Note: Descriptions are shown in the official language in which they were submitted.
CA 02759830 2011-11-24
TITLE: GEOCELL FOR LOAD SUPPORT APPLICATIONS
BACKGROUND
This application is a divisional of Canadian patent application 2,733,055
filed September 29, 2008.
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.
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.
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.
1
CA 02759830 2011-11-24
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/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.
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.
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.
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
2
CA 02759830 2011-11-24
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.
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.
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
3
CA 02759830 2011-11-24
increases, and higher horizontal loads are developed, providing increased hoop
stress in the cell wall.
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.
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.
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.
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.
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.
4
CA 02759830 2011-11-24
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.
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.
BRIEF DESCRIPTION
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.
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.
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.
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.
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.
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
5
CA 02759830 2011-11-24
plant ash, fly ash, coal ash, iron blast furnace slag, cement manufacturing
slag,
steel slag, and mixtures thereof.
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.
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.
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.
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.
These and other embodiments are described in more detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
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. I 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.
6
CA 02759830 2011-11-24
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
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.
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.
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
7
CA 02759830 2011-11-24
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.
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.
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
mm. The strip is then stretched by moving the clamps away from each other at
25 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
30 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
8
CA 02759830 2011-11-24
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.
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.
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%.
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.
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.
9
CA 02759830 2011-11-24
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.
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.
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.
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.
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
CA 02759830 2011-11-24
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
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 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.
A typical HDPE geocell, when subjected to the PRS SIM procedure, can
barely provide a long term design stress of 2.2 MPa.
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.
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
11
CA 02759830 2011-11-24
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.
The polymeric strip may include a polyethylene (PE) polymer, such as
HDPE, MDPE, or LDPE, which has been modified as described further below.
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 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.
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
12
CA 02759830 2011-11-24
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.
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.
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.
13
CA 02759830 2011-11-24
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.
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.
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.
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.
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
14
CA 02759830 2011-11-24
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).
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.
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.
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).
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,
CA 02759830 2011-11-24
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.
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.
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.
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.
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.
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
16
CA 02759830 2011-11-24
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.
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.
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 twin-screw extruder or single screw extruder with
enough
17
CA 02759830 2011-11-24
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.
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.
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
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.
The present disclosure will further be illustrated in the following non-
limiting
working examples, it being understood that these examples are intended to be
18
CA 02759830 2011-11-24
illustrative only and that the disclosure is not intended to be limited to the
materials, conditions, process parameters and the like recited herein.
EXAMPLES
Some geocells were made and tested for their stress-strain response, DMA
properties and their impact on granular material bearing capacity.
Generally, the tensile stress-strain properties were measured by the Izhar
procedure previously described.
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 I
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
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
19
CA 02759830 2011-11-24
(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 II
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 lzhar 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
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
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 lzhar procedure and is shown in Table 3.
Table 3.
CA 02759830 2011-11-24
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
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%.
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.
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
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
21
CA 02759830 2011-11-24
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.
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
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.
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.
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 1 2 3 >10 >15 >20 >20 >20 >20 >20
(mm)
Deflection with cell of 0.7 1.3 2 2.5 3 4 5 >10 >15 >20
Comparative Example 1
22
CA 02759830 2011-11-24
(mm)
Deflection with cell of 0.6 1 1.1 1.7 2 2.5 2.9 4 5 7
Example 2 (mm)
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.
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
A polymeric strip was made according to Example 2.
As a control, an HDPE strip of 1.5 mm thickness according to Comparative
Example 1 was provided.
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
23
CA 02759830 2011-11-24
temperature range of about -65 C to about 120 C. The control strip was heated
at
C/min and the force, displacement, storage modulus, and tan delta were
measured.
FIG. 8 is a graph of the storage (elastic) modulus and Tan Delta versus
5 temperature for the control HDPE strip.
FIG. 9 is a graph of the storage (elastic) modulus and Tan Delta versus
temperature for the polymeric strip of Example 2.
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.
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
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
24
CA 02759830 2011-11-24
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 2 Oriented Example
Example 1 2
LTDS (MPa) 2.2 3 3.6
As seen here, Example 2 and Oriented Example 2 both had higher LTDS
compared to Comparative Example 1.
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.
