Note: Descriptions are shown in the official language in which they were submitted.
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GEOCELL FOR LOAD SUPPORT APPLICATIONS
BACKGROUND
[0001] The present disclosure relates to a cellular confinement system, also
known
as a CCS or a geocell, which is suitable for use in supporting loads, such as
those
present on roads, railways, parking areas, and pavements. In particular, the
geocells of
the present disclosure retain their dimensions after large numbers of load
cycles and
temperature cycles; thus the required confinement of the infill is retained
throughout the
design life cycle of the geocell.
[0002] A cellular confinement system (CCS) is an array of containment cells
resembling a "honeycomb" structure that is filled with granular infill, which
can be
cohesionless soil, sand, gravel, ballast, crushed stone, or any other type of
granular
aggregate. Also known as geocells, CCSs are mainly used in civil engineering
applications that require little mechanical strength and stiffness, such as
slope
protection (to prevent erosion) or providing lateral support for slopes.
[0003] CCSs differ from other geosynthetics such as geogrids or geotextiles in
that
geogrids / geotextiles are flat (i.e., two-dimensional) and used as planar
reinforcement.
Geogrids / geotextiles provide confinement only for very limited vertical
distances
usually 1-2 times the average size of the granular material) and are limited
to granular
materials having an average size of greater than about 20 mm. This limits the
use of
such two-dimensional geosynthetics to relatively expensive granular materials
(ballast,
crushed stone and gravel) because they provide hardly any confinement or
reinforcement to lower quality granular materials, such as recycled asphalt,
crushed
concrete, fly ash and quarry waste. In contrast, CCSs are three-dimensional
structures
that provide confinement in all directions (i.e. along the entire cross-
section of each
cell). Moreover, the multi-cell geometry provides passive resistance that
increases the
bearing capacity. Unlike two-dimensional geosynthetics, a geocell provides
confinement and reinforcement to granular materials having an average particle
size
less than about 20 mm, and in some cases materials having an average particle
size of
about 10 mm or less.
[0004] Geocells are manufactured by some companies worldwide, including
Presto.
Presto's geocells, as well as those of most of its imitators, are made of
polyethylene
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(PE). The polyethylene (PE) can be high density polyethylene (HDPE) or medium
density polyethylene (MDPE). The term HDPE refers hereinafter to a
polyethylene
characterized by density of greater than 0.940 g/cm3. The term medium density
polyethylene (MDPE) refers to a polyethylene characterized by density of
greater than
0.925 g/cm3 to 0.940 g/cm3. The term low density polyethylene (LDPE) refers to
a
polyethylene characterized by density of 0.91 to 0.925 g/cm3.
[0005] Geocells made from HDPE and MDPE are either smooth or texturized.
Texturized geocells are most common in the market, since the texture may
provide
some additional friction of the geocell walls with the infill. Although HDPE
theoretically
can have a tensile strength (tensile stress at yield or at break) of greater
than 15
megapascals (MPa), in practice, when a sample is taken from a geocell wall and
tested
according to ASTM D638, the strength is insufficient for load support
applications, such
as roads and railways, and even at a high strain rate of 150%/minute, will
barely reach
14 MPa.
[0006] The poor properties of HDPE and MDPE are clearly visible when analyzed
by
Dynamic Mechanical Analysis (DMA) according to ASTM D4065: the storage modulus
at 23 C is lower than about 400 MPa. The storage modulus deteriorates
dramatically
as temperature increases, and goes below useful levels at temperatures of
about 75 C,
thus limiting the usage as load support reinforcements. These moderate
mechanical
properties are sufficient for slope protection, but not for long term load
support
applications that are designed for service of more than five years.
[0007] Another method for predicting the long term, creep-related behavior of
polymers is the accelerated creep test by stepped isothermal method (SIM)
according to
ASTM 6992. In this method, a polymeric specimen is subjected to constant load
under
a stepped temperature program. The elevated temperature steps accelerate
creep.
The method enables extrapolation of the specimen's properties over long
periods of
time, even over 100 years. Usually, when PE and PP are tested, the load that
causes
plastic deformation of 10% is called the "long term design strength" and is
used in
geosynthetics as the allowed strength for designs. Loads that cause plastic
deformation
greater than 10% are avoided, because PE and PP are subject to second order
creep
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above 10% plastic deformation. Second order creep is unpredictable and PE and
PP
have a tendency to "craze" in this mode.
[0008] For applications such as roads, railroads and heavily loaded storage
and
parking yards, this strength of barely 14 MPa is insufficient. In particular,
geocells with
these moderate mechanical properties tend to have relatively low stiffness and
tend to
deform plastically at strains as low as 8%. The plastic deformation causes the
cell to
lose its confining potential, essentially the major reinforcement mechanism,
after short
periods of time or low numbers of vehicles passing (low number of cyclic
loads). For
example, when a strip taken from a typical geocell in the machine direction
(perpendicular to seam plane) is tested according to ASTM D638 at a strain
rate of 20
%/minute or even at 150 %/minute, the stress at 6% strain is less than 13 MPa,
at 8%
strain is less than 13.5 MPa, and at 12% strain is less than 14 MPa. As a
result, HDPE
geocells are limited to applications where the geocell is under low load and
where
confinement of load-bearing infill is not mandatory (e.g. in soil
stabilization). Geocells
are not widely accepted in load support applications, such as roads, railways,
parking
areas, or heavy container storage areas, due to the high tendency of plastic
deformation
at low strains.
[0009] When a vertical load is applied to a substrate of a granular material,
a portion
of that vertical load is translated to a horizontal load or pressure. The
magnitude of the
horizontal load is equal to the vertical load multiplied by the coefficient of
horizontal
earth pressure (also known as lateral earth pressure coefficient or LEPC) of
the
granular material. The LEPC can vary from about 0.2 for good materials like
gravel
and crushed stone (generally hard particles, poorly graded, so compaction is
very good
and plasticity is minimal) to about 0.3 to 0.4 for more plastic materials like
quarry waste
or recycled asphalt (materials that have a high fines content and high
plasticity). When
the granular material is wet (e.g. rain or flood saturating the base course
and sub-base
of a road), its plasticity increases, and higher horizontal loads are
developed, providing
increased hoop stress in the cell wall.
