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Contents :-

Introducation Soil as a dispersed system - The Active Soil fraction Surface behavior of clay particles Physical behavior of soil water system Hydration of clay Viscosity of colloidal clays Swelling of colloidal clays Soil consistency Forms of Soil consistency Consistency of Moist & wet soils Soil Plasticity Soil Structure – classification and genesis Soil as a Three Phase System. Conclusion Bibliography

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Introduction One of the fundamental subject of Soil science physics which deals with the study of mechanics heat, optics as they relate to soil. The properties related to the mechanical behavior of the soil mass are referred to as the physical properties of soils. The study of these properties is known as soil physics, Soil physics may thus be defined as a branch of Soil Science dealing with the state and movement of matter and transformation of energy in the soil. The purpose of Soil physics is to bring together the information on the physical of soils and their effects on bearing capacity of Soil.

From Soil Particle receive essential element, water, oxygen, and mechanical support. Soil temperature controls the intensity of biophysical biochemical and microbiological processes in the soil. Soil physics deals with soil water, soil air, soil temperature and mechanical support. In order to understand al these factors, study of soil physics is very essential. Thus soil physics aims not in biospheres but also to manage the soil property by means of excavation, constriction soil testing and Geotechnical mechanism, soil aeration and regulation of soil temperature.

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Soil as a dispersed system

Dispersion is a phenomenon by which secondary soil participles such as aggregates break into primary soil particles such as aggregates break into primary soil particles like clay is distributed though the dispersion medium such as water. Hydration of exchangeable cations is the dominant factor for dispersion. Monovalent cations like lithium sodium and potassium have high dispersing power because they are highly hydrated ions. Clary saturated with these monovalent cations causes dispersion due to the expansion of double layer resulting saturated clays and minimum in potassium saturated clays. These variations are due to the variations in their degree of hydration the dispersion of clays by monovalent cations is affected by nature of their compounds (Salts) that are present in solution. For example soils contacting sodium on the exchangeable complex and an excess of sodium salts like NaCl or Na2SO4 in solution decreases the disparity because the disparity because the presence of sodium salts causes repression of the thickness of electrical double layer of the adsorbed sodium ions resulting flocculation of the colloidal material. But when the excess salts are leached out, sodium saturated colloidal material hydrolyzes to from NaOH and Na2 CO3 and such system in highly dispersed State. 4

Hydrodynamic dispersion :-It is a non steady irreversible process of transport of

dissolved ions (Salts) through soil pores of various sizes and shapes. As the flow in the large pores is factor than the flow in small pores and is much faster at the centre of the pore than near the surface, the solute particles stay longer near the surface than in the centre during the transport and form a peak – like appearance. Besides this, due to tortuosity, the flow velocity of solution in soil varies in both magnitude and direction from point to point. This variation in velocity and erratic flow patterns along with the tortuous flow paths and between adjacent flow path causes mechanical dispersion of the solute present in the displacing solution into the displaced liquid (water present originally). This spreading phenomenon has been called the hydrodynamic dispersion or miscible displacement in a porous medium. The effect of molecular diffusion on the over all dispersion is more significant at low dispersion. As the displacing and displaced solutions do not generally have identical densities nor viscosities, the dispersion is further increased.

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The Active Soil fraction

After complete dispersion of soil separates, soil particles are graded into number of size groups for knowing their percentage distribution in the soil mass and the process involved for this is termed as fractionation. The coarser particles such as sand fractions are estimated through direct sieving and the finer particle such as silt and clay fractions are determined by sedition methods such as pipette method, hydrometer method, elutriation method and centrifugation method of which pipette method and hydrometer method are commonly adopted. Direct sieving :

In this method the dispersed soil suspension in passed though a nest of sieves having different openings, the particles of different sizes will retain on different sieves. The rangeof diameter of sieve’s opening will be the range of diameter of soil separates. Sieving is usually used for fractionation of particles which are larger than 0.05 mm diameter.

