LITERATURE REVIEW
2.1 THEORY AS APPLIED TO COMPRESSIBILITY DETERMINATIONS
For a saturated soil mass, carrying a static load, causing a total vertical stress of σ bond by the soil mass in two components, i.e (i) σo or Po, being the effective stress or the inter-granular stress, and (ii) u, being the pore pressure.
Let an additional load (or incremental load ∆P) be applied on it. The stress or pressure caused by this additional load is immediately taken up by the water present in the pores of the soil mass, as excess hydrostatic pressure is released and the water starts expelling out of the soil, therefore transferring the pressure to the soil grains. In other words, the pressure on the soil grains would increase, and the volume of the soil mass will decrease due to the decrease in the voids and expulsion of water, which also amounts to reduction in void ratio, e.
Let PO represent the original stress on the soil mass (i.e σo), and ∆P represent the additional stress. Also assume that eo is the original void ratio of the soil mass, and let ∆ℓ be the reduction in the void ratio that occurs after full consolidation takes place due to load ∆P.
Then is defined as the coefficient of the compressibility
of the soil mass, and is represented by av. This is also called
the coefficient of compression
av = ---------------------(1)
av has the dimension of inverse pressure, i.e M2/KN. av is a measure of compressibility of compression deformation or compression of soil. (Garg, 2009)
2.1.2 COEFFICIENT OF VOLUME COMPRESSIBILITY
Another coefficient to measure compression is also used, and it’s called the modulus or the coefficient of volume change or the coefficient of volume compressibility, and is represented by Mv.
It is given by the equation
Mv = av ---------------------(2)
1+eo
Mv has the same dimension as av, i.e M2/KN, and both Mv and av are not constant for a given soil, but depends on the stress range considered both decreasing with increase pressure range.
Let eo represent the initial void ratio i.e the void ratio at the start of increased pressure ∆P let e, represent the void ratio achieved at the end, i.e when full compression has taken place with load ∆P.
Now, the initial porosity of soil no is given by
no = ----------(3)
After full compression has taken place, the porosity referred to the original volume of the soil (n) is given by
n = ----------(4)
Hence the change in porosity per unit original volume of the soil is obtain by subtracting equation (4) from equation (3)
no - n = ----------(4)
Or
∆n = ----------(5)
But from equation (1), we have av =
Or ∆e = av ∆P
:. Equation (5) becomes
∆n = ----------(6)
But Mv = av = coefficient of volume decrease or
1 + eo = coefficient of volume compressibility, as defined in equation (2), Equation (6) becomes then
∆n = Mv.∆P ---------(7)
Substituting this value of ∆n in equation (5), we get
Mv∆P = = Mv.∆P ----------(8)
Now using fig 2.1, we can say that = e
i.e void ratio =
But from equation (8) we have = Mv. ∆P
Or ∆H = Mv.∆P. Ho -----------(11)
This equation can be easily used to work out the settlement ∆H, that takes place with an additional load of ∆P. Mv corresponds to the original pressure po or σo. Mv (i.e coefficient of volume compressibility sometimes called compressibility, is hence defined as the compression of the layer per unit of original thickness due to a unit increase of the pressure. The reciprocal of modulus of volume change (Mv) is called the compression modulus, and is represented by Ec. It has the unit of KN/m2. It is analogous to young’s modulus
Ec =
Value of Ec are specified for different kinds of soils using Ec in equation (11), we get
∆H = = Mv .∆P.Ho ----------------- (12)
Equation (11) can be used to determine the settlement ∆H of a soil, caused by an incremental loading ∆P. But to determine ∆H, it is necessary to determine Mv at the start of the incremental loading. In order to determine such factors, the soil sample is tested in the labouratory or an apparatus called consolidometer. (Garg, 2009).
2.2 COMPACTION OF SOILS
Compaction means pressing the soil particle close to each other by mechanical methods. Air during compaction is expelled from the void space in the soil mass and, therefore, the mass density is increased.