[0010] When the granular material is confined by a geocell, and a vertical
load is
applied from the top by a static or dynamic stress (such as pressure provided
by a
vehicle wheel or train rail), the horizontal pressure is translated to hoop
stress in the
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geocell wall. The hoop stress is proportional to the horizontal pressure and
to the
average cell radius, and is inversely proportional to the thickness of the
cell wall.
HS = VP*LEPC*r
d
wherein HS is the average hoop stress in the geocell wall, VP is the vertical
pressure
applied externally on the granular material by a load, LEPC is the lateral
earth pressure
coefficient, r is the average cell radius and d is the nominal cell wall
thickness.
[0011] For example, a geocell made of HDPE or MDPE having a cell wall
thickness
of 1.5 millimeters (including texture, and the term "wall thickness" referring
hereinafter to
the distance from peak to peak on the cell wall cross-section), an average
diameter
(when infilled with granular material) of 230 millimeters, a height of 200
millimeters, filled
with sand or quarry waste (a LEPC of 0.3), and a vertical load of 700
kilopascal (kPa),
would experience a hoop stress of about 16 megapascals (MPa). As seen from the
hoop stress equation, larger diameter or thinner walls - which are favored
from a
manufacturing economy point of view - are subjected to significantly higher
hoop
stresses, and thus do not operate well as reinforcement when made of HDPE or
MDPE.
[0012] Vertical loads of 550 kPa are common for unpaved roads. Significantly
higher
loads, of 700 kPa or more, may be experienced in roads (paved and unpaved) for
heavy
trucks, industrial service roads, or parking areas.
[0013] Because load support applications, especially roads and railways, are
generally subjected to millions of cyclic loads, the geocell wall needs to
retain its original
dimensions under cyclic loading with very low plastic deformation. Commercial
usage
of HDPE geocells is limited to non load-bearing applications because HDPE
typically
reaches its plastic limit at about 8% strain, and at stresses below typical
stresses
commonly found in load support applications.
[0014] It would be desirable to provide a geocell that has increased stiffness
and
strength, lower tendency to deform at elevated temperatures, better retention
of its
elasticity at temperatures above ambient (23 C), reduced tendency to undergo
plastic
deformation under repeated and continuous loadings, and/or long service
periods.
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BRIEF DESCRIPTION
[0015] Disclosed in embodiments are geocells which provide sufficient
stiffness and
can accept high stresses without plastic deformation. Such geocells are
suitable for
load support applications such as pavements, roads, railways, parking areas,
airport
runways, and storage areas. Methods for making and using such geocells are
also
disclosed.
[0016] In some embodiments is disclosed a geocell formed from polymeric
strips, at
least one polymeric strip having a storage modulus of 500 MPa or greater when
measured in the machine direction by Dynamic Mechanical Analysis (DMA)
according
to ASTM D4065 at 23 C and at a frequency of 1 Hz.
[0017] The at least one polymeric strip may have a storage modulus of 700 MPa
or
greater, including a storage modulus of 1000 MPa or greater.
[0018] The at least one polymeric strip may have a stress at 12% strain of
14.5 MPa
or greater when measured according to the Izhar procedure at 23 C, including a
stress
at 12% strain of 16 MPa or greater or a stress at 12% strain of 18 MPa or
greater.
[0019] The at least one polymeric strip may have a coefficient of thermal
expansion
of 120 x 10.6 / C or less at 25 C according to ASTM D696.
[0020] The geocell may be used in a layer of a pavement, road, railway, or
parking
area. The geocell can be filled with a granular material selected from the
group
consisting of sand, gravel, crushed stone, ballast, quarry waste, crushed
concrete,
recycled asphalt, crushed bricks, building debris and rubble, crushed glass,
power plant
ash, fly ash, coal ash, iron blast furnace slag, cement manufacturing slag,
steel slag,
and mixtures thereof.
[0021] In other embodiments is disclosed a geocell formed from polymeric
strips, at
least one polymeric strip having a storage modulus of 150 MPa or greater when
measured in the machine direction by Dynamic Mechanical Analysis (DMA)
according
to ASTM D4065 at 63 C and at a frequency of 1 Hz.
[0022] The at least one polymeric strip may have a storage modulus of 250 MPa
or
greater, including a storage modulus of 400 MPa or greater.
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[0023] In yet other embodiments is disclosed a geocell formed from polymeric
strips,
at least one polymeric strip having a long term design stress of 2.6 MPa or
greater,
when measured according to the PRS SIM procedure.
[0024] The at least one polymeric strip may have a long term design stress of
3 MPa
or greater, including a long term design stress of 4 MPa or greater.
[0025] These and other embodiments are described in more detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The following is a brief description of the drawings, which are
presented for
the purposes of illustrating the exemplary embodiments disclosed herein and
not for the
purposes of limiting the same.
[0027] FIG. 1 is a perspective view of a geocell.
[0028] FIG. 2 is a diagram showing an exemplary embodiment of a polymeric
strip
used in the geocells of the present disclosure.
[0029] FIG. 3 is a diagram showing another exemplary embodiment of a polymeric
strip used in the geocells of the present disclosure.
[0030] FIG. 4 is a diagram showing another exemplary embodiment of a polymeric
strip used in the geocells of the present disclosure.
[0031] FIG. 5 is a graph comparing the stress-strain results of various cells
of the
present disclosure against a comparative example.
[0032] FIG. 6 is a graph showing the stress-strain diagram for the geocells of
the
present disclosure.
[0033] FIG. 7 is a graph showing the results of a vertical load test for an
exemplary
cell of the present disclosure against a comparative example.
[0034] FIG. 8 is a graph of the storage modulus and Tan Delta versus
temperature
for a control strip.
