Geogrids

Abstract

A geogrid in the form of an integral, mesh structure includes molecularly orientated polymeric material, with the integral, mesh structure being formed of interconnecting mesh defining elements including elongate tensile elements. The molecular orientation of the integral, mesh structure is uniform throughout the extent thereof.

Claims

1. A geogrid comprising an integral, mesh structure, the integral, mesh structure comprising molecularly orientated polymeric material and being formed of interconnecting mesh defining elements including elongate tensile elements, the molecular orientation of the integral, mesh structure being uniform throughout an extent thereof, and said geogrid having a thickness of 0.1 to 3 mm.

2. The geogrid as claimed in claim 1, wherein the cross-section of the elongate tensile elements is uniform along a length thereof.

3. The geogrid as claimed in claim 2, wherein the cross-section of the elongate tensile elements is rectangular.

4. The geogrid as claimed in claim 1, wherein the polymeric material is uniaxially oriented.

5. The geogrid as claimed in claim 4, wherein the geogrid has a Creep Reduction Factor (RF.sub.CR) determined in accordance with PD ISO/TR 20432:2007 based on a Static Creep Test in accordance with BE EN ISO 13431:1999 and Stepped Isothermal Method creep testing in accordance with ASTM D6992-03 of at least 55%.

6. The geogrid as claimed in claim 4, wherein the geogrid has a stretch ratio of at least 4:1.

7. The geogrid as claimed in claim 4, wherein the geogrid has a tensile strength of at least 30 kN/m.

8. The geogrid as claimed in claim 4, with the interconnecting mesh defining elements comprising: (i) a plurality of the elongate tensile elements extending parallel to each other in a direction of molecular orientation, and (ii) a plurality of connector elements integral with the elongate tensile elements and each serving to connect adjacent elongate_tensile elements together, the connector elements connecting any two of the elongate tensile elements together being spaced from each other in the direction of molecular orientation, thereby defining, with the elongate tensile elements, elongate apertures extending parallel to the elongate tensile elements.

9. The geogrid as claimed in claim 8, wherein the elongate tensile elements have a width of 2 to 50 mm, the elongate apertures have a length of 40 to 400 mm and a width of 5 to 100 mm, and the connector elements have a width as measured in a longitudinal direction of the elongate tensile elements of 2 to 20 mm.

10. The geogrid as claimed in claim 9, wherein the elongate tensile elements have a width of 5 to 40 mm, the elongate apertures have a length of 40 to 250 mm and a width of 10 to 80 mm, and the connector elements have a width of 6 to 18 mm.

11. The geogrid as claimed in claim 8, wherein the connector elements are arranged as a plurality of sets in which the connector elements of any one set are aligned with each other in a direction transverse to the elongate tensile elements and the sets are spaced from each other in the direction of molecular orientation.

12. The geogrid as claimed in claim 8, wherein the geogrid has integral beads of the polymeric material on a face of the geogrid at least partly around peripheral edges of the apertures.

13. The geogrid as claimed in claim 12, wherein the integral beads are formed along the ends of the apertures and reduce to zero height along the elongate edges thereof.

14. The geogrid as claimed in claim 4, wherein: (a) the elongate tensile elements comprise a plurality of generally parallel rib structures extending in a direction of molecular orientation, and (b) the interconnecting mesh defining elements further comprise a plurality of spaced, generally parallel bar structures extending transversely to the rib structures, said rib structures and said bar structures being interconnected by junctions at spaced locations along respective lengths thereof, whereby the rib structures are sub-divided along their lengths into alternating junctions and rib segments and the bar structures are sub-divided along their lengths into alternating bar segments and junctions.

15. The geogrid as claimed in claim 14, wherein the rib structures have a width of 2 to 50 mm, the apertures have a length of 40 to 400 mm and a width of 5 to 100 mm, and the bar structures have a width of 2 to 20 mm as measured in a longitudinal direction of the rib structures.

