CHAPTER II
LITERATURE RIVIEW
2.1
Theory of Compaction
Compaction is the densification of a soil by mechanical means. It is
determined by measuring the in-place density of the soil and comparing it to the
results of a laboratory compaction test. It is not a measure of the bearing capacity.
Soil compaction occurs when soil particles are pressed together, reducing pore
space between them (Figure 2.1).
Figure 2.1: Effects of compaction on pore space. (Holtz, 1981)
Heavily compacted soils contain few large pores and have a reduced rate
of both water infiltration and drainage from the compacted layer. This occurs
because large pores are the most effective in moving water through the soil when
it is saturated. In addition, the exchange of gases slows down in compacted soils,
causing an increase in the likelihood of aeration-related problems (Holtz, 1981).
Finally, while soil compaction increases soil strength-the ability of soil to resist
being moved by an applied force-a compacted soil also means that roots must
exert greater force to penetrate the compacted layer. Soil compaction changes
pore space size, distribution, and soil strength. One way to quantify the change is
by measuring the bulk density. As the pore space is decreased within a soil, the
bulk density is increased . Soils with a higher percentage of clay and silt, which
naturally have more pore space, have a lower bulk density than sandier soils. For
the purpose of defining the physical and index properties of soil it is more
convenient to represent the soil skeleton by a block diagram or phase diagram
(Figure 2.2).
Figure 2.2 : Weight - Volume Relationships (Lee and Singh 1981)
2.1.1
Forces of Compaction
Compaction is the process of compressing a material from a given volume
into a smaller volume. This is done by exerting force and movement over a contact
area, causing particles within the material to move closer together. The voids between
the particles – air, water or a combination of both – are expelled by the combination
of force and movement. Four forces, static pressure, manipulation, impact and
vibration are used in compaction (D’ Appolonia, et al., 1969).
.
2.1.2
Static Pressure
In static compaction, weighted loads, applied by rollers, produce shear
stresses in the soil or asphalt that cause the individual particles to slide across each
other. Compaction occurs when the applied force causes individual particles to break
their natural bonds to each other and move into a more stable position within the
material. Static smooth-wheeled rollers, static sheep foot (or pad-foot) and tamping
foot rollers work on this principle. Four factors influence compaction performance on
static rollers. They are axle load, drum width, drum diameter and rolling speed.
2.1.3
Manipulation
Manipulation, the second compactive force, rearranges particles into a more
dense mass by a kneading process. The process is especially effective at the surface
of the lift material. The longitudinal and transverse kneading action is essential when
compacting heavily stratified soils such as clay type soils. It is also the desired
process for the compaction of the final wearing surface of an asphalt pavement.
Manipulation helps to close the small, hairline cracks through which moisture could
penetrate and cause premature pavement failure. Sheep foot rollers and staggered
wheel, rubber tired rollers are specifically designed to deliver this type of compactive
force.
2.1.4
Impact
Impact creates a greater compaction force on the surface than an equivalent
static load. This is because a falling weight has speed, which is converted, to energy
at the instant of impact. Impact creates a pressure wave, which goes into the soil from
the surface. Impacts are usually a series of blows. Impact blows of 5 to 600 blows per
minute are considered low frequency ranges and are used on impact hammers and
hand tampers. Impact blows of 1400 to 3500 blows per minute are high frequency
and are used on vibratory compactors.
2.1.5
Vibration
Vibration is the final and most complex compactive force. It produce a rapid
succession of pressure waves, which spread in all directions. By reducing the air
voids, more soil can be added to the block. When moisture is added to the block the
soil particles will slip more on each other causing more reduction in the total volume,
which will result in adding more soil and, hence, the dry density will increase,
accordingly (Figure 2.3). The vibratory pressure waves are useful in breaking the
bonds between the particles of the material being compacted. When pressure is
applied, the particles tend to reorient themselves in a more dense (fewer voids) state.
To understand how vibratory compactors work, it is necessary to know about
centrifugal force, amplitude and frequency (Roman D and Thomas G.T.1973).
