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Geotechnical & Geoenvironmental Engineering CIVL4121 Part 2: Soil Compaction
CIVL 4121: Compaction: 1
GEOTECHNICAL
&
GEOENVIRONMENTAL
ENGINEERING
CIVL 4121
Part 2:
Soil CompactionMartin Fahey
School of Civil and Resource Engineering
CIVL 4121: Compaction: 2
What is Soil Compaction?
Compaction is the densification of soils by the application of
mechanical energy to reduce air void spaces in the three
phase soil model
it reduces the air content, but not the water content
cant compact saturated soil (almost always true)
Compaction refers to the mechanical bashing together of
unsaturated soil to form a denser soil
Do not confuse soil compaction with consolidation (long term
reduction of void ratio of a given soil).
Consolidation refers to slow squeezing out water from a saturated
soil, by application of a static load
Principal difference:
Compaction is direct & immediate
Consolidation is a time-dependent process
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Where/when is Compaction Used
Any time soil is used as a construction material, it is
compacted to improve its engineering properties:
compaction of sand pad for house foundations
compaction of soil/gravel/crushed rock/asphalt in road construction
compaction of soil in earth dams
compaction of soil behind retaining walls
compaction of soil backfill in trenches
In this unit, compaction will be referred to in:
pavement construction
dam construction
construction of clay liners for waste storage areas
construction of tailings dams
ground improvement
CIVL 4121: Compaction: 4
What does compaction achieve
At most basic level, compaction increases the dry unit weight
For soil containing (clayey) fines, well-compacted soil has
high negative pore pressures (suctions)
high effective stress, even when at ground surface
high strength
Good compaction results in:
higher stiffness (less compressible = less settlement)
higher strength = higher bearing capacity
reduced permeability (more later...)
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CIVL 4121: Compaction: 5
Type of Soil and Compaction Equipment
The desired level of compaction is best achieved by matchingthe soil type and the compaction method. Other factors mustbe considered as well, such as compaction specifications and
job site conditions
Clayey soils:
At the water content required for construction, clayey soil tends to bein the form of semi-dry hard clods
These need to be broken up (kneaded) to force the soil into a denserpacking (otherwise the compacted soil will still consist of clods withlarge voids between them)
The kneading action of a sheepsfoot roller (combined with
vibration) is the best means of doing thisGranular soils: The particles require a shaking or vibratory
action to move them; vibrating rollers (or vibratory platecompactors for small scale) are usually the best choice
CIVL 4121: Compaction: 6
Types of Compaction
There are four types of compaction effort on soil or asphalt:
Pressure alone (from the weight of the roller)
Vibration (+ pressure)
Kneading working the soil to break up lumps
Impact
Wide variety of field compaction equipment, so correct
choice of equipment (or mix of equipment) is vital forachieving the required result at the best possible cost (=usually in the minimum possible time)
Smooth-wheeled steel drum rollers
Pneumatic tyred rollers
Sheepsfoot rollers
Impact rollers
Vibrating rollers
Hand-operated vibrating plate and rammer compactors
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CIVL 4121: Compaction: 7
Smooth-Wheeled Steel Drum Rollers
Self-propelled or towed steel rollers ranging from 2 - 20
tonnes
Suitable for: well-graded sands and gravels; silts and clays of
low plasticity
Unsuitable for: uniform sands; silty sands; soft clays
CIVL 4121: Compaction: 8
Pneumatic-tyred Rollers
Usually a container on two axles, with rubber-tyred wheels.
Wheels aligned to give a full-width rolled track.
Dead load (water) is added to give masses of 12-40 tonnes.
Suitable for: most coarse and fine soils.
Unsuitable for: very soft clay; highly variable soils.
