CH711908A2 - Self-supporting retaining wall. - Google Patents

Abstract

The invention relates to a prefabricated reinforced concrete self-supporting retaining wall (1), comprising: an upwardly pointing wall portion (2) for retaining an embankment, the wall portion (2) having a front face (20) intended to be free and a rear face (21) against which the embankment is intended to be supported; an anchoring element (3) projecting from said rear face and intended to anchor the retaining wall in the embankment, the anchoring element (3) having a bearing surface (313) parallel to said portion of wall (2) and intended to rest against the edge of a foundation (6). This wall allows for reduced excavation compared to a traditional L-shaped retaining wall.

Description

Description TECHNICAL FIELD [0001] The present invention relates to the field of civil engineering and land development. In particular, it relates to a self-supporting retaining wall of prefabricated reinforced concrete.

STATE OF THE ART [0002] Each uneven surface (embankment, embankment, trench) tends to move downwards under the influence of gravity. The shear resistance of the soil opposes this tendency to movement. If the soil shear strength is insufficient, the soil will slip gradually or abruptly.

To limit this risk, it is necessary to consolidate embankments or embankments whose slope is important. For example, solutions are known for retaining an embankment by means of a retaining wall at the edge of a road or a railway for example.

It is important to ensure that the anchor wall itself remains stationary. For this purpose, FIG. 8 illustrates an example of a retaining wall 1 of the prior art which is retained in the embankment 61 by means of anchoring cables 64. This anchorage requires a significant amount of space behind the wall. The volume to be excavated to place these anchoring cables, then backfill, is important. This solution is not available when the available width is insufficient to attach the anchor cables.

Also known consolidation walls whose stability is ensured primarily by the weight of the wall. These walls are called wall-weights. An example is described in patent application FR2566814 which illustrates a prefabricated retaining element for embankments, banks, trenches, wells, tunnels, etc.

A weight wall cast on site is necessarily heavy and therefore expensive and unattractive. In this application, the wall is further maintained by anchoring reinforcement sealed in the ground. This wall therefore requires a firm ground to retain these anchors. This solution is not suitable when it comes to remembering an embankment of loose earth or rubble in which it is impossible to anchor a frame.

[0007] FIG. 9 illustrates a retaining wall 1 of the prior art with an L shape. The stability is obtained by the own weight of the wall as well as by the weight of the embankment material on a foot 65 to the square of the wall 1. These walls have a lighter appearance than wall-weights, and require a reduced mass of concrete. The installation time is reduced thanks to the use of prefabricated square elements. The width of the necessary foundation is however important, particularly towards the rear, in order to position the foot 65. In addition, it is not possible to plant trees just above the foot of the wall. The angled walls also support at most a load of 5 kN for backfill.

[0008] EP 1 228 275 discloses an assembly of concrete panels for coating a wall, for example a wall. The panels are held by means of anchoring elements sealed in the face to be covered. This solution can only be implemented in the presence of a sufficiently strong wall to retain anchoring elements and panels.

[0009] Another example of a self-supporting precast concrete retaining wall is described in US Pat. No. 7,001,110, the contents of which are incorporated by reference. Such a wall has an apparent face and a face against which rests a slope that the wall aims to retain. In this patent, the retaining wall is linked to the portal of a tunnel.

The retaining wall is retained by the gantry, by a reinforced concrete base, and by an anchor element projecting on the rear face of the wall. This anchor element is cantilevered to enter the embankment behind the wall. The metal reinforcement of the concrete makes it possible to link the anchoring element to the vertical portion of the wall.

This type of prefabricated retaining wall can significantly reduce the implementation time compared to the walls cast on site. The durability of the wall remains insufficient for many applications. Slip and tilt stability and the prevention of shear soil failure are not optimal. In addition, the risk of wall damage, or even collapse under the weight of soil and water accumulated behind the back, increases after a few decades. For these reasons, such prefabricated retaining walls remain little used on the edges of roads or railroads frequenting. They are suitable only for applications in which at least one side of the wall is firmly attached to a tunnel portal frame or other rigid support point. They are not suitable for long walls made up of many walls juxtaposed; the collapse of a wall causes domino effect that of its neighbors.