Test sieves consist of a woven wire screen ith square apertures mounted in a frame. The detailed specifications of sieve series showing the relationship of size opf sieves, sieve number (mesh per inch) sieve opening and wire diameter are given

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Size of Sieve

Sieve number (mesh per inch )

Sieve opening (mm)

Wire diameter

(mm)4000200011901000840500250210177149745337

051016182035607080

100200270400

40002000119010000.8400.5000.2500.2100.1770.1490.0740.0530.037

1.3700.9000.6500.5250.5100.3150.1800.1520.1310.1100.0530.0370.025

Specification for Sieves Series (U.S. Bureau of standards )

Sieving may be done by hand or in sieve shaking machine. Usually coarse sand faction ISSS and all sand fraction of USDA are fractionated by sieving and their amount is estimated by taking the dry weight of the portion retained on different sieves.

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Sedimentation methods :-

Sedimentation means the settling of particles in a fluid under the influence of centrifugation the amount of material above or below a specified size is determined either by abstraction of an aliquot of suspension that is then dried an the residue weighed, by measuring the change in the density of suspension or by measuring the amount of sediment that has settled in a suitable container after a certain time. The important sedimentation methods are pipette method, hydrometer method, centrifugation method elutriation method of which pipette method and hydrometer method are commonly adopted.

Pipette method : For fraction of size fraction smaller than 63 um (slit and clay particles) by pipette method the dispersed soil suspension obtained after sieve analysis is transferred to a stoppered 1000\ ml measuring cylinder and the volume is made up to one liter. The suspension is stlrred thoroughly allowed to stand and sampled at different time intervals. The different size particles will fall through the fluid at clay and clay, a measured amount of aliquot is removed by pipette from the definite depth below the surface of the suspension and taken in a previously weighed container, dried in an oven and weighed and the amount of silt plus clay and clay are estimated.

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Silt frction is determined from the differences between silt plus clay and clay. Fine sand fraction of ISSS is estimated by differences or may be determined by elutriation (washing) method.

(b) Htydrometer method:

for fractionation of finer (Silt and clay ) particles by hydrometer method, the dispersed soil suspension after passing though the desired siees is transferred to the specified cylinder supplied with the hydromettter and the volume is made up to the mark. After though string two drop of octan -2-01 are added to control the frothing caused by agitation. Then the hydrometer is immersed and the hydrometer reading are noted at different time intervals for estimation of ailt plus and clay content of the spoil, silt fraction is determined from the difference between silt plus clay and clay. Fine sand fraction of ISSS is estimated by differences or may be determined by elutriation method.

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Surface behavior of clay particles

The Specific surface may be defiende as the total surface area per unuit mass of dry soil (Sm) or the total surface area per unit Volume of Soil (Svt) Sm is expressed as square centimeter of surface per gram of dry soil ; Svs is expressed as square centimeter of surface per cubic centimeter of soil solids and Svt is expressed as square centimeter of surface per

So

Sm = SA/MS Svs = SA/VS

Svt= SA/VI

Where, SA is the total surface area Ms is the mass of soil or dry soil Vs is the volume of soil solids and Vt is the total volume of soil.

Specific surface is a very important characteristic because, most of the physical and chemical reactions occur at the surface and therefore, the amount of reaction is approximately proportional to the specific surface.

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Factors affecting the specific surface of soil particles

Specific surface of soil particle depends mainly on there factors such as

(a) size of particle (b) shape of particle, (c) type of particle.

(a) Size of particle: Specific surface mainly arises from the size of the

particle. As the particle size decreases specific surface increases. A cube of l cm

sides or sphere of l cm diameter has a specific surface of 6 cm 2/cm/cmk3 a cube

of 0.1 cm sides or sphere of 0.1 cm diameter has a specific surface of 60

Cm2/cm3. Thus hen the sides of a cube or radius of sphere is reduced to 1/10th

the specific surface per unit volume incrassates 10 times. A disk or platy particle

