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ANJUMAN COLLEGE OF ENGINEERING & TECHNOLOGY MANGALWARI BAZAAR ROAD, SADAR, NAGPUR - 440001.

DEPARTMENT OF CIVIL ENGINEERING

UNIT – V6. Consolidation: Compression of laterally confined soil, Terzaghis 1-D consolidation theory (formation of Differential equation), Determination of coefficient of consolidation, Degree of consolidation. Determination of preconsolidation pressure, Settlement, Rate of settlement.7. Compaction: Mechanism of compaction, factors affecting compaction, standard & modified proctor Tests, field compaction equipments, quality control, Advance compaction Techniques, Nuclear density meter.

Prof. Mrs. Rashmi G. Bade1



1.1 INTRODUCTION

When a compressive load is applied to soil mass, a decrease in its volume takes place. The decrease in the volume of soil mass under stress is known as compression and the property of a soil mass pertaining to its susceptibility to decrease in volume under pressure is known as compressibility. In a saturated soil mass having its voids filled with incompressible water, decrease in volume or compression can take place when water is expelled out of the voids. Such a compression resulting from a long term static load and the consequent escape of pore water is termed as ‘consolidation’.

1.2 INITIAL, PRIMARY AND SECONDARY CONSOLIDATIONThe consolidation of a soil deposit can be divided into 3 stages:

1) Initial Consolidation:- When a load is applied to a partially saturated soil, a decrease in volume occurs due to expulsion and compression of air in the voids. A small decrease in volume also occurs due to compression of solid particles.

2) Primary Consolidation:- After initial consolidation, further reduction in volume occurs due to expulsion of water from voids. When a saturated soil is subjected to a pressure, initially all the applied pressure is taken up by water pressure, as water is almost incompressible as compared with solid particles. A hydraulic gradient develops and the water starts flowing out and a decrease in volume occurs. The decrease depends upon the permeability of the soil and is, therefore, time dependent. This reduction in volume is called primary consolidation.

3) Secondary Consolidation:- The reduction in volume continues at a very slow rate even after the excess hydrostatic pressure developed by the applied pressure is fully dissipated and the primary consolidation is complete. This additional reduction in the volume is called secondary consolidation.

1.3 SPRING ANALOGY FOR PRIMARY CONSOLIDATIONThe process of primary consolidation can be explained with the help of the spring analogy given by Terzaghi. Fig. 1 (a) shows a cylinder fitted with a tight-fitting piston having a valve. The cylinder is filled with water and contains a spring of specified stiffness. Let the initial length of the spring be 100mm and the stiffness of spring be 10 mm/N. Let us assume that the piston is weightless and the spring and water are initially free of stress.

When a load’P’ (say 1 N) is applied to the pistin, with its valve closed, the entire load is taken by water. Fig.1(b). The stiffness of the spring is negligible compared with that of water, and consequently, no load is taken by the spring. From equilibrium,




Fig.1.1 Spring Analogy.

Pw + Ps = P …………..(i)

where Pw = load taken by water, Ps = load taken by spring, and P = total load. For P = 1 N, eq. (i) becomesPw + Ps = 1.0 ………….(ii)

Initially (t=0) when valve is closed, Ps = 0.00. therefore, Pw = 1.0If the valve is now gradually opened, water starts escaping from the cylinder. The spring

starts sharing some load and a decrease in its length occurs. When a portion (ΔP) of the load is transferred from the water to the spring, eq.(ii) becomes

ΔP + (1.0 – ΔP) = 1.0

As more and more water escapes, the load carried by the spring increases. Fig. 1.2 shows the transfer of the load from the water to the spring. Eventually, when the steady conditions are established, the water stops escaping. Finally, at time t = t f, the entire load is taken by spring. Thus, Pw = 0 and Ps = 1.00.

This load causes decreases in length of the spring by 10 mm. The final length is 90 mm, as shown in fig.1.1(c).

1.4 COMPRESSION OF LATERALLY CONFINED SOILIf a remoulded soil is laterally confined in a consolidometer, consisting of a metal ring,

and porous stones are placed both at its top and bottom faces, the compression or consolidation of soil sample takes place under a vertical pressure applied on the top of porous stones. The porous stones provide free drainage of water and air from or into the soil sample. Under a given applied pressure a final settlement and equilibrium voids ratio is attained after certain time. At



DEPARTMENT OF CIVIL ENGINEERINGthe equilibrium stage, the applied pressure naturally becomes the effective pressure σ’ on the soil. The pressure can then be increased and a new equilibrium void ratio is attained. Thus, a

relationship can be obtained between the effective pressure σ’ and the equilibrium voids ratio e in the form of curve.

