Compaction
Purposes of Compaction
Compaction is the application of energy to soil to reduce the
void ratio
– This is usually required for fill materials, and is sometimes used for
natural soils
Compaction reduces settlements under working loads
Compaction increases the soil strength
Compaction makes water flow through soil more difficult
Compaction can prevent liquefaction during earthquakes
Factors affecting Compaction
Water content of soil
The type of soil being compacted
The amount of compactive energy used
Laboratory Compaction tests
Equipment Handle
collar (mould
extension)
Sleeve guide
Cylindrical
soil mould
Hammer for
compacting soil
Base plate
Laboratory Compaction tests
Equipment Handle
collar (mould
extension)
Sleeve guide
Cylindrical
soil mould
Hammer for
compacting soil
Base plate
Mould Hammer Hammer
volume mass drop
Standard 1000 2.5 300
Modified 1000 4.9 450
Presentation of results
The object of compaction is to reduce the void ratio, or to
increase the dry unit weight.
Gs w
dry
1 e
Presentation of results
The object of compaction is to reduce the void ratio, or to
increase the dry unit weight.
Gs w
dry
1 e
In a compaction test bulk unit weight and moisture content are
measured. The dry unit weight may be determined as follows
W Wt of Solids Wt of Water Ws Ww
bulk
V TotalVolume V
Presentation of results
The object of compaction is to reduce the void ratio, or to
increase the dry unit weight.
Gs w
dry
1 e
In a compaction test bulk unit weight and moisture content are
measured. The dry unit weight may be determined as follows
W Wt of Solids Wt of Water Ws Ww
bulk
V TotalVolume V
Ww
1 W s
Ws
b u lk
V
Presentation of results
The object of compaction is to reduce the void ratio, or to
increase the dry unit weight.
Gs w
dry
1 e
In a compaction test bulk unit weight and moisture content are
measured. The dry unit weight may be determined as follows
W Wt of Solids Wt of Water Ws Ww
bulk
V TotalVolume V
Ww
1 Ws
Ws
b u lk (1 m ) dry
V
Presentation of Results
(d ry)ma x
Dry unit weight
mo p t
Moisture content
From the graph we determine the optimum moisture
content, mopt that gives the maximum dry unit weight,
(dry)max.
Presentation of results
To understand the shape of the curve it is helpful to develop
relations between dry and the percentage of air voids, A.
Va
A (%) 100
V
Presentation of results
To understand the shape of the curve it is helpful to develop
relations between dry and the percentage of air voids, A.
Va
A (%) 100
V
A V Vs
1 w
100 V
Presentation of results
To understand the shape of the curve it is helpful to develop
relations between dry and the percentage of air voids, A.
Va
A (%) 100
V
A V Vs
1 w
100 V
A
(Ws Ww ) (1 )
bulk Ws Ww 100
dry
1 m V (1 m) (Vs Vw ) (1 m)
Presentation of results
To understand the shape of the curve it is helpful to develop
relations between dry and the percentage of air voids, A.
Va
A (%) 100
V
A V Vs
1 w
100 V
A
(Ws Ww ) (1 )
bulk Ws Ww 100
dry
1 m V (1 m) (Vs Vw ) (1 m)
Ws Ww mWs
Vs Vw
Gs w w w
Presentation of results
To understand the shape of the curve it is helpful to develop
relations between dry and the percentage of air voids, A.
Va
A (%) 100
V
A V Vs
1 w
100 V
A
(Ws Ww ) (1 )
bulk Ws Ww 100
dry
1 m V (1 m) (Vs Vw ) (1 m)
Ws Ww mWs
Vs Vw
Gs w w w
A Gs w
dry (1 )
100 Gs m 1
Presentation of results
Gs w
If the soil is saturated (A = 0) and dry
G m 1
s
Presentation of results
Gs w
If the soil is saturated (A = 0) and dry
G m 1
s
Impossible
Dry unit weight
Zero-air-
voids line
S = 90%
S = 50% S = 75%
Moisture content
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.
As moisture content increases, the air content decreases and
the soil approaches the zero-air-voids line.
The soil reaches a maximum dry unit weight at the optimum
moisture content
Because of the shape of the no-air-voids line further increases
in moisture content have to result in a reduction in dry unit
weight.
Effects of varying compactive effort
inc re a s ing c o mp a c tive
e ne rg y
Dry unit weight
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 mopt has little effect.
Effects of soil type
Typical Values
3
dry )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 variable
Field specifications
During construction of soil structures (dams, roads) there is
usually a requirement to achieve a specified dry unit weight.
Accept
Dry unit weight
Reject
Moisture content
(a) > 95% of (modified) maximum
dry unit weight
Field specifications
During construction of soil structures (dams, roads) there is
usually a requirement to achieve a specified dry unit weight.
Accept Reject Accept
Dry unit weight
Dry unit weight
Reject
Moisture content Moisture content
(a) > 95% of (modified) maximum (b) >95% of (modified) maximum dry
dry unit weight unit weight and m within 2% of mopt
Compaction equipment
Equipment Most suitable soils
Smooth wheeled rollers, 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
Sands and Gravels
For (cohesionless)soils without fines alternative specifications are
often used. These are based on achieving a certain relative density.
emax e
Id
emax emin
e = current void ratio
emax = maximum void ratio in a standard test
emin = minimum void ratio in a standard test
Sands and Gravels
For (cohesionless)soils without fines alternative specifications are
often used. These are based on achieving a certain relative density.
emax e
Id
emax emin
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
Sands and Gravels
We can write Id in terms of dry because we have
Gs w
e 1
dry
Sands and Gravels
We can write Id in terms of dry because we have
Gs w
e 1
dry
dry ( dry drymin )
Id max
dry ( dry max
drymin )
Sands and Gravels
We can write Id in terms of dry because we have
Gs w
e 1
dry
dry ( dry drymin )
Id max
dry ( dry max
drymin )
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
Sands and Gravels
We can write Id in terms of dry because we have
Gs w
e 1
dry
dry ( dry drymin )
Id max
dry ( dry max
drymin )
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
