3.1 CompactionIntroduction | unit 3 compaction and stress distribution

Unit - 3

Compaction and Stress Distribution

3.1 Compaction-Introduction:

In case the bearing capacity is less than the required value at a site, there are two solutions. The it is to go at a greater depth for foundation. This involves expenditure and change in design. The other method is to improve the bearing capacity of soil by various procedures. This chapter deals with the modification of soil properties for better performance.

Compaction of Soils:

This is the simplest method of increasing the performance, characteristics, especially bearing capacity of soil.

In this method, by the application of force or vibrations, the soil particles are more closely packed thereby increasing the density and hence bearing capacity of the soil.

Concept of Compaction:

Increasing the density of soil by application of mechanical energy is called compaction. Compaction is also defined as the process where the density is increased by reducing air voids. It may involve modification of water content or gradation of soil or both.

The theory of compaction was first developed by R.R. Proctor while building a dam in the USA.

The principles of compaction developed by him were published in a series of articles in Engineering News record in 1933.

Purpose of Compaction:

Compaction of soil is undertaken for a number of purposes. These are listed below:

To increase density and thereby shear strength and bearing capacity of soil, this is required in the case of slope stability improvement.

To decrease the permeability of soil, this is required for earth dams.

To reduce the settlement of structures after construction.

To reduce danger of piping, this is required for seepage control of earth dam.

To increase resistance towards erosion of soil by rain and other causes.

Key Takeaways:

This is the simplest method of increasing the performance, characteristics, especially bearing capacity of soil. In this method, by the application of force or vibrations, the soil particles are more closely packed thereby increasing the density and hence bearing capacity of the soil

3.2 Comparison between Compaction and Consolidation:

Sr.No.	Compaction	consolidation
1	Instant compression of soil under dynamic load is called compaction.	Gradual compression under a steady load is called consolidation.
2	Takes place before building of structure.	Takes place after building of structure.
3	Fast process	Very slow process.
4	Carried out for improving soil property.	Occurs naturally due to load of structure. Does not improve soil property.
5	Settlement is prevented due to compaction.	Settlement takes place due to consolidation.
6	Artificial process.	Natural process.
7	Pore water pressure not very important.	Pore water pressure very important.
8	Does not go on indefinitely.	Goes on indefinitely.

3.3 Compaction test:

3.3.1 Standard Proctor test:

The stepwise procedure of standard proctor test is given below:

Preparation of specimen:

Take 16 kg of air-dried sample passing through 20 mm 15 sieve.

Apply water to bring water content to about 10 percent, less than the estimated optimum water content.

Keep the soil in an air tight tin for about 20 hours to ensure thorough mixing of the water with the soil.

Divide the sample in six equal parts.

Compaction test procedure:

Clean the mould and weigh it to nearest gram.

Apply grease to inside of mould, base plate and collar.

Assemble the mould and base plate together on the floor.

Take one part of sample and fill the mould in 3 layers giving 25 blows to each layer with the 2.6kg hammer dropping from 310 mm.

Scratch with spatula each layer before putting in the next layer.

Remove the collar and trim the compacted soil flush with the top of mould with a straight edge.

Weigh the mould with the soil to nearest gram. Extract the soil from mould with the excruder.

Cut the soil sample in the middle and take representative sample in an air tight container from middle of the cut surface.

Determine the water content.

Calculate bulk density.

Calculate dry density using bulk density and water content values by using,

12. Repeat steps 4 to 11 by taking 2 to 3% more water than preceding test.

13. For all six samples, record the readings and plot moisture content against dry density.

14. Find out dry density corresponding to the maximum point of the curve and corresponding moisture content.

15. This dry density is known as maximum dry density (MDD) and the corresponding water content is known as the optimum moisture content (OMC).

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Fig.3.1: Rammer for light compaction for SPT

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Fig.3.2: Mould for compaction of SPT

Uses of the Test:

To obtain OMC and MDD of a given soil sample.

To get relation between dry density and moisture content for a soil sample.

To achieve controlled compaction of soil in the field.

Key Takeaways:

Preparation of specimen

Compaction test procedure

3.3.2 Modified Proctor Test:

This Proctor test is used for situations where heavy compaction is indicated.

The test procedure is almost identical except the three factors, viz: number of layers, weight of hammer and the distance through which the hammer falls. A comparison of standard and modified tests is given below:

Sr no.	Feature	Standard proctor test	Modified proctor test
1	Weight of hammer.	2.6 kg	4.89 kg
2	No. of layers in which soil is filled in the mould.	3	5
3	Vertical drop of hammer before striking the soil	310mm	450mm

Mould size in both tests is, 12.73 mm height and 100 mm diameter cylindrical mould and number of blows given in tests is 25.