[0035] FIG. 9 is a graph of the storage modulus and Tan Delta versus
temperature
for a polymeric strip used in the geocells of the present disclosure.
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DETAILED DESCRIPTION
[0036] The following detailed description is provided so as to enable a person
of
ordinary skill in the art to make and use the embodiments disclosed herein and
sets
forth the best modes contemplated of carrying out these embodiments. Various
modifications, however, will remain apparent to those of ordinary skill in the
art and
should be considered as being within the scope of this disclosure.
[0037] A more complete understanding of the components, processes and
apparatuses disclosed herein can be obtained by reference to the accompanying
drawings. These figures are merely schematic representations based on
convenience
and the ease of demonstrating the present disclosure, and are, therefore, not
intended
to indicate relative size and dimensions of the devices or components thereof
and/or to
define or limit the scope of the exemplary embodiments.
[0038] FIG. I is a perspective view of a single layer geocell. The geocell 10
comprises a plurality of polymeric strips 14. Adjacent strips are bonded
together by
discrete physical joints 16. The bonding may be performing by bonding, sewing
or
welding, but is generally done by welding. The portion of each strip between
two joints
16 forms a cell wall 18 of an individual cell 20. Each cell 20 has cell walls
made from
two different polymeric strips. The strips 14 are bonded together to form a
honeycomb
pattern from the plurality of strips. For example, outside strip 22 and inside
strip 24 are
bonded together by physical joints 16 which are regularly spaced along the
length of
strips 22 and 24. A pair of inside strips 24 is bonded together by physical
joints 32.
Each joint 32 is between two joints 16. As a result, when the plurality of
strips 14 is
stretched in a direction perpendicular to the faces of the strips, the strips
bend in a
sinusoidal manner to form the geocell 10. At the edge of the geocell where the
ends of
two polymeric strips 22, 24 meet, an end weld 26 (also considered a joint) is
made a
short distance from the end 28 to form a short tail 30 which stabilizes the
two polymeric
strips 22, 24.
[0039] The geocells of the present disclosure are made polymeric strips that
have
certain physical properties. In particular, the polymeric strip has a stress
at yield, or at
12% strain when the polymeric strip has no yield point, of 14.5 MPa or greater
when
measured in the machine direction (perpendicular to seam plane in the geocell
cell) at a
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strain rate of 20 %/minute or 150 %/minute. In other embodiments, the
polymeric strip
has a strain of 10% or less at a stress of 14.5 MPa, when measured as
described. In
other words, the polymeric strip can withstand stresses of 14 MPa or greater
without
reaching its yield point. Other synonyms for the yield point include the
stress at yield,
the elastic limit, or the plastic limit. When the polymeric strip has no yield
point, the
stress is considered at 12% strain. These measurements relate to the tensile
properties
of the polymeric strip in the machine direction, at 23 C, not its flexural
properties.
[0040] Because many geocells are perforated, measuring the stress and strain
according to the ASTM D638 or ISO 527 standards is generally impossible. Thus,
the
measurements are taken according to the following procedure, which is a
modified
version of said standards and is referred to herein as "the Izhar procedure".
A strip 50
mm long and 10 mm wide is sampled in the direction parallel to ground level
and
perpendicular to the seam plane of the cell (i.e. in the machine direction).
The strip is
clamped so that the distance between clamps is 30 mm. The strip is then
stretched by
moving the clamps away from each other at a speed of 45 millimeters (mm) per
minute,
which translates to a strain rate of 150%/minute, at 23 C. The load provided
by the strip
in response to said deformation is monitored by a load cell. The stress
(N/mm2) is
calculated at different strains (the strain is the increment of length,
divided by original
length). The stress is calculated by dividing the load at specific strain by
the original
nominal cross-section (the width of the strip multiplied by the thickness of
the strip)
Since the surface of the geocell strip is usually texturized, the thickness of
the sample is
measured simply as "peak to peak" distance, averaged between three points on
the
strip. (For example, a strip, having an embossed diamond like texture, and
having a
distance between the uppermost texture of top side and the lowermost texture
of the
bottom side of 1.5 mm, is regarded as 1.5 mm thick.) This strain rate of
150%/minute is
more relevant to pavements and railways, where each load cycle is very short.
[0041] In other embodiments, the polymeric strip may be characterized as
having:
a strain of at most 1.9% at a stress of 8 MPa;
a strain of at most 3.7% at a stress of 10.8 MPa;
a strain of at most 5.5% at a stress of 12.5 MPa;
a strain of at most 7.5% at a stress of 13.7 MPa;
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a strain of at most 10% at a stress of 14.5 MPa;
a strain of at most 11 % at a stress of 15.2 MPa; and
a strain of at most 12.5% at a stress of 15.8 MPa.
The polymeric strip may also have, optionally, a strain of at most 14% at a
stress of 16.5
MPa; and/or a strain of at most 17% at a stress of 17.3 MPa.
[0042] In other embodiments, the polymeric strip may be characterized as
having a
stress of at least 14.5 MPa at a strain of 12%; a stress of at least 15.5 MPa
at a strain of
12%; and/or a stress of at least 16.5 MPa at a strain of 12%.
[0043] In other embodiments, the polymeric strip may be characterized as
having a
storage modulus of 500 MPa or greater at 23 C, measured in the machine
direction by
Dynamic Mechanical Analysis (DMA) at a frequency of 1 Hz. As with the tensile
stress-
strain measurement, the thickness for the DMA analysis is taken as "peak to
peak"
distance, averaged between three points. The DMA measurements described in the
present disclosure are made according to ASTM D4065.
[0044] In other embodiments, the polymeric strip may be characterized as
having a
storage modulus of 250 MPa or greater at 50 C, measured in the machine
direction by
Dynamic Mechanical Analysis (DMA) at a frequency of 1 Hz.
[0045] In other embodiments, the polymeric strip may be characterized as
having a
storage modulus of 150 MPa or greater at 63 C, measured in the machine
direction by
Dynamic Mechanical Analysis (DMA) at a frequency of 1 Hz.
