Drilled shafts are reinforced
concrete columns, which, for the most part, are built below the surface of the
ground. They are designed to provide a
foundation for structures and carry the entire load of the structure. They are sometimes referred to in the field
as caissons.
Prior to installing drilled
shafts, the Contractor is required to submit a written installation plan to the
Engineer. This plan should be closely
reviewed for conformance with the specifications. Among other things, the plan
should describe how the Contractor proposes to excavate the hole, clean out the
hole, and place the concrete. The plan needs to include procedures for
maintaining correct horizontal and vertical alignment of the excavation. The
plan should also include procedures and the proposed equipment required to deal
with the possible presence of and subsequent removal of underground obstructions
within the hole excavations.
If a permanent casing is
specified, the casing should be installed to the prescribed depth before
excavation begins. In some cases, the
Contractor may not have the required equipment to completely install the casing
prior to excavation. If the Contractor
is not able to completely install the casing prior to excavation, he is allowed
to either excavate the material within the casing, or excavate a pilot hole
ahead of the casing. If the Contractor
proposes to excavate the material within the casing to aid in the installation,
it is important that the excavation does not proceed beyond the casing.
If the Contractor proposes to
either pump or tremie the concrete under water while
utilizing a temporary casing, his plan should describe how he proposes to
remove the casing while not disconnecting or breaking apart the tremie or pump hose.
In order to ensure that the end of the pump or tremie
hose is always embedded into the concrete, his plan should detail how he
proposes to monitor the level of the top of the concrete and the bottom of the
pump or tremie hose.
If the Contractor does not include these provisions in his plan, and
encounters water in the field, he should be required to stop and resubmit a
plan containing the necessary information.
He should not proceed with verbal approval as it is too difficult to
document what was said versus what may have been intended.
Drilled shaft installation
can be very complicated since a large amount of work is performed in an area with
very little access. The plan should be
very detailed and site specific. A
generic or “canned” plan should not be accepted.
There are two types of
drilled shafts:
1. End bearing.
2. Friction.
End bearing drilled shafts
derive most of their capacity through end bearing on a hard substrate, such as
bedrock.
Friction type drilled shafts
derive most of their capacity through a combination skin friction with the soil
along the perimeter of the drilled shaft and end bearing on the substrate
immediately below the drilled shaft. To obtain the required skin friction, it
is important that the integrity of the soil be maintained during the drilling
operation and prior to placing the concrete.
There are several different
methods used to stabilize the sides of the excavation during the construction
of the drilled shaft. Factors that
impact the method chosen are types of soil, the elevation of the ground water,
types of drilled shafts, plan requirements, and equipment utilized by the
Contractor.
The dry construction method
is accomplished by excavating the hole without the use of steel casing. The sides and bottom of the excavation should
remain stable and should not experience any caving, sloughing, or
swelling. It should be possible to
visually inspect the excavation prior to the placement of concrete.
The excavation should be done
in a relatively dry condition with very little ground water present. The flow rate of any water that might enter
the excavation should be such that the elevation does not change by more than
12 inches (300 mm) per hour. At the time
of concrete placement, there should be no more than 3 inches (75 mm) of water
in the bottom of the excavation. Both
the flow rate test and the amount water in the bottom of the hole should be
documented.
The wet construction method
should be used at sites with or without casing and where a dry excavation
cannot be maintained. This method
consists of using either water or slurry to contain or prevent the seepage of
ground water into the drilled shaft.
With the use of slurry, this method may be used in place of a temporary
casing to maintain the stability of the perimeter of the hole while advancing
the hole to its final elevation.
If this method is used to
excavate a hole for a friction-type drilled shaft, it is important to not
compromise the integrity of the soil along the perimeter of the drilled shaft
through the seepage of ground water. It
is not only important to prevent the seepage of ground water into the excavation
after it is complete, but it is important to prevent ground water from seeping
into the excavation during the drilling process. To prevent this, it will be necessary to
continually pump either water or slurry into the hole during the drilling
operation to maintain an elevation slightly higher than the elevation of the
static water table.
Either a tremie
or a concrete pump will be used to place the concrete when the wet construction
method is used.
Unless waived by the
Engineer, it is required for the Contractor to use a temporary surface casing
to prevent soil at the top of the casing from sloughing and falling into the
excavation. This casing should never be shorter than 10 feet (3.0 m) long. The temporary casing aids in the proper
alignment and positioning of the drilled shaft.