Figure 2.3 : Mechanism of Soil Compaction (Johnson and Sallberg, 1969)
2.1.6
95% Compaction
The term 95 % compaction is usually found in the contract specifications
of a project. It is a value recommended by the structural or geotechnical engineer
that is necessary to ensure adequate mechanical consolidation of structural soil
backfill. This value can differ for specific areas of a project site. In some cases
98% or 100% may be specified, while in others 85% or 90% may be required
(Lee and Singh 1971).The value refers to a percentage of the oven dry density of
a particular uniform soil material as determined in the laboratory using one of the
specified test methods.
2.2
Field Compaction
The compaction process may be accomplished by rolling, tamping or
vibration. The compaction characteristics are first determined in the laboratory by
various compaction tests. The main aim of these tests is to arrive at a standard
which may serve as a guide and a basis of comparison for field compaction
(Johnson and Sallberg, 1969).
Many factors influence the choice of compaction equipment. The type of
equipment selected for a project is sometimes chosen based on the contractor’s
previous experience, by the type of soil or by method specifications. Other
considerations are how well a machine will conform to the hauling and spreading
operation. Climatic and traction conditions are also important. Standardization of
equipment sometimes plays a role in the decision-making process.
The loose thickness of each layer before compaction is determined by the
soil type. When using large rollers, silt/clays and sandy silty clays require a loose
layer of 0.2m or less. Coarser material such as bank run gravel or crushed stone can
be placed in 0.3m to 0.35m loose layers. When using mechanical hand tampers or
small rollers, the thickness of each loose layer before compaction should be
decreased to between 0.1m and 0.15m.
Each layer must be thoroughly rolled/compacted, tested, and verified before
additional fill can be placed. It can take anywhere from four to eight passes to attain
the specified degree of compaction. This is easier said than done, since better than
80% of all soils cannot be sampled by this method. They contain larger aggregate
sizes that would deform the thin-walled sampler. Once a sample is obtained it must
be removed from the sampler without compression or deformation, and usually
trimmed so that it can be measured, and weighed. This process is not conducive to
field testing. There is no one compactor that will do all things on all jobs. Each
type has a definite material and operating range on which it is most economical.
2.2.1 Pneumatic Tire Compactors
Pneumatic tire compactors (Figure 2.4) are used on small to medium sized
compaction jobs, primarily on bladed, granular base materials. Pneumatics is not
suited for high production, thick lift embankment compaction projects.
The compactive forces (pressure and manipulation) generated by the rubber
tires work from the top of the lift down to produce density. The amount of
compactive force can be varied by altering the tire pressure (the normal method) or
by changing the weight of the ballast (done less frequently). The kneading action
caused by the staggered tire pattern helps seal the surface.
One advantage that pneumatic compactors have is that there is little bridging
effect between the tires. Therefore, they seek out soft spots that may exist in the fill.
For this reason, they are sometimes referred to as "proof" rollers (D’ Appolonia, et
al., 1969).
Another advantage is that pneumatic rollers can be used on both soil and
asphalt so a road building contractor can save by having one compactor for both
stages of construction – base and asphalt.
Figure 2.4: Pneumatic Tyre Roller (D’Appolonia, et al., 1969)
2.2.2
Sheep foot Roller
Sheep foot rollers (Figure 2.5) got their name from the fact that early
Roman road builders used to herd sheep back and forth over base material until the
road was compacted. The word "sheep foot" became a generic term to describe all
types of padded drums. In reality, a sheep foot roller is very different from a padded
drum or tamping foot roller.
A sheep foot pad is cylindrical; usually 203 mm long. The pad face is
circular and will range in size from 17.5 cm2 to 60 cm2. The pads on tamping foot
or padded drums are tapered with an oval or rectangular pad shape. The pad face is
smaller that the face of the pad, an important difference.
The pads on sheep foot drums penetrate through the top lift and actually
compact the lift below. When a pad comes out of the soil, it licks up or fluffs
material. The result is a loose layer of material on top. When more fill is spread, the
top lift will be fluffed and the previous layer will be compacted. A sheep foot roller
compacts from the bottom up. Using a sheep foot compactor has one definite
benefit. Because the top lift of soil is always being fluffed, the process helps aerate
and dry out wet clays and silts.
