CFA Method of Drilled Shaft Construction - Government Engineering by panniuniu


									CFA Method of Drilled
Shaft Construction
Continuous flight augering can reduce costs and                                                     By Michael Zeman
accelerate project schedules.
          FA is a method in which shafts      with the exception that the finished        and concrete placement, as well as many

 C        are drilled using a continuous
          flight auger with concrete
          placement occurring by pump-
ing through the auger’s hollow stem
                                              product is much more like that of a con-
                                              ventional drilled shaft. This is for two
                                              reasons: First, high strength concrete
                                              containing coarse aggregate is used in
                                                                                          other drill functions, to ensure that piles
                                                                                          are constructed to the proper depth and
                                                                                          that concrete is placed to the full diam-
                                                                                          eter of the specified pile. The quality
while it is extracted. There are numerous     lieu of the sand/cement grout mixes         control of these piles is extremely high
benefits to this approach, including          common to ACIP piles. Second, CFA           and can be monitored instantly not only
immediate replacement, zero subsi-            piles utilize full length, standard rebar   by the drill operator, but also by the
dence, lack of free water within the          cages instead of short top cages and cen-   engineers through offsite links to the
shaft, computer monitoring of depth           ter bars. These differences are signifi-    computer in the drill rig.
and concrete placement, and no need           cant, as they increase the load capacity       To better understand the CFA
for casing/slurry. This is the most com-      of the piles, allow use in applications     method and how it can be implemented
mon drilling method in the UK for             with high lateral loads or seismic design   to reduce cost and schedule duration of
small to medium diameter shafts and is        criteria, and do not require significant    public projects, it is important to under-
being heavily used by D. J. Scheffler— a      modification to standard drilled shaft      stand the method in which a CFA shaft
specialist in deep foundation drilling        designs or specifications. In addition to   is constructed, as well as how it differs
and earth shoring work in difficult envi-     the use of high strength concrete and       from a conventional drilled shaft. At the
ronments—in both California and the           full length rebar cages, CFA piles also     beginning of a shift, the pumping lines
Northwest.                                    differ from ACIP piles in that the          connecting the concrete pump to the
   The CFA installation method is simi-       onboard computer in the drill rig’s cab     rig’s auger are primed and pre-charged
lar to auger cast piling systems (ACIP),      continuously measures depth, rotation,      with concrete. The drill rig is then posi-
                                                                                          tioned 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 con-
                                                                                          tinues until the preset tip elevation is
                                                                                             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 com-
                                                                                          plete replacement. During this part of
A steel element—either a cage or steel beam—is lowered into the fluid concrete by a       the process, the soil is removed continu-
support crane. Note the heavy plastic wheel centralizers designed to center the steel     ously from the auger as it is extracted. A
within the shaft.                                                                         support excavator keeps the area around

22   ■   GOVERNMENT ENGINEERING    ■ JULY-AUGUST   2008                                             
                                                                                                                                                                   6                            Another significant difference is that
                                                                                                                                                               SOUTH WALL
                     MIRABELLA,                                                                        Contract Number: 0800                                                                 there is no free water within a CFA
                     Length 10.04 m
                     610 mm diam CFA
                                                             7.04 MJ
                                                                                          Vol :
                                                                                       Excess :
                                                                                                    3.7 m
                                                                                                    26.12 %
                                                                                                                           Rig        R516
                                                                                                                           Operator : STEVE                     29/04/2008
                                                                                                                                                                                             shaft. In a conventional drilled shaft in
                     24in foundation
                                                                                                                                                                                             wet conditions, free water enters the
           Penetration per rev               Torque               (MJm³)                     Extraction Rate           Oversupply            Pressure               Flow Profile
                (mm/rev)                     (kN.m)                   Drill Resistance            (m/min)                   (%)               (bars)                    (mm)                 shaft and drilling is performed under a
         0                     100     0      20        40    0           2     4        0             5        10     0       20     0       1    2       3        -305   0           305   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
        4                                                                                                                                                                                    immediately as the auger is withdrawn,
                                                                                                                                                                                             preventing the accumulation of free
                                                                                                                                                                                             water in the shaft. This eliminates the
                                                                                                                                                                                             need for collection, storage, and dispos-
                                                                                                                                                                                             al of contaminated drilling fluids,
                                                                                                                                                                                             important concerns on many shaft proj-
       10                                                                                                                                                                                       From an efficiency standpoint, the
        Depth (m)