Modified Proctor test gives 5 to 10% higher value of dry density and 2 to 3% less OMC as compared to standard test provided that both tests are performed on the same soil.

Key Takeaways:

It is used for situations where heavy compaction is indicated. The test procedure is almost identical except the three factors, viz: number of layers, weight of hammer and the distance through which the hammer falls.

3.4 Zero Air Void Line:

If the soil is assumed to be 100% saturated and different dry densities are calculated for 100% saturation, then the resulting line on the compaction curve is called the 100% saturation line or the Zero air-voids line.

This line shows that actual dry density cannot reach this theoretical value even after the heaviest of compaction. Fig. shows the compaction curve and its features.

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Fig.3.3: Compaction Curve

3.5 Factors affecting Compaction:

Following factors affect the compaction of soil and the maximum dry density which can be achieved by compaction:

Type of Soil:

For the same compactive effort, a well graded coarse-grained soil can be compacted to higher MDD than a uniformly graded soil. As the grain size decreases the OMC values goes on increasing and the MDD values goes on decreasing.

Amount of compaction:

If the compactive effort is increased, MDD increases and OMC decreases. But the increase in MDD is not linear with increase in energy.

Water content:

As is evident, if water content goes on increasing the maximum density of compacted soil goes on increasing up to a certain water content. If water content is further increased, the density goes on decreasing.

Admixtures:

Various admixtures like lime, calcium chloride, aggregates in various proportions etc. are used to improve the compaction properties of soil. Lime can increase the dry density by about 5 to 10%.

3.6 Effects of Compaction on soil properties:

The objective of compaction test is to improve some desirable properties of the soils such as reduction of compressibility, water absorption, permeability, increase in soil strength, bearing capacity etc. and change in swelling and shrinkage characteristics.

Compressibility:

Saturated clayey sample, compacted on the wet side of optimum, is more compressible than another sample of the same soil, having the same voids ratio, but compacted dry of optimum, when the applied pressure is in low pressure range. This is so because the sample compacted dry of optimum has flocculated structure and requires extra pressure to cause parallel orientation of particles. However, in the high-pressure range, a sample compacted dry of optimum is more compressible than the one compacted wet of the optimum.

Water absorption:

As optimum moisture content is achieved in soil, its further water absorption characteristic reduces.

Permeability:

As the density of soil increases, permeability decreases. This is due to increase in compactive efforts and reduction in void ratio. For the same density, fine grained samples compacted dry of optimum are more permeable than those compacted wet of optimum. This is so because these soils have flocculated structure when compacted dry of optimum, and have dispersed structure when compacted wet of optimum.

Shear strength:

In general, at low strains, strength of cohesive soils compacted dry of optimum is higher than those compacted wet of optimum. Shear strength of compacted clays depend upon dry density. moulding w/c, soil structure, compaction method, strain used to define strength, drainage condition and type of soil.

Shrinkage:

Sample compacted dry of optimum shrink less than that of wet of optimum because the soil particles having parallel orientation can pack more efficiently.

Swelling:

A clayey soil sample compacted dry of optimum water content has high water deficiency and flocculated structure, and hence exert greater swelling pressure and swell to higher water content than the sample of the same density obtained from wet side compaction.

Key Takeaways:

Effects of Compaction on soil properties are as follows:

Compressibility

Water absorption

Permeability

Shear strength

Shrinkage

Swelling

3.7 Field compaction methods:

Field compaction is carried out by various equipment’s. These equipment’s basically use three different procedures or methods of compaction.

These methods are: Rolling, tamping or ramming and vibration.

Rolling:

In this method, rollers of different types pass over the soil and due to their heavy weight, the compaction takes place.

Ramming or Tamping:

This method is very slow. In this method tampers or rammers are used to give repeated blows to the soil to compact it.

Watering may be done during ramming.

Vibration:

In this method the soil is vibrated by equipment known as vibrators.

Due to vibration, the soil particles are rearranged and more dense packing is possible. This increases the MDD of soil.

Key Takeaways:

Field compaction methods are as follows:

Rolling

Ramming or Tamping

Vibration

3.8 Compaction equipment for different types of soil:

For rolling, following equipment’s are used:

Smooth wheeled rollers: These have two rear wheels and one large drum in front. The weight 10 to 16 tons and they are suitable for coarse-grained cohesionless soils.

Pneumatic/rubber tyred rollers: These have 9 to 11 tyres on two axles in staggered fashion. These are suitable for both cohesive and non-cohesive soil.

