GB2251203A - Procedure for fabrication of prefabricated voussoirs - Google Patents

Abstract

Process for the manufacture of a prefabricated concrete arch unit comprising two cylindrical curved faces (11, 12) and at least four side faces (13, 14, 15, 16). According to the invention the arch unit is cast upright on one of its side faces (15) which corresponds to the bottom (35) of the mould, so that the two outer (11) and inner (12) faces of the arch unit (1) are coffered directly on two side walls (31) (32) of the mould, which are given a corresponding curvature and, at the end of the casting, at least one plate (5) is placed in the casting opening (30) and is held, during the casting, in a specific position in relation to the mould, so that the said plate (5) thus sealed forms, after demoulding, a distribution sole plate embodying a reference support plane obeying the geometry to be imparted to the arch unit (1). The invention applies especially to the production of tunnels of large dimensions. <IMAGE>

Description

j####U...-##.r'L #(?4Cjt(tLffii# I The subject of the invention is a procedure for fabrication of prefabricated voussoirs in concrete, for the production of a tunnel lining, and equally covers the voussoirs thus produced as well as the moulds and accessories permitting their fabrication.

For the production of a subterranean gallery, it is almost always necessary, after having bored the tunnel, to re-cover its wall with a lining which, nowadays, is generally produced in reinforced concrete.

Such a lining can be produced by means of sliding shuttering which is put in place after having bored the tunnel along a certain length, and which delimits the internal profile of the tunnel. The concrete is injected between this shuttering and the wall of the excavation, after, perhaps, putting an iron framework in place, and, after the concrete has set, the shuttering is detached then advanced for shuttering a following section.

But it is often preferable to produce the lining in prefabricated elements, supporting themselves on one another. When the cross-section of the tunnel is fairly large, the tunnel generally has a circular cross-section and the lining is produced by successive rings each composed of a series of "voussoirs" each covering an angular sector.

Each voussoir thus comprises two cylindricallycurved faces, respectively inner wall and outer wall, centred on an axis, and, in the most usual case, four lateral faces placed in planes respectively parallel and perpendicular to the axis.

When the tunnel is produced in a good, solid rock, the excavation can be hollowed out along a certain length, producing the corresponding lining in the form of one or more rings, concrete can be injected between the wall of the excavation and the lining which has just been installed, then a new length of tunnel can be bored.

The dimensions of the voussoirs must strictly comply with the fixed geometry in such a fashion that the opposing lateral faces fit well against one another.

To this end, it has already been proposed, in the Patent FR-A-693,495, to pour each voussoir in a dismantleable mould comprising a flat bottom, two cylindrically-curved faces with a vertical axis and two vertical lateral faces. In that way, of the six faces of the voussoir, three of the flat faces and the two curved faces are formed on the walls and the bottom of the mould, the fourth lateral flat face corresponding to the upper opening of the mould and capable of being levelled exactly. In the same way, it is possible to arrange on the walls of the mould accessories permitting, after removal from the mould, indented or protruding parts perfectly positioned on the different faces of the voussoir.

Such a procedure undoubtedly permits an improvement in the precision and, above all, the speed of fabrication, the moulds being mounted on small mobile wagons. However, a mould for horizontal pouring is much simpler and less expensive and this old procedure was thus not generally used.

Furthermore, when galleries of great dimensions are being produced, there is always the fear of cave-ins and water ingress which is why the boring is generally carried out by means of an apparatus called a "@tunneller", which comprises a cutter disc forming a front wall fitted with tools for boring and driven in rotation around a substantially horizontal axis, and a metallic tubular wall which provides the support of the soil between the cutter disc and the concrete lining already produced.

When the rock displays a certain behaviour and there is no fear of significant water ingress, it can be envisaged that part of the tubular protection wall be dismantled in order to put the voussoirs in place one after the other, in such a fashion as to produce a new ring of lining.

