Unit Iv Compaction Ppt.pdf

UNIT IV COMPACTION EFFECTIVE STRESS PRICIPLE VERTICAL STRESS

UNIT IV COMPACTION

COMPACTION Compaction is the application of mechanical energy to a soil so as to rearrange its particles to a closer packing by pressing it and thus to reduce the void ratio. Air present in the soil is expelled during this process of pressing and hence the mass density of soil is increased It is applied to improve the properties of an existing soil or in the process of placing fill such as in the construction of embankments, road bases, runways, earth dams, and reinforced earth walls. Compaction is also used to prepare a level surface during construction of buildings

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COMPACTION The objectives of compaction are: •To increase the shear strength and therefore its bearing capacity and stability of slopes •To reduce compressibility and subsequent settlement under working loads •To reduce soil permeability making it more difficult for water to flow through •Compaction can prevent liquefaction during earthquakes

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CONSOLIDATION Consolidation is also a process of treatment given to soil, wherein the void space in the soil in reduced due to squeezing out of water under sustained static load. The difference between compaction and consolidation are as follows.

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Difference between compaction and consolidation COMPACTION

CONSOLIDATION

Rapid process

Gradual process (time (time dependent)

Momentary mechanical load such Sustained static load is applied as rolling, tamping and vibration for a long time is applied (dynamic load) Air present in the voids of the (partially saturated) soil is released or expelled

Water present in the (completely saturated) soil is released or squeezed

It is an artificial process

Mostly, it is a natural process

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Factors affecting Compaction 

Water content of soil



The type of soil being compacted



The amount of compactive energy used

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



 

Effects of water content Adding water at low moisture contents makes it easier for particles to move during compaction, and attain a lower void ratio. As a result increasing moisture content is associated with increasing dry unit weight till OMC. As moisture content increases, the air content decreases and the soil approaches the zerozero-air air-voids line. The soil reaches a maximum dry unit weight at the optimum moisture content Because of the shape of the nono-air air--voids line further increases in moisture content have to result in a reduction in dry unit weight. 8

Compaction curves for different soils

Dry density

Well graded sand

Low plasticity silt

Low plasticity clay

High plasticity clay

Water content, (%) 9

Effects of soil type Typical Values 3





gdry )max (kN/m )

mopt (%)

Well graded sand

SW

22

7

Sandy clay

SC

19

12

Poorly graded sand

SP

18

15

Low plasticity clay

CL

18

15

Non plastic silt

ML

17

17

High plasticity clay

CH

15

25

Gs is constant, therefore increasing maximum dry unit weight is associated with decreasing optimum moisture contents Do not use typical values for design as soil is highly 10 variable

Range of optimum moisture content (OMC, %) Sand

sandy silt or Silt silty sand

Clay

6-10

8-12

14--20 14

12--16 12

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Effects of varying compactive effort Dry unit weight

inc re a s ing c o m p a c tive e ne rg y

ze ro -a irvo

id s

lin e

Moisture content   

Increasing energy results in an increased maximum dry unit weight at a lower optimum moisture content There is no unique curve. The compaction curve depends on the energy applied Use of more energy beyond OMC has little effect 12

Presentation of results If the soil is saturated and

g dry

 Gsg w    G w  1  s 

Impossible Dry unit weight

Zero-airvoids line S = 100%

S = 50%

S = 75%

S = 90%

M oisture content 13

Field specifications During construction of soil structures (dams, roads) there is usually a requirement to achieve a specified dry unit weight.

Dry unit weight

Accept

Reject Moisture content

(a) > 95% of (modified) maximum dry unit weight

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Field specifications During construction of soil structures (dams, roads) there is usually a requirement to achieve a specified dry unit weight. Reject

Accept

Dry unit weight

Dry unit weight

Accept

Reject Moisture content

(a) > 95% of (modified) maximum dry unit weight

Moisture content

(b) >95% of (modified) maximum dry unit weight and w within 2% of OMC 15

Sands and Gravels For (cohesionless)soils without fines alternative specifications are often used. These are based on achieving a certain relative density.

Id

e max  e  e max  e min

e = current void ratio emax = maximum void ratio in a standard test emin = minimum void ratio in a standard test

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Sands and Gravels For (cohesionless)soils without fines alternative specifications are often used. These are based on achieving a certain relative density.

