 Welcome to lecture number 5 of module 1 of advanced geotechnical engineering. In the previous lecture we understood about different clay minerals like kaolinite, illite and mantumonite. In this lecture we will discuss about clay particle water interaction and index properties. So as we discuss, if you recollect and review once again, all clay particles are said to be natively charged and they are basically having a platey shape or they are called as platey particles. The electrical nature of clay particle is said or attributed to electric charges and of two phases of all platey particles have a negative charge. This is resulting mostly due to the isomorphous substitution that is not neutralized by interlayer cation bonding. The edge of the clay particles usually have a positive charge at low to moderate pH value. With increasing pH the edges of the clay particles can actually have negative charges and the net charge of clay particles is always negative. This particular example of the formation of bentonite cake in the borehole walls. In soil investigation projects we use bentonite which is predominantly having sodium based mantumonite having different grades. If you leave the bentonite for some time, it actually forms a thin cake and actually supports the borehole walls from collapsing. It is said that it is actually the pressure which can actually exert on the borehole walls by the hardened bentonite is about of the order of 10 kPa. This particular phenomenon is due to or it is said as electroporosis. Here what happens is that the negatively charged clay particles always have a tendency to move towards the positively charged particles which are there in the surrounding particles along the peripheries of the borehole walls. This is also visible in the phenomenon called electroporosis. The electrical nature of clay particles is mainly attributed to the negatively charged which is available along the two phases of the plate shaped particles and at low to moderate pH values the edges actually generally have positive charges but with increasing pH that can lead to have negative charge. The another reason is that exchangeable ions. Since the soil must be electrically neutral the negative phases attract exchangeable cations like sodium plus, calcium 2 plus, mg 2 plus etc. So the positive edges attract exchangeable anions or cations if negatively charged in case of a high pH. Another reason is that the surface charge density which is actually called as sigma 0 and which is nothing but number of charges per unit area. If you look into this table for a clay mineral kaolinite the cation exchange capacity that is milligram equivalent per 100 grams which is 5 and the specific surface area is about 15 meters square per gram. So by using sigma 0 is equal to CEC divided by SSA the kaolinite actually have 0.3 to charges per meter square whereas sodium basal montymerlite with CEC of 100 milligram equivalent per 100 gram with a SSA of 800 meter square per gram the surface charge density is about 0.18 charges per square meter. This is low because of the higher surface area. So the structure of the clay soils this makes actually the clay soils have to have some different patterns of structures especially for fine-grained soils forces between clay mineral particles if the two particles which are plate shaped particles approach each other in suspension the forces acting on them are predominantly the van der Waals forces of attraction and the repulsion between the two positively charged ions absorbed water layers that is if you have got a absorbed water layers the ions in between the two positively charged absorbed the repulsion between the two positively charged ionized absorbed water layers. So here in this figure the clay particle which is with negative charge and with the cations this is absorbed layer which is actually shown here. So this makes actually a very small separations the van der Waals forces are always larger and particles which approach sufficient closely will adhere however the van der Waals forces decrease rapidly with increasing separation. If the absorbed layer is thick the repulsion will be large at a distance from the surface which the van der Waals forces are small. So this makes actually because of this the particles will remain in a state called dispersed. This particular they settle independently and the particles will be very difficult to bring together because of the net repulsion which is actually happening between the two clay particles. So here you are seeing a plate shaped clay particles and this is absorbed layer and here because of the net repulsion which is actually happening in the cations present in the soil then they actually particles remain in the dispersed state and then they settle independently. If you wanted to the contact between the dispersed particles will only be established if an external force is applied particularly in case of clay soil if this possible only if an expansive soil is if an external force is applied which is large enough to overcome the net repulsive forces. So in order to bring the particles closer you need to apply higher external forces so that to overcome these repulsive forces. If the absorbed layer is thin and there will be little or no net repulsion at any distance and random movements of the particles will be enough to bring them in contact. So this process of is called flocculation. So here what will happen is that because of the prevalent positive charges along the edges of the plate shaped particles and some resulting attraction between the edges of the clay particles and the faces of the clay particles face to edge and some face to face orientation takes place. So because of this what will happen is that the groups of