ITEM 524 DRILLED SHAFTS
524.01
Description
524.02
Materials
524.03
Contractor’s Installation Plan
524.04 Hole Excavation
524.05 Friction
Type Drilled Shafts
524.06 Casings
524.07 Slurry
524.08 Excavation
Inspection
524.09
Reinforcing Steel for Drilled Shafts
524.10 Concrete
for Drilled Shafts
524.11 Free
Fall Concrete Placement
524.12 Tremie
524.13 Pumped
Concrete
524.14
Construction Tolerances
524.15
Inspection Records
524.16 Method
of Measurement
524.17 Basis of
Payment
524.01 Description. This work consists of
furnishing and installing drilled shafts. The lengths of the drilled shafts
shown on the plans are estimated from available subsurface information.
Furnish the proposed drilled shafts according to plan requirements, with the
understanding that the actual length required is based on conditions
encountered during construction and may differ from the estimated length shown
on the plans.
524.02 Materials. Furnish material conforming to:
Concrete, Class QC
2................................................... 511
Epoxy coated reinforcing
steel ..................................... 509
524.03 Contractor’s Installation Plan. Submit,
for the Engineer’s acceptance, a written installation plan of procedures to
follow when excavating the hole, placing the concrete, and monitoring the
concrete placement. Submit the installation plan at least 14 Calendar
Days before constructing the drilled shafts. Include the following
information:
A. Details of the sequence proposed for
the overall drilled shaft construction operation.
B. Procedures for maintaining correct
horizontal and vertical alignment of the excavation.
C. Procedure and the proposed equipment
required to deal with the possible presence of and subsequent removal of
underground obstructions within the hole excavations..
D. If using a casing, method to advance
the casing.
E. If using a temporary casing, details
of the methods to extract the temporary casing and to maintain the concrete
slump to keep concrete workable by adding admixtures such as retarders or superplasticizers.
F. If using slurry, details of the
methods to mix, circulate, and de-sand the slurry. For polymer slurry,
submit the manufacturer’s recommendations for use of the slurry.
G. Details of methods to clean the shaft
excavation.
H. Details of reinforcement placement
including support and centralization methods.
I. Details of concrete placement including
proposed operational procedures for free fall, tremie,
or pumping methods.
J. A list of proposed equipment to
be used such as cranes, drills, augers, bailing buckets, final cleaning
equipment, de-sanding equipment, slurry pumps, tremies,
concrete pumps, casings, etc.
Acceptance
of the installation plan will not relieve the Contractor of the responsibility
for obtaining the required results.
524.04
Hole
Excavation.
Take precautions to prevent damaging existing structures and utilities.
Precautions include, but are not limited to, selecting construction methods and
procedures that will prevent excessive caving of the shaft excavation,
and monitoring and controlling the vibrations from the driving of casing or
sheeting or drilling of the shaft.
When
encountered, remove objects such as large boulders. Do not blast unless
shown on the plans or authorized in writing by the Engineer.
Unless otherwise shown on the plans,
where drilled shafts are to be installed in conjunction with embankment
placement, construct shafts after the placement of the fill and completion of
any specified settlement periods.
Excavate
for the shafts to the dimensions and elevations shown on the plans. Use
methods and equipment suitable for the intended purpose and materials
encountered. Use either the dry method, wet
method, temporary casing method, or permanent casing method as necessary to
produce sound, durable concrete foundation shafts free of defects. When a
particular method of construction is required on the plans, that method shall
be used. If no particular method is specified for use, select and use a
method based on site conditions.
If
the excavation operation is stopped, protect the shaft cavity by installing a
safety cover. The Contractor is responsible for the safety of the shaft
excavation, surrounding soil, and the stability of the sidewalls. If
necessary to ensure such safety and stability, use a temporary casing, slurry,
or other methods accepted by the Engineer. Unless cased to the full
depth, do not leave excavations unfilled overnight.
Use
appropriate means, such as a cleanout bucket or air lift, to clean the bottom
of the excavation of all shafts. Promptly notify the Engineer when
unexpected obstructions are encountered.
A. Dry Construction Method. Use the dry construction method only at sites
where the groundwater table and site conditions are suitable to allow construction
of the shaft in a relatively dry excavation, and where the sides and bottom of
the shaft remain stable without any caving, sloughing, or swelling and may be
visually inspected before placing the concrete. The dry method consists
of excavating the drilled shaft hole, removing accumulated water, and loose
material from the excavation, and placing the shaft concrete in a relatively
dry excavation. The rate of flow of water into the hole should not be
more than 12 inches (300 mm) within a 1-hour period. Do not place the
initial concrete if there is more than 3 inches (75 mm) of water in the bottom
of the hole.
