Hello, this is Dr. Kitch.
Welcome to this webcast in my Introduction to Geotechnical Engineering series.
This webcast is the second of two webcasts on earthwork in the field.
This webcast will discuss compaction equipment and the variables which control the compaction
processes.
After viewing this webcast, you should be able to: describe different types of compaction
equipment and their pros and cons, select an appropriate piece of compaction
equipment for a given soil, and describe how the key variables of weight, number of passes,
and lift thickness affect compaction.
We must start this webcast with a short description of the processes active in compaction.
The compactability of soils is highly dependent upon the soil type.
The processes which are effective in compacting clay soils are not the same as those in gravel
soils.
Broadly speaking, there are four different processes active in soil compaction.
The first is pressure.
The weight of the compaction equipment divided by its contact area with the soil generates
high pressures which will compress the soil The second process is impact.
Some compaction techniques, like the one we will use to compact soils in the lab, impart
a series of blows to the soils which compact the soil
Vibration is another process which aids in compaction of soils.
Vibration can help to rearrange soil particles into a denser configuration.
Finally, manipulation, which consists of shearing and kneading the soil, is beneficial in the
compaction of cohesive soils.
For effective and efficient compaction, these processes must be matched to the specific
characteristics of the soil being compacted.
There are many different types of compaction equipment—to many to really be counted.
However, they all can be described by certain characteristics.
First is how the compactors is moved.
Some or are towed behind separate tractors and some are self propelled.
The weight is clearly and important characteristic of soil compactors.
Whether the equipment uses only static loads or includes vibrating or impacting loads is
an important characteristic.
And finally the shape of the compacting surface is important.
This surface can either be smooth or footed.
The best way to describe compaction equipment is with pictures, so that’s what we’ll
do.
This photo shows a self-propelled footed, vibratory compactor.
You can see the feet sticking out of the rollers of the compactor.
When this compactor is driven over a loose lift of fill, the high pressure on the feet
of the rollers pushed the feet into the fill material.
This uses both pressure and manipulation to compact the soils.
When the soil is loose, the feet penetrate fully into the fill.
As the fill gets compacted and stronger, the feel will ride on top of the compacted fill.
This is called walking out.
The rollers are equipped with large vibrators which can be turned on or off to aid in compacting
the fill.
Footed rollers such as this are especially good with cohesive fills since manipulation
is an effective process in compacting clayey soils.
This is a photo of a self-propelled, pneumatic tired roller.
You can see a row of five smooth tires on the front of the machine.
There are four similar tires on the back of the machine, placed in the spaces between
the front tires.
This compactor uses primarily pressure to compact the soil.
The offset tires provide some manipulation, though must less than that produced by a footed
roller.
This type of compactor can be used for both cohesive soils and granular soils, though
it’s not very effective for highly plastic clay soils.
This is also a type of compactor use for compacting hot mix asphalt.
This is an example of a self-propelled, vibratory, smooth drum roller.
It’s very good in compacting granular soils, sands and gravels.
Vibration is very effective in compacting these types of soils.
This type of equipment is often use for the final finishing passes on fill material because
it leaves a smooth surface.
This our first example of a towed compactor.
They yellow piece of equipment is the actual compactor and it’s being towed by a rubber
belted tractor.
This is called a high energy impact roller.
Because the rollers are not round, they rise and fall on the fill material.
In this way, they use pressure, vibration, and impact to compact the soil.
This equipment is very good at compacting coarse gravel material.
Because of its very high energy levels, it can compact thicker lift layers than other
equipment.
The high energy impact roller is not used much in North America, though it is common
in both Europe and Asia.
Compactors come in different sizes for different purposes.
This is a small version of the footed roller we saw earlier.
This one is remotely controlled by the operator and can access small areas.
Like its larger cousin, it’s good for compacting cohesive soils.
When compacting sand in small and tight quarter, a hand controlled vibratory pan compactor,
such as the one shown here, can be very effective.
Again, vibration is good for compacting cohesionless soils.
I’d like to close this webcast by overviewing the field variables that affect the compaction
process.
Before I start discussing the variables, let’s review the process of placing and compacting
fill.
First we dump our fill material at the desired location.
Next we smooth the material out with a dozer to a uniform lift thickness.
If needed, we then moisture condition the soil.
Finally we used a piece of compaction equipment to compress and compact the soil.
There are number of variables in this process.
Let’s discuss them.
The moisture content of a soil is critically important in compaction of clayey or silty
soils.
It’s less critical in compacting clean sands and gravels.
It’s so important to the process that we will have an entire lab class devoted to measuring
the effect of moisture on compaction.
As we just got through discussing, the type of equipment is important in soils compaction,
and the equipment must be properly matched to the type of soil.
Finally, I want to discuss the concept of compaction energy per unit volume.
This will help explain some of the key variables affect compaction in the field.
The first variable is equipment weight.
I can choose a small compactor for my earthwork project, or with a larger and heavier compactor.
It should be obvious that increasing the weight of the compactor will increase the compaction
energy imparted to the soil and will increase compaction.
A second variable is the number of passes used when compacting a given lift.
I don’t have to drive the compactor over the lift just once.
I can drive it over the soil many times.
For a given piece of equipment, if I increase the number of passes, I will increase the
compaction energy imparted to the soil.
The final field variable I can control is the lift thickness, or the thickness of the
soil layer I am compacting.
I can place the soil in a relatively thin lift before compacting, or I can place the
soil in a much thicker lift.
If I keep the compaction equipment and number of passes the same, increasing the lift thickness
will decrease the compaction energy per unit volume, because I’m putting the same amount
of energy in a larger volume of soil.
So the key variable we have in the field to control the compaction process are: compaction
moisture content, type of equipment, weight of equipment, number of passes of equipment,
and the lift thickness.
I hope you’ve enjoyed this webcast on compaction equipment and field variables.
I’ll leave you now with time lapse video of the earthwork done to prepare the site
for the construction of the Hunter Strain Engineering lab at Angelo State University.
Have a great day.