Sheep foot rollers: These rollers have a drum of diameter 0.9 to 1.8 m. This drum has projections of 23 cm length. These projections are called sheep-feet. The contact area of each foot is 35 to 90 cm2. The weight is between 2 to 15 tons. These are suitable only for fine grained cohesive soils. Best suited for compaction of the core of an earth dam.

For ramming power rammers are used. These are suitable where access is difficult for rollers. Vibration equipment consists of vibratory plates of 0.5 to 4.5 m coverage and 90 kg weight vibrating at 1600 cycles per minute. Vibratory plates are suitable for coarse grained soils with less cohesion.

Key Takeaways:

Compaction equipment for different types of soil are as follows:

Smooth wheeled rollers

Pneumatic/rubber tyred rollers

Sheep foot rollers

Rammers

3.9 Placement Water Content:

The water content used in the field compaction is called the placement water content. It may be equal to, lower than or higher than the OMC determined in the laboratory. Following table gives examples of different soils in which placement water content is a required to add as mentioned in the table.

Table 3.9.1: Placement water content for different soils

Sr.no.	Type of soil	Placement water content	Remarks
1	Cohesive subgrades under pavement	Wet of optimum	Addition of water up to wet of optimum may not exhibit large expansions and swelling pressure on submergence.
2	Highway embankment on cohesive soil	Dry of optimum	Addition of water to dry of optimum achieve high strength and resistance to deformation and low volume compressibility.
3	High earth dams	1 to 2.5% less than optimum	It reduces probability of development of high pore pressure
4	Impervious cores of earth dams	Wet-side of optimum	It achieves low permeability, greater safety against cracking due to differential settlements or other causes.

3.10 Field Compaction Control and Use of compaction test results (Proctor needle in field compaction control):

The field compaction control consists of the determination of: (a) water content at which the soil has been compacted (b) Dry density. For proper compaction control, rapid test methods must be used like calcium carbide method and proctor needle method.

The proctor needle consists of a needle point attached to graduated needle shank which in turn is attached to a spring-loaded plunger. The needle point of varying cross-sectional area is available so that a wide range of penetration resistance can be measured. The penetration force is read on a loaded gauge fixed over the handle.

To use the needle in the field, a calibration curve is plotted in the laboratory between the penetration resistance as the ordinate and the water content as the abscissa. It is shown in Fig.

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Fig.3.4: Proctor needle

The laboratory penetration resistance is measured by inserting the proctor needle in the compacted soil the proctor mould. The penetration resistance corresponding to various water contents are thus noted at the end of each proctor compaction, and a calibration curve is plotted. This curve may be used to determine the placement water content. The penetration resistance of the compacted soil in the field is determined with the proctor's needle, and its w/c is read off from the calibration curve.

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Fig.3.5: Water content against penetration resistance curve

Key Takeaways:

3.11 Stress Distribution of soil:

Stresses in soil is caused by the self-weight of soil and /or structural loads, applied at or below the surface.

The study of stress distribution in soils have many applications to field problems where transmission and distribution of stresses is in large and extensive masses of soil. [e.g., transmission of wheel loads through embankments to culverts, pressure of foundation to soil strata below footings, pressures from isolated footings transmitted to retaining walls and transmission of wheel loads through stabilized soil pavements to sub-grades below.] This study is useful to the prediction of settlements of buildings, bridges and embankments.

Key Takeaways:

The study of stress distribution in soils have many applications to field problems where transmission and distribution of stresses is in large and extensive masses of soil.

3.12 Geostatic Stress:

The vertical stress in soil due to its, self-weight is called 'Geostatic Stress'. Consider an element of soil inside soil mass at depth Z where vertical and horizontal stresses are acting.

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Fig.3.6: Stress Component on soil element

Consider a point at depth Z below ground surface and stress acting at that point is,

Where, Y = Unit weight of soil

= Poisson's ratio

3.13 Boussinesq’s theory with assumption for point load and circular load:

Boussinesq Theory for Point Load:

The problem of stress evaluation at a point inside soil mass due to point load acting on the surface of a homogeneous, elastic, isotropic and semi-infinite medium of soil mass has solved by Boussinesq.

Assumptions Made by Boussinesq:

The soil medium is an elastic, homogeneous, isotropic and semi-infinite medium.

The soil medium obeys Hooke's law.

The self-weight of the soil is ignored.

The soil is initially unstressed.

The change in soil volume due to load application is neglected.

The top surface of the soil is free of shear stress and only point load is acting at a point.

Consider a point load Q acting at point O. Consider point 'S' inside soil mass where

is acting.

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Fig.3.7: Vertical stress due to point load

Where r= radial distance of point S from axis of load

= Vertical stress at point 'S'

R =Polar radial co-ordinate

The above Equation (iii) can be written as

= KB where, KB=

KB is Boussinesq's influence factor.