B. Wet Construction Method. Use the wet construction method at sites where
a dry excavation cannot be maintained for placement of the shaft
concrete. The wet method consists of using water or slurry to contain
seepage and groundwater movement and placing concrete using a tremie or concrete pump. Additionally, use this
method to maintain stability of the hole perimeter
while advancing the excavation to its final depth, placing the reinforcing
cage, and placing the shaft concrete. This method also consists of
de-sanding and cleaning the slurry. For drilled shafts that are not
socketed into the bedrock and during drilling operations, maintain a water or
slurry fluid elevation inside the shaft excavation higher than the static water
table. Unless demonstrated to the Engineer’s satisfaction that the
surface casing is not required, provide temporary surface casings to aid shaft
alignment and position and to prevent sloughing of the top of the shaft
excavation. Extend surface casings from the ground surface to a point in
the shaft excavation where sloughing of the surrounding soil does not occur.
C. Temporary Casing Construction Method. Use the temporary casing construction method
when the stability of the excavated hole and/or the effects of groundwater must
be controlled. Remove temporary casings while the concrete remains
workable. As the casing is being withdrawn, maintain a 5-foot (1.5 m)
minimum head of fresh concrete in the casing so that all the fluid trapped
behind the casing is displaced upward without contaminating the shaft
concrete. As necessary, increase the required minimum concrete head to
counteract groundwater head inside the casing. Extract casing at a slow,
uniform rate with the pull in line with the shaft axis. Rotate, tap, push down,
or vibrate the casing when necessary to extract it. Rotate the casing as little
as possible to avoid deforming the reinforcing steel cage.
D. Permanent Casing Construction Method. The permanent casing construction method
generally consists of driving or drilling a casing to a specified depth before
excavation begins. If full penetration of the casing to the specified
depth cannot be attained, the Contractor may either excavate material within
the embedded portion of the casing or excavate a pilot hole ahead of the casing
until the casing reaches the desired penetration. Make the pilot hole no
larger than one-half the diameter of the shaft and center the hole in the
shaft. Do not over ream to the outside diameter of the casing unless
specifically shown on the plans.
Ensure that the
casing is continuous between the elevations shown on the plans. Unless
otherwise shown on the plans, do not use temporary casing instead of or in
addition to the permanent casing.
After installing the
casing and excavating the shaft, place the reinforcing steel, then place the shaft concrete. After filling the
permanent casing with concrete, pressure grout voids between the shaft
excavation and the casing with cement grout. Submit the method of
pressure grouting the voids to the Engineer for approval. Pressure
grouting is required to ensure contact (bearing) between the casing and any
surrounding soil layer that is used for lateral support.
524.05 Friction Type Drilled Shafts. Friction
type drilled shafts are defined as drilled shafts that do not bear on bedrock
and obtain their ability to support load from a combination of end bearing on
the soil and adhesion between soil and concrete along the length of the shaft.
For
friction type drilled shafts, dry construction method may be used for cohesive
soils only. If using a casing for the construction of a
friction type drilled shaft, remove the casing completely or partially as shown
on the plans. If enough water is entering the hole through the
sides and bottom of the hole such that the supporting soils are being eroded,
maintain a positive head of fluid in the excavation hole to ensure that water
is not continuously flowing into the hole.
If
the Engineer determines that the hole sidewall has
softened due to excavation methods, swelled due to delays in concreting, or
degraded because of slurry cake buildup, over ream the sidewall to sound
material. If the concrete is not placed the same day that the excavation
is completed, protect the excavation with a temporary casing, and redrill the hole at least 6 inches (150 mm) larger in
diameter, clean the excavation, and perform slurry test before concreting.
524.06 Casings. Use smooth, watertight, steel
casings of sufficient strength to withstand handling and driving stresses and
the concrete and surrounding earth pressures. Provide an outside diameter
of the steel casing equal to or greater than the plan diameter of the
shaft. If the plan diameter of the bedrock socket is same as the drilled
shaft above the bedrock and a steel casing is used, provide a diameter of the
bedrock socket as shown on the plans. Ensure that the diameter of the
casing is large enough to allow the excavation of the bedrock socket.