Furthermore, the establishment of this type of wall is delicate, especially in the case of walls consisting of several retaining walls juxtaposed. In this case, it is necessary that the front faces of the different walls are perfectly aligned in the same plane, for aesthetic reasons but also to avoid the risk of infiltration of water or soil between two portions of misaligned walls. However, it is difficult to precisely align such walls which, because of their weight, must be moved by lifting them with a crane. Even if the crane manages to place a wall at the precise location that is desired, settlement of the ground in the days following installation, or backfill of the volume behind the wall, frequently causes movements that require realignment. A crane must return to this effect to lift the wall and move a few centimeters. It is not uncommon for this new uprising to damage the wall, especially at the junction between the anchor and the portion of the retaining wall.

An object of the present invention is therefore to provide a prefabricated retaining wall that is easier to install and align with other walls.

Another object of the present invention is to provide a prefabricated retaining wall whose slip stability, resistance to the risk of shearing soil and durability are improved.

An object of the present invention is also to provide a retaining wall whose static resistance, resistance to tilting and puncture resistance are greatly improved.

Another object is to achieve a self-supporting wall, that is to say a wall that must not be moored to a wall, but which allows instead to retain a soft earth fill, rubble, sand or any non-freezing material accumulated behind one side of the wall.

An object of the present invention is also to provide a retaining wall more resistant to corrosion reinforcements.

BRIEF SUMMARY OF THE INVENTION [0018] In order to limit these problems and to achieve the objects of the invention, the self-supporting reinforced concrete prefabricated retaining wall comprises: an upwardly pointing wall portion intended to retain an embankment said wall portion having a front face intended to be free and a rear face against which the backfill is intended to be supported; an anchor element projecting from said rear face and for anchoring the retaining wall in the backfill; the anchoring element comprising a bearing surface parallel to said wall portion and intended to rest against the edge of a foundation.

The bearing surface parallel to the wall portion allows to support and align the retaining wall against a foundation. The alignment of several adjacent walls is thus facilitated. This support also avoids the risk of sliding of the retaining wall forward, since it is retained by the bearing surface against the foundation.

The bearing surface may be vertical. It is thus possible to press against the vertical edge of a simple foundation to achieve.

The bearing surface may be below the bottom level of the wall portion when the wall is in place. It is thus possible to support it against a foundation entirely under the wall and under the ground level.

The anchoring element may comprise a rod (or rod) in a plane oriented upwards and substantially perpendicular to the wall portion; and at least one arm (or tab) extending perpendicular to the plane of the penis, in an upward direction, from the distal end of the penis.

The arm or the anchor the retaining wall, which can not advance without having to cause the earth and materials between the arm or arms and the wall portion.

The arm or arms may extend in an oblique plane relative to the wall portion. In this case, the penis has two lateral faces parallel to each other, perpendicular to the wall portion, oriented upwards, and rectangular trapezoidal shape.

At least one arm of the anchoring element may be curved so as to form an anchoring portion having two main faces facing upwards and a bearing portion having two substantially horizontal main faces. The main faces of each portion are the largest faces of this portion. The bearing surface may be formed by one of the faces of this bearing portion, for example by a vertical face facing the wall portion.

The penis may comprise an upper edge and a lower edge, substantially horizontal when the retaining wall is in place.

The lower face of the support portion is preferably below the underside of the penis.

The support portion may be disposed entirely under the lower edge of the penis.

The support portion under the rod is preferably long enough to avoid forming an acute angle with the ground or with other elements of the retaining wall. The lower face of this support portion is preferably substantially horizontal and flat when the wall is in place. This reduces the risk of breaking this support portion when the crane drops the wall.

In one embodiment, the retaining wall comprises a rod and two arms extending on each side of this rod.

The width of the wall portion may be between 200 and 300 cm. Its height can be between 250 and 600 cm.

The wall is self-supporting; it holds stably even without being sealed, and can hold an earth embankment that leans against its rear face. Thanks to the anchoring element, the weight of the wall and the weight of the backfill both help to prevent the wall from sliding forward, tilting and breaking the floor.

According to one aspect, the invention also starts from the observation that the problems of durability and stability of known retaining walls come in particular from the corrosion of the reinforcement of the concrete.

Corrosion is in particular due to infiltrations of water through the concrete; these infiltrations come in particular from microcracks through the concrete, especially at the connection point between the anchoring element and the wall portion. These microcracks are frequently caused during the transport of the wall after its manufacture, during its installation and during the withdrawal related to the hydration of the concrete. The anchoring element is then very stressed and constraints appear frequently on the connection with the wall.

