WO2022254325A2

WO2022254325A2 - Centring for supporting and consolidating an excavation, and method for installing such a centring inside an excavation

Info

Publication number: WO2022254325A2
Application number: PCT/IB2022/055074
Authority: WO
Inventors: Cristiano Bonomi; Franco TAMBURINI
Original assignee: Officine Maccaferri Italia S.R.L.
Priority date: 2021-05-31
Filing date: 2022-05-31
Publication date: 2022-12-08
Also published as: CA3217172A1; WO2022254325A3; BR112023024610A2; AU2022285194A1; EP4348006A2

Abstract

A centring for supporting and consolidating an excavation comprises a plurality of movable structural elements ( 11, 12, 13, 14 ) which are connected to each other in such a manner that the centring can move from a preassembled configuration, at least partially folded before the installation, to a definitive installation configuration, in which the structural elements are locked with respect to each other in a mutual position which generally defines a centring which is at least formed in an arched manner. The structural elements can be locked with structural continuity with respect to each other in a mutual position which generally defines a centring with the closed-geometry profile which comprises the centring which is formed with a closed arch at the bottom by a structural element which acts as a strut or inverted arch.

Description

CENTRING FOR SUPPORTING AND CONSOLIDATING AN EXCAVATION, AND METHOD FOR INSTALLING SUCH A CENTRING INSIDE AN EXCAVATION

Field of the invention

The present invention relates to the field of excavations, in particular tunnels which are constructed with an underground excavation .

The invention has been developed with regard to a centring for supporting an excavation.

In greater detail, the present invention relates to a centring for supporting and consolidating an excavation and a method for installing a supporting centring inside an excavation .

Technological background

In the field of support structures, it is known to use structural elements which are connected to each other in order to construct the final form of the support structure itself. Such structural elements may have an open section, for example, C-shaped, double-T-shaped, or a closed section. In the case of closed sections, the structural elements are tubular and may have sections of any shape, for example, circular, square, rectangular or triangular.

For supporting and consolidating excavations, such as, for example, road or rail tunnels, it is known to use consolidation arches which are called centrings. In particular, a centring usually comprises a plurality of shaped structural elements which are made from steel and which are connected to each other in accordance with a vault like configuration. Such elements are constituted by open profiles with an H-shaped, IPN-shaped, C-shaped or double-T- shaped cross-section, even if there are many examples of closed profiles for constructing tubular centrings. One particular support centring of the tubular type is described in EP 2354447. Examples of articulated connection members between the structural elements of a centring are described in WO 2015/186029.

In the majority of cases, the profiles are connected to each other in the region of the excavation to be consolidated after being shaped in the workshop. After the assembly thereof, each centring is connected to the adjacent ones via the connection chains, the ends of which engage with welded supports along the body of the profiles of the centrings. The space between two consecutive centrings and the wall of the excavation is temporarily consolidated usually via a precoating which is brought about by means of sprayed concrete (shotcrete). Subsequently, the excavation is completed and consolidated by means of a definitive coating of concrete.

In order to connect two structural elements to each other and to obtain the final form of the support structure, there are usually used a pair of joint plates which are intended to be connected to each other via bolts. A joint system is known from the document EP2354447 which describes a supporting and consolidating centring of an excavation which is constituted by a plurality of structural elements which are connected to each other. In the known joint system, each first joint plate has portions which are welded to a relevant structural element in the region of an end section. Each joint plate, which is usually rectangular, comprises two series of holes for connecting bolts which fix the plates to each other, bringing about the connection between the structural elements so that the structural elements form a continuous structure which develops over the entire extent of the structure. This known joint system has different disadvantages because, for example, the structural elements have to be positioned with precision relative to each other so as to align the holes of the plates in which the bolts are to be inserted. This is difficult, particularly in the case of quite extensive structures, particularly vertical ones, or ones which have to take up final formations with particular shapes, such as, for example, the centrings which have a vault-like formation. Furthermore, the connection cannot be brought about automatically because the mutual positioning of the plates and the insertion of the bolts can be brought about only manually. Therefore, this connection requires a substantial use of time. Furthermore, this connection may be unstable if, for example, the bolts are not fixed correctly. In particular, the instability of the connection can increase over time if the retention of the bolts is not monitored periodically .

In order to solve the problems set out above, the present Applicant has developed the solution described in WO 2015/186029 which provides for an articulated connection system with automatic locking of the structural elements which compose a supporting and consolidating centring which has a vault-like configuration and which generally resembles the configuration of the excavation portion which is intended to be consolidated via the centring itself. The structural elements which compose the centring, which are made from metal material, such as, for example, construction steel, are connected to each other with a rotatable connection, for example, by means of hinges, so as to move from a first position, in which the structural elements are substantially folded one on the other, to a second position, in which they are arranged so as to form at least one substantially continuous portion of the centring.

The system described in WO 2015/186029 has been found to be effective but has characteristics which limit the use thereof. The final position of the centring and particularly of the two lateral feet thereof is not defined with precision and could lead to adjacent centrings not being completely aligned with the base of the vault of the excavation, with resultant non-homogeneity in the wall of the resulting tunnel. Furthermore, the thrusts applied by the wall of the excavation to the centring near the base of the excavation can deform if not even move the position of the feet of the centring, with consequences which it is necessary to remedy with operations for maintaining the tunnel.

Therefore, there is perceived the need for a centring which has improved characteristics, which constitutes an improvement with respect to the known centring described above.

Before the final consolidation of the excavation, the loads applied by the rocky structure can bring about deformations of the section of the tunnel, which lead to the deformation, including significant deformation, of one or more centrings. The deformation of the centrings brings about the reduction of efficiency of the reinforcement with possible risks for the safety of the workers who are operating inside the excavation.

Therefore, there is a need to monitor the state of the tensions which act on the centring following the loads applied thereto by the rocky wall and to provide a system so that such loads do not exceed the point of irremediable deformation of the centring, if not even of bringing about the collapse thereof.

There is further a need to compensate for the loads applied by the rocky structure and to provide a system for compensating for the length of the centring so as to discharge the tensions which act thereon in order to prevent loading levels which could lead to the irremediable deformation of the centring, if not even the collapse thereof, from being reached and exceeded.

Statement of invention

An object of the present invention is to overcome the disadvantages of the prior art.

In the context of the above-mentioned object, an objective may be to obtain a centring for supporting and consolidating an excavation which allows a rapid and secure connection to be obtained between the structural elements thereof so as to be particularly stable when it is installed inside an excavation .

In the context of this object, a different objective may be to provide a method of controlling the state of the tensions which act on the consolidation centrings of an excavation, that is to say, of the thrust applied to the centring by the walls of the excavation. Another objective may be to provide a monitoring and control system for a centring which is simple, economical, easy to implement and to use, including by non-specialist operators. A different objective may be to provide a centring which can compensate for the thrust applied by the walls of the excavation. Another objective may be to reduce the risk of deformation, if not even the collapse of the centring, as a result of the loads which are applied thereto by the wall of the excavation and which can vary over time. Another objective may be to provide a compensation system which operates autonomously, particularly when the loads on the centring exceed a predetermined threshold without it being necessary to apply continuous control of the state of the centring, neither by operators nor by means of remotely controlled electronic systems. Another objective may be to provide a compensation system for the thrust applied to the centring which is simple, economical, easy to implement and which does not require particular maintenance or control operations.

Another objective is to obtain a centring which is relatively economical and simple to manufacture.

Another objective is to obtain a centring which can be installed rapidly and automatically.

Another objective is to obtain a centring which is safe during use.

These objects and other objects and objectives are achieved by a centring and by a method having the features set out in the appended claims.

There is described a centring which can be preassembled and which may comprise a plurality of movable structural elements which are connected to each other in such a manner that the centring can move from an initially restricted and compact configuration before the installation to a definitive supporting and consolidating configuration of the profile of the excavation. One advantage of the present invention is the fact that it allows the immediate and integrated consolidation not only of the walls but also of the base of the tunnel by means of a so-called "inverted arch" or "strut" section which also forms part of the structural elements which are connected to each other in a movable manner so as to be able to be locked in position when the centring is in the final configuration, thereby allowing the installation of the centring at the same positioning time, overcoming the need to operate with successive installation steps even if it is not excluded in all cases that the strut can be added to the centring at a second time.

