Title of Invention

"REINFORCED SOIL STRUCTURE AND METHOD FOR BUILDING IT"

Abstract

The reinforced soil structure comprises a fill, a facing (3) placed along a front face of the structure, main reinforcement strips (2) disconnected from the facing and extending in a reinforced zone of the fill situated behind the front face, and secondary elements (6) connected to the facing and extending in a zone of the fill which exhibits, with the reinforced zone, a common part (z) where loads are transmitted between the main reinforcement strips and the secondary elements by the material of the fill.

Full Text

The present invention relates to reinforced soil structure and method for building it. BACKGROUND OF THE INVENTION The present invention relates to the construction of reinforced soil structures. This building technique is commonly used to produce structures such as retaining walls, bridge abutments, etc. A reinforced soil structure combines a compacted fill, a facing and reinforcements usually connected to the facing. Various types of reinforcement can be used: metal (for example galvanized steel), synthetic (for example based on polyester fibers), etc. They are placed in the earth with a density that is dependent on the stresses that might be exerted on the structure, the thrust of the soil being reacted by the friction between the earth and the reinforcements. The facing is usually made from prefabricated concrete elements, in the form of panels or blocks, juxtaposed to cover the front face of the structure. There may be horizontal steps on this front face between various levels of the facing, when the structure incorporates one or more terraces. In certain structures, the facing may be built in situ by pouring concrete or a special cement. The reinforcements placed in the fill are secured to the facing by mechanical connecting members that may take various forms. Once the structure is completed, the reinforcements distributed through the fill transmit high loads, that may range up to several tons. Their connection to the facing needs therefore to be robust in order to maintain the cohesion of the whole. These connections between the reinforcements entail a risk that the maximum load they can withstand may be exceeded if the soil undergoes differential settlement or in the event of an earthquake. Furthermore, the connecting members exhibit risks of degradation. They are often sensitive to corrosion due to moisture or chemical agents present in or which have infiltrated into the fill. This disadvantage often prevents the use of metal connecting members. The connecting members are sometimes based on resins or composite materials so that they degrade less readily. However, their cost is then higher, and it is difficult to give them good mechanical properties without resorting to metal parts. For example, if the reinforcements are in the form of flexible strips and attach by forming a loop behind a bar secured to the facing (US-A-4 343 571, EP-A-1 114 896), such bar undergoes bending stresses, which is not ideal in the case of synthetic materials. By construction, the prefabricated facing elements have a determined number of locations for connection to the reinforcements of the fill. This results in constraints on the overall design of the structure, particularly in terms of the density with which the reinforcements can be placed. For example, if the prefabricated elements each offer four attachment points, the designer will need to envisage connecting the reinforcements there that many times, or possibly a lower number of times, the number always being a whole number. If structural engineering considerations require, for example, 2.5 pairs of main reinforcements per prefabricated element, it is necessary to provide a substantial surplus of reinforcements, which has an significant impact on the cost. These considerations complicate the design of the structure, since the optimization generally requires reinforcement densities that can vary from one point in the fill to another. In "Design, Construction and Performance of Two Geogrid Reinforced Soil Retaining Walls" (Third International Conference on Geotextiles, 1986, Vienna, Austria, pp. 401-406), R.R. Berg, et al. report an experiment made on a reinforced soil retaining wall having main geogrid layers laid in the soil mass and additional geogrid tabs embedded in each concrete facing panel. Geogrid reinforcements have a two-dimensional layout obtained by arranging one-dimensional plastic elements in a grid configuration. In a part of the wall, there was no mechanical connection between the geogrid panel tabs and the main geogrid reinforcement layers. The paper mentions problems with variability in the movements of the individual panels. It concludes that the method is limited to walls in which panel movement is not constrained and is also limited in terms of admissible wall batter. Such connectionless method does not appear to have been used or further studied since the experimental wall was built in 1985. An object of the present invention is to propose a novel method of connection between the facing and the reinforcements placed in the fill which, in certain embodiments at least, makes it possible to reduce the impact of the above-mentioned problems. SUMMARY OF THE INVENTION The invention thus proposes a reinforced soil structure comprising: a fill; a facing placed along a front face of the structure; main reinforcement strips disconnected from the facing and extending through a reinforced zone of the fill situated behind said front face; and secondary members connected to the facing and extending in a zone of the fill which has, with said reinforced zone, a common part where loads are transmitted between the main reinforcement strips and the secondary members by the material of the fill. This reinforced soil structure has significant advantages. In particular, the structure may have good integrity in the presence of small soil movements. Such