Note: Descriptions are shown in the official language in which they were submitted.
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SEISMIC RESTRAINT HELICAL PILE SYSTEMS AND
METHOD AND APPARATUS FOR FORMING SAME
FIELD OF THE INVENTION
[0001] The invention relates to deep foundation systems, in
particular cased
helical pile foundation systems.
BACKGROUND OF THE INVENTION
[0002] Piles are used to support structures where surface soil is
weak by
penetrating the soil to a depth where a competent load-bearing stratum is
found.
Helical (screw) piles represent a cost-effective alternative to conventional
piles
because of their speed and ease of installation and relatively low cost. They
have an
added advantage with regard to their efficiency and reliability for
underpinning and
repair. A helical pile typically is made of relatively small galvanized steel
shafts
sequentially joined together, with a lead section having helical plates. It is
installed
by applying torque to the shaft at the pile head, which causes the plates to
screw
into the soil with minimal disruption.
[0003] The main drawbacks of helical piles are poor resistance to
buckling and
lateral movement. Greater pile stability can be achieved by incorporating a
portland-cement-based grout column around the pile
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shaft. See, for example, U.S. Patent No. 6,264,402 to Vickars
(incorporated by reference herein in its entirety), which discloses both
cased and uncased grouted screw piles and methods for installing them.
The grout column is formed by attaching a soil displacement disk to the
pile shaft, which creates a void as the shaft descends into which flowable
grout is poured or pumped. The grout column may be reinforced with
lengths of steel rebar and/or polypropylene fibers. A strengthening casing
or sleeve (steel or PVC pipe) can also be installed around the grout
column. However, because the casing segments are rotated as the screw
and the shaft advance through the soil, substantial torque and energy are
required to overcome frictional forces generated by contact with the
surrounding soil and damage to the casing material can result. Further,
cased and grouted helical piles installed using current techniques and
materials cannot necessarily be relied on to maintain their integrity
during and after a cyclic axial and lateral loading event, such as an
earthquake.
SUMMARY OF THE INVENTION
[0004] One aspect of the invention is a method for forming a cased
helical pile that includes a screw pier comprising a first shaft having a
screw near one end thereof followed axially by a radially outwardly
projecting soil displacing member. The method comprises the steps of:
placing the screw in soil and turning the first shaft to draw the screw into
the soil; either before or after the preceding step, placing a cylindrical
first
sleeve around the first shaft with a first end thereof abutting the soil
displacing member, and placing a driving assembly on the first shaft, the
driving assembly having a low-friction drive seat that engages a second
end of the first sleeve; operating the driving assembly to further turn the
first shaft to draw the screw further into the soil, thereby causing the
screw to pull the soil displacing member axially through the soil and to
pull the first sleeve through the soil substantially without rotation thereof;
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and either during or after the immediately preceding step, filling the first
sleeve with a hardenable fluid grout, thereby encasing the first shaft.
[0005] In order to form a deeper pile, the method further comprises
adding shaft extensions and sleeve extensions one by one, preferably
before the grout placement step. A cylindrical sleeve coupling, having two
axially opposed low-friction seats, is placed between the ends of adjacent
sleeve sections. As the shaft is turned to draw the screw further into the
soil, the added extension sleeves are pulled through the soil substantially
without rotating.
[0006] Another aspect of the invention is an apparatus for installing a
cased helical pile. The apparatus comprises a driving assembly having a
rotatable head and a low-friction, axially facing annular drive seat
surrounding a central opening that receives the pile shaft. The seat is
adapted to abut an end of a sleeve and allow the head to rotate relative to
the sleeve as the sleeve is drawn into the soil. The apparatus also
comprises at least one cylindrical sleeve coupling, each sleeve coupling
adapted to surround the shaft and join a pair of adjacent sleeves. Each
sleeve coupling comprises two axially opposed, low-friction, annular
coupling seats, each of the coupling seats adapted to abut an end of one
of a pair of adjacent sleeves and allow the sleeve coupling to rotate relative
to the pair of adjacent sleeves.
