Note: Descriptions are shown in the official language in which they were submitted.
CA 02551216 2010-10-29
METHOD AND APPARATUS FOR PROVIDING
A RAMMED AGGREGATE PIER
FIELD OF THE INVENTION
In a principal aspect, the present invention generally relates to a method of
soil
densification and improvement for purpose of forming a stiffened support pier
in a
cavity within the densified and improved soil.
The present invention additionally relates generally to the field of civil and
construction engineering and, more specifically, is directed to methods and
apparatus
for providing load supporting aggregate piers in the earth capable of
supporting a
multitude of possible structures including, but not limited to, buildings,
roads, bridges
and the like.
BACKGROUND OF THE INVENTION
Many soils are deficient in their capability to incorporate a shallow support
system such as shallow foundations or a shallow mat system. Consequently, when
building a structure, highway embankment or retaining wall, it is often
necessary to
provide a special foundation support for the structure and various techniques
have
been developed to provide adequate subsoil support for such structures to
prevent
excessive settlements and to prevent bearing failures. For example, pilings
may be
driven into the ground to bedrock. Various techniques have also been developed
for
densifying and improving the ground and utilizing the improved ground in
combination
with pilings or stiffened piers or footings constructed therein.
It has been conventional practice for many years to provide vertical,
elongated
cavities in the earth for receiving aggregate to form what is known as "stone
columns".
In one conventional procedure cavities are formed by vertically vibrating a
vibroflot
cylindrical tube into the ground. The vibroflot tube has motor driven
eccentric weights
in its lower end for applying lateral or radial vibrations to the tube and the
short conical
tool. Penetration of the earth by the tube is assisted by either air or water
jetting
means. Older devices of the foregoing type use water jetting means and drop
aggregate, crushed stone or other granular materials into the cavity from the
ground
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surface in what is referred to as a "wet method". More recent variations have
employed air jetting and introduction of stone through the tube.
Major problems with the wet method process are that it adds water to the
cohesive clay soils around the vibroflot so as to soften the soil, and it
produces effluent
containing suspended particles that is often required to be treated.
Unfortunately, the
application of horizontal vibration applied to the stone results in a column
having low
stiffness in comparison to short aggregate piers as discussed in the following
paragraphs.
A more recently employed method of providing short aggregate piers is that of
Fox et al. U.S. Pat. No. 5,249,892, which teaches use of a rotary drill to
form a cavity
typically of 18 to 36 inches in diameter, in the manner discussed in column 5,
of the
patent. Upon completion of the cavity, a thin lift (layer) of aggregate is
placed in the
bottom of the cavity and compacted vertically and outwardly by high energy
impact
devices (hydraulic hammers) applying direct downward and high frequency
ramming
to each thin lift of stone with the procedure then being repeated with
subsequent thin
stone lifts until the cavity is filled to complete the short pier.
Shortcomings of such
procedures include the required use of a casing to stabilize the sidewalls of
the cavity
above its lower end, when installations are in unstable soils which cave in,
such as
sands and sandy silts. Also, instability at the bottom of the cavity in
granular soils with
a high groundwater level is a frequent problem because of the water attempting
to flow
or pipe into the casing so as to create unstable conditions at the bottom of
the cavity.
Moreover, the depth of the cavity is limited to approximately 30 feet because
of
structural limitations of the equipment. A further problem arises in soft,
cohesive or
organic soils in which the load capacity of the pier to support loads is
limited by the fact
that the soft soil provides limited resistance to outward bulging movement of
the stone
piers.
Fox U.S. Pat. No. 6,354,766 discloses a variety of special techniques,
including
pre-loading, chemical treatment and use of mesh reinforcement procedures to
enhance the construction and test the properties of short aggregate piers.
Fox U.S. Pat. No. 6,354,768 discloses the use of expandable bladders for
densifying soil adjacent or below stone piers.
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Another method of forming a stone pier is disclosed in U.S. Pat. No. 6,425,713
in which a lateral displacement pier, also know as a "cyclone pier", is
constructed by
driving a pipe into the ground, drilling out the soil inside the pipe and
filling the pipe
with aggregate. The pipe is then used to compact aggregate in thin lifts by
use of a
beveled edge at the bottom of the pier for compaction. Piers fortified by this
method
can be installed to great depths such as 50 feet and in granular soils.
Limitations of
this approach include the need for a heavy crane for installation and a drill
rig to drill
out the casing. Additionally, the system is cumbersome and slow to install
when the
installation uses a normal crane and pipe having diameters such as listed in
the
patent.
Another system developed by Mobius and Huesker in Germany provides an
encased stone column by pushing a closed-ended pipe into soft ground by use of
a
vibratory pile driving hammer mounted at the top of the pipe. When the lower
end of
the pipe reaches designed depth, a geotextile sock or bag is inserted into the
inside
of the pipe. This sock is then filled with crushed stone poured from the
ground surface.
After the sock is filled a trap-door opens at the bottom of the pipe and the
pipe is
extracted upwardly while the geotextile sock and its contents remain in the
excavation.
The primary advantage of this system is that the geotextile sock prevents the
bulging
of the crushed stone into the surrounding soil when loaded. However, a number
of
disadvantages include the fact that the column is not compacted and does not
have
high stiffness sufficient for supporting buildings and the like. Additionally,
this system
must be installed in very soft or loose soil that can be penetrated by closed-
ended pipe
pile driven with a vibratory pile driving hammer.
Another prior system developed by Nathaniel S. Fox employs a 14 inch to 16
inch diameter tamper head attached to the lower end of an 8 inch to 10 inch
diameter
cylindrical pipe. The pipe is vibrated into the ground and is filled with
crushed stone
once the tamper head is driven to the desired designed depth. The tamper head
is
then lifted to allow stone to fall into the cavity following which the tamper
head is driven
back downwardly onto the stone for densifying the stone.
A deep dynamic compaction system developed by Louis Menard employs a
heavy weight which is dropped from a great height to pound the ground. Each
drop
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creates a crater at the ground surface and generates significant ground
shaking and
causes granular soils to densify for the future support of structures. The
system can
be employed by placing fresh stone in the cavities formed by the dropped
weight and
then tapping the stone downward to form stone pillars used to support vertical
loads.
Similar methods are illustrated in United Kingdom Patent No. 369,816, Italian
Patent
No. 565,012, and French Patent No. 616,470. The disadvantages of these
processes
include the need for a large crane to lift the dropped weight and the
excessive vibration
that is induced during tamping.
Another system for making aggregate piers, involving driving a pointed mandrel
has been used by a contractor in the United Kingdom and is disclosed in a
brochure
of Roger Bullivant Ltd dated June 2002. The disclosed device uses a vibrator
piling
hammer to direct the mandrel into the ground to provide a cavity for receipt
of crushed
stone. The mandrel has a sharply pointed end, which inhibits the compaction of
the
stone at the top of the pier.
Densification of the soil and construction of a stiffened pier column using
the
techniques of the type described in the aforesaid prior art comprises a
mechanical
densification process. Various mechanical means are utilized to alter, densify
and
otherwise improve the characteristics of the soil enabling the soil to
effectively
incorporate support piers. The process also produces a stiffened pier, which
in
combination with the improved adjacent soil, results in an effective
structural support
system for shallow foundations, slabs and mats.
A problem typically arises in sandy soil and other unstable soils in that
drilled
holes often cave in and require expensive preventive measures to prevent the
cave-
ins. Another problem with drilled holes is that cuttings are brought to the
ground
surface and they require disposal. This later problem is particularly onerous
when the
soils being penetrated are contaminated, since disposal of contaminated soils
is
extremely expensive.