movements do not cause the reinforcements to tear away from the facing as in known structures, but may give rise to slight slippage between the main reinforcement strips and the secondary members, through shearing of the fill material situated between them, thus avoiding irreversible damage to the structure. This advantage is particularly obtained when secondary members extend in the fill up to a distance substantially shorter than the main reinforcement strips, with respect to the front face. As the material of the fill contributes to the connecting of the main reinforcement strips to the secondary members and therefore to the facing, they advantageously make it possible to avoid attaching to the main reinforcement strips mechanical connectors that transmit the loads to the facing. It is thus possible to eliminate the corrosion or degradation problems often encountered with such connectors in the prior art. The structure according to the invention allows an overall design of the reinforced soil structure that separately and independently optimizes its two parts: (1) the facing and the secondary members connected thereto, and (2) the zone reinforced by the main reinforcement strips. The latter advantage in itself affords great benefit to the proposed structure, independently of the advantages mentioned hereinabove. The structure can be thought of as being made up of two reinforced-earth massifs, one with the main reinforcement strips and the other with the secondary members connected to the facing, these being nested together to give the whole its cohesion. Separate optimization of these two massifs affords an important economic gain. Preferably, there is substantially no direct contact between the main reinforcement strips and the secondary members. In a preferred embodiment of the structure, the facing comprises prefabricated elements in which the secondary members are partly embedded. These prefabricated elements are typically made of concrete, it being possible for the secondary members to consist of flexible synthetic reinforcement members each having at least one part cast into the concrete of one of the prefabricated elements. The facing may also comprise prefabricated elements each having at least one projecting portion forming one of the secondary members. Such prefabricated elements have, for example, an L-shaped profile. The invention can be applied to the repair of an existing structure, but its preferred application is that of the production of a new structure. A second aspect of the invention thus relates to a method for building a reinforced soil structure, comprising the steps of: positioning a facing along a front face of the structure delimiting a volume to be filled; placing main reinforcement strips in a first zone of said volume, wherein the main reinforcement strips are not permanently connected to the facing and extend through the first zone; placing secondary members connected to the facing in a second zone of said volume, said first and second zones having a part in common; introducing fill material into said volume and compacting the fill material, whereby once the fill material has been introduced and compacted, loads are transmitted between the main reinforcement strips and the secondary members by the fill material situated in said common part. The facing is advantageously produced by assembling prefabricated elements. However, it can also be built in situ. BRIEF DESCRIPTION THE DRAWINGS Figure 1 is a schematic view in lateral section of a reinforced soil structure according to the invention, while it is being built. Figure 2 is a perspective part view of this structure. Figure 3 is a schematic view in lateral section of an alternative embodiment of a structure according to the invention. Figure 4 is a schematic perspective view of a facing element usable in an embodiment of the invention. Figures 5 and 6 are schematic elevation and top views of a facing element usable in another embodiment of the invention. Figure 7 is a schematic elevation view of another embodiment of a structure according to the invention. Figures 8 and 9 are schematic elevation and top views of yet another embodiment of a structure according to the invention. DESCRIPTION OF PREFERRED EMBODIMENTS The figures illustrate the application of the invention to the building of a reinforced soil retaining wall. A compacted fill 1, in which main reinforcements 2 are distributed, is delimited on the front side of the structure by a facing 3 formed by juxtaposing prefabricated elements 4, in the form of panels in the embodiment illustrated in figures 1 and 2, and on the rear side by the soil 5 against which the retaining wall is erected. Referring to figure 2, the main reinforcements 2 are strips of fiber-based synthetic reinforcing material following zigzag paths in horizontal planes behind the facing 3. These may in particular be the reinforcement strips marketed under the trade name "Freyssissol". Such strip advantageously has a width of at most 20 cm. Figure 1 schematically shows the zone Z1 of the fill reinforced with the main reinforcement strips 2. The main reinforcement strips 2 are not positively connected to the facing 3, which dispenses with the need to attach them to specific connectors. To ensure the cohesion of the retaining wall, secondary reinforcements or members 6 are connected to the facing elements 4, and extend over a certain distance within the fill 1. These secondary reinforcements 6 contribute to reinforcing the earth in a zone Z2 situated immediately on the back of the facing 3. The cohesion of the structure results from the fact that the reinforced zones Z1 and Z2 overlap in a common part Z'. In this common part Z', the material of the fill 1 has good strength because it is reinforced by both the reinforcements 2 and 6. It is thus able to withstand the shear stresses exerted as a result of the tensile loads experienced by the reinforcements. This part Z' must naturally be