But it is often necessary to maintain a watertight seal between the tubular wall and the lining, for example when in the water table or also when, by reason of the risk of cave-ins, a fluid under pressure, a bentonite sludge or air has to be injected between the tunneller and the wall of the excavation. In this case, the rear extremity of the tubular part of the tunneller has to remain slipped, in a telescopic fashion into the forward part of the lining already constructed, with the interposition of a watertight sealing joint.

Figure 1 shows, by way of example, such an arran gement in the interior of the tunnel T, the tunneller comprising a boring disc A driven in rotation around the axis 10 of the tunnel by a mechanism B and associated with a wall of tubular shoring C whose extremity C1 remains slipped over the lining R in concrete, with the interposition of a watertight sealing joint D.

The lining R is composed by successive rings R1, R2 ... Rn, each ring being itself composed of voussoirs V placed side by side. Concrete E is injected between the lining R already produced and the wall of the excavation T.

In order to construct a new ring Rn+l, the tunneller T first of all hollows out the excavation, supporting itself on the last ring Rn of the lining by means of jacks. Thus the cutter disc A is advanced along a length at least equal to that of one ring, but care is taken that the rear extremity C1 of the shield remains in contact with the lining already constructed.

Thanks to an apparatus F for manoeuvring and installation, associated with the tunneller, the required number of adjacent voussoirs V are put in place to constitute the new ring Rn+l.

After locking the ring Rn+l thus produced, the tunneller can proceed to advance again.

In step with the advance, concrete E is injected between the lining R already produced and the rocky wall of the tunnel T.

This type of tunneller, which has been in use for quite a time, is now well developed; however, as the tunneller supports itself, in order to advance, on the lining already produced, very significant forces are applied on the lateral faces transverse to the axis, and surface irregularities, even slight, can entail stress concentrations bringing about cracking of the voussoir.

Furthermore, when working in a significant depth of water and when, as a consequence, the pressure of the bentonite sludge or the air pressure must be increased, significant leaks can be caused. To maintain the watertight seal, it is necessary to tighten the joint, but then it wears out quickly. Thus the idea was devised of giving the external face of the voussoir on which the joint slides as smooth a surface as possible, in such a fashion as to diminish the risk of leaks between the joint and the voussoir and to reduce the friction.

The subject of the invention is improvements in the procedures of fabrication of prefabricated voussoirs permitting an easy and economic response to all of these requirements.

The invention thus relates, in a general fashion, to a procedure for fabrication of prefabricated voussoirs in concrete, each comprising two cylindrically-curved faces, respectively the inner wall and outer wall, centred on a longitudinal axis, and at least four lateral faces, each voussoir being poured in a mould comprising a bottom, at least four walls and an upper pouring opening.

In order to obtain an excellent surface quality on the face of the outer wall, the voussoir is poured standing upright on one of its lateral faces which corresponds to the bottom of the mould, in such a fashion that the two cylindrical faces, respectively the inner wall and outer wall of the voussoir, are directly formed on the two walls of the mould which are given a corresponding curvature.

According to the invention, at the end of the pouring, in the pouring opening is placed a distribution bed plate which is held, during the pouring, in a fixed position with respect to the mould, in such a fashion that, after removal from the mould, the said bed plate is embedded into the corresponding face of the voussoir opposite the face formed on the bottom and embodies a reference support plane which complies with the geometry the voussoir is to have.

The invention also covers voussoirs thus produced and the moulds permitting their fabrication.

Such a mould thus comprises two lateral walls curved in such a fashion as to constitute the shuttering for the two cylindrical faces of the voussoir, the upper opening and the bottom corresponding respectively to two of the lateral faces of the voussoir.

For preference, at least the curved wall of the mould serving as shuttering for the face of the outer wall is mounted in such a fashion as to be able to be separated after the concrete has set in order to remove the voussoir from the shuttering.

In a particularly advantageous fashion, such a mould can be fitted with means of positioning, through the pouring opening and at an adjustable level, with accessory members capable of being incorporated, after setting, into the corresponding lateral face of the voussoir, for example means of suspension of the voussoir or means of fixing of a waterproof sealing joint.