                                                                                                                                                                                             largest difference is that the shaft is con-
                               5                                                                                                                                                             structed in a continuous operation. In
                                                                                                                                                                                             CFA shafts, the auger is advanced to tip

                                     12:59          13:00         13:01        13:02           13:03           13:04       13:05     13:06         13:07        13:08          13:09
                                                                                                                                                                                             in a single pass, concrete is placed as the
        Auger Revs

                                                                                                                                                                                             auger is extracted, and the rebar cage is
                                                                                                                                                                                             immediately set into place. This differs
                                                                                                                                                                                             greatly from a conventional shaft in soft
                            12:58    12:59          13:00         13:01        13:02           13:03           13:04       13:05     13:06         13:07        13:08          13:09         or wet ground, in which the process of
                                                                                                                                                                                             drilling a single shaft includes advancing
        Concrete Vol (m3)


                                                                                                                                                                                             casing, taking multiple passes with a
                               1                                                                                                                                                             drill tool, cleaning the shaft bottom,
                            12:58    12:59          13:00         13:01        13:02           13:03           13:04       13:05     13:06         13:07        13:08          13:09
                                                                                                                                                                                             placing reinforcement, placing concrete
                                                                                                                                                                                             via tremie, extracting casing, and pump-
Piling instrumentation and control system.                                                                                                                                                   ing off displaced fluids. The elimination
                                                                                                                                                                                             of these extra steps often allows CFA
the auger free of excavated material. This                                                                     While the final product is nearly                                             shafts to be completed three to five
process continues until the auger is fully                                                                  identical to a conventionally drilled                                            times as quickly as a conventional
extracted from the shaft. The support                                                                       shaft, there are significant differences in                                      drilled shaft.
excavator clears away the remaining                                                                         the installation process, each of which is                                          One of the greatest advantages of the
spoils, and then the crew carefully hand                                                                    important to consider when evaluating                                            CFA method is improved safety for the
excavates any spoils or contaminated                                                                        this method for a given project. Most                                            drill crew and inspection team.
concrete at the top of the shaft until a                                                                    importantly, there is never an open hole                                            “Every year, throughout the U. S.,
perfect circle of concrete is exposed to                                                                    when drilling with the CFA method.                                               there is a worker in our industry lost to
the full diameter of the pile. A support                                                                    With conventional drilling techniques,                                           a cave-in of a drilled shaft during con-
crane then lowers the steel element,                                                                        the drill tool repeatedly enters and exits                                       struction of conventional drilled shafts
whether it is a cage or steel beam, into                                                                    the shaft, each time slowly advancing                                            in less than ideal soil conditions. CFA
the fluid concrete of the shaft. In deep                                                                    towards design tip elevation. To prevent                                         methods significantly reduce these risks
shafts, or shafts drilled in dry granular                                                                   the shaft from caving during this                                                by immediately replacing soil with con-
material, it is often necessary to apply a                                                                  process, slurry and/or casing is used to                                         crete during extraction of the auger,”
small amount of vibration to the steel                                                                      provide support for the sidewalls in soft                                        Dale Scheffler, president, D. J. Scheffler
element to assist with the cage insertion.                                                                  or wet conditions. However, with the                                             ( says. “This is a
The intent of this is not to force the steel                                                                CFA method, there is no open drill                                               much safer method since there is never
into the concrete, but rather to simply                                                                     hole, as it is fully supported by the auger                                      an empty shaft.”
assist the flow of the concrete around the                                                                  and spoils during drilling, and support-