Where
drilled shafts are located in open water areas, extend the casing a minimum of
12 inches (300 mm) above the water to protect the shaft concrete from water
action during placement and curing of the concrete. Cut off the casing at
the plan top of drilled shaft elevation after the concrete has cured. If
practical, install the casing in a manner that produces a positive seal at the
bottom of the casing to prevent piping of water or entry of other material into
the shaft excavation.
If
it becomes necessary to remove a casing and substitute a longer or larger
diameter casing through caving soils, stabilize the excavation with slurry or
backfill before installing the new casing. The Contractor may use other methods
accepted by the Engineer to control the stability of the excavation and to
protect the integrity of the foundation soils.
524.07 Slurry. Slurry used in the drilling
process shall be a mineral or polymer slurry. The mineral slurry shall
have both a mineral grain size that remains in suspension and sufficient
viscosity and gel characteristics to transport excavated material to a suitable
screening system. Ensure that the percentage and specific gravity of the
material used to make the suspension is sufficient to maintain the stability of
the excavation and to allow proper concrete placement. Maintain the level
of the slurry at a height sufficient to prevent caving of the hole.
Thoroughly
premix the mineral slurry with clean fresh water and allow adequate time for
hydration before introduction into the shaft excavation. Agitate,
circulate, and adjust the properties of the slurry to prevent the slurry from
“setting up” in the shaft excavation.
Perform
control tests using suitable apparatus on the mineral slurry to determine
density, viscosity, and pH. Conform to the
acceptable range of values for these physical properties as shown in Table 524.07-1.
TABLE
524.07-1 MINERAL SLURRY SPECIFICATIONS
Range of Values at 68 °F (20 °C)
Property |
Test Method |
Time of Slurry Introduction |
Time of Concreting in Hole |
Density lb/ft³ (kg/m³) |
Density Balance |
64.3 to 69.1 (1030 to 1107) |
64.3 to 75.0 (1030 to 1201) |
Viscosity s/qt (s/L) |
Marsh Cone |
28 to 45 (30 to 48) |
28 to 45 (30 to 48) |
pH |
pH Paper or meter |
8 to 11 |
8 to 11 |
If de-sanding is
required, do not allow the sand content to exceed 4 percent by volume at any
point in the shaft excavation as determined by the American Petroleum Institute
sand content test.
Determine
density, viscosity, and pH values before and during the shaft excavation to
establish a consistent working pattern.
Before placing
shaft concrete, use an approved slurry-sampling tool to take slurry samples from
the bottom and at mid-height of the shaft. Eliminate heavily contaminated
slurry that has accumulated at the bottom of the shaft. Ensure that the
mineral slurry conforms to the requirements specified immediately before shaft
concrete placement.
Only use polymer
slurry after demonstrating to the Engineer that the stability of the hole perimeter can be maintained while advancing the
excavation to its final depth by excavating a trial hole of the same diameter
and depth as that of the production shafts. Use the same polymer slurry
in the trial hole as proposed for the production shafts. If using
different sizes of the shafts at the project, use the same size trial hole as
that of the largest diameter shaft, except the depth of the trial hole need not
be more than 40 feet (12 meters). Only one trial hole per project is
required. Do not use the trial hole excavation
for a production shaft. After completing the trial hole
excavation, fill the hole with sand. The acceptance of the polymer slurry
does not relieve the Contractor of responsibility to maintain the stability of
the excavation. Polymer slurry shall conform to the manufacturer’s
requirements.
524.08 Excavation Inspection. Provide
equipment for checking the dimensions and alignment of each shaft
excavation. Determine the dimensions and alignment. Measure the
final shaft depth after final inspection.
Immediately
before placing concrete, ensure that the bottom of the completed drilled shaft
excavation is as clean as practical. Remove drilling spoils that adhere
to the vertical sides of the bedrock socket.
524.09 Reinforcing Steel for Drilled Shafts. Place the
reinforcing steel cage as a unit immediately after inspection of the excavation
and before placing concrete. If not placing the concrete immediately
after installing the cage, the Contractor may have to remove the cage before
placing the concrete to verify the integrity of the excavated area and to
ensure loose material is removed from the bottom of the hole.