But the disadvantages of sheep foot rollers are numerous. The loose top lift
material can act as a sponge when it rains and slow the compaction process. The
loose material also slows hauling units bringing fill material, so haul cycles are
increased.
Plus, sheep foot compactors can work only at speeds from 6-10 km/h,
which cancels any benefit from impact and vibration. Pressure and manipulation are
the only compactive forces exerted on the soil. Usually 6-10 passes are needed to
get density on 203 mm lifts.
Figure 2 5: Sheep foot Compactor (D’ Appolonia, et al., 1969)
2.2.3 Tamping Foot Compactors
Tamping foot compactors (Figure 2.4) are high speed, self-propelled, nonvibratory rollers. They usually have four steel padded wheels and are equipped with
a dozer blade. Their pads are tapered with an oval or rectangular face.
Like the sheep foot, it compacts from the bottom of the lift to the top. But
because the pads are tapered, the pads can walk out of the lift without fluffing the
soil. Therefore, the top of the lift is also being compacted and the surface is
relatively smooth and sealed.
Because tamping foot compactors are capable of speeds in the 24-32 km/h
range, they develop all four forces of compaction: pressure, impact, vibration and
manipulation. This increases their compaction ability as well as production.
Generally 2 to 3 passes will achieve desired densities in 200-300 mm lifts, although
4 passes may be needed in poorly graded plastic silt or very fine clay.
The main disadvantage or limitation to the use of tamping foot compactors
is that they are best suited for large projects. They need long, uninterrupted passes
to build up the speed that generates high production. They are also considerably
more expensive than single drum vibratory compactors.
Figure 2.6: Tamping foot Compactors (D’Appolonia, et al., 1969)
2.2.4
Vibratory Compactors
Vibratory compactors (Figure2.7) work on the principle of particle
rearrangement to decrease voids and increase density. They come in two types:
smooth drum and padded drum. Smooth drum vibratory compactors generate three
compactive forces: pressure, impact and vibration. Padded drum units also generate
manipulative force. Compaction is assumed to be uniform throughout the lift during
vibratory compaction.
Density results from forces generated by a vibrating drum hitting the
ground. Compaction results are a function of the frequency of these blows as well
as the force of the blows and the time period over which the blows are applied. The
frequency/time relationship accounts for slower working speeds on vibratory
compactors. Working speed is important because it dictates how long a particular
part of the fill will be compacted. For vibratory compactors, a speed of from 3.2 to
6.2 km/h will provide the best results.
Smooth drum vibratory compactors were the first machines introduced and
are most in granular materials, with particle size ranging from large rocks to fine
sand. They are also used on semi-cohesive soils with up to 10% cohesive soil
content. Lift thicknesses vary according to the size of the compactor but, generally,
the lift thickness of granular material should not exceed 600 mm. Whenever large
rock is used in the fill, the lifts may be very thick – up to 1.2 m are not unusual. One
thing to remember when large rocks are in the fill is that the thickness should be
about 305 mm more than the maximum rock size. This permits lift consolidation
without having the large rocks project above the fill surface.
When padded drum machines were made available, the material range was
expanded to include soils with up to 50% cohesive material and a greater
percentage of fines. When the pad penetrates the top of the lift it breaks the natural
bonds between the particles of cohesive soil and better compaction results. The pads
are involuted to walk out of the lift without fluffing the soil and tapered to help
clean them. The typical lift thickness for padded drum units on cohesive soil is in
the 300 mm to 450 mm range.
2.2.5
Factors Which Influence Vibratory Compaction:
Vibratory compaction of soil is a complex process. More than 30 different
factors influence the overall compaction effort. (Johnson and Salberg 1969).
Vibratory compaction involves a drum which is moving up and down very rapidly
and moving forward over a non-homogeneous material. All components influencing
compaction should be considered as a whole, not as separate entities. It is the
combined characteristics of the compactor and of the dirt or asphalt it is attempting
to compact that determines the degree of compaction effort.