[0007] Another aspect of the invention is an installed pile per se having
the following components integrated into the pile structure: a segmented
shaft having a screw near a lower end thereof; a radially outwardly
projecting soil displacing member on the shaft near the screw; a
segmented casing comprising a plurality of serially arranged, cylindrical
sleeves surrounding the shaft, the lowest one of the sleeves disposed
adjacent the soil displacing member; at least one cylindrical sleeve
coupling, each sleeve coupling surrounding the shaft and joining a pair of
adjacent sleeves, each sleeve coupling comprising two axially opposed,
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low-friction, annular coupling seats, each of the coupling seats abutting an
end of one
of the pairs of adjacent sleeves; and grout substantially filling the interior
of said
casing and encasing said shaft.
[0007A] In a broad aspect, the invention pertains to a cased helical
pile installed in
soil, comprising a segmented shaft having a screw near a lower end thereof, a
radially
outwardly projecting soil displacing member on the shaft near the screw, and a
segmented casing including a plurality of serially arranged, cylindrical
sleeves
surrounding the shaft, the lowest one of the sleeves being disposed adjacent
the soil
displacing member. There is at least on cylindrical sleeve coupling, wherein
each
sleeve coupling surrounds the shaft and joins a pair of adjacent sleeves, and
includes
two axially opposed, annular coupling seats. Each of the coupling seats abut
an end
of one of the pair of adjacent sleeves, and each sleeve coupling includes a
center wall
dividing the sleeve coupling into two oppositely facing recesses. Each recess
is
bounded by an annular outer side wall and forms the coupling seats. Each of
the
annular coupling seats includes a self-lubricating washer abutting the center
wall and
a metallic washer abutting the self-lubricating washer and abutting an end of
one of
the sleeves.
[0008] Yet another aspect of the invention focuses on the materials
used in an
installed cased helical pile of the type described above, namely: cylindrical
sleeves
made of fiber-reinforced polymer, and grout reinforced with mixed-in steel
fibers.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Preferred embodiments of the disclosed invention, including the
best mode for carrying out the invention, are described in detail below,
purely by way of example, with reference to the accompanying drawing, in
which:
[0010] Fig. 1 is a schematic view in longitudinal section of the lower
sections of a cased, grouted helical pile according to the invention;
[0011] Fig. 2 is a perspective view in longitudinal section of a soil
displacing coupling and pile shaft segment of the pile of Fig. 1;
[0012] Fig. 3 is an exploded perspective view of a driving assembly
usable to install the pile of Fig. 1;
[0013] Fig. 4 is an exploded perspective view in longitudinal section of
the driving assembly of Fig. 3; and
[0014] Fig. 5 is a longitudinal sectional view of the assembled driving
assembly taken along line 5-5 in Fig. 3.
DETAILED DESCRIPTION OF THE INVENTION
[0015] Referring to Fig. 1, a helical pile according to the invention has a
central screw pier 10 comprising a series of conventional steel shaft
sections with mating male and female ends that are bolted together
sequentially as the pile is installed, in a manner well known in the art.
The shaft cross-section preferably is square, but any polygonal cross-
section or a round cross-section, or a combination of cross-sections, may
be used. The bottom three shaft sections are shown in Fig. 1, it being
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understood that additional shaft sections are installed above those shown
in like manner. A conventional lead shaft 12 at the lower end of the pile
carries helical flights 14 that advance through the soil when rotated,
pulling the pier downward. A first extension shaft 16 is joined to lead
shaft 12 within a soil displacing coupling 20, a second extension shaft 18
is joined to first extension shaft 16, and so on to the top of the pile.
