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 (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/cm
3. The term medium density polyethylene (MDPE) refers to a polyethylene characterized
by density of greater than 0.925 g/cm
3 to 0.940 g/cm
3. The term low density polyethylene (LDPE) refers to a polyethylene characterized
by density of 0.91 to 0.925 g/cm
3.
[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 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
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.
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.
[0015] WO2008/105878 A1 discloses an geotechnical article which has a coefficient of thermal expansion less
than 150 ppm/°C at ambient temperature and a creep modulus of at least 400 MPa at
25°C. This article can be used for a cellular confinement system, a geomembrane or
a geogrid.
[0016] A structure for reinforcing basement is disclosed in
JP H09 228301 A. Plural stacked porous plates are joined in such a manner that phase of joint parts
of adjacent porous plates are shifted from each other in the longitudinal direction
of the porous plates. Thereby, a frame body which can be expanded like a honeycomb
is formed.
[0017] A bonded composite open mesh structural textile is disclosed in
WO96/35833. The textile is formed from at least two complementary polymeric components. It is
used as a structural load bearing element in demanding earthwork construction applications.
[0018] DE 41 37 310 A1 discloses a mat being arranged in grid sections entirely from thermoplastic polymer.
The mat consists of at least two rows crossing over each other, and at the point of
intersection the tapes and/or threads are combined.
BRIEF DESCRIPTION
[0019] 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.
[0020] The invention relates to 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] These and other embodiments are described in more detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] 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.
FIG. 1 is a perspective view of a geocell.
FIG. 2 is a diagram showing an exemplary embodiment of a polymeric strip used in the geocells
of the present disclosure.
FIG. 3 is a diagram showing another exemplary embodiment of a polymeric strip used in the
geocells of the present disclosure.
FIG. 4 is a diagram showing another exemplary embodiment of a polymeric strip used in the
geocells of the present disclosure.
FIG. 5 is a graph comparing the stress-strain results of various cells of the present disclosure
against a comparative example.
FIG. 6 is a graph showing the stress-strain diagram for the geocells of the present disclosure.
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.
FIG. 8 is a graph of the storage modulus and Tan Delta versus temperature for a control
strip.
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.
DETAILED DESCRIPTION
[0031] 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.
[0032] 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.
[0033] FIG. 1 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.
[0034] 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 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.
[0035] 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/mm
2) 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.
[0036] 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;
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.
[0037] 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%.
[0038] According to the invention, the polymeric strip is 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.
[0039] In other embodiments, the polymeric strip may be characterized as having a storage
modulus of 250 MPa or greater at 50°C, meas ured in the machine direction by Dynamic
Mechanical Analysis (DMA) at a frequency of 1 Hz.
[0040] 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.
[0041] 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.
[0042] 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 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.
[0043] 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 MDPE have a CTE of about 200 x
10
-6 /°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 /°C 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.
[0044] 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 |
5 |
58 |
3 |
6 |
65 |
3 |
7 |
72 |
3 |
[0045] 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 µ 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.
[0046] A typical HDPE geocell, when subjected to the PRS SIM procedure, can barely provide
a long term design stress of 2.2 MPa.
[0047] 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.
[0048] 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.
[0049] The polymeric strip may include a polyethylene (PE) polymer, such as HDPE, MDPE,
or LDPE, which has been modified as described further below.
[0050] 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. 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 RA1E10 random copolymer from SABIC Innovative Plastics.
Exemplary ethylene alpha-olefin copolymer based elastomers include Exact® elastomers
manufactured by Exxon Mobil and Tafmer® 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.
[0051] 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.
[0052] 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 polymer is from about 5 to about 30 weight percent
of the polymeric strip, including from about 7 to about 25 weight percent.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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).
[0059] 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.
[0060] 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.
[0061] 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, or PP with (2) a
polyamide or polyester. Each outer layer may have a thickness of from about 50 to
about 1500 micrometers (microns).
[0062] 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 / 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.
[0063] 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.
[0064] 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.
[0065] 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.
[0066] 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.
[0067] 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.
[0068] 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 SACHTLEBEN™
Hombitec RM 130F TN, by Sachtleben, ZANO™ zinc oxide by Umicore, NanoZ™ zinc oxide
by Advanced Nanotechnology Limited and AdNano Zinc Oxide™ by Degussa.
