GB2314802A

GB2314802A - Laminated geogrid

Info

Publication number: GB2314802A
Application number: GB9713359A
Authority: GB
Inventors: Frank Brian Mercer; Keith Fraser Martin
Original assignee: Netlon Ltd
Current assignee: Netlon Ltd
Priority date: 1996-07-02
Filing date: 1997-06-24
Publication date: 1998-01-14
Also published as: GB9713359D0

Abstract

A grid for soil reinforcement is formed by providing a layer (4, 32) with interstices or openings and providing on each face of the layer a respective array of parallel, spaced, molecularly-oriented, side-by-side strips (2,3), the strips being bonded to the layer and the strips on one face of the layer being bonded to respective strips on the other face of the layer through the interstices or openings. The layer may be in the form of a mesh or a fabric of open construction.

Description

LAMINATED GEOGRID Background of the Invention The present invention relates to laminated geogrids and methods of making them. High strength grids are widely used in civil engineering for a variety of geotechnical applications, and are termed geogrids. A geogrid will usually have meshes; however, if it does not have meshes, it will have thinner and weaker zones. A geogrid will have significant strength in at least one direction; a geogrid which is to be used as a multi-layer combination could have a strength of as low as about 5 or 10 kN per metre width (kN per metre width is expressed as kN/m) in at least one direction, but geogrids are usually thought of as having a strength of at least about 10 or 20 or 30 or 40 kN/m in at least one direction, the higher strengths being normally when strength is required primarily in one direction rather. than in two directions at 90'. Polymeric (plastics material) geogrids are now widely used, and are generally manufactured either by stretching apertured sheets of plastics material, eg as in US 4 374 798, EP 0 374 365A and GB 2 256 164A, or by weaving or knitting, eg as marketed under the "Fortrac" trade mark by Akzo and various distributors.

Stretched geogrids are limited in range by a number of features of their manufacturing process; in particular, grid pitches and aperture dimensions are limited by the stretching process and the ultimate strength and possible economy are limited by the known production methods and the scale of known production machinery.

Also, all stretched plastics material geogrids have areas of lesser oriented plastics material, which also reduces their economy.

Generally woven or knitted geogrids can only be manufactured slowly and for stability need to be coated after weaving or knitting.

Attempts have been made to satisfy the market with geogrids formed by joining two sets of orthogonal tapes together by welding where a tape of one set crosses a tape of the other set, for example, products developed in the US by Signode, and products developed in the UK by ICI PLC and promoted and marketed under the trade name "Paragrid". In such processes, high strength tapes are produced from oriented plastics material. In the case of Signode tapes, these are solid polyester tapes coated on one face with low melt temperature polymer.

"Paragrid" is an array of high strength filaments enrobed in low melt temperature polyethlene. Welding the tapes together was achieved by melting the low melt temperature component at the cross over points to enable them to bond together. Such products have only low junction shear strength and when used in geotechnical applications interlock with the soil can cause junctions to peel apart at very low loads.

US 4 618 385 discloses a variation where second strips are laminated to each side of first strips, again being bonded to the first strips only at the junctions (areas of contact) and therefore also suffering very low junction shear and peel strength.

The Invention The present invention provides methods as set forth in Claims 1, 16 or 18, geogrids as set forth in Claims 11, 12 or 17, and a geotechnical construction as set forth in Claim 19. The remaining Claims set forth optional or preferred features of the invention.

The invention can provide good long term creep properties and good economy ratios.

In general terms, the geogrid of the invention is made by taking a layer with interstices or openings therein, and bonding substantially parallel, spaced, molecularly-oriented, side-by-side strips on each face.

The strips on one face are bonded to respective strips on the other face through the interstices or openings in the layer, and can also be bonded to the layer itself, but not necessarily so. Thus the strips on each face are bonded to each other either by direct bonding in openings between filaments, fibres, strips or strands of the layer (ie the strips are in contact or have a minimal thickness of bonding agent between them), or by penetration of a bonding agent right through interstices in the layer so that there is continuous bonding agent from one strip to the other.

For convenience herein, said layer is termed the "intermediate layer", and said strips are termed the "bonded-on strips".

