Patent Application
20170009420
The present invention relates to the technical field of the reinforcement of ground for constructing retaining walls. In particular, it relates to a stabilisation strip, also referred to as a geostrip, which is reinforced and can be used for structures made of reinforced soil or reinforced earth for constructing retaining walls.
A reinforced embankment structure comprises an embankment, a facing and reinforcements, connected or not to the facing.
The embankment is composed of a mixture or assembly that may comprise at least one material from sand, gravel, fine earth, crushed rocks, recycled material such as materials from demolition of buildings or civil engineering structures, industrial residues or binders such as lime or cement.
The facing provides the aesthetic appearance and stability of the structure with respect to erosion by covering the front face of the retaining wall, i.e. the invisible face. It is usually produced using juxtaposed prefabricated concrete elements, in the form of slabs or blocks. It may also consist of metallic welded lattice panels or gabions made of woven metal wires.
The reinforcements may be made of various materials, such as metal (and more particularly galvanised steel) and synthetic materials. They are disposed within the embankment with a density depending on the stresses that may be exerted on the structure, the thrust forces from the land being absorbed by friction between the embankment and the reinforcements.
In the great majority of cases, the reinforcements are provided in the form of stabilisation strips having a length of approximately 3 m to 10 m, although shorter or longer strips may be used. The width of the strips is generally between 4 cm and 10 cm, although it is possible to use strips with a width ranging up to 10 cm or 25 cm, or even more. The thickness varies, for example from approximately 1 mm to a few centimetres and is generally between 1 mm and 6 mm. These stabilisation strips transmit the forces within the embankment, thus distributing the forces.
In particular, it is necessary to transmit the forces between a stabilisation strip and the embankment in which it is placed. Moreover, it is preferable for the stabilisation strip to be capable of transmitting the forces over its entire length.
One solution known to the person skilled in the art consists in using stabilisation strips comprising a longitudinal sheath that interacts with the embankment by friction. The stabilisation strips also comprises a reinforcement composed of a set of fibres disposed longitudinally, parallel to one another and embedded inside the sheath in its central part so as to reinforce the tensile strength. The sheath is generally made of polyethylene, and the fibres of polyester. When necessary, a solution known to the person skilled in the art for increasing friction resistance between the strips and the embankment consists in providing the longitudinal sheath with a central part comprising the reinforcement fibres and projecting lateral parts in order better to interact with the grains constituting the embankment.
Polyester fibres have the drawback of being sensitive to surrounding alkalinity and may degrade when the stabilisation strips that enclose them are used for example in basic soils. This is for example the case with fine soils treated with lime or hydraulic binders for improving their workability and/or stability.
Thus it is advantageous to be able to use other types of fibre that are not very sensitive to the nature of the embankment; for example polyvinyl alcohol fibres.
The inventors have attempted to produce stabilisation strips comprising a polyethylene sheath and a reinforcement composed of a set of polyvinyl alcohol fibres.
During some adhesion tests between these stabilisation strips and the embankment, under strong embankment confinements, it has happened that the fibres slide within the sheath where the strip should keep its integrity and that that the strip is caused to slide with respect to the embankment surrounding it. It was concluded therefrom that the absence of a chemical bond between the polyethylene sheath and polyvinyl alcohol fibres led to insufficient adhesion strength between the fibres and the sheath.
The present invention therefore seeks to overcome the drawbacks of the prior art described above. In particular, the present invention seeks to make it possible to produce stabilisation strips that are not sensitive to their environment (and preferably able to be used for various types of embankment) while having high tensile strength and being able to measure their mechanical properties reliably.
To this aim, the present invention provides a reinforced stabilisation strip for reinforced embankment structures, comprising long reinforcement fibres and a longitudinal sheath surrounding or enclosing the long reinforcement fibres, the sheath being at least partially made from a functionalised polymeric material comprising a functionalised polyolefin.