[0069] 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 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 twinscrew 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 bidirectional), 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.
[0070] 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.
[0071] 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
[0072] 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.
[0073] 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
[0074] Some geocells were made and tested for their stress-strain response, DMA properties
and their impact on granular material bearing capacity.
[0075] Generally, the tensile stress-strain properties were measured by the Izhar procedure
previously described.
[0076] 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
[0077] 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 |
[0078] 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 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
[0079] 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 |
[0080] 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
[0081] A high performance polymeric alloy composition comprising 12 wt% polyamide 12, 10
wt% polybutylene terephthalate, 5% polyethylene grafted by maleic anhydride compatibilizer
(Bondyram® 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 Izhar 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 |
[0082] 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%.
[0083] 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.
[0084] 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
[0085] 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.
[0086] 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 long "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 |
[0087] 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.
[0088] 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.
[0089] 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 Load (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 Comparative Example 1 (mm) |
0.7 |
1.3 |
2 |
2.5 |
3 |
4 |
5 |
>10 |
>15 |
>20 |
Deflection with cell of Example 2 (mm) |
0.6 |
1 |
1.1 |
1.7 |
2 |
2.5 |
2.9 |
4 |
5 |
7 |
[0090] 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.
[0091] 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
[0092] A polymeric strip was made according to Example 2.
[0093] As a control, an HDPE strip of 1.5 mm thickness according to Comparative Example
1 was provided.
[0094] 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 9 1°C. The control strip was heated at 5°C/min and
the force, displacement, storage mod ulus, 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.
[0095] FIG. 8 is a graph of the storage (elastic) modulus and Tan Delta versus temperature for
the control HDPE strip.
[0096] FIG. 9 is a graph of the storage (elastic) modulus and Tan Delta versus temperature for
the polymeric strip of Example 2.
[0097] 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.
[0098] 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
[0099] 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.
Table 6.
Geocell |
Comparative Example 1 |
Example 2 |
Oriented Example 2 |
LTDS (MPa) |
2.2 |
3 |
3.6 |
[0100] As seen here, Example 2 and Oriented Example 2 both had higher LTDS compared to Comparative
Example 1.
[0101] 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.
[0102] The present invention is also concerned with the following embodiments. In an embodiment
the invention provides 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.
[0103] In another embodiment the invention provides 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 freq uency of 1 Hz, wherein the at least one polymeric strip has
a storage modulus of 250 MPa or greater.
[0104] In another embodiment the invention provides 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 freq uency of 1 Hz, wherein the at least one polymeric strip has
a storage modulus of 400 MPa or greater.
[0105] In another embodiment the invention provides 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 freq uency of 1 Hz, wherein the at least one polymeric strip has
a stress at 12% strain of 14.5 MPa or greater when measured according to the Izhar
procedure at 23°C.
[0106] In another embodiment the invention provides 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 freq uency of 1 Hz, wherein the at least one polymeric strip has
a stress at 12% strain of 16 MPa or greater when measured according to the Izhar procedure
at 23°C.
[0107] In another embodiment the invention provides 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 freq uency of 1 Hz, wherein the at least one polymeric strip has
a stress at 12% strain of 18 MPa or greater when measured according to the Izhar procedure
at 23°C.
[0108] In yet another embodiment the invention provides 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.
[0109] In another embodiment the invention provides 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, wherein the at least one polymeric
strip has a long term design stress of 3 MPa or greater, when measured according to
the PRS SIM procedure.
[0110] In another embodiment the invention provides 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, wherein the at least one polymeric
strip has a long term design stress of 4 MPa or greater, when measured according to
the PRS SIM procedure.
[0111] In another embodiment the invention provides 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, wherein the at least one polymeric
strip has a stress at 12% strain of 14.5 MPa or greater when measured according to
the Izhar procedure at 23°C.
[0112] In another embodiment the invention provides 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, wherein the at least one polymeric
strip has a stress at 12% strain of 16 MPa or greater when measured according to the
Izhar procedure at 23°C.
[0113] In another embodiment the invention provides 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, wherein the at least one polymeric
strip has a stress at 12% strain of 18 MPa or greater when measured according to the
Izhar procedure at 23°C.