developed, we do the third dimension mapping also ofthe cracks, and we try to realize that how much portion of this compacted fill has becomedebunked.If you have done structural analysis of the retention schemes particularly the retainingwalls and the retaining walls which are exposed to desiccation cracking.The simple question which is asked is what is the earth pressure before and after developmentof the tension crack.And I am sure you have done this analysis to show once this structural crack develops,soil is not in touch with the retaining wall at all.And hence, the earth pressures are going to be different than the earth pressures whichare going to be under full contact of the backfill with the wall.So, this is where it becomes very important apart from you know, knowing what is the amountof seepage which is getting induced into the subsurface of the backfills because of thecrack formation, and if the depth of the cracks is quite high, the situation might becomequite alarming as well in simple words.If this type of situation occurs somewhere, the structural fill has lost its integrityand hence you cannot accept this type of a situation.Now, if you think about the conceptual model that why soils crack, the tensile stresseswhich develop in the system to be blamed for cracking of the soils and tensile stressesdevelop due to the desiccation of the soil mass or the drying of the soil mass.So, I am sure you must be realizing that this is a situation which does not occur becauseof the mechanical loading of the soils.The situation occurs because of the thermal loading of the soils and thermal loading causesexpulsion of water from the soils or the drying we call this as desiccation also, and thenthe tensile stresses mobilizes if the tensile stresses are higher than the tensile stresseswhich are at the limiting stage of cracking, the soils will crack otherwise they remainwithin the permissible limit.So, this type of models are normally used.You have a thin layer of the soil which is fine grained material clay and this is wherethe tensile stress is developed, because of the heating, because of the stretching ofthe clay layer.There will be primary cracks which were developed, air enters into the system and this systemdoes not remain at equilibrium because of the development of the primary cracks, thesecracks multiply in the form of the secondary and tertiary crack and you know the wholesystem which was compacted as the backfill material becomes quite pervious and losesits structural stability.So, this is a simple model, which is used to define the cracking characteristics ofthe soils.Now, the question is how would you determine the tensile stresses in the material and howwould you link it with the thermal effects and the desiccation effects which might behappening because of the environmental conditions.And that is the reason this subject is now becoming quite important to be studied byall those who are in the practice of geotechnical engineering as well as geoenvironmental engineering.Now, if I change the context and if I say rather than heat drying the soil mass, thereis some chemical activity which is occurring in the system and because of that there couldbe internal heating process which is going on.So, this is equivalent to the situation when the soil mass was getting heated up from theoutside or externally heated.So, truly speaking the source of heat generation is not very important, this could be withinthe system, this could be outside the system, what is important is that how the moisturegets released from the pores and that process causes the cracks to develop.
Video 2: Tensile Strength of the Soil
So, if we talk about the mechanism of mobilization of the tensile strength, this is a complicatedprocess, we have to again start with the 3 the phase system.And if you remember we have talked about the soil water air interface or the interactionall the three phases interacting with each other, which is the main reason for mobilizationof the tensile strength of the fine grained materials and this is where we assume thatin case of the dry soils, we have apparent cohesion, if you remember the shear stressversus normal stress relationship or the failure envelops of the soils.I hope you will realize that the initial portion is a OC material OC response where the C remainsconstant and then there comes a point of sigma p prime pre-consolidation pressure and beyondwhich the material becomes NC.Alright?.So, if you plot tow versus sigma relationship, so, you will get an OC response followed byNC response on the shear strength profile and the point of intersection of these tworesponses is the pre-consolidation pressure.So, for less moisture soil, we consider them as the dry soils and we consider this as theapparent cohesion.Apparent cohesion could be the over consolidation, you know value of the shear strength whichis associated with the soils.However, when we talk about the partially saturated soils.This is where the capillary phenomena takes a important role and we talk about the surfacetension and surface tension is nothing but suction in the soil.So, the best way to model the mechanism of mobilization of the tensile strength wouldbe to bring in the suction parameter.I think now, you can realize that why conventional geomechanics does not talk about the tensilestrength of geomaterials, because there forcefully we have saturated the soil samples, we havemade sure that the soil does not remain under