thick enough to hold the facing 3 properly. In practice, a thickness of one to a few meters will generally suffice. By contrast, the main reinforcement strips 2 may extend far more deeply into the fill 1, as shown by figure 1. The simple connection of short reinforcement members 6 to the back of the facing elements 4 thus allows the facing to be held pressed against fills which may be of large volume. It is preferable to avoid contacts between the main reinforcement strips 2 and the secondary reinforcements 6 in the common part Z'. This is because no reliance is placed on the friction forces between reinforcements for reacting the tensile loads given that it is difficult to achieve full control over these friction forces. By contrast, in the reinforced-earth technique, better control is had over the interfaces between reinforcements and fill, which means that the strength properties of the reinforced fill stressed in shear can be relied upon. In the example depicted, the secondary reinforcements 6 are also synthetic fiber-based strips. They may be connected to the facing 3 in various ways. They may be attached to the facing using conventional connectors, for example of the kind described in EP-A-1 114 896. In a preferred embodiment, these secondary reinforcements 6 are incorporated at the time of manufacture of the facing elements 4. In the frequent scenario where the elements 4 are prefabricated in concrete, part of the secondary reinforcements 6 may be embedded in the cast concrete of an element 4. This cast part may in particular form one or more loops around steel bars of the reinforced concrete of the elements 4, thus firmly securing them to the facing. In the exemplary structure configuration illustrated by figures 1 and 2, the main reinforcement strips 2 and the secondary reinforcements 6 are arranged in horizontal planes that are superposed in alternation over the height of the structure. Just two adjacent planes are shown in figure 2 in order to make it easier to read. As indicated earlier, the main reinforcement strips 2 are laid in a zigzag formation between two lines at which they are folded back. The distance between these two lines is dependent on the volume of the reinforced zone Z1. The pitch of the zigzag pattern depends on the reinforcement density required by the structural engineering calculations. Still in the example of figure 2, secondary reinforcements 6 form a comb-like pattern in each horizontal plane in which they lie, the reinforcement strip forming a loop inside a facing element 4 between two adjacent teeth of the comb. In order to build the structure depicted in figures 1 and 2, the procedure may be as follows: a) placing some of the facing elements 4 so as to be able thereafter to introduce fill material over a certain depth. In a known way, the erection and positioning of the facing elements may be made easier by assembly members placed between them; b) installing a main reinforcement strip 2 on the fill already present, laying it in a zigzag pattern as indicated in figure 2. Slight tension is exerted between the two loop-back lines of the reinforcement strip 2, for example using rods arranged along these lines and about which the strip is bent at each loop-back point; c) introducing fill material over the main reinforcement layer 2 which has just been installed, up to the next level of the secondary reinforcements 6 on the rear side of the facing elements 4. This fill material is compacted as it is introduced; d) placing on the fill the secondary reinforcements 6 situated at said level, exerting slight tension thereon; e) introducing fill material over this level and progressively compacting it until the next specified level for the placement of main reinforcement strips 2 is reached; f) repeating steps a) to e) until the upper level of the fill is reached. It should be noted that numerous alternatives may be applied to the structure described hereinabove and to its method of production. First, the secondary members 6 may adopt very diverse forms, as is done in the reinforced soil technique (synthetic strip, metal bar, metal or synthetic grating in the form of a strip, a layer, a ladder, etc), woven or non-woven geotextile layer, etc. Likewise, all kinds of facings may be used: prefabricated elements in the form of panels, blocks, etc, metal gratings, planters, etc. Furthermore, it is perfectly conceivable to build the facing 3 by casting it in situ using concrete or special cements, taking care to connect the secondary elements 6 therein. In certain embodiments, secondary elements may be of one piece with the constituent elements of the facing 3. Figure 3 schematically illustrates such an embodiment in which the facing 3 is made from prefabricated elements 8 each having an L-shaped profile: the upright part of the L extends along the front face of the structure to constitute the facing 3, while the other part of the L forms a secondary member 9 which projects into the reinforced fill 1 provided with the main reinforcement strips 2. A sufficient overlap Z' between the zone Z1 reinforced by the main reinforcement strips 2 and the zone Z2 into which the secondary members 9 penetrate will then, as before, allow loads to be transmitted between the facing 3 and the reinforcements 2 via the material of the fill. Here again, it is appropriate to avoid placing the main reinforcement strips 2 in contact with the secondary members 9. The three-dimensional configurations adopted for the main reinforcement strips 2 and the secondary elements 6 within the fill 1 may also be very diverse. It is possible to find main reinforcements 2 and secondary elements 6 in the same horizontal plane (preferably avoiding contact with one another). It is also possible to have, in the common part Z\ a varying ratio between the density of the main reinforcements 2 and that of the secondary members 6, 9; etc. Another possibility is to arrange parallel segments of main reinforcement strips