Plastics material mesh structures

A biaxially-stretched substantially uniplanar integral plastics material geogrid for use in a geotechnical construction having a greater strength in a primary direction (PD) than in a secondary direction (SD) substantially at right angles to the PD and having a first face and a second face, made from a starting material which has a thickness of at least about 2 mm, and comprising oriented PD strands and oriented SD strands, interconnected by junctions whose mid-point thickness is substantially greater than the mid-point of any oriented strand entering the junction, adjacent sides of adjacent oriented PD and oriented SD strands being interconnected by crotches whose edges for at least part of their length are oriented with orientation running in the direction around the crotch, said PD and SD strands and said crotches defining mesh openings substantially free of fibrils, the orientation ofthe PD strands extending into and through the junctions, and the junctions comprising a central zone which is thicker than thinner zones in the junction adjacent the ends of oriented PD and oriented SD strands, the junction central zone having on each face thereof a narrow generally diagonally-extending projection at the corner extending outwards between said thinner zones, continuing through the crotch between the oriented PD and oriented SD strands and running into the crotch edge, with no point on a line of maximum thickness on the geogrid from the junction mid-point to the crotch edge having a thickness of less than about 80% of the thickness of the junction mid-point.

A method of producing a biaxially-stretched plastics material geogrid for use in a geotechnical construction, having a greater strength in a primary direction (PD) than in a secondary direction (SD) substantially at right angles to the PD, comprising:

• providing a substantially uniplanar plastics starting material which has a thickness of at least about 2 mm and has a pattern of through-holes on a substantially square or rectangular grid whose axes are substantially parallel to the PD and to the SD respectively, the sides of at least some PD end portions of said holes being defined by crotch-forming zones in which, as seen in plan view as one progresses towards the PD end of the hole, each said crotch-forming zone has:
• (i) a first part having a side which is progressively more inclined to the PD and is defined at least in part by a curve which is concave with respect to the hole;
• (ii) a sccond part having a side which is progressively less inclined to the PD and is defined at least in part by a curve which is convex with respect to the hole;
• and
• (iii) a third part having a side which is progressively more inclined to the PD and is defined at least in part by a curve which is concave with respect to the hole;
• applying PD stretch to form oriented PD strands and to apply some orientation to the junction-formning zones so that orientation extends into and through the junction-forming zones;
• and
• applying SD stretch with an overall stretch ratio of at least about 1.5:1 as measured from the mid-point of one junction-forming zone to the mid-point of the adjacent junction-forming zone in the SD, to form oriented SD strands;
• in the geogrid so produced, the mid-point of the junction zone being substantially thicker than the mid-point of any oriented strand entering the junction zone, and the edge of the length of the crotch interconnecting adjacent sides of adjacent oriented PD and oriented SD strands being oriented in the direction running around the crotch with the orientation ratio decreasing significantly as one passes around the crotch edge either from the oriented PD strand or from the oriented SD strand, the crotch edge either a) having an unoriented part, or b) the thickness of the least oriented part of the crotch edge being reduced, or the length of the least oriented part of the crotch edge being increased, by no more than about 20% by the action of stretching, and the action of stretching not reducing the thickness of any point along generally diagonally-extending lines of maximum thickness on the biaxially-stretched mesh structure from the mid-point of the junction zone to the crotch edges to such an extent that the ratio of finished thickness to starting thickness at that point is less than about 80% of the ratio of finished thickness to starting thickness of the junction zone mid-point.

A method of producing a biaxially-stretched plastics material geogrid for use in a geotechnical construction, having a greater strength in a primary direction (PD) than in a secondary direction (SD) substantially at right angles to the PD, comprising:

• providing a substantially uniplanar plastics starting material which has a thickness of at least about 2 mm and has a pattern of through-holes on a substantially square or rectangular grid whose axes are substantially parallel to the PD and to the SD respectively, the sides of at least some PD end portions of said holes being defined by crotch-forming zones in which, as seen in plan view as one progresses towards the PD end of the hole, each said crotch-forming zone has:
• (i) a first part having a side which is progressively more inclined to the PD axis and is defined at least in part by a curve which is concave with respect to the hole;
• (ii) a second part having a side which is progressively less inclined to the PD and is defined at least in part by a curve which is convex with respect to the hole;
• and
• (iii) a third part having a side which is progressively more inclined to the PD and is defined at least in part by a curve which is concave with respect to the hole;
• applying PD stretch to form oriented PD strands and to apply some orientation to the junction-formning zones so that orientation extends into and through the junction-forming zones;
• and
• applying SD stretch with an overall stretch ratio of at least about 1.5:1 as measured from the nmid-point of on junction-forming zone to the mid-point of the adjacent junction-forming zone in the SD, to form orientated SD strands;
• in the geogrid so produced, the mid-point of the junction zone being substantially thicker than the mid-point of any oriented strand entering the junction zone, and the edge of the length of the crotch interconnecting adjacent sides of adjacent oriented PD and oriented SD strands being oriented in the direction running around the crotch with the orientation ratio decreasing significantly as one passes around the crotch edge either from the oriented PD strand or from the oriented SD strand, the crotch edge either a) having an unoriented part, or b) the thickness of the least oriented part of the crotch edge being reduced, or the length of the least oriented part of the crotch edge increased, by not more than about 20% by the action of the stretching.