unsaturated state.However, the tensile strength is going to get mobilized when the soils become partiallysaturated or they tend to become unsaturated.This is an interesting fact that the soils that swell exhibit higher tensile strength.Alright?.So, that is the arrangement of the water retention system on the on that you know minerals ofthe fine grained materials.So, more the holding capacity of the moisture, more the swelling pressure and once the swellingpressure is very high, the tensile strength of the material is going to be high.Now, what we do is, we try to relate the tensile strength by with you know, so many parameters.So, tensile strength is a function of the moisture content of the soil.It is gamma t, now gamma t basically indicates the total unit weight of the material whichis a function of specific gravity and soil type corresponds to mainly texture and CLis the clay content.So, higher the clay content higher this sigma t value, cation exchange capacity, then plasticityindex, specific surface area and the suction matrix suction alright?.So, all these terms put together would create the sigma t value.Yes, please.That high swelling pressure, swelling soils have more tensile strength means when we studythat suppose compaction curve, so the dry side supposed to have more swelling tendencyso they should have more tensile stress should develop, I think no you are not correct ifyou plot gamma d versus W and the suction pressure with develops as a function of moisturecontent, you will realize the maximum amount of suction is at the OMC.Alright?.So, what you are thinking is a totally reverse process.So, if you plot gamma d versus W and if you plot the porewater pressure versus w.So, if this is the OMC, what you will realize the suction pressure is going to be maximumat the OMC.So, what this indicates is that degree of compaction also plays a very very importantrole.And I hope you can easily follow this concept by the fact that the more the compacted amaterial would be you have expelled out most of the air which was present in the soil massand hence the suction is going to be much more so as you compact the soils the suctionbuilds up.A reverse process is like this, the more compacted the soil the more the suction present in it.But suppose if I fill or if I saturate the entire system then this suction gets lostand hence the shear strength decreases.So, this is the complete cycle which you have to consider.The simple logic is the more I compact the soils the more suction gets build in becauseI have expelled air from the pores.Now, this is the tensile strength which also influences the sensitivity of the soils.In conventional geomechanics, we have talked a lot about the sensitivity and we have ascribedsensitivity to the ratio of the shear strengths.Alright?.We say sensitivity is the shear strength of soils in its undisturbed state to the remoldedstate.So, one of the ways of looking at the tensile strength could be If I use the concept ofsensitivity and if I talk about the shear strength parameters associated with the soilmass in its undisturbed as well as the disturbed state, remolded state.Now, this is where the type of loading becomes very very important.So, when we are talking about the tensile strength, you know, we have to understandwhat type of loading the soil mass is undergoing through.So, there are three types of loading which normally we talk about tensile strength, tensilestresses, compressive stresses and shear stresses.Under no shear condition, if I consider what tensile and compressive stresses do to thesample, this becomes a critical situation and then we have to try to understand youknow, what tensile and comprehensive loading does to the arrangement of the grains in thesoil system.And this is where I would like to introduce the concept of the maximum exposed surfacearea, which is responsible for tensile and compressive loading.And ultimately it gets linked to the tensile strength, if you really want to understandhow exposed surface area influences the tensile strength of the material, we have to go intothe micro mechanics of the system.And the mccro mechanics looks like this.So, if you start with, let us say tensile loading, or the compressive loading, and theseare the grains, which are in the soil mass, and if I stretch it, you know what is goingto happen, when I stretch it, all the particles get repelled from each other.That means, in other words, each particle is now free to exhibit the hundred percentsurface area to the environment.So, this is a case when tensile loading causes, you know, exposure of the grains to the environment.I hope this part is clear to you.So, the more the more you stress the material, the particles get stretched out and they arefree to now interact with the environment.Environment could be water, contaminants, which are present in the liquid phase, gaseousphase, sometimes solid phase also, bacteria.So, all these are the attributes of the environment, what happens in case of the compression loading,I think you can realize when the compression loading occurs, the particles come closerto each other certain amount of their surface is getting hidden because of the compressiveforces.That means, when we talk about the compression loading, we are not doing justice with thematerial modelling, because when you bring particles together to each other, there issort of a shadow effect.So, each particle is shadowing the certain fraction of the surface of