[0046] In other embodiments, the polymeric strip may be characterized as
having a
Tan Delta of 0.32 or less at 75 C, measured in the machine direction by
Dynamic
Mechanical Analysis (DMA) at a frequency of 1 Hz. These novel properties are
beyond
the properties of typical HDPE or MDPE geocells.
[0047] Dynamic Mechanical Analysis (DMA) is a technique used to study and
characterize the viscoelastic nature of polymers. Generally, an oscillating
force is
applied to a sample of material and the resulting cyclic displacement of the
sample is
measured versus the cyclic loading. The higher the elasticity, the lower the
time lag
(phase) between the load and the displacement. From this, the pure stiffness
(storage
modulus) of the sample can be determined, as well as the dissipating mechanism
(loss
modulus) and the ratio between them (Tan Delta). DMA is also discussed in ASTM
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D4065. DMA is the state-of-the-art technology when analyzing (1) time
dependent
phenomena such as creep; or (2) frequency dependent phenomena such as damping,
cyclic loading, or fatigue, that are very common in transportation
engineering.
[0048] Another aspect of the geocell of the present disclosure is its lower
coefficient
of thermal expansion (CTE) relative to current HDPE or MDPE. The CTE is
important
because the expansion/contraction during thermal cycling is another mechanism
that
provides additional hoop stresses as well. HDPE and MOPE have a CTE of about
200
x 10-6 1 C at ambient (23 C), and that CTE is even higher at temperatures
greater than
ambient. The geocell of the present disclosure has a CTE of about 150 x 10-6
/OC or
less at 23 C, and in specific embodiments about 120 x 10-6 / C or less at 23 C
when
measured according to ASTM D696. The CTE of the geocell of the present
disclosure
has lower tendency to increase at elevated temperatures.
[0049] Another aspect of the geocell of the present disclosure is its lower
creep
tendency under constant load. The lower creep tendency is measured according
to
accelerated creep test by stepped isothermal method (SIM), as described in
ASTM
6992. In this method, a polymeric specimen is subjected to a constant load
under a
stepped temperature program (i.e. the temperature is increased and held
constant for a
predefined period). The elevated temperature steps accelerate creep. The
procedure
of SIM test is applied to a sample of 100 mm width and net length of 50 mm
(distance
between clamps). The sample is loaded by a static load and heated according to
a
procedure comprising the steps:
Step T time
Celsius hours
0 23 0
1 30 3
2 37 3
3 44 3
4 51 3
58 3
6 65 3
7 72 3
(0050] This SIM procedure is referred to herein as "the PRS SIM procedure".
The
plastic strain (irreversible increase in length, divided by initial length) at
the end of the
procedure is measured. The plastic strain is measured against different loads,
and the
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load that causes plastic strain of 10% or less is called the "long term design
load." The
stress related to the long term design load (said load, divided by (original
width
multiplied by original)) is the "long term design stress" and provides the
allowed hoop
stress the geocell can tolerate for a long period of time under a static load.
[0051] A typical HDPE geocell, when subjected to the PRS SIM procedure, can
barely provide a long term design stress of 2.2 MPa.
[0052] In some embodiments, the polymeric strip according to the present
disclosure
are characterized by a long term design stress of 2.6 MPa or greater,
including a long
term design stress of 3MPa or greater, or even 4 MPa or greater.
[0053] Unlike HDPE geocells, the geocell of the present disclosure can provide
significantly better properties up to 16% strain and in some embodiments up to
22%
strain. In particular, the geocell can respond elastically to stresses greater
than 14.5
MPa, thus providing the required properties for load support applications. The
elastic
response guarantees complete recovery to original dimensions when the load is
removed. The geocell will provide the infill with a higher load bearing
capacity and
increased rebound to its original diameter under repeated loadings (i.e.
cyclic loads).
Moreover, the geocell of the present disclosure can be used with granular
materials that
generally cannot be used in base courses and sub-bases, as described further
herein.
The geocell of the present disclosure also enables better load bearing and
fatigue
resistance under humid conditions, especially when fine grained granular
materials are
used.
[0054] The polymeric strip may include a polyethylene (PE) polymer, such as
HDPE,
MDPE, or LDPE, which has been modified as described further below.
[0055] The polymeric strip may also include a polypropylene (PP) polymer.
Although
most PP homopolymers are too brittle and most PP copolymers are too soft for
load
support applications, some grades of PP polymers are useful. Such PP polymers
can
be stiff enough for the load support application, yet soft enough that the
geocell can be
folded up. Exemplary polypropylene polymers suitable for the present
disclosure
include polypropylene random copolymers, polypropylene impact copolymers,
blends of
polypropylene with either an ethylene-propylene-diene-monomer (EPDM) or an
ethylene alpha-olefin copolymer based elastomer, and polypropylene block
copolymers.
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Such PP polymers are commercially available as R338-02N from Dow Chemical
Company; PP 71 EK71 PS grade impact copolymer from SABIC Innovative Plastics;
and
PP RA1 E10 random copolymer from SABIC Innovative Plastics. Exemplary ethylene
alpha-olefin copolymer based elastomers include Exact@ elastomers manufactured
by
Exxon Mobil and Tafiner elastomers manufactured by Mitsui. Since PP polymers
are
brittle at low temperatures (lower than about minus 20 C) and tend to creep
under static
or cyclic loadings, geocells of the present disclosure which incorporate PP
may be less
load-bearing and more restricted as to their operating temperatures than
geocells of the
present disclosure which incorporate HDPE.
[0056] The PP and/or PE polymers or any other polymeric composition according
the
present disclosure are generally modified, through various treatment process
and/or
additives, to attain the required physical properties. The most effective
treatment is
post-extrusion treatment, either downstream from the extrusion machine, or in
a
separate process afterwards. Usually, lower crystallinity polymers such as
LDPE,
MDPE, and some PP polymers will require a post-extrusion process such as
orientation,
cross-linking, and/or thermal annealing, while higher crystallinity polymers
can be
extruded as strips and welded together to form a geocell without the need to
apply post-
extrusion treatment.