The polyolefin functionalisation makes it possible to provide the functionalised polymeric material of the sheath with functional groups with which the material of the reinforcement fibres can react, thus creating bonds between the reinforcement fibres and the sheath that prevent disconnection thereof by increasing the adhesion force between the reinforcement fibres and the sheath.
Other optional and non-limiting features are presented below.
The functionalised polyolefin advantageously comprises 0.01 wt. % to 45 wt. % functionalisation.
The functionalised polymeric material may comprise a mixture of non-functionalised polymer and functionalised polyolefin. Preferably, the non-functionalised polymer is a non-functionalised polyolefin. Preferably, the non-functionalised polyolefin is a non-functionalised polyethylene, still preferably a non-functionalised linear low-density polyethylene. The mass ratio between functionalised polyolefin and non-functionalised polymer is between 1:9 and 10:0.
The functionalised polymeric material advantageously has a functionalisation gradient with a maximum at contact with the reinforcement fibres and which decreases gradually and away from the reinforcement fibres.
In a particular embodiment of the invention, the functionalised polyolefin is a polyolefin substituted by a chemical element having a functional group chosen from mono- or di-carboxylic acid anhydrides or on which the chemical element has been grafted. Preferably, the chemical element is a maleic anhydride or a phthallic anhydride group or an acrylic acid.
The sheath may further comprise a non-functionalised zone surrounding or enclosing the functionalised polymeric material. This non-functionalised zone is a non-functionalised polymer, for example the same non-functionalised polymer of the mixture forming the functionalised polymeric material, or other.
The reinforcement fibres are advantageously made of a material chosen from polyvinyl alcohol, polyesters, silica glass, linear or aromatic polyamides and metals. The reinforcement fibres may be in the form of threads, strands or cords; these threads, strands or cords may be spun or woven.
The sheath may further comprise at least one longitudinal edge free from reinforcement fibres and having notches.
The stabilisation strip may have two longitudinal ends joined to each other, thus adopting the form of a loop.
The present invention also proposes a stabilisation layer made at least partly of stabilisation strips as described above. This stabilisation layer may be in the form of a geogrid formed by a warp and a weft composed of stabilisation strips (1), the warp and weft being woven or superimposed one on the other. The stabilisation strips of the warp and weft are connected at certain intersection points by hot welding or adhesive bonding.
The present invention also proposes a reinforced embankment structure comprising:
This reinforced embankment structure may further comprise a facing and connectors for connecting at least some of the stabilisation strips and/or stabilisation layers to the facing. These connectors may also be formed by stabilisation strips, in particular those having a loop form.
Finally, the present invention provides a method for manufacturing a stabilising strip as described above, said method comprising:
The method may further comprise drawing the reinforcement fibres, and shaping the functionalised polymeric material may be carried out by extruding the functionalised polymeric material around the reinforcement fibres.
The method may also comprise heating the non-functionalised polymer and drawing the reinforcement fibres;
in which shaping the functionalised polymeric material is carried out by co-extruding the functionalised polymeric material around the reinforcement fibres and the non-functionalised polymer around the functionalised polymeric material forming the non-functionalised zone of the sheath surrounding or enclosing the functionalised polymeric material.
Drawing the reinforcement fibres is advantageously carried out as the sheath is extruded.
Other objectives, features and advantages will become apparent from a reading of the following detailed description with reference to the drawings given by way of illustration and non-limitatively, among which:
With reference to
This stabilisation strip 1 comprises long reinforcement fibres 12 and a longitudinal sheath 11 surrounding or enclosing the long reinforcement fibres 12. The sheath 11 is at least partially made from a functionalised polymeric material comprising a functionalised polyolefin (Po-f).
A polyolefin is a saturated aliphatic polymer, optionally substituted, obtained from polymerising an olefin (also referred to as an alkene).
The functionalised polyolefin can be chosen from functionalised polyethylenes, functionalised polypropylenes or functionalised olefinic copolymers such as functionalised ethylene vinyl acetate (EVA). Preferably functionalised polyethylenes and in particular functionalised linear low-density polyethylene will be chosen.