The present invention provides new and improved methods and apparatus for
forming aggregate piers.
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The present invention provisions new and improved methods and apparatus for
forming cavities in the earth that maintain their structural integrity during
construction
of stone piers or columns in such cavities.
The present invention also provisions new and improved methods for radially
compacting the side wall of a cavity as it is being formed so as to reduce the
possibility
of side wall deterioration during subsequent construction procedures.
The present invention also provides improved apparatus and methods for soil
densification and improvement in forming a cavity and a stiffened support pier
therein.
The present invention also provides an improvement in the strength and
stiffness of the piers by producing improved methods for aggregate compaction
during
construction of the pier shaft and the top of the pier.
The present invention also provisions vertical impact energy and downward
static forces applied by the top-mounted hammers used for construction.
The present invention also provides an improved method and apparatus for soil
densification and formation of a stiffened structural support pier of
aggregate or
aggregate and cementitious grout in soils of various types, and, in
particular, granular
soils such as sandy soils.
The present invention also provides a method and apparatus for mechanical
densification of the soil and formation of stiffened piers that is more
efficient than prior
techniques and which may be used in a wider range of soils.
The present invention also provides a method and apparatus for soil
densification, wherein a stiffened pier is formed within a passage or cavity
in the soil,
and wherein the pier or support includes either a single stage construction or
multiple
stage construction depending upon the characteristics of the soil being
densified and
on the results needed in design.
The present invention also provides a method for formation of a support pier
in
soils, particularly granular soils and contaminated soils, where the formed
support pier
comprises an aggregate or an aggregate with cementitious grout, within soil
that has
been densified and strengthened by pre-straining and pre-stressing the soil in
the
vicinity of the formed pier.
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Other features and advantages of the present invention will be apparent to
those skilled in the art upon consideration of this specification and the
accompanying
drawings.
SUMMARY OF THE INVENTION
Achievement of the foregoing is enabled by a unique primary mandrel for
forming cavities in the earth which tapers inwardly from its upper end to a
blunt lower
end with the distance between the upper end and the lower end being at least
equal
to the height of the aggregate pier to be formed in a cavity formed by the
primary
mandrel. Typically, the taper or pitch angle of the primary mandrel relative
to the axis
of the mandrel is constant and will fall in the range of about 1.0 to about
5.0 degrees
so that vertical movement of the mandrel which is effected by both vertical
static force
and vertical vibratory force creates essentially lateral radial forces on the
surrounding
earth. These lateral radial forces serve to compact and stabilize the entire
sidewall
surface of the cavity being formed and consequently greatly reduce the
possibility of
subsequent loss of structural integrity of the cavity during the extraction of
the mandrel.
The pitch angle of the primary mandrel is selected for different soil profiles
to achieve
enhanced stability so that the mandrel may be lifted from the cavity without
the need
for temporary casing or drilling fluid to maintain sidewall stability. It is
also
consequently possible to avoid the need for temporary casing or drilling fluid
to
maintain sidewall stability during the deposit and compaction of aggregate
deposited
in the open cavity during subsequent pier building procedures.
Upon completion of the cavity the primary mandrel is removed upwardly from
the bottom of the cavity to enable the beginning of construction of a pier by
deposit of
a layer of aggregate on the bottom of the cavity. The primary mandrel is then
reinserted in the cavity and the mandrel's blunt lower end engages the
previously
deposited aggregate with greater downward static force (crowd force) than
achieved
for cylindrical vibroflot construction to compact both the aggregate and the
soil radially
adjacent and in contact with the aggregate. The primary mandrel is again
removed
from the cavity and another deposit of aggregate is placed upon the previously
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deposited aggregate. This next deposit of aggregate is then compacted as in
the
previous compacting procedure by the blunt lower end of the mandrel and the
aggregate depositing and compacting procedures are repeated until the
aggregate
nears the upper end of the cavity. Final compaction of the aggregate in the
upper end
of the cavity to complete the pier construction may optionally be effected by
use of a
short secondary tamping mandrel having a larger blunt lower end than the
primary
mandrel employed in forming the cavity.
The unique primary mandrel has a hollow shell-frame preferably formed of steel
plate having an octagonal cross-section. However, other cross-sectional shapes
could
be used, including but not limited to square, hexagonal and circular. The
shell-frame
is preferably formed of an upper half-shell component and a lower half-shell
component which are welded together at the mid-point of the primary mandrel to
provide a rugged and effective structure at reduced cost.
The present invention also relates to a method for densification of soil and
forming of a stiffened column of aggregate or aggregate with cementitious
grout, which
comprises a series of steps, including forming a tapered cavity or passage in
the soil,
filling in that passage or at least in part filling it in, with aggregate or
with aggregate
with a cementitious grout, compacting the aggregate and at the same time
displacing
a portion of the aggregate laterally into the adjacent soil to densify and
laterally
prestress the adjacent soil. The method further contemplates the filling of
the passage
with aggregate or with aggregate with cementitious grout upward from the
bottom of
the passage.
A method of forming the passage is to utilize a long, tapered steel or other
hard
material mandrel or probe with larger cross-section top portion and smaller
cross-
section bottom portion. The probe may have a variety of shapes including a
circular
cross-section. The bottom of the probe may be flat, or it may be flat with
beveled sides
with a greater taper than the taper of the sides of the main probe, or it may
have a
different shaped bottom such as a cone point or a convex semi-spherical
bottom.
Different bottom shapes may be preferable in different types of soil.
The elongated tapered mandrel or probe of the present invention is pushed and
optionally vibrated into the ground using a static force, optionally a dynamic
force, and
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optionally a vibrating force, or a combination of these forces. The probe is
pushed until
it reaches. the predetermined depth of improvement desired. The probe is
subsequently raised, either in one movement to the top, or in a series of
intermediate
movements, depending upon the method selected to form the pier.
The method further contemplates densifying the top of the aggregate pier with
a secondary probe that has a greater cross-sectional area at the probe bottom
than
the primary probe.
The method additionally contemplates the use of telltales, uplift anchors and
post grating to measure deflections, resist uplift-loads and reduce the
propensity for
bulging.
As an aspect of the present invention, there is provided a method of forming
an
aggregate pier comprising the steps of (a) driving a downwardly tapered
mandrel
having a blunt lower end surface into the ground by a power driven apparatus
to form
a downwardly tapered cavity to a desired depth for the aggregate pier while
outwardly
compacting the sidewalls of the cavity as the cavity is being formed the
mandrel
having a conduit extending internally to an aperture with the blunt lower end
to deliver
aggregate through the body lower end, and having an outer surface which tapers
inwardly, toward its lower end at a taper of less than about 5 degrees
relative to a
central axis thereof; (b) moving the tapered mandrel upwardly a sufficient
distance to
permit access to the lower end of the cavity; (c) depositing a layer of
aggregate in the
cavity through the conduit; (d) lowering the tapered mandrel downwardly in the
cavity
so that the blunt lower end of the mandrel engages the aggregate in the cavity
and
densifies the aggregate in the cavity by force applied by the blunt lower end
of the
mandrel; and (e) repeating steps (b), (c) and (d) until a pier component of
desired
height is formed.