[0057] In some embodiments, the polymeric strip comprises a blend (usually as
a
compatibilized alloy) of (i) a high performance polymer and (ii) a
polyethylene or
polypropylene polymer. The blend is generally an immiscible blend (an alloy),
wherein
the high performance polymer is dispersed in a matrix formed by the
polyethylene or
polypropylene polymer. A high performance polymer is a polymer having (1) a
storage
modulus of 1400 MPa or greater at 23 C, measured in the machine direction by
Dynamic Mechanical Analysis (DMA) at a frequency of 1 Hz according to ASTM
D4065;
or (2) an ultimate tensile strength of at least 25 MPa. Exemplary high
performance
polymers include polyamide resins, polyester resins, and polyurethane resins.
Particularly suitable high performance polymers include polyethylene
terephthalate
(PET), polyamide 6, polyamide 66, polyamide 6/66, polyamide 12, and copolymers
thereof. The high performance polymer typically comprises from about 5 to
about 85
weight percent of the polymeric strip. In particular embodiments, the high
performance
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polymer is from about 5 to about 30 weight percent of the polymeric strip,
including from
about 7 to about 25 weight percent.
[0058] The properties of the polymeric strips can be modified either prior to
forming
the geocell (by welding of the strips) or after forming the geocell. The
polymeric strips
are generally made by extruding a sheet of polymeric material and cutting
strips from
said sheet of polymeric material, and the modification generally is made to
the sheet for
efficiency. The modification can be done in-line to the extrusion process,
after the melt
is shaped to a sheet and the sheet is cooled to lower than the melting
temperature, or
as a secondary process after the sheet is separated from the extruder die. The
modification can be done by treating the sheet, strips, and/or geocell by
cross-linking,
crystallization, annealing, orientation, and combinations thereof.
[0059] For example, a sheet which is 5 to 500 cm wide may be stretched (i.e.
orientation) at a temperature range from about 25 C to about 10 C below the
peak
melting temperature (Tm) of the polymeric resin used to make the sheet. The
orientation process changes the strip length, so the strip may increase in
length from
2% to 500% relative to its original length. After stretching, the sheet can be
annealed.
The annealing may occur at a temperature which is 2 to 60 C lower than the
peak
melting temperature (Tm) of the polymeric resin used to make the sheet. For
example,
if a HDPE, MDPE or PP sheet is obtained, the stretching and/or annealing is
done at a
temperature of from about 24 C to 150 C. If a polymeric alloy is annealed, the
annealing temperature is 2 to 60 C lower than the peak melting temperature
(Tm) of
the HDPE, MDPE, or PP phase.
[0060] In some specific embodiments, a polymeric sheet or strip is stretched
to
increase its length by 50% (i.e. so the final length is 150% of the original
length). The
stretching is done at a temperature of about 100-125 C on the surface of the
polymeric
sheet or strip. The thickness is reduced by 10% to 20% due to the stretching.
[0061] In other embodiments, a polymeric sheet or strip is cross-linked by
irradiation
with an electron beam after extrusion or by the addition of a free radical
source to the
polymeric composition prior to melting or during melt kneading in the
extruder.
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[0062] In other embodiments, the required properties for the geocell can be
obtained
by providing multi-layer polymeric strips. In some embodiments, the polymeric
strips
have at least two, three, four, or five layers.
[0063] In some embodiments as shown in FIG. 2, the polymeric strip 100 has at
least
two layers 110, 120, wherein two of the layers are made from same or different
compositions and at least one layer is made of a high performance polymer or
polymer
compound having (1) storage modulus of 1400 MPa or greater at 23 C, measured
in the
machine direction by Dynamic Mechanical Analysis (DMA) at a frequency of 1 Hz
according to ASTM D4065; or (2) an ultimate tensile strength of at least 25
MPa. In
embodiments, one layer comprises a high performance polymer and the other
layer
comprises a polyethylene or polypropylene polymer, which may be a blend or
alloy of a
polyethylene or polypropylene polymer with other polymers, fillers, additives,
fibers and
elastomers. Exemplary high performance resins include polyamides, polyesters,
polyurethanes; alloys of (1) polyamides, polyesters, or polyurethanes with (2)
LDPE,
MDPE, HDPE, or PP; and copolymers, block copolymers, blends or combinations of
any two of the three polymers (polyamides, polyesters, polyurethanes).
[0064] In other embodiments as shown in FIG. 3, the polymeric strip 200 has
five
layers. Two of the layers are outer layers 210, one layer is a core layer 230,
and the
two intermediate layers 220 bond the core layer to each outer layer (i.e. so
the
intermediate layers serve as tie layers). This five-layer strip can be formed
by co-
extrusion.
[0065] In other embodiments, the polymeric strip 200 has only three layers.
Two of
the layers are outer layers 210, and the third layer is core layer 230. In
this
embodiment, the intermediate layers 220 are not present. This three-layer
strip can be
formed by co-extrusion.
[0066] The outer layers may provide resistance against ultraviolet light
degradation
and hydrolysis, and has good weldability. The outer layer can be made from a
polymer
selected from the group consisting of HDPE, MDPE, LDPE, polypropylene, blends
thereof, and alloys thereof with other compounds and polymers. Those polymers
may
be blended with elastomers, especially EPDM and ethylene-alpha olefin
copolymers.
The core and/or outer layer can also be made from alloys of (1) HDPE, MDPE,
LDPE,
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or PP with (2) a polyamide or polyester. Each outer layer may have a thickness
of from
about 50 to about 1500 micrometers (microns).