the another particle.Alright.So, what it indicates is the fundamental difference between tensile loading and the compressionloading is that exposure of the surface gets changed and hence we are not really modelingthe grains.The way we should have modeled them for finding out what should be the response of the loadingon the geomaterials particularly the fine grained materials.So, this is a very convincing model which has been used by people to put forth thisanalogy or the philosophy that why tensile loading should be considered for characterizationof the soils.So, truly speaking, the tensile strength is a parameter which can be utilized to characterizethe geomaterials.So, the question is, how would I go ahead, how would I obtain the tensile strength ofthe soils having done all these things with all convincing statements that tensile strengthis the most important governing parameter as far as the geomaterial characteristicsare concerned?You know there are two techniques which can be utilized, one is the direct measurementeither in the laboratory condition or in the field condition.Alright?.So, the type of photographs which I showed you where the entire backfill has crackedI can do the image analysis and I can find out how the tensile strength is getting mobilized,Sometimes we use softwares or the image analysis packages and we get the information.However, this is a very complicated process and one of my PhD scholars who is a facultymember, now at IIT Mandi, Dr. Uday Kala, he was the person who started imaging of thecracks, which he developed in the fine grained materials and this was his PhD thesis, wherewe have taken soil samples, which after desiccation crack or we have accelerated the crackingprocess by creating the environmental conditions in the laboratory.So, we did the simulation.So, when you do these type of software based image analysis thing, we get accurate anddirect measurement of the track patterns and their geometry.Area included in each segment, I will show it to you intersection of the tracks and itsinitiation in a particular soil mass I hope you realize that the whole system is verydynamic, because the cracking pattern is a function of time and the moment soils getsexposed to the environment, desiccation starts evaporation starts and there could be a situationwhere all of a sudden the entire soil mass may crack.So, if you want to capture the whole process of cracking of the geomaterials, you haveto utilize the modern gadgets where the entire sequence of desiccation can be video graphedand then you can discretize the events and then you can stitch them to see how this wholesituation has got developed and created.Another interesting or the big challenge would be having all these parameters known thatis what type of cracking pattern is what type of geometry the cracks follow, what is thearea included in each segment of the crack, you know, what is the line of intersectionof the cracks.Can I link all these parameters to the basic soil properties, because our hypothesis isthat soils are very intelligent materials?.So, they do not just crack for this fun sake.And another thing is the crack pattern is a unique fundamental behavior of a given soilmass provided the environmental conditions are maintained.So, using these hypotheses, what we have done is, we have linked or tried to link the parameterswhich we have listed over here with the physical, chemical and mineralogical characteristicsof the soils.And the way the suction profile gets developed in the Soil mass.Suction profile gets developed in the soil mass when the soil gets dried up expulsionof moisture alright?. and this expulsion of moisture is because of the heating, whichis a coupled phenomena.So, now I am sure you are dealing with a situation which is a dynamic situation.Everything is a function of time here.Until now you have not talked about the material property as a function of time.Here, the moisture content in the soil mass is a function of time, the suction in thesoil mass is a function of time.The crack width and the depth and the geometry of the track pattern itself is a functionof time.What we did is we have selected soils have known properties, exposed them to known environmentalconditions, we call this facility as environmental chamber.Where we can create different types of temperatures and humidities over a period of time.So, I am sure you are getting a feel of how environmental loading can be simulated onthe soil mass.Clear the way you used to do consolation test, where you started with one sigma value andthen you keep kept on doubling it after a certain time when the dial gauge readingsbecomes constant.When we talk about the environmental loading you know we have to have a specialized setupswhere we exposed the soils to different temperatures, humidity, wind velocity, solar cycle and soon.And then we simulate the loading conditions on this and these loading conditions are thethermal loading and of course the size of the sample, because you will realize thatthe analogy is from the consolidation test, if you take the oedometer sample after theconsolidation is over, you know the porewater pressure is not going to be uniform withinthe sample.It is going to be less at the two edges where the porous stone was kept and the porewaterpressures are going to be maximum in the center of the sample isochrone concept.So, sample size plays a very very important role.
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