As another aspect of the present invention, there is provided a mandrel for
forming aggregate piers, comprising a substantially hollow steel shell
elongated body
having an upper end, a central axis, a blunt lower end with a sacrificial cap
to prevent
clogging of the mandrel during soil penetration and to be left in place at the
bottom of
the pier during pier connection, an outer surface with a continuous outer
perimeter and
which tapers inwardly from adjacent its upper end to its lower end at a taper
of less
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than about 5.0 degrees relative to the central axis and a conduit extending
internally
of the body from the body upper end to an aperture in the body lower end
generally
along the central axis, the conduit being configured to deliver aggregate
through the
body lower end and to obviate withdrawal of the mandrel prior to the delivery
of the
aggregate, the elongated body configured to form an elongated hole in a ground
surface with densifying soil in sidewall surfaces of the hole through direct
engagement
of the outer surface with the side wall surfaces, and further configured to
compact the
aggregate placed in the hole such that the aggregate penetrates the sidewall
surfaces
through direct engagement of the mandrel with the aggregate.
As another aspect of the present invention, there is provided a method of
forming a stiffened pier comprising in combination the steps of (a) forming a
cavity in
the soil by inserting a tapered probe to displace the soil the tapered probe
having a
conduit extending internally to a bottom end thereof at an opening thereof;
(b) filling
the cavity, at least in part, with aggregate or aggregate with cemetitious
grout while
lifting the probe at least partially out of the cavity and discharging the
aggregate from
the conduit in the probe; and (c) re-introducing the probe at least once into
the
aggregate discharged into the cavity to compact the aggregate and to displace
a
portion of the aggregate laterally into the adjacent soil to densify the soil,
wherein the
probe is moved only partially up the cavity length while concurrently
discharging
aggregate following which the probe is pressed downwardly into the aggregate
to
compact and displace the aggregate laterally into the adjacent soil and
repeating the
foregoing procedure at different elevations within the pier being formed until
the full
length of the pier has been formed.
As another aspect of the present invention, there is provided a mandrel system
for forming aggregate piers comprising a substantially hollow shell elongated
primary
mandrel and a short secondary tamping mandrel the primary mandrel having a
central
axis, a blunt lower end with a sacrificial bottom cap configured to prevent
clogging of
the mandrel during soil penetration and to be left in place at the bottom of
the pier
during pier construction, an outer surface with a continuous outer perimeter,
at least
a major portion of the outer surface tapering inwardly toward its lower end by
between
about 1.0 and about 5.0 degrees relative to the central axis and a conduit
extending
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internally of the shell from the upper end to an aperture with the lower end
generally
along the central axis, the conduit being configured to deliver aggregate
through the
lower end and to obviate withdrawal of the mandrel prior to the delivery of
the
aggregate, the primary mandrel configured to form an elongated hole in a
ground
surface, densify soil in a sidewall surface of the hole to provide a self-
supporting
sidewall, and compact the aggregate placed in a lower portion of the hole, the
secondary tampering mandrel configured to copact aggregate in an upper portion
of
the hole and having a larger compacting surface lower end than the blunt lower
end
of the primary mandrel.
As another aspect of the present invention, there is provided a mandrel for
forming an aggregate pier of thin lifts, comprising a substantially hollow
shell elongated
body having an upper end, a central axis, a blunt lower end with a flat bottom
surface
with an upwardly beveled edge portion configured to compact sidewalls of a
cavity
formed therewith, an outer surface with a continuous outer perimeter tapering
inwardly
from adjacent the upper end to the lower end at a taper of less than about 5.0
degrees
relative to the central axis, and a conduit extending internally of the
bodyfrom the body
upper end to an aperture in the body lower end generally along the central
axis, the
conduit being configured to deliver aggregate through the body lower end and
to
obviate withdrawal of the mandrel prior to the delivery of the aggregate, the
elongated
body configured to form an elongated hole in a ground surface with a self-
supporting
sidewall and to sequentially compact the aggregate delivered into the hole so
as to
form a vertically compacted series of the thin lifts which penetrate the
sidewall by
direct engagement of the mandrel with the aggregate.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is better understood by reading the following Detailed
Description
of the preferred embodiments with reference to the accompanying drawing
figures,
which are not necessarily to scale, and in which like reference numerals refer
to like
elements throughout, and in which:
Figure 1 is a front elevation of a first embodiment earth penetrating primary
mandrel employed in practice of the present invention;
Figure 2 is a top plan view of the mandrel taken along lines 2-2 of Figure 1;
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Figure 3 is a sectional view of the mandrel taken along lines 3-3 of Figure 1;
Figure 4 is a sectional view taken along lines 4-4 of Figure 1;
Figure 5 is an exploded top view of end portions of the two lower quarter-
shell
components of the mandrel shell for the mandrel of Figure 1;
Figure 5(a) is a plan view of a lower bulkhead juncture plate for the mandrel
of
Figure 1;
Figure 5(b) is a pre-assembly exploded side view of the two lower quarter-
shell
components of the mandrel shell for the mandrel of Figure 1, illustrating an
initial step
in the assembly of the lower half-shell component;
Figure 5(c) is a side view of the two lower quarter-shell components of Figure
5(b) in assembled relationship forming the lower half-shell component;
Figure 6 is encircled portion 6 of Figure 1 comprising a front elevation
partial
section view illustrating the connection structure between the upper and lower
half-
shell components;
Figure 6(a) is an exploded pre-assembly side view of the two upper quarter-
shell components of the mandrel of Figure 1, illustrating an initial step in
the assembly
of the upper half-shell components;
Figure 6(b) is a side view of the two upper quarter-shell components of Figure
6(a) illustrating their assembled relationship forming the upper half-shell
component;
Figure 7 is a front elevation of a secondary tamping mandrel used for tamping
stone previously positioned near the top of a cavity formed by the mandrel of
Figure
1;
Figure 8 is a lower plan view of a blunt bottom plate of the mandrel of Figure
1;
Figure 9 is a perspective view illustrating association of the primary mandrel
of
Figure 1 with a conventional supporting and driving device for driving the
mandrel into
the earth;
Figure 10 is a vertical section of the earth illustrating completion by the
primary
mandrel of Figure 1 of a cavity in which an aggregate pier is to be
constructed;
Figure 11 is a vertical section showing the primary mandrel of Figure 1 in a
second position assumed subsequent to the Figure 10 position to permit deposit
of
aggregate in the bottom of the cavity;
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Figure 12 is a vertical section showing the primary mandrel of Figure 1 in an
aggregate densifying position assumed subsequent to the Figure 11 position;
Figure 13 is a vertical section showing completion of a pier by densifying the
uppermost aggregate portion by the secondary tamping mandrel of Figure 7:
Figure 14 is a front elevation of a modified mandrel embodiment which includes
structure for injecting concrete or grout into aggregate in the cavity;
Figure 15 is a vertical section illustrating concrete injection into aggregate
in the
cavity by the embodiment of Figure 14;
Figure 16 is a plan view of a rear brace plate provided near the upper end of
the
mandrel of Figures 1 or 14;
Figure 17 is a plan view of a front brace plate provided near the upper half
of
the mandrel of Figures 1 or 14;
Figure 18 is a front elevation view of the drive and support plate provided in
the
upper end of the mandrel of Figures 1 or 14;
Figure 19 is a graphic illustration of stress (psf) and resultant deflection
measure for three test piers formed in accordance with the present invention,
as
measured at the tops of the piers and at lower pier areas by telltales;
Figure 20 is a plot of the stiffness modulus (ratio of applied stress to
deflection)
for increasing values of pier stress values for the three test piers of Figure
19;
Figure 21 illustrates SPT-N values for different distances from piers
constructed
according to the present invention; and
Figure 22 illustrates the ratio of SPT-N values for piers constructed using
the
present invention to the SPT-N values in the soil prior to construction of the
piers.
