ZA200300864B

ZA200300864B - Method for building retaining structure.

Info

Publication number: ZA200300864B
Application number: ZA200300864A
Authority: ZA
Inventors: Jean-Marc Jailloux; Fahim Ziai
Original assignee: Jailloux Jean Marc Ziai Fahim
Priority date: 2001-06-11
Filing date: 2003-01-30
Publication date: 2003-09-03
Also published as: WO2002101158A1; EP1395712A1; JP2004521213A; FR2825730A1; FR2825730B1

Description

J! "

Method for building retaining structure

The invention relates to a method for building a retaining structure built against a face of an existing structure, such as an embankment, a wall or a rock face, a face in which distributed anchoring points are provided. Such a face is usually either substantially vertical or inclined.

Numerous old retaining structures which bear traffic routes or static loads were constructed in some cases more than one hundred years ago for traffic in respect of which the load and the frequency were much less than they are today. This results in overloading and fatigue of these structures, due in particular to vibrations, which frequently results in breaking up or deformation.

These retaining structures frequently consist of a pile of stones positioned with or without mortar. Their cross-section is most often trapezoidal, wide at the base and reducing up the height of the wall. These structures function as gravity walls. In other words, the weight of the assembly of stones balances the thrust of the land. Numecrous disturbances affect these old structures: deformation, local collapse of the stones or even ruining of a portion of the structure by falling down or slipping.

Various techniques have been used in order to recreate the conditions of use of the land- retaining walls. A recent technique is known in particular which is called nailing and consists of driving in or embedding resistant linear elements, commonly called nails, which are capable of forming a composite material with the ground by friction between the ground and the external surface of the nail or its cladding. This technique gives the ground new mechanical properties. It is defined in well in documents such as the "Recommandations

Clouterre".

According to the current technique which 1s most frequently used, the heads of the nails are connected by a welded lattice. The whole of the face is covered with shotcrete in order to form a new shotcrete facing which does no more than follow the shapes of this face. The concrete face is generally somewhat inclined with respect to the vertical, often more than the original face of stones. Its appearance is not very pleasing. This is why, in order to restore the attractive appearance of the stones, it is necessary to construct a new face of stones or of pebbles in front of the shotcrete wall.

Finally, the area of level ground at the head of the wall is equal to or smaller than the original area due to the shrinkage of the top of the wall.

The invention relates to a method for building a retaining structure built against a face of an existing structure, such as an embankment, a wall or a rock face, which remedies these drawbacks. This method must be economical and quick to carry out. It must make it possible to construct a retaining structure which is steeper, that is to say closer to the vertical, than the original wall if it is necessary to enlarge the area of level ground.

Finally, in certain variants this method must make it possible to maintain an external appearance of the retaining structure which is close to the original appearance of the wall of stones.

These objects are achieved according to the present invention in that: - a facing structure is erected which has fixing points at a certain distance from the face in such a way as to delimit a space between this face and the facing structure; - the anchoring points of the face are successively connected to the fixing points of the facing structure by at least one continuous flexible connection; and - the space included between the face and the facing structure is backfilled.

This method offers numerous advantages. First of all, at the moment of construction the flexible connection adapts easily to the variations in distance between the anchoring points of the face and the fixing points of the facing structure. In fact, it is not easy to embed the nails in the ground at precise locations. Consequently the position of the heads of the nails in three dimensions is imprecise. The flexible connection adapts easily to this imprecision of the position of the nail heads.

Moreover, the flexible connection offers a high tensile strength for a modest price. It constitutes an advantageous alternative relative to mechanical connections with controllable traction of a known type, such as reverse-pitch screw links.

The method according to the invention also offers the advantage that it does not necessitate a foundation.

Another advantage of the method according to the invention resides in the fact that the retaining face can easily be steeper than the slope of the original structure. That makes it possible to enlarge the area of level ground which is available if this is felt to be necessary.

The facing structure can be formed in one piece, particularly if the dimensions of the structure are not very great. However, the facing structure preferably consists of modular elements.

These elements are assembled with one another, which makes it possible to form a structure of large dimensions.

The facing structure is preferably erected by successive banks of the backfill of the space between the face and the facing structure. In other words, first of all a first row of modular elements is constructed on the ground, the space contained between this first row and the face is filled in, then a second row of modular elements is placed above the first and so on until the construction is completed.