According to a first aspect, there is described a centring for supporting and consolidating an excavation which may comprise two or more structural elements. The structural elements are elongate in the sense that they have one much larger dimension with respect to the other two dimensions which define the cross-section of the structural elements.

The structural elements may be connected to each other. The structural elements may be connected to each other in a movable manner. The connection may be such that the centring can move from a preassembled configuration before the installation to a definitive installation configuration. The preassembled configuration can be at least partially folded. In the definitive configuration, the structural elements can be locked with respect to each other. The structural elements can be locked with structural continuity, that is to say, resisting the forces applied by the walls of the excavation as if it were a single element or as if it were locked with fixed connection members, such as bolts and the like. The structural elements can be locked in a mutual position which generally defines a centring with the closed geometric profile or a centring with an arched profile. According to a particular aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, the structural elements can be mounted to be articulated, hinged or oscillating with respect to each other. The structural elements can be connected to each other by means of joint members which comprise hinges.

According to another particular aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, a base structural element acting as a strut or inverted arch may have an end which is slidingly mounted on a different structural element, which is lateral with respect to the excavation. According to another particular aspect, the centring may comprise at least three structural elements, preferably at least four or five structural elements, preferably exactly three or exactly four or exactly five structural elements.

According to another particular aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, a base structural element acting as a strut or inverted arch may have one or two end connectors for the connection in one or two seats which are constructed on one or both of the lateral structural elements of the centring.

It is thereby possible to definitively install in the excavation a centring which is configured in an arched manner, and to subsequently complete it by adding the base structural element which brings about the strut or inverted arch and allows the structure of the centring to be generally closed by constructing a closed geometric profile. According to a variant, the strut or inverted arch can also be installed separately, immediately after the positioning of the arched centring, that is to say, the elements of the centring which will be forming the sides and the vault. The connection of the base structural element, that is to say, the strut or inverted arch, can be brought about rapidly, immediately or separately, including after a specific time with respect to the arched centring structure, including as a result of the possibility that one or both of the end connectors is/are constructed to be orientable, for example, using one or two ball joints, which are not therefore subjected to any torsions or rotations of the arched centring and the lateral structural elements thereof with respect to the installation plane of the base structural element.

According to another aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, there is described a centring in which a base structural element acting as a strut or inverted arch may be constructed in two different portions. The two portions of the base structural element can be articulated to two respective lateral structural elements of the centring. The two portions of the base structural element may be joined to each other by means of a joint member. The joint member which joins the two portions of the base structural element can be of the snap-fitting type in order to thereby obtain a definitive and irreversible configuration of the centring with the closed geometric profile.

According to another particular aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, there is described a centring in which one or more of the structural elements can be subdivided into two or more sections, between which corresponding resilient joints can be inserted. The resilient joints allow the absorption, by reacting resiliently, of the thrusts which, as time passes, can be applied by the walls of the excavation to the centring.

According to another aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, there is described a method for installing a centring inside an excavation. The method may comprise the step of providing a plurality of structural elements of a centring. The method may comprise the step of connecting the structural elements to each other in order to obtain a preassembled centring. The connection may be a movable connection. The method may comprise the step of folding the centring, preferably at least partially, before the installation thereof. The method may comprise the step of transporting the centring, preferably in the at least partially folded state, inside the excavation. The method may comprise the step of installing the centring in a definitive installation configuration in a mutual position which generally defines a centring with the closed geometric profile or with the arched profile. Preferably, in this step, the structural elements can be locked with respect to each other, preferably with structural continuity. According to another aspect, the method may provide for the structural elements to be locked with respect to each other in an irreversible manner.

According to another aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, there is described a centring for supporting an excavation which may comprise two or more structural elements. The structural elements are elongate in the sense that they have one much larger dimension with respect to the other two dimensions which define the cross-section of the structural elements. The structural elements are joined consecutively in the direction of the extent thereof in order to define at least one arch-like support portion of the wall of the excavation. The centring may further comprise at least one control device for controlling the thrust applied to the centring by the walls of the excavation. The control device may be interposed between two adjacent structural elements. The control device may be connected to at least one pressure sensor. There is used in the centring a sensor which is configured to measure a pressure value which is representative of the force applied to the centring by the walls of the excavation. The sensor which is configured can be the at least one sensor which is connected to the control device. The pressure sensor measures the pressure of a fluid. The fluid may be an incompressible fluid, for example, water. The pressure value may be representative of the force which is applied by the adjacent structural elements to the monitoring and control device which is interposed between them following the thrust applied by the walls of the excavation to the arch portion defined by these adjacent structural elements.

According to a particularly advantageous aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, the control device which is interposed between the two structural elements may comprise at least one hydraulic thrustor. The hydraulic thrustor may be connected to a source of pressurized fluid and may be configured to act counter in an adjustable manner, by means of a hydraulic pressure which can be measured by the pressure sensor, to the force applied by the structural elements to the control device following the thrust of the walls of the excavation to the structural elements.

According to another particularly advantageous aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, the hydraulic thrustor may comprise a piston which may be connected to one end of one of the two elongate structural elements. The piston may be able to slide in a fluid-tight manner inside a liner. The liner may be connected to one end of the other of the two elongate structural elements.

According to a different aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, the centring may comprise at least two, preferably four or in any case an even number of control devices. These control devices can be arranged symmetrically with respect to a vertical plane of symmetry of the centring.

According to another aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, the centring may comprise a connection joint between at least two elongate structural elements. The control device which can be interposed between the two elongate structural elements can be directly connected to the connection joint, at one side. At the other side, the control device can be directly connected to the end of one of the two elongate structural elements.

According to another aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, there is described a method for controlling the thrust applied by the wall of an excavation to a centring of the above-mentioned type. More generally, the method may provide for defining a threshold pressure value. There may be provision for monitoring the pressure value which is measured by the at least one pressure sensor which can be connected to the control device. There may be provision for activating a control signal if the pressure value measured exceeds the threshold pressure value. There may further be provision for, when verifying this condition, the pressure being able to be automatically discharged.

According to a different aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, the method for controlling the thrust which is applied by the wall of an excavation may provide for defining a minimum threshold pressure value and a maximum threshold pressure value. It is possible to install the centring behind the wall of the excavation and subsequently it is possible to monitor continuously or at predetermined intervals the pressure value measured by the at least one pressure sensor which can be connected to the control device. Therefore, it is possible to supply a pressurized fluid to a hydraulic thrustor of the control device if the measured pressure value is less than the minimum threshold pressure value. It is possible to activate the discharge of fluid from the hydraulic thrustor if the measured pressure value exceeds the maximum threshold pressure value.

The control which is able to be carried out by means of the centring and the method incorporating the most salient features indicated above is particularly effective for verifying the state of the tensions in the excavation and for intervening in good time by controlling the thrust applied to the centring, thereby preventing the damaging deformations, the consequences of which have been mentioned above.

The centring may be arranged so as to be controlled remotely. To this end, there can be used electronic pressure sensors which are connected - for example, via wires or wirelessly - to a local electronic control unit. The local electronic control unit can send a message to a remote node, such as, for example, but in a non-limiting manner, a server, a server cluster, a cloud service, so as to have in a remote manner immediate control of all the positioned centrings.

According to another aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, there is described a supporting centring for an excavation which may comprise at least two elongate structural elements. These structural elements may be joined consecutively in the sense of the extent thereof in order to define at least one supporting arch portion of the wall of the excavation. The centring may comprise at least one compensation device for the thrust which is applied by the walls of the excavation. The compensation device may be interposed between at least two structural elements of the centring. The compensation device may be movable between an extended configuration and a reduced configuration thereof in the sense of the extent of the centring, or the structural elements which compose it. There may be arranged in the compensation device at least one compressible member which is able to offer resistance to the movement of the compensation device from the extended configuration to the reduced configuration. The compensation device may therefore compensate for the thrusts applied by the wall of the excavation to the centring without there being any risks of being subjected to significant deformations. The compensation system is particularly simple and economical to construct by using a variety of compressible members which are readily commercially available at a low cost and/or can be readily constructed .

It is described, for example, that the compressible member may be a resiliently compressible member. The resistance to the movement of the compensation device from the extended configuration to the reduced configuration may be proportional to the extent of the compression thereof. The resilience of a compressible member allows absorption of the thrusts which are applied to the centring by the walls of the excavation, gradually reducing the extension of the centring with increases in the loads, at the same time supporting the excavation with the resilient reaction which acts counter to the compression.

