AU2011247163B2

AU2011247163B2 - A diaphragm wall with prefabricated facing

Info

Publication number: AU2011247163B2
Application number: AU2011247163A
Authority: AU
Inventors: Michel Guicherd; Xavier Iltiss; Patrick Lemaire
Original assignee: Soletanche Freyssinet SA
Current assignee: Soletanche Freyssinet SA
Priority date: 2010-04-26
Filing date: 2011-04-20
Publication date: 2016-08-04
Anticipated expiration: 2031-04-20
Also published as: FR2959253B1; WO2011135235A1; FR2959253A1; PL2563975T3; ES2525082T3; EP2563975B1; EP2563975A1

Abstract

The present invention relates to a method for creating a molded wall, said method including at least the steps that involve: forming, in the ground, an elongate excavation (14), having vertical walls that mark the border of an area (18) to be cleared; positioning, in the excavation (14), an assembly that includes at least one reinforcement cage (26) having an L-shaped profile and comprising a first portion (30), having a width suited to the width of the excavation (14) and intended to be positioned in a manner parallel to the base of the excavation (14), and a second portion (32), having a width less than that of said first portion (30) and intended for standing against the longitudinal wall (16a) of the excavation (14) located on the side opposite the area to be cleared (18), said assembly also including at least one piece of prefabricated cladding (44) intended to be positioned against the longitudinal wall (16b) of said excavation (14) located on the side of the area (18) to be cleared; and pouring concrete into said excavation (14) so as to form a molded wall including said prefabricated cladding (44). The present invention also relates to a molded wall capable of being created by means of such a method.

Description

A DIAPHRAGM WALL WITH PREFABRICATED FACING

The present invention relates to the field of special works in the ground. It relates more particularly to a diaphragm wall, and to a method of making such a diaphragm wall.

The diaphragm wall of the invention and the corresponding method of making it are particularly adapted to building quays, car parks, metro stations, tunnels, and more generally any works in which it is necessary to clad a wall.

Diaphragm walls in the ground have been well known for a long time. An excavation of elongate shape, and of width and depth equal to the width and depth of the wall that is to be obtained is initially formed in the ground. While it is being excavated, the trench is stabilized by filling the excavation with a liquid known as "mud", generally based on bentonite. The mud forms a waterproof deposit on the walls of the excavation, thereby preventing the mud from percolating into the soil and preventing the walls from collapsing. When the excavation has reached the desired depth, a reinforcing cage is put into place therein, and then the excavation is filled progressively with concrete by means of a dip tube, filling beginning from beneath the mud at the bottom of the excavation.

Under certain circumstances (wall incorporated in the final work, optimizing wall thickness, special waterproofing requirements, regulations), prefabricated walls constitute an advantageous alternative to diaphragm walls. In particular when access to the wall is impossible or dangerous, it is not possible, after ground has been dug away, to act on the diaphragm wall for the purpose of finishing off the wall. Prefabricated walls have been well known since the beginning of the 1970s, and they constitute a solution to that problem. In known methods, an excavation is made in the ground in the same manner as that described above for diaphragm walls. Prefabricated panels are then lowered into the excavation that has previously been filled with bentonite drilling mud. A support device resting on guide strips serves to hold the prefabricated panels in place. The drilling mud is then replaced by a hardening slurry slip based on cement that serves to seal the prefabricated panel permanently in the soil.

Prefabricated panels present finish that, under certain circumstances, allows them to constitute the final facing element of the work. No burdensome finishing action is then needed in order to obtain satisfactory visible appearance for the visible portion of the wall.

Nevertheless, prefabricated walls present drawbacks that prevent them from being used in certain types of work, and in particular in works of large dimensions. At present, there do not exist any convenient means for butt-joining two prefabricated panels. The dimensions of prefabricated panels are limited to the capacity of transportation means and of hoisting means, and they are difficult to use for works of great height. Furthermore, given the weight of the panels, the concrete reinforcement in the anchoring can require metalwork that is very dense and very complex.

There is a need to provide wall that is suitable for being adapted to works of large dimensions, that is strong, and that, once constructed, presents visual appearance that is sufficiently satisfactory for subsequent finishing work not to be essential.

It is an object of the present invention to at least substantially satisfy the above need.

An aspect of the present invention provides a method of making a diaphragm wall, the method being characterized in that it comprises at least the following succession of steps: • forming in the ground an excavation of elongate shape, having vertical walls, marking the frontier of a zone to be dug away; • positioning, in the excavation, a reinforcing cage of L-shaped profile having a first portion of width matching the width of the excavation and intended to be positioned parallel to the bottom of the excavation, and a second portion of width less than the width of said first portion and intended to be backed against the longitudinal wall of the excavation that is situated on its side remote from the zone to be dug away, in such a manner that an empty gap is retained between said second portion and the longitudinal wall of said excavation situated beside the zone to be dug away; and • positioning at least one prefabricated facing element in said empty gap, against the longitudinal wall of the excavation situated beside the zone to be dug away, and casting concrete into said excavation in such a manner as to form a diaphragm wall including said prefabricated facing element.