[00671 The intermediate (tie) layers can be made from functionalized HDPE
copolymers or terpolymers, functionalized PP copolymers or terpolymers, a
polar
ethylene copolymer, or a polar ethylene terpolymer. Generally, the HDPE and PP
copolymers I terpolymers contain reactive end groups and/or side-groups which
allow
for chemical bond formation between the intermediate layers (tie layers) and
the outer
layer. Exemplary reactive side-groups include carboxyl, anhydride, oxirane,
amino,
amido, ester, oxazoline, isocyanate or combinations thereof. Each intermediate
layer
may have a thickness of from about 5 to about 500 micrometers. Exemplary
intermediate layer resins include Lotader resins manufactured by Arkema and
Elvaloy , Fusabond , or Surlyn resins manufactured by DuPont.
[0068] The core and/or outer layer may comprise a polyester and alloys thereof
with
PE or PP, a polyamide and alloys thereof with PE or PP, and blends of
polyester and
polyamide and alloys thereof with PE or PP. Exemplary polyamides include
polyamide
6, polyamide 66, and polyamide 12. Exemplary polyesters include polyethylene
terephthalate (PET) and polybutylene terephthalate (PBT). The core and/or
outer layer
may have a thickness of from about 50 to about 2000 micrometers.
[0069] In other embodiments as shown in FIG. 4, the polymeric strip 300 has
three
layers: a top layer 310, a center layer 320, and a bottom layer 330. The top
layer is the
same as the outer layer previously described; the center layer is the same as
the
intermediate layer previously described; and the bottom layer is the same as
the core
layer previously described.
[0070] Geocells are generally embossed (texturized by pressing the semi-solid
mass
after extrusion against a texturized roll) to increase friction with granular
infill or with soil.
Geocells may also be perforated to improve friction with granular infill and
water
drainage. However, both embossing and perforation reduce the stiffness and
strength
of the geocell. Since these friction aids are usually present, it is necessary
to provide
enhanced strength and stiffness to the geocell, by altering its polymer
composition
and/or morphology.
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[0071] The polymeric strip may further comprise additives to attain the
required
physical properties. Such additives may be selected from, among others,
nucleating
agents, fillers, fibers, nanoparticles, hindered amine light stabilizers
(HALS),
antioxidants, UV light absorbers, and carbon black.
[0072] Fillers may be in the form of powders, fibers, or whiskers. Exemplary
fillers
include a metal oxide, such as aluminum oxide; a metal carbonate, such as
calcium
carbonate, magnesium carbonate, or calcium-magnesium carbonate; a metal
sulfate,
such as calcium sulfate; a metal phosphate; a metal silicate - especially
talc, kaolin,
mica, or wollastonite; a metal borate; a metal hydroxide; a silica; a
silicate; an; an
alumo-silicate; chalk; talc; dolomite; an organic or inorganic fiber or
whisker; a metal;
metal-coated inorganic particles; clay; kaolin; industrial ash; concrete
powder; cement;
or mixtures thereof. In some embodiments, the filler has an average particle
size of less
than 10 microns, and in some embodiments, also has an aspect ratio of greater
than
one. In specific embodiments, the fillers is mica, talc, kaolin, and/or
wollastonite. In
other embodiments, the fibers have a diameter lower than 1 micron.
[0073] Nanoparticles can be added to the polymeric composition for various
purposes. For example, inorganic UV-absorbing solid nanoparticles have
practically no
mobility and are therefore very resistant against leaching and/or evaporation.
UV-
absorbing solid nanoparticles are also transparent in the visible spectrum and
are
distributed very evenly. Therefore, they provide protection without any
contribution to
the color or shade of the polymer. Exemplary UV-absorbing nanoparticles
comprise a
material selected from the group consisting of titanium salts, titanium
oxides, zinc
oxides, zinc halides, and zinc salts. In particular embodiments, the UV-
absorbing
nanoparticles are titanium dioxide. Examples of commercially available UV-
absorbing
particles are SACHTLEBENTM Hombitec RM 130F TN, by Sachtleben, ZANOTM zinc
oxide by Umicore, NanoZTM zinc oxide by Advanced Nanotechnology Limited and
AdNano Zinc OxideTM by Degussa.
[0074] The polymeric strips from which the geocell is formed are made by
various
processes. Generally, the process comprises melting a polymeric composition,
extruding the composition through an extruder die as a molten sheet, forming
and
optionally texturizing the resulting sheet, treating the sheet as needed to
obtain the
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desired properties, cutting the sheet to strips, and welding, sewing, bonding,
or riveting
strips formed from the sheet together into a geocell. First, the various
components,
such as the polymeric resins and any desired additives are melt kneaded,
usually in an
extruder or co-kneader. This can be done in, for example, an extruder, such as
a twin-
screw extruder or single screw extruder with enough mixing elements, which
provides
the needed heat and shearing with minimal degradation to the polymer. The
composition is melt kneaded so that any additives are thoroughly dispersed.
The
composition is then extruded through a die, and pressed between metal
calendars into
sheet form. Exemplary treatments provided downstream of the extruder die
include
texturing the surface of the sheet, perforating the sheet, orientation (uni-
directional or bi-
directional), irradiation with electron beam or x-rays, and thermal annealing.
In some
embodiments, the sheet is heat treated to increase crystallinity and reduce
internal
stresses. In other embodiments, the sheet is treated to induce cross linking
in the
polymeric resin by means or electron beam, x-ray, heat treatment, and
combinations
thereof. Combinations of the above treatments are also contemplated.
[0075] Strips can be formed from the resulting sheet and welded, sewed, or
bonded
together to form a geocell. Such methods are known in the art. The resulting
geocell is
able to retain its stiffness under sustained load cycling over extended
periods of time.
[0076] The geocells of the present disclosure are useful for load support
applications
that current geocells cannot be used for. In particular, the present geocells
can also
use infill materials that are typically not suitable for load support
applications for base
courses, subbases, and subgrades
[0077] In particular, the geocells of the present disclosure allow the use of
materials
for the infill that were previously unsuitable for use in load support
applications, such as
base courses and subbases, due to their insufficient stiffness and relatively
poor fatigue
resistance (in granular materials, fatigue resistance is also known as
resilient modulus).