The bonded-on strips on respective faces of the intermediate layer are preferably substantially in register. However, it would be possible for instance to have a 75% overlap on width of two bonded-on strips of equal width on the respective faces, and bonded-on strips of different widths on the respective faces could be used, for instance with two narrow bonded-on strips on one face bonded to a broad bonded-on strip on the other face, or even a broad bonded-on strip on one face could be bonded to a single narrow bonded-on strip on the other face, within the width of the broad strip.

The bonded-on strips, or the strips of the intermediate layer referred to below, can be termed tapes, particularly when they are thin and flexible. They will usually be of generally rectangular cross-section, much wider than they are thick, eg at least four times wider, though the cross-section may taper down to the edges.

The strips need not be of uniform consistency - for instance they can have a core of polyester filaments which are enrobed in low density polyethylene.

The strength can be adjusted as appropriate. For instance, if a pair of bonded-on strips, one on each face of the intermediate layer, should have a strength of about 10 kN, then the pair could be made of two bonded-on strips each having a strength of about 5 kN, or alternatively of two bonded-on strips with respective strengths of about 8 kN and about 2 kN.

When reinforcing soil embankments, under certain circumstances, products having a strength of about 200 kN/m or greater are desirable and it is economically advantageous to have such products as wide as possible, eg 4 metres or greater. The invention is a very convenient method for producing very wide structures having a high axial strength.

In certain applications, it is advantageous to use geogrids which have a machine direction and transverse direction strength in excess of about 60 kN/m with the strength in each direction being equal. The invention is a convenient method for producing such structures.

In general, the finished geogrid may have the same axial and transverse strength, or for certain applications it may have a higher strength in one direction.

In certain applications, a particular combination of strengths in the two directions and/or a particular spacing of strands and/or a particular aperture size may be desired. The invention is a convenient way of meeting such requirements.

The intermediate layer may for instance be a fabric such as a non-woven fabric having suitable interstices, but in a general sense will comprise filaments, fibres, strips or strands. However, in a preferred arrangement, the intermediate layer comprises an array of generally parallel, spaced, side-by-side strips, the spaces between the strips providing said openings and the bonded-on strips being cross-laminated to the strips of the intermediate layer, preferably but not necessarily at generally 90.. The bonded-on strips are preferably bonded at least to some degree to the respective faces of the strips of the intermediate layer where the bonded-on strips cross over the strips of the intermediate layer, and the bonded-on strips are preferably bonded to each face of the strips of the intermediate layer. By bonding the bonded-on strips to respective faces of the strips of the intermediate layer, and by bonding the bonded-on strips together throughout their entire length (except possibly where they pass over the strips of the intermediate layer), the integrity of the structure is greatly increased.

For any given plan area of junction and given method of bonding (or joining), the shear strength of the junction created by bonding the bonded-on strips to respective faces of the strips of the intermediate layer will be about twice that of a single-sided junction. Whether or not bonding of respective faces at cross-overs occurs, the peel failure of single-sided junctions cannot occur.

The strips of the intermediate layer can be of any suitable construction, and can already be linked together before applying the bonded-on strips. Examples which provide suitable intermediate layers are: (a) A mesh structure formed by stretching a plastics starting material having a pattern of holes or depressions to form strips extending in the stretch direction, stretching being continued until the strips are of fairly uniform thickness throughout their lengths.

(b) A mesh structure formed by uniaxially stretching a plastics starting material having a pattern of holes or depressions, so that the strips are interconnected by oriented, small connecting strips, as described in US 4 618 385 (Mercer).

(c) A mesh structure formed by biaxially or uniaxially stretching a plastics material having a pattern of holes or depressions, generally in accordance with GB 1 250 478 (Netlon) or US 3 252 181 (Hureau).

(d) A uniaxially oriented or biaxially oriented mesh structure generally in accordance with US 4 374 798 (Mercer) or a mesh structure generally in accordance with EP 0 374 365A (R.D.B.).

(e) An array of spaced, unconnected tapes, which need not be of uniform consistency, eg having high strength filaments enrobed in a lower-strength material; the filaments could be for instance of plastics material, eg polyester, or metal or glass they could all be enrobed in plastics materials but by way of example, glass could be enrobed in bitumen. The tapes may be knitted, woven or braided or any other construction.

(f) A fabric which may be woven, non-woven or knitted.

When such a fabric is used, either it would be of sufficiently open construction for the bonded-on strips to bond at openings in the structure or it would be sufficiently thin for the bonding agent bonding the two parts of the bonded-on strips together to penetrate right through the fabric.