1.5 TERZAGHI’S 1-D CONSOLIDATION THEORYThe theoretical concept of the consolidation process was developed by Terzaghi (1923). In the development of the mathematical statement of the consolidation process, the following simplifying assumptions are made:

1. The soil is homogeneous and fully saturated2. Soil particle and water are incompressible 3. The deformation of the soil is due entirely to change in volume.4. Darcy’s law for the velocity of flow of water through soil is perfectly valid.5. Coefficient of permeability is constant during consolidation.6. Load is applied in one direction only and deformation occurs in the direction of the load

application i.e the soil restrained against lateral deformation.7. Excess pore water drains out only in the vertical direction.8. The boundary is a free surface offering no resistance to the flow of water from the soil.9. The change in thickness of the layer during consolidation is insignificant.10. The time lag in consolidation is due entirely to the permeability of soil, and thus the

secondary consolidation is disregarded.

Fig.1.4 Plot between ef andfig shows a clay layer of thickness H, sandwiched between two layer of sand which serves as drainage faces. When the layer is subjected to a pressure increment Δσ, excess hydrostatic pressure is set up in the clay layer. At the time t0, the instant of pressure application whole of the consolidating pressure Δσ is carried by the pore water so that the initial excess hydrostatic pressure ữ0 is equal to Δσ, and is represented by a straight line ữ0= Δσ on the pressure distribution dig. The straight line ADB joining the water levels in the piezometric tubes represent this distribution . As water starts escaping into the sand. The excess hydrostatic Prof. Mrs. Rashmi G. Bade

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DEPARTMENT OF CIVIL ENGINEERINGpressure at the pervious boundaries drops to zero and remain so at all times. After a very great time tf , the whole of the excess hydrostatic pressure is dissipated so that ữ=0, represented by line CFE. At an intermediate time t, the consolidating pressure Δσ is partly carried by water and partly by soil, and the following relationship is obtained :

the distribution of excess hydrostatic pressure ữ at any time t is indicated by the curve CDE,joining water levels in the piezometric tubes; this curve is known as isochrones and a number of such isochrones can be drawn at various time intervals t1, t2,t3 etc. the slope of isochrones at any point at a given time indicates the rate of change of ữ with depth.

At any time t, the hydraulic head h corresponding to the excess hydrostatic pressure is given by

Hence the hydraulic gradient i is given by

Thus , the rate of change of ữ along the depth of the layer represents the hydraulic gradiebt . the velocity with which the excess pore water flows at the depth z is given by Darcy’s law

The rate of change of velocity along the depth of the layer is then given by

Consider a small soil element of size dx, dz and of width dy perpendicular to the xz plane. If v is the velocity of water at the entry into the elements,the velocity at the exit will be equal to

The quantity of water entering the soil element =v.dx.dy.

The quantity of water leaving the soil element =

Hence the net quantity of water dq squeezed out of the soil element per unit time is given by

The decrease in the volume of soil is equal to the volume of water squeezed out. However, from




Where V0= volume of soil element at time t0=dx .dy dz

Change of volume per unit time is given by

From above equation we get

Now

Therefore from above eq we get

1.6 LABORATORY CONSOLIDATION TEST

The laboratory consolidation test is conducted with an apparatus known as consolidometer consisting essentially of a loading frame and a consolidation cell in which the specimen is kept. Porous stones are put on the top and bottom ends of the specimen. Fig.1.5 (b) and 1.5 (c) show the fixed ring cell and floating ring cell, respectively. In the fixed ring cell, only the top porous stone is permitted to move downwards as the specimen compresses. In the floating ring cell, both top and bottom porous stones are free to compress the specimen towards the middle. Direct measurement of permeability of the specimen at any stage of loading can be made only in the fixed ring type. However, the floating ring cell has the advantage of having smaller effects of friction between the specimen ring and the soil specimen.

Fig.1.5 Consolidation Test.




Fig.1.6 Plot between ef and

The loading machine is usually capable of applying steady vertical pressure upto 800 or 1000 kN/m2 (kPa) to the soil specimen. During the test, the specimen is allowed to consolidate under a number of increments of vertical pressure, such as 10, 20, 50, 100, 200, 400, 800, 1000 kN/m2 (kPa) and such pressure increment is maintained constant until the compression virtually ceases, generally means of a dial gauge. Dial gauge readings are taken after the application of each pressure increment at the following total elapsed times: 0.25, 1.00, 2.25, 4.00, 6.25, 9.00, 12.25, 16.00, 20.25, 25, 36, 49, 60 minutes, and 2, 4, 8 and 24 hours. The dial gauge readings showing the final compression under each pressure increment are also recorded. After the completion of consolidation under the desired maximum vertical pressure, the specimen is unloaded and allowed to swell. The final dial reading corresponding to the completion of swelling is recorded and the specimen is taken out, and dried to determine its water content and the weight of soil solids. The consolidation test data are then used to determine the following:

i) Voids ration and co-efficient of volume change.ii) Coefficient of consolidation, and iii) Coefficient of permeability.

1.7 CALCULATION OF VOIDS RATIO AND COEFFICIENT OF VOLUME CHANGE

The equilibrium voids ratio or final voids ratio at the end of each pressure increment can be calculated by two methods:

i) ‘Height of solids’ methods.ii) ‘Change in voids ratio’ method.



