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Soils have two main forms saturated or unsaturated this depend on the degree of saturation (the water content). If the degree of saturation is about 100% the soil is saturated and if it is much lesser than 100% the soil is unsaturated. Therefore, saturated soil has just two phases solid (soil) and water, whereas the unsaturated soil has three phases solid, water and air and there is another phase has been added which is called contractile skin which is the interface between air and water (Fredlund and Morgenstern, 1977; Fredlund, 2006).
This difference in composition of saturated and unsaturated soil leads to a difference in the mechanical behaviour of these soils. Because of the fact that unsaturated soils represent the majority of surface soils on the earth and thus most of the foundations of the constructions are placed on this soil. For this reason engineers have to understand the mechanical behaviour of unsaturated soils.
Knowing the behaviour of unsaturated soil enables engineers to predict the risk or the failure when buildings or roads are constructed on such these soils.
the effective test that can be used to get fundamental understanding of the mechanical behaviour of unsaturated soil is triaxial test (Bishop and Donald (1961); Matyas and Radhakrishna (1968); Alonso et al.(1987); Fredlund and Rahardjo (1993); Sharma (1998); Gallipoli et al.(2003); Wheeler et al.(2003); Lu and Likos (2004); Sharma et al.(2004)).
The fabric in unsaturated soil is more important than in saturated soil, because in saturated soil the fabric will be damaged immediately if the soil is subjected to compression or shear force. Whereas, the suction will support the fabric in unsaturated soil if it is subjected to shear or compression force this had been provided by (Toll, 1990).
The pore water in an unsaturated soil will take three forms (bulk water, adsorbed water and meniscus water). The adsorbed water is the water coating the soil particles. The water is flooded in the void space is called bulk water and the water which surrounds the particle contacts is called meniscus water. Wheeler and Karube (1996)
The water meniscus which is shaped between neighbouring particles of unsaturated soil is subjected to negative pore water pressure (tensile stress) this will produce a normal force between the neighbouring particles, This phenomenon, is called soil suction, which can improve the stability of earth structures (Kayadelen et al., 2007).
An inter-particle contact force within bulk water is likely to be different within the meniscus water Wheeler et al. (2003).
The saturated soils become unsaturated when the rate of evaporation is more than the rate of rain fall and thus water table will fall down and the soil above the water table becomes unsaturated. In addition, unsaturated soil can be formed by compaction.??
The soil near to the ground surface which is in a dry atmosphere or subject to compaction processes this leads to unsaturated conditions and as a thus subjected to negative pore water pressure (Fredlung and Rahardjo,1993).
The mechanical behaviour of the soil can be divided for two parts by the ground water table as it is shown in the figure (1). The first part is the soil below the water table (the saturated soil) which it's behaviour is depend on effective stress and the second part is the soil above the t ground water table (unsaturated soil) which it's behaviour depend on independent matric suction and stress normal stress (Jennings and Burland, 1962, and Fredlund and Morgenstern, 1977).
The definition of capillary phenomena is the movement of water in the connected void spaces as a result of cohesion, the forces of adhesion, and surface tension.
The capillary is difference in pressure between the air and water acting on the interface between air and water (the contractile skin)
The capillary force which is produced by the surface tension happening between water and air leads to keeping water close to the contact point of two particles. The capillary phenomena is seen when a small tube is put in water. The water rises in the tube due to the surface tension. As it is shown in the following figure
The height of water in the tube depend on the radius of the tube, the value of the surface tension and the contact angle
At vertical force equilibrium:
Where: is the height of water in the tube, is the surface tension of water, is the density of water, is gravitational acceleration, is the radius of the tube and is the contact angle of the surface tension.
The previous equation can be written in the following form:
Where: is the radius of curvature.
The matric suction is the difference between the pore air pressure and pore water pressure acting on the interface between air and water (the contractile skin),this parameter is associated to capillary tension in the pore water (Fredlund and Rahardjo, 1993).