Compaction is an entirely different process than consolidation. It is important to note the following basic difference between the two processes, even though both the processes cause a reduction in the volume,
Consolidation is a gradual process of reduction of volume under sustained, static loading; whereas compaction is rapid process of reduction of volume by mechanical means such as rolling tamping and vibration.
Consolidation causes a reduction in volume of a saturated soil due to squeezing out of water from the soil, whereas in compaction, the volume of a partially saturated soil decreases because of the expulsion of air from the voids at the unaltered water content (Sohail,2012).
Consolidation is a process which occurs is nature when the saturated soil deposits are subjected to static loads caused by the weight of the buildings and other structures. In contrasts, compaction is an artificial process which is done to increase the density (unit weight) of the soil to improve its properties before it is put to any use. (Sohail, 2012).
2.2.1 JODHPUR MINI COMPACTOR TEST
The jodhpur minicompactor test was developed by prof Alam Singh (1965). A small mould of internal diameter 79.8mm (cross-sectional area = 500mm2) effective height 60mm and a capacity of 300ml is used. The rammer used is of 2.5kg mass and is known as the dynamic ramming tool (DRT). The mass slides down a stem through a height of 250mm and falls over a foot of 40mm diameter ad 75mm height and compacts the soil. The test is suitable for both fine-grained soils and coarse–grained soils (minus 4.75mm sieve).
The procedure for conducting the test is similar to that in the standard proctor test, but the soil is compacted only in 2 layers. Each layer is compacted by 15 blows of the dynamic ramming tool uniformly distributed over the soil surface. The compactive effort is approximately equal to that in the standard proctor test. It is recommended that, for fine-grained soils, a fresh soil sample shall be taken for each test after allowing a suitable maturing time (Garg, 2009).
2.2.2 HARVARD MINIATURE COMPACTION TEST
In Harvard miniature compaction test, compaction is done by the kneading action of a cylindrical tamping foot of 0.5 inch (12.7mm) diameter. The tamping foot operates through a pre-set compression spring to give the taming force to a predetermined value.
The mould used is of 15/10 inch (33.34mm). the capacity if the mould is 1/456 cubic foot (62.4ml). the number if layers, the tamping force and the number of tamps per large are selected depending upon the type of the soil and the amount of compaction required (Garg, 2009).
2.2.3 ABBOT COMPACTION TEST
In the abbot compaction test, a metal cylinder (mould) of 5.2cm internal diameter and 40cm effective height is used. The cylinder is damped to the base. The soil is taken in the cylinder and compacted by a 2.5kg rammer having a circle face of 5cm diameter. The rammer is lifted up and dropped inside the cylinder through a height of 35cm above the base (Garg, 2009).
2.2.4 FACTORS AFFECTING COMPACTION
The dry density of the soil is increased by compaction. The increase in the dry density depends upon the following factors.
WATER CONTENT: At low water content the soil is stiff and offers more resistance to compaction. As the water content is increased, the soil particles get lubricated. The soil mass becomes more workable and the particles have closer packing. The dry density of the soil increase with an increase in the water content title the optimum water content is reached. At that stage, the air voids attain approximately a constant volume with further increase in water content, the air voids do not decrease, but the total voids (air plus water) increase and the dry density decreases. The effect of water content on the dry density of the soil can also be explained with the help of electrical double layer theory. At low water content, the forces of attraction in the adsorbed water later are large and there is more resistance to movement of the particles (Faruq, 2009). As the water content is increased, the electrical double layer expands and the interparticle repulsive force increase. The particles easily slide over one another.
AMOUNT OF COMPACTION: The effect of increasing the amount of compactive effort is to increase the maximum dry density and to decrease the optimum water content. At a water content less than the optimum, the effect of increased compaction is more predominant. At a water content more that the optimum, the effect of increased compaction is more predominant. At a water content more than the optimum. The volume air voids becomes almost constant and the effect of increased compaction is not significant. (Krishna, 2002).