having 1x10-4 cm thickness and 1.155x10-4 cm radius has a volume of 4.2x10-

12 cm and specific is 37142cm2/cm3 a disk of 1x10-5 cm thick and 3.65x10-4

cm radius has a volume 4.2x10-12 cm3 and specific surface is 37142 cm2/cm3

a disk of 1x10-5 cm thick and 3.6510-4 \ \ cm radius has a volume 4.2x10-12 cm

and the specific surface is 204524 cm-2/cm-3 thus for the same volume (4.2x 10-

12 cm3) of disks when the thickness is reduced to 1/10th the specific surface per

unit volume increases to 5.5 times. 11

b. Shape of particles : Specific surface varies with the shape of the particles. Spherical shaped particles has the smallest specific, for the some volume the specific surface of a cube is 1.239 time larger than that a sphere. If a sphere is deformed into a disc or plate the specific surface increases. Platy or disks hoped particles exhibit the greatest specific surface. For the same volume, when the thickness of a disk is same as the radius of the sphere the specific surface per unit volume of a disk increases 1.238 time (or 23.8 percent) over that of a sphere. Flattened or elongated particles have greater specific surface per unit volume or per unit mass than do equal dimensional particles like cubical or spherical particles

c. Type of particle : two types of particles are present in the soil. One is swelling types i.e. they expand on imbibing water and the other is non swelling type i.e. they do not expand on imbibing water Both types have external surface but swelling type of particles exhibit surface between the individual sheets in addition to the external surface. The amount of specifrice surface in between the individual sheet is much more than that in the external surface So the specific surface of swelling types particles is much larger as compared to that of nopn swelling particles.

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Physical behavior of soil water system

The Concept of potential was first applied to soil water by Buckingham (1907) the concept of soil instead of the absolute amount of potential energy contend in soil water. It expresses the potential energy of soil water in a particular location in the profile as compared to that of pure water at standard pressure and temperature in a standard reference state. The standard reference state is usually assumed as a hypothetical reservoir of pure and free water and atmospheric pressure at the same temperature as the soil water (or at some specific temperature) and at a given or constant elevation. Since the elevation of the of the hypothetical reservoir depends on will, the potential as determined by comparison with the standard reference state cannot be an absolute potential but it can be the relative level of the specific potential energy of water it different location within the soil

Determination of the absolute potential energy level of soil water is a very difficult task. Actually there is no need of absolute potential energy level of soil water because the relative values of soil water potential energy in different location within the soil needed to predict the rate and direction of movement of water in the soil, the difference in potential energy levels between the pure water I the reference state and sate of soil water is termed as soil water potential. If all soil water potential values have lower soil water potential. 13

The soil water potential is due to several forces, each of which is component of total soil water potential the most important forces acting on soil water are (a) gravitational forces which tends to pull the wither downward (b) adsorption and capillary forces resulting from the affinity of the water to the solid particles (matrix) including its process and particle surface together, (c) osmotic forces caused by the attrition of ions or other solutes for water and the corresponding potentials are gravitational potential (øg) metric potential (øg) and osmotic potential (ø0) respectively. Another factor which may affect the forces acting on the soil water is due to change in the pressure of the ambient air which has been termed as pneumatic potential (øm) and plenum aci potential (øa) and in saturated condition a potential that is due to weight of water is known as pressure potential (øp). The sum of these potentials is termed as total soil water potential (ø1) So the soil water potential (øt) is

øt= øg + øp + ø0 + …………(1)

Under unsaturated condition ø1 is :

øt= øg + øm + øA + ø0 -----------(2)

Under unsaturated condition ø1 is : øt= øg + øs + ø0 ----------- (3)

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Where øs is the submergence potential which depends on te submergence depth. The dots on the right side signify the additional terms like overburden potential (øп) or other terms which are theoretically possible. The sum of gravitational (øg) and pressure potential (øp) is called hydraulic potential (øh)

So Øh= øg+ øp

Defferences between hydraulic potential at different places in the soil provied the driving force for the movement of Soil water referred to as water.

The some of matric pueumatic and osmotic potentials is referred to as water potential (øw)

Øw= øm + øa + øo

Water potential is dircectly related to the relative humidity of vapour in equilibrium with the liquied phase in soil and plant øw is an important measure of plant water status and is also important in saline soil. Where the osmotic potential of the soil solution of sufficient magnitude to influence plant water uptake.