In fact, such a voussoir must sometimes be fitted with a peripheral waterproof sealing joint extending along its lateral faces.

Such a joint comprises four sections placed end to end and extending respectively along the four lateral faces of the voussoir, each section comprising an external part projecting towards the outside of the corresponding face of the voussoir and an internal part for fixing to the said face, in the thickness of the voussoir.

In a first embodiment, the external parts of the three sections of the joint, corresponding to the formed lateral faces of the voussoir, are mounted so as to be removable, before the pouring, on the bottom and the corresponding lateral walls of the mould, in such a fashion that the internal parts of the joint project towards the interior of the mould and the external part of the fourth section of the joint is held across the pouring opening and positioned in such a fashion that its internal part is below the level of the corresponding face of the voussoir, the said internal parts of the four sections of the joint being integrated, after the pouring and the setting of the concrete, into the corresponding faces of the voussoir.

In another embodiment, the bottom and the lateral walls of the mould serving as shuttering for the lateral faces of the voussoir, are each fitted, before the pouring, with a straight edge standing proud towards the interior of the mould, and a fourth straight edge held positioned below the level of the corresponding face of the voussoir is placed across the pouring opening, the said gauges each leaving, after setting, in the corresponding face of the voussoir, an imprint capable of constituting a groove for removable fixing of the external part of the corresponding section of the joint.

In a preferential embodiment, the external part and the internal part of the joint are composed of two different materials, the external part being composed of a material which is specially adapted to ensure the watertight seal with sliding movement on the voussoir, and the internal part being composed of a compressible material such as an elastomer and engaging in a removable fashion, by clipping for example, into a groove let into the length of the corresponding face of the voussoir. The external part of the joint can also be fixed by clipping onto the internal part.

But the invention will be better understood through the following description of certain particular embodiments given by way of example and represented on the attached drawings.

Figure 1 represents schematically the embodiment of a tunnel lining in the form of prefabricated voussoirs, Figure la showing, in perspective, a voussoir.

Figure 2 represents, in vertical section, the pouring of an element.

Figure 3 shows, in perspective, the removal of an element from the mould.

Figure 4 is a detail view showing accessory members being put in place.

Figure 5 is a detail view, in transverse section, of a watertight sealing joint.

Figure 6 is a detail view showing other accessory members being p#ut in place.

Figure 7 is a view in perspective of a prefabricated voussoir according to the invention.

On Figure 1 is represented schematically, and by way of a simple example, the production of the cylindrical lining of a tunnel of circular cross-section composed of a series of consecutive tubular segments R1, R2 ...

Rn, each segment being itself composed of a series of voussoirs 1 each covering an angular sector. Each voussoir 1, represented in perspective on Fig. la, is composed of an element prefabricated in reinforced concrete comprising six faces, that is to say two cylindrically-curved faces centred on the axis 10 of the lining, respectively an outer wall face 11 and an inner wall face 12, and four flat lateral faces, respectively two rectangular faces 13, 14, situated in planes parallel to the axis 10 and passing, generally, through the latter, and two circumferential faces 15, 16, situated in planes perpendicular to the axis 10.

In a general fashion, a prefabrication mould delimits a pouring cavity comprising a substantially horizontal bottom, set on a moulding table, and lateral faces susbtantially perpendicular to the bottom, and defined by the walls of the shuttering fixed on the table.

When, as is the case here, the prefabricated element has the form of a slab of substantially constant thickness very much less than the other dimensions, it seems normal to pour this slab horizontally in such a fashion as to limit the height of the concrete. In this case, the moulding table has a convex part comprising the bottom of the mould and having a curvature equal to that of the inner wall 12 face and an upper opening, open wide, through which the concrete is poured from a hopper after a reinforcement has been put in place in the cavity of the mould. The outer wall 11 face of the voussoir is then produced by smoothing the concrete by means of a straight edge.