The term “functionalisation” will be understood in the context of the present invention to designate a modification of the polyolefin by substituting it with a chemical element comprising a functional group or an insaturation or by grafting the chemical element onto the polyolefin. Modification of the polyolefin may also lead to the creation of an insaturation in the polyolefin chain. The functional group for its part is capable of reacting with the material of the reinforcement fibres 12, by creating covalent bonds or hydrogen bonds with it.
In particular, the functional group may be chosen from mono- or di-carboxylic acid anhydrides.
For example, the chemical element substituting a hydrogen atom in the carbon chain of the polyolefin may be: a maleic anhydride or phthalic anhydride group or an acrylic acid; the maleic anhydride group being the most generally used.
It is not necessary to provide a large number of functional groups in the Po-f. Indeed functionalisation of the Po-f is generally between 0.01 wt. % and 45 wt. %. Beyond 45 wt. % functionalisation, it is no longer a polyolefin. Advantageously, the functionalisation degree is between 0.01 wt. % and 30 wt. %, preferably between 0.01 wt. % and 15 wt. %, or preferably between 0.01 wt. % and 5 wt. %, still preferably between 0.1 wt. % and 2 wt. %.
The functionalisation degree of the Po-f must be understood as the ratio between the mass of functional groups that have reacted with the polyolefin and the total mass of functionalised polyolefin Po-f. It can also be calculated by the gain in mass between the non-functionalised initial polyolefin (Po-nf) and the functionalised polyolefin Po-f. For example, if 10 g of maleic anhydride has reacted with a polyolefin and the total mass of the Po-f is 100 g, then the degree of functionalisation is 10 wt. % by mass. The functionalised polymeric material may comprise 100 wt. % of Po-f or a mixture of Po-f and non-functionalised polymer. This non-functionalised polymer is a polymer compatible with the functionalised polyolefin, that is to say their mixture is stable over time and no phase separation is observable. They are said to be totally miscible. The non-functionalised polymer is preferably chosen from non-functionalised polyethylenes (PE-nf), non-functionalised polypropylene (PP-nf) or olefinic copolymers such as ethylene vinyl acetate (EVA-nf). The preferred non-functionalised polymer is linear low-density polyethylene (LLDPE-nf).
Advantageously, the mass ratio between Po-f and non-functionalised polymer is between 1:9 and 10:0. The mass ratio between Po-f, preferably LLDPE-f and non-functionalised polymer, preferably LLDPE-nf, may be between 1:4 and 1:1.
The functionalised polymeric material 111 may have a functionalisation gradient with a maximum at contact with the reinforcement fibres 12 and which decreases gradually and away from the reinforcement fibres 12. The gradient may be continuous or in steps.
The sheath 11 may also comprise a non-functionalised zone 112 surrounding or enclosing the functionalised polymeric material 111. Thus the cost of the sheath 11 can be reduced since, in general, non-functionalised polymer (or Po-nf) is less expensive than Po-f. The polymer of the non-functionalised zone may be the same as that of the mixture becoming the functionalised polymeric material or different, and chosen from those mentioned above for the mixture.
One or more channels 13 may be formed inside the sheath 11, the reinforcement fibres 12 being drawn inside these channels 13. Increasing the number of channels 13 makes it possible to increase the surface area of contact between the long reinforcement fibres 12 and the sheath 11, and consequently the interaction strength between these two constituent elements. Preferably, the number of channels is between 5 and 20.
The reinforcement fibres 12 consist of any material enabling increase in the tensile strength of the stabilisation strip. They are advantageously made of material chosen from polyvinyl alcohol (PVAL), polyesters, silica glass, linear or aromatic polyamides (also referred to as aramids) and metals, or a mixture thereof. If two or more materials are used, the reinforcement fibres made of one given materials may be grouped together, or the composition of reinforcement fibres in each of the channels 13 may be different from that of another, but preferably the composition of reinforcement fibres is the same in each of the channels 23.