The characteristic of the material to be compacted plays a part in the
dynamics of compaction. Type, gradation, texture, initial density, moisture content,
aggregate strength characteristics, layer thickness, subsoil base and its supporting
capability all influence compaction. The sum effect of these properties is termed
mass stiffness and damping (Punmia, et al., 1994).
Figure 2.7: Vibratory Compactor (D’ Appolonia, et al., 1969)
2.2.6
Cohesionless Soil
Vibratory compaction with smooth drum machines is especially suitable
and economical on sand and gravel. High densities can be achieved in few passes
with the lift thickness determined by the size of the compactor (D’ Appolonia, et
al., 1969).
Free-draining sand and gravel that contains less than 10% fines are easily
compacted, especially when water saturated. When high density is required and
the lifts are thick, water should be added. This water will drain out of the lift
during the compaction process.
If the sand and gravel contains more than 10% fines, the soil is no longer
free draining and may become elastic when the water content is high. For this
type of soil, there will be an optimum moisture content at which maximum
density can be reached. Drying of the wet soil may be necessary to reach the
optimum moisture content.
On poorly graded sand and gravel, it is difficult to achieve high density
close to the surface of the fill. There is low shear strength in poorly graded soils
and the top layer tends to rise up behind the drum. This is not a problem when
multiple lifts are being compacted. The previous top layer will be compacted
when the next layer is rolled. However, the difficulty of compacting the surface
should be kept in mind when testing for density.
2.2.7
Cohesive Soil
Silts are non-plastic fines that are usually compacted with smooth drum
vibratory rollers. They can be spread and rolled in thick lifts. Like all fine-grained
soils, their compactability is dependent on moisture. For best compaction results,
the water content should not vary much from the optimum moisture content. If too
much water is present, silts rapidly approach the fluid state and compaction is
impossible. This means that the lifts may have to be aerated with discs, mixed
with drier soil (an expensive procedure) or the borrow pit has to be better drained.
Silty soils that also contain clay may have considerable cohesion. On these soils,
padded drum, tamping foot or pneumatic rollers will give the best results.
Clays have plastic properties which mean that the compaction
characteristics are highly dependent on moisture content. When the water content
is low, clay becomes hard and firm. Above the optimum moisture content, clay
becomes more and more plastic and difficult to compact. The main problem in
clay compaction is very often the need to adjust the water content. The addition of
water by using water trucks, discs or soil stabilizers is time-consuming. Water
infiltration into the borrow pit may be a better alternative. Drying wet clay can be
done only in warm and dry conditions. Prolonged rolling with sheep foot rollers is
sometimes done to lower the moisture content.
Even at the optimum moisture content, clay requires a higher compactive
effort and a lower lift thickness compared to non-cohesive soils. Padded drum
rollers work best because as the pads penetrate the soil, they break the natural
cohesive bonds between the particles. Pneumatic tire compactors can be used on
clays with a low to medium Plasticity Index. On projects where high production is
a requirement and clay is used as fill, good results can be obtained by using
tamping foot compactors in conjunction with vibratory padded drum compactors.
Tamping foot compactors equipped with dozer blades are efficient at spreading
the fill and breaking large, hard lumps of clay often found in clay borrow
material. These machines perform the first passes. Final density is reached by
vibratory padded drum compactors.
2.3
Measurement of Soil Density
2.3.1
Laboratory Compaction Tests
Depending on the project site, and the availability of fill materials, a
laboratory compaction test is required for each soil type or borrow source being
used on the project. The maximum density is affected by the grain size
distribution, and chemical/mineralogical makeup of the soil. A fully saturated soil
has zero air content. In practice, even quite wet soil will have small air content
The maximum dry density is controlled by both the water content and the
air-voids content. Curves for different air-voids contents can be added to the d /
w plot. The air-voids content corresponding to the maximum dry density and
optimum water content can be read off the d/w plot.
2.3.1.1 Effect of soil type
Well-graded granular soils can be compacted to higher densities than
uniform or silty soils. Clays of high plasticity may have water contents over 30%
and achieve similar densities (and therefore strengths) to those of lower plasticity
with water contents below 20%. As the % of fines and the plasticity of a soil
increases, the compaction curve becomes flatter and therefore less sensitive to
moisture content. Equally, the maximum dry density will be relatively low.