By taking the influence of matric suction in to account, the properties of unsaturated soil could be better interpreted (Fredlund 2000).
Total suction is equal to:
is the matric suction.
is pore air pressure and is pore water pressure.
is osmotic suction.
The osmotic suction is related to concentration of the dissolved salts in the pore water. Osmotic suction is very important for knowing soil properties. However, the effect of osmotic suction is usually neglected. Therefore, the change in total suction is equal to the change in matric suction (Fredlund 1989, Fredlund 1991).
Osmotic pressure can be measured by using thermodynamic principles. Because of the osmotic suction is function of ionic concentration, the saturated and unsaturated soils can be presented by using this suction (Robinson and Stokes 1968).
All studies have shown that osmotic suction can have an influence on the mechanical behaviour of active clay materials. This influence of osmotic suction is related alteration of effective stresses (Graham et al.1988, Barbour and Fredlund 1989).
There are two main factors which control the change of matric suction in clays, the factors are adsorption and capillarity (Richards 1974).
The relation between matric suction and surface tension
The fourth phase on the unsaturated soil which is the contractile skin is supposed to has a surface tension which is related to the forces among the inter-molecular. Therefore, the contractile skin will behaves as an elastic membrane as it is shown in the figure (2).
This behaviour is similar to a balloon inflated with air which is the pressure inside this balloon is greater than the pressure outside it. Because of the difference between the air pressure and water pressure (matric suction) the contractile skin takes the curve shape. The following equation is Kelvin's capillary equation:
From this equation it can be clearly seen that when the matric suction increases the radius of contractile skin will reduces and the radius of contractile skin will g to infinity when the matric suction is equal to zero.
The relation between the degree of Saturation and pore water Pressure and in Unsaturated Soil
Voids in the unsaturated soil can be filled with water or air (water-filled or air-filled). These voids can be considered as a tube which has radius (r).Therefore, the matric suction will be equal to:
Where: is the matric suction and is the surface tension in the tube as it is shown in the following figure:
From the above equation it is clear that the matric suction is proportional to the surface tension and inversely proportional to the radius of the tube. The voids in an unsaturated soil it is not normally to be filled partially with water and filled partially with air. Therefore, the voids will be filled with water or air (Sharma 1998).
it is very important to measure and control suction in laboratory test in order to understand the mecanical behaviour of unsaturated soils. Suction is considered one of the two factors that control the behavior of unsaturated soils Fredlund et al, 1994; Lu and Likos, 2004).
The problem of unsaturated soil is that the pore water pressure is negative with respect to atmospheric pressure. Achieving the same condition in the laboratory will produce then cavitation phonmona which is the creation of bubble in water pressure these bubbles will make an erro of applying and measuring the pressure. In addition, the measurement of the volume of water will be incorrect due to these bubbles.
There are some methods have been made to improve the suction measurement.
Axis translation technique
The principle of axis translation technique is that increase pore water pressure, pore air pressure and total pressure by equal amounts. Therefore, the cavitations phenomenon will be prevented because pore water pressure will become positive pressure Hilf (1956).
As it is shown in the figure (1) the total stress will be increased from which is the total stress in the laboratory to which is the total stress in the field, the pore air pressure will be increased from (pore air pressure in the laboratory) to (pore air pressure in the field) and the pore water pressure will be increased from (the negative pore water pressure in the laboratory) to (the positive pore water pressure in the field). Therefore, the net stress ( and the matric suction ( will not be changed.
One the other hand, the prevention of the cavitations is considered as disadvantage of this technique because the cavitations phenomena in the field have a consequence axis which will not be included in the laboratory by using translation technique.
The requirement of axis translation is achieving continuity of air space in the specimen this can be achieved by using saturated high air entry such as ceramic disk which will make isolation between water and air in the soil because this disk will allow the entrance of water and prevent the entrance air. In addition, this effetivance of this technique requires the degree of saturation less 70 %. Bocking and Fredlund(1980).