Advantageously, the compressible member can be constructed in the form of a prismatic or cylindrical block which is completely or partially constructed with one or more resilient materials. There are commercially available various resilient materials of the type and with different characteristics and which can be selected in accordance with the cost and/or the desired performance levels. Preferably, the compressible member can be completely or partially constructed from a technical polymer, a rubber or other material of the type.

According to a different example, what is described as the compressible member may be a plastically compressible or collapsible member.

The compressible member can act counter to the movement of the compensation device from the extended configuration to the reduced configuration until exceeding a threshold compression value, in addition to which it becomes plastically deformed or collapsed, acting counter to a minimum resistance to the movement of the compensation device. The plastic deformation or the collapse of the compressible member allow the construction of a type of "fuse", that is to say, a member which also visually shows an initial state and a final state which can clearly be distinguished so as to provide an operating indication. A periodic control of the state of the compensation devices can provide an indication of the deformation over time of the walls of the excavation, it being possible to distinguish and mark over time the compensation devices which are still in the initial state with respect to the ones which have been activated, changing to the final state.

Advantageously, the compressible member of the collapsible type can be constructed with at least one tube which is arranged in the compensation device so as to be subjected to an axial compression load, also called the "peak load". It is known that the resistance of a tube to such an axial compression load can be determined with reasonable precision, it being possible to predetermine with this the intervention threshold of the compensation device following the collapse of the tube when the peak load thereon exceeds the threshold value. Furthermore, it is very practical and economical to use a tube with a function as a sensitive element in the compensation device. It may be preferable to use a tube made of steel.

According to a particular aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, it is described how the tube which can be used to construct the sensitive element in a compensation device can be provided and processed in order to obtain a predetermined peak load resistance (that is to say, a resistance to "buckling") . To this end, the tube may comprise a plurality of elongate slots in the longitudinal direction in a manner parallel with the axis of the tube, which can constitute a predetermined weakening of the peak load resistance of the tube with respect to the same integral tube. Advantageously, it is described how the slots can be distributed regularly over the lateral wall of the tube in a circumferential direction. In this manner, a specific symmetry of the tube is maintained with respect to the individual longitudinal axis with the result that the collapse of the tube when exceeding a threshold peak load value is brought about in an orderly and possibly symmetrical manner.

According to a particular aspect, which is not necessarily subordinate or dependent with respect to what is indicated above, it is described how the compensation device may comprise a telescopic group, in which one or more compressible members of the type indicated above can be received. The telescopic group is also simple to produce in a metal workshop, is economical and reliable. Furthermore, any compressible member which is received therein remains protected and, for example, is not weakened or compromised by the concrete which is sprayed on the centring during the installation thereof in the excavation.

Brief description of the drawings

Additional features and advantages will be appreciated from the following detailed description of a preferred embodiment with reference to the appended drawings which are given by way of non-limiting example and in which:

- Figure 1 is a schematic view of the cross-section of an excavation in which a first example of a centring which incorporates elements of the present invention is positioned, in a configuration before the definitive installation;

- Figure 2 is a schematic view of the same cross-section of the excavation of Figure 1, with the centring in a virtually definitive configuration, which produces the integral support of the entire section of the excavation,

- Figure 3 is a perspective view of an embodiment of a foot of the centring;

- Figure 4 is a front view of another embodiment of a foot of the centring;

- Figure 5 is a side view according to the arrow V of Figure

4;

- Figure 6 is a perspective view of an example of the articulated connection between two structural elements of the centring;

- Figure 7 is a section along the plane of the line VII-VII of Figure 6;

- Figure 8 is a perspective view of an example of the connection between two non-articulated ends of two structural elements of the centring, for example, between the end of the strut and the end of a lateral element of the centring;

- Figure 9 is a view of the connection of Figure 8 in the connected configuration in which the two ends of the structural elements of the centring are locked against each other;

- Figure 10 is a detailed view of the end of one of the structural elements of Figures 8 and 9, for example, the end of the strut;

- Figures 11 and 12 are side views of two steps of the connection between the two structural elements of Figures 8 and 9, respectively, in a not-yet connected configuration and in a connected configuration corresponding to the one of Figure 9;

- Figure 13 shows an example of a resilient joint, between two sections of a structural element of the centring, which is intended to absorb and oppose over time the deformations of the wall of the tunnel; Figure 14 is a longitudinal section of the resilient joint of Figure 13;

- Figure 15 is a perspective view of an articulated connection variant between two structural elements of the centring, which can be used as an alternative to the articulated connection of Figure 6;

- Figures 16 and 17 are side views of two configurations of the articulated connection, respectively, in the not-yet connected configuration of Figure 15 and in a connected configuration;

- Figure 18 is a perspective view of another articulated connection variant between two structural elements of the centring, in a connected configuration;

- Figure 19 is a perspective view of an example of a connection between two adjacent centrings, also called the "chain" in the sector;

- Figure 20 is a perspective view of another example of a connection between two adjacent centrings, also called the "chain" in the sector;

- Figure 21 is a schematic front view of another example of a centring which brings about the integral support of the entire section of the excavation, incorporating elements of the present invention;

- Figure 22 is a perspective view of a connection between two structural elements of the centring of Figure 21, for example, between the end of the lower strut and the end of a lateral structural element;

- Figure 23 is a perspective view of the end of one of the two structural elements of Figure 22, for example, of the lower strut;

- Figure 24 is a perspective view of a variant of a connection between two structural elements of the centring of Figure 21, for example, between the end of the lower strut and the end of a lateral structural element; - Figure 25 is a schematic front view of another example of a centring which brings about the integral support of the entire section of the excavation, incorporating elements of the present invention;

- Figure 26 is a perspective view, drawn to an enlarged scale, of a section of the centring of Figure 1, comprising an articulation joint beside a hydraulic control and regulation thrustor;

- Figure 27 is a longitudinal section of the hydraulic control thrustor of Figure 26;

- Figure 28 is a schematic view of the cross-section of an excavation, in which a variant of the centring illustrated in Figure 2 is positioned;

- Figure 29 is a perspective view, drawn to an enlarged scale, of a section of the centring of Figure 28, comprising an articulation joint beside a compensation device for the thrust applied by the walls of the excavation, in a manner partially sectioned to show the interior thereof; and

- Figure 30 is a perspective view similar to Figure 29, which illustrates another embodiment of the compensation device for the thrust applied by the walls of the excavation, also in a manner partially sectioned to show the interior thereof.

Detailed description

In the following embodiments, there are described features which allow the invention to be carried out. The features described can be combined with each other in various manners and are not necessarily limited to the precise embodiment to which the drawings and the relevant description refer. In other words, a person skilled in the art of the sector who reads the following description will know how to obtain the useful information in order to know the way to achieve one or more of the features described by combining it with one or more of the other features described without the particular formulation of the description, the paragraphs, the phrases or the drawings constituting a limit to the possibility of isolating one or more of the features described and illustrated in order to combine it with one or more of any one of the other features described and illustrated. In greater detail, in the present description any combination of any two features which are expressly described must be understood to be expressly described, including if the features are extracted individually from the specific context, in which they can be juxtaposed or combined with other different features, taking into account the competences and knowledge of a person skilled in the art of the sector who understands the possibility of functionally combining the features without it being necessary to functionally provide the other different features. Unless otherwise specified, each and any element, member, means, system, component, object described and illustrated in the present description must be understood to be individually described and able to be autonomously modified, and separable from and/or able to be combined with each and any other element, member, means, system, component, object described and illustrated. The materials, forms and functions described and illustrated do not limit the present invention but are merely specified in order to allow a person skilled in the art to understand and carry out the invention according to preferred though non exclusive embodiments.

Now with reference to Figures 1 and 2, there is schematically illustrated a cross-section of an excavation S, for example, the section of a road or rail tunnel. The excavation comprises a vault V and a base F. The base F can be composed by two lateral planar zones M with the objective of supporting the centring structure and by a central arcuate zone C which forms the so-called "inverted arch".

Inside the excavation S, there is illustrated a supporting and consolidating centring 10 which incorporates aspects of the present invention. Figure 1 illustrates the centring 10 in a configuration before the definitive installation.