Figure 23 is a vertical section of the earth illustrating completion of a pier
receiving cavity by a third embodiment tapered mandrel having a radially
extending
flange at its upper end;
Figure 24 is a vertical section of the earth illustrating completion of a pier
receiving cavity by a further embodiment mandrel having a straight untapered
sided
top portion and a tapered lower portion;
Figure 25 illustrates anothertapered mandrel having an internal perforated
pipe
axially positioned therein;
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Figure 26 illustrates a mandrel following insertion in the earth for the
initiation
of forming a pier;
Figure 27 illustrates the position of the components effected subsequent to
the
Figure 26 position and in which the mandrel is elevated to permit deposit of
aggregate
in the cavity;
Figure 28 illustrates the position subsequent to the position illustrated in
Figure
27 in which the mandrel has been reinserted to compact aggregate previously
deposited in the cavity as shown in Figure 27;
Figure 29 illustrates the condition assumed subsequent to removal of the
mandrel from the cavity as shown in Figure 28 with the perforated pipe
remaining in
the cavity for enabling post-grouting of the aggregate;
Figure 30 illustrates a first alternative secondary tamping mandrel;
Figure 31 illustrates a second alternative secondary tamping mandrel;
Figure 32 is a diagrammatic view of a first step in the formation of a pier
using
the single stage method;
Figure 33 is a diagrammatic view of a subsequent step to the step of Figure 32
in formation of a pier using the single stage method;
Figure 34 is a diagrammatic view of a further step subsequent to the step of
Figure 33 using the single stage method;
Figure 35 is a diagrammatic view of the finished pier formed in accordance
with
the steps of Figures 32 through 34 using the single stage method;
Figure 36 comprises a diagrammatic view of a first step of the formation of a
pier using the multiple stage method;
Figure 37 is a diagrammatic view of a second step subsequent to the step of
Figure 36 in formation of a pier using the multiple stage method;
Figure 38 is a diagrammatic view of a further step subsequent to the step of
Figure 37 using the multiple stage method;
Figure 39 is a diagrammatic view of the finished pier formed in accordance
with
the steps illustrated in Figures 36 through 37 using the multiple stage
method.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In describing preferred embodiments of the present invention as illustrated in
the drawings, specific terminology is employed for the sake of clarity.
However, the
invention is not intended to be limited to the specific terminology so
selected, and it is
to be understood that each specific element includes all technical equivalents
that
operate in a similar manner to accomplish a similar purpose. It should also be
understood that the directional and positional descriptions such as above,
below, front,
rear, upper, lower and the like are based upon the relative positions of the
structural
components illustrated in Figures 1, 2 and 3.
The present invention achieves the foregoing objects in a preferred
embodiment by employment of a unique primary ground penetrating downwardly
tapered mandrel, generally designated 20 (Figure 1), which is typically about
10 to
about 20 feet long and has a longitudinal axis 100. Primary mandrel 20 is
often
octagonal in cross-section and continuously tapers inwardly with a taper angle
of about
1.0 to about 5.0 degrees from its upper end surface 24 to its lower end 22
terminating
in a blunt bottom plate 23. Upper end surface 24 of primary mandrel 20 is
preferably
about 12 to about 30 inches in maximum width and blunt bottom plate 23 has a
maximum width of preferably about 4 to about 10 inches. A drive and support
plate 60
has its lower portion fixedly mounted in primary mandrel 20 and is supported
at its
upper end by a conventional pile driving rig generally designated 26 (Figure
9), which
applies both a downward static force and vertical vibratory force for
effecting
penetration of the earth by the mandrel 20 to form a unique cavity having
stable
sidewalls in which an aggregate pier is subsequently constructed. Alternately,
a
downward impact hammer may be used to achieve penetration.
In its preferred form, the main component of primary mandrel 20 is a rigid
steel
plate shell having a lower half-shell steel plate component 28 and an upper
half-shell
steel plate component 30. The lower half-shell component 28 is formed of a
first
quarter-shell component generally designated 28(a) and a second quarter-shell
component generally designated 28(b) (Figures 5 and 5(b)). The upper half-
shell
component 30 is similarly formed of upper quarter-shell components 30(a) and
30(b)
(Figures 6(a) and 6(b)). Half-shell components 28 and 30 are octagonal in
cross-
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section, are coaxially positioned and are joined and welded together at
juncture plane
52 (Figure 1).
Lower quarter-shell component 28(a) is formed with four upwardly and
outwardly flaring planar panels A, B, C and D, and lower quarter-shell
components
28(b) are formed in like manner with upwardly and outwardly flaring panels E,
F, G and
H (Figure 4). The lower quarter-shell components 28(a) and 28(b) are of
identical
construction and are formed of two respective steel plates each of which is
bent by
conventional bending apparatus at bend areas 131, B2 and B3 in quarter-shell
28(a)
to form panels A, B, C and D and at bend areas B4, B5 and B6 in quarter-shell
28(b)
to form panels E, F, G and H as shown in Figures 4 and 5. The lower quarter-
shell
component 28(a) has linear side surfaces 41 which face and are welded to
linear side
surfaces 42 of lower quarter shell component 28(b). Lower quarter-shell
components
28(a) and 28(b) are identical mirror images of each other as shown in Figure 5
and the
resultant lower half-shell 28 is of octagonal transverse cross-section.
The upper quarter-shell components 30(a) and 30(b) are identical mirror images
of each other and are similarly formed from two sheets of steel plate by
conventional
bending procedures so that they are octagonal in transverse cross-section when
assembled together to form upper half-shell 30. Upper half-shell component
30(a)
includes upwardly and outwardly flaring panels A', B', C' and D' and upper
half-shell
component 30(b) includes upwardly and outwardly flaring panels E', F', G' and
H'
(Figure 3). The panels A' through H' of upper half-shell 30 are tapered at the
same
angle from axis 100 as panels A through H of lower half-shell 28. Panels A'
through
H' also have their lower ends respectively aligned with the upper ends of
corresponding panels A through H of the lower half-shell component 28. The
upper
end surface 50 (Figure 6) of the lower half-shell 28 faces, but does not
engage, the
lower end surface 79 of the upper half-shell 30. All of the panels A, A', etc.
are
oriented at a taper angle of about 1.0 to about 5.0 degrees relative to axis
100 of the
primary mandrel with the amount of taper depending upon the type of soil in
which the
mandrel is intended for use.
Assembly of the preferred embodiment can begin with the fabrication of lower
half-shell 28 by connection of the lower quarter-shell components 28(a) and
28(b) to
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CA 02551216 2010-10-29
form the lower half-shell component 28. Such assembly begins with positioning
of the
lower mid-bulkhead juncture plane 53 in the upper end of the lower quarter-
shell 28(a)
with its upper surface 54 above the upper end surface 50 of lower quarter-
shell 28(a)
where it is held in the position shown in Figure 5(b) by welding WL (Figure
6).
Typically, the upper surface 54 is approximately 0.5 inches above surface 50.
The
other lower quarter-shell component 28(b) is then positioned in alignment with
the
lower quarter-shell component 28(a) with surfaces 41 and 42 being in facing
contact.
Facing surfaces 41 and 42 are then welded together. Blunt bottom plate 23 is
then
welded on the lower end of lower half-shell component 28. Lower half-shell
component
28 is then ready for connection to the upper half-shell component 30.