particles will flock together and this is said as this result this is resulted because of the resulting net attraction. So the group of particles settle together or flock together and this state is actually is structure is resulting as a flocculated structure. So here it is explained in this graph the net force between two particles in a suspension. In case of a if you take the distance between the crystal faces with close to the face with an increase in the ion concentration what is actually happening is that the net attraction will keep on decreasing close to the face it is very high. But in case of low ion concentration the net repulsion is very high. So as I said the dispersive structure of the clay soils which is resulted predominantly because of the net repulsion the net forces of repulsion are greatest in the case of particles approaching face to face. This is an example for this is that Lacostrine clays deposited in fresh lakes generally have a dispersive structure. In this case few of the particles are in direct contact most being separated by the adsorbed water layers. So here schematically the dispersive structure of clay soils is shown here the net forces of repulsion are greatest in the cases of particles approaching face to face. So this is actually predominant in Lacostrine clays basically they are deposited in fresh water lakes and generally they have a dispersive structure. Another structure which is called as flocculated structure which results because of the net attraction. This is predominantly happens for the marine soil deposits particularly for marine clays deposited in seawater in which ion concentration is high. So that the adsorbed water layers are thin and generally have a flocculated structure with edge to edge orientation or edge to face orientation. So this results in a void space formation a truss like structure and where the flocculated structure has a open structure with large void spaces and with particles attracted to each other with edge to the edge or edge to face contacts. So this is resulting as a flocculated structure of the clay soils and this is predominantly occurs in marine clays deposited in seawater in which ion concentration is high. So here once again the structure of the clay soils fine grained soils a typical arrangement of the plated particles is shown here with the low ion concentration when the pH is actually less than 7 there is a possibility of the prevalence of the dispersed structure with high ion concentration because of the edge to face attraction or the resulting of the net attractive forces between the particles there is a possibility of the flocculated structure. So structure of the clay soils here predominantly undistributed flocculated structure of the marine clay is shown typically a clay with an undistributed flocculated structure will possess large wide opening spaces and here in between the silt particles are shown here schematically. So this is an undistributed flocculated structure of a marine clay which predominantly has large wide openings and it has edge to face orientations and some sort of edge to edge attractions here which are actually shown in this soil structure. When plated particles are carried into fresh water lake they do not flocculate and settle along with silt particles as they do in salt water. So the remolding of the flocculated structure results in dispersed structure. So if you remold the flocculent structure the orientations of the clay particles takes place and it results in the dispersed structure. So remolded or dispersed structure of the fresh water deposit which is actually shown here. So here typical clay structures along with dispersed which is shown here flocculated which is shown here and these are also which are involved they are book house structures where you have got several particles attracted to each other and they have a similar to a flocculated structure and then you also have the turbo static clay structures. And here schematically once again a natural clay structure which is shown here with the large particles which are actually silt and then you have the flocculations which actually have taken place and these are the voids shown in the particular arrangement. So in order to identify this particular arrangement there are many methods are there like one of the predominant method which is actually used is by ordinary microscope basically used for coarse grained soils only when you look you can see the coarse particles. But if you wanted to look into the particular arrangement and possible fabric of this different types of soils one should actually use scanning electron microscopy SCM it is popularly called and this is ideally suited for clay soils as the resolution is sufficiently high and it is possible to go for higher magnifications where up to 10 to the power of 5 times magnification is possible. So SCM is a type of electron microscope that images a sample by scanning it with a beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing the signal and then that contains information about the sample surface topography composition and other properties such as electrical conductivity etc. So here the basically the output results in arrangement of the particles. So in this slide scanning electron microscopy of a coal ash blended with sodium bentonite is shown here. So as you all know the coal ash particles are predominantly called as xenospheres and they are basically spherical particles. And sodium bentonite which is actually having about 80% of mineral is mantleite which is actually having a gel like structure. So you can see here the combination of coal ash blended with sodium bentonite up to 15% by dry weight also is actually shown with the magnification of 650 