Figure 2.8: Effect of different soil type in compaction (Lee and Singh, 1971).
2.3.1.2 Standard Proctor Test
This method covers the determination of the relationship between the
moisture content and density of soils compacted in a mould of a given size with a
2.5 kg rammer dropped from a height of 30 cm.
2.3.1.3 Modified Proctor Test
The Modified Compaction method requires 4.5 times more effort than
required by the Standard Compaction method. It therefore densifies the soil to a
greater degree, reducing the air voids and increasing the in-place density by a
nominal 5% over that attained by using the Standard Compaction effort
(Figure2.9). The Modified Compaction method is required where foundations are
to be placed in the soil backfill, and where minimal or no settlement can be
tolerated by the structure. A 4.5 kg hammer is dropped from a height of 450 mm.
The soil sample is compacted in five layers with 25 blows per layer. The
compaction energy is 4.5 times larger than the Standard Proctor test.
Figure 2.9: Difference between Standard and Modified Proctor Curve (Holtz and
Kovacs, 1981)
The compactive effort will be greater when using a heavier roller on site or a
heavier rammer in the laboratory. With greater compactive effort:i.
Maximum dry density increases
ii.
Optimum water content decreases
iii.
Air voids content remains all the same
2.3.1.4 Harvard Miniature Test
In this test (Punmia, et al., 1994) the soil is compacted by kneading action
of a cylindrical tamping foot 12.7mm in diameter. The soil is compacted in a
small cylindrical mould having a capacity of 62.4ml with an internal diameter of
50mm and a height of 65mm. The tamping foot operates in a preset compression
spring so that the tamping force does not exceed appreciably a predetermined
value. The no of layers, no of tamps per layer and the tamping force are variables
depending upon the type of soil and the amount of compaction required.
2.3.1.5 Dietert Compaction Test
The apparatus consists of a 50mm diameter mould supported on a metal
base by two pegs. The soil is compacted by means of a piston on which a
cylindrical weight of 8kg fall through a height of 50mm. The weight is operated
by means of a cam. An air dried sample, weighing 150g and passing through a
2.36mm sieve, is mixed with water and then compacted in the mould by the
application of 10 blows of the weight. The mould is then inverted and further 10
blows are applied. The weight and the length of the compacted soil cylinder are
then measured to know its volume and bulk density. A specimen is put in the oven
for its water content determination. The same procedure is adopted with the soil
having varying water contents and the curve between water content and dry
density is plotted (Punmia, et al., 1994).
2.3.1.6 Abbot Compaction Test
The apparatus consists of a metal cylinder 5.2 cm in internal diameter and
40cm in effective height, clamped to the base. Oven dried soil; weighing 200g is
mixed with water and compacted in the cylinder with the blows of 2.5kg rammer
with a 5cm circular face falling through a height of 3.5cm above the base. The no
of blows to be used for the compaction are decided by calibration test either with
respect to proctor’s compaction or field compaction methods. The upper portion
of the stem of the rammer is graduated in millimeters. The height of the
compacted specimen may be determined from the reading on the graduated stem
of the rammer. The volume of the compacted specimen is calculated from the
known values of its height and cross section. Knowing the dry mass (200g) of the
sample, the dry density is known. A number of such specimens are compacted at
varying water contents, and a graph is plotted between dry density and water
content(Punmia, 1994).
2.3.2 Field Density Test
Compaction tests are required to verify and certify that the required
densification has been achieved. Testing frequency is usually one test for every
2000 sq. ft. (185 sq.m.) for each 6 in. (15 cm.) compacted lift. An In-Place density
test is just what you would expect - the determination of the density of the
material in-place. The classical method is to remove an undisturbed soil sample,
weigh it and measure it to determine its volume and then determine moisture
content by drying it (Lambe, 1969). From the measured values the dry density can
be calculated.
The required percentage of compaction is attained by conditioning the soil so
that the moisture content is within +/-3% of the optimum moisture. It is very
important to proof roll the site before placing any backfill to verify that there are
no soft or unstable areas (D’ Appolonia, et al., 1969). Soft and unstable virgin sub
grades prevent attaining the required degree of compaction. Periodic field-testing
is done to insure that the two important elements – target density and moisture
content – are being maintained throughout the particular construction job. These
tests can also indicate the effectiveness of the compaction equipment and
construction methods being used.