It has, however, been noted that, in this case, the outer wall 11 face does not have a surface quality sufficient to maintain the watertight seal correctly when the tunneller is working under high pressure.

In the procedure according to the invention, on the other hand, each voussoir 1 is poured in a mould 3 standing upright and comprising two cylindrical walls 31, 32, with a vertical axis, as shuttering for the outer wall 11 and inner wall 12 faces of the voussoir, and two flat vertical walls 33, 34 as shuttering for the lateral faces 13, 14, parallel to the axis.

Thus is obtained an excellent surface quality on the outer wall 11 face which is formed and vibrated along the vertical wall 31, which permits a very good watertight seal to be achieved with the possibility of sliding movement and without excessive wear of the joint D, placed in the rear of the tunneller, along the external face of the lining.

On Figure 2, which is a transverse sectional view, and on Figure 3, which is a view in perspective, are represented respectively the pouring of a voussoir element 1 in such a mould 3, and its removal from the shuttering.

The mould 3 comprises a fixed part constituting the bottom and three of the lateral faces of the pouring cavity 30, and a movable part constituting the fifth lateral face which can advantageously be constituted by an openable door 31, as can be seen on Figure 5. Given that, in the example represented, the lateral walls 33, 34, converge radially, it is advantageous, in order to facilitate removal from the shuttering, that the openable door constitute the wall 31 for shuttering the convex face, that is to say the outer wall 11 of the voussoir, the fixed part thus comprising the wall 32 for shuttering the concave face 12, the two sides 33, 34 and the bottom 35.

In order to produce a voussoir, as has been represented on Figure 2, after having closed the door 31, a prefabricated iron framework 17 is first of all put in place well in the centre with regard to the two walls, then the concrete 18 is poured, preferably while being vibrated. The mould 3 can, to this end, be associated with one or more vibrators.

After the concrete has set, the door 31 can be opened and the voussoir 11 can be removed from its mould 3 by being lifted up, as has been represented on Figure 5, by the use of slings 28 which hook onto manipulating members 4 fixed on the upper face 16 of the element 1.

Thanks to such a mode of pouring, a perfect surface quality of the outer wall 11 face of the voussoir can be achieved. On the other hand, it is the lateral face 16, located at the level of the pouring opening 30, which must then be smoothed, and as a result there is a relative uncertainty as to the geometric positioning of the face 16.

Moreover, in order to carry out the pouring of the voussoir into a mould placed upright, in good conditions, it is obviously preferable that the axis of the cylindrical walls 11 and 12 should be vertical and, in this case, the two opposite lateral faces corresponding respectively to the bottom of the mould and the pouring opening are composed of the two circumferential faces 15 and 16, transverse to the longitudinal axis of the voussoir. Now, as the tunneller must support itself, in order to advance, on the lining in concrete already produced, very significant thrust is applied to the circumferential faces 15 and 16 which support themselves on each another. That being so, surface irregularities, even quite slight ones, on the opposed faces of two adjacent voussoirs, can entail stress concentrations bringing about a cracking of the voussoir.

It is for that reason, according to one of the essential characteristics of the invention, that the upper face 16 of the voussoir has on the one hand a perfect geometry, and on the other hand a surface quality such that there need be no fear of stress concentration.

To this end, the face 16 is fitted, during the pouring, with at least one distribution plate 5, in metal or in synthetic material, which is fixed into the concrete in such a fashion as to define precisely a plane of assembly of the voussoir with the adjacent voussoir along the lateral face 16.

Each support plate 5 is fitted, on its internal face, with anchoring members 53 and is embedded into the lateral face 16 in the fashion represented schematically on Figure 4.

The mould 3 is fitted, in its upper part, with a raisable device composed of at least two arms 7, articulated around a horizontal axis which, when they are lowered, straddle the mould and come to be fixed, by their extremity 72, onto the upper edge of the convex wall 31.

During the greater part of the pouring, the arms 7 are raised, as indicated on Figure 2, in such a fashion as to clear the pouring opening 30. Moreover, the plate or plates 5 which have to be embedded into the face 16 are mounted in advance onto support members 51, 52, fixed to the arms 7.