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Geotechnical & Geoenvironmental Engineering CIVL4121 Part 2: Soil Compaction
CIVL 4121: Compaction: 9
Sheepsfoot Roller ('tamping roller' ; pad-foot roller)
Self propelled or towed units, with drum fitted with
projecting club-shaped feet high contact stress,
kneading action (and sometimes vibrating as well)
Mass range from 5-8 tonnes
Suitable for: fine grained soils; sands and gravels, with
>20% fines; good for breaking down soil clods
Unsuitable for: very coarse soils; uniform gravels
CIVL 4121: Compaction: 10
Impact Roller
Compaction by static pressure, combined with the impact of
the 5-sided roller
Higher impact energy breaks up soil clods, achieving better
compaction (like a sheeps-foot roller in some ways)
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CIVL 4121: Compaction: 11
Impact Roller,
Mandurah area
Effective to 2-4 m depth
(?) in Perth
Three-sided version
CIVL 4121: Compaction: 12
Vibrating Drum Roller
Vibratory compactor: Fitting a vibrating mechanism to a
drum (or sheepsfoot) roller can increase its efficiency for
many soils. It also levels and smoothens any rutting that may
have been caused by tyre-roller.
Sheepsfoot roller may also have vibration mechanism
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CIVL 4121: Compaction: 13
Vibration Mechanisms
Same direction of rotation
gives forward-backward
vibration (as well as
vertical) discomfort to
operator? Counter-rotating masses
vertical vibration only
Vibrating mechanisms consist of internal rotating eccentric
masses typically rotating at up to 30 Hz
CIVL 4121: Compaction: 14
Plate and Rammer Compactors
Vibrating plate compactors
used for smaller confined areas
common in house construction in Perth sand
Rammer compactors used for backfilling (trenches)
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CIVL 4121: Compaction: 15
Factors Affecting Field Compaction (Soils)
Soil type: Grain size distribution, shape of the particles,
specific, gravity, quantity of clay in the soil
Water content (CRUCIAL)!
Compaction Effort: Controlled by the type of the equipment,
thickness of the lift, and properties of the soil or mix.
Layer (Lift) thickness: For soil, the thinner the layer is the
better compaction, but more costly.
Number of passes of the equipment and its speed: For soil,
more passes lead to better compaction results
Mix properties: Aggregate gradation, surface texture, and
angularity of the particle surfaces.
Environmental Effects (for asphalt): Air temperature,
humidity, wind, temperature of the surface under the mix
CIVL 4121: Compaction: 16
LABORATORY COMPACTION
Aim of laboratory compaction:
Simulate field procedures, aid in
the control of placement
conditions.
Two common types of test:
Standard compaction test, steel
rammer dropped on loose soilplaced in a mold
Modified compaction test
similar, but heavier rammer, and
more layers used
AS 1289 5.1.1 & 5.2.1 1993
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Geotechnical & Geoenvironmental Engineering CIVL4121 Part 2: Soil Compaction
CIVL 4121: Compaction: 17
Standard (or Proctor) Compaction
Mould is 105 mm diam. x 115.5 mm high (1 litre) & removable collar
Hammer is 2.7 kg, drop height 300 mm
Soil placed in mould in 3 layers, each compacted using 25 blows
Total energy delivered = 596 kN.m/m3
Layers judged so that at the end of compaction, soil is just above the top
of the lower mould
Remove collar
Strike off excess
Weigh mould
determine wet density
get water content get dry density
Repeat at different water contents
Plot dry density versus water content
CIVL 4121: Compaction: 18
Modified Compaction
Standard compaction test too light to represent modern
field compaction equipment (Standard test is from 1930s)
Modified compaction test uses:
heavier hammer (4.9 kg)
greater drop height (450 mm)
same mould (1 litre)
5 layers
25 blows per layer
Energy = 2703 kN.m/m3
Standard = 596 kN.m/m3
4.5 times more energy
Otherwise, procedure is the
sameVarious hammers
Automatic compaction machine
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CIVL 4121: Compaction: 19
Maximum Possible Compaction
Compaction involvesdriving out the air
Curves showsmaximum possible dryunit weight for givenwater content fordegree of saturation(Sr) = 100%, 95%,90% and 85%
These represent
maximum possibledensity for zero airvoids (ZAV), and airvoids (A) of 5%, 10%and 15%
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
0 5 10 15 20
Water Content w (%)
DryDensity
d
(t/m3)
Sr = 1
Sr = 0.95
Sr = 0.9Sr = 0.85
For G s = 2.65
( )( ) sssr srd G.wA1GA1
G.wS
GS
+=+=
CIVL 4121: Compaction: 20
Actual Compaction Curve (Example)
For a given compactionenergy, curve achievedshows:
a maximum dry densityd max (MDD) correspondingto an optimum moisturecontent (OMC)
near saturation on the wetside of OMC(Sr = 95% in this case)
low degrees of saturation onthe dry side of OMC
(dry unit weight d ratherthan dry density d can beplotted)
1.4
1.5
1.6
1.7
1.8
1.9
2
5 10 15 20
Water Content w (%)
DryDensity
d
