EP0897035A1 - Geosynthetic reinforcement for soil with high settling risk - Google Patents

Abstract

The material comprises at least one layer of parallel and alternating reinforcing strands from two groups: the first group is made with fibres having a high tension module and a breaking elongation of about 5%, while the second is made with fibres having a lower tension module and a breaking elongation of at least 12%. Preferably the fibres in the first group of strands are of polyvinyl acetate, while those in the second group are of polyester or polypropylene. The material can be made from a knitted fabric associated with a non-woven fabric.

Description

The invention relates to a geosynthetic for reinforcement earthworks.

Geosynthetics and more particularly geotextiles are more and more frequently used as reinforcements in the construction of earthworks, for roadways or railway.

Some geotechnical difficulties can be relatively identified and be subject to appropriate and final treatment. Other geotechnical difficulties are more complex to identify and sometimes lead the engineer to manage a risk.

This is particularly the case when the land includes underground cavities, of natural origin (karst phenomenon) or anthropogenic (sapes, marnières ...) not listed and difficult to identify. The construction of an earthen structure on this type of land is sometimes inevitable and therefore involves a geotechnical risk linked to the random formation of land collapses (fontis) in infrastructure footprint, incompatible with level of service required for the work.

The technical solution goes through a reinforcement geotextile "parachute" type used as a preventive measure. The characteristics of this geotextile depend on its position, surface or in depth, of the importance of the fontis to be treated as well as the maximum deflection permissible on the surface, permanent and dynamic, depending on the time intervention for the repair of the structure. You have to take into account the service life of the reinforcement after the appearance of the fontis traffic function on the track above the structure.

Various parameters are involved in defining the module of geotextile traction to choose.

The aim of the invention is to propose a geosynthetic which allows normal operation during use of the structure and which provides a parachute type safety solution in ground collapse.

The invention therefore relates to a geosynthetic which comprises at least minus one layer of parallel reinforcing strands.

This geosynthetic is characterized by the fact that the strands of reinforcement are divided into a first group of strands made with a fiber with a high tensile modulus and a value of elongation at break close to 5%, and in a second group of strands made with a fiber having a tensile modulus lower than fiber traction module of the first group of strands, and a breaking elongation value at least equal to 12%.

Advantageously, the fiber of the strands of the first group is made of PVA (polyvinyl acetate) which has an elongation at the rupture of around 5%. It should be noted that other fibers may be used, such as glass or aramid, which have value 2 or 2.5% maximum elongation at break, which is too little high for font areas.

The fibers of the strands of the second group are preferably made of polyester or polypropylene.

In the absence of serious disorder in the soil, all of the strands of the two groups actively participates in the recovery of constraints, up to the deformation value of 5%.

From this value, which corresponds to the beginning of a disorder serious, the strands of the first group begin to break, and the resumption efforts are made, moreover, only by the strands of the second group, which results in greater deformation of the geosynthetic, parachute type, but the strands of the second group with a breaking strain value greater than 12% can support the work, despite the great disorder in the ground.

In practice, the geosynthetic according to the invention may advantageously be associated, during the construction of the structure with a second geosynthetic equipped with wires that can break at predetermined elongation values, for example, 4 and 8% and connected to warning devices, this second geosynthetic being arranged on the ground and separated from the geosynthetic according to the invention by a layer of soil which, under its own weight, causes a distortion of the second geosynthetic by 5%, for example, which causes one of the warning devices to activate.

la figure 1 montre la courbe caractéristique de l'allongement du géosynthétique selon l'invention en fonction des forces latérales exercées sur lui par unité de longueur ;

la figure 2 montre un exemple de la structure du géosynthétique selon l'invention ;

la figure 3 est une coupe transversale du géosynthétique de la figure 2 selon la ligne III-III ;

la figure 4 montre un exemple d'utilisation du géosynthétique selon l'invention dans un ouvrage au-dessus d'une zone susceptible de s'effondrer ;

la figure 5 est une coupe transversale de l'ouvrage de la figure 4 au-dessus d'une zone effondrée.