At the end of the pouring, the arms 7 are lowered, and the support plate or plates 5 are placed through the pouring opening 30 of the mould. Then the vertical position of the plate 5 is adjusted precisely in such a fashion that its upper level corresponds exactly to the theoretical dimensions of the voussoir. To this end, each support member 51, 52 advantageously comprises a system for fine adjustment, for example a screw and nut.

In this way, the lateral face 16 of the voussoir can be fitted with one or more distribution plates 5 which constitute perfectly positioned areas of transmission of the thrust, and define a reference support plane corresponding exactly to the geometry of the voussoir.

Generally, the two lateral circumferential faces 15 and 16 are parallel to each other and perpendicular to the axis of the cylindrical faces 11 and 12, the distribution bed plates 5 being horizontal. In this case, as is represented on Figure 1, the junction planes between the different tubular segments are perpendicular to the axis.

However, the junction planes can also be slightly inclined with respect to the axis, for example for a change of direction of the tunnel. In this case, the two circumferential faces 15 and 16 are no longer parallel, and moreover, their distance can vary as a function of the position of the voussoir. The arrangements according to the invention permit this problem to be easily solved, for it is possible to adjust at will the level and the orientation of each bed plate 5 with respect to the bottom 35 of the mould on which is formed the opposite face 15. To this end, each bed plate 5 can be suspended from two stalks 52 whose lengths are adjusted differently, the support 51 of the bed plate 5 being capable of comprising an articulation in order to adjust the orientation of the bed plate.

The inclination of the face 16 with respect to the horizontal will generally be quite small in order not to present problems on pouring. In the case of a greater inclination, it could be envisaged that the whole of the mould 3 be inclined in such a fashion that the face 16 would be horizontal.

The arrangements which have just been described for the holding of the distribution bed plates 5 can advantageously be utilised to embed other accessory members at the level of the upper face 16 of the voussoir.

In particular, in many cases, it is necessary to provide, between the opposed faces of the adjacent voussoirs, a watertight sealing joint which, in the example represented, will comprise four sections extending respectively along the four lateral faces 13, 14, 15, 16 of the voussoir.

Such a joint 6 comprises, in a general fashion, an external part 60 projecting outside and an internal part 61 for fixing in the voussoir.

On Figure 5, is represented, by way of example, such a watertight sealing joint 6 in an elastic material such as an elastomer, fixed in a removable fashion in a groove 62 to let into the lateral face of the voussoir 1.

In this case, the joint 6 is in two different materials, the external part 60 being produced in a material specially adapted to ensure the watertight seal with the outer face of the voussoir while permitting sliding movements, while the internal part 61 is simply provided to be engaged in a removable and watertight fashion in the groove 62.

For example, the part 60 will advantageously constitute a "hydro-inflating" joint which inflates itself on contact with liquids, and which is "clipped" onto the internal part 61 which constitutes a fixing base. The latter may advantageously have a trapezoidal cross-section, the groove 62 having a corresponding dovetail cross-section. Moreover, the part 61 can contain hollows which facilitate its deformation.

The joint 6, as well as the groove 62, are composed of four sections, corresponding respectively to the four lateral faces 13, 14, 15, 16 of the voussoir.

To produce the three sections of groove on the formed faces 13, 14, 15 of the voussoir, it is sufficient to place on the corresponding walls 33, 34, 35 of the mould 3 a straight edge 63 in three sections which stand proud on the faces of the shuttering in such a fashion as to be incorporated into the voussoir 1 on pouring. After the element has been removed from the shuttering, each section of straight edge 63 is withdrawn, the imprint thus left constituting the groove 62. Clearly, having regard to the dovetail profile of the groove, it is necessary to permit the withdrawal of the straight edge 63, for example by producing it in several parts, but this forms part of known problems and it is known how to solve them in the techniques of prefabrication.