Id

e max  e  e max  e min

e = current void ratio emax = maximum void ratio in a standard test emin = minimum void ratio in a standard test

Id = 1 when e = emin and soil is at its densest state Id = 0 when e = emax and soil is at its loosest state

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Sands and Gravels We can write Id in terms of gdry because we have Gs g w e   1

g dry

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Sands and Gravels We can write Id in terms of gdry because we have Gs g w e   1

Id



g dry g dry

max

(g dry  g dry min )

g dry (g dry

max

 g dry min )

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Sands and Gravels We can write Id in terms of gdry because we have Gs g w e   1

Id



g dry g dry

max

(g dry  g dry min )

g dry (g dry

max

 g dry min )

The terms loose, medium and dense are used, where typically loose

0 < Id < 0.333

medium

0.333 < Id < 0.667

dense

0.667 < Id < 1

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Sands and Gravels We can write Id in terms of gdry because we have Gs g w e   1

Id



g dry g dry

max

(g dry  g dry min )

g dry (g dry

max

 g dry min )

The terms loose, medium and dense are used, where typically loose

0 < Id < 0.333

medium

0.333 < Id < 0.667

dense

0.667 < Id < 1

The maximum and minimum dry unit weights vary significantly from soil to soil, and therefore you cannot determine dry unit 21 weight from Id

LABORATORY COMPACTION TEST Laboratory Compaction The variation in compaction with water content and compactive effort is first determined in the laboratory. There are several tests with standard procedures such as: Indian Standard Light Compaction Test (similar to Standard Proctor Test) Indian Standard Heavy Compaction Test (similar to Modified Proctor Test)

Indian Standard Light Compaction Test Soil is compacted into a 1000 cm3 mould in 3 equal layers, each layer receiving 25 blows of a 2.6 kg rammer dropped from a height of 310 mm above the soil. The compaction is repeated at various moisture contents.

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Indian Standard Heavy Compaction Test It was found that the Light Compaction Test (Standard Test) could not reproduce the densities measured in the field under heavier loading conditions, and this led to the development of the Heavy Compaction Test (Modified Test). The equipment and procedure are essentially the same as that used for the Standard Test except that the soil is compacted in 5 layers, each layer also receiving 25 blows. The same mould is also used. To provide the increased compactive effort, a heavier rammer of 4.9 kg and a greater drop height of 450 mm are used. To assess the degree of compaction, it is necessary to use the dry unit weight, which is an indicator of compactness of solid soil particles in a given volume. The laboratory testing is meant to establish the maximum dry density that can be attained for a given soil with a standard amount of compactive effort.

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Laboratory Compaction tests 

Equipment

Handle collar (mould extension) Sleeve guide

Cylindrical soil mould Hammer for compacting soil Base plate

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Laboratory Compaction tests 

Equipment

Handle collar (mould extension) Sleeve guide

Cylindrical soil mould Hammer for compacting soil Base plate

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Comparison of Laboratory compaction tests Type of test

Mould Volume (cm3)

Standard 1000 Proctor (Light compaction) Modified 1000 Proctor (heavy compaction)

Mass of hammer (kg)

Drop of hammer (mm)

Number of layers

2.6

310

3

4.5

450

5

Energy delivered in J/m3

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Influence of water content and compactive energy on compaction •Increasing the water content •Increasing the compactive effort Effect of Increasing Water Content As water is added to a soil at low moisture contents, it becomes easier for the particles to move past one another during the application of compacting force. The particles come closer, the voids are reduced and this causes the dry density to increase. As the water content increases, the soil particles develop larger water films around them. This increase in dry density continues till a stage is reached where water starts occupying the space that could have been occupied by the soil grains. Thus the water at this stage hinders the closer packing of grains and reduces the dry unit weight. The maximum dry density (MDD) occurs at an optimum water content (OMC), and their values can be obtained from the plot. 27

Effect of Increasing Compactive Effort The effect of increasing compactive effort is shown. Different curves are obtained for different compactive efforts. A greater compactive effort reduces the optimum moisture content and increases the maximum dry density.

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An increase in compactive effort produces a very large increase in dry density for soil when it is compacted at water contents drier than the optimum moisture content. It should be noted that for moisture contents greater than the optimum, the use of heavier compaction effort will have only a small effect on increasing dry unit weights. It can be seen that the compaction curve is not a unique soil characteristic. It depends on the compaction effort. For this reason, it is important to specify the compaction procedure (light or heavy) when giving values of MDD and OMC.

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Presentation of results In the test, the dry density cannot be determined directly, and as such the bulk density and the moisture content are obtained first to calculate the dry density as,

where

= bulk density, and w = water content

A series of samples of the soil are compacted at different water contents, and a curve is drawn with axes of dry density and water content. The resulting plot usually has a distinct peak as shown. Such inverted ?V? curves are obtained for cohesive soils (or soils with fines), and are known as compaction curves.