In accordance with the method according to the invention, the facing structure is disposed at a certain distance from the wall. This distance can vary from approximately 0.5 metres to several metres.

In accordance with the method according to the invention: - a first row of elements of the structure is fixed at anchoring points of the face by at least one continuous flexible connection;

- the space between the face and elements of the structure is backfilled; and - the same operations are repeated in order to produce successive superimposed rows until the structure is completed.

It is necessary to retain the modular elements of the facing structure until the backfilling is achieved. They can be retained in different ways. According to a first variant of the method, the elements of the structure are kept spaced from the face by at least one element called a "buffer" which is withdrawn after the space between the face and the elements of the structure has been backfilled.

According to a variant of the method, the elements of the facing structure are kept spaced from the inclined face by an external guide attached to the elements of the structure of the row immediately below; the space between the face and the elements of the facing structure is backfilled; and a continuous flexible connection is placed on the upper edge of the elements of the facing structure.

The anchoring points can be formed by any structure capable of withstanding the forces required, for example an embankment, an existing wall made from concrete which may or not be reinforced, a rock face or any other structure. However, in a preferred embodiment the anchoring points provided in the face are formed by nails driven into the ground to be strengthened. The nails are put in place either by ramming, that is to say driving in by force, or by boring, that is to say putting a nail in place in a borehole and sealing with a slurry.

Instead of nails, it is equally possible to use tie rods anchored at their opposite end to the head of the nail.

In a particular embodiment the anchoring points, which may for example be nails or tie rods, have at least one curved element serving as a support for the flexible connection. This curved element can be a curved bar or even curved moulded head with several branches. The curvature of the bar or of the moulded head has the function of retaining the connection whilst preventing it from slipping.

When the nails are very spaced, or when they are spaced irregularly, they have on their external end fixing elements such lugs to which distribution bars are fixed. Fixing elements such as stirrups can then be mounted on these distribution bars at chosen locations.

The fixing points of the connection on the facing structure can be formed in numerous ways.

By way of non-limiting example they can be formed by bars which are supported on the facing structure or preferably by stirrups attached to the facing structure. These latter can be sufficiently elongated to pass through an external zone of mechanically aggressive backfill for the flexible connection, such as for example a layer of pebbles.

In the same way, the continuous flexible connection may take very varied forms. It may be metallic, for example made from galvanised steel or stainless steel. It may also be produced from a material chosen from amongst polymers with high toughness, polyamides, aramids or even glass or carbon fibres. The flexible connection preferably takes the form of a tape or a strip. This form is advantageous because, with the same resistance as a wire or cable of round cross-section, a connection with a flat cross-section turns much better about anchoring points and fixing points. The stresses are more balanced in a flat cross-section than in a circular cross-section.

In a particular embodiment the flexible connection is formed by a strip formed by polyester cores surrounded by a polyethylene cladding.

Finally, numerous solutions are offered to the person skilled in the art in order to form the facing structure. The modular facing elements may be formed by welded lattice panels.

These panels may be assembled with a layer of stones or pebbles which gives an appearance close to that of the original facing. The modular facing elements may also be formed by parts made from concrete, by boarding on a framework, or even by a pile of prefabricated blocks forming plant-holders.

Other characteristics and advantages of the invention will become apparent from a reading of the following description of embodiments which is given by way of illustration with reference to the accompanying drawings. In these drawings:

- Figure 1 is a general perspective view which illustrates the method of construction according to the invention; - Figure 2 is a lateral view in elevation of the embankment shown in Figure 1; - Figures 3 and 4 illustrate a particular variant of the method according to the invention; - Figures 5 and 6 illustrate another particular variant of the method according to the invention; - Figures 7 to 9 show various embodiments for anchoring of the flexible connection in the face by means of nails driven into the embankment; and - Figures 10 to 13 show various embodiments for fixing of the flexible connection of the facing structure.

In Figure 1 a general perspective view is shown which illustrates the method according to the invention. The reference 2 designates an existing structure to be strengthened, in this example a piece of ground. This piece of ground is limited on its upper part by a level area 4 which constitutes the original level area which supports a load such as a traffic route or a construction. The level area 4 is limited laterally by a surface 6 which is very inclined with respect to the horizontal 8. In the illustrated example the surface 6, which may also be called the face, is not doubled by a wall, for example a wall of stone. It therefore constitutes a simple embankment. The ground 2 could equally be supported by a pile of dry or built-up stones.