It may be mentioned that the maximum dry density does not go an increasing with an increase in the compactive effort. For a certain increase in the compactive effort, the increase in the dry density becomes smaller and smaller. Finally, a stage is reached beyond which there is no further increase in the dry density with an increase in the compactive effort.
The line of optimum which joins the peaks of the compaction curves of different compactive effort follows the general trend of the zero-air void line. This line corresponds the air voids of about 5%.
TYPE OF SOIL: The dry density achieved depends upon the soil. In general coarse –grained soils can be compacted to higher dry density than fine-grained soils. The same quantity of fines to a coarse – grained soil, the soil attains a much higher dry density for the same compactive effort.
However if the quantity of fines is increased to a value more than that required to fill the voids of the coarse–grained soils, the maximum dry density decrease.
2.2.5 EFFECT OF COMPACTION ON PROPERTIES OF SOILS
The engineering properties of soil are improves by compaction. The desirable properties are achieved by proper selection of the soil type, the mode of placement and the method of compaction. The effect of compaction on various soil properties are discussed below. In the following discussions, the dry of optimum means when the water content is less than the optimum, and the wet of optimum means when the water is less than the optimum, and the wet of optimum means when the water content is more than the optimum. (Garg, 2009).
a) SOIL STRUCTURE
The water content at which the soil is compacted plays an important role in the engineering properties of the soil. Soil compacted at a water content less than the optimum water content generally have a flocculated structure, regardless of the method of compaction.
Soil compacted at a water content more than the optimum water content usually have a dispersed structure if the compaction induces large shear strains and a flocculated structure if the shear strains are relatively small
Water content (%)
Fig. 2.7: Soil structure in compacted soils
In fig. 2.7, at point A on the dry side of the optimum, the water content is so low that the attractive forces are more predominant than the repulsive forces, this result in a flocculated structure. As the water content is increased beyond the optimum, the repulsive forces and the particles get oriented into a dispersed structure, if the compactive effort is increased, there is a corresponding increase in the orientation of the particles and higher dry densities are obtained, as shown by the upper curve.
b) PERMEABILITY
The permeability of a soil depends upon the size of void. The permeability of a soil decreases with an increase in water content on the dry side of the optimum water content. There is an improved orientation of the particles and a corresponding reduction in the size of voids which cause a decrease in permeability. The minimum permeability occurs at a slightly above the optimum water content. After the stage, the permeability slightly increases, but it always remains much less than that on the dry side of the optimum (Garg, 2009).
SWELLING
A soil compacted dry of the optimum water content has high water deficiency and more random orientation of particles. Consequently, it imbibes more water than the sample compacted wet of the optimum and has therefore, more swelling consequently, the soils on the dry side are less compressible.
Therefore, the compressibility of soil depends upon a number of other factors. It increases with an increase in the degree of saturation. The compressibility of a soil compacted on the wet side of the optimum is also influenced by the method of compaction.
If the compaction is of kneading or impact type, it creates a more dispersed structure with a corresponding increase in the compressibility. If the compaction causes very large stress, the compressibility increases due to breakdown of the structure and greater orientation of the particles (Lav, 1999).
PORE WATER PRESSURE
A sample compacted dry of the optimum has low water content. The pore water pressure developed for the soil compacted dry of the optimum is therefore less than that for the same soil compacted wet of the optimum.
SHRINKAGE
Soil compacted dry of the optimum shrink less on drying compared with those compacted wet of the optimum. The soils compacted wet of the optimum shrink more because the soil particles in the dispersed structure have nearly parallel orientation of particles in the dispersed structure have nearly parallel orientation of particles and can pack more efficiently. (Lav 1999).
f) COMPRESSIBILITY
The flocculated structure developed on the dry side of the optimum offers greater resistance to compression than the dispersed structure on the wet side.