CFA Method of Drilled Shaft Construction CFA Method of Drilled

CFA Method of Drilled
Shaft Construction
Continuous flight augering can reduce costs and
accelerate project schedules.
C
FA is a method in which shafts
are drilled using a continuous
flight auger with concrete
placement occurring by pumping
through the auger’s hollow stem
while it is extracted. There are numerous
benefits to this approach, including
immediate replacement, zero subsidence,
lack of free water within the
shaft, computer monitoring of depth
and concrete placement, and no need
for casing/slurry. This is the most common
drilling method in the UK for
small to medium diameter shafts and is
being heavily used by D. J. Scheffler— a
specialist in deep foundation drilling
and earth shoring work in difficult environments—in
both California and the
Northwest.
The CFA installation method is similar
to auger cast piling systems (ACIP),
with the exception that the finished
product is much more like that of a conventional
drilled shaft. This is for two
reasons: First, high strength concrete
containing coarse aggregate is used in
lieu of the sand/cement grout mixes
common to ACIP piles. Second, CFA
piles utilize full length, standard rebar
cages instead of short top cages and center
bars. These differences are significant,
as they increase the load capacity
of the piles, allow use in applications
with high lateral loads or seismic design
criteria, and do not require significant
modification to standard drilled shaft
designs or specifications. In addition to
the use of high strength concrete and
full length rebar cages, CFA piles also
differ from ACIP piles in that the
onboard computer in the drill rig’s cab
continuously measures depth, rotation,
A steel element—either a cage or steel beam—is lowered into the fluid concrete by a
support crane. Note the heavy plastic wheel centralizers designed to center the steel
within the shaft.
By Michael Zeman
and concrete placement, as well as many
other drill functions, to ensure that piles
are constructed to the proper depth and
that concrete is placed to the full diameter
of the specified pile. The quality
control of these piles is extremely high
and can be monitored instantly not only
by the drill operator, but also by the
engineers through offsite links to the
computer in the drill rig.
To better understand the CFA
method and how it can be implemented
to reduce cost and schedule duration of
public projects, it is important to understand
the method in which a CFA shaft
is constructed, as well as how it differs
from a conventional drilled shaft. At the
beginning of a shift, the pumping lines
connecting the concrete pump to the
rig’s auger are primed and pre-charged
with concrete. The drill rig is then positioned
with its auger carefully centered
over the pin marking the shaft center
and the mast and auger are checked for
verticality. Drilling commences and as
the auger penetrates the ground, a small
quantity of soil rises to the surface. The
operator monitors the drilling process
using the real-time information on the
overhead screen; this information
includes drill resistance, torque, depth,
and penetration rate. The process continues
until the preset tip elevation is
met.
Once the auger has reached the pile
tip, concrete is immediately pumped
through the auger. The auger is slowly
raised as concrete continues to flow, with
the operator monitoring the concrete
pressure and oversupply to ensure complete
replacement. During this part of
the process, the soil is removed continuously
from the auger as it is extracted. A
support excavator keeps the area around
22 ■ GOVERNMENT ENGINEERING ■ JULY-AUGUST 2008 www.govengr.com

0 100
0
2
4
6
8
10
Length 10.04 m Revs
610 mm diam CFA
24in foundation
Penetration per rev
(mm/rev)
Depth (m)
Auger Revs
Concrete Vol (m3)
MIRABELLA, Contract Number: 0800
Torque
(kN.m)
Energy
7.04 MJ
(MJm³)
Drill Resistance
0 20 40 0 2 4 0 5 10 0 20 0 1 2 3 -305 0 305
the auger free of excavated material. This
process continues until the auger is fully
extracted from the shaft. The support
excavator clears away the remaining
spoils, and then the crew carefully hand
excavates any spoils or contaminated
concrete at the top of the shaft until a
perfect circle of concrete is exposed to
the full diameter of the pile. A support
crane then lowers the steel element,
whether it is a cage or steel beam, into
the fluid concrete of the shaft. In deep
shafts, or shafts drilled in dry granular
material, it is often necessary to apply a
small amount of vibration to the steel
element to assist with the cage insertion.