The method of at least a preferred embodiment the diaphragm wall technique with the prefabricated wall technique. The diaphragm wall obtained in this way is provided with a prefabricated facing element that covers a portion of one of its longitudinal faces, and a portion that is made by casting concrete in situ, which portion presents a generally L-shaped profile and has the reinforcing cage embedded therein, possibly together with starter bars of the prefabricated facing element. This diaphragm wall thus benefits from the advantages of both techniques. It is constructed in conventional manner using alternating or successive individual panels and it can be constructed over very great lengths. Furthermore, because of the presence of the prefabricated facing element on one of the faces of the wall, a visible portion presenting the required visual appearance can be obtained once the wall has been constructed and without requiring any burdensome subsequent action. Under such circumstances, in addition to its appearance function, it is also possible for the prefabricated facing element to take up a fraction of the forces exerted on the structure.

Generally, it may be considered that the reinforcing cage does not need to be positioned inside the excavation with very great accuracy. However, in order to guarantee that the wall is of good appearance, it is important for the prefabricated facing element(s) to be properly positioned, and when there are two or more facing elements, for them to be in alignment with one another.

In at least a preferred embodiment of the present invention, the reinforcing cage is thus initially lowered into the excavation (as a single segment or as successive segments, as described in greater detail below), and then once the cage is in position inside the excavation, the prefabricated facing element is lowered separately into the space retained between the narrower portion of the cage (i.e. the second portion) and the longitudinal wall of the excavation situated beside the zone to be dug away.

By means of these provisions, it is easier to position the reinforcing cage inside the excavation, particularly when the height and weight of the cage are large. Similarly, since the facing element is originally independent of the reinforcing cage, it is easier to handle, and can thus be positioned more easily in the trench.

In at least a preferred embodiment of the present invention, two guide strips marking the location desired for the diaphragm wall are made in the form of low walls in the ground that is to be excavated. Excavation then takes place vertically between the two guide strips. The guide strips situated beside the zone that is to be dug away is generally removed after the diaphragm wall has been concreted. During construction, these guide strips constitute an accurate reference for use in supporting markers for locating the elements and for positioning them vertically. They ensure that surface soil is stabilized and they serve as rests for means that hold the reinforcing cage once it has been lowered into the excavation, and also for other pieces of equipment.

In an implementation, the reinforcing cage and the facing element are inserted into the excavation as follows: the cage is inserted into the excavation while supported by grip loops provided in its top portion. Its position is stabilized temporarily in the excavation by means of bars passed through the grip loops and bearing transversely on the guide strips. Support of the reinforcing cage is then taken over by means of a lifting beam that is shaped to leave free the empty gap for insertion of said prefabricated facing element. Finally, the bars are removed and the prefabricated facing element is inserted into the empty gap of the excavation.

For works of very great height, and in particular if a reinforcing cage made as a single piece would be too heavy for handling by the hoisting plant, it can be advantageous for the cage to be made up of a plurality of segments that are lowered into the excavation in succession.

According to another preferred embodiment of the present invention, the surface of the prefabricated facing element that is to face towards the zone to be dug away is coated with protection prior to the prefabricated facing element being positioned in the excavation. The protection may be removed after ground has been dug away. It serves to avoid the visible face of the prefabricated facing element being dirtied, and in particular it serves to prevent any concrete that seeps around from adhering thereto.

In at least a preferred embodiment of the present invention, the bottom portion of the prefabricated facing element is chamfered lengthwise, and the prefabricated facing element is positioned in the excavation in such a manner that the chamfered portion faces towards the inside of the diaphragm wall. This provision enables the concrete to flow more easily level with the bottom portion of the prefabricated facing element, thereby avoiding poor contact between the concrete and the prefabricated facing element in this zone. In addition, the concrete also tends to urge the facing element against the guide strip towards the zone to be dug away and not towards the inside of the excavation. This serves to limit the chances of the facing element being moved out of position.

According to at least a preferred embodiment of the present invention, the prefabricated facing element includes starter bars on its bottom end face, and when the prefabricated facing element is positioned in the excavation, these starter bars extend between the bars of the first portion of the reinforcing cage. This prevents the facing element from becoming separated from the concreted portion of the diaphragm wall.

In at least a preferred embodiment of the present invention, a shuttering element is positioned at least one end of the excavation prior to concreting. In its face facing towards the inside of the excavation, the shuttering element may include a slot extending over a height that is not less than the height of the diaphragm wall and in which there is inserted a sealing tongue that projects outwards. In this way, the portion of the tongue that projects outwards is set in the concrete during concreting. These provisions serve to ensure good sealing between the various panels making up the diaphragm wall, and in particular between two individual panels that are molded in succession in the ground (in particular between the respective concreted portions of these two panels).

In at least a preferred embodiment of the present invention, starter bars project from the top face of the diaphragm wall after concreting. In addition, on its top face, the prefabricated facing element includes positioning members such as loops. In order to finish off the top portion of the work, in particular when building a quay, it is possible to make a capping beam on the top face of the diaphragm wall so that the starter bars of the diaphragm wall and the positioning members of the prefabricated facing element are included in the capping beam.