Upper half-shell 30 can be assembled in a similar manner as lower half-shell
28 with the initial step being welding of upper mid-bulkhead juncture plate 77
to the
inner surface of the lower end of the upper quarter-shell 30(b) by welding WH
so that
the bottom surface 78 of upper mid-bulkhead juncture plate 77 is positioned
below
lower end surface 79 of upper half-shell 30. Again, the bottom surface 78 is
typically
positioned about 0.5 inches below surface 79. The upper shell components 30(a)
and
30(b) are then positioned in facing relationship with their longitudinal edges
43 and 44
in facing contact where they are welded together to complete upper half-shell
30 which
is then ready for welding to lower half-shell 28.
Connection of the half-shells 28 and 30 begins with positioning of the upper
end
of the lower half-shell 28 in alignment with the lower end of the upper half-
shell 30 and
with the upper surface 54 of plate 53 being in face-to-face contact with the
lower face
79 of juncture plate 77 as shown in Figure 6. A circular weld W is effected in
the
peripheral groove surrounding the outer surfaces of bulkhead juncture plates
53 and
77 between surfaces 50 and 79 to complete the strong connection of the upper
half-
shell 30 and the lower half-shell 28. Welding of the juncture plates together
is made
possible because the upper surface 54 of lower juncture plate 53 is positioned
above
upper surface 54 of half-shell 28 and the lower surface 79 of upper mid-
bulkhead
juncture plate 77 is below lower end surface 79 of upper half-shell 30. The
vertical
spacing between surfaces 54 and 79 provides the peripheral groove, preferably
about
one inch, in which welding W is provided, as shown in Figure 6, to bond
juncture plate
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53 and juncture plate 77 as well as the lower end 79, upper half-shell 30, the
upper
end 50 and lower half-shell 28 into a unitary rigid structure.
Drive and support plate 60 (Figure 18) is preferably about 1.5 inches thick
and
about 48 inches long. Drive and support plate 60 has parallel vertical upper
side edges
extending downwardly from its upper end 60U to termination line 63' aligned
with
upper end surface 24 of half-shell 30. Lower inwardly tapering edge surfaces
60T
extend downwardly below line 63' and are machined to provide planar contact
with the
inner surface of half-shell 30 in a face-to-face relationship with panels D'
and H', which
enables welding of portions 60T to such inner surfaces as shown in Figure 2.
The
upper end 60U of drive and support plate 60 is preferably positioned about 18
inches
above the upper end surface 63 of upper half-shell 30, and the lower end 60L
is
preferably about 30 inches below upper end surface 63.
Additionally, bracing for vertical drive and support plate 60 is provided by
horizontal rear brace plate 64 having peripheral surfaces 81, 82, 83, 84, 85
and 66
(Figure 16) and horizontal front brace plate 68 having peripheral surfaces 91,
92, 93,
94, 95 and 69 (Figure 17). Plates 60, 64 and 68 are all preferably formed of
1.5 inch
steel plate. Brace plates 64 and 68 are perpendicular to plate 60 and are
preferably
positioned about 4 inches below upper end surface 63. Front surface 66 of
brace plate
64 engages and is welded to rear face 61 of drive and support plate 60, and
rear face
69 of brace plate 68 engages and is welded to front surface 60F of drive and
support
plate 60.
Side surfaces 81, 82, 83, 84 and 85 of brace plate 64 are machined to engage
the inner surfaces of the half-shell 30 in a face-to-face manner. Similarly,
brace plate
68 has surfaces 91, 92, 93, 94 and 95 which engage the upper half-shell 30 in
a face-
to-face manner. All of the contacting surfaces of brace plates 64 and 68 are
welded
to the half-shell 30 surfaces which they contact. Additional bracing for drive
and
support plate 60 is provided by a rear center plate 74 having a front surface
welded
to the rear surface 61 of drive and support plate 60, a lower surface welded
to the front
surface of plate 64 and a rear vertical surface welded to the inner surface of
panel B'.
Similarly, a forward vertical brace plate 70 is welded to the inner surface of
panel F',
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the upper surface of front brace plate 68 and front surface 60F of drive and
support
plate 60.
In use, primary mandrel 20 is lifted by cable hooks in ear brackets 78 and 80
welded to upper half-shell 30 so that drive and support plate 60 is vertically
positioned
and securely held between clamping means C and Cof conventional pile driving
rig
26 (Figure 9). Rig 26 is capable of applying downward direct constant static
force
and/or vibratory force provided by either a vibratory piling hammer or
hydraulic impact
hammer to drive and support plate 60. Primary mandrel 20 is consequently
prepared
to be driven vertically downwardly into the ground to form a cavity in which
an
aggregate pier is to be constructed. The supporting rig 26 provides both
static and
vibratory pressure or impact force downwardly on drive and support plate 60 to
effect
full length movement of the mandrel downwardly into the earth E to form a
cavity C as
shown in Figure 10.
Movement of primary mandrel 20 from the surface to the Figure 10 position
results in a combination of radial and vertical forces exerted against the
surrounding
earth to compact the cavity wall CW. This compaction serves to increase the
structural
integrity of the surrounding earth sufficiently to preclude wall collapse or
other failures
during subsequent operations in forming a pier in the cavity C.
Once the cavity C is formed, the primary mandrel 20 is partially or fully
withdrawn to the upper end of the cavity as shown in Figure 11, and a quantity
of loose
aggregate A is deposited into the bottom end of the cavity as shown in Figure
11.
Primary mandrel 20 is then reintroduced into the cavity and downward static
and
vibratory or impact forces are applied to the drive and support plate 60 so
that the
blunt bottom plate 23 on the lower end 23 of the mandrel engages and
compresses
the previously deposited aggregate as shown in Figure 12. Operation of the
blunt
bottom plate 23 on the lower end of primary mandrel 20 consequently densifies
the
aggregate vertically providing for the construction of a strong and stiff pier
and the
tapered mandrel creates radial outward forces which act on the aggregate to
push it
into the surrounding sidewalls of the cavity and further compact the
surrounding earth
to densify the soil surrounding the pier to provide additional strength.
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The foregoing steps are repeated with deposit of additional layers of
aggregate
followed by subsequent densification of each layer by primary mandrel 20. When
the
top of the aggregate is near the upper portion of the pier as shown in Figure
13 the
optional larger diameter short length secondary tamping mandrel 20' of Figure
7, which
is powered by either an impact hydraulic hammer or a vibratory hammer, may
optionally be employed for tamping and compressing the upper aggregate portion
to
complete formation of the pier. Large diameter tamping mandrel 20' has a lower
end
plate 23' which is preferably at least 75% of the diameter of the top of the
pier being
formed and is consequently substantially larger than blunt bottom plate 23 of
the
primary mandrel 20. Tamping mandrel 20' is supported by its drive and support
plate
60' which is clamped in position on pile driving rig 26 which applies vertical
static and
vibratory force to plate 60' for densifying the aggregate in the upper 3 to 5
feet of the
cavity previously formed with primary mandrel 20. Alternatively, a secondary
rig with
an impact hammer may be used to power the secondary mandrel.
Figure 30 illustrates another alternative secondary tamping mandrel 360 having
a hollow shell, a smaller diameter bottom guide portion 362 and a top
cylindrical
portion 364 having a diameter exceeding the diameter of the upper end of
primary
mandrel 20. Smaller diameter portion 362 is connected to top portion 364 by an
outwardly flared canted portion 366. The small diameter lower portion 362 has
a
transverse smaller lower end surface 365. The diameter of portion 362 is
approximately the same as the diameter of the top of the cavity formed by the
upper
end of primary mandrel 20 which is shown by the dashed lines extending
downwardly
below mandrel 360.
Figure 31 illustrates a further secondary mandrel 370 having a conical surface
372 facing downwardly to engage the upper end of a previously formed cavity
illustrated by the dashed lines in Figure 31. This shape is advantageous in
that it forms
a larger diameter top-of-pier shape so as to provide resistance to soil heave
and also
provides increased confinement.