DEPARTMENT OF CIVIL ENGINEERINGThe change in voids ratio method is used only for fully saturated specimens, while the

height of solids method is applicable for both saturated as well as unsaturated samples.i) Height of solids method:- The height Hs of the solids of the specimen is calculated

from the following expression:

where Hs = height of solids (cm), Md – mass of dried specimen (g), Wd = weight of dried specimen, A = cross-sectional area of specimen (cm2) and G specific gravity of soil.

The voids ratio is calculated from the following relation:

where,

H = specimen height (cm) at equilibrium under various applied pressures = H0 + ∑ΔH = H1 + ΔHH0 = initial height of specimen.ΔH = change in the specimen thickness under any pressure increment.H1 = height of specimen at the beginning of the load increment.

Knowing the voids ratio at the beginning and at the end of the test, the corresponding water contents and degrees of saturation can be calculated.

ii) Change in voids ratio method:- Assuming the specimen to be fully saturated, the voids ratio ef at the end of the test is determined from the relationef = wf.Gwhere ef = final voids ratio, at the end of the test.

wf = final water content, at the end of the test.The change of voids ratio Δe under each pressure increment is calculated from the relationship:

where Hf = final height of specimen, at the end of the test.Knowing Δe and working backwards from the known value of ef, the equilibrium voids ratio

corresponding to each pressure can be evaluated.1.8 UNLOADING AND RELOADING PLOT

In fig.1.4, the curve AB indicates the decrease in void ratio with an increase in the effective stress. After the sample has reached equilibrium at the effective stress

, as shown by point B, the pressure is reduced and the sample is allowed to take up water and

swell. The curve BEC is obtained in unloading. This is known as the expansion curve or swelling curve. It may be noted that the soil cannot attain the void ratio existing before the start of the test, and there is always some permanent set or residual deformation.




Fig.1.7 Loading, unloading and reloading plot.If the specimen which has swelled to the point C is reloaded, the recompression curve CFD is

obtained. As the load approaches the maximum value of the load previously applied corresponding to point B, there is reversal of curvature of the curve and then the plot DG continues as an extension of the first loading curve AB. However, the reloaded specimen remains at a slightly lower void ratio at point D than that attained at B during the initial compression for the same load.

1.9 BASIC DEFINITIONS1) Coefficient of Compressibility:- The coefficient of compressibility (av) is defined as decrease in void ratio per unit increase in effective stress. It is equal to the slope of the e – σ curve at the point under consideration.

Thus,

As the effective stress increases, the void ratio decreases, and therefore, the ratio is negative. The minus sign makes av positive. For convenience, the coefficient of compressibility av is reported as positive.

As the value of av is different at various effective stresses, while reporting its value, the effective stress to which that value corresponds must be mentioned. The coefficient of compressibility decreases with an increase in the effective stress. In other words, the soil becomes stiffer (less compressible) as the effective stress is increased and the curve becomes flatter.

The coefficient of compressibility (av) has the dimensions of [L2/F]. The units are m2/kN. It may be noted that the units are inverse of that for pressure.2) Coefficient of Volume change:- The coefficient of volume change (or volume compressibility) is defined as the volumetric strain per unit increase in effective stress. Thus

………..(i)where mv = coefficient of volume change, V0 = initial volume,Prof. Mrs. Rashmi G. Bade

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DEPARTMENT OF CIVIL ENGINEERINGΔV = change in volume Δ σ = change in effective stress.The reader should note that the coefficient of volume change is inverse of the bulk

modulus used in solid mechanics and fluid mechanics. For most clays, mv -= 1 x 10-3 to 1 x 10-4

m2/kN.The volumetric strain (ΔV/V0) can be expressed in terms of either void ratio or the

thickness of the specimen as explained under:Let e0 be the initial void ratio. Let the volume of solids be unity. Therefore, the initial

volume V0 is equal to (1+e0). If Δe is the change in void ratio due to change in volume ΔV, we have ΔV = Δe. Thus

Therfore eq(i) becomes,

3) Compression Index:- The compression index (Cc) is equal to the slope of the linear portion of the void ratio versus log σ plot. Thus

where σ0 = initial effective stress, σ = final effective stress, Δe = change in void ratio.a) For undisturbed soils, Cc = 0.009 (wl – 10)b) For remolded soils, Cc = 0.007 (wl – 10)

where wl = liquid limit (%).4) Expansion Index:- The expansion index or swelling (Cc) is the slope of the e-log σ plot

obtained during unloading

5) Recompression Index:- Recompression is the compression of a soil which had already been loaded and unloaded. The load during recompression is less than the load to which the soil has been subjected previously. The slope of the recompression curve obtained during reloading.

2.0 TERZAGHI’S THEORY OF CONSOLIDATION(1) Assumptions:- Terzaghi (1925) gave the theory for the determination of the rate of

consolidation of a saturated soil mass subjected to a static, steady load. The theory is based on the following assumptions:

The soil is homogeneous and isotropic. The soil is fully saturated.



DEPARTMENT OF CIVIL ENGINEERING The solid particles and water in the voids are incompressible. The consolidation occurs

due to expulsion of water from the voids. The coefficient of permeability of the soil has the same value at all points, and it remains

constant during the entire period of consolidation. Darcy’s law is valid throughout the consolidation process. Soil is laterally confined, and the consolidation takes place only in axial direction.