2.3.2.1 Sand Replacement Method
The sand Replacement method is a multi-step procedure which is more
time consuming than the nuclear density method, but has had proven accuracy
(Anon, 1984). It is sometimes used in conjunction with the nuclear method to
verify the calibration of the nuclear density meter.
First, a test site away from operating equipment (so vibrations do not
disturb the test) is selected and leveled. The unit’s base plate is laid on the
compacted surface and material is excavated through the hole in the plate to a
depth of about six inches (150 mm).
This wet material is weighed, dried in an oven and weighed again to
determine the moisture content.
The volume of the hole is measured by filling it with dry, free-flowing
sand from a special sand-cone cylinder. Since the density of the sand is known,
the volume of the hole can be calculated.
The density (wet unit weight) of the compacted sample is found by
dividing the weight of the material by the volume of the hole. Dry unit weight can
be found by dividing the wet unit weight by one plus the moisture content
(expressed as a decimal). For example, if the moisture content is 9%, the wet unit
weight would be divided by 1.09 to find dry density.
2.3.2.2 Water Balloon Method
The water balloon method is also called the Washing Denso meter Test.
The test’s first three steps – excavating a sample, weighing it and drying it – are
the same as performed in the sand-cone method (Punmia et al.,1994). In this
manner, moisture content is calculated.
However, in place of the sand-cone step to measure the volume of the
excavated hole, a Washington Denso meter is used. The Denso meter, a fluidfilled device is placed over the hole, and a balloon attached to the base plate is
placed in the hole. A valve is opened on the side of the Densometer and calibrated
fluid is forced into the balloon. As the balloon is filled, it takes on the shape of the
hole. The Denso meter is calibrated so the tester can read the volume of fluid and
thus the volume of the hole.
The density (wet unit weight) is found by dividing the weight of the
excavated sample by the volume of the hole – just as with the sand-cone method.
Dry unit weight also can be calculated by dividing the wet unit weight by one plus
the moisture content.
Limitations to the water balloon method are, again, the length of time
needed to get results and the fact that accuracy depends on the ability of the
balloon to conform to any irregularities along the sides of the hole.
2.3.2.3 Nuclear Density Method
Nuclear density meters emit radiation into the soil being tested and
counter measures both moisture content and density. The test is quick and can be
performed without disturbing the material.
There are two basic methods of measuring density – backscatter and direct
transmission. The direct transmission method gives the best accuracy, least
composition error and least surface roughness error. It can be used for testing over
a range of depths from 0.05m to 0.3m. The most important aspect of the direct
transmission method is that the operator has direct control over the depth of
measurement.
The backscatter method eliminates the need to create an access hole in the
compacted soil because the unit rests on the surface. However, accuracy is less
and composition errors are likely. This method works best in shallow depths –
0.05m to 0.75m.
Still another method offers an improvement in composition error and can
be used in either the direct or the backscatter mode. This is known as the air-gap
method. The testing device is raised above the test surface to lessen the
composition error, but accuracy will still not match the direct transmission
method.
The limitations for nuclear testing equipment are the precautions that must
be observed when handling radioactive material, and the fact that false readings
are sometimes obtained from organic soils or materials high in salt content.
Nuclear density gauges can be used to test all types of fill materials
including cohesive and cohesionless soils, granular fills, and fly-ash treated or
lime-stabilized fill materials. Nuclear density gauges are also commonly used for
non-destructive testing of asphaltic concrete. These gauges are particularly useful
in testing fly ash treated fills, lime stabilized fills, and asphaltic concrete.
Quickly-obtained compaction test results will allow reconditioning of fills in a
very timely manner. This capability can help minimize strength loss in fly-ash
treated or lime-stabilized fills. Compaction test results are also available rapidly
enough to re-roll lifts of asphaltic concrete before the material cools below
acceptable compaction temperatures. Each soil or aggregate has its own particular
chemical/mineralogical makeup. The nuclear densitometer reads the rebounding
radiation from the source to the detector. It does not know how the
chemical/mineralogical makeup is dispersing or absorbing the radiation.