16. The geogrid as claimed in claim 15, wherein the rib structures have a width of 5 to 40 mm, the apertures have a length of 40 to 250 mm and a width of 10 to 80 mm, and the bar structures have a width of 6 to 18 mm.

17. The geogrid as claimed in claim 1, wherein the polymeric material is biaxially orientated, optionally the geogrid having a stretch ratio of at least 1.5:1 and/or further optionally the geogrid having a tensile strength of at least 10 kN/m.

18. A method of strengthening a particulate material, the method comprising embedding in the particulate material the geogrid as claimed in claim 1.

19. A strengthened particulate material, the strengthened particulate material comprising: a particulate material having embedded therein the geogrid as claimed in claim 1.

20. A geoengineering construction comprising a mass of particulate material having embedded therein the geogrid as claimed in claim 1.

21. The geoengineering construction as claimed in claim 20, wherein the geoengineering construction is selected from the group consisting of embankment foundation, railway track ballast and/or sub ballast, road bed foundation, bridge abutment, retaining wall, steep >20 degrees slope, slip repair, steel mesh face, wraparound face, terraced wall, wall and slope, vegetated face, non-vegetated face, modular blocks, concrete panel, marine unit, and gabion face.

22. The geoengineering construction as claimed in claim 20, wherein the geogrid imparts to the geoengineering construction an improvement, as compared to the construction absent said geogrid, in at least one property selected from the group consisting of strength, stabilization, layer thickness, life, bearing capacity, control of differential settlement, ability to cap weak deposits, and ability to span voids of and/or beneath the particulate material and/or geoengineering construction.

23. The geoengineering construction as claimed in claim 20, comprising the mass of particulate material improved in at least one property selected from the group consisting of strength, stabilization, layer thickness, life, bearing capacity, control of differential settlement, ability to cap weak deposits, and ability to span voids of and/or beneath the particulate material and/or geoengineering construction, by embedding therein the geogrid.

24. A method of forming a geoengineering construction for at least one purpose selected from the group consisting of strengthening, stabilizing, reducing layer thickness, increasing the life of, increasing bearing capacity, controlling differential settlement, capping weak deposits, and spanning voids of and/or beneath the particulate material and/or geoengineering construction, the method comprising embedding the geogrid as claimed in claim 1 within a particulate material.

25. A geogrid comprising an integral, mesh structure, the integral, mesh structure comprising polymeric material and comprising elongate tensile elements interconnected by junctions in the integral, mesh structure, wherein the junctions and the elongate tensile elements have a same mean thickness, the molecular orientation of the integral, mesh structure being uniform throughout an extent thereof, and said geogrid having a uniform thickness of 0.1 to 3 mm.

26. A geogrid comprising an integral, mesh structure, the integral, mesh structure comprising molecularly orientated polymeric material and comprising elongate tensile elements interconnected by junctions in the mesh structure, wherein there is no thickening of the junctions caused by stretching of the polymeric material, the molecular orientation of the integral, mesh structure being uniform throughout an extent thereof, and said geogrid having a uniform thickness of 0.1 to 3 mm.

27. A uniax geogrid comprising an integral, mesh structure, the integral, mesh structure comprising polymeric material which is uniaxially oriented, the molecular orientation of the integral, mesh structure being uniform throughout an extent thereof, and the uniax geogrid having a Creep Reduction Factor (RF.sub.CR) determined in accordance with PD ISO/TR 20432:2007 based on a Static Creep Test in accordance with BE EN ISO 13431:1999 and Stepped Isothermal Method creep testing in accordance with ASTM D6992-03 of at least 55%.

28. A method of producing a geogrid, the method comprising the steps of: (a) stretching an elongate polymeric starting sheet having a thickness of from 2 to 12 mm to form a geogrid precursor comprising molecularly orientated polymer, the geogrid precursor being of essentially uniform thickness and having a thickness of 0.1 to 3 mm, and (b) converting the geogrid precursor into a geogrid by forming apertures in the geogrid precursor to define an integral mesh structure formed of interconnecting mesh defining elements including elongate tensile elements, the molecular orientation of the integral mesh structure being uniform throughout an extent thereof.