Tie
and support the reinforcing steel so it remains within the required
tolerances. Securely tie spacers at quarter points around the cage
perimeter and space at intervals not to exceed 5 feet (1.5 m) along the
length of the cage. If the size of the longitudinal reinforcing steel equals
or exceeds 1-inch (25 mm) in diameter, the Contractor may increase the minimum
spacing of the spacing devices to 10 feet (3 m). Use spacers of adequate
dimensions to ensure a minimum annular space between outside of cage and side
of hole or casing of 3 inches (75 mm) for shaft diameters up to 4 feet (1.2 m)
and 6 inches (150 mm) for shaft diameters larger than 4 feet (1.2 m). The
Contractor may use round plastic spacers.
Maintain
the top of the reinforcing steel cage no more than 6 inches (150 mm) above and
no more than 3 inches (75 mm) below the required position. If the
reinforcing steel cage is not maintained within tolerances, make acceptable
corrections and do not construct additional shafts until the method of
reinforcing steel cage support has been approved.
When
approved by the Engineer, the Contractor need not provide the reinforcing steel
for the extended length of the drilled shaft if it is determined in the field
that the Contractor must drill the shaft deeper than the estimated length.
524.10 Concrete for Drilled Shafts. For all
drilled shafts, use Class QC 2 concrete according to Item 511
except as modified and supplemented as follows. The required slump is
6 ± 1 inch (150 ± 25 mm). Achieve the additional slump over 4 inches (100
mm) by using chemical admixtures conforming to 705.12,
Type F or G. The accepted JMF’s maximum water
cementitious ratio shall not be exceeded. If placing concrete under
water, add 10 percent more cement to the concrete mix. If placing
concrete using a tremie, further increase the slump
to 8 ± 1 inch (200 ± 25 mm), by using chemical admixtures.
For
wet method construction, place concrete in one continuous operation from bottom
to top of the shaft. After the concrete has reached the top of the
drilled shaft, continue pumping and remove all contaminated concrete until
acceptable quality concrete is evident at the top of the shaft. Do not
vibrate concrete with a vibrator. Carefully remove the casing so that the
reinforcing steel cage is not deformed by the force of the downward flowing
concrete.
Do
not place concrete in any drilled shaft excavation without acceptance from the
Engineer. Inspect the drilled shaft excavation immediately before placing
the concrete. Provide a light powerful enough to thoroughly inspect the
reinforcing steel cage, the sides, and the bottom of the drilled shaft.
The inspection for the wet construction method consists of only probing and
measuring.
If
the elevation of the top of the shaft is below ground at the time of concrete
placement, use a casing to prevent caving of materials into fresh concrete.
524.11 Free Fall Concrete Placement. The
Contractor may place the concrete in a dry drilled shaft excavation using the
free fall method provided the concrete falls to its final position through air
without striking the sides of the hole, the reinforcing steel cage, or any
other obstruction. Use a centering drop chute, at least 3 feet (1 m) long
with the free fall method. Unless shown on the plans, there is no limit
to the height of free fall. If the concrete placement
causes the shaft excavation to cave or slough or if the concrete strikes the
rebar cage or sidewall, reduce the height of free fall or the rate of concrete
flow into the excavation, or both.
If
the Engineer determines that dewatering is not practical or placement by free
fall method cannot be accomplished, place the concrete using a tremie or a concrete pump.
524.12 Tremie. The
Contractor may use a gravity tremie to place concrete
placement instead of a concrete pump in either wet or dry holes. For
uncased wet holes, maintain the drilled shaft excavation full of slurry or
water to such a depth that water does not flow into the shaft excavation at any
time. To place concrete, use tremies consisting
of a tube of sufficient length, weight, and diameter to discharge concrete at
the shaft base elevation. If the tremie
contains aluminum parts, do not allow these parts to contact the
concrete. Use tremies with an inside diameter
of at least 10 inches (250 mm). Ensure that the inside and outside
surfaces of the tremie are clean and smooth to allow
both flow of concrete and unimpeded withdrawal during concreting. Use tremies with a wall thickness adequate to prevent crimping
or sharp bends that restrict concrete placement.
For
concrete placement, use water-tight tremies. Do
not begin underwater placement until the tremie is
placed to the shaft bottom elevation. Use valves, bottom plates, or plugs
so concrete discharge can begin within one tremie diameter
of the base. Either remove plugs from the
excavation or use plugs of an Engineer approved material that does not cause a
defect in the shaft if not removed. Construct the discharge end of the tremie to allow the free radial flow of concrete during
placement operations. Immerse the tremie
discharge end at least 10 feet (3 m) in concrete at all times after
starting the flow of concrete.
If
the tremie line orifice is removed from the fluid
concrete column during the concrete pour, and discharges concrete above the
rising concrete level, consider the drilled shaft defective.