In order to retain the ground 2 a retaining structure is constructed according to the invention.

This structure includes a face 10 known as a facing structure which is spaced from the face 6 (embankment) in such a way as to delimit a certain space between the face and the structure 10. This distance 12, measured at the foot of the structure 10, can vary from about 0.5 metre to several metres. It will also be noted that the angle of inclination of the structure 10 is closer to the vertical than the angle of the embankment 6. In that way the distance between the structure and the embankment increases the further up one goes. It will be noted in particular that the level area 14 between the upper part of the embankment 6 and the upper part of the facing structure 10 is wider than the distance 12. Thus the space available on the level ground 4 is increased.

In order to balance the weight of the backfill 6, the facing structure 10 must be anchored to the ground 2 by anchorages provided on the surface of the embankment 6. In the illustrated embodiment, the anchorages are formed by the heads 18 of nails 20 driven into the ground 2.

The nails 20 are formed by mechanically resistant linear elements such as bars or tubes of steel. They can be driven in forcibly by ramming. It is equally possible to bore holes in the ground 2, introduce the nails into the bore and seal them with a slurry. These nails are distributed as regularly as possible over the surface of the embankment 6. They are for example disposed in a series of horizontal rows. The presence of the nails improves the properties of mechanical resistance of the ground 2 and contributes to its reinforcement.

The facing structure 10 has fixing points 22 situated facing the anchorages 18. The fixing points 22 are distributed over the surface of the facing structure in horizontal rows preferably situated at the same level as the rows of heads 18 of the nails 20. A continuous flexible connection 24 connects the anchorages 18 of the ground 2 to the fixing points 22 of the structure 10. The connection 24 is made from one or several materials which are resistant to the mechanical stresses imposed by the construction phases, permanent loading and overloads over a period of several decades and even of the order of a century. It is equally resistant to corrosion. It may be made from a metal such as galvanised steel or stainless steel. It may equally be made from plastic material, for example from polymer (polyester with high toughness, polyamide or aramid) or from glass or carbon fibres. The connection 24 is continuous over several metres, even over several tens of metres. It is stretched between the heads 18 of the nails 20 and the fixing points 22. It has the advantage that at the moment of construction it adapts to the differences in distance and position between the face 10 to be created and the heads 18 of the nails. For this reason it is easy to put in place. It also has the advantage of being inexpensive.

After the facing structure 10 has been put in place and has been fixed by means of the connections 22, the space between the embankment 6 and the structure 10 is backfilled with a material having characteristics chosen in order to complete the structure. In a variant, it is possible to provide a layer 26 of pebbles or dry stones just below the facing structure 10 in order to give the structure a pleasant appearance.

Figure 2 shows a side view in elevation of the embankment shown in Figure 1. Although the facing structure may be produced in one single panel it is preferably formed by assembled modular panels. In Figure 2 a method according to the invention is illustrated in which the backfill 16 is produced in six banks of earth formed one after the other until the height of the embankment is reached. The height of each bank is equal to the height of a modular element 30. In a first step the embankment is strengthened by nailing. The nailing is preferably carried out from the top downwards, starting with the upper part of the embankment 6. Work then progresses downwards by removing stones or unstable elements (dash-dot line 32) in such a way as to obtain an embankment having a surface which is firm and stable in the short term.

Once the operation of nailing and rectification of the embankment is terminated, a first row of modular elements 30 is arranged, consisting for example of welded lattice panels placed at the foot of the structure. The panels 30 are placed at a distance 12 from the lower part of the backfill, as explained previously (Figure 1). They are anchored to the embankment by two flexible connections. A lower connection 24, connects the heads 18, of the nails 20, of the lower row to the fixing point 22, situated on the lower part of the elements 30, of the first row. In the same way, a flexible connection 24, successively connects the heads 18, of the nails 20, of the second row to the fixing point 22, provided on the upper part of the modular elements 30, of the first row. The space between the modular elements 30, of the first row and the lower part of the embankment is then backfilled. The elements 30, are then retained, on the one hand, by the pressure of the earth and, on the other hand, by the strong traction exerted on the connections 24, and 24,. It is then possible to proceed to the production of the second bank of backfill. For this purpose a second row of modular panels 30, is disposed above the first, the lower part of these panels being supported on the inner edge of the panels of the first row. The upper part of the panels of the second row is retained by means of a flexible connection 24,. It is not necessary to provide a new connection to the lower part of these panels since they are held supported on the upper edge of the panels of the first row.