Other advantages and characteristics of the invention will emerge on reading the following description given by way of example and with reference to the appended drawings in which:

FIG. 1 shows the characteristic curve of the elongation of the geosynthetic according to the invention as a function of the lateral forces exerted on it per unit of length;

Figure 2 shows an example of the geosynthetic structure according to the invention;

Figure 3 is a cross section of the geosynthetic of Figure 2 along the line III-III;

FIG. 4 shows an example of the use of geosynthetics according to the invention in a structure above an area liable to collapse;

Figure 5 is a cross section of the structure of Figure 4 above a collapsed area.

In Figure 1 we have shown the characteristic curve (C) of the elongation of the geosynthetic according to the invention, according to the forces exerted on this geosynthetic in the longitudinal direction or transverse reinforcement strands equipping this geosynthetic. This curve (C) is given for information with regard to the tensile strength. This can advantageously be modulated in according to the needs of the site.

When the elongation is less than 5%, the slope of the curve (C) is high. Up to a 5% extension, efforts are resumed by the reinforcing strands having the high tensile modulus and a breaking elongation value of 5%, which are made of preferably in PVA (polyvinyl acetate), as well as by the other strands reinforcement that have a significantly lower tensile modulus and a value of elongation at break greater than 12%, and preferably equal to 18%, these other strands being made, for example, polyester or polypropylene.

When the elongation exceeds 5%, the PVA strands break and the load is then taken up by the polyester strands.

When the elongation reaches 5.5%, all the PVA strands are broken, and the efforts are taken up only by the strands in polyester.

The OA portion of curve C corresponds to an operation normal geosynthetic in a work. The AB portion corresponds to an alert situation, and the BD portion corresponds to the use of geosynthetics in parachute mode in case land collapse under a structure equipped with geosynthetics according to the invention, for example.

Figures 2 and 3 show a geosynthetic 10, produced in knitted woven technology, with a combination of nonwoven which comprises, in its transverse direction, a plurality of strands or cables 11, 12 parallel, distributed in a first group of strands 11 made of PVA fiber and a second group of strands 12 made polyester or polypropylene fibers.

The strands 11 and 12 are integrated, during the manufacture of the geosynthetic 10, with a perfectly rectilinear arrangement which does not cause any ripple or shearing effect as in other textile surfaces.

In FIGS. 2 and 3, the strands 11 in PVA and the strands 12 in polyester or polypropylene are arranged alternately. It is obvious that the density of the two types of strands may be different and calculated according to the use of geosynthetics 10.

Figures 4 and 5 show an advantageous use of the geosynthetic 10 in a civil engineering structure 20 arranged on a ground 21 comprising internal cavities 22 separated from the surface 23 of the ground 21 by arches 24. Above the surface 23 there is a first geosynthetic 25 which includes wires capable of break when the elongation coefficient of the first geosynthetic 25 is 4%, for example, in order to signal a first alert level, and wires likely to break when the coefficient of elongation of the first geosynthetic is 8%, by example, to provide a second level of alert.

Between geosynthetic 10 according to the invention and the first geosynthetic 25 described above there is an H layer of soil, which is calculated in such a way that the first geosynthetic 25 undergoes a deformation greater than 4% under the weight of this layer H.

When a collapse occurs under the first geosynthetic 25, the weight of layer H causes the wires to break with a low fracture elongation coefficient, and the provision of a first alert level.

In this condition, the geosynthetic 10 according to the invention supports only the weight of the work 20. Beyond a certain deformation of geosynthetic 10 (5 to 6%), it will transmit additional efforts to the first geosynthetic 25 which can lead to the breaking of wires with a breaking elongation coefficient high of the first geosynthetic 25, and the provision of the second alert level. This second level of alert will be reached for a deformation of geosynthetic 10 of the order of 6 or 7%, which is significantly lower than the breaking strain level of the strands in geosynthetic polyester 10.

Claims (4)

Geosynthetics of the type comprising at least one ply of parallel reinforcing strands,
characterized by the fact that said reinforcing strands are divided into a first group of strands made with a fiber having a high tensile modulus and a value of elongation at break close to 5%, and into a second group of strands made with a fiber having a tensile modulus lower than the tensile modulus of the fiber of the first group of strands, and a value of elongation at break equal to at least 12%. Geosynthetics according to claim 1, characterized in that that the fiber of the strands of the first group is made of polyvinyl acetate. Geosynthetic according to any one of claims 1 or 2, characterized in that the strand fiber of the second group is polyester or polypropylene. Geosynthetic according to any one of claims 1 to 3, characterized in that it is made of knitted woven with nonwoven association.

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