29. The method as claimed in claim 28, wherein the polymeric starting sheet has a mean thickness of 4 to 10 mm.

30. The method as claimed in claim 28, wherein the apertures are formed such that the elongate tensile elements are generally rectangular as seen in cross-section at right angles to a longitudinal extent of the elongate tensile elements, the length sides of rectangular cross-section being along faces of the geogrid.

31. The method as claimed in claim 30, wherein the apertures are formed such that the elongate tensile elements have a width on opposite sides of the geogrid of 2 to 20 mm.

32. The method as claimed in claim 31, wherein the width of the elongate tensile elements is from 6 to 18 mm.

33. The method as claimed in claim 31, wherein the apertures are formed to have a length of 40 to 250 mm and a width of 5 to 80 mm.

34. The method as claimed in claim 33, wherein the apertures have a length of 50 to 200 mm and a width of 5 to 50 mm.

35. The method as claimed in claim 28, wherein the stretching in step (a) is effected in a single direction to provide a geogrid precursor in which the polymeric material is uniaxially orientated.

36. The method as claimed in claim 35, wherein, in step (a), the polymeric starting sheet is stretched to a stretch ratio of at least 4:1.

37. The method as claimed in claim 36, wherein said stretch ratio is at least 7:1.

38. The method as claimed in claim 37, wherein said stretch ratio is from 7:1 to 12:1.

39. The method as claimed in claim 35, wherein the apertures are formed such that the elongate tensile elements extend parallel to the stretching direction and the apertures are elongate and also extend parallel to the stretching direction.

40. The method as claimed in claim 39, wherein the apertures are formed such that the mesh structure produced in step (b) comprises: (i) a plurality of the elongate tensile elements extending parallel to each other, and (ii) a plurality of connector elements integral with the elongate tensile elements and each serving to connect adjacent elongate tensile elements together, the connector elements connecting any two of the elongate tensile elements being spaced from each other in the stretching direction thereby defining, with the elongate tensile elements, the elongate apertures.

41. The method as claimed in claim 40, wherein the apertures are formed such that the connector elements are arranged as a plurality of sets in which the connector elements of any one set are aligned with each other in a direction transverse to the elongate tensile elements, and wherein the sets are spaced from each other in the longitudinal direction of the elongate tensile elements.

42. The method as claimed in claim 40, wherein the apertures are formed such that the elongate tensile elements have a width of 2 to 50 mm, the apertures have a length 40 to 400 mm and a width of 5 to 100 mm, and the connector elements have a width as measured in the longitudinal direction of the elongate tensile elements of 2 to 20 mm.

43. The method as claimed in claim 42, wherein the elongate tensile elements have a width of 5 to 40 mm, the apertures have a length of 40 to 250 mm and a width of 10 to 80 mm, and the connector elements have a width of 6 to 18 mm.

44. The method as claimed in claim 39, wherein the apertures are formed such that the interconnecting mesh defining elements produced in step (b) comprises: (i) the elongate tensile elements comprising a plurality of generally parallel rib structures extending in the direction of uniaxial orientation, and (ii) a plurality of spaced, generally parallel bar structures extending transversely to the rib structures, said rib structures and said bar structures being interconnected by junctions at spaced locations along their respective lengths whereby the rib structures are sub-divided along their lengths into alternating junctions and rib segments, and the bar structures are sub-divided along their lengths into alternating bar segments and junctions.

45. The method as claimed in claim 44, wherein the apertures are formed such that the rib structures have a width of 2 to 50 mm, the apertures have a length of 40 to 400 mm and a width to 5 to 100 mm, and the bar structures have a width as measured in a longitudinal direction of the rib structures of 2 to 20 mm.