within the massif. Such possibility is interesting where the strip-shaped reinforcements are relatively rigid (e.g. made of welded metal wires). In the embodiment illustrated in figure 3, the facing element 14 is equipped with a reinforcement strip which follows a C-shaped path 15 when seen in a vertical section. The strip (not shown to display the shape of the path) is embedded in the concrete as it is poured into the manufacturing mould. It preferably passes around one or more metallic rods 16 used to reinforce the concrete element. The ends of the C-shaped path 15, at the level at the rear side of the facing element, guide the projecting sections of the strip in horizontal directions. Such strip sections provide a pair of secondary members which emerge from the facing element 14 into the fill 1 at vertically offset positions. This arrangement takes advantage of the soil/plastic friction on both sides of each strip section, thus optimizing the use of the reinforcement material in zone 72. In the alternative embodiment illustrated in figures 5 and 6, the strip 26 forms a loop around a metallic reinforcement rod 27 of the concrete facing element 24. Its two projecting sections 26A, 26B emerge on the rear side of the facing element 24 in substantially the same horizontal plane. But in that plane (figure 6), their angles with respect of the rear surface of the element are different. The two strip sections 26A, 26B are laid at the same time on a level of the fill by keeping the angle between them. This oblique layout also takes full advantage of the soil/plastic friction on both sides of each strip section. One of the significant advantages of the proposed structure is that it makes it possible to adopt very varied configurations and placement densities for the main reinforcement strips 2 and the secondary members 6, 9, because the transmission of loads by the fill material situated between them eliminates most of the constructional constraints associated with the method of connection between the main reinforcements and the facing. It will thus be possible to find, within one and the same structure, regions where the relative densities of main reinforcements and/or of secondary elements 6 vary significantly, while they are optimized individually. An important advantage of the use of strips as the main reinforcements 2 is that it provides a very large capacity to adjust the density of the main reinforcements: it is possible to vary as desired not only the vertical spacing of the reinforcement layers and their depths behind the facing, but also their density in a horizontal plane (e.g. by varying the pitch of the zigzag paths). Such adjustment is not constrained by the predefined spacing of connectors behind the facing panels. A full 3D optimization of the amount of reinforcement is virtually achieved, which provides a very significant advantage in terms of cost of the reinforced soil structure. In addition, strip-shaped main reinforcements ensure a good control of the friction properties at the soil/reinforcement interface. In the embodiment shown in figure 7, the facing is made of blocks 44 of relatively small dimensions. These blocks are individually connected to the stabilized soil structure by means of secondary members 6. Such arrangement ensures the individual stability of the blocks, and avoids offsets between adjacent blocks without requiring strong positive connections between the blocks. As shown in the figure, the density of the main reinforcement strips 2 in zone Z1 may be lower than that of the secondary members 6 in zone 72. Since, in this application, the reinforcement density in zone Z2 is set by the dimensions of the blocks 44, it is seen that the invention enables to optimize the amount of main reinforcement strips to be used, which is an important economic advantage. The invention is also interesting in reinforced soil structures whose facing is made of deformable panels, as illustrated in figures 8 and 9. Such panels 54 may consist of a mesh of welded wires to which soil reinforcements 56 are connected, directly or via intermediate devices. Usually, the deformation of such wire mesh facing is limited by increasing the number of connection points and reinforcements. Again, the requirement to consolidate the facing leads to a higher expenditure for the reinforcements to be used. This problem is circumvented by the present invention since it permits to design the reinforcement of zone Z1 by means of the main reinforcement strips 2 independently of that of the facing connection zone 22 by means of the soil reinforcements 56 used as secondary members. When a main reinforcement strip 2 is being placed on a level of the fill (step b above), it is possible to connect this reinforcement strip 2 to the facing by means of temporary attachments intended to break as the structure is gradually loaded with the overlying fill levels. Such temporary attachments, which are optional, make correct positioning of the main reinforcements easier, but are not relied upon to transmit load at the facing/fill interface once the structure is completed. We claim: 1. A reinforced soil structure comprising: -a fill (1); - a facing (3) placed along a front face of the structure; - main reinforcement strips (2) extending through a reinforced zone (Zl) of the fill situated behind said front face; and characterized in that the main reinforcement strips (2) are disconnected from the facing, the reinforced soil structure further comprising: - secondary members (6, 9, 26) connected to the facing and extending in a zone (Z2) of the fill which has, with said reinforced zone, a common part (Z') where loads are transmitted between the main reinforcement strips and the secondary members by the material of the fill. 2. The structure as claimed in Claim 1, wherein the secondary members (6, 9, 26) extend into the fill (1) up to a distance substantially shorter than the main reinforcement strips (2), with respect to the front face. 3. The structure as claimed in Claim 1 or 2, wherein the facing (3) comprises prefabricated elements (4, 14, 24) in which the secondary members (6, 26) are partly embedded. 