In order to produce the groove on the upper face 16 of the element, a horizontal straight edge 64 is utilised, which is held at the required level by supports 71 fixed onto the support arms 7 for the plates 5 and whose lengths can be adjusted in such a fashion as precisely to position the straight edge 64.

By way of example, on Figure 6 is represented another embodiment of the joint 6. The latter, which is produced in a compressible elastomer material, comprises a fixing part 65 intended to be sealed into the concrete.

To this end, the joint 6 is fixed in a removable fashion onto the extremity of the support 71 whose length can be adjusted, in order that the upper part of the joint 6 overhang above the level of the opening 30. When the mould has been poured, the joint 6 is embedded into the concrete by its part 65 and so remains fixed along the face 16 when the supports 71 are withdrawn.

The walls 33, 34, 35 of the mould can be equipped in the same fashion in order to produce the sections of the joint corresponding respectively to the faces 13, 14, 15.

Moreover, the voussoir 1 can be equipped, as has been seen, with hooks 4 onto which are hooked slings for the removal from the mould and the manipulation of the voussoir and these hooks have to be able to be withdrawn at the moment of installation in order entirely to clear the support face 16.

For this reason each manipulating hook 4 can advantageously be fitted with a threaded part which is engaged in a socket 41, internally threaded and embedded into the voussoir 1 during pouring.

As is represented in Figure 6, the sockets 41, which are fitted on their external face with anchoring parts, are screwed onto supports 42 mounted on the arms 7 and whose length can be adjusted. Thus, when the arms 7 are lowered, the sockets 41 are placed at the required level and are immersed in the upper part of the element 1 when the concrete is poured. After setting, the supports 42 are simply unscrewed, each socket 41 remaining embedded into the concrete for the removable fixing of a hook 4.

Clearly, the invention is not limited to the details of the embodiments which have just been described and which could be the subject of other variants, each voussoir element 1 being capable of being fitted with other accessory members placed in the mould in advance.

Thus, on Figure 7, which shows an example of a voussoir produced according to the invention, the inner face 12 of the voussoir is fitted with housings 43 into which open orifices 44 serving to let foundation bolts pass for the fixing together of the voussoirs, the housings 43 and the orifices 44 being reserved at the time of moulding by appropriate inserts placed in the mould 3.

Moreover, the invention has been described for the case of voussoirs having a general rectangular form, comprising four flat lateral walls, respectively parallel and perpendicular to the axis, but the same arrangements could be applied to voussoirs of other forms having lateral faces which are possibly convex or concave.

The reference marks, inserted after the technical characteristics mentioned in the claims, have the sole aim of facilitating the understanding of the latter, and in no way limit its scope.

Claims (23)

1. Procedure for fabrication of a prefabricated voussoir in concrete, comprising two cylindrically-curved faces, respectively the outer wall (11) and inner wall (12), centred on a longitudinal axis, and at least four lateral faces (13, 14, 15, 16), by pouring of the concrete into a mould (3) comprising a bottom, four lateral walls and an upper pouring opening, the voussoir being poured upright on one of its lateral faces (15) which corresponds to the bottom (35) of the mould (3), in such a fashion that the two cylindrical faces, respectively the outer wall (11) and inner wall (12) of the voussoir (1) should be directly formed on the two lateral walls (31) (32) of the mould which are given a corresponding curvature, characterised by the fact that in the pouring opening (30) is placed at least one plate (5) which is held, during the pouring, in a fixed position with respect to the mould, in such a fashion that the said plate (5) is embedded into the corresponding face (16) of the voussoir opposite the face (15) formed on the bottom (35) and forms, after removal from the mould, a distribution bed plate embodying a reference support plane complying with the geometry the voussoir (1) is to have.

2. Procedure for fabrication according to Claim 1, of a prefabricated voussoir for the production of a lining of a tunnel, characterised by the fact that the level and the orientation of the plate or plates (5) can be adjusted with respect to the bottom (35) of the mould (3) in such a fashion as to adjust the length of the voussoir (1) and the orientation of the lateral faces (15) and (16) as a function of the position of the voussoir (1) in the lining.