46. The method as claimed in claim 45, wherein the rib structures have a width of 5 to 40 mm, the apertures have a length of 40 to 250 mm and a width of 10 to 80 mm, and the bar structures have a width of 6 to 18 mm.

47. The method as claimed in claim 39, wherein the apertures are provided by a punching operation.

48. The method as claimed in claim 47, wherein the punching operation forms integral beads of a polymeric material that comprises the geogrid on a face of the geogrid and at least partly around peripheral edges of the apertures.

49. The method as claimed in claim 48, wherein the beads are formed along ends of the apertures and reduce to zero height along the elongate edges thereof.

50. The method as claimed in claim 28, wherein the elongate polymeric starting sheet is stretched in two mutually perpendicular directions to produce a geogrid precursor in which the polymeric material is biaxially orientated.

51. A geogrid comprising: a geogrid produced by the method as claimed in claim 28.

52. A method of producing a geogrid, the method comprising the steps of: (a) providing a geogrid precursor as a polymeric starting sheet comprising polymeric material uniformly molecularly orientated throughout an extent of the sheet, the geogrid precursor being of essentially uniform thickness and having a thickness of 0.1 to 3 mm, and (b) converting the geogrid precursor into a geogrid by forming apertures in the geogrid precursor to define an integral mesh structure formed of interconnecting mesh defining elements including elongate tensile elements, the molecular orientation of the integral mesh structure being uniform throughout an extent thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be further described by way of example only with reference to the following non-limiting Examples and the accompanying drawings also described herein.

(2) In the figures referred to herein some of the reference numbers refer to the following elements: 1 denotes a geogrid generally; 2 denotes rib structures; 3 denotes bar structures; 4 denotes elongate apertures; 5 denotes junctions; 6 denotes a rib segment or strand; 7 denotes a bar segment; and 10 denotes a pressure mark.

(3) FIG. 1 shows a portion of a uniax geogrid in accordance with the invention. Certain dimensions are denoted in FIG. 1 by labels where “c” refers to the length of the elongate apertures, “d” refers to the width of the bar structures and “e” refers to the width of the rib structures. For one specific geogrid illustrated by FIG. 1, c is 210 mm; d is 16 mm and e is 9.5 mm.

(4) FIG. 2 schematically illustrates, to a much enlarged scale, a portion of the underside of a geogrid as shown in FIG. 1.

(5) FIG. 3a is a sectional view on the line A-A of FIG. 2.

(6) FIG. 3b is a sectional view on the line B-B of FIG. 2.

(7) FIG. 4 is a photograph of a sample of the geogrid produced in Example 1, shown next to a ruler for scale.

(8) FIG. 5 is a graph of Tensile Strength (y-axis) vs Tensile Strain (x-axis) for both a geogrid produced in accordance with Example 1 and a comparative strength a conventional uniax geogrid. FIG. 5 compares the short term tensile strength behaviour of a geogrid of the invention (the top line of the graph labelled 11) with a conventional HDPE uniaxial reinforcing geogrid (shown as the bottom line of the graph labelled 13).

(9) FIG. 6 shows creep data for a geogrid in accordance with the invention and a conventional uniax geogrid, both tested at 60% load and 20° C. In FIG. 6, the (top line of) data, generally labelled 15 and indicated by diamonds, is data generated from the conventional uniax geogrid available commercially from Tensar under the trade designation RE560, where the geogrid ruptured at the time indicated by label 19 in FIG. 6. The bottom line of data, generally labelled 17 and indicated by triangles, is data generated from a sample according to the invention where the geogrid was still live after the duration of the test indicated by label 21 in FIG. 6.