Drainage of water also occurs only in the vertical direction. The time lag in consolidation is due entirely to the low permeability of the soil. There is a unique relationship between the void ratio and the effective stress, and this

relationship remains constant during the load increment.(2) Derivation of Differential Equation:- The basic differential equation of one-

dimensional consolidation can be derived as under:Let us consider a saturated clay layer of thickness 2d (= H) sandwiched between two

layers of sand. When a uniform pressure of Δσ is applied on the surface of the top sand layer, the total stress developed at all points in the clay layer is increased by Δσ.

As explained in the spring analogy model, initially the whole of the pressure is taken up by water, and the hydrostatic excess pressure of Δσ/γw develops. Fig. 1.8 shows the excess hydrostatic pressure diagram on the right side. It is assumed that various points along the thickness of the clay layer are connected by flexible tubes to the pirzometers. At time t – 0, just after the application of the load, the excess hydrostatic pressure

is equal to Δσ/γw throughout the clay layer. This is represented by the horizontal line AB. The

excess hydrostatic pressure is independent of the position of the water table. For convenience, The water table

Fig.1.8 Excess Hydrostatic pressureis assumed at the level of the surface of the clay layer.

Water starts escaping towards the upper and the lower sand layers due to excess hydrostatic pressure developed. The hydrostatic excess pressure at the top and the bottom of the clay layer, indicated by points C and E in the pressure diagram, drops to zero. However, the excess hydrostatic pressure are hnown as isochrones. The isochrones CDE indicates the distribution of excess hydrostatic pressure at time t.



DEPARTMENT OF CIVIL ENGINEERINGAs the consolidation progresses, the excess hydrostatic pressure in the middle of the clay

layer decreases. Finally at time t = tf, the whole of the excess hydrostatic pressure has been dissipated, and the pressure distribution is indicated by the horizontal isochrones CFE.

Let us consider the equilibrium of an element of the clay at a depth of z from its top at time t. The consolidation pressure Δσ is partly carried by water and partly by solid particles as

where = is the pressure carried by solid particles,

and is the excess hydrostatic pressure (pressure units)Thus,

………………..(a)

where is the excess hydrostatic pressure at depth z.The expression for the hydraulic gradient i can also be obtained as under. Let us consider

a thin slice of clay layer Δz at depth z fig. 1.9. The pressure difference (Δ ) across this thickness is given by

The unbalanced head across the thickness is given by

The gradient becomes

(…..same as (a))

Fig.1.9 Pressure difference on a Thin Slice.From Darcy’s law, the velocity of flow at depth z is given by

The velocity of flow at the bottom of the element of thickness Δz can be written as




The discharge entering the element Qin is

where Δx and Δy are the dimensions of the element in plan.The discharge leaving the element Qout is

Therefore, the net discharge squeezed out of the element is given by,ΔQ = Qout - Qin

……………(b)As the water is squeezed out, the effective stress increases and the volume of the soil

mass decreases.

where V0 = initial volume of soil mass ( = Δx Δy Δz)

and = increase in effective stress.

The decrease in volume of soil per unit time

…………………(c)As the decrease in volume of soil mass per unit time is equal to the volume of water

squeezed out per unit time, Eqs. (b) and (c) give

……………………(d)For a given increment, δΔσ = 0. Therefore,

Therefore, Eq. (d) becomes,

Equating Two values of δv/δz,

where cv is the coefficient of consolidation and is given by




…………(e)Eq. (e) is the basic differential equation of one-dimensional consolidation. It gives the

distribution of hydrostatic excess pressure with depth z and the time t.

2.1 DETERMINATION OF COEFFICIENT OF CONSOLIDATIONThe curve between dial gauge reading and time t obtained in the laboratory by testing the

soil sample is similar in shape to the theoretical curve between U and Tv obtained from the consolidation theory. This similarity between the laboratory curve and the theoretical curve is used for the determination of the coefficient of consolidation (cv) of the soil. The methods are known as the fitting methods. The following two methods are commonly used.

1) Square – root of time method:- The method, devised by Taylor, utilizes the theoretical

relationship between U and . The relationship is linear up to the value of U equal to 60%. It

has been further established that at U=90%, the value of is 1.15 times the value obtained by

the extension of the initial straight line portion. (fig. 2.0 (a))The sample of the soil whose coefficient of consolidation is required is tested. For a

given load increment, the dial gauge readings are taken for different time intervals. A curve is

plotted between the dial gauge reading(R), as ordinate, and the as abscissa (fig. 2.0 (b)). The

curve ABCDE shows the plot. The curve begins at the dial gauge reading R0 at time t0, indicated by point A.

As the load increment is applied, there is an initial compression. It is obtained by producing back the initial linear part of the curve to intersect the dial – gauge reading axis at point A’. This corresponds to the corrected zero reading (Rc). The consolidation between the dial gauge reading R0 and Rc is the initial compression. The Terzaghi theory of consolidation is not applicable in this range.