In at least a preferred embodiment of the present invention, at least two prefabricated facing elements are positioned in the excavation. The first of the prefabricated facing elements to be positioned in the excavation includes a groove of keyhole-shaped profile in its side face facing towards the inside of the excavation and extending over the full height of the facing element, and the second facing element that is intended to be positioned beside the first facing element includes a blade of profile complementary to the profile of the slot and fastened at the bottom end of its side face facing towards the first facing element. In order to connect the second prefabricated facing element to the first facing element, the blade of the second facing element is engaged progressively in the slot of the first facing element until the top faces of both facing elements are at the same height. The blade serves to guide the second facing element while it is being inserted in the excavation and to position it accurately relative to the first facing element. The blade also enables a sealing gasket to be inserted to connect together the slots of the first and second facing elements.

In at least a preferred embodiment of the present invention, after concreting, said first excavation is extended at least one of its ends by means of a second excavation. Under such circumstances, the last facing element of the first excavation, situated at the end that is to be extended, includes a slot of keyhole-shaped profile in its side face facing towards said end, extending over the full height of the facing element, and closed by breakable or detachable protection. The protection serves to prevent concrete dirtying or clogging the keyhole-shaped slot of said facing element. The first facing element to be positioned in the second excavation has a blade of profile complementary to that of said slot, which blade is fastened at the bottom end of the side face of the facing element that faces towards the first excavation. In this way, the blade of the first facing element of the second excavation can be engaged progressively in the slot of the last facing element of the first excavation until the top faces of those two facing elements are at the same height. The sliding of the blade in the keyhole-shaped slot of the last facing element of the first excavation enables the protection of said slot to be detached or destroyed.

In at least a preferred embodiment of the present invention, after the diaphragm wall has been concreted and after a capping beam, if any, has been made, the zone to be dug away is indeed dug away so as to uncover at least part of the surface of the prefabricated facing element that faces towards the zone to be dug away.

At least a preferred embodiment of the invention provides a diaphragm wall having a portion made of reinforced concrete cast in situ and comprising at least one reinforcing cage of L-shaped profile made up of a base (or first) portion and of a vertical (or second) portion, and at least one prefabricated facing element situated in the gap above said base portion and secured to the reinforced concrete portion.

Other characteristics and features of the present invention appear on reading the following description of embodiments given by way of non-limiting illustration. The description refers to the sheets of the accompanying drawings, in which: • Figures 1A to IF, 2, 3, and 4 are diagrammatic views of different steps of the method of making a diaphragm wall in an implementation of the invention; • Figure 5 shows a variant implementation of the method of the invention for making a diaphragm wall; • Figure 6 shows the construction of a diaphragm wall of the invention in the form of successive panels; • Figure 7 shows the top end of a prefabricated facing element suitable for use in the present invention;

Figure 8 shows the bottom end of the Figure 7 prefabricated facing element; • Figure 9 shows an example of shuttering suitable for use in the present invention; • Figure 10 shows an example of a joint suitable for use in the present invention; • Figure 11 shows the connection between two prefabricated facing elements; • Figure 12 shows a quay made using the method of the present invention; • Figures 13 A, 13B, and 13C show a variant implementation of the method of the present invention for making a diaphragm wall; and • Figure 14 shows yet another variant implementation of the method of making a diaphragm wall.

By way of example, a diaphragm wall 10 of the present invention may be used for making a quay as shown in Figure 12. Nevertheless, the invention may be implemented for any other suitable works, and in particular any works requiring a good finish for the visible portions of walls.

With reference to Figures 1A to IF and 2 to 4, there follows a description of the steps in one implementation of the method of the invention for making a diaphragm wall 10. A first step of the method, shown in Figure 1 A, consists in making guide strips 12a, 12b in the form of low walls that serve to mark the location of the future diaphragm wall. In this example, two mutually parallel guide strips 12a and 12b define a gap of constant width t corresponding to the width desired for the future diaphragm wall. These guide strips 12a and 12b are generally made of reinforced concrete and they present a height of about 1 meter (m) to 1.50 m. They have the function of stabilizing surface soil, of constituting leveling marks, and of providing rests for the hoisting plant needed for constructing the wall. The guide strips 12a, 12b usually constitute temporary works, for destruction once the wall 10 has been completed.

In a second step, an excavation 14 of height H and of width i corresponding to those desired for the diaphragm wall 10 is dug vertically between the two guide strips 12a and 12b. The excavation 14 is of elongate shape that extends over a length L. As shown in Figure 2, the excavation has two longitudinal walls 16a and 16b that are spaced apart by a constant distance, each of them extending vertically in line with a respective one of the two guide strips 12a, 12b. The excavation 14 marks the future location of the diaphragm wall 10, and thus constitutes the frontier of a zone 18 to be dug away. For a quay, the zone 18 to be dug away extends from the longitudinal wall 16b of the excavation 14 facing towards the stretch of water, and into the stretch of water.

Depending on the soil and on the specifications, various types of tooling may be used for digging the excavation 14, e.g. cable grabs, Kelly grabs, hydraulic cutters, etc.