(t/m3)
For Gs = 2.65
Air voids (%) 15 10 5 0
ZAV
MDD = 1.87 t/m3
O
MC=14.4%
Optimum Moisture Content (OMC): The moisture content at which the
maximum possible dry density is achievedfor a particular compaction energy
or compaction method
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CIVL 4121: Compaction: 21
Reasons for the Shape of the Curve
On the dry side of OMC, clayey soil shows high suction, hard
strong lumps, difficult to break down = difficult to compact
Increasing the water content reduces the suction, softens the
lumps, lubricates the grains = easier to compact
As water content increases, higher dry densities result, until
we start approaching full saturation (say at Sr = 90-95 %)
Now nearly impossible to drive out the last of the air
further increase in water content results in reduced dry
density (curve follows down parallel to the maximum
possible density curve the Zero Air Voids curve) (Note, values of MDD and OMC depend on the compaction
energy they are not unique soil properties)
For sand, suction at low water contents also prevents
compaction (but not if completely dry)
CIVL 4121: Compaction: 22
Cohesionless soil (clean sand)
Cohesionless soils d max achieved either completely dry, orcompletely saturated
at low water content, grains held together by suction (water at grain
contacts only)
this prevents compaction
Laboratory test for d max for sand requires fully saturatedsample, and involves vibration saturated sand in mould
weight on top (=5 kPa)
vibrate for certain time
measure d max
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CIVL 4121: Compaction: 23
MDD and OMC depend on input energy
As compaction energy increases, MDD (d max) increases andOMC reduces (curves constrained by ZAV line: parallel to
ZAV line)
Modified
Standard
CIVL 4121: Compaction: 24
Compaction affects soil structure
Soil tends to be more flocculated on the dry side; more
dispersed on the wet side
A: flocculated; C: dispersed
More compactive effort tends to disperse the soil
E more dispersed than A
It is these different structures,
in conjunction with the different
dry densities, that give different
properties at different points
on the compaction diagram
Soil structure as defined here
is referred to as soil fabric
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CIVL 4121: Compaction: 25
Compaction and Permeability
Lowest permeability for clayeysoils compacted wet of OMC
Where permeability (rather thanstiffness or strength) isimportant, could be best tocompact wet of OMC
More likely to undergo shrinkageif allowed to dry = crackingpossible, leading to grossreduction in overall permeability
Balance between lowpermeability and avoidance ofshrinkage cracking is aprimary concern in dam (core)construction, and in clay linersfor waste disposal areas
CIVL 4121: Compaction: 26
Suctions
Compacted clay samples shownegative pore pressure (suction)
depends on type of compaction &moulding water content
one of the contributing factors to soilstrength and stiffness (suction =effective stress = strength)
will see later that keeping water awayfrom compacted subgrade is importantfactor in life of pavement
Amount of total shrinkage oncomplete drying varies withmoulding water content and type ofcompaction
has implications for cracking of damcores and clay liners for waste storageareas
1 psi ~7 kPa
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CIVL 4121: Compaction: 27
Range of OMC & MDD for various soils
Different soils show different
compaction curves, even for
the same compaction energy
Slight changes in soil from a
borrow area can change the
compaction characteristics
Frequent checking of the
compaction curve essential, to
ensure that the correct target
values are being used
CIVL 4121: Compaction: 28
Shear strength of compacted samples
For same compaction energy, dry samples are much stronger
than wet samples, even though dry density may be less
Dry samples more brittle
if deformation crackingWet samples ductile
may be an advantage
can tolerate movement
integrity of dam cores, liners
MF
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Slide 28
MF1 Martin Fahey, 6/07/2006
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Geotechnical & Geoenvironmental Engineering CIVL4121 Part 2: Soil Compaction
CIVL 4121: Compaction: 29
Achieving the desired outcome
In many cases (e.g. road construction), aim is to get densest possible state
aim for high % of Modified MDD, and close to OMC (e.g. +0 2%)
In other cases, aim might be to have lowest permeability
compact wet of OMC, perhaps accepting lower density
BUT
soil compacted wet of OMC may undergo excessive shrinkage if allowed to
dry (cracking of dam cores, clay liners
high overall permeability, even if intact permeability is low soil compacted very dry of OMC is strong, but brittle
soil compacted wet of OMC is ductile can accommodate larger
deformations without cracking
Crucial to consider not just properties as compacted, but potentialchanges in properties with time due to exposure to drying, water, etc.