DEPARTMENT OF CIVIL ENGINEERINGFig.2.0 Square – root of Time plot.

From the corrected zero reading A’, a line A’C is drawn such that its abscissa is 1.15 times that of the initial linear portion A’B of the curve. The intersection of this line with the curve at point C indicates 90% of U. The dial gauge reading corresponding to C is shown as R90

and the corresponding abscissa as .

The point D for 100% primary consolidation can be obtained from R90 as,

The consolidation after 100% of primary consolidation, in the range DE, is the secondary consolidation.

The value of the coefficient of consolidation of the soil for that laod increment is

obtained from the value of obtained from the plot. Fot U= 90%, the value of Tv = 0.848.

Therefore, using

2) Logarithm of time method:- The method given by Casagrande uses the theoretical curve between U and Log Tv. The curve consists of three parts: (i) an initial portion which is parabolic in shape, (ii) a middle portion which is almost linear, and (iii) the last portion to which the horizontal axis is an asymptote. It is observed that the point of intersection of the tangent drawn at the point of inflexion on the curve and the asymptote of the lower portion gives the value of 100% consolidation.

The sample of the soil is tested. For a given load increment, a curve is plotted between the dial gauge reading R and log t. Let R0 be the initial dial gauge before the application of the load increment. The corrected zero reading (Rc) is obtained using the fact that the initial portion of the curve is parabolic. Two points B and C are selected corresponding to some arbitrary time t1

and 4t1, respectively, and having the vertical intercept represents the corrected dial gauge reading Rc corresponding to zero primary consolidation.




Fig.2.1 Logarithm of time plot.The final portion of the experimental curve is linear. The point F corresponding to 100%

consolidation is obtained from the intersection of the two linear parts. The values of R100 and t100

are obtained corresponding to point F. The compression between the dial gauge readings Rc and R100 is the primary consolidation, and that between R100 and Rf is the secondary consolidation.Thus

For U = 50%,

2.2 PRECONSOLIDATION PRESSUREThe maximum pressure to which an over consolidated soil had been subjected in

the past is known as the preconsolidation pressure or over-consolidation pressure . When a

soil specimen is taken from a natural deposit, the weight of the overlying material (over-burden) is removed. This causes an expansion of the soil due to a reduction in pressure. Thus the specimen is generally preconsolidated or overconsolidated. When the specimen is loaded in the consolidated test, the initial portion AB of the compression curve ABCD (fig. 2.2) is actually a recompression curve. Consequently, the initial portion AB is relatively flat. It is followed by a straight line CD with a steep slope which indicates the compression of a virgin (normally consolidated) soil.




Fig.2.2 Determination of .

In the transition range BC, the slope gradually changes. The preconsolidation pressure falls

in this range. It can be obtained using the method given by Casagrande.The procedure consists of the following steps:

1) Determine the point E on the curve where the curvature is maximum, i.e., the radius of curvature is minimum.

2) Draw the tangent EF to the curve at E.3) Draw a horizontal line EG at E.4) Bisect the angle between the tangent EF and the horizontal EG, and draw the bisector EH.5) Produce back the straight line portion CD of the curve and determine the point of intersection P

of the bisector EH and the backward extension of CD.

6) Draw the vertical PJ through P which cuts the log -axis at J. The point J indicates the

preconsolidation pressure .

7) The vertical PJ cuts the curve at point K. The portion ABK of the curve represents the recompression curve and the portion KCD as the virgin compression curve.

COMPACTIONIt is a process by which the soil particles are artificially rearranged and packed together in to a closer state of contact by mechanical means in order to decrease the porosity of soil and thus increase its dry density. The compaction process may be accomplished by means of rolling, tamping or vibration. Compaction is somewhat different from consolidation. An example of compaction is the reduction in voids produced in a layer of the sub-grade by a rubber-tyred of steel tyred roller during construction.




In 1933, Proctor showed that there existed a definite relationship between the soil water content and degree of dry density to which a soil might be compacted and that for a specific amount of compaction energy applied on the soil there was a water termed the ‘optimum water content’ at which a particular soil attained maximum density. Compaction test are based on any one of the following methods or types : dynamic or impact, kneading, static and vibration. Some of the usual compaction tests used in the laboratory to determine water-density relationship of soil are: Standard and Modified Proctor tests, the main aim of these tests is to arrive at a standard which may serve as a guide and a basis of comparison for field compaction.

Standard Proctor Test:- The Standard Proctor test was developed by R.R.Proctor (1933) for the construction of earth fill dams in the state of California. The test equipment consist of(i) cylindrical metal miuls , having an internal diameter of 4 inches (10cm) an internal effective height of 4.6 inches (11.7 cm) and a capacity of 1/30 cu.ft(0.945 litre) (ii) detachable base plate(iii) collar 2in (5cm) in effective height (iv) rammer 5.5 lb(2.5kg) in mass falling through a height of 12 in (30.5cm). the test consist in compacting soil at various water content in the mould , in three equal layer each layer being given 20blows of the 5.5lb rammer dropped from a height of 12in. the dry density obtained in each test is determined by knowing the mass of the compacted soil and its water content.

the compactive energy used for this test is 6065kg cm per 1000ml of soil. IS :2720(Part VII) 1965 recommends a mould of 1000 ml capacity with an internal diameter 100mm and internal effective height of 127.5mm. the rammer has a mass of 2.6 kg with a drop of 310mm.