Casing sleeve sections 22, 24, etc. surround the shaft sections 16, 18, etc.
above soil displacing coupling 20, each pair of adjacent sleeves being
joined by a sleeve coupling 30, which also functions as a centralizer for
the shaft. Grout G completely fills the casing to encase the screw pier.
[0016] Referring to Fig. 2, soil displacing coupling 20 is made of steel
and comprises a tapered central body 26, a bottom square elevation tube
28 and a top cup-shaped recess 32 formed by a cylindrical wall 34 and an
annular inner web 36, which has a square hole 38 for passage of and
rotational engagement with extension shaft 16. A bolt 40 through
elevation tube 28, extension shaft 16 and lead shaft 12 (not shown)
secures those three parts together. Cup-shaped recess 32 forms a seat
for the end of sleeve 22. The seat optionally may have a low-friction insert
comprising a self-lubricating (e.g., Teflon) washer 42, which abuts inner
web 36, and a metallic (e.g., steel) washer 44, which is sandwiched
between self-lubricating washer 42 and sleeve 22. Central body 26
optionally may be provided with one or more helical plates 42, which
provide additional thrust when rotated to help advance the pier through
the soil. The location of the bolt hole along elevation tube 28 is selected
to properly position helical plate(s) 42 relative to the helical flights 14 on
lead shaft 12.
[0017] Enhanced strength and durability of the pile, especially for
seismically active locations, is afforded by selecting the proper grout
formulation, by uniformly including certain reinforcing elements in the
grout mix at a certain concentration, and by using a certain type of
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reinforced casing material, which increases bending resistance. The grout
preferably is high performance, Portland cement based and shrinkage
compensated. A preferred grout is PT Precision Grout, manufactured by
King Packaged Materials Company, Burlington, Ontario, Canada.
Another suitable grout is MASTERFLOW 1341, manufactured by BASF
Construction Chemicals, LLC, Shakopee, Minnesota. The grout
reinforcing elements preferably are round-shaft cold drawn steel wire
fibers, preferably on the order of 0.7 mm in diameter and 30 mm long,
and preferably having flat ends that anchor well within the grout mix. A
suitable example is NOVOCON FE 0730 steel fibers, manufactured by SI
Concrete Systems, Chattanooga, Tennessee, which conform to ASTM
A820/A820M Type 1. The preferred grout mix contains about 1.00% of
steel fibers by volume. The casing material (sleeve) is a fiber reinforced
polymer (FRP), preferably constructed on continuous glass fibers wound
in a matrix of aromatic amine cured epoxy resin in a dual angle pattern
that takes optimum advantage of the tensile strength of the filaments. A
suitable example is BONDSTRAND 3000A fiberglass pipe manufactured
by Ameron International Fiberglass Pipe Group, Burkburnett, Texas, in
accordance with ASTM D2996 Specification for RTRP. Such a pipe sized
for use in helical piles would have a wall thickness on the order of about
2.0 to 3.0 mm. Greater bending resistance would be afforded by using
custom-manufactured pipe as the casing.
[0018] Testing of sample piles that combined FRP sleeves with the
specified steel fiber reinforced grout as described in the preceding
paragraph demonstrated assured integrity of the pile system during and
after cyclic loading, allowing the pile system to sustain its axial capacity.
See Y. Abdelghany and M. El Naggar, "Full-Scale Experimental and
Numerical Analysis of Instrumented Helical Screw Piles Under Axial and
Lateral Monotonic and Cyclic Loadings - A Promising Solution for Seismic
Retrofitting," presented June 28, 2010 at the Sixth International
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Engineering and Construction Conference in Cairo, Egypt, which may be
referred to for further details. This testing demonstrated the above-
described pile system as appropriate for highly seismic areas as it will
maintain serviceability after severe lateral loading events.