CHOOSING THE CORRECT COMPACTION STRATEGY
REQUIRES CAREFUL CONSIDERATION OF ALL THESE ISSUES
CIVL 4121: Compaction: 30
Drying Back
Common technique used in road construction in WA
Compact at close to OMC, to close to MDD
Leave exposed to drying
for a period (weeks)
reduces water content,
but not density
may even increase density
achieves much stiffer,
much stronger result
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CIVL 4121: Compaction: 31
Suitability of
Soils to
Compaction
Relative Desirability for Various Uses(1=best; 14=least desirability)
Rolled EarthFill Dams
CanalSections
Foundations Roadways
GroupSymbol
* if gravelly** erosion critical*** volume change critical
- not appropriate for thistype of use
Soil Type
GWWell-graded gravels, gravel/sand mixes, little or no fines
- - 1 1 - - 1 1 1 3
GPPoorly-graded gravels, gravel/mixtures, little or no fines
- - 2 2 - - 3 3 3 -
GMSilty gravels, poorly-gradedgravel/sand/silt mixtures
2 4 - 4 4 1 4 4 9 5
GCClay-like gravels, poorlygraded gravel/sand/claymixtures
1 1 - 3 1 2 6 5 5 1
SWWell-graded sands, gravellysands, little or no fines
- - 3* 6 - - 2 2 2 4
SPPoorly-graded sands, gravellysands, little or no fines
- - 4* 7* - - 5 6 4 -
SMSilty sands, poorly-gradedsand/ silt mixtures
4 5 - 8* 5** 3 7 6 10 6
SCClay-like sands, poorly-graded sand/clay mixtures
3 2 - 5 2 4 8 7 6 2
ML
Inorganic silts and very finesands, rock flour, silty or clay-like fine sands with slightplasticity
6 6 - - 6** 6 9 10 11 -
CL
Inorganic clays of low tomediumplasticity, gravelly clays,sandyclays, silty clays, lean clays
5 3 - 9 3 5 10 9 7 7
OLOrganic silts and organic silt-clays of low plasticity
8 8 - - 7** 7 11 11 12 -
MNOrganic silts, micaceous ordiatomaceous fine sandy orsilty soils, elastic silts
9 9 - - - 8 12 12 13 -
CHInorganic clays of highplasticity, fat clays
7 7 - 10 8** 9 13 13 8 -
OHOrganic clays of medium highplasticity
10 10 - - - 10 14 14 14 -
CIVL 4121: Compaction: 32
1.4
1.5
1.6
1.7
1.8
1.9
2
5 10 15 20
Water Content w (%)
DryDen
sity
d
(t/m3)
For Gs = 2.65
ZAV
95% MDD
OMC + 1%OMC - 1%
MDD
OMC
Meets specification
Control/Monitoring of Field Compaction
Field compaction is normallyspecified in terms of themaximum dry density obtainedfrom the laboratory
Example: Must achieve 95% ofMDD, and OMC 1%
In the diagram, tests fallinginto yellow zone meet these two
requirements often, only minimum density
ratio specified (e.g. 95%MDD)
Adequate compaction requirescompaction in layers (generally
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Sand Replacement Method Find sand loose density (lab.) - min Known weight of sand in bottle
Calibrate how much left after opening
cone on level surface (A)
Dig hole collect, weigh and dry the soil
removed (B)
Fill hole with sand weigh what is left in
bottle (C)
Slow (costly), accurate
A
B
C
CIVL 4121: Compaction: 34
Coring
Drive coring tube into ground surface using special hammer
and protective collar
Dig out the coring tube, trim the ends, weigh the contents
Obtain water content
work out the dry density
Used also for obtaining samples fordetermining the in situ CBR value
(California Bearing Ratio)
discuss later in the pavements section
100 mm
130mm
Driving collar (dolly)
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CIVL 4121: Compaction: 35
Nuclear Density Meter