At least one injection pipe may be provided near the junction between said wall portion and said anchor member to inject a microcement, a mortar and / or a resin to fill any cracks there. in the concrete.

The wall may comprise self-sealing sealing strips to seal the surface of the junction between the wall portion and the anchoring element. Thus, the water can not enter the cracks.

The armature of the wall can be made at least partially of stainless steel.

The concrete may be an ultra-high performance fiber concrete (UHPC), which has the advantage of almost perfect sealing, and high ductility reducing the risk of microcracks.

To reduce corrosion, the concrete may include a calcium nitrate type inhibitor for example.

The reinforcing bars may be coated with epoxy, or covered with a protective coating.

The retaining wall may comprise a metal frame and a plurality of corrosion detectors of this frame. Thus, it is possible to detect corrosion quickly and replace or repair a portion of wall before it collapses.

Corrosion detectors can be embedded in the thickness of the wall and provided with a radio frequency interface.

Corrosion detectors may be moisture detectors.

The corrosion detectors may be based on an electrical measurement, electrochemical, a humidity measurement, or potential fields.

The retaining wall may be connected to other identical retaining walls, to support a portion of embankment longer than the width of a single wall. It is important to maintain the adjacent walls between them, so that a valid wall prevents a neighboring wall in less good condition from collapsing, and then dragging it into its fall and the downfall of the embankment.

For this purpose, the wall portion may comprise two side edges oriented upwards. At least one of these slices, but preferably both, have a profile that allows a mortise and tenon type retention with an adjacent wall.

The rear face of the wall may further be provided with rails for fixing a holding plate with an adjacent wall. The rail mounting compensates for the variability of the play between two adjacent walls.

The rear face may include cable attachment points for fixing holding cables with an adjacent wall.

We have seen that the microcracks between the wall portion and the anchor element often occur during transport and the establishment of the retaining wall. In order to reduce this risk, in one aspect, the wall is provided with means for lifting and moving it without creating tensions or cracks in that area.

For this purpose, the rear face of the retaining wall may include anchor points for lifting the wall, for example by passing a cable in the anchor point, or by screwing a pin to pass a cable .

At least one anchor point may be provided on the anchoring element, for example on the upper edge of the anchoring element.

Advantageously, these anchor points allow to hold the wall in a balanced manner with the portion of the horizontal wall.

Advantageously, these anchor points allow to hold the wall with the portion of the vertical wall, and a cable holding the anchoring element.

To further reduce the risk of cracks during handling and transport of the wall, it is preferably provided fixing points on the rear face and on the anchoring element, which allow to fix a frame holding means for holding the anchoring element during transport, and avoiding any movement relative to the wall portion. The fixing points advantageously make it possible to fix a rectangular frame held in at least three points to the wall portion and to the anchoring element.

The armature of the wall is designed to further reduce the risk of slits at the interface between the rear face of the wall and the anchor element. This reinforcement also contributes to the support and the increase of the resistance of the wall. For this purpose, the wall portion preferably comprises a metal frame with U-shaped and U-shaped elements turned upside down.

The anchoring element may comprise a metal frame with substantially horizontal elements in C and C returned.

The retaining wall may comprise both reinforcing elements in the wall portion and wall elements in the anchoring element. These reinforcing elements can be interwoven to further secure the wall portion and the anchoring element.

In one embodiment, the wall may comprise several anchoring elements, for example two anchors with, for each, a rod in a vertical plane and one or more arms.

The anchoring element may comprise a drainage hole. A drainage hole may be parallel to the plane of the wall portion. A drainage hole can cross the wall portion.

Drainage may be provided at the rear of a foundation under the retaining wall.

A drainage may be provided at the rear of the wall portion.

The retaining wall may comprise a sill, that is to say a concrete beam, for example prefabricated, on the top of the wall. This sill improves the attachment of the retaining wall embankment and / or adjacent walls.

For the same purpose, the retaining wall may comprise a sill or a metal plate on the rear face. BRIEF DESCRIPTION OF THE FIGURES [0064] Examples of implementation of the invention are indicated in the description illustrated by the appended figures, which illustrate:

Fig. 1 a side view of a retaining wall according to one embodiment of the invention.