Secondary tamping mandrels 360 and 370 are used in the same manner as
secondary tamping mandrel 20' as described above to form the top of the cavity
in
accordance with their specific shapes when such shapes conform with the
structural
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requirements of particular piers to be constructed. If desired, telltales
comprised of flat
steel plates embedded in lower portions of piers and connected to upwardly
extending
steel bars which extend upwardly to the surface can be installed to provide an
indication of any movement or bulging of the piers. Typically, the steel
plates are
installed on the bottom of the cavity and the bars extend either within the
cavity or
along the sidewalls of the cavity to the ground surface. Any movement of such
steel
plates will consequently result in observable displacement of the upper end of
one or
more of the steel bars so as to provide notice of bulging or other pier
movement.
If desired, uplift anchors comprised of flat steel plates embedded in lower
positions of the pier and connected to upwardly extending steel bars which
extend
upwardly to the surface can be installed to resist uplift loads.
A second embodiment of the present invention is illustrated in Figures 14 and
and is directed to a primary mandrel generally designated 220. Mandrel 220 is
identical to the first embodiment mandrel 20, but differs by the additional
inclusion of
15 a concrete injection pipe 222 extending axially along the mandrel's length
and having
a sacrificial pop-off cap 224 at its lower end. In use, the mandrel 220 is
employed for
forming concrete foundations and similar structures. Construction of such
foundations
is effected by driving the mandrel 220 to the desired depth. Concrete or grout
is then
forced downwardly through injection pipe 222 to initially force the
sacrificial cap 224
from the lower end of the mandrel and inject the concrete or grout. The
concrete or
grout is forced into the sidewalls of the cavity so as to increase load
bearing capacity.
The mandrel 220 is then slowly withdrawn from the cavity while continuing to
inject
concrete or grout until the mandrel is fully retracted. Additionally, the
mandrel can then
be reinserted to force the concrete further into the sidewalls of the cavity
so as to
increase load capacity.
Referring, therefore, to Figures 32 through 39, there is illustrated two
typical
examples of implementation of the soil densification and stiffened pier
forming
procedures of the present invention.
As depicted in Figure 32, a passage or cavity having a cavity wall CW is
formed
lo in the earth by statically pushing, while optionally vibrating, a tapered
probe 420
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having an axial passageway 421 of sufficient size to permit the flow of
aggregate into
the soil matrix 422.
Upon completion of the cavity, the single stage method of forming the pier is
begun by completely withdrawing probe or mandrel 420 from cavity 400 and
raising
it to the ground level or near ground level as shown in Figure 33. The upper
end of
probe or mandrel 420 can be supplied with aggregate and/or cementitious grout
by
means such as disclosed in patent application Ser. No. 10/728,405 of co-
inventor
Nathaniel S. Fox or by different conventional means. Aggregate 430 or
aggregate with
cementitious grout is then discharged down through probe or mandrel 420 to
completely fill cavity 400. The aggregate is discharged typically from the
bottom of
probe 420 through a clam valve, a sliding valve or other type of conventional
mechanical opening device as the probe is raised. Another alternative is for
the bottom
of the probe to remain open without a valve. A further option is to discharge
aggregate
by means of a plunger apparatus in the probe where a preset volume of
aggregate is
discharged by pushing the plunger separately relative to the probe.
The probe apparatus is then re-introduced into the aggregate-filled cavity,
and
has displaced the aggregate laterally into the soil adjacent to the cavity as
shown in
Figure 34.
The probe apparatus may be withdrawn from the cavity and aggregate
deposited to fill the void created by removal of the probe. The probe
withdrawal,
aggregate deposit and probe reintroduction steps may be repeated a plurality
of times
to create a larger effective pier diameter and greater soil densification of
granular soils
resulting in the outwardly bulging configuration as shown in Figure 35.
The multitude stage method of forming a pier, passage or cavity having a
cavity
wall is formed by pushing and optionally vibrating a tapered probe 420 into
the ground
in the manner illustrated in Figure 32. Probe 420 is then partially raised
while
discharging aggregate or aggregate and cementitious grout 431 only into the
bottom
portion of the cavity as illustrated in Figure 36.
The probe is then re-introduced into the aggregate in the bottom end portion
of
the cavity to compact the aggregate and displace a portion of the aggregate
and
surrounding soil to form bulges as shown in Figure 37 extending into the
adjacent soil.
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Removal of the probe upwardly from the Figure 37 position results in a void in
the
space previously occupied by the probe. The next deposit fills in the void and
a portion
of the cavity above the prior-created upper surface of aggregate. The
aggregate
deposits and compaction are then repeated a plurality of times in like manner
to
provide completed pier 450 as illustrated in Figure 39.
It is also possible to use the mandrel 220 to effect compaction grouting below
the bottom of the mandrel. In this method, the mandrel is advanced to the
design tip
elevation and low-slump grout is pumped at high pressure from pipe 222. The
compaction grout bulb is used to strengthen and stabilize soil at the tip of
the mandrel.
The presence of the mandrel during compaction grouting operation also provides
confinement for the grouting operation. After grouting, conventional concrete
or grout
may be pumped through the pipe to fill the cavity as the mandrel is extracted,
or the
cavity may be filled with aggregate in the manner described above.
Figure 23 illustrates a modified mandrel 200, which is similar to mandrel 20,
but
is provided with an optional peripheral flange 202 at its upper end. Flange
202 is
circular and extends completely around the top of the mandrel. It thus acts to
inhibit
upward movement of surficial soil during mandrel penetration to the fully
embedded
position shown in Figure 23. During manual penetration of mandrels not having
a
radial flange, the surficial soil may be displaced laterally and may also
heave upwardly.
Such lateral displacement and upward heaving is a particularly acute problem
with
cohesive soils. During penetration, the radial flange engages the heaving soil
and
forces it downwardly so as to compact the soil and provide additional
confinement to
the upper portions of the tapered mandrel shaft so as to reduce or eliminate
heaving.
Flange 202 also acts to provide a larger cavity at the top of the pier which
can
be filled with aggregate to create a larger top-of-pier diameter which is cost
advantageous when the pier is to support thin building floor slabs. Such cost
benefits
result from reducing the floor slab span between piers so that the
construction costs
of the slab can be reduced. While an alternative for reducing the pier-to-pier
floor slab
span would be to make the entire length of the pier of greater diameter from
top to
bottom, such procedure would be much more costly than having a top-of-pier
large
diameter portion.
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Figure 24 illustrates a further mandrel embodiment 208 formed with a tapered
lower section 280 and a straight-sided untapered upper section 300. The
straight/tapered mandrel 208 is advantageous in the stabilization of soil
profiles that
consist of cohesive soils in the upper portion of the profile and granular
soils in the
lower portion of the profile. The tapered bottom section of the mandrel is
advantageous for keeping the granular soils stabilized during construction.
However,
the tapered shape is not needed for stability of the upper level cohesive
soils. An
advantage of the straight-sided section at the top of the mandrel is that a
fairly narrow
cavity may be constructed through the cohesive soils thus reducing the amount
of
energy required for installation relative to the amount of energy required by
a mandrel
that is tapered from bottom to top.
Figure 25 illustrates a mandrel 350 similar to the mandrel of Figure 1, but
which
has been modified to include a hollow core extending axially along the length
of the
mandrel with a perforated pipe 352 being loosely positioned within the core.
The lower
end of pipe 352 is connected to a bottom plate 354 that covers the annulus of
the
bottom of the mandrel.
