Hello, this is Dr. Kitch.
Welcome to this webcast in my Introduction to Geotechnical Engineering series.
This webcast is the first of two webcasts on earthwork in the field.
This webcasts will introduce the earthwork process.
As an introductory webcast, most of the learning objectives are simple and knowledge based.
When you’ve completed this webcast, you should be able to: define cut and fill, list
and described the steps in earthwork, use 3-phase diagrams so show changes in the soil
during earthwork, and list equipment used in earthwork.
Many, many civil engineering project include earthwork in them.
In fact, almost every one does.
This particular project shows a roadway project.
As you can see in the background, this area used to be a hill.
The original ground surface was somewhere near the red line shown.
The soil beneath that has all been removed, or cut from this area.
Therefore, we call this a cut area of the roadway.
In foreground the original ground surface was much lower than the current roadway, as
shown by the yellow lines.
Soil has been placed in this area to raise the ground surface.
We call this a fill area of the roadway.
This photo illustrates the construction of an earth dam.
You can see that the entire area where the dam is to be placed has been cut.
In the center of cut zone you can see fill operations in progress to build the dam.
Here is an example of the construction of an earth pad to support a new home.
This is entirely fill material which has been brought in from another location.
The location original source of the original source material is call the borrow area or
borrow pit.
I’m not sure why we call it a borrow pit as we never take soil back it—it’s not
borrowed at all.
But that is a term that we use.
The earthwork process generally follows a common set of steps.
First we cut or excavate the source material from a borrow or borrow pit.
The borrow, or cuttings are then transported or hauled from the borrow pit.
They are often stockpiled at a location someplace between the borrow pit and the eventual fill
area.
When the material is needed it is then transported again to the fill area.
In some cases a stockpile isn’t needed.
If there’s not stockpile the material is transported directly from the borrow pit to
the fill area.
The fill process itself has three steps.
First, the soil must be properly placed at the fill site.
It isn’t just dumped there.
Each placement is called a lift and the material is no longer borrow but fill.
After the soil is place, it is usually moisture conditioned by either adding or removing water.
By now you should have experience how soil behavior can be dramatically affected by moisture
content.
We will learn more about how moisture content affects compaction in a later lesson.
For now you only need to know that it’s important to compaction.
After the fill material is placed and moisture conditioned, it is compacted to increase its
strength and reduces its compressibility.
The next series of photos will illustrate the earthwork process.
Here you see the borrow area for an earth dam construction site.
The excavator is digging up the borrow material and placing it in a large dump truck for hauling.
In the background you can see a motor grader (or road grader).
Graders are generally used in the placement process.
In this case, the grader is being used to maintain the unpaved haul road.
This photo shows the stockpile area of the project.
The dump truck from the borrow area deposits the material in the rock crusher just to left
of this photo.
This piece of machinery both crushes the large cobbles in the borrow and sorts the material
into different sizes.
The conveyors transport the material around the site into various stockpiles of different
sized material from coarse gravels to sands.
In the background, you can see a loader filling up a dump truck from a stockpile of sand.
This dump truck will transport the sand to the fill area.
Here you see the dump truck from the previous photo dumping the fill material at its intended
location.
You’ll note that that at this point the fill isn’t placed very smoothly.
Before the fill can be compacted, it must be smoothed out to a uniform thickness.
Here you see dozer smoothing out the soil dumped by the dump truck.
In this image the dozer is actually moving from right to left, dragging the soil on the
back side of its blade.
The dozer leaves behind a relatively smooth layer of soil called a lift.
I supposed it’s called a lift because it lifts or raised the surface of the fill, though
that doesn’t really make sense to me.
The thickness a lift is typically 8 to 12 inches or 20 to 30 centimeters.
If the fill material is not at the optimal moisture content for compaction, then it is
moisture conditioned.
If the soil is too dry, water must be added.
Here you see a water truck spraying water across the a fill area to increase the moisture
content of the soil.
If the soil is too moist, it is disked to expose it to the wind and sun so it can dry
out.
Here you see an agricultural type disk set, but the ones used for construction are essentially
the same.
They are pulled by a separate tractor.
The moisture conditioning can occur before or after the fill material is smoothed out
by the dozer.
It depends upon the details of the particular construction operation.
After the moisture conditioning, the fill can be compacted.
Here you see two smooth drum vibratory compactors working in tandem to complete a lift.
The soil undergoes a number of changes during the earthwork process.
I hope by now you’ve learned how important three-phase diagrams are for understanding
soils.
We will use three phase diagrams to illustrate how a soil changes during the earthwork process.
In its natural state at the borrow pit, a given amount of soil will contain certain
amounts of mineral solids, water, and air, as you have already learned.
When we excavate this soil in preparation for hauling, it will undergo some changes.
The amount of solids and water won’t change as we excavate the soil and place it in the
dump truck.
But its total volume will increase because the excavation process will loosen the soil.
If the solids and water don’t change, the only way the total volume can increase is
if the volume of the air increases.
We call this phenomenon bulking.
After dump truck in the borrow area is full, it transports its load to the stockpile area
where it is dumped or sorted into one or more stockpiles.
The total volume of solid doesn’t change significantly from transport to being stockpiled.
And certainly the volume of solid material doesn’t change in the stockpile.
The water, however, is a different matter.
The water content may or may not change while the soil is stockpiled.
If the weather is hot and dry, water may evaporate from the soil, reducing the water content.
If there is precipitation, the amount of water may increase.
We don’t really know for sure how the water content will change in the stockpile.
It depends upon the weather and how long the soil is stockpiled.
When the material is needed for fill, it is transported from the stockpile to the fill
area for placement.
There is no significant change in total volume or amount of solids in the process.
Recall, we want the soil to be at an optimal water content for compaction before starting
the compaction process, this will often require water conditioning.
If the stockpile material is drier that the optimal compaction water content, the water
must be added by spraying the fill with a water truck.
If the stockpile material is moister than the optimal compaction water content, then
it must be disked and allowed to dry out.
After moisture conditioning, the soil is compacted.
There will be no change in the volume of the solids or the volume of water during compaction.
However, the total volume of the soil will be significantly decreased by compaction.
Since the water and solids aren’t changing, the only way there can be volume change is
if the volume of the air is decreased.
This volume decrease from stockpile to compacted soil is called shrinkage, which, isn’t a
really good term.
The soil isn’t shrinking, we’re pounding the heck out of with some really heavy equipment.
Still that the term that’s used to describe this volume change.
Notice that only thing that doesn’t change during the earthwork process is the volume
of the solids.
The water content will increase and decrease during the process
The total volume will definitely increase from its natural state during excavation.
And, it will definitely decrease from its state in the stockpile to its final compacted
volume.
Generally, the total volume of the compacted fill will be less than its total volume in
its natural state as shown here.
However, if the natural soil is very dense, the compacted fill might have about the same
volume or even more than the original volume.
However, that’s not often the case.
You should study these three-phase diagrams to be sure you understand how the soil changes
during earthwork.
Before we finish this webcast let’s quickly review the learning objectives.
You should now be able to define cut and fill and explain how they are used in civil engineering
projects.
You should be able to describe the earthwork process and all its steps.
You should be able to use three-phase diagrams to illustrate the changes that occur in soil
during earthwork.
And, finally, you should be able to list they kinds of equipment used in earthwork.
I hope this webcast has been helpful in learning the basics of earthwork.
In part 2 of this earthwork series, we will discuss compaction equipment in more detail
and we’ll identify the key variables which control the compaction process in the field.
See you in earthwork, part 2