Osmotic control of matric suction
The principle of osmotic control of matric suction is based on the concept of osmosis. For example, semi-permeable membranes locate between the sample of soil and osmotic solution such as molecules of polyethylene glycol. The membrane is porous to ions in the soil and water. However, the membrane is impermeable to large solute molecules and soil particles (Zur, 1966).
The most important advantage of using the osmotic control for matric suction is that the cavitation due to the negative pore water pressure is allowed to occure as the existent field condition. moreover, there is no need to raise the cell pressure as in the axis translation technique to reach a high (Delage et al. 1998).
On the othere hand the osmotic control technique has some disadvantages. One of these disadvantages is that the difficulty of achieving continuous variation of suction, due to the varieation of the polyethylene glycol concentration. The second drawback is the semi-permeable membrane is very weak if it is subjected to excessive stress (Delage and Cui, 2008).
Effective stress(Ïƒ') it is a essential state variable to describe the state of stresses within the soil (Fredlund and Rahardjo, 1993).
Therzaghi (1943) used the stress state variable to control the soil behaviour and considered the effective stress acting on saturated soil are the difference between the total stress, and the pore water pressure.
Where:is the effective stress, is the total stress and is the pore water pressure.
The theory of the effective stress expressed that the t deformation behaviour of the soil is controlled by effective stress .However, for unsaturated soil there are two additional parameter must be considered (1)the pore air pressure(ua), which is the stress within the air phase and (2) the matric suction(Ïˆ), which is the difference between the pore air pressure(ua) and the pore water pressure(uw), (Lu and Likos, 2004).
(Fredlund and Rahardjo, 1993) stated that Bishop (1959) has modified Therzaghi's effective stress theory to explain the effective stress in unsaturated soil as it is shown in the following equation:
Where: is the effective stress, is the pore air pressure, is the total stress, and is the pore water pressure,
is the effective stress parameter which is related to the degree of saturation of the soil.The value of the effective stress parameter for saturated soil is equal to one unit and it is equal to zero for dry soil.
Aitchison (1961) expressed the relationships between the matric suction and the effective stress factors in the following equation:
Where: Ïˆ is parameter which his value between from 0 to 1 and is the pressure deficiency,
Jennings (1961), used the following equation to express the effective stress for an unsaturated soil:
Where: is parameter which his value between from 0 to 1 and p" is pore water pressure which is taken as positive value.
Richards (1966), added the solute suction factor into the effective stress equation:
Where: is effective stress parameter for matric suction, is matric suction, is effective stress parameter for solute suction, and is solute suction.
Aitchison (1973), proposed the following equation for the effective stress of unsaturated soil which is similar to Richards equation :
Where: is matric suction, is solute suction, and and are soil parameter which have value between 0 to 1 depend on the stress path.
All previous equation of effective stress for unsaturated soil based on incorporated the soil parameter by using a single value of effective stress. The magnitudes of the soil parameter is different for different types of soil, different stress paths and different problems such as volume change versus shear strength (Jennings and Burland, 1962; Coleman, 1962; Bishop and Blight, 1963).
Researchers suggested to use two independent stress state variables to describe the mechanical behaviour of unsaturated soils after they examined the proposed effective stress equations because the use of independent stress state variables has resulted in more meaningful explanation of unsaturated soil behaviour .(Bishop and Blight, 1963; Matyas and Radhakrishna,1968).
When soil are subjected to a shear stresses and normal, the strength of the soil increases up to a point where it can no longer resist shear stresses.
Terzaghi (1936) used effective stress variable in Mohar-Coulomb theory to predict the shear strength of saturated soils. The equation of the shear strength of saturated soil is linear function as follow:
is the shear strength.
is the effective cohesion.
is the effective angle of internal friction.
is the total normal stress on the plane of failure.
is the pore water pressure.
is the effective normal stress on the plane of failure.