About 3kgof air-dried and pulverized soil, passing a 4.75mm sieve, is mixed thoroughly with a small quantity of water. The mixture is covered with wet cloth, and left for a maturing time of about 5 to 30 minutes to permit proper absorption of water. The quantities of water to be added



DEPARTMENT OF CIVIL ENGINEERINGfor first test depend upon the probable optimum water content for the soil. The initial water content may be taken 40% for coarse grained soil and 10% for fine grained soils. The empty mould attached to the base plate is weight without collar. The collar is then attached to the mould. The mixed and mature soil is placed in the mould and compacted by giving 25blows of the rammer uniformly distributed over the surface , such that the compacted height of soil is about 1/3 the height of the mould. Before putting the second layer of soil, the top of the first compacted layer is scratched with the help of any sharp edge. The second and third layers are similarly compacted, each layer being given 25 blows. The last compacted layer should project not more than 6mm into the collar. The collar is removed and the excess soil s trimmed off to make it level with the top of mould. The weight of the mould, base plate and the compacted soil is taken. A representative sample is taken from the centre of the compacted specimen and kept for water content determination. The bulk density γ and the corresponding dry density ρd for the compacted soil are calculated from the following relation:

M= mass of wet compacted specimen (g)

w= water content (ratio) V= volume of the mould, The compacted soil is taken out of the mould, broken with hand and remixed with raised water content (by 2 or 4 percent).after allowing for the maturing time, the soil is compacted in the mould in three equal layer , as describe above and the corresponding dry density ρ d and water content w are thus determined. The test is repeated on soil sample with increasing water contents, and the corresponding dry density ρd obtained .A compaction curve is plotted between the water contents as abscissa and the corresponding dry densities as ordinates. The dry density goes on increasing as the water content is increased, till maximum density is reached. The water content corresponding to the maximum density is called the optimum water content wo .




Zero air voids line:- A line which shows the water content dry density relation for the compacted soil contain9ng a constant percentage air voids is known as an air-voids line, and can be established from the following relation :

Where na= percent air voids w= water content of compacted soil ρd =dry density corresponding to w G =specific gravity ρw =density of water – 1 g/cm3

The theoretical maximum compaction for any given water content corresponds to zero air voids condition (na =0). The line showing the dry density as a function of water content for soil containing no air voids is called the zero air voids line or the saturation line, and is established by the equation :

Alternatively , a line showing the relation between water content and dry density for a constent and dry density for a constant degree of saturation Sr is established from equation:

For Sr = 100% the air voids line or degree of saturation lines when drawn across a compaction curve give a direct indication of the percentage air voids or of the degree of saturation existing at different points on the curve.

Modified Proctor Test:- Higher compaction is needed for heavier transport and military aircraft. The modified proctor test was developed to give a higher standard of compaction. This test was standardized by the American Association of State Highway Officials and is known as the modified AASHO test. In this test, the soil is compacted in the standard Proctor mould (capacity 1/30 cu. Ft or945 ml), but in five layers, each layer being given 25 blows of a 10lb rammer dropped through a height of 18 Prof. Mrs. Rashmi G. Bade

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DEPARTMENT OF CIVIL ENGINEERINGinches. The compactive energy given to the soil in this test is 27260 kg- cm per 1000 cm3 of soil , which is about 4 ½ times that of the standard proctor test IS: 2720(part VIII0-1965 recommends the use of a 4.89 kg rammer with a drop of 45 cm for heavy compaction. In the modified proctor test. The water content dry density curve lies above the standard proctor test curve, and has its peak relatively placed toward the left. Thus for a same soil the effect of heavier compaction is to increase in the maximum dry density and to decrease the optimum water content.

Fig. Compaction Curves of Standard Proctor Test and Modified Proctor TestFactors affecting compaction:- The various factor which affect the compacted density are as follows: (i) water content (ii) Amount and type of compaction (iii) Type of soil (iv) Addition of mixture.1) Water content:-It has been seen by laboratory experiment that as the water content is increased , the compacted density goes on increasing till a maximum dry density is achieved after which further addition of water decreases the density. When only a relatively small amount of water is present in soil, it is firmly held by the electrical forces at the surface of soil particles with a high concentration of electrolyte which prevent the diffuse double layer surrounding the particles from developing fully. The double layer depression leads to a low inter-particle repulsion and the particle do not move over one another easily when compactive energy is applied and hence high percentage air voids and low density is achieved. the increase in water content result in an expansion of double layer and reduction in the net attractive forces between particles which permit the particle to slide more easily past one another into a more oriented and denser state of packing together, and hence higher density. 2) Amount of compaction:- The amount of compaction greatly affects the maximum dry density and optimum water content of a given soil. The effect of increasing the compaction energy result in an increase in the maximum dry density and decrease in the optimum water content.However the increase in maximum dry density does not have a liner relationship with increase of compaction effort.3) Method of compaction:-The density obtained during compaction, for given a soil, greatly

depends upon the type of compaction or the manner in which the compaction effort is applied. The various variable in this aspect are (i) weight of the compacting equipment(ii)



DEPARTMENT OF CIVIL ENGINEERINGthe manner of operation such as dynamic or impact , static, kneading or rolling and (iii) time and area of contact between the compacting element and the soil.

