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A group of engineers, experts in geotechnical
theories, were chatting and came up with several different "most
important" aspects of compaction. "Everybody's become an expert
in compaction," commented one of the group, rather testily when
his own conclusions about density, moisture, and stiffness were
not accepted by all the others. "Even the city clerk and my brother's
accountant know all about compaction." That elicited a good laugh
from everybody. We imagine that teachers would say the same thing
about education; authors might say the same about budding journalists.
When a procedure is recognized as a science rather than just a
job, experts seem to appear from everywhere. Some are promoting
their own knowledge, others the products of their company. What
is a contractor to do?
From many conversations, we deduce that a
compaction contractor should be clear about what is required in
the specifications. Then he should perform the compaction and
have a way to show that he actually achieved what was specified.
It is not a question of eyeballing or stamping on the ground;
there are "scientific" requirements involved. The contractor himself
need not fully comprehend tests for moisture condition, impact
soil testers or cone penetrometers, Shelby tubes, the dangers
and benefits of nuclear density meters, or the traditional sand
replacement method.
What is the soil you are engaged to compact?
Forget gardening! Soil at construction sites is not the stuff
in which you grow dahlias and roses. There are places that identify
soil with symbols like GW, SC, CH, and ML—more than a dozen types—but,
for this article, we can practically place the soil found at construction
sites into three categories: rockfill, coarse aggregate, and fine
aggregate. Rockfill is cobbles and boulders and is used sometimes
for the foundations of buildings but especially for such projects
as dam construction and embankments for roads, railroad tracks,
and even airports. (Cobbles are considered greater than, say,
2.5 inches and boulders bigger than 4 inches.) To compact such
stones you need high compaction energy and you would probably
use heavy, single-drum rollers or heavy plates.
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| There's
a compaction tool for every application. |
Coarse
aggregate is sand and gravel, with the sand up to about 0.1 inch
and the gravel up to 2.5 inches. It is used frequently for sub-bases
and bases for roads, foundations for buildings, and trenches and
backfills. Its compactability depends on the grading. "A well-graded
soil consists of a wide range of particle sizes with the smaller
particles filling voids between larger particles," quotes Peter
Cannon for Multiquip. The equipment used to compact coarse aggregate
could be single-drum rollers (with a smooth drum), vibratory tandem
rollers, or even medium and heavy plates. Perhaps the most difficult
to compact—in part because its compactability depends on its
water content—is fine aggregate. It looks like the easiest,
doesn't it? Both the clay and silt materials are tiny and the cohesion
is great. To compact this aggregate you need high compaction energy
and you would expect to see trench compactors and heavy plates working
at it and heavy and medium single-drum rollers (with either the
smooth or the padfoot drum) on suitable jobs. Let's mention some
manufacturers here who have helped us with our research: Bomag,
Vibromax, Caterpillar, Hamm, Stone Construction Equipment, Sakai,
Wacker, Ingersoll-Rand, Hypac, and Ammann. They have experts with
understandable answers.
Enough Is Enough and Too Much Won't Help
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| Padfoot
configurations can be exactly right for some jobs. |
There are dangers in
both overcompaction and undercompaction. The greatest danger to
the contractor may be that he has to do the job twice because the
first effort did not meet specifications. The second time is pure
loss, isn't it? The chief aim of compaction is to provide the best
density for the particular soils or materials so the ground becomes
the ideal base for whatever it is going to support, be it a building,
parking lot, street, or playground. If the compaction is insufficient
(undercompaction), there will be settling. For buildings, this can
mean weakening cracks in the foundations and walls or, more obviously
to users of the building, doors that won't open and close properly
and windows that seem to be stuck most of the time. On roads, streets,
and parking lots, undercompaction will cause the driving surface
to sink or collapse. Overcompaction is no more helpful and may be
more difficult to correct. If you continue to use a roller on correctly
compacted soil, the particles of material will start to break down
and the structural integrity of the ground will be lost. After some
overcompaction errors, contractors have had to dig up the subgrade
and start over.