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Viscosity of colloidal clays :-

We simulate clay-like colloids, for which in many cases the attractive Van-der-Waalsforces are relevant. They are sometimes called“ peloids” (Greek: clay-like). In soil mechanics there is a need to better understand materials containing particles of μm size and below. As a model system we have chosen a suspensionof Al2O3 particles with a diameter of0.37 μm. Al2O3 is not only a cheap testing materialfor investigations related to soil mechanics, but it is also an important material for ceramics.In process engineering one of the basic questions is, how to obtain components of a predefined shape. Wet processing of suspensions,followed by a sinter process is a common practice here. Nevertheless, to optimize the production process and to improve the homogeneity and strength of the fabricated work piece one has to understand the complex rheological behaviorof the suspension and its relation to the microscopic structure. This knowledge in turncan be applied to soil mechanics. Shear thinning as observed in our simulations and experiments is a very important mechanism for the dynamics of landslides making them more dangerous. In this paper we present our simulation results of sheared suspensions of Al2O3 particles. The overall behavior is strongly determined bythe effective interaction potential between the particles in the suspension. The potentials canbe related to experimental conditions within Debye-H¨uckel theory, and thus we can compare

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our simulation results to experimental data. Our paper is organized as follows: first we

shortly describe our MD implementation followed by a short sketch of the SRD simulation method, and a description of how we have implemented our shear cell. The simulation method is described in detail in ref. [1]. Then we describe the 2pK model which relates our simulation parameters with the pH-value adjusted in the experiment. A short description of the simulation setup and of the experiments carried out follows. After that we present our simulation results and compare them to the experimental data. Finally a summary is given. II. MOLECULAR DYNAMICS In the MD part of our simulation we include effective electrostatic interactions and van der Waals attraction, a lubrication force and Hertz an contact forces. The electrostatic and van der Waals potential are usually referred to as DLVO potentials[2, 3, 4], which capture quite well the static properties of colloidal particles in aqueous suspensions.

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The effective surface potential is the electrostatic potential at the border between thediffuse layer and the compact layer, it may therefore be identified with the -potential. It includes the effect of the bare charge of the colloidal particle itself, as well as the charge of the ions in the Stern layer, where the ions are bound permanently to the colloidal particle. In other words, DLVO theory uses a renormalized surface charge, which we determine by the model described in Sec. IV. is the inverse Debye length defined by 2 = 8`BI. The Bjerrum length `B := e2 4"0"r measures the distance at which the electrostatic interaction of two elementary charges amounts −1 = kBT. "0 is the permittivity of the vacuum,"r the relative dielectric constant of the solvent (we use 81 for water, `B = 7°A for room temperature). Long range hydrodynamic interactions are taken into account in the simulation for the fluid as described below. This can only reproduce interactions correctly down to a certain level. On shorter distances, a lubrication force has to be introduced explicitly in the molecular dynamics simulation

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Phase diagramDepending on the experimental conditions one can obtain three different phases: A clustered region, a suspended phase and a repulsive structure. The charge regulation model allows us to quantitatively relate the interaction potentials to certain experimental conditions. A schematic picture of the phase diagramis shown in Fig. 1. Close to the isoelectric point (pH = 8.7), the particles form clusters for all ionic strengths since they are not charged. At lower or higher pH values one can prepare a stable suspension for low ionic strengths because of the charge, which is carried by the colloidal particles. At even more extreme pH values, one can obtain a repulsive structure due to very strong electrostatic potentials (up to = 170mV for pH = 4 and I = 1mmol/l, according to our model). The repulsive structure is characterized by an increased shear viscosity. Three states, on which we focus in the following are marked with the symbols A–C: State A (pH = 6, I = 3 mmol/l) is in the suspended phase, state B (pH = 6, I = 7 mmol/l) is a point already in the clustered phase, but still close to the phase border, and state C (pH = 6, I = 25mmol/l) is located well in the clustered phase. Some typical examples for the different phases are shown in Fig. 2. In the suspended case (a), the particles are mainly coupled byhydrodynamic interactions. One can find a linear velocity profile and a slight shear thinning.If one increases the shear rate ˙ > 500/s, theparticles arrange in layers.