4) Type of soil:- the maximum dry density achieved corresponding to a given compaction energy largely depend upon the type of soil. Well graded coarse- grained soils attain a much higher density and lower optimum water contents then fine grained soils which require more water for lubrication because of the greater specific surface.fig shows dry density water content curves for a range of soil type.

5) Addition of addmixture :- the compaction properties of a soil can be modified by a number of admixture other than soil material. These admixture have special application in stabilized soil construction.

Field compaction equipments

The necessary compaction of subgrades of roads, earth fills, and embankments may be obtained by mechanical means. The equipment that are normally used for compaction consists of1. Smooth wheel rollers2. Rubber tired rollers3. Sheeps foot rollers4. Vibratory rollers Laboratory tests on the soil to be used for construction in the field indicate the maximum drydensity that can be reached and the corresponding optimum moisture content under specifiedmethods of compaction. The field compaction method should be so adjusted as to translate Soil Improvement. Laboratory condition into practice as far as possible. The two important factors that are necessary to achieve the objectives in the field are1. The adjustment of the natural moisture content in the soil to the value at which the field compaction is most effective.2. The provision of compacting equipment suitable for the work at the site.The equipment used for compaction are briefly described below:

Smooth Wheel Roller



DEPARTMENT OF CIVIL ENGINEERINGThere are two types of smooth wheel rollers. One type has two large wheels, one in the rear and a similar single drum in the front. This type is generally used for compacting base courses. Theequipment weighs from 50 to 125 kN .The other type is the tandem roller normally usedfor compacting paving mixtures. This roller has large single drums in the front and rear and theweights of the rollers range from 10 to 200 kN.

Rubber Tired RollerThe maximum weight of this roller may reach 2000 kN. The smaller rollers usually have 9 to 11tires on two axles with the tires spaced so that a complete coverage is obtained with each pass. The tire loads of the smaller roller are in the range of 7.5 kN and the tire pressures in the order of200 kN/m2. The larger rollers have tire loads ranging from 100 to 500 kN per tire, and tire pressures range from 400 to 1000 kN/m2.

Sheepsfoot RollerSheepsfoot rollers are available in drum widths ranging from 120 to 180 cm and in drum diameters ranging from 90 to 180 cm. Projections like a sheeps foot are fixed on the drums. The lengths of these projections range from 17.5 cm to 23 cm. The contact area of the tamping foot ranges from 35 to 56 sq. cm. The loaded weight per drum ranges from about 30 kN for the smaller sizes to 130 kN for the larger sizes




Vibratory RollerThe weights of vibratory rollers range from 120 to 300 kN. In some units vibration is produced by weights placed eccentrically on a rotating shaft in such a manner that the forces produced by the rotating weights are essentially in a vertical direction. Vibratory rollers are effective for compacting granular soils Selection of Equipment for Compaction in the FieldThe choice of a roller for a given job depends on the type of soil to be compacted and percentageof compaction to be obtained. The types of rollers that are recommended for the soils normallymet are:Type of soil Type of roller recommendedCohesive soil Sheepsfoot roller or Rubber tired rollerCohesionless soils Rubber-tired roller or Vibratory roller.

Method of Compaction The first approach to the problem of compaction is to select suitable equipment. If theProf. Mrs. Rashmi G. Bade

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DEPARTMENT OF CIVIL ENGINEERINGcompaction is required for an earth dam, the number of passes of the roller required to compactthe given soil to the required density at the optimum moisture content has to be determined byconducting a field trial test as follows: The soil is well mixed with water which would give the optimum water content asdetermined in the laboratory. It is then spread out in a layer. The thickness of the layer normallyvaries from 15 to 22.5 cm. The number of passes required to obtain the specified density has to be found by determining the density of the compacted material after every definite number of passes.The density may be checked for different thickness in the layer. The suitable thickness of the layer and the number of passes required to obtain the required density will have to be determined. In cohesive soils, densities of the order of 95 percent of standard Proctor can be obtained with practically any of the rollers and tampers; however, vibrators are not effective in cohesive soils. Where high densities are required in cohesive soils in the order of 95 percent of modified Proctor, rubber tired rollers with tire loads in the order of 100 kN and tire pressure in the order of 600 kN/m2 are effective.