4. The structure as claimed in Claim 3, wherein the prefabricated elements (4, 14, 24) are made of concrete and the secondary members (6, 26) comprise flexible synthetic reinforcement members each having at least one part cast into the concrete of one of the prefabricated elements. 5. The structure as claimed in Claim 4, wherein the cast part of the flexible synthetic reinforcement member (6, 26) follows a loop within said one of the prefabricated elements (4, 14, 24), so that said flexible synthetic reinforcement member has two sections projecting into the second zone (Z2) of the fill (1). 6. The structure as claimed in Claim 5, wherein the loop is arranged in said one of the prefabricated elements (14) so that the two sections of said flexible synthetic reinforcement member emerge from the facing into the fill (1) at vertically offset positions. 7. The structure as claimed in Claim 5, wherein the loop is arranged in said one of the prefabricated elements (24) so that the two sections (26A, 26B) of said flexible synthetic reinforcement member emerge at different angles from the facing into the fill (1) in substantially the same horizontal plane. 8. The structure as claimed in any one of Claim 4 to 7, wherein the flexible synthetic reinforcement members (6, 26) are strip-shaped. 9. The structure as claimed in Claim 1 or 2, wherein the facing (3) comprises prefabricated elements (8) each having at least one projecting portion (9) forming one of the secondary elements. 10. The structure as claimed in Claim 1 or 2, wherein the facing comprises wire mesh panels (54) to which soil reinforcements (56) are connected as secondary members. 11. The structure as claimed in any one of Claim 1 to 10, wherein the main reinforcement strips (2) each have a width of at most 20 cm. 12. The structure as claimed in any one of Claim 1 to 11, wherein the main reinforcement strips (2) are arranged along zigzag paths in the first zone (Zl). 13. A method for building a reinforced soil structure as claimed in any one of claims 1 to 12, comprising the steps of: - positioning a facing (3) along a front face of the structure delimiting a volume to be filled; - placing main reinforcement strips (2) in a first zone (Zl) of said volume, wherein the main reinforcement strips extend through the first zone; - introducing fill material (1) into said volume and compacting the fill material, whrein the main reinforcement strips (2) are not permanently connected to the facing, the method further comprising: - placing secondary members (6, 9, 26) connected to the facing in a second zone (Z2) of said volume, said first and second zones having a part in common (Z'), whereby once the fill material has been introduced and compacted, loads are transmitted between the main reinforcement strips and the secondary members by the fill material situated in said common part. 14. The method as claimed in Claim 13, wherein the secondary members (6, 9, 26) are installed up to a distance substantially shorter than the main reinforcement strips (2) with respect to the front face. 15. The method as claimed in Claim 13 or 14, wherein the facing (3) comprises prefabricated elements (4, 8, 14, 24) incorporating secondary members (6, 9, 26). 16. The method as claimed in Claim 15, wherein the prefabricated elements (4, 14, 24) are made of concrete, wherein the secondary members (6, 26) comprise synthetic flexible reinforcement members each having at least one part cast into the concrete of one of the prefabricated elements, and wherein the cast part of the flexible synthetic reinforcement member (6, 26) follows a loop within said one of the prefabricated elements (4, 14, 24), so that said flexible synthetic reinforcement member has two sections projecting into the second zone (Z2) of the fill (1). 17. The method as claimed in Claim 16, wherein the loop is arranged in said one of the prefabricated elements (14) so that the two sections of said flexible synthetic reinforcement member emerge from the facing into the fill (1) at vertically offset positions, and wherein the step of placing the secondary members comprises laying the two sections of said flexible synthetic reinforcement member in two distinct horizontal planes. 18. The method as claimed in Claim 16, wherein the loop is arranged in said one of the prefabricated elements (24) so that the two sections (26A, 268) of said flexible synthetic reinforcement member emerge at different angles from the facing into the fill (1), and wherein the step of placing the secondary members comprises laying the two sections of said flexible synthetic reinforcement member in substantially the same horizontal plane. 19. A method according to Claim 15, wherein at least some of the prefabricated elements (8) have at least one projecting portion (9) forming one of the secondary elements. 20. The method as claimed in Claim 13 or 14, wherein the facing comprises wire mesh panels (54) to which soil reinforcements (56) are connected as secondary members. 21. The method as claimed in any one of Claim 13 to 20, wherein the step of placing the main reinforcement strips (2) comprises arranging the main reinforcement strips along zigzag paths in the first zone (Zl). 22. The method as claimed in any one of Claims 13 to 21, further comprising the step of determining independently an optimal configuration and density of the main reinforcement strips (2) in said first zone (Zl) and an optimal configuration and density of the secondary members (6, 26) in said second zone (Z2). 23. The method as claimed in any one of Claims 13 to 22, further comprising the step of connecting at least some of the main reinforcement strips (2) to the facing (3) by means of temporary attachments designed to break in the step of introducing and compacting the fill material.