If (a), (b), (c) or (d) is used, there will normally be thicker portions at the junctions or at the small, interconnecting strands. It is desirable, though not essential, that the bonded-on strips should extend between, rather than over, such thickened portions.

It is preferable for speed and economy of manufacture that the strips of the intermediate layer (if present) are in accordance with (a), (b), (c) or (d) above.

The bonded-on strips can be of any suitable construction, eg as described above for the intermediate layer strips in (d) or individually extruded and oriented flat plastics material tapes or tapes slit from extruded and oriented sheet or fibre glass tapes (which are effectively molecularly oriented).

If a bonded-on strip is formed of two or more materials (before bonding-on), the materials may be combined as the bonded-on strips are bonded to the intermediate layer or at any other time.

The bonded-on strips can be bonded together in register in any suitable manner, eg by coextruding the strips with a layer of low melting point resin on one face thereof and (if required) coextruding the strips of the intermediate layer with a layer of low melting point resin on each side thereof, and bonding by heating and melting the low melting point resin, for instance by flame bonding. Hot melt adhesives could be used.

Each bonded-on strip is preferably bonded along substantially the whole of its length, alternate zones being bonded to the bonded-on strip on the other face of the intermediate layer and to a strip of the intermediate layer; however, there may be small triangular gaps between the bonded-on strips where they separate to pass over a strip of the intermediate layer, and the bonded-on strips need not be bonded to the strips of the intermediate layer.

Preferably, the bonded-on strips on one face of the intermediate layer are flat or nearly flat, the bonded-on strips on the other face being suitably contoured where they pass over the strips of the intermediate layer; some deviation from flatness may be difficult to avoid in manufacture, but it is preferred that the extension of the flat or nearly flat bonded-on strip merely due to straightening (after slitting the structure apart) is not more than about 10%, and more preferably not more than about 5% or not more than about 2, or about 1, or about 1/2%; more simply, the departure from flatness can be measured by laying a rule along the face of the bonded-on strip - it is preferred that the average depth of the dips from one face is at least about three times that on the other (nearly flat) face, and more preferably at least about five or about ten times. It is preferred that the flat or nearly flat bonded-on strip has a greater strength than the facing bonded-on strip, more preferably much greater, eg about 2x or about 3x or up to even about 9x or more.

Any suitable materials can be used for the strips (and the bonded-on strips need not be made of the same materials as the strips of the intermediate layer), preferred materials being oriented polymers, eg polypropylene, high or low density polyethylene, polyester, polyamide and PVC. The bonded-on strips on the two faces need not be made of the same material, provided they have sufficient strength to resist peel and provided the bonding surfaces are compatible; for instance, the strips on one face could be of polyester and the strips on the other face of polypropylene.

In a manufacturing process, it is preferred to have the intermediate layer strips running in the transverse direction and to apply the bonded-on strips in the machine direction.

The Drawings The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an isometric, schematic view of a first geogrid in accordance with the invention; Figure 2 is a longitudinal section through a second geogrid in accordance with the invention; Figure 3 is a plan view of a third geogrid in accordance with the intention; Figure 4 is a plan view of a fourth geogrid in accordance with the invention; Figure 5 is an isometric view of a fifth geogrid in accordance with the invention; Figure 6 is a plan view of a sixth geogrid in accordance with the invention; Figure 7 is a schematic view of a first method for making the geogrids; Figure 8 is a schematic view of a second method for making the geogrids; Figures 9 to 13 are vertical sections through different geotechnical constructions incorporating the geogrid of the invention; and Figure 14 is an isometric view of a seventh geogrid or reinforced geotextile in which the function of the first strips is performed by a non-woven fabric.

Figure 1 A geogrid 1 is formed by cross-laminating bonded-on strips 2, 3 to strips 4 forming an intermediate layer.

Each strip 1, 2, 3 is like a tape, of roughly rectangular cross-section. In each set, the strips 1, 2, 3 are parallel, spaced, and side-by-side. The bonded-on strips 2, 3 on respective faces of the array of strips 4 are bonded together in register between the strips 4, and are bonded to respective faces of the strips 4 where the strips 2 cross over the strips 4.

If bonding is achieved using a layer of low melting point resin on the strips 2, 3, the thickness of the layer on both strips 2, 3 is the minimum possible to enable a good continuous bond to be produced.