Shear stress at point S due to point load Q is,

Trz=^5/2

The typical values of KB are as follows:

Table: Values of influence factor for r/z ratios

r/z	KB	r/z	KB	r/z	KB
0.00	0.4775	0.40	0.3294	0.80	0.1386
0.10	0.4657	0.50	0.2733	0.90	0.1083
0.20	0.4329	0.60	0.2214	1.00	0.0844
0.30	0.3849	0.70	0.1762	2.00	0.0085
				3.00	0.0015

Boussinesq's Theory for Circular Load:

Consider the loaded circular area of radius a, 'q' is uniformly distributed load intensity on circular area. Consider elemental ring of radius 'r' and thickness dr.

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Fig.3.8: Vertical stress on circular area

Consider point 'P' at depth Z from the centre of loaded area.

The vertical stress at a depth. 'Z' under the centre of circular area of diameter 'a' is,

Key Takeaways:

3.14 Pressure distribution diagram on horizontal and vertical plane:

Pressure distribution diagram on horizontal plane:

Consider a horizontal plane at a depth Z from point of application of load Q.

Hence the vertical stress on horizontal plane due to point load acting at surface is

Where =Vertical stress on horizontal plane

Z=Depth from the point of application of load

r=horizontal distance of a point from a point load

Q = point load

Pressure Distribution Diagram on a Vertical Plane:

Consider a vertical plane at radial distance r from a point load acting on the surface of soil mass, The equation is useful to calculate stress on a vertical plane.

e.g., Let Q= 150 KN and r=3m

Following table gives the values of a, for different depths.

Z(m)	(KN/m2)
1m	0.295
2m	0.94
3m	1.407
4m	1.467
5m	1.328
6m	1.1388

3.15 Pressure Bulb and its significance:

Pressure bulb is also known as Isobar.

'Isobar' is a line or contour joining points of equal vertical stress inside soil mass.

Isobar is a spatial curve which shows the same vertical pressure at all points in a horizontal plane at equal radial distances from the load. This stress isobar is bulb in shape hence it is called as pressure bulb. The area outside the pressure bulb is assumed to have negligible stresses. The zone within which the stresses have a significant effect on the settlement of structures is known as the pressure bulb.

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Fig.3.9: Isobar

The Boussinesq Equation for vertical stress due to point load is,

Stress isobars for different vertical stress intensity can be plotted.

Key Takeaways:

Pressure bulb is a spatial curve which shows the same vertical pressure at all points in a horizontal plane at equal radial distances from the load. This stress isobar is bulb in shape hence it is called as pressure bulb.

3.16 Westergaard’s theory:

Boussinesq's theory for determination of vertical stress in a soil mass is for isotropic soils. But if soil deposits like sedimentary deposits are generally made up of thin layers of sand and homogeneous clay strata (i.e., anisotropic) Westergaard's theory for solution of stresses is related to such soils only. This theory is more suitable for stratified soil layers (i.e., sedimentary deposits)

Assumptions In Westergaard's theory:

The soil mass is an elastic medium.

The modulus of elasticity (E) of soil is constant.

The soil mass is semi-infinite in extent i.e., boundary surface is ground level itself but extends infinitely in all other directions.

The soil mass is anisotropic i.e., properties are not same throughout the soil mass.

The vertical stress from Fig.

^3/2

Where ɳ=

The value of 0.2 to 0.3 in above equation is quite realistic for many types of stratified soil deposits. In many cases, for practical purposes, ju is taken as zero.

=KW

where, Kw is called Westergaard influence factor.

Key Takeaways:

Westergaard’s theory is more suitable for stratified soil layers (i.e., sedimentary deposits).

3.17 Equivalent Point load method:

This is an approximate method to find the vertical stress at any point inside a soil mass due to a uniformly distributed load acting on an area of any shape.

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Fig.3.10: Equivalent point load

In this method, the loaded area is divided into a number of smaller areas. The total load acting on each area unit is considered to be a concentrated load acting at its centroid.

Q₁ Q2 Q3 Q4 = Load acting on unit areas.

F₁, F2, F3, F4=Radial distances of points of load acting from point P

The vertical stress acting at point P due to uniformly distributed load acting on the soil surface is,

Key Takeaways:

Equivalent point load method is an approximate method to find the vertical stress at any point inside a soil mass due to a uniformly distributed load acting on an area of any shape.

3.18 Approximate stress distribution method:

Numerical:

A point load of 800 KN acts at the ground surface. calculate the vertical stress at 9m depth on the axis of the load and at 2m away from the axis. Use Bossinesq’s analysis.

Solution: =