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The same can be observed if the Debye-screening length of the electrostatic potential is increased (b), which means that the solvent contains less ions (I < 0.3 mmol/l) to screen the particle charges. On the other hand, if one increases the salt concentration, electrostatic repulsion is screened even more and attractive van der Waals interaction becomes dominant (I > 4 mmol/l). Then the particles form clusters (c), and viscosity raises. A special case called “plug flow” can be observed for high shear rates, where it is possible to tear the clusters apart and smallerparts of them follow with the flow of the solvent (d). This happens in our simulations for I = 25 mmol/l (state C) at a shear rate of˙ > 500/s. However, as long as there are only one or two big clusters in the system, it is too small to expect quantitative agreement with experiments. In these cases we have to focus onstate B (I = 7mmol/l) close to the phase border. In our simulations we restrict ourself to the region around pH = 6 where we find the phase border between the suspended region and the clustered regime at about I = 4mmol/l in the simulations as well as in the experiments. Also the shear rate dependence of the viscosity is comparable in simulations and experiments

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FIG. Schematic phase diagram for volume fraction = 35% in terms of pH-value and ionic strength involving three different phases: A clustering regime due to van der Waals attraction, stable suspensions where the charge of the colloidal particles prevents clustering, and a repulsive structure for further increased electrostatic repulsion. This work concentrates on state A (pH = 6, I = 3mmol/l) in the suspended phase, state B (pH = 6, I = 7mmol/l) close to the phase border, but already in the clustered phase, and state C (pH = 6, I = 25mmol/l) in the clustered phase.

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FIG. : Images of four different cases. For better visibility we have chosen smaller systems than we usually use for the calculation of the viscosity. Thepotentials do not correspond exactly to the cases A–C in Fig. 1, but they show qualitatively the differences between the different states: a) suspension like in state A, at low shear rates, b) layer formation, which occurs in the repulsive regime, but also in the suspension (state A) at highshear rates, c) strong clustering, like in state C, so that the singlecluster in the simulation is deformed d) weak clustering close to the phase border like in state B, where the cluster can be broken into pieces, which follow the flow of the fluid (plug flow)

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Swelling of colloidal clays

The production branch of the petroleum industry is interested in the swelling properties of clays because these minerals occur widely in oil producing rocks (Nahin, Merrill, Grenall and Crog, 1951). With their relatively large surface areas the clays are capable of affecting significantly the flow of oil to producing wells. In primary production it is important to maintain the greatest oil permeability in the maximum drainage volume immediately surrounding the borehole; in ~secondary recovery, the aim is to obtain optimum injectivity of flood water through the injection wells. In both operations the degree of hydration of the clays in the vicinity of the borehole affects the efficiency of the process, tt is important therefore to know how, and to what extent, the various fluids introduced into wells during their drilling, maintenance, or use as secondary injectors affect the physical volume of the clays.

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As is well known, there is considerable literature devoted to the claywater relation and swelling (Brindley, 1951 ; Cardwell, 1954 ; Foster, 1953 ; Grim, 1953; Hermans, 1949; Hendricks, Nelson and Alexander, 1940; Katz, 1933 ; Kister, 1947 ; Marshall, 1949 ; Mering, 1946 ; Overbeek, 1952 ; Trans. Faraday Soc., 1946). However, nearly all of this work concerns systems at atmospheric pressure and under conditions far removed from those thought to exist in a petroleum reservoir. The closest published approach to the type of swelling study described in this paper was that by Power, Towle and Plaza (1942). They measured the pressure developed by a confined plug of Wyoming bentonite as a function of the amount of fluid, at atmospheric pressure, imbibed through a paper blotter. This is the classical swelling experiment of Posnjak (1912) in which the gel volume is held constant. In the present study of clay swelling as related to oil reservoirs it appeared necessary to recognize that in the reservoir both clay and fluids exist in a high pressure isobaric environment. Furthermore, because the volume of a gel is less than the volume sum of the separate phases (Katz, 1933) and, secondly, because pressure favors diminution of volume, according to the law of van't Hoff and Le Chatelier, simultaneous pressure on both liquid and gel should increase swelling. The swelling system described herein was designed to realize this process.The main purpose of this paper is to report the development

