FR2767344A1 - GEOSYNTHETIC FOR DEVICE FOR REINFORCING SOILS AT RISK OF COLLAPSE - 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

i 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 earthen structures, 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 ground includes underground cavities, of natural origin (karst phenomenon) or anthropic (sapes, marnières ...) not listed and difficult to define. 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 the grip of the infrastructure, incompatible with the level of service.

required for the work.

  The technical solution goes through a reinforcement geotextile type "parachute" used as a preventive measure. The characteristics of this geotextile depend on its position, surface or in depth, the size of the fonts to be treated as well as the maximum permissible deflection on the surface, permanent and dynamic, depending on the intervention time for repairing the work. The lifespan of the reinforcement must be taken into account after the appearance of the

  traffic function on the track above the structure.

  Various parameters are involved in defining the module

  of geotextile traction to choose.

  The object of the invention is to provide a geosynthetic which allows normal operation during the period of use of the structure and which provides a safety solution of parachute type

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 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%. Advantageously, the fiber of the strands of the first group is made of PVA (polyvinyl acetate) which has an elongation at break of the order of 5%. It should be noted that other fibers can be used, such as glass or aramid, which have a maximum breaking elongation value of 2 or 2.5%, 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 the strands of the two groups actively participate in the resumption of constraints,

  up to the deformation value of 5%.

  From this value, which corresponds to the beginning of a serious disorder, the strands of the first group begin to break, and the recovery of efforts is made, moreover, only by the strands of the second group, which results in a deformation greater of the geosynthetic, parachute type, but the strands of the second group having a value of breaking strain 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 which can break at predetermined elongation values, for example, 4 and 8% and connected warning devices, this second geosynthetic being placed on the ground and separated from the geosynthetic according to the invention by a layer of earth which, under its own weight, causes a deformation of the second geosynthetic of 5%, for example, which

  causes one of the warning devices to activate.

  Other advantages and characteristics of the invention will emerge

  on reading the following description given by way of example and in

  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 work of Figure 4

above a collapsed area.

  FIG. 1 shows the characteristic curve (C) of the elongation of the geosynthetic according to the invention, as a function of the forces exerted on this geosynthetic in the longitudinal direction or

  transverse reinforcement strands equipping this geosynthetic.

  This curve (C) is given for information as regards 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 an elongation of 5%, the forces are taken up by the reinforcing strands having the high tensile modulus and a breaking elongation value of 5%, and which are preferably made of PVA (polyvinyl acetate), as well than by the other reinforcing strands which have a significantly lower tensile modulus and an elongation value at break greater than 12%, and preferably equal to 18%, these other strands being produced, 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 forces are taken up only by the polyester strands. The OA portion of curve C corresponds to normal geosynthetic functioning in a structure. The AB portion corresponds to an alert situation, and the BD portion corresponds to the use of geosynthetics in parachute mode in the event of land collapse under a structure equipped with geosynthetics

according to the invention, for example.

  Figures 2 and 3 show a geosynthetic 10, made in knitted woven technology, with association 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 in 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 undulation or shearing effect as in

other textile surfaces.

  In FIGS. 2 and 3, the strands 11 made of PVA and the strands 12 made of 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.

  FIGS. 4 and 5 show an advantageous use of geosynthetics 10 in a civil engineering structure 20 placed on a ground 21 comprising internal cavities 22 separated from the surface 23 of the ground 21 by arches 24. Above the surface 23 one has a first geosynthetic 25 which has wires capable of breaking when the elongation coefficient of the first geosynthetic 25 is 4%, for example, in order to signal a first level of alert, and wires capable of breaking when the coefficient d elongation of the first geosynthetic is 8%, by

  example, to provide a second level of alert.

  Between the geosynthetic 10 according to the invention and the first geosynthetic 25 described above, there is a layer H 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 the H layer causes the breaking of the wires with a low breaking elongation coefficient, and the provision of a

first alert level.

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

geosynthetic polyester 10.

Claims (4)

  1. Geosynthetics of the type comprising at least one ply of parallel reinforcing strands, characterized in that said reinforcing strands are distributed in a first group of strands made with a fiber having a high tensile modulus and an elongation value of failure close to%, and in 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 at less equal to 12%.   2. Geosynthetics according to claim 1, characterized by the fact   that the fiber of the strands of the first group is made of polyvinyl acetate.   3. Geosynthetics according to any one of claims 1 or   2, characterized in that the strand fiber of the second group is polyester or polypropylene.   4. Geosynthetics according to any one of claims 1 to   3, characterized in that it is made of knitted woven with nonwoven association.