A method of producing a biaxially-stretched plastics material geogrid for use in a geotechnical construction having a greater strength in a primary direction (PD) than in a secondary direction (SD) substantially at right angles to the PD, comprising:

• providing a substantially uniplanar plastics starting material which has a thickness of at least about 2 mm and has a pattern of through-holes on a substantially square or rectangular grid whose axes are substantially parallel to the PD and to the SD respectively, and wherein the SD punch-out is less than about 60%, the sides of at least some PD end portions of said holes being defined by crotch-forming zones in which, as seen in plan view as one progresses towards the PD end of the hole, each said crotch-forming zone has:
• (i) a first part having a side which is progressively more inclined to the PD axis and is defined at least in part by a curve which is concave with respect to the hole;
• (ii) a second part having a side which is progressively less inclined to the PD and is defined at least in part by a curve which is convex with respect to the hole;
• and
• (iii) a third part having a side which is progressively more inclined to the PD and is defined at least in part by a curve which is concave with respect to the hole;
• applying PD stretch to form oriented PD strands and to apply some orientation to the junction-forming zones so that orientation extends into and through the junction-forming zones;
• and
• applying SD stretch with an overall stretch ratio of at least about 1.5:1 as measured from the mid-point of on junction-forming zone to the mid-point of the adjacent junction-forming zone in the SD, to form orientated SD strands;
• in the geogrid so produced, the mid-point of the junction zone being substantially thicker than the mid-point of any oriented strand entering the junction zone, and the edge of the length of the crotch interconnecting adjacent sides of adjacent oriented PD and oriented SD strands being oriented in the direction running around the crotch with the orientation ratio decreasing significantly as one passes around the crotch edge either from the oriented PD strand or from the oriented SD strand, the crotch edge either a) having an unoriented part, or b) the thickness of the least oriented part of the crotch edge being reduced, or the length of the least oriented part of the crotch edge increased, by not more than about 20% by the action of the stretching.

A substantially uniplanar biaxially-stretched integral plastics material mesh structure geogrid for use in a geotechnical construction, having a greater strength in a primary direction (PD) than in a secondary direction (SD) substantially at right angles to the PD and having a first face and a second face, made from a starting material which has a thickness of at least about 2 mm, and comprising oriented PD strands and oriented SD strands, interconnected by junctions whose mid-point thickness is substantially greater than the mid-point of any oriented strand entering the junction, adjacent sides of adjacent oriented PD and oriented SD strands being interconnected by crotches whose edges for at least part of their length are oriented with orientation running in the direction running around the crotch, said PD and SD strands and said crotches defining mesh openings substantially free of fibrils, the orientation of the PD strands extending into and through the junctions, and the junctions comprising a central zone which is thicker than thinner zones in the junction adjacent the ends of oriented PD and oriented SD strands, the junction central zone having on each face thereof a narrow generally diagonally-extending projection at the corner extending outwards between said thinner zones, continuing through the crotch between the oriented PD and oriented SD strands and running into the crotch edge, with no point on a line of maximum thickness on the geogrid from the junction mid-point to the crotch edge having a thickness of less than about 80% of the thickness of the junction mid-point.

A method of producing a biaxially-stretched plastics material geogrid for use in a geotechnical construction, having a greater strength in a primary direction (PD) than in a secondary direction (SD) substantially at right angles to the PD, comprising:

• providing a substantially uniplanar plastics starting material which has a thickness of at least about 2 mm and has a pattern of through-holes on a substantially square or rectangular grid, the SD punch-out being less than about 60%, the axes of said grid being substantially parallel to the PD and to the SD respectively, the sides of at least some PD end portions of said holes being defined by crotch-forming zones in which, as seen in plan view as one progresses towards the PD end of the hole, each said crotch-formning zone has:
• (i) a first part having a side which is progressively more inclined to the PD and is defined at least in part by a curve which is concave with respect to the hole;
• (ii) a second part having a side which is progressively less inclined to the PD and is defined at least in part by a curve which is convex with respect to the hole;
• and
• (iii) a third part having a side which is progressively more inclined to the PD and is defined at least in part by a curve which is concave with respect to the hole;
• applying PD stretch to form oriented PD strands and to apply some orientation to the junction-forming zones so that orientation extends into and through the junction-forming zones;
• and
• applying SD stretch with an overall stretch ratio of at least about 1.5:1 as measured from the mid-point of one junction-forming zone to the mid-point of the adjacent junction-forming zone in the SD, to form oriented SD strands;
• the geogrid so produced having a first face and a second face, and in said geogrid, the junction zone forming a junction which interconnects two oriented PD strands entering the junction from opposite sides and two oriented SD strands approaching or entering the junction from two other opposite sides, the junction comprising a central zone which is thicker than thinner zones in the junction adjacent the ends of the oriented PD and oriented SD strands and the central zone being substantially square or rectangular and having a narrow projection at each corner thereof extending outwards between said thinner zones, continuing through the crotch between the respected oriented PD and oriented SD strands and running into the crotch edge, the mid-point of the junction zone being substantially thicker than the mid-point of any oriented strand entering the junction zone, and the edge of the length of the crotch interconnecting adjacent sides of adjacent oriented PD and oriented SD strands being oriented in the direction running around the crotch with the orientation ratio decreasing significantly as one passes around the crotch edge either from the oriented PD strand or from the oriented SD strand, the crotch edge either a) having an unoriented part, or b) the thickness of the least oriented part of the crotch edge being reduced, or the length of the least oriented part of the crotch edge being increased, by no more than about 20% by the action of stretching, and the action of stretching not reducing the thickness of any point along generally diagonally-extending lines of maximum thickness on the biaxially-stretched mesh structure from the mid-point of the junction zone to the crotch edges to such an extent that the ratio of finished thickness to starting thickness at that point is less than about 80% of the ratio of finished thickness to starting thickness of the junction zone mid-point.