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MATERIALS, APPARATUS AND PROCEDURE

The "Wyoming" bentonite from Belle Fourche, South Dakota, used in all of the tests as a "typical montmorillonite" was obtained from the Baroid Division of National Lead Company. A less-than 0.2-micron fraction was prepared from a 2 percent suspension of the clay in distilled water by centrifugation in a Sharples supercentrifuge. Analysis of this fraction indicated a magnesium-sodium-calcium ion exchange clay with 39, 38 and 17 milliequivalents per 100 gm., respectively. This exchange composition might be expected to typify the clay surface in an oil reservoir containing a medium-hard connate water. Total cation exchange capacity of the clay by the ammonium acetate method is 94 meq./100 gin. The samples used in the swelling tests consist of a mixture of 4.50 gm. each of 105 ~ C dried powdered clay and smaller-than 50 mesh ball-milled Selas 06 microporous porcelain. The latter, of maximum pore radius 0.30- micron, serves as fluid distribution medium and is obtained by crushing 3-inch diameter x ~g-inch thick discs supplied by Boder Scientific Company, Pittsburgh, Pennsylvania. The weighed, thoroughly mixed sample is compacted in the swelling cell (Fig. 2) at 1,820 psi by means of a calibrated

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SWELLING OF CLAY UNDER FRESSURE

heavy duty spring compressed by a manually operated hydraulic jack. The filled-cell is then depth-gauged for calculation of sample volume. The porosity is determined by helium displacement following which the cell is connected to the swelling unit (Fig. 1). A thin surgical rubber or neoprene diaphragm separates the mercury-clay interface. The sampIe is evacuated to 0.001 ram. mercury pressure and, as is indicated in Figure 1, nitrogen is introduced to pressure the system to 1,500 psig, the swelling agent is admitted to the sample through the porous Selas 06 disc (Part D, Fig. 2), and the height of the mercury column in the calibrated stainless steel buret is recorded at the beginning of the run and usually daily thereafter until an apparent equilibrium is reached. In operation at 10,000 psig the method is similar except the storage cylinder of a pressure intensifier is charged with about 11,500 psi of nitrogen which, over a period of less than a minute,is leaked to the swelling unit previously pressured to 1,500 psig from astandard nitrogen bottle.

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Upon admission of the swelling fluid the clay begins to swell, apparently immediately. Progress of the swelling is manifested by the displacement of a xta-inch diameter soft iron ball lying atop the column of mercury in the buret. The position of the ball is located by a magnetic induction coil sliding on the steel buret. The fg-inch thick 1ı89 diameter coil of No. 34 magnet wire wound on a lucite spool forms one arm of an 11,000 cycles per second impedance bridge. Balance is shown by null indication on a Hewlett- Packard Model 400C vacuum tube voltmeter. The ball can be located to within 4-I mm. ; calibration values for the eight burets used varied in the range 20-23 cm./cc. ; displacements ranged from about 2-70 cm.

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whereVu= bulk volume of clay-Selas mixtureVd= volume of Selas solid phaseVi= volume of internal pores of Selas not available to clay gelVHe= helium displacement of dry sample before swelling (sum of solidphase volumes of clay and Selas)Md= weight crushed Selas.

Effect of PressureThe effect of hydrostatic pressure on swelling of Wyoming bentonite in0.171N solutions of hydrochloric acid and three common salts is shown inFigure 4. Concentration of 0.171N was chosen because this correspondsapproximately to the ionic strength of a 10,000 ppm sodium chloride brine,characteristic of certain California reservoir waters. However, it must benoted that with the recorded average fluid imbibidon of 3 to 4 cc., this

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Soil Consistency

• The rupture resistance is a field measure of the ability of the soil to withstand an applied stress or pressure as applied using the thumb and forefinger.