Figure 2 illustrates the centring 10 in a configuration similar to the definitive installation except for the definitive extension of the lateral feet. In particular, in Figure 2 the centring 10 is completely extended so as to occupy and support the entire circumferential development of the excavation S. However, the principles of the invention apply equally well to any type of centring, including of the open type with a configuration with an open arch or with an inverted U-shape. One of the lateral feet of the centring, in particular the foot illustrated on the left in Figure 2, is already extended so as to move into abutment against the corresponding lateral planar zone M on the left. The foot of the centring illustrated on the right in Figure 2 is still in the retracted position, however. Following the extension of the foot of the centring on the right in Figure 2 being supported on the corresponding lateral planar zone M on the right, there is also obtained the definitive configuration of the positioning of the centring for supporting the section of excavation S.

The centring 10 is formed by a plurality of structural elements, for example, four structural elements 11, 12, 13, 14. The structural elements 11, 12, 13, 14 are preferably made from metal material, such as, for example, construction steel (Fe 430 or the like). According to a preferred embodiment, each structural element 11, 12, 13, 14 is formed by a body which is constituted by a profile with an open cross-section, for example, H-shaped or C-shaped or double-T- shaped, for example, a European standardized profile, such as HEA, HEB, HEM, IPE, etc. However, it is not excluded that one or more of the structural elements 11, 12, 13, 14 is/are constructed with different profiles, for example, tubular profiles with a circular, elliptical, square, rectangular section, etc. In particular, it is not excluded that the structural element 14 which forms the base (or "inverted arch") of the centring 10 may have a different section from the other structural elements 11, 12, 13 which form the sides and the top of the centring 10 and which preferably, even if not necessarily, have mutually identical profiles.

The centring 10 has a vault-like or arch-like configuration which is substantially symmetrical with respect to a plane of symmetry P. The centring 10 can be formed by a plurality of structural elements, for example, four structural elements: a first lateral structural element 11, a central structural element 12, which is also called the upper or top structural element and which is connected to the first lateral structural element 11, a second lateral structural element 13 which is connected to the central structural element 12, and a bottom or base structural element 14, which is also called the "strut" and which is connected to the second lateral structural element. The three structural elements 11, 12, 13 form the sides and the top of the centring 10. The inverted arch at the bottom can be defined by one or more structural elements, for example, by the single structural element 14 or, as will be seen below with reference to Figure 25, by the two structural elements 14a, 14b. The symmetrical vault-like configuration of the centring 10, in the virtually definitive configuration shown in Figure 2, generally reproduces the section of the excavation portion S which is intended to be consolidated via the centring itself.

The centring 10 which is illustrated in the Figures comprises a first lateral structural element 11, a central structural element 12, which is connected to the first lateral structural element 11, a second lateral structural element 13 which is connected to the central structural element 12, and a bottom structural element 14 which is connected to the second lateral structural element 13. As illustrated, the first lateral structural element 11 and the second lateral structural element 13 are arranged in accordance with a position which is substantially specular with respect to the plane of symmetry P of the centring 10. In other words, in this exemplary configuration of the centring 10, the first lateral structural element 11 and the second lateral structural element 13 are lateral structural elements of the centring 10. The central structural element 12 and the bottom structural element 14 preferably develop symmetrically between the first lateral structural element 11 and the second lateral structural element 13 with respect to the same plane of symmetry, at opposite sides with respect to a horizontal plane T which intersects the first structural element 11 and the second lateral structural element 13. In the example of the centring 10, the central structural element 12 is an upper structural element which is also called the upper or top structural element, while the bottom structural element 14 is a lower structural element which is also called the base or "inverted arch" structural element.

The first lateral structural element 11 has a first end 111 which is operatively connected to a first end 121 of the central structural element 12. The connection between the first end 111 of the first lateral structural element 11 and the first end 121 of the central structural element 12 is brought about via first joint members 151. In the vicinity of a second end 112 of the first lateral structural element 11, there is provided a first support element 161 of the centring 10 which incorporates and defines a foot of the centring.

Similarly to the first lateral structural element 11, the second lateral structural element 13 has a first end 131 which is operatively connected to a second end 122 of the central structural element 12. The connection between the first end 131 of the second lateral structural element 13 and the second end 122 of the central structural element 12 is brought about via second joint members 152. In the vicinity of a second end 132 of the second lateral structural element 13, there is provided a second support element 162 for the centring 10 which incorporates and defines another foot of the centring.

A second end 132 of the second lateral element 13 is operatively connected to a first end 141 of the bottom structural element 14. The connection between the second end 132 of the second lateral structural element 13 and the first end 141 of the bottom structural element 14 is brought about via third joint members 153.

The base structural element 14 has a second end 142 which is intended to be connected during installation, in the definitive configuration of the centring 10 which is illustrated in Figure 2, at the second end 112 of the first lateral structural element 11. The connection between the second end 142 of the base structural element 14 and the second end 112 of the first lateral structural element 11 is brought about via fourth joint members 154. Advantageously, the first 151, second 152 and/or third 152 joint members are movable joint members and comprise, for example, an articulated, oscillating or rotatable connection, which can, where applicable but not necessarily, be locked definitively and irreversibly in the definitive configuration of the centring illustrated in Figure 2. In some variants which are not illustrated, the joint members can be fixed and can comprise, for example, a connection with bolted flanges.

Preferably, the first 151, second 152 and/or third 152 joint members are hinges. The structural elements which are connected by means of these joint members can therefore move from a first installation position of the centring 10, in which the structural elements are substantially folded one on the other, as can be seen in Figure 1, to a second definitive position of the centring 10, as can be seen in Figure 2, in which the structural elements are arranged so as to form at least one substantially continuous portion of the centring.

In a preferable though non-limiting manner, in the installation configuration of the centring 10, the second end 142 of the base structural element 14 is constructed in such a manner as to be slidingly connected to the body of the first lateral structural element 11. In this manner, there is also obtained assembled continuity between the structural elements of the centring 10 in the folded installation form illustrated in Figure 1. It is thereby particularly easy and advantageous to transport the centring 10 folded about itself in the installation configuration inside the excavation S, wherein the structural elements 11, 12, 13, 14 are joined together continuously without any of the ends thereof being freely variable.

Generally, in the definitive or virtually definitive form of the centring illustrated in Figure 2, the structural elements 11, 12, 13, 14 form a closed figure when viewed from the front. In this configuration, the first 161 and the second 162 support elements project from the closed figure of the centring 10 externally and laterally, that is to say, the two feet. The support elements 161, 162 can preferably be extended and can extend by an adjustable amount to be supported on the ground, in particular on the respective lateral planar zones M of the excavation S. As Figure 2 shows, the support element 161, on the left in the Figure, is elongate and is supported on the ground, while the support element 162, on the right in the Figure, is illustrated in a retracted position before being extended to be supported on the ground in the definitive configuration of the centring 10.

Figure 3 illustrates an embodiment of one of the support elements 161 or 162, that is to say, of one of the feet of the centring 10. The support element 161 or 162 comprises a fixed structure 16 and a movable structure 17 which is mounted in an extensible manner with respect to the fixed structure 16. The fixed structure 16 is fixed to a corresponding lateral structural element 11 or 13, preferably in the vicinity of the second end 112 or 132 thereof, respectively. The fixed structure 16 can be fixed to the lateral structural element 11 or 13 by means of any system of the known type, for example, by means of welding or with bolts or rivets or other connection members of the type. The fixed structure 16 is preferably though not necessarily made from the same metal material as the lateral structural element 11 or 13, such as, for example, construction steel (Fe 430 or the like). Preferably, the fixed structure 16 is constructed with a profile with an open cross-section, for example, H-shaped or C-shaped or double-T-shaped, for example, a European standardized profile, such as HEA, HEB, HEM, IPE, etc. Preferably, the fixed structure 16 is constructed with a profile having the same section and the same dimensions as the profile of the lateral structural element 11 or 13, as can clearly be seen in the example of Figure 3.