However for unsaturated soil the shear strength can be illustrated by using any two of these three stress state variables:, and by(Fredlund and Morgenstern, 1977).
The following equation for shear strength of unsaturated soil has been proposed by (Fredlund et al 1978).
is the angle which point out the raise of shear strength with respect to a change in matric suction when and are used as two state variables.
is the angle which point out the raise of shear strength with respect to a change in matric suction when and are used as two state variables.
The above equation shows a linear relationship between matric suction and shear strength. However, there were studies have shown that the increases of shear strength in unsaturated soils is not linear with an increase in matric suction.the explanation of these different results is that, at the beginning, the shear strength increases linearly with the increase of matric suction up to value above this value the increases of strength turn out to be non-linear.Moreover, the strength is tend to be constant or drop dowan with high matric suction (Fredlund and Rahardjo 1993).
Lamborn (1986) presented a shear strength equation for unsaturated soils as follows:
Where is the volumetric water content which is equal to ratio of the volume of water to the total volume of the soil. The volumetric water content reduces as matric suction raises.
Peterson (1988) expressed the shear strength for soil that has a degree of saturation less than 85% in the following equation:
Where: is the cohesion because of the suction. The effect of soil suction on shear strength in the above equation is considered as raise in the cohesion of the soil. The cohesion due to suction is reliant on the water content of the soil.
The analysis of stress theory for an unsaturated soil expressed that the independent stress variable,, and can be used to describe the volume change behaviour and the shear strength behaviour of an unsaturated soil (Fredlund and Morgenstern 1977).
The volume change is one of the most dangerous features of the soil which affect the infrastructure. The volume of unsaturated soil either increase or decrease when it is subjected to wetting, this behaviour is due to the applying stress level (Gens, 1996).
Because of the fact that the voids between particles in saturated soil are filled with water. Therefore, the volume change of a saturated can be measured by calculate the flow of water coming in or coming out of the sample(assuming the volume of the solid soil is constant). However, the volume change of unsaturated cannot be measured by the same way because the voids between the particles are filled with air and water. Therefore, in order to knowing the volume changes of unsaturated soil based on measuring the volume change of the phases of unsaturated soil. The key points about the measurment of volume change in unsaturated soil by using triaxial test is summrised in the following points by (Sharma, et al. (2006)):
1.in the triaxil test for unsaturated soil the measurment of volume change can not be achieved by montoring the water flow or outflow from the spicement.
2. the measurment of the volume change for unsaturated soil depend on knowing the volume of each phase of the soil, the overall volume change of the speciment and kowing the changes of water volume in the speciment as well as assuming the volume of the solid soil is constant.
3. the volume of water changes in the spicement of unsaturated soil is similar to the measurment for spicement of saturated soil by measuring the water inflow or water outflow from the soil speciment.
There are two effective way two measure the volume change of unsaturated which are internal local strain measurments and the inner cell technique (Cabarkapa and Cuccoillo, (2006)):
The princible of using the inner cell technique for measuring the volume change of unsaturated soil is depend on the measurment of the flow of water coming in or coming out of the cell.The addvantage of using this technigue is that the values of overall volume change is very precise. On the other hand, using the inner cell technique for measuring the volume change of unsaturated is considerd to be very sensitive for any change in pressure and temprature (Sivakumar (1993),Cabarkapa and Cuccoillo, (2006)).
The volume change of unsatutated soil can be measured by using internal local strain measurment which measures the overall volume change by measuring the strain in two direction vertical and radial directly by placing instruments in these direction to the soil sample. Therefore, this technique gives a very accurate values of the volume change of the sample. On the othe hand, internal local strain measurment has a drawback because this technque can be valid for rigid speciments which does not represent the natural soil condition in the field (Cabarkapa and Cuccoillo, (2006)).