Fig. 2 a top view of a retaining wall according to one embodiment of the invention.

Fig. 3 a top view of two retaining walls according to one embodiment of the invention, linked to each other.

Fig. 4 a side view of a retaining wall according to one embodiment of the invention, mounted on a concrete base as a foundation, and provided with two longitudinal members.

Fig. 5 a side view of a retaining wall according to one embodiment of the invention, mounted on a concrete base as a foundation, and provided with a sill and drains.

Fig. 6 a side view of a retaining wall according to one embodiment of the invention, the armature being shown in broken lines.

Fig. 7 a sectional view of a retaining wall according to one embodiment of the invention, after installation.

Fig. 8 a sectional view of a retaining wall according to the prior art, after installation.

Fig. 9 a sectional view of a retaining wall according to the prior art, after installation.

Example (s) of Embodiment of the Invention [0065] A retaining wall 1 made of precast concrete is illustrated in side view in FIG. 1 and in plan view in FIG. 2. It generally comprises a wall portion 2 with a front face 20 and a rear face 21. The two faces 20, 21 are preferably parallel to each other and substantially vertical, or in any case oriented upwards, when the wall is installed.

The front face 20 is intended to be visible while the rear face 21 is intended to retain an embankment 61 pressed against this face, as shown schematically in FIG. 10. This retaining wall 1 is for example intended to retain the earth or rubble embankment 61 to prevent them from running down a road 60 or a railway in front of the front face 20. One or more drainage openings 24 can be provided through the wall portion 2, to allow the water accumulated behind the wall to pass through the wall and reduce the water pressure on the rear face 21.

The retaining wall 1 is retained on the one hand by its own weight, and on the other hand by at least one anchoring element 3 on the rear face 21 of the wall. In one embodiment, the anchoring element 3 shown from above in FIG. 2 has an anchor shape with a rod (or rod) 30 and two arms (or tabs) 31 which extend laterally on each side from the distal end of this rod. The penis 30 has two lateral faces 32, 34 shaped trapezoidal rectangle, substantially perpendicular to the rear face 21 of the wall, and substantially vertical when the retaining wall 1 is in place. The upper edge of this penis is referenced with the number 302 and the lower edge with the number 301.

The two arms 31 extend in an inclined plane perpendicular to the plane of the penis 30, and oblique. The lower end of these arms 31 forms a bearing portion 312 curved towards the wall portion 2, and extends in part below the lower edge 301 of the trapezoidal rod. The lower end 314 of the bearing portion 312 is flat and linked by obtuse angles to the rest of the anchoring element, which limits the risk of breaking during transport or installation of the wall of support. The upper face 318 of the bearing portion 312 is also flat. The length of the support portion 312 also reduces the risk of breakage.

The bearing portion 312 of each arm 31 has a substantially vertical bearing surface 313 for supporting the wall directly against the vertical edge of a concrete sole as a foundation 6. This arrangement avoids the risk of sliding wall 1 forward and position it accurately relative to this foundation and relative to adjacent walls.

It is also possible to provide the rear face 21 of the wall portion 2 of several anchoring elements 3.

The rear face 21 of the wall portion 2 is further provided with anchor points 210 for lifting the retaining wall by means of a crane, for example. The anchor points allow the introduction of studs, for example spherical or ring-shaped studs, for lifting by means of cables or straps. In an advantageous embodiment, four anchoring points 210 are provided on the rear face 21, distributed so as to lift the wall with the portion 2 horizontally and a balanced tension on the four straps or cables. At least one additional anchor point 310 may be provided on the upper edge 302 of the penis or on the upper edge of the arms 31, in order to lift the wall vertically by means of cables or straps connected to the anchor points 210. higher and at this or these points of anchorage of the anchoring element.

As illustrated in FIG. 2, one or more injection pipes 36 are advantageously provided in order to be able to inject microcement, mortar and / or resin in order to fill any cracks in the fastener between the anchoring element 3 and the rear face 21. One end of each pipe emerges on the rear face 21, or possibly on one of the lateral faces of the shank 30, in order to allow the injection of microcement, mortar and / or resin under high pressure, by example with a pressure greater than 50 bar, preferably greater than 100 bar. The injection pipe may be provided with perforations allowing the injected product to be introduced into any microcracks to fill them.