The first step in the use of mandrel 350 is insertion of the mandrel into the
earth
to the position shown in Figure 26. Mandrel 350 is then lifted upwardly to an
elevated
position as shown in Figure 27; however, perforated pipe 352 is not lifted
upwardly
with mandrel 350 but remains in the cavity. Aggregate A is deposited in the
lower end
of the cavity and the mandrel 352 is then re-inserted downwardly to compact
the
aggregate as shown in Figure 28. Sequential depositing of aggregate and
compaction
are continued until the aggregate fills the pier as shown in Figure 29 with
the
perforated pipe remaining in the aggregate that has previously been densified
by the
mandrel. The pier may then be post-grouted by connecting the top of the pipe
to a
grout hose 356 into which grout is pumped to flow downwardly through pipe 352
and
exit from the perforations 357 in the lower end of the pipe. In this way,
specific areas
of the pier may be post-grouted quickly and efficiently. Such post-grouting is
particularly advantageous for soils such as peat that are susceptible to pier
bulging
when placed under load. It should be understood that in all instances where
grout is
used, the grout may be enhanced by the addition of additives and agents such
as
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chemicals or fillers, recycled concrete or slag for strengthening,
accelerators for
controlling the rate at which solidification will occur or other materials
deemed
desirable for a particular project.
An alternate method of construction is illustrated in Figures 32 to 39. The
tapered probe or mandrel assembly is pushed into the ground to enable
simultaneous
densification and improvement of soil adjacent the cavity or passage to permit
creation
of a stiffened pier or pile within the passage in the densified soil. The
alternate process
contemplates discharge of aggregate or aggregate with cementitious grout into
the
cavity formed as the probe is raised from the bottom of the formed cavity and
then
pushing the probe back into the aggregate-filled (or aggregate-with-grout-
filled)
passage to densify and displace the aggregate into the adjacent soil. This
process
may be performed as a single stage process, wherein the probe is raised the
full
length of the cavity and then re-introduced into aggregate that has been
discharged
into the cavity, or it may be performed as a multiple stage process, wherein
the probe
is raised only a portion of the cavity length, and then re-introduced and
pushed into the
aggregate to compact the aggregate and displace it into the adjacent soil in a
plurality
of steps. Aggregate may be discharged from the bottom of the probe from an
opening
at the bottom created by a clam-valve apparatus, a sliding valve, or other
mechanical
or hydraulic means of opening and then closing the bottom of the probe
apparatus. An
alternative is to leave the opening of the bottom of the probe open with no
closing and
opening valves. Aggregate may also be discharged by being injected into the
cavity
by a plunger-type apparatus which would essentially dictate the volume of
aggregate
being discharged.
For all of the embodiments described above, the aggregate may be aggregate
of various size ranges, may be aggregate alone or may be aggregate with the
addition
of a cementitious grout. The grout may include numerous additives and agents
such
as chemicals or fillers for strengthening, accelerators for controlling the
rate at which
the fluid material will solidify and other additives.
For all of the embodiments described above, the bottom of the tapered probe
may be flat, or it may be flat with beveled sides with a taper greater than
the taper of
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the probe sides, or it may have another shape such as conical or convex semi-
spherical.
Field tests reflected in Figures 19, 20, 21 and 22 indicate the stiffness of
the
pier when load-tested and indicate the increase in soil density that is
achieved by pier
construction. More specifically, Figure 19 is a graphic illustration of stress
applied to
and resultant deflection of test piers "A", "B" and "C" which were
respectively
constructed by specific different, but similar, construction procedures.
Specifically, test pier "A" was constructed by using a single blunt-ended
tapered
primary mandrel 20 having a taper angle of 5 degrees to form the cavity and
then to
densify all of the aggregate forming the entire pier up to the ground surface
(grade).
This means that all of the aggregate in the entire pier was compacted using
the blunt
bottom plate 23 that has a small cross-sectional area compared to the cross-
sectional
area of the top pier and mandrel portions. The mandrel was driven downwardly
by
constant static pressure and concurrent vertical vibration supplied by a
vibratory piling
hammer using rotating weights driven at approximately 2,400 revolutions per
minute
to create vertical high frequency (up and down) vibratory energy applied to
compact
and densify each lift of aggregate.
Test pier "B" was constructed using the same drive means used for pier "A" to
drive blunt-ended tapered primary mandrel 20 to form a cavity and densify
aggregate
from the bottom of the cavity up to a position approximately four (4) feet
below the
surface of the earth. The remaining portions of the pier above the four (4)
foot depth
were constructed upwardly to the surface of the earth using a widened blunt-
end
tamping mandrel 20' of Figure 7 which was driven by static force and the same
vibratory piling hammer used for pier "A". The tamping mandrel 20' had a cross-
sectional area approximating the cross-sectional area of the top of the pier
which is
substantially greater in area than the blunt bottom plate 23 of tapered
primary mandrel
20.
Test pier "C" was constructed using the blunt-end tapered primary mandrel 20
to form a cavity and densify aggregate upward to a location four (4) feet
below grade
in the same manner as pier "B". However, the upper pier portion extending
upwardly
from the position four (4) feet below grade was constructed using a
conventional
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beveled tamper such as tamper 10 disclosed in U.S. Pat. No. 5,249,892. The
beveled
tamper was driven by a conventional hydraulic impact hammer applying
relatively low
frequency blows at approximately 500 blows per minute applied concurrently
with
static downward pressure. The conventional hydraulic impact hammer was part of
excavation-mounted rig 26 and employed a ram lifted hydraulically and then
smashed
downwardly internally on a striker plate to drive the beveled tamper
downwardly.
Figure 19 illustrates the results of load tests of piers "A", "B" and "C"
which were
each tested by placing a concrete cap over the full diameter of the pier at
ground level.
Loads were applied to the pier by pushing down on the concrete caps. The
stress
applied to the pier was calculated by dividing the applied load in pounds by
cross-
sectional area of the top of the pier in square feet. Readings TOG reflect
deflection
readings taken at the tops of the piers and readings TT reflect below grade
telltale
deflection for each of the three piers.
The construction procedures used in forming pier "A" resulted in a pier with
excellent load carrying capacity and stiffness (Figure 20). The improved
results flow
from the unique construction procedures which resulted in significantly
strengthening
and stiffening of the matrix soil in which the piers were constructed and from
the blunt
end of the primary mandrel used to achieve compaction.
Pier "B" was constructed by use of the wider tamping mandrel 20' to compact
the top portion of the pier and the strength and stiffness of the pier was
somewhat
better than for pier "A". Such strength increase is demonstrated by Figure 19
in which
equivalent deflections for test piers "A" and "B" reveal that test pier "B"
allows for
greater applied stresses at the same deflection level. This means that test
pier "B" can
support greater loads than test pier "A". In other words, fewer "B" piers than
"A" piers
could be used to support a given load while achieving the same performance.
Alternatively, "B" piers will result in less settlement than "A" piers at the
equivalent
applied stress.
The procedures used in constructing test pier "C" resulted in the construction
of a pier having even greater strength and stiffness than piers "A" and "B".
The plots of Figure 21 reveal that SPT-N values in the soil at various
distances
from the piers constructed in accordance with the present invention were
enhanced
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by the forces exerted on the matrix soils during installation of the piers.