Chemically bound water of hydration affects the moisture probe reading, usually
reading it as free water.
2.4
Moisture Content
Field moisture is the single most important factor, after the compaction
methods, in controlling the outcome of the compactive effort - PROPERLY
COMPACTED SOIL. When a soil sample is compacted in the laboratory, a
"Proctor" curve is developed by plotting the oven-dry density vs. the % moisture.
As the moisture increases the density increased until a maximum value is reached.
Upon increasing the moisture content further the density begins to decrease due to
saturation of the soil mass. The moisture content at which the maximum density is
reached is called the Optimum Moisture.
The recommended field practice is to try to get the field moisture within
±3% of the Optimum value by either adding water or spreading it out to dry. If the
soil is too dry or too wet it cannot be compacted to the required density.
2.5
Previous Work on Field Density Testing
Previous correlation using Nuclear Densometer has been studied by many,
in this chapter research carried out by the following researchers/Institution will be
discussed
i.
Dr. Pedro Romero Phd. P.E. of The University of Utah, titled
“Correlation of Nuclear Densometer with Core samples in Ashpalt
testing” 1999, as presented in October 2001 at the 77th Annual Meeting of
the North Eastern States Materials Engineers Association, Albany, New
York.
ii.
Dr. Murad Abu Farsakh Phd. P.E.of The Louisiana Transportation
Research Centre, titled “Assesment of In-Situ Test Technology for
Construction Control of Base Courses and Embankments” 2001 as
published in the LTRC Project Capsule 02 - 1GT.
iii.
Clean Washington Centre, titled “Density Testing of Cullet Aggregate
Using Nuclear Densometer” 1994, as published in the CWC Report GL –
94: 1994.
iv.
Thomas B. Randrup and John M. Lichter of The Royal Veterinary and
Agricultural University, Copenhagen Denmark, titled “Measuring Soil
Compaction – A Review of Nuclear Gauges” 2001 as published in the
Journal of Arboriculture 27(3): May 2001.
v.
Saskatchewan Highways and Transportation, titled “Correlation of
Nuclear Gauge density and Laboratory Core density for Asphalt Mixes”
2003, as published in the SHT journal STP 204-26 May 2003.
2.5.1
Research by Dr. Pedro Romero
The objectives of Dr. Romero’s research is to evaluate the Nuclear
Densometer in obtaining field compaction of Asphaltic pavement against the
traditional core sampling which is very time consuming in carrying out. The
results will only be known after the entire pavement work is completed and
usually if the core sample fails no correction can be made on the field and the
work stands to be rejected. With the use of Nuclear Densometer it is hoped that
further compaction in the field can be made immediately and rejection of
completed pavement works can be avoided.
A road construction site in Utah where pavement work was in progress has
selected. A total of 20 tests each were carried out using the nuclear densometer
and cored sample. The Average method and graphical method were used to
correlate the results. The results show that the value obtained using core method
was lower than Nuclear Densometer method and the mathematical parameters did
not favour the Nuclear Densometer since it obtained low slope and low
correlations. It was concluded that Nuclear Densometer can be used in obtaining
pavement densities but subject to supplement with core measurements, the device
needs to be calibrated to project conditions, adjust specification to account for
new density measuring device and training in proper usage of device is critical.
2.5.2
Research by Dr. Murad Abu Farsakh
The objective of Dr. Abu Farsakh’s study in this research was to assess the
use of Nuclear Densometer in order to evaluate the Field Compacted Density of
Highway materials for application in the QC/QA procedures during construction
of embankments.The research presented a thorough investigation and identified
an accurate and reliable assesment of the Nuclear Gauge. He has used core
samples to correlate the field densities against densities obtained using Nuclear
Densometer.
Three sites were selected and each site consisted of three samples. The
Nuclear Densometer was used to test followed by core sample which was
collected 0.15m away from the test point.