(10) FIG. 7 is plot of creep data (plotted as log.sub.10(time) on the ordinate versus log.sub.10(load) on the abscissa) to compare creep performance of prior art geogrids (a conventional HDPE uniaxial reinforcing geogrid) the data denoted generally by label 23 and plotted as the dashed line to the left in FIG. 7 and geogrids in accordance with the invention denoted generally by label 25 and plotted as the solid line to the right in FIG. 7. In data set 25, the creep data of the invention, a cross indicates a log.sub.10(load)=1.86 or 72% of ultimate tensile strength (UTS) after 10.sup.6 hours. In data set 23, the creep data of, the conventional HDPE uniaxial geogrid, a cross indicates a log.sub.10(load)=1.68 or 47.5% of UTS after 10.sup.6 hours.

(11) FIG. 8 is a photograph of a rib element of a geogrid of the invention before and after a reversion test used to determine the draw ratio i.e. degree of molecular orientation of that element. The photograph FIG. 8 has been redrawn in FIG. 8A (longer rib element before reversion) and FIG. 8B (shorter rib element after reversion) so that the dimensions seen in the photograph and referred to herein are indicated clearly. The scalloping of the side edges of the rib element before the test can be seen in the photograph (FIG. 8) but for clarity are omitted in the corresponding drawing (FIG. 8A). The dimensions of a rib element as photographed in FIG. 8 are, for the oriented element pre-test (as shown in FIG. 8A) labelled by: “f” (rib starting length); by “g” (rib starting width); and by “h” (bar starting width); and for the reverted element post-test (as shown in FIG. 8B) labelled by: “i” (rib finishing length); by “j” (rib finishing width); and by “k” (bar finishing width). For the specific rib element shown in the photograph of FIG. 8: pre-test (FIG. 8A): f is 108 mm; g is 14 mm; and h is 6 mm; and post-test (FIG. 8B): i is 11 mm; j is 15 mm; and k is 0.6 mm. However it will be appreciated that different values of dimensions “f” to “k” may also be obtained in a reversion test of other rib elements of geogrids according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(12) Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

(13) FIG. 1 illustrates a portion of a uniaxially oriented geogrid 1 in accordance with the invention which has been produced by stretching a plastics sheet starting material in a single direction MD as indicated by the arrow in FIG. 1 (MD being an abbreviation for “machine direction”) so as to molecularly orient the material in that direction and subsequently forming elongate apertures in the stretched material. The geogrid 1 comprises rib structures 2 which extend generally parallel to the machine direction MD. The rib structures 2 are transversely spaced from each other and connected at regularly spaced intervals by bar structures 3 extending in the transverse (TD) direction whereby a plurality of longitudinal extending, elongate apertures 4 with radiused corners are defined in the geogrid 1. As further shown in FIG. 1, the rib structures 2 and bar structures 3 meet at junction regions 5 of the geogrid 1. Each rib structure 2 is continuous throughout the geogrid 1 as are the bar structures 3. Thus the junctions 5 are considered simultaneously to be both part of a rib structure 2 and a bar structure 3. As represented in FIG. 1, each rib structure 2 is comprised of an alternating arrangement of rib segments or strands 6 and junctions 5 whereas each bar structure 3 is comprised of an alternating arrangement of junctions 5 and bar segments 7.

(14) Expressed alternatively, the geogrid 1 shown in FIG. 1 may be considered to comprise the rib structures 2 with the bar segments 7 acting as “connectors” between adjacent rib structures.

(15) There are a number of points to note about the geogrid 1. Firstly, the geogrid is of generally uniform thickness. Any deviation from non-uniformity is likely to result for the case where the web material (from which the geogrid is produced) is unrestrained in the width direction during the stretch in the length direction, in which case the marginal edge regions of the stretched web may be slightly thicker than the central region (these marginal edges may be removed from the commercial product). There may also be some localised non-uniformity in thickness around parts of the apertures. Secondly, the degree of orientation in the MD direction is the same throughout the geogrid.