times. So here in this as a typical example SCM of coal ash blended with sodium bentonite is shown. In this slide for understanding SCM of locally available silt which is actually shown here in natural form when we see it will be like this. But when you look into the SCM through with the help of the SCM you can actually see the large grain particles in the soil with a 500 times magnification. Now having discussed about the particular arrangement particularly because resulting due to clay water interaction and also we also understood that clays are predominantly consists of different minerals and their properties are actually influenced by the mineral present in the clay particles. See if you wanted to identify there are three different methods are there for the identifying clay minerals. The first method being X-ray diffraction it is called XRD method. The second method is differential thermal analysis DTA and third method which is also used by using Casagrande's plasticity chart but we will discuss about the Casagrande's plasticity chart in the later part of the lectures. One thing we must understand is that no one method is satisfactory for identification this is partly because of the interference of minerals in a mixer and the range of composition and crystal structure of clays from different sources. So these methods will give an indication about the type of the clay minerals present but no one method is satisfactory for identification. This is attributed to the interference of minerals in a mixer and the range of composition and crystal structure of clays from different sources. Let us try to understand about the X-ray diffraction method. The most widely used method of identification of clay minerals is from an X-ray diffraction pattern basically of a powdered sample of the clay size fractions of a soil. Minerals can usually be identified from diffraction lines so the basic principle of the XRD is minerals with regular or repeating patterns of crystal structures diffract X-rays. So this is actually used and based on that the different types of minerals are identified. So the schematic diagram of the X-ray diffraction unit for crystal identification is shown in this slide. So here the sample is actually kept and this is called as the Giger-Muller counting tube and this is that angle of the counter that is called as 2 theta. So with this the resulting you will get the XRD spectra which is actually shown here along this axis degrees 2 theta and then this here the intensity of the reflection on the y axis is plotted. So you have different peaks here. So this is the typical XRD pattern of kaolinite, montumonite and illite. So this is for kaolinite and this is for montumonite and this is for illite mineral. So you can see different types of the XRD patterns for different minerals present in the soils. So different minerals with different crystalline structures will have different X-ray diffraction patterns that is shown here and in fact these different patterns help to identify different minerals. So this is actually used to identify different minerals. So in this slide a typical XRD pattern of locally available expansive soil and which is also called as black cotton soil is shown here. On the y axis the intensity of reflection is shown and here on the x axis 2 theta which is plotted. So the interpretation of this XRD pattern reads as mt that is montumonite has 48 to 50% and quads about 30 to 32%, calcite 15 to 16% and ta say about 1 to 2%. So this is a typical XRD pattern of locally available expansive soil which actually has got 50% of mineral, predominant mineral is montumonite. So this could be one of the reason for expansive soil for exhibiting shrink and swell characteristics that means that in case of exposed to water the expansive soil tend to increase its volume and in case of divide of water then there is a possibility of shrinking of the soil. Here in this slide the XRD diffraction spectra for bentonite is shown here and here the intensity of reflection in counts this is about 400 counts with the different diffraction pattern it can be seen here. So the montumonite and megamite and ametite and entase and calcite is found to have very low peaks of calcite that is low peaks of kaolinite that is kaolin traces are actually reported. So these peaks which are predominant peaks are one peak and second peak montumonite and then here another this is megahemite. This indicates that about 80% of the mineral clay mineral present in this bentonite is montumonite then followed by megahemite and ametite and entase and calcite and some traces of kaolin. So what we have seen is that we actually try to understand how XRD diffraction can be used to identify different clay minerals with an example of expansive soil as well as some bentonite with a grade A which is called which is actually having about 85% of montumonite. As I said earlier the demerits which are reported they are not suitable for soils with the mixers of clay minerals, organic matter and non clay mineral constituents and basically they have the inability to specify proportions of each mineral in a mixer. So the typical or we can put the demerits of XRD method as follows not suitable for soils with the mixers of clay minerals, organic matter and non clay mineral constituents and inability to specify the proportions of each mineral in a mixer. Then the second method which is differential thermal analysis method DTA method which determines the temperature at which the changes occur in a mineral when it is heated continuously to a higher temperature. So the intensity of change is proportional to the amount of mineral present in the soil. So the clay lose water or go through the phase changes at specific temperatures. So