Then the space between the panels of the second row and the lower part of the embankment is backfilled, and the same cycle is repeated until the structure is completed. It will be noted that the last row of panels 30; is retained by a flexible connection 24, which is fixed to the same nail 20 as the flexible connection 24, which retains the panels 30, of the row below.

While the operations described with reference to Figure 2 are being carried out, it is necessary for the modular panels 30, to be temporarily kept spaced from the face of the embankment 6 until they can be held naturally by receiving the thrust of the backfill. Several methods make it possible to achieve this result. In Figures 3 and 4 an elongated element 34, called a "buffer", is interposed between the embankment 6 and the face of the panel 30 which is directed towards the embankment, in such a way as to maintain the desired spacing.

Simultaneously the upper part of the panels 30 is held against the buffers 34 by the flexible connection 24,. Thus the panels 30, are held temporarily until the backfill is completed (Figure 4), and as the thrust of the backfill is exerted on the inner face of the panels 30 (see arrows) the buffers 34 can be withdrawn and can be reused in order to block the panels of the upper row.

Figures 5 and 6 show a second method of provisionally holding the panels 30. Once the lower row has been produced a guide 36, for example a plank of wood, having a hook 38 is attached to the exterior of the panels of the facing structure. The panels 30, of the second row are supported on the guide 36 during the backfilled of the space between the embankment 6 and the second row of panels. Once the backfill is finished, the flexible connection 24, is placed between the anchorages 18; formed by the heads of the nails 20, driven into the embankment and the fixing points 22, provided on the upper part of the panels 30,. The thrust of the backfill is then balanced by the traction of the connection 24,. The guide 36 can then be detached reused in order to produce the following level of the backfill.

Figures 7 to 9 shows various embodiments of the anchorage of the connection in the ground 2 of the embankment. In Figure 7 the nails are formed by tubes driven into the ground by ramming, that is to say by striking their free end. The head 18 of the nail 20 has a curved bar which is simply welded to the end of the tube 20. As will be noted, the flexible connection 24 passes over the bar 18 twice. However, in the general case the connection is only fixed once to a given anchorage of a tube 20. It will also be noted that the connection 24 is in the form of a tape which is easily adapted to the external cylindrical diameter of the welded bar.

In Figure 8 the nail 20 is formed by a solid bar which is threaded at its free end. A moulded head 18 with two curved arms is fixed to the threaded end of the bar 20 by means of a bolt 37.

The flexible connection 24 passes over each of the two arms of the moulded head.

In Figure 9 a perspective view is shown of an embodiment which is preferably used when the spacing between the nails 20 is very great, in particular when it is greater than the spacing between the fixing points of the facing structure 10. Each nail 20 has a welded lug 40 at its free end. The lug 40 ends in a tab 42 which is folded round and fixed by a bolt 44 in such a way as to delimit a hole into which a distribution bar or tube also known as a stringer can be introduced. As will be noted in Figure 9, the stringer 46 has a considerable length, for example several metres, in such a way that it can be retained by several nails. Stirrups can then be attached to the stringer 46 between two successive nails 20. For example in Figure 9 two stirrups forming two anchorages 18 are interposed between the two nails 20. Thus the number of anchorages 18 can be equal to the number of fixing points 22 of the facing structure, in such a way that the flexible connection 24 has a regular shape between the fixing points 22 and the anchorages 18. In Figure 9 the fixing points of the facing structure are equally formed by stirrups attached to the principal bars of the welded lattices forming the modular elements 30.