Among the fibres mentioned, PVAL fibres are preferred. This is because, unlike polymers, glass, linear polyamides, aramids and metals, these materials are not sensitive to the nature of the fill (and particularly the pH of the soil forming part of the composition of the fill).
The reinforcement fibres 12 are advantageously disposed in the sheath 11 parallel to the length thereof, and parallel to one another. They may be raw, i.e. not spun.
The reinforcement fibres 12 may also be present in the form of threads parallel to one another. A “thread” results from the spinning of fibres. That is to say the fibres are all oriented to the same direction and twisted together. A thread composed of fibres has a higher tensile strength than all the fibres merely put alongside one another; indeed, the spinning reinforces the mechanical properties of the fibres.
The reinforcement fibres 12 may also be present in the form of strands or cords parallel to one another, as described in document EP 2171160. The spinning or braiding of several threads together gives a “strand”. The spinning or braiding of several strands together gives a “cord”. Because of the fact that the strand and cord result from an assembly of a plurality of threads, the appearance of their surface is not as smooth as that of fibres or threads. Consequently a strand or cord has a surface relief, i.e. their surface has recesses and bulges. The functionalised polymeric material surrounding or enclosing the reinforcement fibres 12 fits to the shape of these recesses and bulges, thus making it possible to add mechanical strength to the tensile strength, further increasing the adhesion force between the reinforcement fibres 12 and the sheath 11.
The reinforcement fibres 12 may also be composed of a mixture comprising at least two elements from raw fibres, threads, strands and cords.
The sheath 11 may comprise at least one high-adhesion longitudinal edge 113 free from the reinforcement fibres and having notches 114 (see
In general terms, and as already mentioned above, the stabilisation strip 1 has a length of approximately 3 m to 10 m, although longer or shorter stabilisation strips 1 may also be provided. The width of the stabilisation strip 1 is between 4 cm and 6 cm, although it is possible to manufacture strips with a greater width ranging up to 10 cm or even 25 cm. The thickness of the stabilisation strip 1 varies between 1 mm and a few centimetres, but preferentially between 1 mm and 6 mm.
The stabilisation strip 1 may have two longitudinal ends joined to each other, thus taking the form of a loop. Such a loop made of a stabilisation strip may be used as a connector for connecting the stabilisation strips to the facings of the reinforced embankment structure. Preferably, the circumference of the loop is between 40 cm and 80 cm.
A plurality of stabilisation strips 1 may form at least part of a stabilisation layer 10, advantageously in the form of a geogrid formed by a warp comprising stabilisation strips and a weft also comprising stabilisation strips. The warp and weft are superimposed (
The stabilisation strips 1c of the warp and the stabilisation strips 1t of the weft may be fixed by hot welding or adhesive bonding. The known welding methods for the polyolefin sheaths are hot air welding, mirror welding, hot wedge welding, ultrasonic welding and infrared welding.
The stabilisation strip 1 described above is used in the construction of reinforced embankment structures 2 (
The fill 21 generally comprises a mixture or assembly that may comprise at least one material from sand, gravel, fine soil, crushed rocks, recycled materials such as materials from demolition of buildings or civil-engineering structures, industrial residues or binders such as lime or cement.
Generally, such a reinforced embankment structure 2 also comprises a facing 22 and connectors 23 for connecting at least some of the stabilisation strips 1 to the facing 22. The facing 22 may be produced from prefabricated juxtaposed elements 221 made from concrete, in the form of slabs or blocks. It may also consist of metal welded lattice panels or gabions made of woven metal wires.
The stabilisation strips 1 may be used as they are, that is to say they are individually disposed during construction of the reinforced embankment structure 2.
When the stabilisation strips 1 are in the form of stabilisation layers 10, a whole set of stabilisation strips 1 is disposed in one operation during construction of the reinforced embankment structure 2. The advantage is a saving in time for laying the stabilisation strips 1 compared with individual placing. Another advantage is an easier laying since the distance between the stabilisation strips 1 is defined in advance during manufacturing of the stabilisation layer 10.