different when the clay soils have different minerals they actually have got they change their phases at different temperatures. So the temperature at which these reactions occur are the characteristics of the mineral and can therefore be used for identification. So this principle is used for identifying clay minerals by using the differential thermal analysis. So in this slide the DTA apparatus is shown here and associated with the recording and controlling mechanisms. For DTA measurement a sample of clay and a sample of inert material are slowly heated in a furnace which is actually shown here schematically. And calcined aluminum oxide or ceramic are used as the reference material. So the reference materials here are calcined aluminum oxide or ceramic basically they are used as the reference material. So when a temperature is reached at which the clay loses water by vibration the sample temperature will drop below the inert material and that is actually used as a reference indication. So when you look into the review basically we try to understand about the particulate arrangement in coarse grained soils and fine grained soils. And we said that in the particulate arrangement for coarse grained soils because of the large particles they actually have bulk structures. When it comes to fine grained soils two predominant structures which are actually possible are dispersed structure and flocculant structure or dispersed or flocculated structures. And then because these particularly bulk structures can also have loose bulk structure or dense bulk structure and so a parameter called relative density has been defined. And forces between the clay mineral particles depending upon the ion concentrations they can actually have net attraction or net repulsion. Net attraction results in a flocculant structure, net repulsion results in a dispersed structure. So the dispersed and flocculant structures are actually discussed. Now having introduced understood about the clay minerals and their so called we understood that fine grained soils are strongly influenced by the presence of the minerals or the influenced by the type of mineral. In case of a sandy soil the type of mineral may not have much influence. Now in order to classify the soil basically we need to understand about the index properties then particularly with two different types of soil again here large particle sizes and fine particle sizes. So the index properties refers to those properties of a soil that indicate the type and condition of soil. They basically provide a relationship to the structural properties such as the strength that is capacity to take the load and compressibility or tendency for swelling, how swellable they are and permeability which is nothing but a property of the soil which indicates the ease with which soil will allow the water to flow. So these index properties can be broadly classified into two heads, one is soil grain properties other one is soil aggregate properties. The development of the availability to think of soils in terms of numerical values of their index properties should be one of the foremost aims of every engineer who deals with soil mechanics. So here once we have this index property values it is possible to understand about the type of soil or one of the foremost aim for every engineer who deals with the soil mechanics. So the soil grain properties are the properties of individual particles of which the soil is composed without reference to manner in which these particles are arranged in a soil mass. Particularly example mineralogical composition that we just now understood specific gravity of solids and size and shape of the grains. So soil grain properties are the properties of the individual particles of which the soil is composed without reference to the manner in which these particles are arranged in the soil mass. Aggregate properties or soil aggregate properties which are dependent on the soil mass as a whole and thus represents the collective behavior of the soil. The soil aggregate properties are function of stress history that is the type of loading the soil has been subjected, mode of soil formation the type of deposit and the soil structure that is the particular arrangement that is whether dispersed structure or flocculated structure or whether it is a bulk structure in loose condition or bulk structure in a dense condition. If this aggregate refers to the soil itself it may differ in porosity, retub density, water and air content and consistency. Although the soil grain properties are commonly used for identification purpose the engineer should realize that the soil aggregate properties have greater influence on the engineering behavior of soil because the engineering structures are founded on the natural deposits or undisturbed soil mass. So although the soil grain properties are commonly used for identification purpose the engineer should realize that the soil aggregate properties have a greater influence on the engineering behavior of a soil mass because the engineering structures are founded on natural deposits or undisturbed soil mass. So the index properties are broadly divided into the grain size distribution particularly the gradation of the soil and consistency limits and in order to get the particle size distribution the two heads one is the mechanical analysis or sieve analysis which is actually done for coarse grain particles and hydrometer analysis which is basically done for fine grain particles. Generally in a given soil if the percentage fines that is particles passing 75 micron that is 0.075 mm if there are more than 12 percent then it is mandatory to do hydrometer analysis basically to assess the percentage fines because the presence of present as fines