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COMPACTION CURVE

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Dry density can be related to water content and degree of saturation (S) as

Thus, it can be visualized that an increase of dry density means a decrease of voids ratio and a more compact soil. Similarly, dry density can be related to percentage air voids (na) as

The relation between moisture content and dry unit weight for a saturated soil is the zero air-voids line. It is not feasible to expel air completely by compaction, no matter how much compactive effort is used and in whatever manner. 32

Engineering Behaviour of Compacted Soils The water content of a compacted soil is expressed with reference to the OMC. Thus, soils are said to be compacted dry of optimum or wet of optimum (i.e. on the dry side or wet side of OMC). The structure of a compacted soil is not similar on both sides even when the dry density is the same, and this difference has a strong influence on the engineering characteristics. Soil Structure For a given compactive effort, soils have a flocculated structure on the dry side (i.e. soil particles are oriented randomly), whereas they have a dispersed structure on the wet side (i.e. particles are more oriented in a parallel arrangement perpendicular to the direction of applied stress). This is due to the well-developed adsorbed water layer (water film) surrounding each particle on the wet side.

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Swelling Due to a higher water deficiency and partially developed water films in the dry side, when given access to water, the soil imbibes more water and will soak and then swell more. Shrinkage During drying, soils compacted in the wet side tend to show more shrinkage than those compacted in the dry side. In the wet side, the more orderly orientation of particles allows them to pack more efficiently. Construction Pore Water Pressure The compaction of man-made deposits proceeds layer by layer, and pore water pressures are induced in the previous layers. Soils compacted wet of optimum will have higher pore water pressures compared to soils compacted dry of optimum, which have initially negative pore water pressure. Permeability The randomly oriented soil in the dry side exhibits the same permeability in all directions, whereas the dispersed soil in the wet side is more permeable along particle orientation than across particle orientation. Compressibility At low applied stresses, the dry compacted soil is less compressible on account of its truss-like arrangement of particles whereas the wet compacted soil is more compressible. 34

The stress-strain curve of the dry compacted soil rises to a peak and drops down when the flocculated structure collapses. At high applied stresses, the initially flocculated and the initially dispersed soil samples will have similar structures, and they exhibit similar compressibility and strength

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Field Compaction and Specifications To control soil properties in the field during earthwork construction, it is usual to specify the degree of compaction (also known as the relative compaction). This specification is usually that a certain percentage of the maximum dry density, as found from a laboratory test (Light or Heavy Compaction), must be achieved. For example, it could be specified that field dry densities must be greater than 95% of the maximum dry density (MDD) as determined from a laboratory test. Target values for the range of water content near the optimum moisture content (OMC) to be adopted at the site can then be decided, as shown in the figure.

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For this reason, it is important to have a good control over moisture content during compaction of soil layers in the field. It is then up to the field contractor to select the thickness of each soil lift (layer of soil added) and the type of field equipment in order to achieve the specified amount of compaction. The standard of field compaction is usually controlled through either end-product specifications or method specifications.

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End-Product Specifications In end-product specifications, the required field dry density is specified as a percentage of the laboratory maximum dry density, usually 90% to 95%. The target parameters are specified based on laboratory test results

The field water content working range is usually within ± 2% of the laboratory optimum moisture content It is necessary to control the moisture content so that it is near the chosen value. From the borrow pit, if the soil is dry, water is sprinkled and mixed thoroughly before compacting. If the soil is too wet, it is excavated in advance and dried. In the field, compaction is done in successive horizontal layers. After each layer has been compacted, the water content and the in-situ density are determined at several random locations. These are then compared with the laboratory OMC and

the sand replacement method, or the core cutter method

MDD using either of these two methods:

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Method Specifications Procedure for the site is specified giving: Type and weight of compaction equipment Maximum soil layer thickness Number of passes for each layer They are useful for large projects. This requires a prior knowledge of working with the borrow soils to be used

Field Compaction Equipment There is a wide range of compaction equipment. Compaction achieved will depend on the, thickness of lift (or layer) the type of roller the number of passes of the roller Speed of the roller and the intensity of pressure (contact pressure) on the soil The selection of equipment depends on the soil type as indicated.