(16) It will therefore be appreciated that the uniax geogrid 1 of FIG. 1 differs from conventional uniax geogrids in that it is uniformly “flat” rather than having thickened junctions. Furthermore, the orientation in the MD direction is uniform along the length of the rib structures 2 and throughout the geogrid. Thus in contrast to conventional uniax geogrids, there is no variation in orientation along a rib structure (going in the MD direction). Additionally, and relatedly, the geogrid of the invention avoids a disadvantage of prior uniax geogrids where unoriented polymer does not contribute significantly to the strength of the geogrid but is encapsulated in the thickened junctions and bar segments of the prior uniax geogrids.

(17) Reference is now made to FIG. 2 which schematically illustrates, to a much enlarged scale, a portion of the underside of a geogrid 1 of the type shown in FIG. 1 and also to FIG. 3a which is a sectional view on the line A-A of FIG. 2. Shown in FIG. 2 are the rib structures 2 providing the tensile elements, the connector elements 7 extending transversely to (and connecting) adjacent rib structures 2, and the elongate apertures 4. The geogrid illustrated in FIG. 2 has been produced by a punching operation under conditions such that a pressure mark 10 has been formed along the end regions of the apertures 4 on one side of the geogrid 1. As will be appreciated from FIGS. 2, 3a and 3b, the pressure mark 10 is, in effect, a bead of the polymer that forms the geogrid 1, where the polymer is standing proud of the surface on which it is provided. The pressure mark 10 has its maximum height along its extent transverse to the rib structures 2 and becomes of decreasing height as it turns around the corners of the apertures 4 so as to reach nil height after only a very short extent along the edges of the rib structures —see particularly FIG. 3b.

(18) The pressure mark 10 provides reinforcement at the ends of the apertures and as such inhibits tearing of the connector 7 (bar segment) from one edge to the other. Although not illustrated in FIGS. 2, 3a and 3b, it will be appreciated that pressure marks 10 are provided on the same side of the geogrid at each end of the apertures.

Example 1 (FIGS. 1, 2, 3a, 3b & 4)

(19) In this Example, the method of the invention was used for producing a geogrid from an extruded, initially unoriented sheet of High Density Polyethylene (HDPE) having an indefinite length, a width of 1515 mm and a nominal thickness of 6.35 mm (giving a cross-sectional area of approximately 9620 mm.sup.2).

(20) First Step

(21) In a first step of the process, the sheet of unoriented HDPE was heated to a temperature of about 105° C. and then drawn (in the length direction, LD or MD) at a nominal draw ratio of about 10:1 prior to cooling. No restraint on width was applied to the web during the drawing step. Samples of the oriented web were then cut for further processing in the second step of the process (see below).

(22) The width of the oriented web was 1249 mm (the reduction as compared to the starting width of 1515 mm being due to lack of width restraint during the draw process) and it was noted that the oriented web was somewhat thicker at the outer marginal regions (about 50 mm inbound of each edge) than at the centre.

(23) The average thickness of the oriented web was determined to be 0.76 mm, giving a cross-sectional area of about 949 mm.sup.2. This compares with a cross-sectional area of about 9620 mm.sup.2 for the starting material, thus confirming the anticipated draw ratio of about 10:1.

(24) Second Step

(25) In the second step of the process, samples of the sheet of oriented HDPE obtained from the first step were perforated to produce a geogrid 1 as shown in FIG. 1 in which the apertures 4 had a length of approximately 210 mm and a width of approximately 9.5 mm. The transverse connectors 7 had a width (i.e. its dimension perpendicular to the MD direction) of approximately 16 mm.

(26) Perforating was effected by feeding the samples (in the length direction, LD or MD) through a perforating station provided with side-by-side matched punches and dies having a length of 25.4 mm. The punches had radiused ends and formed waists where the width at their mid-points was less than at their ends. To produce the geogrid the punching station was programmed to make a single stroke followed by 13 consecutive 14.6 mm index strokes followed by a larger single index of 41.28 mm to form the transverse bar. The punch tool formed an aperture that formed slight waists of narrower widths intermediate to the width at its ends (the punch has a corresponding waist shape).