As illustrated in FIG. 2, the risk of introducing water into this zone can be further reduced by means of a surface seal, for example by means of self-adhesive sealing strips 33 glued to each side of the rod 30, so as to cover the connection with the dorsal surface 21. The width of these strips is advantageously between 15 and 30 centimeters.

The risk of corrosion can be further reduced by manufacturing the retaining wall with ultra-high performance fiber concrete (UHPC). This concrete offers the advantage of an almost perfect seal, limiting the risk of introducing water through the concrete. Its high ductility also reduces the risk of microcracks and the risk of brittle fractures.

The concrete may also be provided with an additive based on calcium nitrate (saltpetre) as a corrosion inhibitor.

For retaining walls 1 difficult to maintain or replace, it will be advantageous to use a steel reinforcement of stainless steel.

Corrosion detectors not shown can also be embedded in the concrete mass of the wall 1, in order to quickly detect the corrosion of the frame and allow an intervention before the retaining wall is at risk of s 'collapse. The detectors may be passive or active RFID detectors, that is to say with or without a battery. They can be powered by induction from an external reading device. They may include a moisture detector to detect water intrusions into the concrete. They can be equipped with voltage detectors and connected to the metal armature of the wall, to detect voltage variations between two points, which can occur in case of corrosion or reduction of the diameter of the armature. They can also be based on an electrochemical measurement or a measurement of potential fields. Several detectors may be provided in the wall portion and / or in the anchor member. The detectors advantageously allow a reading from the front face 20 of the wall, which remains accessible.

The retaining wall 1 can be mounted next to other similar walls to retain a backfill of great length. The width of each wall is advantageously between 2 and 3 meters, for optimal transport by truck. The height can be between 2.5 and 6 meters, depending on the needs.

It is important to retain the retaining walls 1 to each other in order to ensure a perfect alignment, and especially to avoid that the possible collapse of a wall does not lead to that of adjacent walls as soon as the embankment falls through the opening thus created. The link between adjacent retaining walls also helps to retain a damaged wall by means of its valid neighbors.

In the embodiment of FIG. 3, the maintenance between adjacent retaining walls is provided by a profile forming a connection of the type mortise 26 - mortise 27 on the side edge of the walls 1. A foam strip 214 can be glued between the walls to improve the seal and protect the profile.

One or more metal holding plates 213 may also be attached to the rear face 21 of the walls 1, to connect two adjacent walls. The plates can be screwed into rails 212 to compensate for the variability of play between two neighboring walls.

In a variant not shown, the adjacent walls are secured to each other by means of stainless cables connected to the rear face 21. Each cable can link two or more adjacent walls. Several cables can be provided between each pair of walls.

FIG. 4 illustrates a variant in which adjacent retaining walls 1 are maintained by means of a sill 5A on the wall head, and or one or more sills on the rear face 21.

FIG. 5 illustrates an embodiment in which a drainage aperture 24 is provided through the shank 30, to prevent the accumulation of water between the wall portion 2 and the arms 31 of the anchoring element 3 .

The durability of the wall also depends on the rigidity of the connection between the vertical wall portion 2 and the anchoring element 3. This rigidity is particularly given by the metal frame. For this purpose, the wall is advantageously made in two steps.

In a first step, the anchoring element 3 is molded with the horizontal rod 30 around an armature comprising irons 70, 71 in C and C returned. These irons protrude at the end of the rod facing the wall 2. The irons are illustrated in FIG. 6.

In a second step, the anchoring element is mounted on the holding frame 4, then returned to a vertical position above a table for casting the wall portion 2. The frame of this portion wall is placed on the molding table. This armature advantageously comprises elements 72, 73 in U and U returned, oriented vertically in the normal position of the wall. These elements are inserted through the reinforcing portion which protrudes from the anchoring element, and attached to this element. RFID sensors can be attached to the armature. Additional reinforcing elements, in bulk, can be added. The wall portion 2 is then cast around this frame.

FIG. 7 illustrates a retaining wall 1 in place in front of a fill 61 to protect a road 60. Tests have shown that such a wall manages to retain a load of 20kN or more.