In cohesionless sands and gravels, vibrating type equipment is effective in producingdensities up to 100 percent of modified Proctor. Where densities are needed in excess of 100percent of modified Proctor such as for base courses for heavy duty air fields and highways, rubber tired rollers with tire loads of 130 kN and above and tire pressure of 1000 kN/m2 can be used to produce densities up to 103 to 104 percent of modified Proctor.Field Control of CompactionMethods of Control of DensityThe compaction of soil in the field must be such as to obtain the desired unit weight at the optimum moisture content. The field engineer has therefore to make periodic checks to see whether the compaction is giving desired results. The procedure of checking involves:1. Measurement of the dry unit weight, and2. Measurement of the moisture content.

There are many methods for determining the dry unit weight and/or moisture content of thesoil in-situ. The important methods are:1. Sand cone method,2. Rubber balloon method,3. Nuclear method, and4. Proctor needle method

Nuclear MethodThe modern instrument for rapid and precise field measurement of moisture content and unitweight is the Nuclear density/Moisture meter. The measurements made by the meter arenon-destructive and require no physical or chemical processing of the material being tested. The instrument may be used either in drilled holes or on the surface of the ground. The main advantage of this equipment is that a single operator can obtain an immediate and accurate determination of the in-situ dry density and moisture content.




Proctor Needle MethodThe Proctor needle method is one of the methods developed for rapid determination of moisture contents of soils in-situ. It consists of a needle attached to a spring loaded plunger, the stem ofwhich is calibrated to read the penetration resistance of the needle in lbs/in2 or kg/cm2. Theneedle is supplied with a series of bearing points so that a wide range of penetration resistancescan be measured. The bearing areas that are normally provided are 0.05, 0.1, 0.25, 0.50 and 1.0sq. in. The apparatus is shown fig . A Proctor penetrometer set is shown in Fig. 21.15(ASTMD-1558).Laboratory Penetration Resistance CurveA suitable needle point is selected for a soil to be compacted. If the soil is cohesive, a needle witha larger bearing area is selected. For cohesionless soils, a needle with a smaller bearing area willbe sufficient. The soil sample is compacted in the mold.The penetrometer with a known bearing area of the tip is forced with a gradual uniform push at a rate of about 1.25 cm per sec to a depth of 7.5 cm into the soil. The penetration resistance in kg/cm2 is read off the calibrated shaft of the penetrometer. The water content of the soil and the corresponding dry density are also determined. The procedure is repeated for the same soil compacted at different moisture contents. Curves giving the moisture-density and penetration resistance-moisture content relationship are plotted as shown in Fig.. To determine the moisture content in the field, a sample of the wet soil is compacted into the mold under the same conditions as used in the laboratory for obtaining the penetration resistancecurve. The Proctor needle is forced into the soil and its resistance is determined. The moisturecontent is read from the laboratory calibration curve. This method is quite rapid, and is sufficiently accurate for fine-grained cohesive soils. However, the presence of gravel or small



DEPARTMENT OF CIVIL ENGINEERINGstones in the soil makes the reading on the Proctor needle less reliable. It is not very accurate in cohesionless sands.

Advance compaction Techniques

The advance compaction techniques are (1).dynamic compaction (2) vibrofloation

Dropping of a Heavy WeightThe repeated dropping of a heavy weight on to the ground surface is one of the simplest of themethods of compacting loose soil.The method, known as deep dynamic compaction or deep dynamic consolidation may beused to compact cohesionless or cohesive soils. The method uses a crane to lift a concrete or steel block, weighing up to 500 kN and up to heights of 40 to 50 m, from which height it is allowed to fall freely on to the ground surface. The weight leaves a deep pit at the surface. The process is then repeated either at the same location or sequentially over other parts of the area to be compacted. When the required number of repetitions is completed over the entire area, the compaction at depth is completed. The soils near the surface, however, are in a greatly disturbed condition. The top soil may then be levelled and compacted, using normal compactiing equipment. The principal claims of this method are:1. Depth of recompaction can reach up to 10 to 12 m.2. All soils can be compacted.3. The method produces equal settlements more quickly than do static (surcharge type) loads.The depth of recompaction, D, in meters is approximately given by Leonards, et al., (1980) as



DEPARTMENT OF CIVIL ENGINEERINGwhere W = weight of falling mass in metric tons ,h = height of drop in meters.




VibroflotationThe Vibroflotation technique is used for compacting granular soil only. The vibroflot is a cylindrical tube containing water jets at top and bottom and equipped with a rotating eccentric weight, which develops a horizontal vibratory motion as shown in FigThe vibroflot is sunk into the soil using the lower jets and is then raised in successive small increments, during which the surrounding material is compacted by the vibration process. The enlarged hole around the vibroflot is backfilled with suitable granular material. This method is very effective for increasing the density of a sand deposit for depths up to 30 m. Probe spacings of compaction holes should be on a grid pattern of about 2 m to produce relative densities greater than 70 percent over the entire area. If the sand is coarse, the spacings may be somewhat larger.In soft cohesive soil and organic soils the Vibroflotation technique has been used with gravel as the backfill material. The resulting densified stone column effectively reinforces softer soils and acts as a bearing pile for foundations.

Fig Compaction by using vibroflot.


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