Exemplary granular infill materials that may now be used include quarry waste
(the fine
fraction remaining after classification of good quality granular materials),
crushed
concrete, recycled asphalt, crushed bricks, building debris and rubble,
crushed glass,
power plant ash, fly ash, coal ash, iron blast furnace slag, cement
manufacturing slag,
steel slag, and mixtures thereof.
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[0078] The present disclosure will further be illustrated in the following non-
limiting
working examples, it being understood that these examples are intended to be
illustrative only and that the disclosure is not intended to be limited to the
materials,
conditions, process parameters and the like recited herein.
EXAMPLES
[0079] Some geocells were made and tested for their stress-strain response,
DMA
properties and their impact on granular material bearing capacity.
[0080] Generally, the tensile stress-strain properties were measured by the
Izhar
procedure previously described.
[0081] The load at different deflections was measured or translated to Newtons
(N).
The deflection is measured or translated to millimeters (mm). The stress was
calculated
by dividing the load at a specific deflection by the original cross-section of
the strip
(original width multiplied by original thickness, wherein thickness is the
nominal peak-to
peak distance between upper face and bottom face). The strain (%) was
calculated by
dividing the specific deflection (mm) by the original length (mm) and
multiplying by 100.
COMPARATIVE EXAMPLE 1
[0082] A geocell made from high density polyethylene (HDPE) commercially
available from Presto Geosystems (Wisconsin, USA) was obtained and its
properties
tested. The average cell wall thickness was 1.5 mm and the strip had a texture
of
diamond like vertical cells. The geocell was non-perforated. Its stress-strain
response
according to the Izhar procedure and is shown in Table 1.
Table 1.
Stress (MPa) 7.874 10.499 12.336 13.386 13.911 14 14 14
Strain (%) 2 4 6 8 10 12 14 16
[0083] At strain of about 8% and a stress of about 13.4 MPa, the Comparative
Example began undergoing severe plastic deformation and actually reached its
yield
point at about 8% strain. In other words, after the release of stress, the
sample did not
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recover its original length, but remained longer permanently (permanent
residual
strains). This phenomenon is undesirable for cellular confinement systems for
load
support applications - especially those subjected to many (10,000-1,000,000
and more
cycles during the product life cycle) and is the reason for the poor
performance of HDPE
geocells as load supports for pavements and railways.
EXAMPLE 1
[0084] An HDPE strip was extruded, and embossed to provide a texture similar
to
Comparative Example 1. The strip had a thickness of 1.7 mm, and was then
stretched
at a temperature of 100 C (on the strip surface) so that the length was
increased by
50% and the thickness was reduced by 25%. The stress-strain response of this
HDPE
strip was measured according to the Izhar procedure and is shown in Table 2.
Table 2.
Stress (MPa) 8 10.8 12.5 13.7 14.5 15.2 15.8 16.5 17.3
Strain (%) 1.9 3.3 4.8 6 6.6 7.6 8.8 10.5 12
[0085] The strip of Example 1 maintained an elastic response up through 12%
strain
without a yield point and without reaching its plastic limit and at stresses
greater than 17
MPa. The recovery of initial dimensions, after release of load, was close to
100%.
EXAMPLE 2
[0086] A high performance polymeric alloy composition comprising 12 wt%
polyamide 12, 10 wt% polybutylene terephthalate, 5% polyethylene grafted by
maleic
anhydride compatibilizer (Bondyram(D 5001 manufactured by Polyram), and 73%
HDPE
was extruded to form a texturized sheet of 1.5 mm thickness. The stress-strain
response of a strip formed from the composition was measured according to the
lzhar
procedure and is shown in Table 3.
Table 3.
Stress (MPa) 8 10.8 12.5 13.7 14.5 15.2 15.8 16.5 17.3
Strain (%) 1.9 3.6 5.2 6.8 7.9 8.9 10 12 14
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[0087] The strip of Example 2 maintained an elastic response up through 14%
strain
and at stresses greater than 17 MPa, without a yield point and without
reaching its
plastic limit. The recovery of initial dimensions, after release of load, was
close to
100%.
[0088] FIG. 5 is a graph showing the stress-strain results for Comparative
Example
1, Example 1, and Example 2. An additional point at (0,0) has been added for
each
result. As can be seen, Example 1 and Example 2 have no sharp yield point, and
maintained increase in stress without yield up to 12-14% strain at stresses of
greater
than 17 MPa, while the Comparative Example 1 reached its yield point at 8-10%
strain
and a stress of about 14 MPa. This translates into a greater range at which an
elastic
response is maintained. The fact that no yield point was observed for Example
1 and
Example 2 is important when cyclic loading is expected and the ability to
return to the
original dimensions (and thus the maximal confinement of infill) is crucial.
[0089] FIG. 6 is a graph showing the difference between the stress-strain
result of
Comparative Example 1 and a polymeric strip of the present disclosure which is
characterized as having a strain of at most 1.9% at a stress of 8 MPa; a
strain of at
most 3.7% at a stress of 10.8 MPa; a strain of at most 5.5% at a stress of
12.5 MPa; a
strain of at most 7.5% at a stress of 13.7 MPa; a strain of at most 10% at a
stress of
14.5 MPa; a strain of at most 11% at a stress of 15.2 MPa; a strain of at most
12.5% at
a stress of 15.8 MPa; a strain of at most 14% at a stress of 16.5 MPa; and a
strain of at
most 17% at a stress of 17.3 MPa. The area to the left of the dotted line
defines the
combinations of stress-strain according to the present disclosure.
EXAMPLE 3
[0090] Two cells were tested to demonstrate the improvement in granular
material
reinforcement and increased load-bearing capacity. These cells were a single
cell, not
a complete geocell. As a control, one cell corresponding to Comparative
Example 1
was used. For comparison, a cell was made from a composition according to
Example
2, texturized, and had a thickness of 1.5 mm,
[0091] The walls of each cell were 10 cm high, 33 cm between seams, embossed,
non perforated, and had a thickness of 1.5 mm. The cell was opened so that its
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"radius" was about 260 mm and its short radius was about 185 mm. A sandbox of
800
mm length and 800 mm width was filled to 20 mm depth with sand. The sand
gradation
distribution is provided in Table 4,
Table 4.