In order to ensure that the excavation 14 is stable during the boring operation, and in particular in order to avoid the walls 16a and 16b collapsing, while the excavation 14 is being excavated, it is filled with mud 24, generally a mud based on bentonite.

In a third step, as shown in Figure 1B, a reinforcing cage 26 of L-shaped profile is inserted progressively into the excavation 14 by means of a hoist 28.

The reinforcing cage 26 is shown in greater detail in Figure 2, where it can be seen that it has a first portion 30 of width corresponding substantially to the width l of the excavation 14, surmounted by a second portion 32 of smaller width.

Slings 34 from the hoist 28 hold the reinforcing cage 26 by means of grip loops 36 provided at the top end of its second portion 32.

Once the reinforcing cage 26 is in position in the excavation 14, its first portion 30 is positioned parallel to the bottom 38 of the excavation 14 at a distance from the bottom 38 that is sufficient for the end bars of the cage 26 to be properly embedded in concrete when the excavation 14 is concreted, as described in greater detail below in the present description.

The second portion 32 of the reinforcing cage 26, which extends above said first portion 30, is itself backed against the longitudinal wall 16a of the excavation 14 situated on the side remote from the zone 18 to be dug away.

An empty gap 40 is thus conserved above the first portion 30 of the reinforcing cage 26, between the second portion 32 and the wall 16b of the excavation 14 situated beside the zone 18 to be dug away.

In the example shown, the first portion 30 is of a width t\ and of a length LI corresponding substantially to the width l and the length L of the excavation 14, and it is of a height HI. The second portion 32 presents a width 12 that is less than £ 1, the same length LI as the first portion 30, and a height H2. In this embodiment, the reinforcing cage 26 is made as a single piece. Other embodiments are described below, in particular with reference to Figures 13A, 13B, and 13C.

As mentioned above, it is necessary for the reinforcing cage 26 to remain a certain distance from the bottom 38 of the excavation 14 so that it does not bear against said bottom 38, in order to avoid it being deformed and in order to make it possible subsequently to embed its reinforcing bars in its bottom portion in satisfactory manner.

To do this, in a fourth step, as shown in Figure 1C, the reinforcing cage 26 is stabilized temporarily in position in the excavation 14 by means of bars 42 that are passed through its grip loops 36 and that bear transversely on the guide strips 12a and 12b. Putting the bars 42 into place enables the reinforcing cage 26 to be held in position after its grip loops 36 have been released from the slings 34 of the hoist 28.

The drawback of bars 42 extending transversely from one guide strip to the other over the excavation 14 is that they prevent access to the excavation 14 for elements of great length. As described in greater detail below in the present application, a prefabricating facing element 44 of length that is substantially equal to the length of the excavation 14 needs to be lowered into the empty gap 40 in the excavation 14 situated above the first portion 30 of the reinforcing cage 26 and between the second portion 32 of said cage 26 and the wall 16b of the excavation situated on the side 18 to be dug away.

In order to enable the prefabricated facing element 44 to be inserted (as shown in Figure IF), in a fifth step, a lifting beam 46 of length longer than the length L of the excavation 14 is installed on the guide strips 12a and 12b and is positioned in such a manner that its two legs 46a and 46b are far enough apart from each other to avoid obstructing the opening of the excavation 14 (Figure ID). The function of the lifting beam 46 is to hold the reinforcing cage 26 in position by means of hoisting tackle 48, for example, connected to the grip loops 36. Once the cage 26 is held by the lifting beam 46, it is possible to remove the bars 42 from the grip loops 36 of the cage 26 and thus to clear the opening of the excavation 14 completely so as to enable the prefabricated facing element 44 to be inserted (Figure IE).

In a sixth step, as shown in Figures IF and 2, the prefabricated facing element 44 is brought over the excavation 14 so as to be inserted into the empty gap 40 defined between the second portion 32 of the reinforcing cage 26 and the wall 16b of the excavation 14 situated beside the zone 18 to be dug away, and so as to be placed therein while bearing against the guide strip 12b situated on that side.

In reality, as shown in Figure 2, a flat iron spacer 50 is advantageously provided at the top end of the guide strip 12b in order to enable the prefabricated facing element 44 to be finely positioned relative to the guide strip 12b. This flat iron spacer 50 bears against the top face of the guide strip 12b and extends at right angles along the longitudinal face of the strip 12b that faces towards the excavation 14. It is of very small thickness, being about 1 centimeter (cm) thick, and it enables sufficient clearance to be conserved relative to the prefabricated facing element 44 so that when the guide strip 12b is destroyed after the wall 10 has been constructed, the visible face of the facing element 14 is not damaged (i.e. there is no damage to its face that faces towards the zone 18 to be dug away).

An example of prefabricated facing suitable for the present invention is shown in greater detail in Figures 7 and 8.

Figure 7 shows the top end of the prefabricated facing element 44 having starter bars 52 projecting therefrom for a purpose that is described in greater detail below with reference to Figure 4, and positioning members 54 enabling the facing element 44 to be handled by the hoist 28. These positioning members may be loops, as shown in Figure 7, or any other hanger means.