The movable structure 17 can preferably be constructed with a tubular body, for example, with a square or rectangular section, and in any case configured and dimensioned so as to slide linearly with respect to the fixed body 16, preferably telescopically, without any possibility of rotation. At the lower end of the movable structure 17, which is intended to be supported on the ground, a support plate 18 is fixed. The support plate 18 has dimensions greater than the section of the movable structure 17 in order to form a support base which is sufficiently extensive to distribute the weight over a relatively great surface of the lateral planar zone M of the excavation S. The support plate 18 preferably mainly extends towards the interior of the centring 10, that is to say, towards the end 112 or 132 of the structural element 11 or 13, respectively, where it forms a wing 19. Some reinforcements 20, also called connection plates, preferably of a planar metal sheet, are fixed, preferably welded, between the support plate 18 and the movable structure 17 in order to reinforce the connection between these two elements and to stiffen the support plate 18.

From the installation configuration which is shown in Figure 1 or with reference to the foot on the right, in Figure 2, the movable structure 17 can be extended with respect to the fixed structure 16 up to the definitive configuration illustrated with reference to the foot on the left in Figure 2. The extension of the movable structure 17 can be brought about with direct movement while the possibility of backward movement is prevented, that is to say, the foot being shortened. To this end, there may advantageously be provided a ratchet gear 21 which comprises, for example, a toothed rack 22 with saw teeth. In the teeth of the toothed rack 22 there is directed the end 23a of a flexible metal tongue 23 which acts as a latch. The toothed rack 22 can be fixed, for example, welded or screwed, to a side of the movable structure 16. The other end 23b of the metal tongue 23 can be fixed to the upper portion of the movable structure 17.

Figures 4 and 5 illustrate another embodiment of one of the support elements of the centring 10. In this case, the support element 161' or 162' comprises a structure 25 which is hinged in an oscillating manner about a horizontal rotation axis on the lateral structural element 11 or 13 by means of a pin 26. The structure 25 is preferably, though not necessarily, made from the same metal material as the lateral structural element 11 or 13, such as, for example, construction steel (Fe 430 or the like). The structure 25 comprises two walls 25a, 25b which are connected by cross members 27. Each wall 25a, 25b comprises two branches 28a,

28b which extend from the hinging zone of the pin 26 as far as a circle-arc-like element 29. A tooth arrangement 30 with saw teeth at the outermost edge of one or both of the circle- arc-like elements 29 contributes to the construction of a ratchet gear 31. The end of a flexible metal tongue 32, which acts as a latch and which is fixed to the structural element 11 or 13, is directed into the tooth arrangement 30.

From the installation configuration, which is depicted with broken lines in Figure 4, the structure 25 can be rotated about the horizontal axis of the pin 26 as far as the definitive configuration illustrated with solid lines in Figure 4. As a result of the ratchet gear 31, the rotation of the structure 25 is allowed with direct movement, that is to say, in the clockwise direction in Figure 4, while the possibility of backward movement, in the counter-clockwise direction in Figure 4, is prevented.

Advantageously, the joint members 151, 152, 153 for connecting the structural elements 11, 12, 13, 14 may comprise at least one engagement element which is associated with at least one resilient member. The resilient members of the joint members 151, 152, 153 allow a snap-fit joint to be constructed in the definitive position during installation of the centring. The snap-fit connection determines the precise relative position of each structural element with respect to the adjacent structural elements so as to construct a centring having a geometric and dimensional configuration which is substantially predetermined as intended, and stable and resistant to the thrusts applied thereto by the walls of the excavation S.

Figures 6 and 7 illustrate an example of the joint member 151 which is also similarly applicable to one of the other joint members 152 and/or 153 which connect the structural elements 11, 12, 13, 14 to each other in an articulated manner, as described above.

The joint member 151 illustrated in Figures 6 and 7 comprises two portions 151a, 151b which are hinged to each other, for example, by means of a pin 40, and which are fixed to one and the other of the ends 111, 121 of the structural elements 11, 12 to be connected, respectively. The first portion 151a comprises two walls 42a, 42b which are joined by a base wall 43 with two bored extension pieces 44, in which the pin 40 is inserted. The first portion 151a has a generally U-shaped or rectangular box-shaped section, as can be seen in Figure 7. The second portion 151b can be received in the first portion 151a. The second portion 151b can be constructed by means of a tubular extension piece 48 which projects from the end 121 of the structural element 12. The tubular extension piece 48 preferably has a square or quadrangular section with two lateral walls 49a, 49b which, in the definitive configuration illustrated in Figures 6 and 7, are beside the internal surface of the walls 42a, 42b of the first portion 151a.

Two respective holes 50 which, in the definitive configuration illustrated in Figures 6 and 7, are aligned with two corresponding holes 51 which are formed in the lateral walls 49a, 49b of the tubular extension piece 48, are formed in the walls 42a, 42b of the first portion 151a. These holes 50, 51 are passed through by two pegs 52 which are received inside the tubular extension piece 48 and which are pressed outwardly by a spring 54 which is contained in a guide tube 55. Initially, when the centring 10 is in the installation configuration of Figure 1, the pegs 52 can be retained inside the second portion 151b, for example, by means of two thin metal plates (not illustrated) which are welded externally to the two lateral walls 49a, 49b so as to cover the holes 51. By forcing the second portion 151b inside the first portion 151a, the metal plates which retain the pegs 52 are torn from the walls 42a, 42b in such a manner that the pegs 52, which are urged outwards by the spring 54, initially move into abutment against the internal surface of the walls 42a, 42b and subsequently, by continuing the relative rotation of the two portions 151a, 151b, they engage with the holes 50 in the walls 42a, 42b, definitively locking with a snap-fit and in a substantially irreversible manner the structural elements 11, 12 in the definitive configuration of the centring 10. The joint members 154 for connecting the base structural element 14 to the first lateral structural element 11 can also comprise at least one engagement element which is associated with at least one resilient member. This resilient member allows the construction of a snap-fit joint in the definitive position during installation of the centring. The snap-fit connection determines the precise relative position of the base structural element 14 with respect to the adjacent lateral structural element so as to construct a centring having a geometric and dimensional configuration which is substantially predetermined as intended and which is stable and resistant to the thrusts applied thereto by the walls of the excavation S.

As Figure 8 shows, an example of a joint member 154 for connecting the base structural element 14 to the first lateral structural element 11 comprises two portions 154a, 154b which are fixed to one and the other of the ends 112,

142 of the structural elements 11, 14 to be connected, respectively. The first portion 154a comprises two opposite walls 62 which are joined by a base wall 63. Two respective holes 64, which are preferably aligned coaxially, are formed in the walls 62. Generally, therefore, the first portion 154a has a U-shaped cross-section. As shown in Figure 9 which shows the joint member 154 in the connected condition, two recesses 61 project inwardly from the walls 62.

The second portion 154b can be received in the first portion 154a. As can also be seen in Figure 10, which shows only the second portion 154b when viewed from the opposite side with respect to the view of Figure 8, the second portion 154b may comprise two opposite walls 66 which project from the end 142 of the structural element 14. The walls 66 are joined by an end wall 65 which is fixed to the end 142 of the base structural element 14. Two respective holes 67 which are preferably coaxially aligned are formed in the walls 66. An engaging body 68 is mounted between the two walls 66. The engaging body 68 comprises two walls 69. The two walls 69 are joined by an upper wall 72. At the side opposite the upper wall 72, the two walls have an arcuate edge 69a, as can be clearly seen in Figures 11 and 12. Two respective holes 70, which are preferably coaxially aligned and each of which is coaxially aligned with the hole 67 which is formed in a respective wall 66 facing the wall 69, are formed in the walls 69. In a zone around the holes 70, the walls 69 comprise a portion 71 with a greater thickness which defines a step 73, against which, in the connected position illustrated in Figure 9, the recesses 61 of the first portion 154a of the joint member 154 move into abutment so as to prevent the movement apart thereof and detachment from the second portion 154b.

The holes 70 are passed through by two pegs 77 which are received inside the engaging body 68 and pressed outwardly by a spring 74 which can be contained in a preferable though non-limiting manner in a guide tube 75, in a manner substantially similar to what has been seen above with reference to the connection member 151.