"Many believe that more is better for compaction,"
observes Steve Wilson, manager for marketing services at BOMAG
Americas. "That is erroneous thinking. Proper soil compaction
involves chemistry with the different mixtures of soil types.
It concerns how moisture or additives can affect their compactive
properties. It involves physics, with the effects of weight and
vibration on certain materials, and how air voids can influence
results. The fact is that soil compaction is a science in itself."
Most of us are not scientists and cannot afford to hire one for
every job. Don't despair; help is on the way. There can be few
products that have made such positive difference to any construction
work as roller-mounted density-measuring devices. Several manufacturers
offer them today; they have been going for about 10 years. Instruments
that tell you the same details have been available longer than
that, but those are used off the machine. It is the "onboard"
aspect that has changed most, to the advantage of the user. Though
each manufacturer tends to have its own method, what the devices
do is similar. They measure the stiffness of the compacted material,
noting the changes and guiding the operator to the desired density.
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| Compactors
can quickly turn rough ground into good foundations. |
"At BOMAG we have the
Terrameter," explains Wilson. "This uses a set of accelerometers
to measure the speed and reaction of the drum while it is compacting.
When the material is soft, in the first stages of the compaction,
most of the drum's energy is directed into the ground and there
is little reaction read by the accelerometers. As the material stiffens,
it is indicating that it is more compacted and dense and less energy
is transmitted into the ground and more is coming back into the
equipment [like a reflection]. The result is a more violent or reactive
response by the drum. The accelerometers read that. It is translated
at once as a higher density reading [omega value] by the microprocessor
in the cab." In the Terrameter system, the operator receives this
information via an analog omega meter, computer printout, and LED
console. The signal goes to the operator that the required compaction
has been achieved for the area surveyed and it's time to stop rolling
or to move into another area. "It seems simple," adds Wilson, "but
the onboard Terrameter system gives results that are as dependable
and accurate as those from onsite testing devices. The advantage
with the onboard system is that the operator gets the results on
the fly and does not have to stop to see how he's doing."
New Techniques = New
Benefits
We have seen sites where conscientious contractors
have been using onsite compaction meters after every pass of the
roller. It is effective, of course, but the operator seems to
wait for each new measurement before completing another pass or
going on to the next bit. That must be slower than using an onboard
device. Are these measurement systems on the roller difficult
to understand? What if your operator is not experienced? The indications
of progress are visual, and most systems have an LED indicator
that changes color when the desired density has been reached.
Achieving good results doesn't get much simpler than that on any
machine.
A bonus from onboard
density-measuring devices is that they have facilitated a new, useful
technique in soil compaction. It's known as "proof rolling." At
the beginning of a project the roller operator goes over the entire
job site…on his roller. This trip (it reminds us of warming
up for a race with gentle laps) gives the operator a printout for
the ground to be compacted. Where there are lower lines on the chart
(= low omega values) it indicates that there is weakness below;
where the lines are higher (= high omega values), the area is more
compacted, naturally. It's like getting a photo of the site's condition;
the contractor/operator can decide from the picture how he will
proceed with the actual compaction.
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| Many
compaction jobs are small assignments. |
There's another not-so-obvious
benefit. Contractors follow the specifications of the bid or contract.
Sometimes there are disputes. Onboard density-measuring devices
give readouts that help the operator on the job, but they also provide
valuable documentation of the whole project. "Engineering core samples
and onsite measuring devices can prove definitive compaction results
where the test was administered," counsels Steve Wilson of BOMAG.
"The onboard device yields printouts that indicate the compaction
achieved for the entire length of the lane traveled by the roller
and for the entire width of the drum. They can show what you achieved
for 100% of the compacted area. The printouts show interested parties
a progression of compaction, from one pass to the next. They also
include a time/date stamp to indicate the job's pace." If something
should go wrong and the public authorities (federal, state, county,
or city) come to the contractor for answers, the density-measurement
printout lets the contractor show the compaction results for every
square inch of the project.