The intent of this is not to force the steel
into the concrete, but rather to simply
assist the flow of the concrete around the
cage or beam. It should be noted that
centralizers, either rebar skids or heavy
plastic wheels, are used to centralize the
steel within the shaft and to ensure proper
steel coverage.
99.5
Vol :
Excess :
3.7 m
26.12 % Operator :
Extraction Rate
(m/min)
Rig
Oversupply
(%)
R516
Pressure
(bars)
6
SOUTH WALL
29/04/2008
Flow Profile
0
1
2
3
4
5
6
7
8
9
10
12:58 12:59 13:00 13:01 13:02 13:03 13:04 13:05 13:06 13:07 13:08 13:09
104
95
86
77
68
59
50
41
32
23
14
5
12:58 12:59 13:00 13:01 13:02 13:03 13:04 13:05 13:06 13:07 13:08 13:09
3
2
1
0
12:58 12:59 13:00 13:01 13:02 13:03 13:04 13:05 13:06 13:07 13:08 13:09
time
13:04
Piling instrumentation and control system.
STEVE
(mm)
While the final product is nearly
identical to a conventionally drilled
shaft, there are significant differences in
the installation process, each of which is
important to consider when evaluating
this method for a given project. Most
importantly, there is never an open hole
when drilling with the CFA method.
With conventional drilling techniques,
the drill tool repeatedly enters and exits
the shaft, each time slowly advancing
towards design tip elevation. To prevent
the shaft from caving during this
process, slurry and/or casing is used to
provide support for the sidewalls in soft
or wet conditions. However, with the
CFA method, there is no open drill
hole, as it is fully supported by the auger
and spoils during drilling, and supported
by the concrete as the auger is
extracted. This eliminates the need for
casing and slurry, both of which significantly
add to the time and cost of shafts
in soft or wet conditions.
Another significant difference is that
there is no free water within a CFA
shaft. In a conventional drilled shaft in
wet conditions, free water enters the
shaft and drilling is performed under a
water or slurry head. However with
CFA, very little soil is removed from the
shaft until concrete placement begins,
only enough to displace the volume of
the auger flights. Concrete is placed
immediately as the auger is withdrawn,
preventing the accumulation of free
water in the shaft. This eliminates the
need for collection, storage, and disposal
of contaminated drilling fluids,
important concerns on many shaft projects.
From an efficiency standpoint, the
largest difference is that the shaft is constructed
in a continuous operation. In
CFA shafts, the auger is advanced to tip
in a single pass, concrete is placed as the
auger is extracted, and the rebar cage is
immediately set into place. This differs
greatly from a conventional shaft in soft
or wet ground, in which the process of
drilling a single shaft includes advancing
casing, taking multiple passes with a
drill tool, cleaning the shaft bottom,
placing reinforcement, placing concrete
via tremie, extracting casing, and pumping
off displaced fluids. The elimination
of these extra steps often allows CFA
shafts to be completed three to five
times as quickly as a conventional
drilled shaft.
One of the greatest advantages of the
CFA method is improved safety for the
drill crew and inspection team.
“Every year, throughout the U. S.,
there is a worker in our industry lost to
a cave-in of a drilled shaft during construction
of conventional drilled shafts
in less than ideal soil conditions. CFA
methods significantly reduce these risks
by immediately replacing soil with concrete
during extraction of the auger,”
Dale Scheffler, president, D. J. Scheffler
(www.djscheffler.com) says. “This is a
much safer method since there is never
an empty shaft.”
Design Considerations
Typically, a CFA pile of equal depth
and diameter will have equal or slightly
better load capacity when compared to a
conventional drilled shaft. On projects
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Among the reasons CFA was used at
Pacific City was the developer was
concerned about the use of the driven pile
foundation system due to noise and
vibrations so close to existing homes and
the adjacent hotel.
that have already been designed for
drilled shafts, there is no need to switch
engineering methodology in order to
benefit from the use of the CFA methods.
Even in cases where redesign is
required, the design effort is often more
than offset by the cost and time savings
during actual construction.