3. Procedure for fabrication of a voussoir according to one of Claims 1 or 2, characterised by the fact that each voussoir is fitted, during the pouring, with means of fixing of a watertight sealing joint comprising four sections extending respectively along the four lateral faces (13, 14, 15, 16) of the voussoir and placed end to end in such a fashion as to form a continuous joint, each section comprising an external part (60) extending outwards from the corresponding face of the voussoir (1) and an internal part (61) for fixing into the said face.

4. Procedure for fabrication of a voussoir according to Claim 3, characterised by the fact that the bottom (35) of the mould (3) and the lateral walls (33, 34) of shuttering for the lateral faces (13, 14) of the voussoir (1), are each fitted, before the pouring, with a straight edge (63) standing proud towards the interior of the mould (3), and in that, a fourth straight edge (64) held positioned below the corresponding lateral face (16) of the voussoir (1) is placed across the pouring openeing (30), the said straight edges (63) (64) each leaving, after setting, in the corresponding face (13, 14, 15, 16) of the voussoir (1), an imprint capable of constituting a groove (62) for removable fixing of the external part (60) of the corresponding section of the joint (6).

5. Procedure for fabrication of a voussoir according to Claim 3, characterised by the fact that the external parts (60) of the three sections of the joint (6), corresponding to the formed lateral faces (13, 14, 15) of the voussoir (1), are mounted so as to be removable, before the pouring, on the bottom (35) and the corresponding lateral walls (33, 34) of the mould (3), in such a fashion that the internal parts (61) of the joint (6) project towards the interior of the mould and in that the external part (60) of the fourth section of the joint (6) is held across the pouring opening (30) and positioned in such a fashion that its internal part (61) is below the level of the corresponding face (16), the said internal parts (61) of the four sections of the joint (6) being integrated, after the pouring and the setting of the concrete, into the corresponding faces (13, 14, 15, 16) of the voussoir (1).

6. Procedure for fabrication of a voussoir according to one of the preceding claims, characterised by the fact that, members (41) for anchoring of means of suspension (4) permitting, after setting, the manipulation of the voussoir are incorporated into the upper face (16) of the voussoir (1) corresponding to the pouring opening (30) during the pouring.

7. Prefabricated voussoir in concrete, capable of being associated with other adjacent voussoirs for the production of the lining (R) of a subterranean tunnel (T) and comprising two cylindrically-curved faces, respectively the outer wall (11) and inner wall (12), and at least four lateral faces (13, 14, 15, 16) for support on the adjacent voussoirs, the said voussoir (1) being produced in the interior of a mould (3) comprising a bottom and four lateral walls for the shuttering of five faces of the voussoir, and an upper opening for the pouring of the concrete, corresponding to the sixth face, characterised by the fact that the two outer wall (11) and inner wall (12) cylindrical faces constitute two faces formed in the mould (3) and in that the upper face (16) of the voussoir corresponding to the pouring opening (30) is fitted with at least one plate (5) perfectly positioned during the pouring in such a fashion as to form a distribution bed plate capable of embodying a reference support plane on an adjacent voussoir.

8. Prefabricated voussoir according to Claim 7, characterised by the fact that the level and the orientation of the plate (5) with respect to the bottom of the mould are adjusted as a function of the position of the voussoir (1) in the lining (R) in such a fashion that the reference support plane should be inclined at an angle other than 900 with respect to the axis of the tunnel.

9. Prefabricated voussoir according to one of Claims 6 or 7, characterised by the fact that it is fitted with a continuous watertight sealing joint comprising four sections extending respectively along four lateral faces (13, 14, 15, 16) of the voussoir (1) and each comprising an external part (60) projecting towards the exterior of the corresponding face of the voussoir (1) and an internal part (61) for fixing onto the said face, in the thickness of the voussoir (1).