cage or beam. It should be noted that                                                                       ed by the concrete as the auger is                                               Design Considerations
centralizers, either rebar skids or heavy                                                                   extracted. This eliminates the need for                                            Typically, a CFA pile of equal depth
plastic wheels, are used to centralize the                                                                  casing and slurry, both of which signifi-                                        and diameter will have equal or slightly
steel within the shaft and to ensure prop-                                                                  cantly add to the time and cost of shafts                                        better load capacity when compared to a
er steel coverage.                                                                                          in soft or wet conditions.                                                       conventional drilled shaft. On projects                                                                                                                                                                   GOVERNMENT ENGINEERING   ■ JULY-AUGUST    2008   ■   23
                                             eter shafts are often best constructed        an invaluable benefit to Caltrans and the
                                             using conventional methods.                   local community as a whole.
                                                                                              Pacific      City       Development,
                                             Case Studies                                  Huntington Beach, CA, Makar
                                                The Scheffler companies have suc-          Development, 2007. Pacific City is a
                                             cessfully completed numerous projects         large mixed use development located
                                             using the CFA method, and are prepar-         directly across from the famous
                                             ing to start construction on several oth-     Huntington Beach. The original foun-
                                             ers within the next 12 months. The fol-       dation design for the multiple structures
                                             lowing are just a few examples, but they      was based upon the use of precast con-
                                             provide a range of applications under         crete piles driven into the dense sands
                                             which CFA is an ideal method to con-          located at a depth of about 40 ft from
                                             struct drilled shafts.                        ground surface. However, before con-
                                                PCH Emergency Stabilization,               struction of the residential portion of
                                             Malibu, CA, Caltrans, 2005. Heavy             the site, the developer became con-
                                             runoff compromised a 50-year old              cerned about the use of the driven pile
                                             drainage pipe located 45 ft below grade,      foundation system due to noise and
                                             creating a depression in the Pacific Coast    vibration levels in close proximity to
                                             Highway. D. J. Scheffler was selected to      existing homes and the adjacent hotel.
                                             construct a secant pile wall with tiebacks    Scheffler evaluated the load require-
                                             along the centerline of the historic road-    ments of the original foundation sys-
                                             way. A series of overlapping 30-in. diam-     tem, as well as the geotechnical condi-
Among the reasons CFA was used at            eter piles were drilled to depths of up to    tions of the site and determined that
Pacific City was the developer was           65 ft using the CFA method, creating a        while a conventional drilled shaft foun-
concerned about the use of the driven pile   continuous wall without the need for          dation would be cost and schedule pro-
foundation system due to noise and           wood lagging. In five working days, 61        hibitive, piles drilled using the CFA
vibrations so close to existing homes and
                                             total piles were installed, less than one-    method could easily achieve the
the adjacent hotel.
                                             third the time it would have taken with       required capacity at a lower cost than
that have already been designed for          conventional methods. In addition, the        the original driven foundation system.
drilled shafts, there is no need to switch   company installed 32 tiebacks, each with      After a series of pile tests, it was deter-
engineering methodology in order to          an average length of 60 ft. Given the crit-   mined not only that the CFA piles
benefit from the use of the CFA meth-        ical nature of this roadway and the poten-    exceeded the capacity of the driven piles
ods. Even in cases where redesign is         tial impacts to motorists, the time in        by a factor ratio of 2:1 (30-in. diameter,
required, the design effort is often more    which this wall was constructed provided      55-in. depth), but that they also signifi-
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 suit-
ed for conventional methods. Third, the
diameter of the shaft needs to be con-
sidered. CFA is commonly used on
drilled shafts up to 48 in. in diameter,
and occasionally up to 60 in. in diame-
ter. Beyond that, concrete supply limita-
tions typically reduce the efficiency of     As opposed to auger cast piling systems, CFA delivers a finished product that is much
the method. For this reason, large diam-     more like that of a conventional drilled shaft.

24   ■   GOVERNMENT ENGINEERING   ■ JULY-AUGUST   2008                                               
                                                                                             ing the poor quality, saturated soils pres-
                                                                                             ent in the upper 40 ft of the site.
                                                                                             Installation of roughly 800 piles, rang-
                                                                                             ing in depth from 40 to 55 ft, began this
                                                                                                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 con-
                                                                                             tain 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
At Pacific City the use of CFA easily achieved the required capacity at less cost than the   working days.                           GE
original driven foundation system.                                                           Mr. Zeman is president of Scheffler
                                                                                             Northwest, a partner company of D. J.
cantly exceeded the calculated capacity        sion to 1500 kips, with no sign of rap-       Scheffler, Inc. located in Vancouver, WA. He
of conventional drilled shafts. Thirty-in.     idly increasing settlement or other fail-     can be reached at 360-818-0070 or
diameter piles were tested in compres-         ure characteristics, remarkable consider-                                                                 GOVERNMENT ENGINEERING     ■ JULY-AUGUST      2008   ■   25

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