In Figure 1, each bonded-on strip is of similar thickness to the facing bonded-on strip 3. This causes strips 2, 3 to be contoured or crimped to mate with the contour of the strip 4 at the junctions, ie where the strips 2, 3 pass over a strip 4.

Figure 2 Figure 2 illustrates a geogrid 1 which is similar to that of Figure 1; but the bonded-on strip 5 is substantially thicker than the facing bonded-on strip 6, preferably at least twice as thick. The strip 5 is flat where it passes over a strip 4, the strip 6 being contoured to mate with the contour of the strip 4. The strip 5 is the high strength element and there is no crimping to be straightened out so that the strips 5 would not elongate under load due to this effect. The actual strain under load would be dependent upon the material of the strips 5.

Figure 3 In Figure 3, the intermediate layer strips 4 have been provided by making a mesh structure as described in GB 2 124 965A. The strips 4, which are oriented in their longitudinal direction, are interconnected by small interconnecting strands 7 which are oriented in their own longitudinal direction. Where the interconnecting strands 7 meet the strips 4, there are small lumps or nodules 8, and the bonded-on strips 2, 3 lie between these nodules 8. The strips 2, 3 can be formed as shown in Figure 1 or, preferably, as shown in Figure 2.

Figure 4 In Figure 4, an intermediate layer has been formed by biaxially or uniaxially stretching an apertured plastics material mesh structure as described in US 4 374 798 or by biaxially or uniaxially stretching a mesh structure manufactured in accordance with GB 1 250 478 or US 3 252 181. The intermediate layer has oriented strands 9 interconnected by oriented strands 10 meeting at junctions which contain thicker lumps or nodules 11, or by unoriented strands or bars 10. The bonded-on strips 2, 3 preferably lie between the strands 10. The bonded-on strips 2, 3 can be formed as in Figure 1 or as in Figure 2.

Figure 5 In Figure 5, the intermediate layer strips 4 have been formed by stretching apertured plastics starting material manufactured in accordance with US 3 252 181 set up in the particularly advantageous way that the strips 4 have an "aerofoil" cross-section as shown in Figure 5 tapering down towards the edges. This cross-section form enables both parts of the bonded-on strips 2, 3 to be easily contoured to mate with the contours of the strips 4, with minimal triangular zones of unbonded material.

Figure 6 In Figure 6, intermediate layer strips 4 have been formed by stretching an apertured plastics starting material so that they are of substantially uniform thickness throughout their length, but are interconnected by short bars 12 whose centre portions have not been orientated and are therefore relatively thick. The bonded-on strips 2, 3 lie between the bars 12. The strips 2, 3 can be formed as in Figure 1 or as in Figure 2.

Figure 7 Figure 7 shows two laminating rolls 21 associated with flame heads or hot air jets or high intensity radiant heaters 22. The intermediate layer strips 4 extend in the transverse direction and can be of any suitable type. They are fed between the rolls 21 as indicated by the arrow. If, as is preferred, the strips 4 have a low melting point resin on both faces, there can be further flame heads or hot air jests or high intensity radiant heaters 23. The bonded-on strips 2, 3 before bonding have a low melting point resin on that face which is towards the flame heads-or hot air jets or high intensity radiant heaters 22.

If the intermediate layer strips 4 are manufactured as a continuous assembly, eg in accordance with US 4 374 798 (uniaxially or biaxially oriented), GB 1 250 478 or US 3 252 181, it may be advantageous to install the laminating assembly of Figure 6 at the exit to the stretching machine which is forming the strips 4. At this position, laminating may be carried out whilst the strips 4 are still hot from being stretched, and thereby save energy by eliminating items 23.

Figure 8 Figure 8 illustrates an arrangement similar to that of Figure 6, but where the bonded-on, reinforcing strips 2, 3 are provided by high strength filaments, fed through reeds (not shown) in order to provide proper spacing of the filaments, which are enrobed by unoriented plastics material strips 24; the filaments are bonded in register with each other between the intermediate layer strips 4 and to the respective faces of the strips 4 where they pass over the strips 4.

In this embodiment, the high strength filaments need not be fed in equal quantities to both sides of the strips 4 and the plastic material strips 24 do not need to be of equal thickness, provided each strip 2, 3 has sufficient strength to resist peel.

Figures 9 to 13 Figures 9 to 13 are schematic illustrations of known geotechnical structures, in which geogrids 25 in accordance with the invention are used in place of earlier geogrids.