• Soil consistency is defined as the relative ease with which a soil can be deformed use the terms of soft, firm, or hard.

• Consistency largely depends on soil minerals and the water content.

Cohesion & Adhesion

• Cohesion is the attraction of one water molecule to another resulting from hydrogen bonding (water-water bond).

• Adhesion is similar to cohesion except with adhesion involves the attraction of a water molecule to a non-water molecule (water-solid bond).

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Consistency Description –

•Engineering/Environmental–Rupture Resistance –Moist and Dry Consistency–Stickiness –Wet Consistency–Plasticity-Wet Consistency•Geophysical–Manner and Type of Failure –Penetration Resistance

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Soil Structure – Classification & Genesis Soil conditions and characteristics such as water movement, heat transfer, aeration, and porosity are much influenced by structure. In fact, the important physical changes imposed by the farmer in ploughing, cultivating, draining, liming, and manuring his land are structural rather than textural. Definition of Soil Structure: The arrangement and organization of primary and secondary particles in a soil mass is known as soil structure. Soil structure controls the amount of water and air present in soil. Plant roots and germinating seeds require sufficient air and oxygen for respiration. Bacterial activities also depend upon the supply of water and air in the soil. Formation of soil structure: Soil particles may be present either as single individual grains or as aggregate i.e. group of particles bound together into granules or compound particles. These granules or compound particles are known as secondary particles. A majority of particles in a sandy or silty soil are present as single individual grains

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while in clayey soil they are present in granulated condition. The individual particles are usually solid, while the aggregates are not solid but they possess a porous or spongy character. Most soils are mixture of single grain and compound particle. Soils, which predominate with single grains are said to be structure less, while those possess majority of secondary particles are said to be aggregate, granulated or crumb structure. Mechanism of Aggregate Formation: The bonding of the soil particles into structural unit is the genesis of soil structure. The bonding between individual particles in the structural units is generally considered to be stronger than the structural units themselves. In aggregate formation, a number of primary particles such as sand, silt and clay are brought together by the cementing or binding effect of soil colloids. The cementing materials taking part in aggregate formation are colloidal clay, iron and aluminium hydroxides and decomposing organic matter. Whatever may be the cementing material, it is ultimately the dehydration of colloidal matter accompanied with pressure that completes the process of aggregation. Colloidal clay: By virtue of high surface area and surface charge, clay particles play a key role in the formation of soil aggregates. Sand and silt particles can not form aggregates as they do not possess the power of adhesion and cohesion.

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These particles usually carry a coating of clay particles; they are enmeshed in the aggregates formed by the adhering clay particles. Colloidal particles form aggregates only when they are flocculated. There is vast difference between flocculation and aggregation. Flocculation is brought about by coalescence of colloidal particles and is the first step in aggregation. Aggregation is some thing more than flocculation involving a combination of different factors such as hydration, pressure, dehydration etc. and required cementation of flocculated particles. The cementation may be caused by cations, oxides of Fe and Al, humus substances and products of microbial excretion and synthesis. Clay particles form aggregates only if they are wetted by a liquid like water whose molecules possess an appreciable dipole moment.

Clay - - +Water - - +Cation+ - -Clay - - +Water - - +Cation+ - -Clay -The aggregation also depends upon the nature of clay particles, size and amount of clay particles, dehydration of clay particles, cations like calcium and anions like phosphate. Fe and Al oxides: The colloidal Fe oxides act as cementing agent in aggregation. Al oxides bind the sand and silt particles. These act in two ways. A part of the hydroxides acts as a flocculating agent and the rest as a cementing agent.