Initially, when the centring 10 is in the installation configuration of Figure 1, the pegs 77 can be retained inside the engaging body 68, for example, by means of two thin metal plates 76 which are welded externally to the two walls 69 so as to cover the holes 70. By forcing the second portion 154b inside the first portion 154a, the metal plates 76 which retain the pegs 77 are torn from the walls 69 in such a manner that the pegs 77, which are urged outwards by the spring 74, initially move into abutment against the internal surface of the walls 62 of the first portion 154a and subsequently, by continuing the insertion of the second portion 154b into the first portion 154a, they engage with the holes 64 in the walls 62, definitively locking with a snap-fit and in a substantially irreversible manner the structural elements 11, 14 in the definitive configuration of the centring 10. This definitive position illustrated in Figures 9 and 11 being reached can be controlled by visually verifying through the holes 67 in the walls 66 of the second portion 154b that pegs 77 are in the correct engaging position in the holes 64 in the walls 62.

As described above, the base structural element 14 can be connected slidingly to the lateral structural element 11 before being brought into the blocked configuration of Figure 9. Figures 11 and 12 illustrate as a side view the two positions taken up by these two structural elements slightly before and slightly after the definitive configuration is reached, in a state locked by means of the joint member 154. In the example of the Figures, the lateral structural element 11 is constructed with an H-shaped beam which comprises two planar portions 80, 81 which are connected by a central web 82. There can be optionally fixed to the walls 66 of the second portion 154b two spikes 84 which project inwardly and which remain engaged under the planar portion 81 of the beam which constitutes the lateral structural element 11. The arcuate edge 69a of the walls 69 instead slides on the external surface of the planar portion 8. In this manner, the planar portion 81 acts as a type of linear guide for the end 142 of the base structural element 14 and in particular for the second portion 154b of the joint member 154. The second portion 154b can therefore slide on the planar portion 81 while the centring 10 moves from the installation configuration to the definitive configuration illustrated in Figure 12. The optional presence of the spikes 84 contributes to preventing the second portion 154b from sliding on the planar portion 81 without any risk of moving apart therefrom.

Figures 13 and 14 illustrate an example of a resilient joint 90 which can be inserted between two sections of a structural element 11, 12, 13 and/or 14 of the centring 10. In particular, the use of one or more resilient joints 90 allows the absorption, by reacting resiliently, of the thrusts which, as time passes, can be applied by the walls of the excavation S to the centring 10. The resilient joint 90 can be introduced into one or more of the structural elements 11, 12, 13, 14 even if it is more preferably introduced beforehand into the central structural element 12 and/or into both the lateral structural elements 11, 13. A plurality of resilient joints 90 can also be introduced into one or more of the structural elements.

Taking as a non-limiting example the central structural element 12 (as mentioned above, similar considerations can be made with reference to the other structural elements), there is inserted between the two portions 12a, 12b thereof a resilient joint 90 which comprises two portions 90a, 90b. The two portions 90a, 90b are connected to each other in a sliding manner, preferably in a telescopic manner, along a longitudinal axis, and are preferably blocked with respect to a mutual rotation about this longitudinal axis. The two portions 90a, 90b are preferably in the form of two cylindrical sleeves which are inserted one inside the other. In order to block the portions 90a, 90b in terms of mutual rotation, there may be provided at least one spike 91 which engages in a corresponding slot 92. There are preferably provided a pair of spikes 91 which are preferably arranged diametrically opposite and each of which engages in a corresponding slot 92. The at least one spike 91 can be mounted on the outermost sleeve 90b while the at least one slot 92 is formed on the innermost sleeve 90a, as can be seen in Figure 13. A first spring 93 which is supported against respective end walls 94a, 94b of the sleeves 90a, 90b is inserted inside the sleeves 90a, 90b. A second spring 95, which is shorter than the first spring 93, is mounted inside a tubular housing 96 which is fixed to one of the two end walls 94a, 94b of the sleeves 90a, 90b. In the extended condition of the resilient joint 90, which normally corresponds to the definitive configuration of the centring 10 which has just been mounted in the excavation S, the first spring 93 is supported against respective end walls 94a, 94b of the sleeves 90a, 90b and is compressed as soon as the two portions 12a, 12b of the central structural element 12 move towards each other following a compression thrust which is applied by the walls of the excavation S to the centring 10. As the compression gradually increases, the end walls 94a,

94b move together, compressing the first spring 93 which applies an increasingly great resilient force. If the compression further increases, the two walls 94a, 94b move together to such an extent as to also bring into action the second spring 95, the resilient thrust of which is added to the thrust of the first spring 93 so as to support a greater compression thrust by the walls of the excavation S on the centring 10.

Figure 15 illustrates as a perspective view a joint member 180 which can be used as an articulated connection variant between two structural elements of the centring as an alternative to the joint member illustrated in Figure 6. The joint member 180 can therefore serve to connect the structural elements 11, 12, 13 and/or 14 of the centring 10. In the Figure, there is taken, for example, the articulated connection between the lateral structural element 11 and the central structural element 12. At the first end 111 of the lateral structural element 11 there is fixed a transverse plate 181, from which a pair of mutually parallel external walls 182 project orthogonally. Similarly, at the first end 121 of the central structural element 12 there is fixed a transverse plate 183, from which a pair of mutually parallel internal walls 184 project orthogonally. The internal walls 184 are spaced apart from each other so as to be inserted between the external walls 182. In this manner, the external surface of each internal wall 184 is beside the internal surface of a corresponding external wall 182. A pin 185 extends transversely through both the external walls 182 and the internal walls 184 for the articulated joint of the lateral structural element 11 and the central structural element 12. The pin 185 may, on the one hand, have an enlarged head and, on the other hand, it may be fixed in position by known means, such as cotter pins, resilient rings and other generally known systems.

A resilient blocking member of the joint member 180 which provides for locking the structural elements in the definitive position of the centring 10 is fixed to one of the two structural elements, for example, the central structural element 12 in the non-limiting example of Figure 15. In the example of Figure 15, the resilient element is formed by a metal plate 186 which is fixed at one side, for example, by welding, to one of the two structural elements, in the example the central structural element 12.

As illustrated in Figure 16, in the non-blocked installation configuration of the joint member 180, the plate 186 is forced to flex in such a manner that the free end presses on the external walls 182 of the joint member 180. In particular, two end teeth 187 press against the edges 182' of the external walls 182 as a result of the resilience of the plate 186 which tends to take up a planar configuration.

There are formed in the edges 182' of the external walls 182 two respective notches 188, in which following a mutual rotation of the lateral structural element 11 and central structural element 12, the teeth 187 will become engaged in a snap-fitting manner, locking the joint member 180 in the definitive configuration of the centring 10 which is illustrated in Figure 17.

Figure 18 illustrates as a perspective view a joint member 190 which can be used as a variant of an articulated connection between two structural elements of the centring, as an alternative to the joint member illustrated in Figure 6 or in Figure 15. The joint member 190 can therefore serve to connect the structural elements 11, 12, 13 and/or 14 of the centring 10. The joint member 190 comprises a first transverse plate 191 which is fixed to an end of a structural element of the centring 10. Two mutually parallel external walls 192 project orthogonally from the first plate 191. The joint member 190 comprises a second transverse plate 193 which is fixed to an end of another structural element of the centring 10. Two mutually parallel internal walls 194 project orthogonally from the second plate 193. The internal walls

194 are spaced apart from each other so as to be inserted between the two external walls 192. In this manner, the external surface of each internal wall 194 is beside the internal surface of a corresponding external wall 192. A pin

195 extends transversely through both the external walls 192 and the internal walls 194 for the articulated joint of the two structural elements to which the first plate 191 and the second plate 193 are fixed. The pin 195 may have, on the one hand, an enlarged head and, on the other hand, it may be fixed in position by known means, such as cotter pins, resilient rings and other generally known systems.