The modification for Triaxil apparatus
The apparatus of the Triaxial test is used to measure many parameters of saturated soil such as the permeability of soil shear strength characteristics, consolidation characteristics. This apparatus has been modified to measure unsaturated soil parameters by controlling pore-water, pore-air pressures and volume changes
There are some modification has been done on conventional Triaxial apparatus for unsaturated soils which will allow controling both pore-air and pore-water phases of a test sample, and thus examine the effect of matric suction on the behaviour unsaturated soil. The matric suction will stay stable under undrained conditions for soils having a low degree of saturation and subjected to a low stress. It was agreed that by using soil with an initial degree of saturation about 32 %, this will make the transition stress to be about 50 kPa. Wulfsohn et. al. (1998).
The modern triaxial system can measure the coefficient of permeability of a saturated soil with a coefficient of permeability as low as 5 Ã- 10-11 m/s. whereas, the measurement of coefficient of permeability can be much lower than 5 Ã- 10-11 m/s for unsaturated soil. There is a slight decrease in the coefficient of permeability of the soil with the increase of matric suction from (0) up to the air-entry. However, the coefficient of permeability will decline sharply when the value matric suction surpass the air-entry value.( Huang, (1996))
In the triaxial testing the volume change of a soil sample in is an essential parameter, which has to be calculated and it is very important for understanding the volumetric compression of the soil. The volume change due to consolidation of saturated sample in Triaxial testing is equal to the volume of water coming out of the sample. As a result, the best way to calculate the volume change is by measuring the volume of water coming out from the sample. Whereas, the volume change due to consolidation of unsaturated sample in Triaxial testing is not equal to the volume of water coming out of the sample. Therefore, using the volume of water coming out from the sample to measure the volume change is not valid in unsaturated soil.
The first successful attempt to measure the volume change was by Bishop and Donald (1961). They put an open inner cylindrical container in a conventional cell. The inner cell was filled with mercury and the space between the inner and outer cell was filled with water and thus the volume changes of the unsaturated soil sample can be measured by monitoring the vertical movement of a steel ball which is floating on the surface of the mercury.
Yin (1998) used the same idea of adding another cell to a conventional cell. The inner cell was filled with distilled water to specific position and the space between the inner and outer cell was filled with air and thus the volume changes of the unsaturated soil sample can be measured by monitoring the vertical movement of the water in the inner cell. However, this way of measuring the volume change in the soil sample is not accuracy due to the using of naked eye for mentoring the change of water position.
Wheeler (1988) has developed Bishop and Donald (1961) by using double cell in the triaxial system for testing unsaturated soils with large gas bubbles. a water burette was used to measure the volume of the inner cell and a local proving ring was used to measure the vertical axial load. By using a diaphragm between inner cell top shaft and the loading piston the potential of leakage can be minimized.
The effect of reinforcement on the behaviour of unsaturated soil
The behaviour of soil with fiber reinforcement has been studied over the last two decades. Reinforce the soil by fiber is considered as workable method for geotechnical engineering problems. It was found that the shear strength of the soils increases with the increase of fiber content in the sample. This increase of shear strength was proportional to fiber content or area ratio (3, 5, 6, 7, and 8). But, over 8 the increase in the strength was not proportional to the reinforcement concentration. In addition, the cohesion and angle of internal friction in the soil with fiber reinforcement will increase the increase of fibers content.( Wei Chen, (2006)).
The first difficulty for dealing with unsaturated soil is the measurement of negative pore water pressures that because water cavitations at a pressure more or less of -1 atmosphere. There are some techniques have been invented to overcome the cavitations phenomenon such as axis translation technique
The principle of axis translation technique is increasing the ambient air pressure by a certain amount, and increasing the pore-water pressure by an the same amount, that will keep matric suction constant (Hilf ,1956).
Wulfsohn et. al. (1998) the samples can be undistributed filed samples or remoulded specimens prepared in the laboratory. However, there is small difference which is undisturbed samples present the field conditions better than remoulded samples. The preparation of remoulded samples is compacting a soil sample to specific water content and density in a specific mould.
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