Documents:

2583-DELNP-2005-Abstract-(20-07-2011).pdf

2583-DELNP-2005-Abstract-(26-03-2012).pdf

2583-delnp-2005-abstract.pdf

2583-DELNP-2005-Claims-(20-07-2011).pdf

2583-DELNP-2005-Claims-(26-03-2012).pdf

2583-delnp-2005-claims.pdf

2583-DELNP-2005-Correspondence Others-(20-07-2011).pdf

2583-delnp-2005-Correspondence Others-(21-07-2011).pdf

2583-DELNP-2005-Correspondence Others-(26-03-2012).pdf

2583-delnp-2005-correspondence-others.pdf

2583-DELNP-2005-Description (Complete)-(26-03-2012).pdf

2583-delnp-2005-description (complete).pdf

2583-DELNP-2005-Drawings-(20-07-2011).pdf

2583-delnp-2005-drawings.pdf

2583-DELNP-2005-Form-1-(20-07-2011).pdf

2583-DELNP-2005-Form-1-(26-03-2012).pdf

2583-delnp-2005-form-1.pdf

2583-delnp-2005-form-18.pdf

2583-DELNP-2005-Form-2-(20-07-2011).pdf

2583-DELNP-2005-Form-2-(26-03-2012).pdf

2583-delnp-2005-form-2.pdf

2583-delnp-2005-form-26.pdf

2583-DELNP-2005-Form-3-(20-07-2011).pdf

2583-delnp-2005-form-3.pdf

2583-DELNP-2005-Form-5-(20-07-2011).pdf

2583-delnp-2005-form-5.pdf

2583-delnp-2005-form-6.pdf

2583-delnp-2005-Petition-137-(21-07-2011).pdf