[0019] A pile driving assembly, usable to install a pile, will now be
described with reference to Figs. 3-5. Driving assembly 50 is shown
interfaced with a generic pier shaft section X and generic sleeve sections
Y, which are the particular shaft and sleeve sections being driven at any
given state of pile installation. The same pertains to sleeve coupling and
centralizer 30. A driving cap 52 has an annular end wall 54 and a
depending annular side wall 56. An annular low-friction drive seat is
formed in driving cap 52 by a self-lubricating (e.g., Teflon) washer 58,
which abuts end wall 54, and a metallic (e.g., steel) washer 60, which is
sandwiched between self-lubricating washer 52 and an end of upper
sleeve section Y. The upper sleeve section Y may optionally be a short
length of sleeve material or other pipe repeatedly used as a tool as
successive shafts and sleeve sections are installed. Sleeve coupling 30
essentially resembles two driving caps 52 placed back-to-back, except
that there is only a single annular central wall 62 that divides the
coupling into two oppositely facing recesses bounded by annular side wall
64. Each recess has an annular low-friction drive seat similarly formed
by a self-lubricating (e.g., Teflon) washer 66, which abuts central wall 62,
and a metallic (e.g., steel) washer 68, which is sandwiched between self-
lubricating washer 66 and an end of the adjacent sleeve section Y. A
conventional square drive shaft tool 70, shown pinned to shaft X in Fig. 5,
is adapted to be coupled to a conventional rotary tool head (not shown).
[0020] Pile installation using the above driving assembly proceeds as
follows. Lead shaft section 12 is screwed almost completely into the soil
by a rotary tool head coupled to drive shaft tool 70. (Alternatively, initial
soil penetration can be done with lead screw 12, soil displacement
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coupling 20 and sleeve 22 preassembled as shown in Fig. 1.) Tool 70 is
then uncoupled, and first extension shaft 16 and soil displacing coupling
20 are bolted at 40 to the protruding upper end of lead shaft 12. A sleeve
section 22 is then placed around extension shaft 16 and seated in cup-
shaped recess 32 of the soil displacing coupling. (Sleeve section 22
should be short enough so as not to hamper connection of the next
extension shaft 18.) Driving cap 52 is then placed over the upper end of
sleeve section 22 and tool 70 is connected to shaft extension 16 and
rotated to advance the pier and the sleeve into the soil as the soil
displacing coupling creates a cylindrical void in its wake. Tool 70 is then
uncoupled and the next extension shaft 18 is coupled to the upper end of
the first extension shaft 16. A sleeve coupling 30 is then placed over the
upper end of sleeve 22 followed by extension sleeve 24, which is seated in
the opposite side of coupling 30. Driving cap 52 is then placed over the
upper end of sleeve section 24 and tool 70 is connected to shaft extension
18 and rotated to advance the assembly into the soil. The process is
repeated with subsequent shaft extensions, sleeves and sleeve couplings
until a competent load-bearing stratum is reached. Grout is poured or
pumped into the casing, preferably after all the sleeves are installed.
Alternatively, the grout may be placed in the casing in batches: one batch
after each sleeve section is installed.
[0021] Whenever a sleeve section is placed in an annular low-friction
seat, the seat interfaces preferably are lubricated with grease or other
suitable lubricant to enhance the slipperiness of the interfaces. The low-
friction characteristics of the annular seats may be provided by
arrangements other than Teflon and steel washers, such as roller thrust
bearings. The ability of the driving cap 52 and the sleeve couplings 30 to
substantially freely rotate relative to the sleeve sections during pile
installation advantageously enables the sleeve sections to be drawn into
the soil by the lead screw (and pushed by the drive head, if necessary)
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substantially without rotation of the sleeve sections. This avoids the
otherwise high frictional forces generated by constant rotational sleeve
contact with the surrounding soil, reducing the amount of torque and
energy needed for shaft rotation and minimizing abrasion of the sleeve.
[0022] While preferred embodiments have been chosen to illustrate the
invention, it will be understood by those skilled in the art that various
changes and modifications may be made without departing from the
scope of the invention as defined by the appended claims.