Temporary casing may be used
at sites where dry excavation cannot be maintained and the Contractor elects
not to use slurry.
It is important that the
Contractor begins removal of the temporary casing while the concrete remains
workable. Failure to remove the casing
could result in a drilled shaft that is not capable of supporting the design
load.
When the casing is withdrawn,
there is a possibility that fluid trapped behind the casing will contaminate
the concrete. To prevent this, it is
important to maintain a head of concrete at least 5 feet (1.5 meter) in the
casing. This minimum head may need to be
increased to counteract any ground head that might be in the casing at the time
it is withdrawn. Casing should be
removed by pulling at a slow uniform rate.
However, if the casing gets stuck, the Contractor may rotate, vibrate,
or tap the casing to facilitate extraction.
Rotating the casing may twist the reinforcing cage, so only rotate the
casing enough to get it unstuck.
Friction-type drilled shafts
derive much of their capacity through the adhesion of concrete with the
surrounding soil. If the Contractor
elects to use a temporary steel casing and fails to remove it, or he fails to
protect the integrity of the soil adjacent to the drilled shaft, much of the
capacity of the drilled shaft could be lost.
When drilled shafts extend
below the top of the water table, it is important that the water or slurry fluid
inside the shaft excavation be maintained higher than the top elevation of the
water table at all times. To accomplish this, it is not only important for the
Contractor to add water or slurry fluid after the excavation is complete, but
it is also important for him to add water or slurry fluid during the drilling
operation. If this is not done, the
surrounding ground water will begin to enter the excavation and erode the
soil. This will result in the reduced
capacity of the drilled shaft.
The dry construction method
can be used in construction of friction-type drilled shafts. It should be used when the bottom of the
drilled shaft is above the water table and the excavation can be made without
the sides or bottom of the excavation experiencing any caving, sloughing, or
swelling. If the dry construction method
results in the sidewall becoming softened or swelling, the Contractor shall
over ream the sidewall to sound material.
If the Contractor elects to
use slurry, a delay in placing the concrete could result in the sidewalls
degrading due to slurry cake buildup.
Any slurry cake buildup shall be corrected by reaming the sidewalls to
sound material.
If a temporary casing is not
used, and concrete is not placed the same day that the excavation is complete, the
excavation shall be re-drilled 6 inches (150 mm) larger in diameter immediately
prior to the placement of the concrete.
If a temporary casing is
used, it should be smooth and free of dried concrete and other foreign
materials that might contaminate the fresh concrete. While the strength and thickness of the steel
casing is not specified, it should be strong enough to withstand handling,
installation, and extraction stresses as well as the pressures exerted on it by
the fresh concrete and surrounding earth.
The outside diameter of the
casing should be at least equal to the plan diameter of the drilled shaft. Many times the Contractor will elect to use a
casing larger than the specified casing.
Oversized casings are acceptable; however, all additional costs
associated with the oversized casings should be borne by the Contractor.
Typically, the diameter of
the bedrock socket will be less than the diameter of the remainder of the
drilled shaft. When the diameter of the
bedrock socket is the same as the remainder of the drilled shaft, the diameter
of the drilled shaft may need to be increased to permit the excavation of the
bedrock socket. Again, increasing the
diameter of the drilled shaft should be done at no additional cost to the
state.
One potential method of
excavating a hole through unstable or caving soils is through the use of
slurry. The slurry should be added to
the excavation during the drilling process and replace the material that is
being removed. This is accomplished by
mixing the slurry with the material to be removed. The combination of slurry and soil is then
pumped from the hole while clean slurry is added. The slurry that was pumped from the hole is
then cleaned of foreign material and placed back into the hole. This process is continued until the original
soil has been removed.
There are two different types
of materials used to produce slurries.
One type of material produces mineral slurry and the other type of
material produces polymer slurry.
If the Contractor elects to
use polymer slurry, he must first demonstrate the slurry's ability to prevent
caving of the hole. If the slurry is not
capable of stabilizing the perimeter of the hole while the hole is being
excavated, it should not be allowed. This should be accomplished by the use of
a separate trial hole. This trial hole
should not be one of the production shafts and no separate payment should be
made for the trial hole. The trial hole should be the same size and diameter as
the largest production drilled shaft except the depth of the hole need not be
more than 40 feet (12 meters). The
slurry used in the trial hole should be the same as that used in the production
shafts.
An important factor in the
performance of the drilled shaft is the cleanliness of the hole
excavation prior to the placement of the reinforcing steel and the concrete.