can influence the properties of the soil. So mechanical analysis particularly the coarse grain for soil, coarse grains for coarse grain soils it actually has got dry method and wet method and the consistency limits basically liquid limit, plastic limit and the plasticity index and shrinkage limit. So these are the nothing but water contents at different physical states of the soil. These are basically for fine grain soils and if you have these properties then it is possible for us to group the soils having identical properties. So when grouping of the soil takes place it is easy for soil engineer or a geotechnical engineer to understand and use these soils according to their function and requirement. So the grain size distribution GSD in soil mechanics it is virtually always useful to quantify the size of grains in a type of soil. Since in a given soil will often be made up of grains of many different sizes sizes are measured in terms of grain size distribution and the soil can actually have a possibility of wide range of particles. So the distribution particularly the particle size is indicated on the plotted on the semi logarithmic plot which is on the x axis particle size which is plotted on the logarithmic scale and on the y axis the percentage finer is plotted. So that this is basically done to considering the wide range of the particles in the soil and GSD assists in providing the rough estimates of soil engineering properties. So if you have the GSD characteristics it is possible to estimate some rough estimation of the engineering properties of the soil. And nowadays the active research interest is that the accurate prediction of the soil properties based largely on the GSDs and wide ratio and soil particle characteristics. So nowadays is being attempted in unsaturated soil mechanics that by using grain size distribution the properties of soils are being attempted to be predicted. So when measuring GSDs for soils two methods are generally used for grains larger than 0.075 mm sieving method or sieve analysis is used for grains in the range of 0.075 to 0.5 micropeters the hydrometer analysis is used. So in the sieve analysis the known mass of soil which is actually taken and the soil is placed in the set of sieves having different sizes. It is not possible exactly to determine the shape of the particles because the size which is actually predominantly nothing but the size of the mesh which is actually used for sieving the particles. So here the particle size is nothing but the approximate size which is actually prevalent in the particular soil mass. So if this soil particle is smaller than the mesh use then there is a possibility of the soil particle passing through the particular sieve. If the particle size is larger than the mesh use then there is a possibility mesh size use there is a possibility that it will actually retain there. So here the set of sieves run from higher sizes to lower sizes and here the percentage retained on the 75 micron if it is less than 12% then there is a possibility of one can actually ignore in performing sieve analysis in performing hydrometer analysis but in otherwise it is required to carry out the hydrometer analysis to assess the percentage points. So here typical grain size distribution curves are shown here the soil particles which are actually sieved through set of sieves here it is said that the grain sizes are larger this side and the smaller sizes are here that means that the fine particle sizes are somewhere down the line here and this is a well graded soil basically the well graded soils are very much required for construction and this is uniformly graded soil and this is a gap graded soil or is also called as a bimodal soil. So gap graded soil where some size of the grains are tend to miss so that in the gap graded soil which is also called as the bimodal soil and a full earth erotical curve which is actually shown here in the idealized fuller packing it says that in the large spaces which are actually filled by the small particles so in the process actually what will happen is that there is a possibility of having this type of full earth erotical curve. So the particle sizes actually vary from large size to small size and the soil mass actually can have different range of particles so if the soil mass has uniformly sized particles then it is called as poorly graded or uniformly graded soil and if the soil that means that the slope of this curve is almost equal to unity and when the slope is actually very flat here then that means that there is a well distribution of all the particles and when the nature of this curve is like this then it is called gap graded or bimodal soil. In order to determine the finer fraction of the soil that is passing 75 micron seam the sedimentation analysis is widely used. So this is basically based on the principle of the Stokes law. It is assumed as a first approximation that fine grained soil particles can be visualized as small spears. Though we actually have discussed the soil particles are platey shaped particles but here it is assumed as the fine grained particles as spears and spherical particles falling in a liquid of infinite extent and all particles have the same unit weight. So the particles are actually assumed to have identical unit weight and the spherical particles are assumed to fall in a liquid in a having infinite extent. So that means that without any boundaries the particles reach at constant terminal velocity within few seconds and afterwards after it is allowed to fall that means that once the particles are suspended in a suspension the particles reach constant terminal velocity within few seconds after it is allowed to fall. So although the clay particles are far from