(27) FIG. 4 is a photograph of a geogrid obtained by this method which shows that the side edges of apertures 4 have a slightly scalloped profile. This is due to overlap of the relative positions (in the MD) of the waist shaped punches and the sheet in successive index strokes as sheet travels through the perforating station in the MD to form the elongate apertures 4.

Example 2 (FIG. 5)

(28) For a uniax geogrid intended for soil reinforcement applications such as in walls or slopes, two properties of the material of the geogrid are especially useful. The first is the short-term tensile strength and the second is the percentage of the short-term tensile strength available for the long-term “creep” performance of the product.

(29) This Example demonstrates short term tensile testing of rib segments cut from the geogrid produced in accordance with Example 1 and compares the results with those obtained for a conventional uniax geogrid commercially available from Tensar International Ltd under the designation RE560. Tensile test specimens in accordance with ISO10319 were cut from the geogrid produced in accordance with Example 1. Tensile testing was carried out according to ISO10319 on a testing machine available from Instron, with the jaws drawn relatively apart at a rate of 20% of the specimen gauge length in accordance with the ISO10319 Standard. The results are shown in the following Table.

(30) TABLE-US-00001 Strain at Strength at Strength at Strength At Max Load 2% Strain 5% Strain Max Load Ex (%) (kN/m) (kN/m) (kN/m) 1 5.9 43.66 75.65 85.51 2 6.33 41.64 73.4 88.06 3 5.63 43.22 76.3 83.22 4 5.96 40.7 72.7 83.12 5 6.03 42.94 75.6 86.95 Mean 5.97 42.44 74.7 85.37

(31) For the purposes of comparison FIG. 5 shows a composite plot of the results of the above tensile tests with those obtained for rib segments of the same length cut from a uniax geogrid produced by Tensar, which is produced from the same polymer (HDPE) and which, of the uniax geogrids Tensar produce, provides the closest match in terms of tensile strength to the product of Example 1. Once the 6.35 mm sheet was oriented to 10:1 the resulting punched geometry had a strength similar to the conventional uniax product produced from 4.05 mm sheet.

(32) FIG. 5 shows a composite plot of tensile results for the test specimens cut from the geogrid produced in accordance with the invention (top, solid line 11) in comparison to a set of population average data for the prior art RE560 product (bottom, dashed line, 13).

(33) The data in FIG. 5 shows that the short-term tensile strength material efficiencies (potential material efficiency benefit) of the two materials tested can be calculated by dividing the short-term tensile strength by the mass per unit area.
Invention=(85.4/0.50)=171(kN/m)/(kg/m.sup.2)
RE560=(94.0)/0.62=152(kN/m)/(kg/m.sup.2)

(34) Thus the gain in short-term tensile efficiency of the geogrid of the invention over an equivalent strength conventional uniax product is about 12.5% on the basis of weight of polymer.

(35) Furthermore, FIG. 5 clearly demonstrates that the tensile curves for the product of the invention are much “stiffer”—for the same percentage strain the tensile strength is 20% to 30% higher for the geogrid of the invention (plot 11) than for the conventional equivalent strength uniax (plot 13). Strain at maximum load is also lowered by a significant percentage but the ultimate tensile strength is within 10%, i.e. 85.4 kN/m for the product of the invention as compared to 94.0 kN/m for the conventional equivalent uniax product.

Example 3 (FIG. 6)

(36) This Example demonstrates the creep properties of a geogrid produced according to Example 1 in comparison with those of a conventional equivalent strength uniax product.

(37) A sample of geogrid produced in accordance with Example 1 was subjected to a static creep test according to BS EN ISO 13431:1999 at 20° C. using a load corresponding to 60% of short term tensile strength. For comparison an example of a conventional equivalent strength uniax geogrid (RE560) was subjected to the same 20° C. temperature and load corresponding to 60% of its short-term tensile strength. The results are shown in FIG. 6 which is a plot of strain (y-axis) vs time (x-axis) where the top plot (diamonds, 15) is data generated by the prior art geogrid RE560 and the bottom plot (triangles, 17) is data generated by the geogrid of Example 1.