The connectors 23 may be stabilisation strips 1, in particular in the form of loops made by winding and assembling. In such case, the adhesion between the threads and the sheath is essential in order to ensure the strength of the loop.
With reference to
The method for manufacturing a stabilisation strip as described above comprises:
The activation temperature is the temperature at which the functional group is activated and depends on the nature of the chemical element functionalising the polyolefin. For example, the activation temperature for maleic anhydride is 180° C. Thus, advantageously, the polymeric material functionalised by maleic anhydride is heated to approximately 180° C. for a few seconds in an extruder or mixer.
The method advantageously comprises drawing the reinforcement fibres in a drawing direction. Shaping the functionalised polymeric material is carried out by extruding the functionalised polymeric material around reinforcement fibres in the drawing direction.
This embodiment is advantageously used for a uniform sheath, i.e. a sheath without any non-functionalised zones.
In a variant, shaping the functionalised polymeric material is carried out so as to form a functionalised gradient in the polymeric material with a maximum at contact with the reinforcement fibres and which decreases gradually away from the reinforcement fibres.
In addition, the method may comprise heating a non-functionalised polymer, preferentially PE-nf, and more preferentially still LLDPE-nf. Shaping the functionalised polymeric material is carried out by co-extruding the functionalised polymeric material around reinforcement fibres and the non-functionalised polymer around functionalised polymeric material in order to form the non-functionalised zone of the sheath surrounding or enclosing the functionalised polymeric material.
Drawing the reinforcement fibres may be carried out as the sheath is extruded, affording a saving in time and a saving in space for manufacturing the stabilisation strip.
Prior to the drawing of the reinforcement fibres, the latter may have been spun into threads. The threads may have been spun or braided into strands and the strands may have been spun or braided into cords.
Alternatively, the reinforcement fibres are already supplied in the form of threads, strands or cords, optionally previously drawn. Optionally, the fibres supplied in the form of threads or strands may be spun or braided to give respectively strands or cords.
The test presented below is carried out on stabilisation strips comprising a sheath and a functionalised polymeric material comprising a mixture of LLDPE-nf and LLDPE-f in proportions as indicated in table 1. The LLDPE-f has a functionalisation degree estimated at approximately 1 wt. % with maleic anhydride elements. A control example is also provided for comparison. The sheath in this control example comprises 100% LLDPE-nf.
The stabilisation strips 1 comprise a PVAL reinforcement in the form of strands present inside the sheath. The PVAL reinforcement fibres are distributed in five channels 13 inside the sheath 11, the central channel 131 of which is 7 mm wide and 2 mm high. The shape and constitution of the stabilisation strips are identical for all the tested mass ratios of LLDPE-f to LLDPE-nf and for the control example.
A cut En is made on two opposite metal edges of the stabilisation strip, leaving only the central channel 131 intact. At 10 cm from the cut edge, a transverse incision In is made through the central channel 131 over the whole of its width, thus severing the strands situated in the central channel 131. Thus, between the cut En and the incision In, the stabilisation strip 1 is left intact over 10 cm (see
Each stabilisation strip 1 thus prepared is disposed on a uniaxial traction bench. The two ends of the strip are fixed to the bench and a traction force is applied between both ends so as to separate both ends at a speed of 200 mm/min. The force needed for the separation at 200 mm/min is recorded.
Thus, it was possible to measure the adhesion force between the reinforcement fibres and the sheath 11 over a length of 10 cm corresponding to a contact surface between the reinforcement fibres and the sheath 11 of 1800 mm2. The results are recorded in following table 2:
It is thus observed that, compared with a stabilisation strip with a sheath made entirely of LLDPE-nf, the gain in adhesion force is already 56% for a sheath with a functionalised polymeric material comprising 25% LLDPE-f. This gain climbs to more than 110% for stabilisation strips with a sheath with a functionalised polymeric material comprising 50%, 75% and 100% LLDPE-f.
| Number | Date | Country | Kind |
|---|---|---|---|
| 14 00193 | Jan 2014 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2014/053577 | 12/30/2014 | WO | 00 |