spherical though we said that in this sedimentation analysis we assume clay particles as spherical particles the application of Stokes law based on equivalent diameters provide a basis for arriving at the GSD of the fine grained particles is appears to be sufficiently realistic. So here when you consider a particle a spherical particle of diameter D when it is released in a medium having certain dynamic viscosity and then it is subjected to two forces predominantly one is settling force that is because of the self weight of the particle or in this case the bionic weight of the particle and it is opposed by a drag force which is actually acting on the periphery of the particle. So the drag forces act along the periphery of the particles. So according to Stokes law the viscous drag force Fd on a spherical body moving through a laminar fluid at a steady velocity V is given by Fd is equal to 3 pi mu Vd where D is the diameter of the sphere which is representing the particle size and V is the steady velocity of the body and mu is the dynamic viscosity of the fluid in which the particle is suspended. Now if you drop a grain of soil into a viscous fluid it eventually achieves a terminal velocity V where there is a balance of forces between the viscous drag forces that is gravity weight forces and bionic forces. So once you have that one Fg-Fb the net force is nothing but 1 6th of Gs-1 into gamma W into pi Dq. So for equilibrium of the soil grain Fd which is that is drag force is equal to Fg is nothing but the self weight that is gravity force Fb is nothing but the bionic force. See from the equation if you solve for the equilibrium or terminal velocity V of the soil grain can be obtained as V is equal to Gs-1 gamma W divided by 18 mu D2. So this is said as Stokes law this is after George Stokes which is given in 1891. So this indicates that the larger the soil grain the faster it settles that means V is proportional to D2. In Stokes law the terminal velocity of the particle is proportional to particle size square. So the larger the soil grain the faster it settles in water. This critical fact is used in the hydrometer analysis to obtain the grain size distribution for the fine grain soil. So that means that if you have got a finer particles particularly having say bentonite the terminal velocity is proportional to it will be very very low that means that they have a tendency of taking large time for undergoing sedimentation. See the process of sedimentation of dispersal specimen the theory of this sedimentation is based on the fact that large particles in suspensionally liquid settle more quickly than small particles because we said that V is proportional to D2 assuming that all particles have similar densities and shapes. If all the particles were of single size with effective diameter D by knowing the terminal velocity we can determine time it will take for the particle to settle that is Td can be given as 18 mu Z Gs-1 into gamma W D2. So if you see here Td is inversely proportional to Td is directly proportional to 1 by D2. So this indicates that the larger the particle size the less time it takes the smaller the particle size the more time it takes to sedimentation. The limitations of Stokes law can be understood the soil particles are not truly spherical and sedimentation is done in a jar having D greater than 0.2 mm and causes turbulence in water and D less than 0.002 mm that is 0.2 micron causes Brownian movement the two small velocities of settlement can be eliminated with less concentrations that means that the soil particles are basically are not truly spherical and the sedimentation is done in a jar with a confined walls and for this causes turbulence in water and the Brownian movement occurs and the two small velocities of settlement can be eliminated this can be eliminated with less concentration this Brownian movement particularly for bentonite can be eliminated with less concentration and another limitation we said there is that flock formation due to inadequate dispersion the because of the attraction of the these particles net attraction of the particles falsely some flocks are actually formed generally in doing sedimentation analysis it is before preparing the soil specimen for hydrometer test the soil is adequately dispersed and unequal specific gravity of the all particles also is possible that means that when you have got a clay particle like for example we have discussed about coal ash blended with bentonite so we have got one particle one type of particle is actually having very low specific gravity that is coal ash and another side we have bentonite which is actually having a specific gravity of 2.7 or 2.8 or so so but this is found to be insignificant for soil particles with the fine fraction. So in the present in this lecture we try to understand about the clay particle interaction and the index properties particularly about how to actually perform you know see particularly the dry sieve analysis and wet sieve analysis for getting the large particle sizes which are for the force for the coarse grained soils. So in the next lecture we try to understand about the hydrometer analysis and different errors which are actually used different corrections which are actually used in interpreting these hydrometer analysis results and the reasons for the same and then we will try to look into you know for assessing or understanding about the different index properties namely liquid limit plastic limit and plasticity index and the shrinkage limit once you have these things then we will try to attempt in the next lecture after having discussed these issues we will try to do some couple of problems as well as attempts to do the discuss about the soil classification systems.