The Contractor must provide equipment to check the dimensions, alignment and
cleanliness of the hole excavation.
Reinforcing should be placed
just prior to concrete placement. It
should be placed as one continuous cage.
If a casing is not used, care should be taken when lowering the reinforcing
steel cage into the shaft so it does not drag down the face of the shaft and
compromise the integrity of the exposed soil surface.
Spacing devices,
commonly referred to as “donuts,” need to be installed at quarter points around
the shaft to ensure that the required concrete cover is obtained. On the bottom of the shaft, the Contractor
can use plastic “shoes” to keep the reinforcing cage at the proper
elevation. These shoes are normally 6
inches (152 mm) to 8 inches (203 mm) tall and about as big around as a soda
can. In the past, mortar blocks were wired to the end of longitudinal steel to
accomplish this task, but were unstable and the cage often fell off the blocks.
The concrete used in the
drilled shaft is Class QC 2. In order to
aid the consolidation of the concrete without vibration, it is necessary to
increase the slump to 6 inches (150 mm) ± 1 inch (25 mm). If the concrete is placed using a tremie, the slump should be increased to 8 inches (200 mm)
± 1 inch (25 mm). The accepted JMF’s maximum water-cementitious ratio shall not be
exceeded. It may be necessary to achieve the additional slump through the use
of a super-plasticizer.
If the Contractor uses the
wet method or places concrete under water or slurry, increase the cement
content by 10 percent and place the concrete by either tremie
or concrete pump.
If a temporary casing is
used, it should be removed slowly and carefully. As the casing is removed, concrete that has
been previously placed will fill the void left by the casing thus causing the
top level of the concrete in the excavation to lower. As the level of the concrete drops, the
concrete will tend to pull down on the reinforcing steel. If the casing is removed too quickly, the
downward force of the concrete on the reinforcing steel will cause the reinforcing
steel to be displaced.
A tremie
may be used to place concrete in a wet hole.
If concrete is placed in a wet hole, it is important that the concrete
not be placed into moving water. If
concrete is placed into moving water, the water will have a tendency to wash
the cement off of the sand and aggregate.
To prevent moving water in the excavation, the level of water or slurry
in the excavation must be equal to or higher than the level of the ground
water.
The tremie
must not contain aluminum parts that will come into contact with the
concrete. In order for the concrete to
pass freely through the tremie, the minimum diameter
of the tremie shall be at least 10 inches (250
mm). It is important that the tremie be clean, smooth, and free of built-up concrete and
other foreign material.
Prior to placing the tremie tube into the water, it is important to plug the end
of the tremie to prevent the intrusion of water into
the tremie.
The tremie can be placed into the excavation
after the plug is in place. After the tremie is filled with concrete, it should be raised up no
more than one diameter of the tube. This
allows the plug to be displaced and the concrete to begin flowing into the
excavation. If the tremie
is not plugged, the tube will fill with water.
When the concrete is dropped through the tube, it would drop through the
water which separates the cement from the sand and gravel.
During the placement of the
concrete, the end of the tremie should always be at
least 10 feet (3 meters) below the surface of the concrete to prevent the water
from contaminating the fresh concrete.
It is important to devise a method to determine elevation of the top of
the concrete and the bottom of the tremie since the
concrete will be under water and not visible.
This method should be determined and agreed upon with the Contractor
prior to the concrete’s delivery.
In order to prevent air voids
in the concrete when a tremie or pump is used, place
the concrete in one continuous operation.
If the Contractor is allowed to break apart the tremie
tube or pump hose to facilitate the removal of temporary casing, the tremie tube or pump hose could get air voids in them that
will be forced down into the drilled shaft concrete. If the end of the tremie is pulled out of the concrete prior to completely
placing all the concrete, the drilled shaft will contain concrete that will be
contaminated by water. As a result, the
drilled shaft may not have the required strength and should be considered
defective.
After the concrete placement
has been completed, there will be a layer of concrete at the top of the drilled
shaft called, “laitance,” that has been contaminated with water. This concrete should be removed either by
overfilling the drilled shaft and causing the contaminated concrete to flow out
of the drilled shaft or by shoveling off the concrete. If the contaminated concrete is shoveled off,
the Contractor must place additional concrete to replace the concrete that was
shoveled off.