Despite the advantages of the CFA
method, there are some things that need
to be considered before specifying or
utilizing this method on a particular
project. First, the method does not allow
for efficient drilling though hard
bedrock conditions or large boulders.
Second, there is an effective limit in how
deep full length steel elements can be
wet set into a CFA shaft. While
advanced mix designs do allow cage
installation as deep as 80 ft, shafts
beyond this depth are typically best suited
for conventional methods. Third, the
diameter of the shaft needs to be considered.
CFA is commonly used on
drilled shafts up to 48 in. in diameter,
and occasionally up to 60 in. in diameter.
Beyond that, concrete supply limitations
typically reduce the efficiency of
the method. For this reason, large diameter
shafts are often best constructed
using conventional methods.
Case Studies
The Scheffler companies have successfully
completed numerous projects
using the CFA method, and are preparing
to start construction on several others
within the next 12 months. The following
are just a few examples, but they
provide a range of applications under
which CFA is an ideal method to construct
drilled shafts.
PCH Emergency Stabilization,
Malibu, CA, Caltrans, 2005. Heavy
runoff compromised a 50-year old
drainage pipe located 45 ft below grade,
creating a depression in the Pacific Coast
Highway. D. J. Scheffler was selected to
construct a secant pile wall with tiebacks
along the centerline of the historic roadway.
A series of overlapping 30-in. diameter
piles were drilled to depths of up to
65 ft using the CFA method, creating a
continuous wall without the need for
wood lagging. In five working days, 61
total piles were installed, less than onethird
the time it would have taken with
conventional methods. In addition, the
company installed 32 tiebacks, each with
an average length of 60 ft. Given the critical
nature of this roadway and the potential
impacts to motorists, the time in
which this wall was constructed provided
an invaluable benefit to Caltrans and the
local community as a whole.
Pacific City Development,
Huntington Beach, CA, Makar
Development, 2007. Pacific City is a
large mixed use development located
directly across from the famous
Huntington Beach. The original foundation
design for the multiple structures
was based upon the use of precast concrete
piles driven into the dense sands
located at a depth of about 40 ft from
ground surface. However, before construction
of the residential portion of
the site, the developer became concerned
about the use of the driven pile
foundation system due to noise and
vibration levels in close proximity to
existing homes and the adjacent hotel.
Scheffler evaluated the load requirements
of the original foundation system,
as well as the geotechnical conditions
of the site and determined that
while a conventional drilled shaft foundation
would be cost and schedule prohibitive,
piles drilled using the CFA
method could easily achieve the
required capacity at a lower cost than
the original driven foundation system.
After a series of pile tests, it was determined
not only that the CFA piles
exceeded the capacity of the driven piles
by a factor ratio of 2:1 (30-in. diameter,
55-in. depth), but that they also signifi-
As opposed to auger cast piling systems, CFA delivers a finished product that is much
more like that of a conventional drilled shaft.
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At Pacific City the use of CFA easily achieved the required capacity at less cost than the
original driven foundation system.
cantly exceeded the calculated capacity
of conventional drilled shafts. Thirty-in.
diameter piles were tested in compression
to 1500 kips, with no sign of rapidly
increasing settlement or other failure
characteristics, remarkable considering
the poor quality, saturated soils present
in the upper 40 ft of the site.
Installation of roughly 800 piles, ranging
in depth from 40 to 55 ft, began this
spring.
BNSF Rail Expansion, Vancouver,
WA, 2008. This project includes the
construction of a 200-ft long secant pile
wall with 48-in. diameter primary and
secondary piles placed on 60-in. centers.
The ground conditions generally consist
of loose sandy silts over dense saturated
gravels. The primary secant piles contain
full length reinforcement cages with
up to 24 #11 bars and #5 spiral with
five-in. pitch. The reinforced piles range
from 35 ft to 80.5 ft in depth.
Construction began in May 2008 and
the use of CFA is expected to reduce the
construction schedule by roughly 40
working days.
GE
Mr. Zeman is president of Scheffler
Northwest, a partner company of D. J.
Scheffler, Inc. located in Vancouver, WA. He
can be reached at 360-818-0070 or
mzeman@schefflernw.com.
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