After analysing the results obtained from site, Dr Abu Farsakh concluded
that the Nuclear Densometer can be used as effective tool in determining field
compaction density since it produced almost similar results as the core samples,
furthermore results were immidiately known and adjustment to the field can be
made accordingly without having to wait as in the case of the Core samples. He
further recommended that larger scale field testing is required for comprehensive
analysis to carry out further correlation so that specifications for QC/QA in
evaluating field densities can be developed.
2.5.3
Research by Clean Washington Centre
The objective of CWC’s research is to investigate the suitability of
Nuclear Densometer in determining field compacted density of cullet aggregate
used as granular fill in construction of embankments. The study compared density
measurements obtained using a densometer with those obtained using a sand
cone. The latter is a physical test that determines of compacted material by
measuring its volume and weight. The Nuclear Densometer tests included the
backscatter which measures the density near the surface, and direct transmission
mode with the source probe extending to depths of 0.15m to 0.3m.
The study concluded that Nuclear Densometer could be used for testing of
cullet aggregate. No correction to the density is required and the procedyres can
be the same as those used for natural materials. The test frequency is
recommended to be the same at one test per lift per 250m2 of fill, but not less than
one per lift. To get the most accurate reading, it is recommended that four
measurements be obtained at each test locations with Nuclear Densometer and the
average reading should be used. The backscatter mode of the Nuclear Densometer
should be avoided as this test mode measures the upper portion of the lift only. It
is recommended that the test be performed using the direct transmission mode
with probe extended the full length of the lift.
2.5.4
Research by Thomas B. Randrup and John M.Lichter
The main objective of the study was to review the techniques of
determining natural soil compaction in urban areas using Nuclear
Densometer.Trial tests were performed to correlate Nuclear Densometer with the
traditional core sampling. Three site were selected in an urban park in the city of
Ringsted, Denmark. At all the sites, the soil was a clay loam. The soil was
levelled, and measurements with a Nuclear Densometer were made from the soil
surface to depths of 0.3m, at 0.1m intervals. One measurement was performed at
each depth at each test site, with 12 measurements in all. A standard measurement
time of 1 minute was used for each measurement. Right after the Nuclear
measurement, a core sampler was used to evaluate bulk density at each site. Three
cores were taken at each depth, 12 cores in each hole, 36 cores at all three test
sites.
The results and difference in bulk densities were tabulated. In general,
measurements with Nuclear Densometer provided a higher bulk density than
found with core sampling. The difference varied between –2.4% and 18.66%,
with an average +/- 5.91%.
The high variability between core sampling and Nuclear Densometer
measurements maybe due to the high difference between the three core samples
that were used to describe each test. It was difficult to obtain three valid core
samples in each depth due to rocks and artifacts. Rock did not seem to cause any
problems to Nuclear Densometer. The core sampling technique was far more time
consuming than the Nuclear Densometer. The tendency of lower bulk densities
with with core sampling maybe because of disturbed samples. It was concluded
that correlation of Nuclear Densometer with Core method could not be achieved
in clay loams and the core method may be regarded as unreliable for measuring
soil compaction on urban site, if the soil is stony.
2.5.5
Research by Saskatchewan Highways and Transport
The objective of the research by Saskatchewan Highways and Transport
was to correlate the density results of Asphalt Concrete pavements obtained with
a Nuclear Density Gauge and with a laboratory test on a cored sample.The test
was performed at the beginning of a paving contract for each lift, for every
change in lift thickness and at the change in the job mix formula to calibrate the
density in place by nuclear gauge with the density obtained from the cored
samples. Test locations was identified and marked followed by density in place
measurement using Nuclear Gauge obtained. The cored samples were obtained in
the exact same location as the Nuclear Gauge density readings were taken.
The results from both the test was analysed mathematically and
graphically. The value of coefficient r calculated was 0.9347 which is close to 1.
This indicates that there is a strong linear correlation between the equation and
data. The equation of the line and the data points was graphed on the correlation
chart to ensure the equation fits the data and that the data is in fact linear. An
equation to adjust in-place density by nuclear gauge readings was computed. It
was given as Y= 0.7096(x) +621.904, where Y=Adjusted Nuclear Density and x=
the actual nuclear density reading.