Figures 10 to 13 show different variants of the fixing points of the flexible connection 24 on the facing structure 10. In Figure 10 the flexible connection 24 is fixed by a stirrup 22 to the principal bars 48 of the metal lattices forming the facing structure, in the same way as is illustrated in Figure 9. However, in this variant the stirrup has a greater length such that its end over which the flexible connection 24 passes is located outside a zone of pebbles 26 (see

Figure 1) placed against the facing structure 10. The purpose of the pebbles 26 is to give an attractive appearance to the retaining structure, but they are aggressive for the connection 24 and they could damage it. This is why the stirrups are designed so as to be sufficiently elongated that the fixing of the connection is located in the loose ground of the backfill.

In Figure 11 the flexible connection 24 is simply retained by a rigid bar 22 placed close to a principal bar 48 of the welded lattice structure forming the facing 10. The bar does not have to be fixed. It is then simply held against the lattice by the traction exerted by the flexible connection 24.

Figures 12 and 13 show fixing points adapted to a rigid panel, for example to a panel of concrete. In Figure 12 two rings 50 are sealed into the concrete face. The rings 50 are disposed in a horizontal plane. A bar forming a fixing point 22 is slide inside the two rings 50 and the flexible connection 24 winds around the bar 22. The bar is held by the traction exerted by the flexible connection, without other fixing.

In Figure 13 two rings 52 disposed in a horizontal plane and at the same level are sealed into the concrete wall which forms the facing structure 10. A stirrup 22 similar to the stirrups used in the previously described variants is attached by its two ends to each of the rings 52.

The flexible connection 24 passes over the stirrup 22.

Claims

1. Method for building a retaining structure built against a face (6) of an existing structure (2), a face in which distributed anchoring points (18) are provided, characterised in that: - a facing structure (10) is erected which has fixing points (22) at a certain distance (12) from the face (6) in such a way as to delimit a space between this face (6) and the facing structure (10); - the anchoring points (18) of the face (6) are successively connected to the fixing points (22) of the facing structure by at least one continuous flexible connection (24); and - the space included between the face (6) and the facing structure (10) is backfilled.

2. Method as claimed in Claim 1, characterised in that the facing structure (10) is formed by elements of modular structure (30), and that the facing structure (10) is preferably erected by successive banks of the backfill of the space between the face (6) and the facing structure (10).

3. Method as claimed in Claim 2, characterised in that: - a first row of elements of the structure (30,) is fixed at anchoring points (18;, 18,) of the face (6) by at least one continuous flexible connection (24,, 24,); - the space between the face (6) and the first row of elements of the structure (30,) is backfilled; and - the same operations are repeated in order to produce successive superimposed rows (30,) until the structure is completed.

4, Method as claimed in Claim 3, characterised in that for each row of elements of the structure (30) the elements (30) are kept spaced from the face (6) by at least one element (34) called a "buffer" which is removed after the space between the face (6) and the elements of the structure (30) has been backfilled.

5. Method as claimed in Claim 3, characterised in that for each row of elements of the structure (30): - the elements of the structure are kept spaced from the face (6) by an external guide (36) attached to the elements of the structure (30) of the row immediately below; - the space between the face (6) and the elements of the structure (30) is backfilled; - a continuous flexible connection (24) is placed on the upper edge of the elements of the structure (30); and - the guide (36) is removed.

0. Method as claimed in one of Claims 1 to 5, characterised in that the anchoring points (18) provided in the face (6) are formed by nails (20) embedded in the existing structure (2).

7. Method as claimed in one of Claims 1 to 6, characterised in that the anchoring points (18) have at least one curved element which serves as a support for the flexible connection (24).

8. Method as claimed in Claim 7, characterised in that the curved element is a moulded head with several arms.

9. Method as claimed in one of Claims 1 to 8, characterised in that the fixing points (22) of the facing structure (10) are formed by stirrups attached to the facing structure.

10. Method as claimed in Claim 9, characterised in that the stirrups (22) are provided sufficiently elongated to pass through an external backfill zone (26) which is mechanically aggressive for the continuous flexible connection.

11. Method as claimed in one of Claims 1 to 10, characterised in that the continuous flexible connection is made from a material chosen from the group comprising galvanised, steel, stainless steel, polymers with high toughness, polyamides and aramids, glass and carbon fibres, and that the flexible connection is in the form of a tape or strip.

12. Method as claimed in one of Claims 1 to 11, characterised in that the modular facing elements (30) are chosen from amongst welded lattice panels and parts made from concrete.