Figure 8 shows the bottom end of the prefabricated facing element 44, showing a portion 56 that is chamfered in the long direction of the facing element. When it is inserted into the excavation 14, the prefabricated facing element 44 thus takes up a position such that the chamfered portion 56 faces towards the inside of the excavation 14 and not towards the zone 18 to be dug away.

It can also be seen from Figure 8 that the prefabricated facing element 44 includes starter bars 58 on its chamfered bottom end face 56. When the prefabricated facing element 44 is in position in the empty gap 40 of the excavation 14, bearing against the flat iron spacer 50, with its top face substantially level with the guide strips, these starter bars 58 are engaged between the reinforcing bars of the first portion 30 of the reinforcing cage 26, as shown in Figure 3.

The face of the prefabricated facing element 44 that is to face towards the zone 18 to be dug away is advantageously covered in a protective sheet 60, e.g. in the form of an adhesive sheet, prior to the facing element 44 being lowered into the excavation 14. The face of the facing element 44 is naturally thoroughly cleaned before being covered in said adhesive sheet 60. This protection prevents concrete that seeps around the facing element 44 from dirtying its visible portion. As shown in Figure 12, the sheet is removed after the excavation 14 has been concreted and the zone 18 has been dug away.

Other characteristics of the facing element shown in Figures 7 and 8 are described in greater detail below in the description.

In a seventh step, once the facing element 44 has been positioned in the empty gap 40, a shuttering element 62 is positioned at each end of the excavation 14.

An example of suitable shuttering 62 is described in greater detail with reference to Figure 9. This shuttering 62 comprises a shaped central portion 64 of profile that corresponds to the concave indentation that it is desired in this example to impart to the end of the diaphragm wall 10, two plane side margins 66a and 66b situated on either side of the central portion 64, and an angle iron 68 welded along the junction line between the central portion 64 and the side margin 66a that is to be positioned facing the end of a facing element. The function of the angle iron is described in greater detail with reference to Figure 5. In this example, the shuttering 62 includes a slot 67 in the face of its central portion 64 that faces towards the inside of the excavation 14, said slot 67 extending over the full height of the shuttering 62. This slot 67 receives a sealing tongue 70 that projects towards the inside of the excavation 14 to perform a function that can be seen more clearly in the light of Figure 6 that shows a diaphragm wall of the invention made up from successive panels. The sealing tongue is for ensuring sealing between two successive panels in such a diaphragm wall.

In an eighth step, once the shuttering elements have been put into place at the ends of the excavation 14, concrete 72 is cast, beginning from under the bentonite mud 24 at the bottom end of the excavation 14 by using the concreting technique that involves a tube dipping under the mud. The concrete 72 thus progressively embeds the reinforcing bars of the cage 26 and also the starter bars projecting from the chamfered bottom face 56 of the prefabricated facing element 44 until the in situ concreted portion 74 reaches the level of the top face of the prefabricated facing element. The chamfered portion 56 of the facing element 44 serves to avoid poor contact between the concrete 72 and the prefabricated facing element 44 in this zone. Furthermore, under the effect of this chamfered shape, the concrete 72 tends to push the facing element 44 towards the guide strip 12b, thus preventing the facing element 44 being deflected towards the inside of the excavation 14.

In general, and in particular when the top end of the diaphragm wall 10 is to present good finish, it is necessary, as shown in Figure 4, to cover the diaphragm wall 10 with a capping beam 76 of reinforced concrete and of width corresponding to the width of said wall 10. Under such circumstances, provision is made for starter bars 78 of the reinforcing cage 26 to project from the top face of the concreted portion 74 of the diaphragm wall 10, after concreting. Thus, the capping beam 76 cast onto the top face of the diaphragm wall 10 covers all of the starter bars 78, the starter bars 52 provided on the top face of the prefabricated facing element 44 (as described with reference to Figure 7), and also the positioning loops 54 on the same prefabricated facing element 44.

This capping beam 76 that is generally needed on the work thus serves to improve the finishing of the work and to enhance the mechanical strength of the prefabricated facing element 44 and of the concreted portion 74 of the diaphragm wall 10.

In another implementation, and as an alternative to the capping beam 76, it is also possible to make provision at the top end of the prefabricated facing element 44 for the starter bars 52 to be bent through 180° and to extend towards the excavation so as to dip into the concreted portion 74. These starter bars enable the prefabricated facing element 44 to be secured better to the concreted portion 74 of the diaphragm wall 10, avoiding potential separation therebetween.

In the implementation described with reference to Figures 1A to 4, the diaphragm wall 10 has only one reinforcing cage 26 and only one prefabricated facing element 44, both of which are of a length that is substantially equal to the length of the excavation 14.

In an alternative, it is possible to lower a number N1 of reinforcing cages of L-shaped profile into the excavation, together with a number N2 (not necessarily equal to Nl) of prefabricated facing elements that can be inserted in the empty gap retained between the N1 reinforcing cages and the side wall 16b of the excavation 14 situated beside the zone 18 to be dug away.