Sieve aperture (mm) 0.25 0.5 0.75 1 2 4
Cumulative Passing % 10-20 35-55 50-70 60-80 80-90 90-100
[0092] The cell was placed on the surface of this sand and filled with the
same sand.
The expanded cell had a roughly elliptical shape, about 260 mm on the long
axis and
about 180 mm on the short axis. Additional sand was then placed into the
sandbox to
surround the cell and bury the cell so that a top layer of 25 mm covered the
cell. The
sand was then compacted to 70% relative density.
[0093] A piston of 150 mm diameter was placed above the center of the cell and
the
load was increased to provide pressure on the sand surface in 50 kPa
increments (i.e.
the pressure was increased every 1 minute by 50 KPa). The deflection
(penetration of
piston into the confined sand) and pressure (vertical load divided by piston
area) were
measured.
[0094] The piston was used on (1) sand only, (2) a cell of Comparative Example
1;
and (3) a cell of Example 2. The results are shown in Table 5.
Table 5.
Vertical toad (kPa) 100 150 200 250 300 350 400 450 500 550
Deflection in sand only (mm) 1 2 3 >10 >15 >20 >20 >20 >20 >20
Deflection with cell of 0.7 1.3 2 2.5 3 4 5 >10 >15 >20
Comparative Example 1
(mm)
Deflection with cell of 0.6 1 1.1 1.7 2 2.5 2.9 4 5 7
Example 2 (mm)
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[0095] The cell of Example 2 continued to perform elastically at pressures
greater
than 400 kPa, whereas the cell of Comparative Example 1 did not. Due to the
yielding
of the HDPE wall, poor confinement was observed in the cell of Comparative
Example
1. The yield point for Comparative Example 1 was at vertical pressure of about
250
KPa, and if the average hoop stress is calculated (average diameter of cell is
225 mm)
at that vertical pressure, a value of about 13.5 MPa is obtained. This number
is in very
good agreement with the yield point values obtained by the stress-strain
tensile
measurements according to the Izhar procedure. The results showed there was a
strong and significant correlation between the stiffness and resistance to
yield (ability to
carry hoop stresses greater than 14 MPa) and the ability to support a large
vertical load.
It should be noted that this test only provided a single load, whereas in
practical
applications the load to be supported is cyclic. As a result, the resistance
to plastic
deformation is very important and was not present in the cell of Comparative
Example
1.
[0096] FIG. 7 is a graph showing the results in Table 5. The difference in
resistance
to penetration (i.e. how well the cell supported the vertical load) is very
clear.
EXAMPLE 4
[0097] A polymeric strip was made according to Example 2.
[0098] As a control, an HDPE strip of 1.5 mm thickness according to
Comparative
Example 1 was provided.
[0099] The two strips were then analyzed by Dynamic Mechanical Analysis (DMA)
at
a frequency of 1 Hz according to ASTM D4065. The control HDPE strip was tested
over a temperature range of about -150 C to about 91 C. The control strip was
heated
at 5 C/min and the force, displacement, storage modulus, and tan delta were
measured.
The polymeric strip of Example 2 was tested over a temperature range of about -
65 C
to about 120 C. The control strip was heated at 5 C/min and the force,
displacement,
storage modulus, and tan delta were measured.
[0100] FIG. 8 is a graph of the storage (elastic) modulus and Tan Delta versus
temperature for the control HDPE strip.
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[0101] FIG. 9 is a graph of the storage (elastic) modulus and Tan Delta versus
temperature for the polymeric strip of Example 2.
[0102] The storage modulus of the HDPE decreased more rapidly than the storage
modulus of Example 2. The storage modulus for the strip of Example 2 was
almost
three times higher than the storage modulus for the HDPE strip at 23 C. To
obtain the
same storage modulus as the HDPE strip had at 23 C, the strip of Example 2 had
to be
heated to almost 60 C, i.e. the strip of Example 2 maintained its storage
modulus
better.
[0103] The Tan Delta for the HDPE strip increased exponentially starting at
around
75 C, indicating a loss of elasticity (i.e. the material became too plastic
and would not
retain sufficient stiffness and elasticity), so that the strip was viscous and
plastic. This is
undesirable, as geocells can be heated even when placed underground (such as
in a
road). The Tan Delta for the strip of Example 2 maintained its properties at
temperatures as high as 100 C. This property is desirable as it provides an
additional
safety factor. Since performance at elevated temperatures is a way to predict
long term
performance at moderate temperatures (as described in ASTM 6992), the fact
that
HDPE began losing its elasticity and thus its load support potential at about
75 C within
seconds, provides some insight about its poor creep resistance and tendency to
plastically deform. Unlike HDPE, the composition according to the present
disclosure,
kept its elasticity (low Tan Delta) at very high temperatures, thus suggesting
that it has
the potential to retain its properties for many years and many loading cycles.
EXAMPLE 5
[0104] Three strips were tested according to the PRS SIM procedure to
determine
their long term design stress (LTDS). As a control, one HDPE strip was made
according to comparative example 1. The first test strip was one made
according to
Example 2. The second test strip was one made according to Example 2, then
oriented
at 115 C to increase its original length by 40%). The results are shown in
Table 6
below.
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Table 6.
Geocell Comparative Example 2 Oriented Example 2
Example 1
LTDS (MPa) 2.2 3 3.6
[0105] As seen here, Example 2 and Oriented Example 2 both had higher LIDS
compared to Comparative Example 1.
[0106] While particular embodiments have been described, alternatives,
modifications, variations, improvements, and substantial equivalents that are
or may be
presently unforeseen may arise to applicants or others skilled in the art.
Accordingly,
the appended claims as filed and as they may be amended are intended to
embrace all
such alternatives, modifications variations, improvements, and substantial
equivalents.
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