Briefly: Figure 9 shows a retained wall 26 formed of full-height panels which can be vertical or inclined by up to 45 to the vertical. The geogrids 25 are in parallel layers and fixed to the wall 26.

Figure 10 shows a retained wall 27 formed of discrete blocks or incremental panels, which wall 27 can be vertical or inclined by up to 45 to the vertical. The geogrids 25 are fixed to the blocks or panels or secured between them in any suitable manner, eg by friction or using mortar or pins or special fixings.

Figure 11 shows the face of a retained embankment 28 with the geogrids 25 in parallel layers and the end of each geogrid 25 taken up at the face and brought back into the soil, the brought-back part 29 being in contact with the geogrid 25 of the next layer up.

Figure 12 shows a simpler retained embankment 30 construction, with the geogrids 25 terminating at the face.

The geogrids 25 in the geotechnical structures illustrated in Figure 9 to 12 are required to carry tensile loads in the direction shown across the page. Geogrids used in such applications would typically have strengths of from about 40 kN/m in that direction up to about 200 kN/m or more, depending on the height, style and construction of the geotechnical structure and the loads to be imposed upon it. In the direction at right angles to the page, the geogrid may in some applications have very low strength, just sufficient to ensure that the geogrid does not rupture while being installed.

Figure 13 shows a geotechnical structure 31 whose base is stabilised by a geogrid 25. There may be multiple, vertically-spaced layers of geogrid 25 in any suitable arrangement, above the geogrid 25 shown. The structure 31 may be an embankment, a road, a parking area or for any other purpose. The ratio of height to width shown in Figure 13 is illustrative of one condition only and is not limiting.

If the geotechnical structure shown in Figure 13 is a road or parking area for light vehicles, eg passenger cars only, then a geogrid with a strength of about 10-20 kN/m in each direction may be adequate. At the other extreme, geogrids of strengths of about 200 kN/m or more in one direction (or in both orthogonal directions) may be needed for the basal reinforcement of embankments over soft ground.

Figure 14 shows a geogrid 1 in which the function of the intermediate layer is performed by a non-woven fabric 32. The bonded-on strips 2, 3 before bonding had a sufficiently thick layer of adhesive material or low-melt-temperature polymer on the faces to be bonded for this adhesive or low-melt-temperature polymer to penetrate right through the fabric 32 to form a joint 33 between the strips 2, 3.

Example 1 A grid was made generally as in Figures 2 and 3.

The strips 4 and interconnecting strands 7 were made generally as described in US 4 618 385, and specifically as follows: Starting material: (a) Main core (80%) - polypropylene homopolymer (b) Surface layers (2x10%) - polypropylene low molecular weight copolymer - Thickness (total) .................. . 2.4 mm - Hole shape ....... Rectangular - Hole size (stretch direction) 12. 7 mm - Hole size (direction at 90 ) ........ . 6. 35 mm - Hole pitch (stretch direction) ... 14. 3 mm - Hole pitch (direction at 90 ) .23.8 mm mm - w: d ratio, main zone ................... 7. 3: 1 - Temperature of stretching ..... .. 105 C - Stretch ratio on main strands ........... 8. 8. 8: 1 - Main strand width reduction (measured at mid-point) ... ....... 64% - Maximum opening size measured at 90 to stretch direction ............. 17.5 mm - Calculated stretch ratio on interconnecting strands 7 .... ....... 2. 8: 1 - Longitudinal strand thickness (mid-point = thinnest point) . .... 0.48 mm - Longitudinal strand width (mid-point = narrowest part) ............ 6. 3 mm - w: d ratio longitudinal strand 13: 1 - Interconnecting strand thickness (mid-point) ................. ..... 0. 5 mm - Interconnecting strand width (mid-point) ............................. 0.25 mm - Thickness of thickest point of structure ............................ 1. 1 mm - Pitch of main strands 4 ................. 23. 8 mm - Pitch of interconnecting strands 7 126 mm The bonded-on strips 5, 6 were as follows: Strips 5 (a) Main core (95%) - polypropylene homopolymer (b) One surface layer ( 5%) - polypropylene low molecular weight copolymer - Thickness as stretched - about 1 mm Strips 6 (a) One face (75%) - polypropylene homopolymer (b) The other face (25%) - polypropylene low molecular weight copolymer - Thickness as stretched - about 0. 25 mm For both strips 5, 6: - Thickness as stretched (total) .......... 1.25 mm - Width as stretched ...................... 50 mm - Temperature of stretching ............... 105 C - Stretch ratio ........................... 8: 1 - Pitch ................................... 126 mm Example 2 A geogrid was made generally as in Figure 14.