10. Prefabricated voussoir according to Claim 9, characterised by the fact that the external part (60) of the joint (6) is composed, at least partially, of a material specially adapted for producing the watertight seal with the possibility of sliding movement, in particular of the hydro-inflating type.

11. Prefabricated voussoir according to Claim 9, characterised by the fact that the external part (60) and the internal part (61) of the joint (6) are composed of two different materials, the external part (60) being composed of a material (65) which is specially adapted to permit the sliding movement and maintaining of the watertight seal and the internal part (61) being composed of a compressible material (66) such as an elastomer, and having a form adapted to be engaged in a watertight and removable fashion into a groove (62) let in along the corresponding face of the voussoir (1).

12. Prefabricated voussoir according to Claim 11, characterised by the fact that the external part (60) is fixed by clipping onto the internal part (61).

13. Mould for fabrication of a voussoir (1) in concrete, comprising two cylindrically-curved faces, respectively the outer wall (11) and inner wall (12), centred on an axis, and at least four substantially flat lateral faces (13, 14, 15, 16), the said mould comprising a horizontal bottom (35), two lateral walls (31, 32) curved in such a fashion as to constitute shuttering for the two cylindrical faces (11, 12) of the voussoir (1), and an upper opening (30), characterised by the fact that it is fitted with means (51, 52) for positioning, across the pouring opening (30) and at an adjustable level, of at least one plate (5) fitted with means (53) of being embedded into the concrete and positioned in such a fashion as to constitute, at the level of the lateral face (16) of the voussoir (1), a distribution bed plate for thrust forces, perfectly complying with the geometry the voussoir is to have.

14. Mould according to Claim 13, characterised by the fact that the means of positioning of the plate (5) comprise at least one stalk (51, 52) of adjustable length fitted at one extremity with a removable means of fixing for the plate and fixed at its other extremity onto at least one arm (7) extending across the pouring opening (30) and movably mounted onto the mould (3) in such a fashion as to be able to clear the pouring opening (30).

15. Mould according to Claim 14, characterised by the fact that the means (51, 52) for positioning of the plates (5) are organised in such a fashion as to permit the adjustment of the level and of the orientation of each plate (5) with respect to the bottom (35) of the mould (3).

16. Mould according to one of Claims 14 or 15, characterised by the fact that the arm (7) is fitted with means (71) (42) of positioning, through the pouring opening (30) and at an adjustable level, of accessory members capable of being incorporated, after setting, into the corresponding lateral face of the voussoir (1).

17. Mould according to Claim 16, characterised by the fact that the arm (7) is fitted with means of positioning (71) supporting a straight edge (64) held at the level of the lateral face (16) of the voussoir in such a fashion as to make in the latter an imprint capable of constituting a groove (62) for removable fixing of a watertight sealing joint (6).

18. Mould according to Claim 16, characterised by the fact that the said means of positioning (71) support a watertight sealing joint (6) fixed onto the said means (71) by an external part (60) extended by an internal part (61) and positioned in such a fashion that the external part (60) is located above the level of the lateral face (16) and that the internal part (61) should be embedded into the concrete.

19. Mould according to Claim 16, characterised by the fact that the arm (7) is fitted with means of positioning (42) onto which are fixed in a removable fashion anchoring members (41) capable of being embedded into the voussoir (1) below the level of the lateral face (16) and onto which can be fixed in a removable fashion the members (4) for manipulating the voussoir (1).

20. Mould according to one of Claims 13 to 19, characterised by the fact that at least the curved wall (31) of the mould (3) serving as shuttering for the outer wall face (11) is removably mounted in such a fashion as to be able to be separated after the concrete has set in order to remove the voussoir (1) from the shuttering.

21. A method for fabrication of a prefabricated voussoir in concrete, substantially as hereinbefore described with reference to the accompanying drawings.

22. A prefabricated concrete voussoir substantially as hereinbefore described with reference to the accompanying drawings.

23. A mould for fabrication of a concrete voussoir substantially as hereinbefore described with reference to the accompanying drawings.