A pump may be used to place
concrete in a wet hole. If concrete is
placed in a wet hole, it is important that the concrete not be placed into
moving water. If concrete is placed into
moving water, the water will have a tendency to wash the cement off of the sand
and aggregate. To prevent moving water
in the excavation, the level of water or slurry in the excavation shall be
equal to, or higher than, the level of the ground water.
Due to the adverse reaction
of concrete with aluminum, the pump must not contain aluminum parts that will
come into contact with the concrete.
In order to allow the
concrete to pass freely through the pump, the minimum diameter of the pump pipe
must be at least 4 inches (100 mm).
During the pumping operation,
the pipe used to convey the concrete to the bottom of the drilled shaft must be
anchored to the steel casing or other suitable stationary object to prevent the
pipe from undulating. Otherwise, the
tendency of the pipe to undulate could cause it to pull out of the concrete that
was previously placed.
In order to lubricate the
pump equipment, grout should be first pumped through the hose prior to pumping
the concrete. The grout should not be
placed in the drilled shaft. This
process does not need to be repeated as long as the process is continuous.
Prior to placing the pump
pipe into the water, it is important to plug the end of the pipe to prevent the
intrusion of water into the pipe. After
the plug is in place, the pipe can be placed into the excavation. When the pipe is filled with concrete, the
pressure of the concrete will dislodge the plug. If the pipe is not plugged, and the concrete
drops through the water, the water would separate the cement from the sand and
aggregate.
During the placement of the
concrete, the end of the pump pipe should always be at least 10 feet (3 meters)
below the surface of the concrete to prevent the water from contaminating the
fresh concrete. It is important to
devise a method to determine elevation of the top of the concrete and the
bottom of the pipe since the concrete will be under water and not visible. This method should be determined and agreed
upon with the Contractor prior to the concrete’s delivery to the project.
In order to prevent air voids
in the concrete when a tremie or pump is used, place
the concrete in one continuous operation.
If the Contractor is allowed to break apart the tremie
tube or pump hose to facilitate the removal of temporary casing, the tremie tube or pump hose could get air voids in them that
will be forced down into the drilled shaft concrete.
If the end of the pipe is
pulled out of the concrete prior to completely placing all the concrete, the
drilled shaft will contain concrete that will be contaminated by water. As a result, the drilled shaft may not have
the required strength, and should be considered defective.
After the concrete placement
has been completed, there will be a layer of concrete at the top of the drilled
shaft called, “laitance,” that has been contaminated with water. This concrete should be removed either by
overfilling the drilled shaft and causing the contaminated concrete to flow out
of the drilled shaft or by shoveling off the concrete. If the contaminated concrete is shoveled off,
the Contractor must place additional concrete to replace the concrete that was
removed.
It is the Contractor's
responsibility to provide the Engineer with all the necessary labor and
equipment to obtain measurements of the drilled shaft. Since it is not possible to obtain these
measurements after the concrete is placed, it is necessary to obtain these
measurements prior to placing concrete.
Due to the risks involved, at
no time should the Engineer ever go down into a drilled shaft for inspection or
any other purpose.
A copy of form CA-S-1
should be filled out and submitted to the Office
of Geotechnical Engineering.
The pay length of the drilled
shaft is the required, accepted length measured along the axis of the
shaft. It should be measured from the
required bottom of the shaft to the proposed top plan elevation. Any over excavation below the required bottom
of the shaft should not be measured for payment.
Drilled shafts that extend
into bedrock should be divided into two sections. The lower section is the length of the
drilled shaft that extends into the bedrock or the bedrock socket. The upper section is the length of drilled
shaft above the bedrock. If the top
elevation of the bedrock is lower than indicated on the plans, the additional
upper section or length of drilled shaft above bedrock should be measurement
for payment. The Contractor should not
be paid for any over excavation of the bedrock unless he is ordered to do so by
the Engineer.
1. Review Contractor installation plan.
2. Holes accurately located to line and spacing.
3. Documentation of flow rate of ground water into shaft
to validate Dry Construction Method.
4. Fill out form CA-S-1.
5. Document drilling method: dry, wet, temporary casing,
or permanent casing.
6. Slurry use per 524.07.
7. Shaft excavation clean on bottom.
8. Reinforcing steel cleaned.
9. Placement of reinforcing steel, center alignment with
spacers, clearances, plumbness, etc.
10. Note concrete placement method: pump, tremie, or free fall. (524.10
- 524.13).
11. Notify Engineer when unexpected obstructions are encountered.
12. Measure and pay per 524.16
and 524.17.