ENGINEERING MATERIAL PROPERTIES (CE1303)

1
ENGINEERING MATERIAL PROPERTIES(CE1303)
Compaction
Ms Ikmalzatul
2
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.
3
The main objective of compaction is to improve
the engineering performance of the soil and
compaction achieves this by -

• 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.
• Decreasing the compressibility of the soil
• large voids can lead to the soil compacting under
  the imposed loads which results in settlement
• Decreasing the void ratio
• reduces the permeability of the soil usually
  desirable in most construction operations
• 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

4
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.
5
CompactionSpecification and Site Procedures
It is difficult on site to control the water
content of the soil being compacted and this
should therefore be its 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.
6
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

7
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.
8
FORMULAE
Note remember w and Av must be entered in
decimal form.
9
Standard British Laboratory Test Procedure
Remove all particles greater than 20mm in size
from the soil sample. Determine the mass of the
empty mould without its collar. Compact the
soil sample in three layers into a standard mould
of volume 1000 cm3 with collar attached.
10
Standard British Laboratory Test Procedure
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
11
Standard British Laboratory Test Procedure
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.
12
Standard British Laboratory Test Procedure
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.
13
Standard British Laboratory Test Procedure
Other compaction tests available included BS
Heavy Test Standard Proctor Test Modified
Proctor Test Vibrating Hammer Test MCV Test
14
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.
15
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 soil 2)
decrease water absorbtion and hence
permeability 3) decrease settlements under loads.
16
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
17
(No Transcript)
18
Assume Gs 2.7
Water Content () Dry Density, Dry Density,
Water Content () Av 0 Av 5
15 1922 1826
16 1885 1791
17 1851 1758
18 1817 1726
19 1785 1695
20 1753 1666
19
Av 5
Av 0