In an embodiment shown in Figure 5, two reinforcing cages 261 and 262 are positioned in succession in the excavation 14. Each reinforcing cage 261 and 262 is of L-shaped profile similar to that of the reinforcing cage 26 described with reference to Figures 1A to 4, however it is of a length that is substantially equal to half the total length L of the excavation 14.

As in the above-described implementation, each reinforcing cage 261 and 262 is positioned in the excavation 14 in such a manner that its first portion extends substantially along the bottom of the excavation 14, and its second portion is backed against the wall 16a of the excavation 14 that is situated remote from the zone 18 to be dug away. Thereafter, two prefabricated facing elements 441 and 442 are positioned in succession in the excavation 14, one beside the other, in the empty gaps defined respectively by the reinforcing cages 261 and 262. In the example of Figure 5, where the diaphragm wall 10 is a straight wall, the two facing elements 441 and 442 are positioned substantially in the same plane.

As shown in Figure 5, each prefabricated facing element 441 and 442 includes a respective cavity (or setback) 80 in each of its side faces, these cavities in this example being in the form of a "keyhole-shaped" slot extending over the full height of the facing element. One of these cavities 80 that faces towards the end of the excavation 14 is closed by protection, such as a layer of polystyrene 84 that is to prevent concrete from dirtying said cavity during concreting.

After the prefabricated facing elements 441 and 442 have been put into place, installing shuttering 62 at the ends of the excavation 14 enables the side margin 66a of each shuttering element to be pressed properly against the adjacent polystyrene layer 84. As can be seen in Figure 5, the angle bar 68 of each shuttering element 62 is then positioned facing the polystyrene layer 84 covering the end of the adjacent prefabricated facing element. The angle bar enables the tongue of concrete that is formed during concreting over the layer of polystyrene to be thin enough to be broken off when removing the shuttering.

Nevertheless, it should be observed that the keyholes 80 in the prefabricated facing element(s) provided at the ends of the excavation are needed only when those facing elements are to be connected to other similar facing elements inserted subsequently into adjacent excavations. This is described below in greater detail with reference to Figure 6.

Each prefabricated facing element 441 and 442 also includes, at the bottom end of one of its side faces, a metal blade 82 of profile that is complementary to the profile of the cavities 80. One such blade 82 is shown in Figure 8. In this example, and for reasons that are explained below, the second facing element 442 presents a metal blade 82 at the bottom end of its side face that is to be positioned beside the first facing element 441.

After the first facing element 441 has been inserted into the excavation 14 with its side face that is covered in polystyrene 84 facing towards the end of the excavation 14, the second facing element 442 is placed over the empty gap adjacent to the first facing element 441 and is then lowered into the empty gap (as shown in Figure 11) in such a manner that its blade 82 is received progressively in the slot 80 of the first facing element 441, with this continuing until the top faces of both facing elements 441 and 442 are at the same height.

The function of the blade 82 is to guide and position the second facing element 442. It also enables a sealing gasket 86 to be slid between the two facing elements 441 and 442, in particular a gasket of the inflatable "water stop" type. Such a gasket 86 positioned in the cavities 80 of two adjacent facing elements is shown in detail in Figure 10. It comprises two inflatable hollow tubes 88a and 88b each for insertion into a respective cavity 80, and an intermediate portion connecting these two tubes 88a and 88b together. Sealing is provided when a slurry of cement is injected into each of the tubes so as to cause it to be inflated sufficiently to obtain close contact between the concrete and the gasket.

For a better understanding of the general principles of the method of the invention, the description above is limited to a diaphragm wall 10 that has only one panel. In the vast majority of works, the dimensions desired for the diaphragm wall are such that it needs to be constructed in conventional manner by individual panels either in succession or in alternation.

The construction of a diaphragm wall using a succession of individual panels is described below with reference to Figure 6.

As shown in the right-hand portion of Figure 6, a primary panel 101 is made in a first excavation 141 using the method described above with reference to Figure 5, and a second excavation 142 is dug to the left of the first excavation 141. The shuttering 62 is removed from the left of the first excavation 141 after digging the excavation 142. As shown in the Figure, the primary panel 101 presents a continuous generally-concave indentation over its entire height at its end. The sealing tongue 70 of the shuttering 62 projects from this concave end face (as described above with reference to Figure 9), being secured to the first panel 10 when the concrete sets.

As for the primary panel 101, two reinforcing cages 26 Γ and 262' and two prefabricated facing elements 44 Γ and 442' have been positioned in the second excavation 142. Shuttering 62 identical to that shown in Figure 9 is placed at the end of the second excavation 142 remote from the primary panel 101. Only one shuttering element 62 is needed since, at its other end, the second excavation 142 is defined by the concreted end face of the primary panel 101.

As described above with reference to Figure 5, the facing element 442 of the first excavation 141 having an end face adjacent to the second excavation 142 (i.e. in this example the second facing element 442 of the first excavation 141) includes a keyhole-shaped slot 80 in this end face that extends over the full height of the facing element 442 and that is closed by a layer of polystyrene 84.