The fabric 32 was a needle-bonded non-woven polypropylene fabric with a weight of approximately 110 g/m2 and a compressed thickness of approximately 0.45 mm.

Each bonded-on strip 2, 3 was 20 mm wide x 1. 25 mm thick, and comprised a 1 mm thick layer of longitudinally-oriented polypropylene homopolymer and on one face a 0. 25 mm thick surface layer of polypropylene low-molecular-weight copolymer.

The strips 2, 3 were assembled at a pitch of 100 mm on to the fabric 32 to give a product with a weight of approximately 520 g/m2 and a strength in the direction of the strips 2, 3 of approximately 200 kN/m.

* * * * * The present invention has been described above purely by way of example, and modifications can be made within the spirit of the invention. The invention also consists in any individual features described or implicit herein or shown or implicit in the drawings, or any combination of any such features or any generalisation of such features or combination.

Claims

Claims 1. A method of making a laminated geogrid, comprising: providing a layer with interstices or openings therein; and providing on each face of said layer a respective array of substantially parallel, spaced, molecularly-oriented, side-by-side strips, said strips being bonded to said layer and said strips on one face of said layer being bonded to respective said strips on the other face of said layer through said interstices or openings.
2. The method of Claim 1, wherein said layer comprises an array of generally parallel, spaced, side-by-side strips, the spaces between the strips of said layer providing said openings, said bonded-on strips being cross-laminated to said strips of said layer.
3. The method of Claim 2, wherein respective said bonded-on strips are bonded to each face of the strips of said layer.
4. The method of Claim 2 or 3, wherein said bonded-on strips comprise material oriented in the longitudinal direction of the bonded-on strips.
5. The method of any of the preceding Claims, wherein said layer is formed by stretching a plastics starting material having a pattern of holes or depressions.
6. The method of any of Claims 1 to 4, wherein said layer is formed by the method of Claim 1 or Claim 3 of GB 2 124 965B or is the mesh structure of Claim 25 of GB 2 124 965B.
7. The method of any of the preceding Claims, wherein the bonded-on strips and/or the strips of said layer comprise individual filaments.
8. The method of any of the preceding claims, wherein the thickness or strength of the bonded-on strips on one face of said layer is substantially greater than that of the bonded-on strips on the other face of said layer.
9. The method of Claim 8, wherein the thicker or stronger bonded-on strip is substantially flat, the thinner or weaker bonded-on strip being contoured to mate with contours of said layer.
10. The method of Claim 1, wherein said layer is a non-woven fabric.
11. A laminated geogrid made by the method of any of the preceding Claims.
12. A laminated geogrid, comprising: a layer which had interstices or openings therein; and on each face of said layer, a respective array of substantially parallel, spaced, molecularly-oriented, side-by-side strips, said strips being bonded to said layer and the strips on one face of said layer being bonded to respective strips on the other face of said layer through said interstices or openings.
13. The geogrid of Claim 12, wherein said layer comprises an array of generally parallel, spaced, side-by-side strips, the spaces between the strips of said layer providing said openings, said bonded-on strips being cross-laminated to said strips of said layer.
14. The geogrid of Claim 13, wherein said bonded-on strips are bonded to each face of the strips of said layer.
15. The geogrid of any of Claims 12 to 14, wherein on one face of said layer, said bonded-on strips are substantially flat and are thicker or stronger than said bonded-on strips on the other face of said layer, the bonded-on strips on the other face of said layer being contoured where they pass over the strips of said layer.
16. A method of making a laminated geogrid, substantially as herein described with reference to any of Figures 1 to 8 and 14 of the accompanying drawings.
17. A laminated geogrid, substantially as herein described with reference to, and as shown in, any of Figures 1 to 6 and 14 of the accompanying drawings.
18. A method of strengthening soil, comprising embedding in the soil the geogrid of any of Claims 11 to 15 and 17.
19. A geotechnical construction, comprising a mass of particulate material strengthened by embedding therein the geogrid of any of Claims 11 to 15 and 17.