Reference numbers used in figures [0089] 1 Retaining wall 2 Wall portion facing up 20 Front face of wall portion 21 Back side of wall portion 210 Anchor points 211 Mounting points of frame retaining on the wall portion 212 Fixing rails 213 Retaining plate 214 Foam strip 24 Drains 26 Stud profile on the side edge of a wall 27 Mortise profile on the side edge of a wall 3 Anchor element ( or anchor)

Claims (18)

30 Yard in an upward plane 300 Anchor point 301 Lower edge of the yard 302 Upper edge of the yard 31 Anchor arm 310 Anchorage point of the frame on the arms 312 Portion of support of the arms, extending under a portion of the lower portion 301 313 bearing surface 314 lower face of the bearing portion 315 anchoring portion 316 first face of the anchoring portion 317 second face of the portion of anchor 318 Upper side of the support portion 32 First vertical face of the yard 33 Self-adhesive sealing strips 34 Second vertical face of the yard 36 Injection pipes 5A Longitudinal edge at the top of the wall portion 5B Longitudinal side of the wall 6 Foundation 60 Route 61 Embankments 64 Anchoring Cables 65 L-shaped wall feet 70, 71 C-shaped horizontal bars 72, 73 Vertical U-shaped bars 1. Prefabricated reinforced concrete self-supporting retaining wall (1), comprising: an upwardly directed wall portion (2) for retaining a fill (61), the wall portion (2) having a front face (20); ) intended to be free and a rear face (21) against which the embankment is intended to be supported; an anchoring element (3) projecting from said rear face and intended to anchor the retaining wall in the embankment (61), the anchoring element (3) having a bearing surface (313) parallel to said wall portion (2) and intended to rest against the edge of a foundation (6). 2. retaining wall according to claim 1, the anchoring element (3) comprising: a rod (30) in an upwardly directed plane and substantially perpendicular to the wall portion (2); and at least one arm (31) extending in a plane perpendicular to the plane of the penis and oblique with respect to the wall portion (2), from the distal end of the penis (30). 3. retaining wall according to claim 2, said at least one arm (31) being curved so as to comprise an anchoring portion (315) with two main faces (316, 317) facing upwards and a portion of support (312) with two substantially horizontal main faces (314, 318), said bearing surface (313) being formed by one of the faces of said bearing portion (312). 4. retaining wall according to claim 3, said rod having an upper portion (302) and a lower portion (301), said bearing portion may be disposed under the lower portion (301). 5. retaining wall according to one of claims 3 to 4, the lower face (314) of the bearing portion (312) being substantially flat and horizontal. 6. retaining wall according to one of claims 3 to 5, the lower face (314) of the bearing portion (312) being below the underside (301) of the rod (30). 7. retaining wall according to one of claims 2 to 6, comprising a rod (30) and said two arms (31) extending on each side of the rod. 8. Retaining wall according to one of claims 1 to 7, comprising at least one injection pipe (36) near the junction between said wall portion (2) and said anchoring element (3) and allowing inject a microcement or a resin to fill any cracks in the concrete. 9. retaining wall according to one of claims 1 to 8, said wall portion (2) having two lateral edges oriented upwards and at least one of which is provided with a profile (26; 27) allowing a maintenance of type tenon (26) - mortise (27) with an adjacent retaining wall. 10. Retaining wall according to one of claims 1 to 9, said rear face (21) having rails (212) for fixing a holding plate (213) with an adjacent wall. 11. Retaining wall according to one of claims 1 to 10, said rear face (21) having cable attachment points for fixing holding cables with an adjacent wall. 12. Retaining wall according to one of claims 1 to 11, said wall portion comprising a metal frame with elements oriented upwards U and U returned, said anchoring element comprising a metal frame with elements substantially horizontally in C and C returned (70, 71), the reinforcing elements in the wall portion (2) and in the anchoring element (3) being intertwined. 13. Retaining wall according to one of claims 1 to 12, said anchoring element (3) having a drainage hole (24). 14. Retaining wall according to one of claims 1 to 12, the wall portion (2) having a drainage hole (24) between the front face (20) and the rear face (21). 15. Retaining wall according to one of claims 1 to 14, comprising a sill (5A) on the top of the wall or on said rear face. 16. A set comprising a retaining wall according to one of claims 1 to 15 and a foundation (6) on which said wall portion (2) and against which rests said bearing surface (313). 17. The assembly of claim 16, said foundation (6) having a drain. 18. Wall comprising a plurality of retaining walls according to one of claims 1 to 15, at least two adjacent walls being juxtaposed and rigidly connected to each other by at least one sill (5A, 5B).