Through-holes 196, preferably but in a non-limiting manner four of them in each wall, are formed in the external walls 192 and in the internal walls 194. The holes 196 in each external wall 192 are axially aligned with the holes in each internal wall 194 which are adjacent only in the definitive configuration of the structural elements which are connected by the joint member 190. This position corresponds to the one illustrated in Figure 18. In this position, the aligned holes

196 in each wall 192, 194 are engaged by a corresponding peg

197 which is pressed outwardly by a spring 198 which is inserted in a tube 199 which is mounted between the two internal walls 194 similarly to what has been seen above with reference to the snap-fit lock which is mounted at the end of the base structural element and which is illustrated in Figure 7. In order to prevent the pegs 197 from locking the joint member 190 in an angular position different from the definitive position illustrated in Figure 18, the four holes 196 in each wall are each arranged at a radial distance which is different from the centre of mutual rotation of the external walls 192 and internal walls 194 which is defined by the pin 195. In this manner, in any mutual angular position of the internal and external walls which is not the definitive position, the pegs 197 which are received in the holes 196 in the internal walls 194 strike against the internal surface of the external walls 192. Only when the joint member 190 reaches the definitive configuration do the holes 196 in the internal and external walls become aligned coaxially and can the pins 197, under the thrust of the springs 198, engage with the corresponding holes 196 in the external walls 192, blocking the mutual rotation of the structural elements which are fixed to the joint member 190. The centrings 10 are connected in situ to each other by means of connection members, also known in the field as "chains". Figure 19 illustrates an example of a connection member 200 between a centring 10 and an adjacent centring 10'. The connection member 200 comprises a bar 201 which is fixed at one end 202 thereof to the centring 10. There is provided on the centring 10, for example, a housing 203, in which the end 202 of the bar 201 which is blocked by a pin, spike or bolt 204, is inserted. The bar 201 projects from the centring 10 in a substantially transverse direction and terminates at the other side with a folded section 205 which is intended to be inserted in an eyelet 206 which is fixed to the other centring 10'. The folded section 205 is provided with a resilient locking member 207 which locks it in a snap-fitting manner in the eyelet 206, preventing it from leaving with a backward movement. The resilient locking member 207 is formed, for example, by a flexible metal tongue which is fixed to the end of the folded section 205, in the form of an arrow, as clearly illustrated in Figure 19.

Figure 20 illustrates another example of a connection member 400 between a centring 10 and an adjacent centring 10'. The connection member 400 comprises a bar 401, for example, but in a non-limiting manner, a quadrangular tubular bar such as the one illustrated in the Figure. The bar 401 is fixed in an oscillating manner at one end 402 thereof to the centring 10. To this end, there may be provided on the centring 10, for example, two walls 403, between which there is inserted the end 402 of the bar 401, which is passed through by a pin, spike or a bolt 404. The bar 401 projects from the centring 10 in a substantially transverse direction and terminates at the other side with a hook-like section 405 which is intended to be inserted in a slot 406 which is fixed to the other centring 10'. The hook-like section 405 is provided with an end tooth 407 which is locked in a snap-fitting manner on an oscillating plate 408 which is forced by a spring 409 which, for clarity of illustration, is shown in the Figure with reference to the centring 10 in a state provided with a similar slot for engaging with the preceding centring. A metal plate 410 urges the bar 401 in a direction counter to the insertion of the end tooth 407 into the slot 406.

In order to engage the centring 10 with the centring 10', the hook-like section 405 of the bar 401 is urged into the slot 406, acting counter to the thrust applied by the metal sheet 410 to the bar 401. The end tooth 407 raises the oscillating plate 408 acting counter to the thrust of the spring 409 until, the end tooth 407 being passed, the oscillating plate 408 snap-fits under the end tooth 407, locking the hook-like section 405 in the slot 406. The thrust of the metal sheet 410 on the bar 401 ensures the retention of the connection between the centring 10 and the centring 10'.

Figure 21 illustrates a variant of the centring 210 which differs from the centring described above in that the base structural element 214 is initially separated from the lateral structural elements 211, 213 and is fixed thereto only after they have taken up a substantially definitive configuration. The lateral structural elements 211, 213 are articulated to the central structural element 212 in a similar manner to that previously described, via the joint members 151, 152. The base structural element 214 is fixed by means of end connectors 220 which engage in seats 221 which are formed in the lateral structural elements 211, 213 near the support elements 161, 162. Figure 22 shows in greater detail an example of connection of an end 214' of the base structural element 214 which is provided with the end connector 220 to a lateral structural element, for example, the lateral structural element 211. The end of the lateral structural element 211 is configured to define the seat 221 of the end connector 220. In particular, the seat 221 comprises two lateral walls 222. Two respective through-holes 223 which are axially aligned with respect to a transverse axis are formed in the lateral walls 222. At the end of the lateral walls 222, there are provided two respective returning lips 224 which define a notch 225.

The end connector 220, which is also illustrated in Figure 23, comprises a neck 228 which projects from the end 214' of the base structural element 214. A ball joint 229 is fixed to the neck 228. The ball 230 of the ball joint carries two opposite pegs 231 which are urged outwards by a spring (not visible) which is received inside the ball 230. The ball joint 229 allows compensation for the rotations and torsions of the structural element 211 and particularly the seat 221 with respect to the base structural element 214. In the locked configuration of the end connector 220 in the seat 221, the neck 228 is inserted in the notch 225 and the pegs 231 are inserted in a snap-fitting manner in the through- holes 223 in the lateral walls 222. The structure of the end connector 220 and the seat 221 is repeated in a substantially specular manner at the other end of the structural element 214 for connection to the other lateral structural element 213.

Figure 24 shows in greater detail another example of connection of an end 214' of the base structural element 214 which is provided with an end connector 240 to a lateral structural element, for example, the lateral structural element 211. The end of the lateral structural element 211 is configured to define a seat 241 of the end connector 240. The seat 241 comprises two lateral walls 242. There are formed in the lateral walls 242 two respective through-holes 243 which are axially aligned with respect to a transverse axis. The lateral walls 242 have an end edge 244 which is preferably configured so as to be substantially vertical with respect to the definitive position of the lateral structural element 211. The end connector 240 comprises a plate 245 which defines two surfaces 246 which are orientated substantially parallel with the end edges 244 of the lateral walls 242. Two walls 247 which are connected by a wall 248 project orthogonally from the plate 245. At the opposite side to the wall 248, the walls have an arcuate edge 247a which facilitates the sliding on the upper portion of the lateral structural element 211, similarly to what is seen above in relation to the connection member illustrated in Figures 11 and 12. Two holes 249 which are axially aligned along a transverse axis are formed in the walls 247. There are inserted in the holes 249 two corresponding pegs 250 which are urged outwardly by a spring 251 which is arranged inside the walls 247. Before the end connector 240 engages with the seat 241, the pegs 250 are retained by disposable plates or bars 252 which are removed or discarded when the walls 247 are introduced inside the walls 242. After this introduction, the pegs 250 are free to engage with the holes 243 and thereby lock the connection. Advantageously, the sliding action of the edge 244 of the walls 242 on the surfaces 246, preferably in the vertical direction, substantially facilitates the connection and makes the connection easy, economical and simple to produce and to put in action.

Naturally, the connections illustrated in Figures 22, 23 and 24 may be applied equally well to the case in which the base structural element is articulated at one side to a lateral structural element, as illustrated in Figure 1, or in the case in which it is separated therefrom, as in the case of Figure 21. In one case, this involves bringing about the connection at a single end of the base structural element or at both ends.

Figure 25 illustrates a variant of the centring 310 which differs from the centrings described above in that the base structural element 14 is divided into two portions 14a, 14b. In the example illustrated in Figure 25, the two portions 14a, 14b of the base structural element 14 are each connected in an articulated manner to a respective second end 112, 132 of a lateral structural element 11, 13. In the example illustrated in Figure 25, therefore, there are present four articulated joint members 151, 152, 153, 153' while the two portions 14a, 14b of the base structural element 14 can be connected by means of a joint member 353 which is substantially identical to the one described and illustrated above with reference to Figures 8 to 12. Alternatively, the two portions 14a, 14b can be connected to each other by means of a joint member which is substantially identical to the one described and illustrated with reference to Figures 21 and 22. Naturally, it is not excluded that the connection of the two portions 14a, 14b may be brought about in any other manner, for example, with a conventional bolted connection system. Figure 25 illustrates the centring 310 both in the installation configuration with the structural elements folded and in the definitive open configuration for supporting the walls of the excavation S.

Now with reference to Figures 26 and 27, there can be arranged along the development of the centring 10 one or more monitoring and control devices. Preferably, the centring 10 may comprise more than one of these monitoring and control devices. Preferably, the monitoring and control devices can be arranged symmetrically with respect to the plane of symmetry P. Preferably, but in a non-limiting manner, the monitoring and control devices can be arranged adjacent to one or more of the joint members between the structural elements of the centring 10. The embodiment of these devices illustrated in Figures 26 and 27 is the one with hydraulic thrustors arranged between two sections of a centring structural element.

