ENGINEERING MATERIAL ENGINEERING MATERIAL PROPERTIESPROPERTIES

(CE1303)(CE1303)

CompactionCompaction

Ms IkmalzatulMs Ikmalzatul

WHAT IS COMPACTION?

Compaction is the application of energy, mechanical energy, to a soil in order to rearrange the soil particles and thus get them to pack closer together - reducing the void ratio. The smallest possible void ratio is generally aimed for when undertaking construction works on or in a soil or when placing fill material.

Increasing the shear strength of the soil which• leads to improvements in the stability of embankments• increases the bearing capacity of foundations, road pavements, etc.

ii. Decreasing the compressibility of the soil• large voids can lead to the soil compacting under the imposed loads

which results in settlement

iii. Decreasing the void ratio• reduces the permeability of the soil – usually desirable in most

construction operations

iv. Decreasing the size of any air voids, if these fill with water they may• reduce the shear strength of the soil• increase the potential for swelling of the soil• increase the potential for shrinkage of the soil• increase the potential damage from frost heave

The main objective of compaction is to improve the engineering performance of the soil and compaction achieves this by :-

COMPACTION(Specification and Site Procedures)

Compaction constitutes one of the major processes in many construction operations (i.e. road construction). It is important that the compaction specified and obtained should sufficient to ensure the desired engineering performance of the soil is achieved but is not greater than necessary.

Where soil is being placed as a fill material it is normally compacted by a roller in layers 150 to 300 mm thick. Using a heavy roller will give better compaction however it is usually preferable to select a roller that is easily available and if necessary to improve its compaction performance by either reducing the thickness of the layers, by increasing the number of passes the roller makes over each layer or by reducing the speed of the roller.

The contractor will, whilst keeping within the specification, wish to keep the number of passes to the minimum and the layer thickness and speed of the roller to a maximum. It is necessary therefore necessary to maintain a constant check to ensure that adequate compaction is being achieved and to make agreed adjustments as necessary to number of passes, layer thickness and roller speed.

CompactionSpecification and Site Procedures

It is difficult on site to control the water content of the soil being compacted and this should therefore be it’s natural water content.

The degree of compaction is normally controlled by specifying the compactive effort that is to be applied, known as a method specification. With this type of specification it is usual to specify the relative compaction that has to be achieved on site where Relative Compaction =

Values for relative compaction of between 90 and 100 % are commonly found. Both the plant and material used have a bearing on the relative compactions that are achievable.

When constructing embankments it is often best to undertake a trial compaction using the soil and plant that will be used in the construction of the full size embankment in order to accurately determine the number of passes, layer thickness, etc. A factor which has to be taken into account when determining the rate of construction of an embankment, is the build up and dissipation of pore water pressures.

%100labtheinobtaineddensitydrymaximum

siteonobtainedbetodensitydry

Factors Affecting Compaction

The state of compaction of a soil is measured in terms of the DRY DENSITY of the soil. Dry density is used as this is the mass of solids per unit volume, the higher the dry density achieved the greater the amount of solids in the unit volume. The degree to which any soil can be compacted is affected by three factors:

• the water content of the soil• the amount of compactive effort that is applied to the soil• the type of soil and its grading

Factors Affecting Compaction

In order to determine the ideal compactive effort and water content that is

needed to achieve the maximum dry density for a soil and to determine water content ranges to achieve specified relative compaction values, a compaction test has to be carried out and from the results of this test a graph(s) of water content against dry density plotted. From this graph the maximum dry density achievable, optimum water content, air voids estimates and relative compaction water contents for the soil under test can be read off.

FORMULAE

Note – remember w and Av must be entered in decimal form.

wb

d 1

s

vwsd Gw

AG

1

1

Standard British Laboratory Test Procedure

Layer 1

Layer 3

Layer 2

Surplus soil to be trimmed flush with top of mould

Compacted soil Compacted

soil

Vt = 1000 cm3

Co

llar

Mo

uld

Remove all particles greater than 20mm in size from the soil sample.

Determine the mass of the empty mould without it’s collar.

Compact the soil sample in three layers into a standard mould of volume 1000 cm3 with collar attached.

Standard British Laboratory Test Procedure

Layer 1

Layer 3

Layer 2

Surplus soil to be trimmed flush with top of mould

Compacted soil Compacted

soil

Vt = 1000 cm3

Co

llar

Mo

uld

Compact each layer with 27 blows from the standard rammer (2.5kg) dropping through a height of 300 mm.

Remove the collar and trim the top of the compacted soil sample flush with the top of the mould such that it has a volume of exactly 1000 cm3

Standard British Laboratory Test Procedure

Layer 1

Layer 3

Layer 2

Surplus soil to be trimmed flush with top of mould

Compacted soil Compacted

soil

Vt = 1000 cm3

Co

llar

Mo

uld

Clean the outside of the mould and determine the mass of the mould and compacted soil.

Calculate the bulk density of the compacted soil in the mould.

Determine the water content of the compacted soil and hence calculate its dry density.

Standard British Laboratory Test Procedure

Layer 1

Layer 3

Layer 2

Surplus soil to be trimmed flush with top of mould

Compacted soil Compacted

soil

Vt = 1000 cm3

Co

llar

Mo

uld

Repeat the test for a range of water contents.

Plot the graph of water content against dry density and from this determine the maximum dry density and optimum water content.

On the same graph plot percentage air voids lines such that the air voids at different densities and water contents can be determined.

Standard British Laboratory Test Procedure

Layer 1

Layer 3

Layer 2

Surplus soil to be trimmed flush with top of mould

Compacted soil Compacted

soil

Vt = 1000 cm3

Co

llar

Mo

uld

Other compaction tests available included:BS Heavy TestStandard Proctor TestModified Proctor TestVibrating Hammer TestMCV Test

AIR VOIDS LINE

These lines are plotted on the water content / dry density plot in order to obtain an estimation of the air voids content of the soil under test/ analysis.

The 0% air voids line (also known as the Saturation line) is the theoretical curve of dry density against water content. It can never in practise be achieved due to the impossibility of expelling all of the air entrapped in the voids.

SUMMARY

Soil compaction is the process whereby the soil particles are constrained to pack more closely together, hence there is a reduction in air voids and, up to a certain point, there is an increase in dry density.

When the water content of the soil is low the soil is stiff, difficult to compact and hence the dry density value achievable is low. Adding more water acts as a lubricant, the soil softens and higher dry density values are achievable with less air voids. However, as the air voids are further decreased the air and water combine to hold the soil particles apart and hence the dry density values start to decrease.

Compacted soil under controlled conditions is used in roads, railways, embankments, earth dams, foundations, etc to

1) increase the shear strength of the soil2) decrease water absorbtion and hence permeability3) decrease settlements under loads.

EXAMPLE

Bulk Density(kgm-3)

1952 2006 2069 2099 2091 2081

Water Content

( % )12.5 13.4 14.8 16.2 17.4 18.4

Dry Density(kgm-3)

1735 1769 1802 1806 1781 1758

wb

d 1

1700

1750

1800

1850

10 12 14 16 18 20

Water Content (%)

Dry

Den

sity

(kg

/m3 ) 1809

15.7

Water Content (%) Dry Density,

Av = 0 Av = 5%

15 1922 1826

16 1885 1791

17 1851 1758

18 1817 1726

19 1785 1695

20 1753 1666

vs

wsd A

wG

G

11

Assume Gs = 2.7

1809

15.7

1700

1750

1800

1850

10 12 14 16 18 20

Water Content (%)

Dry

Den

sity

(kg

/m3 )

Av = 5%Av = 0%