The first facing element 441' to be positioned in the second excavation 142 has a metal blade 82 identical to that described with reference to Figures 8 and 11 that is secured to the bottom end of its side face facing towards the first excavation 141. In this way, the blade 82 of the first facing element 441' of the second excavation 142 can be engaged progressively in the cavity 80 of the last facing element 442 of the first excavation 141 until the top faces of both facing elements 44Γ and 442 are at the same height. In reality, before beginning to engage the blade 82 in said cavity 80, an operator manually "scrapes away" the top end of the polystyrene layer 84 so as to uncover the keyhole-shaped slot 80. The blade 82 sliding along the slot 80 then enables the remainder of the protection 84 to be detached or destroyed over the entire height of the facing element 442. The protection is subsequently taken away with the mud.

The protections such as the layer of polystyrene 84 shown in Figures 5,6, and 7 are provided essentially to prevent concrete seeping round during a concreting operation and dirtying the keyhole-shaped slot 80 into which the blade 82 of an adjacent facing element is subsequently to be inserted. Conversely, once two facing elements are already connected together during a concreting operation, in particular by a sealing gasket of the type described above, such protection is no longer needed.

In the example of Figure 6, it can be understood that the first facing element 44 Γ of the second excavation 142 has no protection on any of its end faces since it is to be connected to the two adjacent facing elements 442 and 442' before the concreting operation. In contrast, the second facing element 442' of the second excavation 142 needs to be protected by a polystyrene layer 84 in order to ensure that the cavity 80 provided in its side face facing towards the shuttering 62 is not dirtied by concrete during the concreting operation, since that would make it impossible for it to be connected with an additional facing element placed in a possible third excavation situated to extend the second excavation.

In all of the above-described examples, the reinforcing cage 26,261, or 262 is described as being formed as a single piece. Nevertheless, in certain circumstances, the dimensions desired for the diaphragm wall 10 are too great for the reinforcing cage 26,261,262 to be lowered as a single segment. Consequently, it is possible for each of the above-described reinforcing cages 26, 261,262 to be made up of a plurality of segments that are lowered into the excavation one after another. The positioning of the reinforcing cage is then performed in the manner described with reference to Figures 13A and 13B. In this implementation, the reinforcing cage 26 is made up of two segments 90 and 92. It will readily be understood that the same method could be used for an arbitrary number of segments.

In this example, the first segment 90 of the reinforcing cage 26 in fact corresponds to the first portion 30 of the reinforcing cage 26 that presents a width substantially equal to the width of the excavation 14, while the second segment 92 corresponds to the second portion 32 of the cage 26, of width that is less than that of the first portion 30 and that is to be backed against the wall 16a of the excavation 14 that is situated remote from the zone 18 to be dug away.

The first segment 90 is inserted in a first step into the excavation 14 until its top end is positioned substantially level with the guide strips 12a and 12b. Using the same method as that described with reference to Figure 1C, this first segment 90 is held temporarily in position by means of bars 42 placed transversely and bearing on each of the guide strips 12a and 12b. Using a hoist, the second segment 92 is brought directly over the first segment 90, and the first and second segments 90 and 92 are secured to each other by tie wires or by welding together starter bars of the two segments. The assembly made up of the first and second segments is then lowered into the excavation 14 until it reaches its final position.

As in the above-described examples, the prefabricated facing element is then positioned in the empty gap that is retained between the second portion 32 and the longitudinal wall of the excavation 14 that is situated beside the zone 18 to be dug away (see Figure 13C).

In another example shown in Figure 14, the prefabricated facing element 44 may be secured to the segment of the reinforcing cage 26 that is to be inserted last into the excavation so as to limit the number of hoisting operations while constructing the wall. Thus, after the first portion 30 of the reinforcing cage has been inserted in the excavation (in one or more segments, as the case may be), the assembly made up of the last segment 32 and of the facing element 44 is inserted therein, thereby finishing off the L-shaped structure of the reinforcing cage. Under such circumstances, provision is generally made for the second portion 32 of the reinforcing cage (i.e. the portion that is narrower than the base of the L-shape extending parallel to the bottom of the excavation, and backed against the wall of the excavation remote from the zone to be dug away) is made as a single segment so that the facing element 44 can fill the empty gap situated between the second portion of the reinforcing cage and the wall of the excavation situated beside the zone to be dug away.

Advantageously, prior to being secured to the last segment 92 (i.e. to the second portion 32) of the reinforcing cage 26, the prefabricated facing element 44 can be molded (i.e. cast) together with the segment 92 of the reinforcing cage 26 that is to be inserted last into the excavation 14. In another implementation, the facing element 44 may include starter bars, which starter bars may be secured with tie wires to bars of the last segment 92 of the reinforcing cage 26 so as to secure those two elements together. In all of the implementations shown, the top of the diaphragm wall is substantially level with the working platform (i.e. level with the guide strips). However the invention also makes it possible to make diaphragm walls with a low top, if necessary several meters lower than the working platform. Under such circumstances, each prefabricated facing element is extended at each of its ends by a small post, said post being of sufficient height to reach the working platform once the prefabricated facing element is in position in the excavation, and the members for positioning the prefabricated facing element (loops, threaded bars, etc.) are also lengthened accordingly. The keyhole-shaped cavities in the prefabricated facing elements and the protections, e.g. made of polystyrene, covering these cavities extend over the full height of the small posts. These posts are designed to be destroyed after concreting.