What prevents every compaction contractor
from having rollers with onboard measuring devices? The cost.
(Surprise!) You don't have to purchase a new roller, however,
because the devices can now be installed on existing machines.
You would attach accelerometers to the drum of a roller and bring
communicating cables to the microprocessor in the cab. Even so,
the problem is the cost. Many contractors do not perceive the
advantages being worth the expense. "They do not feel that they
can justify the expense for such devices," confirms Wilson of
BOMAG. "Since they may be getting by with current rolling technology
and onsite testing devices, they see any additional expense as
cutting directly into potential profits. The opposite, in fact,
is true."
The viewpoint of proponents of onboard measuring
devices is that some of the costs of not using them are ignored.
Every pass of the roller has its costs for the owner—in operator
expenses, time, and machine maintenance. Every pass is also earning
money for the owner, if it is necessary. If the operator does
nine passes when five would have been just right, then four of
the passes have contributed standard expense but no profit. To
be really accurate, the four wasted passes have had a negative
effect on the project because they could have been made on an
area where they were not required and have used labor resources
that could have been profitable elsewhere. That's not counting
any problems caused by overcompaction! With onsite (as opposed
to onboard) devices for measuring the density achieved, one worker
generally does the test while the roller operator waits. Then
the roller goes while the tester waits? You can see that this
stop/go rolling for the duration of a long project could add up
to an unhealthy loss of worker productivity.
Tampering With Success
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| Ask
the experts when you want advice about features and benefits. |
Smaller compaction projects
are sometimes forgotten among all the applications using big rollers,
yet there are thousands of tamper/rammer/plate jobs done daily.
They may be during the construction of new buildings, along foundation
walls, over supply lines, and around support pillars. Tampers (or
stompers) work well on those everyday jobs like patios and driveways.
They generally have smaller feet than plates and handle work in
tight areas. Designed to compact cohesive materials (like clays
and silts), tampers compress the little pancakes of material in
cohesive soils and squeeze out the air voids and the water. They
also work in granular materials (sand and gravel), which require
vibration to allow the particles to rearrange themselves to a greater
density, because they do produce vibratory force as well as the
obvious impact force.
Just about every residential job needs some
compaction, and it is usually done with small machines. Think
of the many trenches (for cable and pipe) that need backfilling
and compacting and you will understand the popularity of machines
with names like rammer, reversible-plate, vibratory-plate,
and ride-on vibratory roller. Each of these compaction
devices is especially suitable for certain types of soil. Cohesive
soils respond well to rammers and small padfoot rollers. Granular
soils seem to prefer plates (reversible and vibratory) or those
smallish ride-on vibratory rollers. The particles in granular
soils are affected by vibrations according to their size; the
smaller the particle, the higher the frequency necessary. When
you have soils with larger particles, you will tend to use larger
equipment to get lower frequencies but higher compaction forces.
Finding the right equipment seems to be a
question of asking the experts, who could be fellow contractors,
dealers, or manufacturers. Vibromax, for example, offers rammers,
standard plates, walk-behind rollers, reversible plates, and trench
rollers, a range that has proved useful for building, excavating,
street work, and utility and landscape applications. "Our Series
6 single-drum rollers are available in smooth drum, padfoot drum,
and padfoot drum with a leveling blade configuration," notes Thomas
Meyer for Vibromax. "A padfoot shell is available for smooth-drum
machines. The dual frequencies give the user the ability to match
frequency and amplitude to job requirements and soil types."
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| Don't
underestimate the importance of good compaction for those small
projects. |
How tampering machines
perform depends on a combination of features: impact force, travel
speed, weight, stroke, and balance. The stroke is the height that
the shoe comes off the ground. With adequate weight, you will have
a higher impact force from a high stroke, the impact force being
the energy produced by the tamper when it hits the ground. Good
compaction should follow from a combination of a high-impact force
and a high frequency of blows per minute. The good travel speed
mentioned will be the result of a well-balanced design for the machine
so that its weight distribution gives good forward speed.