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Organic matter: It also plays an important role in forming soil aggregates.•During decomposition, cellulose substances produce a sticky material very much resembling mucus or mucilage. The sticky properly may be due to the presence of humic or humic acid or related compounds produced.•Certain polysaccharides formed during decomposition.•Some fungi and bacteria have cementing effect probably due to the presence of slimes and gums on the surface of the living organisms produced as a result of the microbial activityClassification of Soil Structure: The primary particles sand, silt and clay usually occur grouped together in the form of aggregates.Natural aggregates are called peds where as clod is an artificially formed soil mass. Structure is studied in the field under natural conditions and it is described under three categories1.Type - Shape or form and arrangement pattern of peds 2. Class - Size of Peds 3. Grade - Degree of distinctness of peds

 

 

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Types of Soil Structure: There are four principal forms of soil structurePlate-like (Platy): In this type, the aggregates are arranged in relatively thin horizontal plates or leaflets. The horizontal axis or dimensions are larger than the vertical axis. When the units/ layers are thick they are called platy. When they are thin then it is laminar. Platy structure is most noticeable in the surface layers of virgin soils but may be present in the subsoil.This type is inherited from the parent material, especially by the action of water or ice.Prism-like: The vertical axis is more developed than horizontal, giving a pillar like shape. Vary in length from 1- 10 cm. commonly occur in sub soil horizons of Arid and Semi arid regions. When the tops are rounded, the structure is termed as columnar when the tops are flat / plane, level and clear cut prismatic.Block like: All three dimensions are about the same size. The aggregates have been reduced to blocks. Irregularly six faced with their three dimensions more or less equal. When the faces are flat and distinct and the edges are sharp angular, the structure is named as angular blocky. When the faces and edges are mainly rounded it is called sub

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angular blocky. These types usually are confined to the sub soil and characteristics have much to do with soil drainage, aeration and root penetration.Spheroidal (Sphere like): All rounded aggregates (peds) may be placed in this category. Not exceeding an inch in diameter. These rounded complexes usually loosely arranged and readily separated. When wetted, the intervening spaces generally are not closed so readily by swelling as may be the case with a blocky structural condition.Therefore in sphere like structure, infiltration, percolation and aeration are not affected by wetting of soil. The aggregates of this group are usually termed as granular which are relatively less porous. When the granules are very porous, it is termed as crumb. This is specific to surface soil particularly high in organic matter/ grass land soils.Classes of Soil Structure: Each primary structural type of soil is differentiated into 5 size classes depending upon the size of the individual peds.The terms commonly used for the size classes are:•1. Very fine or very thin•2. Fine or thin•3. Medium•4. Coarse or thick•5. Very Coarse or very thick

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The terms thin and thick are used for platy types, while the terms fine and coarse are used for other structural types.Grades of Soil Structure: Grades indicate the degree of distinctness of the individual peds. It is determined by the stability of the aggregates. Grade of structure is influenced by the moisture content of the soil. Grade also depends on organic matter, texture etc. Four terms commonly used to describe the grade of soil structure are:•Structure less: There is no noticeable aggregation, such as conditions exhibited by loose sand.•Weak Structure: Poorly formed, indistinct formation of peds, which are not durable and much unaggregated material.•Moderate structure: Moderately well developed peds, which are fairly durable and distinct.•Strong structure: Very well formed peds, which are quite durable and distinct.Soil Structure Naming: For naming a soil structure the sequence followed is grade, class and type; for example strong coarse angular blocky, moderate thin platy, weak fine prismatic.

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Soil physics is the study of soil physical properties and

processes. It is applied to management and prediction

under natural and managed ecosystems. Soil physics deals

with the dynamics of physical soil components and

their phases as solids, liquids, and gases. It draws on the

principles of physics, physical chemistry, engineering,

and meteorology. It is especially important in this day and

age because most farmers require an understanding of

agroecosystems. Soil physics applies these principles to

address practical problems of agriculture,ecology, and

engineering.

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Singh Alam,Modern Geotech. Engg. Punmia B.C., Jain A.K. ,soil mechanics & foundations,laxmi

publications pvt. ltd. Matter related to swelling of colloidal clays from, University of

Stuttgart. Saha Arun kumar ,soil physics,kalyani publication pvt ltd.  Arvind V. Shroff, Dhananjay L. Shah, Soil mechanics and

geotechnical engineering. Soil Mechanics and Foundation Engineering by Dr. K.R. Arora Soil Mechanics and Foundation Engineering by Dr. Gopal Ranjan

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