524.13 Pumped Concrete. Pump concrete into either
wet or dry holes. For uncased wet holes, maintain the drilled shaft
excavation full of slurry or water to such a depth that water does not flow
into the shaft excavation at any time. Use concrete pump pipe at least 4
inches (100 mm) in diameter and constructed with watertight joints.
Arrange the concrete pump equipment so no vibrations result that might damage
fresh concrete. Arrange pipes carrying concrete from the pump to the
shaft with a minimum number of bends. Anchor the pipe used to convey the
concrete to the bottom of the drilled shaft excavation to the steel casing or
another suitable stationary object to prevent the pipe from undulating during
the initial placement of the concrete. Do not begin placing concrete
until the pump line orifice is at the shaft base elevation.
Do
not use aluminum pipe as a conveyance for the concrete. Pump an adequate
quantity of grout, mortar, or concrete without coarse aggregate through the
equipment ahead of the specification concrete to provide lubrication to the
pumping system. Do not place the concrete used for lubrication in the
shaft. The lubrication process will not be repeated as long as the
pumping operations are continuous. Operate the pump so a continuous
stream of concrete without air pockets is produced. To prevent the
contamination of the concrete placed initially at the bottom of the shaft, seal
the outlet end of the pumping pipe with a diaphragm or plug that is flushed out
when the hydrostatic pressure from the column of concrete exceeds that of the
water in the shaft. Control the initial rate of concrete placement so not
to lift or displace the cage of reinforcing steel. Use a watertight
conveying system, and leave the outlet end well below the top of the freshly
placed concrete. The preferred concrete placement procedure is to
maintain the outlet end of the pumping system at approximately 10 feet (3 m)
below the top of the fresh concrete. When the concrete reaches the top of
the drilled shaft column, remove all laitance.
If
the concrete pump line orifice is removed from the fluid concrete column during
the concrete pour, and discharges concrete above the rising concrete level, the
drilled shaft shall be considered defective.
524.14 Construction Tolerances. For
shafts supporting single columns, position the drilled shaft within 3 inches
(75 mm) of the plan location in the horizontal plane at the plan elevation for
the top of the shaft. For shafts supporting footings, position the center
within 6 inches (150 mm) of the plan location. Do not allow the vertical
alignment of the shaft to vary from the required alignment by more than 1/4
inch per foot (21 mm/m) of depth. Construct the supported elements
at the plan location. Perform all corrections required to construct the
supported elements.
524.15 Inspection Records. Provide
all necessary equipment and labor needed to obtain measurements for completing
the Inspection Records. Obtain measurements before placing concrete.
524.16 Method of Measurement. The
Department will measure Drilled Shafts by the number of feet (meters), measured
along the axis of the drilled shaft from the required bottom elevation of the
shaft to the proposed top plan elevation. The Department will not measure
the length of reinforcing steel projecting from the drilled shaft into the pier
column or the footing supported on Drilled Shafts as shown on the plans.
If the drilled shaft extends into the bedrock, The Department will divide the
total length of each drilled shaft into two segments. The length of the
lower segment is the length of the bedrock socket, and the length of the upper
segment is the length of the drilled shaft above the bedrock socket.
If
a steel casing extending down to bedrock is used, the Department will measure
the bedrock socket from the bottom of the casing to the bottom of the drilled
bedrock excavation. If the Engineer is assured that a portion of the
metal casing is embedded in solid bedrock, and upon the Engineer’s concurrence,
the Department may include the embedded distance as a part of the bedrock
socket.
524.17 Basis of Payment. Payment
is full compensation for performing required excavation; furnishing and placing
steel casings; furnishing and placing reinforcing steel and concrete by free
fall, pumping, or tremie method; removing casings;
casings left in place; supplying equipment and performing slurry testing;
supplying and disposing of slurry; and disposing excess excavated material.
The
Department will not pay for the cost of performing slurry tests before
concreting.
The
Department will not make separate payment for the trial holes.
The
Department will not pay for corrections required to construct the supported
elements.
The
Department will pay for accepted quantities at the contract prices as follows:
Item
Unit
Description
524
Foot
(Meter)
Drilled Shafts, ___" (___ mm) Diameter,
above Bedrock
524
Foot
(Meter)
Drilled Shafts, ___" (___ mm) Diameter,
into Bedrock
524
Foot (Meter)
Drilled Shafts, ___" (___ mm) Diameter