The Standard
Penetration Tests were performed within soil borings by driving a two-inch
outside
diameter steel tube (called a "spoon") 18 inches into the ground using a 140
pound
hammer with a 30 inch drop. The number of driving blows for each six-inch
increment
are counted, and the N-value is the sum of the last two recordings (or the
number of
blows required to drive the last 12 inches of the spoon). Low N-values
indicate weak
and soft soil. High N-values indicate strong and dense soil. The plot shown in
Figure
21 reveals that increased N-values are found near the installed piers and that
the
installation increases the density of matrix soils (existing soils in place
prior to pier
installation) which results in an increase in penetration resistance (N-value)
and soil
stability. These results are significant because they show that the pier
installations, not
only result in strong and stiff piers, but also they improve the ground around
the piers
so as to enhance their function of limiting settlement below structures
supported by the
piers.
Figure 22 comprises a plot of improvement ratios to depth. The improvement
ratio is a ratio of SPT-N values measured after the piers are installed to the
SPT-N
values of the matrix soil before the piers are installed. The higher the
improvement
ratio, the greater the positive effect of the pier installation on the soils
being treated.
This plot clearly shows improvement ratios exceeding 1.0 which evidence the
beneficial effects of pier installation on the matrix soil which adds to the
pier's
effectiveness at reducing the magnitude of pier settlement.
The above described apparatus and methods provide a number of advantages.
One such advantage is enhanced stability of the sidewalls of the cavity after
the
mandrel penetration forming the cavity. Unlike previous methods of
construction of
stone columns, the continuously tapered mandrel provides stability in both
stable soil
and soil that is otherwise susceptible to collapse. It is consequently
possible for a
simple, fast and economical introduction of aggregate into the cavity to be
accomplished immediately after the mandrel is withdrawn.
A further advantage of the cavity sidewall having enhanced stability is that
it
permits the efficient inspection of the cavity and the placement of the stone
as
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compared to prior art procedures in which the cavity wall and the lower end of
the
cavity are not visible due to the need for wall retaining means.
Another advantage of the present invention resides in the fact that the
enhanced stability of the sidewalls permits installation of telltales with
load test piers.
Such telltales are an important part of load testing because they provide pier
installers
with the ability to ascertain deformations at both the top and bottom of the
pier during
testing.
A further advantage of the enhanced stability of the sidewalls is that it
permits
the installation of uplift anchors at the bottom of the piers. Such anchors
are used as
permanent tie-downs for a variety of structures. The previously known
procedures do
not facilitate the installation of such uplift anchors.
Yet another advantage of the enhanced sidewall stability provided by the
present invention is that it permits the introduction of large aggregate and
heterogeneous durable angular materials within the pier. Pier backfill may
consist of
cobbles, large stone, bricks, recycled concrete columns, soil stabilized with
admixtures
and other types of durable backfill. Portions of the pier maybe filled with
low-slump
concrete, and the backfill materials are not limited to the shape of a pipe
used to feed
the backfill to the bottom of the cavity.
The continuously tapered shape of the cavity is the optimal shape for
achieving
resistance to pier loads that would otherwise cause the piers to bulge
outwardly and
collapse. This is true because conventional cylindrical stone columns are most
susceptible to bulging at the tops of the columns where the confining stresses
of the
surrounding cavity wall are lowest. At greater depths, confining stresses are
higher so
as to inhibit the propensity of the columns to bulge. The construction of the
pier with
the largest cross-sectional area at the top and the smallest cross-sectional
area at the
bottom, as provided by the present invention, results in a column with the
greatest
resistance to bulging at the top and least resistance to bulging at the
bottom. The
resistance profile, combined with the matrix soil confining stress profile,
allows the pier
to have a uniform resistance to bulging with depth thus optimizing the volume
of
aggregate used in construction.
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CA 02551216 2010-10-29
The shape of the blunt-bottom mandrel also provides a more efficient means
for compacting the aggregate in the portions of the pier. Such effectiveness
of
compaction is much greater than for the prior known mandrels having small or
pointed
lower ends. The resultant pier construction will consequently have greater
vertical load
support capability.
The use of vertical vibration or impact energy is much more effective than
conventional horizontally applied vibration energy for compacting aggregate in
the pier.
Vertically applied energy increases the density of the aggregate and increases
the
load carrying capacity of the pier in comparison to stone columns constructed
by prior
known conventional methods.
The vertical vibration energy applied to the mandrel also increases the
density
of matrix granular soil and densities the surrounding soil during installation
and also
during construction of the pier. The densification of the matrix soil during
initial
penetration and during subsequent densification of aggregate lifts the load
carrying
capacity of aggregate piers and increases the stiffness of the matrix soil
surrounding
the pier. This increased matrix soil stiffness increases support capability of
the pier.
The increase in soil density is shown by the increase in post-installation
Standard
Penetration Test N-Values for soil sampled between, adjacent to and far away
from
the installed pier.
The vertically applied energy develops greater penetration capability than
conventional vibration with horizontal oscillators.
The optional use of the larger, secondary mandrel for compaction at the top of
the cavity provides for a great increase in the stiffness of the pier in
comparison to
densifying the entire pier with the tapered conical mandrel used to create the
cavity.
The installation process also allows for an efficient means of installing
concrete
foundation elements, and also allows the further densification of the concrete
by
pushing the mandrel back down into the grout/concrete filled cavity.
It is also possible to form piers by the inventive method which may serve as
drainage elements in cohesive soils if open-graded aggregate is used in the
cavity.
The great ease in placing aggregate in the cavity allows for ease in changing
the type
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CA 02551216 2010-10-29
of aggregate used at various depths of the pier so as to permit optimization
of the
drainage and filtration features of the aggregate.
Another advantage of the tapered sides is to ease the force necessary to raise
the probe and reduce the possibility of the probe becoming "stuck" in the
ground.
Quality control is enhanced because a measured amount of stone is applied to
each lift. A method of continuously measuring aggregate quantity usage in pier
using
sensors to measure and a computer to record elevation of top of aggregate pile
is
possible.
Another advantage is that great flexibility in installation procedures is
enabled
by altering the number of repetitions that are made of raising with
discharging of
aggregate and pushing the probe back into the aggregate to densify and pre-
stress the
adjacent soil following which repeating the procedure at the same approximate
elevation by raising and discharging aggregate into the cavity formed and
pushing the
probe back into the aggregate enables a pier of greater the effective
diameter, greater
the lateral soil stressing especially in granular soils and the greater the
densification
of adjacent soil.
Use of the tapered mandrel also results in a significant change to the in-site
stress field surrounding the pier. Advanced numerical analyses indicate that
the
vertical stresses in the matrix soil are also increased by approximately 10
percent
during mandrel penetration allowing for further compaction of the soil. These
stress
field changes are significant for two reasons. First, in fine-grain cohesive
soil, the
cavity expansion results in the formation of radial tension cracks in the soil
surrounding
the pier. These cracks serve as drainage galleries, increasing the composite
permeability of the matrix soil. Secondly, in granular soil, the increase in
vertical stress
allows for a densification of the soil immediately surrounding the mandrel.
This
densification is a process that provides for enhanced cavity stability during
mandrel
lifting, even in soil subject to caving.
Modifications and variations of the above-described embodiments of the
present invention are possible by those skilled in the art in light of the
above teachings.
For example, the mandrel could be formed using only two half-shells, each of
which
would extend from the lower end to the upper end of the mandrel. Also, it
would be
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CA 02551216 2010-10-29
possible to provide a mandrel having a cross-section other than octagonal;
however,
the octagonal cross-section may be superior in terms of fabrication costs and
operational efficiency. It is therefore to be understood that, within the
scope of the
appended claims and their equivalents, the invention may be practiced
otherwise than
as specifically described and the scope of the claims defines the invention
coverage.
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