When you are comparing models from different
manufacturers, there are details to ask about. Oil-lubricated
tamping legs and nitrated spring cylinders are features that users
have praised (especially when those cylinders may require only
once-a-season oil changes). See if there is a guarantee on the
springs; they should last as long as everything else under normal
working conditions. (BOMAG gives a three-year guaranty on its.)
The bearings in tampers work hard and should be of the best quality
you can afford. Manufacturers point out that inferior bearings
may be cheaper at first but they probably end up costing more
than the high-quality versions. All the features of a tamper are
important, and we recommend that you ask your favorite dealer
about "all the usual suspects": engine, engine-speed control,
clutches, carburetion, air filters, and fuel tanks. You don't
want any inferior small part to ruin what can be a most practical
and profitable tool for your company.
Not Your Ordinary
Compaction
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| Today's
compactors are not difficult for the operator to control. |
Ground consolidation
is a phrase that you may see more frequently in the future. It is
a system used by foundation engineering contractors and seems to
be gaining popularity worldwide. Recently, in Chilliwack, BC, AMEC
Earth & Environmental was retained as a contractor to provide
geotechnical consultant services for the design and construction
of a fire hall and office building, with a rectangular footprint
of approximately 130 by 262 feet. The fire hall will be two stories
high with five apparatus bays and a three-story hose tower. The
office building will be four stories. Rapid Impact Compactors Ltd.
(agent for BSP International Foundations) was chosen to compact
the granular soils as part of the ground-improvement program. They
used a rapid impact compactor in the 7-tonne mode, mounted to a
40-tonne Hitachi 400H hydraulic excavator. At this site it was the
required depth of compaction that was critical.
"We were asked by AMEC to densify the ground
to a depth of 14.75 feet," comments Joe Miller, managing director
of Rapid Impact Compactors. "After testing, densities had actually
been increased to more than 29 feet." For this kind of compaction
(or ground consolidation), BSP's attachment is mounted on a suitably
sized hydraulic excavator. It is more efficient than suspending
a 10- or 15-ton weight from a crane and dropping it from a predetermined
height of 15 to 50 feet. The rapid-impact compactor's foot (diameter
of 1.5 feet) stays in contact with the ground. Energy is imparted
by dropping the weight through a relatively small height of up
to 4 feet at a rate of between 40 and 60 blows per minute. Although
the energy per blow is small in comparison with conventional dynamic
compaction, the rapid blow frequency amply compensates and can
result in a much greater total energy output per unit area of
the site. Quality-control Becker Penetration Testing was done
after completion of the ground improvement work and it confirmed
the rapid impact compactor's success in densifying the granular
soils to a depth of around 19.5 feet. "The use of this compactor
has been very successful," asserts Henrick Kristiansen, AMEC's
project manager. "It is proving to be a very cost-efficient ground-improvement
method."
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| Yes,
compactors can handle slopes. |
Soil compactors can do
more. One common task that we will only mention briefly here is
compaction at landfills where space becomes more precious each month.
At the Magnolia Landfill site in Baldwin County, AL, a key performer
is a Cat 836G compactor. The facility comprises 580 acres (with
80 acres total permitted footprint and 20 acres current lined footprint).
The Cat compactor weighs more than 113,000 pounds; it has 480 horsepower.
"It pushes garbage and does the compaction, spreading and arranging
the garbage in a four-to-one slope," says James M. Ransom Jr., environmental
and development director for the county's Solid Waste Department.
"We are getting excellent compaction with the 836G and its increased
horsepower and weight have really increased the compaction." Among
other big compactors, seen frequently along sites for new highways,
working with scrapers and articulated trucks, is Caterpillar's 815F
soil compactor. Its tamping wheels and tips are designed to promote
productivity. The wheel is designed for four-wheel coverage in two
passes, with front- and rear-wheel tracking to eliminate overlaps
or gaps. You can add a blade to this model, for dozing.
Paul
Hull writes on construction and environmental topics for several
international magazines.
GEC
- September/October 2004
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