(38) A comparison of the two data plots (15, 17) in FIG. 6 clearly demonstrates that the geogrid produced in accordance with the invention (data 17) exhibits much lower strain than the geogrid RE560 of conventional structure (data 15). This is due primarily to the reservoir of unoriented polymer locked into the bars of the conventional geogrid structure. It can also be seen that whilst the conventional geogrid ruptured at approx. 90 hours (datum 19), the geogrid produced in accordance with the invention was still live at approximately 11000 hours, an increase of over two log cycles (datum 21).

Example 4 (FIG. 7)

(39) Conventional static creep loading carried out in accordance with BS EN ISO 13431:1999 formed part of a Time Temperature Superposition (TTS) creep program to establish a creep reduction factor RFcr in accordance with PD ISO/TR 20432:2007. As part of the process of establishing RFcr, in addition to the aforementioned TTS creep program, a Stepped Isothermal Method (SIM) program of creep testing was also carried out in accordance with ASTM D6992-03.

(40) FIG. 7 shows the resulting composite SIM/TTS creep regression plot for the geogrid produced in accordance with the invention (solid line 25), in comparison to a conventional HDPE uniaxial reinforcement geogrid (dashed line 23). The geogrid produced in accordance with the invention has RFcr of 72% at 10.sup.6 hours at 20° C., whilst the conventional uniaxial geogrid has RFcr of 47.5% at 10.sup.6 hours at 20° C.

(41) The data in FIG. 7 show that when the short-term tensile strength is multiplied by the creep reduction factor the potential long-term material efficiency benefit of the two materials tested increases further because of the greater creep resistance of the invention compared to a conventional HDPE geogrid
Invention=(85.4*72%)/0.50)=123(kN/m)/(kg/m.sup.2)
RE560=(94.0*47.5%)/0.62)=72(kN/m)/(kg/m.sup.2)

(42) Thus the gain in long-term creep limited tensile efficiency of the geogrid of the invention over an equivalent strength conventional uniax product is about 60% on the basis of weight of polymer.

Example 5 (FIGS. 8, 8A & 8B)

(43) Shrinkage Reversion test for molecular orientation.

(44) FIG. 8 is a photograph showing a single test of a tensile element (rib and bar) from a geogrid of the invention, the element being made from oriented HDPE (large element on the right of FIG. 8, also drawn as FIG. 8A). The element was held at 150° C. for 60 minutes to revert the polymer so the element shrank and the reverted element is shown on the left of FIG. 8 (also drawn as FIG. 8B). The rib part of the element had a starting length in the machine direction (MD) of 108 mm (the dimension labelled “f” in FIG. 8A) and a length at the end of the test (finishing length) of 11 mm (labelled “i” in FIG. 8B) which is a 9.8:1 draw ratio. The actual draw ratio of the rib during production was 10:1 so this test has an accuracy within 2%. The starting width of the rib in the transverse direction (TD) was 14 mm (labelled “g” in FIG. 8A), with a finishing width of 15 mm (labelled “j” in FIG. 8B) which is a 0.93:1 expansion. This confirms that the polymers in this rib were substantially un-oriented in TD. The dimensions of the bar part of the same element is also measured to have an initial width of 6 mm (labelled “h” in FIG. 8A), in MD direction and a 0.6 mm finishing width (labelled “k” in FIG. 8B), measured using a micrometer, which is a 10:1 draw ratio. This confirms that the bar part of the element had the same orientation in the MD as the rib part of the element. Starting thickness of the bar was 0.8 mm with the finishing thickness being 6.5 mm after reversion which is close to the 6.35 mm nominal thickness of the polymer sheet before it was stretched to orientate the polymer, which also provides confirmation of the draw ratio in MD.

(45) The invention being thus described, it will be apparent that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be recognized by one skilled in the art are intended to be included within the scope of the following claims.