Claims

1. A method of making a diaphragm wall, the method comprising at least the following succession of steps: • forming in the ground an excavation of elongate shape, having vertical walls, marking the frontier of a zone to be dug away; • positioning, in the excavation, a reinforcing cage of L-shaped profile having a first portion of width matching the width of the excavation and intended to be positioned parallel to the bottom of the excavation, and a second portion of width less than the width of said first portion and intended to be backed against the longitudinal wall of the excavation that is situated on its side remote from the zone to be dug away, in such a manner that an empty gap is retained between said second portion and the longitudinal wall of said excavation situated beside the zone to be dug away; and • positioning at least one prefabricated facing element in said empty gap, against the longitudinal wall of the excavation situated beside the zone to be dug away, and casting concrete into said excavation in such a manner as to form a diaphragm wall including said prefabricated facing element.

2. A method of making a diaphragm wall according to claim 1, wherein after concreting, the zone to be dug away is dug away so as to uncover at least part of the surface of the prefabricated facing element that faces towards the zone to be dug away.

3. A method of making a diaphragm wall according to claim 1 or claim 2, wherein said reinforcing cage of L-shaped profile is made up of a plurality of segments lowered successively into said excavation.

4. A method of making a diaphragm wall according to any one of claims 1 to 3, wherein two guide strips marking the desired location for the diaphragm wall are made in the form of low walls in the ground to be excavated, and the excavation is then excavated vertically between these two guide strips.

5. A method of making a diaphragm wall according to any one of claims 1 to 4, wherein said reinforcing cage includes grip loops at its top portion, and said cage is inserted into said excavation while supporting it via said loops, it is temporarily stabilized in position in said excavation by means of bars passed through said loops and bearing transversely on said guide strips, support of the reinforcing cage is taken over by means of a lifting beam shaped to leave clear said empty gap in order to insert said prefabricated facing element, said bars are withdrawn, and said prefabricated facing element is inserted.

6. A method of making a diaphragm wall according to any one of claims 1 to 5, wherein, prior to positioning the prefabricated facing element in the excavation, the surface of said prefabricated facing element that is to face towards the zone to be dug away is covered in protection, preferably an adhesive sheet.

7. A method of making a diaphragm wall according to claims 2 and 6, wherein the protection is removed after digging away.

8. A method of making a diaphragm wall according to any one of claims 1 to 7, wherein the bottom portion of the prefabricated facing element is chamfered lengthwise, and the prefabricated facing element is positioned in the excavation in such a manner that the chamfered portion faces towards the inside of the diaphragm wall.

9. A method of making a diaphragm wall according to any one of claims 1 to 8, wherein the prefabricated facing element includes starter bars on its bottom end face, and when the prefabricated facing element is positioned in the excavation said starter bars extend between the reinforcing bars of the first portion of the reinforcing cage, whereby said facing element is prevented from becoming detached from the concreted portion of the diaphragm wall.

10. A method of making a diaphragm wall according to any one of claims 1 to 9, wherein, prior to concreting, a shuttering element is positioned at at least one end of the excavation.

11. A method of making a diaphragm wall according to any one of claims 1 to 10, wherein, after concreting, starter bars project from the top face of the diaphragm wall, the prefabricated facing element includes positioning members on its top face, and, after concreting, a capping beam is made on the top face of the diaphragm wall in such a manner that said starter bars of the diaphragm wall and said positioning members of the prefabricated facing element are included in said capping beam.

12. A method of making a diaphragm wall according to claim 3 and any one of claims 1 to 11, wherein at least the guide strip situated beside the zone to be dug away is removed after the diaphragm wall has been concreted.

13. A method of making a diaphragm wall according to any one of claims 1 to 12, wherein at least two prefabricated facing elements are positioned in said excavation, the first prefabricated facing element to be positioned in the excavation includes, on its side face facing towards the inside of the excavation, a slot having a keyhole-shaped profile extending over the full height of said facing element, the second facing element intended to be positioned beside the first facing element includes a blade of profile complementary to said slot and fastened to the bottom end of its side face facing towards said first facing element, and in order to connect the second prefabricated facing element to the first facing element, the blade of the second facing element is engaged progressively in the slot of the first facing element until the top faces of the two facing elements are at the same height.

14. A method of making a diaphragm wall according to any one of claims 1 to 13, wherein, after concreting, said first excavation is extended at at least one of its ends by a second excavation, the last facing element of the first excavation that is situated at the end that is to be extended includes, on its side face facing towards said end, a slot of keyhole-shaped profile extending over the entire height of the facing element and closed by breakable or detachable protection, the first facing element to be positioned in the second excavation includes a blade of profile complementary to said slot and fastened to the bottom end of its side face facing towards the first excavation, and the blade of the first facing element of the second excavation is engaged progressively in the slot of the last facing element of the first excavation until the top faces of the two facing elements are at the same height, thereby removing said breakable or detachable protection.

15. A diaphragm wall made in accordance with the method of any one of claims 1 to 14.