Measures absorption of radiation (function of density)
Fast, accurate (after calibration)
Most common method in WA
CIVL 4121: Compaction: 36
Control of Field CompactionField Density Testing Method
Sand ConeBalloon Dens
meterShelby Tube Nuclear Gauge
Advantages* Large sample* Accurate
* Large sample* Direct readingobtained* Open graded material
* Fast* Deep sample* Under pipe haunches
* Fast* Easy to redo* More tests (statisticalreliability)
Disadvantages
* Many steps* Large area required* Slow* Halt Equipment* Tempting to accept flukes
* Slow* Balloon breakage* Awkward
* Small Sample* No gravel* Sample not alwaysretained
* No sample* Radiation* Moisture suspect* Encourages amateurs
Errors
* Void under plate* Sand bulking* Sand compacted* Soil pumping
* Surface not level* Soil pumping* Void under plate
* Overdrive* Rocks in path* Plastic soil
* Miscalibrated* Rocks in path* Surface prep required* Backscatter
Cost * Low * Moderate * Low * High
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CIVL 4121: Compaction: 37
Perth Sand Penetrometer
Developed at UWA (Glick & Clegg)
from Scala Penetrometer- similar but has cone at the
tip (used for CBR testing see later in pavements)
Widely used in Perth for compaction control in sand
house sand pads (every two-bit contractor has one)
backfilling of trenches
N = number of blows for penetration from 150 mm to
450 mm (penetration of 300 mm)
Correlations between N and percentage MDD (Glick
and Clegg, UWA)
for house pads, typically N 7 or 8 Can fit extension rods (up to 3 m) -much abused test
overburden stress increases the N value without any
increase in density
therefore, N = 8 at 3 m depth indicates a much lower
density than N = 8 at surface
6 kg sliding mass,
drop onto anvil
600 mm free
drop height
Count
number of
blows (N)
for 300 mm
penetration
Ignore first
150 mm of
penetration
Fixed anvil
Top stop
16 mm diameter bar
50 mm graduations
CIVL 4121: Compaction: 38
Intelligent Compaction
Based on vibrating drum roller being equipped with
accelerometers, to measure the ground response to
the vibrations. Not yet widely used in Australia, but
will be much more so in the future.
The principle is that the accelerations measured in
the drum depend on the ground stiffness (if you drop
something onto soft ground, the (negative)
acceleration is much lower than if you drop the same
object onto hard ground). As the ground stiffness
increases with ongoing compaction, the acceleration
response changes. By automatically logging the
ground response, and mapping this onto a 2-D plan ofthe ground surface, soft spots can be readily
identified. So, even as a simple indicator of where to
concentrate the compaction effort, the system would
be useful.
The systems in use in Europe go further. The ground
response is used to change the vibration mode of the
drum to improve compaction efficiency, and to
indicate where compaction effort should be
concentrated.
At UWA in the early 1980s, Dr Baden Clegg (since deceased) invented the CLEGG IMPACT HAMMER. This is simply a
modified compaction hammer, equipped with an accelerometer. The hammer is dropped onto the ground surface from a given
height, and the acceleration measured. The CLEGG IMPACT VALUE is an indication of ground stiffness, and hence an
indirect indication of the degree of compaction.
From the Bomag Brochure