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Compaction equipment Equipment Smooth wheeled rollers,

Most suitable soils static or Well graded sand-gravel, crushed rock,

vibrating

asphalt

Rubber tired rollers

Coarse grained soils with some fines

Grid rollers

Weathered rock, well graded coarse soils

Sheepsfoot rollers, static

Fine grained soils with > 20% fines

Sheepsfoot rollers, vibratory

as above, but also sand-gravel mixes

Vibrating plates

Coarse soils, 4 to 8% fines

Tampers, rammers

All types

Impact rollers

Most saturated and moist soils

Also drop weights, vibratory piles

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Various method of compaction Compaction by different equipments Compaction by explosives Compaction piles Precompression Compaction by pounding Vibrofloatation method

Terra probe method

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Vibrofloatation method Thick deposits of loose sandy soils upto 30 m depth Cylindrical tube 2 m dia, fitted with water jets at top and bottom. It contains a rotating eccentric mass which develops a horizontal vibratory motion. The vibrofloat is sunk into the loose soil upto the desired depth using the lower water jet. This water jet creates a momentary quick condition ahead of the vibrofloat and the soil will loose shear strength. After reaching the desired depth, the vibrofloat vibrates laterally and causes compaction in the horizontal direction to a radius of about 1.5 m.

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Suitability of various methods of compaction in the field

For cohesionless soils only Smooth wheel rollers for small thickness Vibratory rollers, vibrofloatation, terra probe, blasting, compaction piles and explosives are used for compaction of large thickness of soil

For cohesive soils only Sheep-foot rollers and (precompression is also effective)

Both cohesionless and cohesive soils Tampers (suitable in confined area) Pneumatic tyres (suitable for all type of soil) Pounding method is heavy tamping, high energy compaction (all types of soil) 43

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45

46

Smooth wheel Rollers 47

Pneumatic- tyred rollers 48

Sheep Foot rollers 49

Vibratory Plate Compactors 50

Vibratory rollers 51

Following are the result of a compaction test Volume of the mould: 950 cc Weight of the mould: 1000 gm Specific gravity of sample G=2.7 Plot the compaction curve and indicate M D D and OMC (optimum moisture content) Plot the zero air void line. Also calculate void ratio, degree of saturation and theoretical max dry density. Calculate the percentage air voids at MDD and OMC.

Mass of mould 2.68 2.85 and wet soil (kg)

2.91

Water content %

16

12

14

2.87 2.87 2.85

18

20

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Solution Mass of mould 2.68 and wet soil (kg) Mass of wet 1.68 soil (kg)

2.85

2.91

2.87

2.87

2.85

1.85

1.91

1.87

1.87

1.85

Water content 12 % (0.12)

14 (0.14)

16 18 20 (0.16) (0.18) (0.2)

22 (0.22)

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Water content , W, (%)

12 (0.12)

14 (0.14)

16 (0.16)

18 (0.18)

20 (0.20)

22 (0.22)

Mass of wet soil (M), kg

1.68

1.85

1.91

1.87

1.87

1.85

Bulk density

b 

M V

Dry density

d 

b 1 w

Void ratio, (e)

 G  e   w   1  d  Deg of saturation

S 

wG e

Theoretical MDD

 d max 

G w 1  Gw 54

Water content , W, (%)

12 (0.12)

14 (0.14)

16 (0.16)

18 (0.18)

20 (0.20)

22 (0.22)

Mass of wet soil (M), kg

1.68

1.85

1.91

1.87

1.87

1.85

Bulk density

1.68/0.95

1.85 /0.95

1.91 /0.95

1. 870.95

1.87 /0.95

1.85 /0.95

= 1.77

= 1. 95

= 2.01

= 1.97

= 1. 97

= 1. 95

1.77 / 1+0.12

1. 95 / 1+0.14

2.01 / 1+0.16

1.97 / 1+0.18

1. 97 / 1+0.2

1. 95 / 1+0.22

= 1.58

= 1.71

= 1.73

= 1.67

= 1.64

= 1.60

0.58

0.56

0.62

0.65

0.69

0.12 x 2.7 /0.71 =0.46

0.65

0.77

0.78

0.83

0.86

2.04

1.96

1.89

1.82

1.75

1.69

b 

M V

Dry density

d 

b 1 w

Void ratio, (e)

 G  e   w   1  d  Deg of saturation

S 

wG e

Theoretical MDD

 d max 

(2.70 x 1 / 1.58) -1 = 0.71

G w 1  Gw 55

MDD – OMC relationship

Scale X- axis: 1 % (0.01) Y-Axis: 0.02 g/cc

MDD= 1.738 g/cc OMC = 15.5 %

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MDD – OMC relationship Zero air void line /100 % saturation line/Theoretical max dry density

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To calculate percentage air voids (na ) at MDD – OMC From the graph, MDD OMC W K T d 

= 1.74 g/cc =15.5 %

(1  na )G w 1  wG

(1  na )G w d  1  wG  d 1  wG   (1  na )G w (1  na ) 

 d 1  wG  G w

  1  wG   (na )   d  1  G w  -na = {1.74 (1+0.155*2.7) / (2.7 x1.0)} – 1 -na = -0.085

or

Percentage air voids (na ) = 8.5 %

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