Figure 26 illustrates a detail of the centring 10 which shows a monitoring and control device 416 which could be interposed between a joint member 152' and the end of one of the structural elements, for example, the end 131 of the structural element 13. At the opposite side to the monitoring and control device 416, the joint member 152' is fixed to another structural element, for example, the structural element 12. The joint member 152' can comprise a pair of walls 418 which are fixed to an end of the structural element 12. The two walls 418 are articulated by means of a hinge 420 to a locking mechanism 422 of the joint member. Two holes, in which two respective spikes or pins 424 of the locking mechanism, which are urged outwardly by a resilient member (not illustrated), are formed in the walls 418. The locking mechanism 422 has a flange 425 for connection to the adjacent structural element or, as in the case illustrated in Figure 26, directly to the monitoring and control device 416. Naturally, a large number of variants are possible for constructing the joint member 152' between the two structural elements 12 and 13, for example, following the teachings of the already cited WO 2015/186029. The monitoring and control device 416 of Figure 26 is in the form of a hydraulic thrustor, as can better be seen in the longitudinal section of Figure 27. A piston 430 is fixed to the end 417 of the structural element 13. The piston 430 comprises a cylindrical member 432 which is preferably hollow and which is closed by a head 434 having a diameter which is slightly greater than that of the cylindrical member 432. At the external surface of the piston, a sealing ring 436 is mounted in a sliding manner. The sealing ring 436 comprises a flange portion 437 which is fixed to a flange 438 at the end of a liner 440 in which the piston 430 slides. The flanges 437 and 438 have a square form or in any case a form other than a circular form, in order to be connected in a form fitting manner to a cover 442 having a cross-section which is square or in any case similar to that of the flanges 437,

438. The cover 442 is fixed to the end 417 of the structural element 13 in such a manner that the form-fitting connection with respect to the flanges 437, 438 prevents the mutual rotation of the piston 430 and the liner 440 about the longitudinal axis X-X, thereby preventing a mutual rotation of the structural elements of the centring which are fixed to the two heads of the hydraulic thrustor. The useful travel of the hydraulic thrustor, that is to say, the travel of the piston 430 in the liner 440, may vary in accordance with the design requirements, the dimensions of the centring 10, the number of hydraulic thrustors which are mounted in total on the centring, and the characteristics of the excavation S, such as, for example, the geomorphological characteristics of the rock in which the excavation is being carried out. By way of example, a reasonable useful travel for hydraulic thrustors used in centrings for consolidating excavations of road or rail tunnels may be estimated at approximately 200 mm. There is defined between the liner 440 and the piston 430 a chamber 444 which communicates via a pipe 446 with a nozzle 448 for connection to a hydraulic circuit 450 for supplying the hydraulic thrustor with a pressurized fluid, for example, water. A manometer 452 which measures the pressure of the fluid in the hydraulic thrustor is inserted in the hydraulic circuit 450, which is schematically illustrated in Figure 27. A valve 454 allows a pressurized fluid to be supplied, for example, water, inside the chamber 444 or, conversely, fluid to be removed from the chamber 444, with a resultant shortening and movement together of the elements 12 and 13, thereby varying the overall diameter of the centring. Naturally, it is possible to provide variants for the hydraulic circuit 450 and the components thereof, for example, by providing pressure sensors in addition to or in place of the manometer 452 so as to transmit pressure data to an electronic system, for example, a data-processing centre or a server. The valve 454 may in turn be a solenoid valve which is controlled, for example, by the same electronic system which processes the pressure data.

Figure 28 illustrates a centring 10 which is a variant of the centring 10 which is illustrated and described above. Identical reference numerals correspond to elements identical to the ones previously described and to which this description will not return for the sake of brevity.

There are arranged along the development of the centring 10 one or more compensation devices 516 for the thrust which is applied by the walls of the excavation. These compensation devices may be resilient joints, for example, of the type described above and illustrated in Figures 13 and 14, which can be inserted between two sections of the structural elements of the centring. The use of one or more resilient joints allows the absorption, by reacting resiliently, of the thrusts which, as time passes, can be applied by the walls of the excavation to the centring.

Preferably, the centring 10 comprises more than one of these compensation devices. Preferably, the compensation devices 516 are arranged symmetrically with respect to the plane of symmetry P. In a preferable but non-limiting manner, the compensation devices 516 are arranged adjacent to one or more of the joint members 151, 152, 153, 154 between the structural elements of the centring 10''. The embodiment of these compensation devices 516 illustrated in Figures 28 to 30 is the one of compressible members which are arranged between two sections of a structural element of the centring.

The compensation devices 516 generally comprise one or more compressible elements in a longitudinal direction, that is to say, in the direction of extension of the structural elements of the centring 10' , which allow the longitudinal extension of the centring to be reduced and the force applied thereto by the excavation S to be reduced or eliminated. The compressible elements illustrated in the examples which follow can comprise one or more elements which are compressible resiliently or plastically. In the case of resilient compressibility, the compensation device progressively shortens under the load applied to the centring by the surrounding rocky wall, acting counter to a resilient force which is proportional to the shortening. In the case of plastic compressibility, however, the compensation device irreversibly gives way, collapsing under the load applied to the centring by the rocky wall when it reaches a predetermined threshold value. Naturally, it is possible for the compensation device to have a combined resilient/plastic behaviour or behaviour which is substantially resilient up to a load value beyond which the behaviour becomes substantially plastic.

Figure 29 illustrates a detail of the centring 10 which shows a first example of a compensation device 516' which is compressible in a substantially resilient or resilient/plastic manner.

The compensation device 516' is interposed between a joint member 153 and an end of one of the structural elements, for example, the structural element 14. The joint member 153 comprises two opposite spikes or pins 524 of a locking mechanism 522 which are urged outwardly by a resilient member (not illustrated). In the condition illustrated in Figure 29, the spikes or pins 524 are retained in a retracted position by two catches 520, for example, two bars which are welded to the body of the locking mechanism 522 and which are removed when there are engaged with the locking mechanism 522 a pair of walls (not illustrated) which are fixed to one end of a structural element of the centring and in which there are formed two holes, in which the two spikes or pins 524 engage. The locking mechanism 522 has a structure 525 for connection to the adjacent structural element or, as in the case illustrated in Figure 29, directly to the compensation device 516'. Naturally, a large number of variants are possible for constructing the joint member 153 between the two structural elements of the centring, as described above, which could, for example, be of the articulated type and in any case of a type constructed following the teachings of the already cited WO 2015/186029.

The compensation device 516' of Figure 29 is in the form of a compressible member, in particular a compressible member in a substantially resilient manner. Preferably, the compressible member has a prismatic form, in particular parallelepipedal or cylindrical and forms a compressible block 530, for example, made of a technical polymer which has the desired resilience properties. Such a compressible block 530 is received in a telescopic group 531. The telescopic group 531 particularly comprises a tubular body 532 which is fixed to the structure 525. A pressing member 534 which is fixed to the end of the structural element 14 can slide telescopically in the tubular body 532. The tubular body 532 and the pressing member 534 preferably have a polygonal cross- section, for example, square, which prevents the mutual rotation thereof about a longitudinal axis which is parallel with the direction of the telescopic sliding. The compressible block 530 is supported at one side against a base 536 of the tubular body 532 and, at the other side, against a head 538 of the pressing member 534. In a variant which is not illustrated, the pressing member does not have any head and is also tubular and the compressible block 530 is supported against a base thereof.

A projection 540 which is formed on the base 536 of the tubular body 532 can be engaged in a relevant seat in the compressible block 530 in order to keep it centred with respect to the tubular body 532. To this end, there can also be provided more than one projection, which projections engage with one or both of the ends of the compressible block 530.

Figure 30 illustrates a variant of the compensation device which is generally designated 516^,f. Identical reference numerals correspond to identical elements in Figures 29 and 30. A telescopic group 551 comprises a first tubular member 552 in which a second tubular member 554 can slide telescopically. There is arranged inside the telescopic group 551 a collapsible member 560 which is formed so as to give way, reducing its own length, when there is exceeded a predetermined load which is applied to the two opposite ends thereof which are supported on a base 552a of the first tubular member 552 and a base 554a of the second tubular member 554, respectively. The base 554a of the second tubular member extends externally, forming a flange 556 which acts as an abutment for the edge 558 of the first tubular member 552 when the telescopic group 551 reaches the minimal extension configuration thereof.

The collapsible member 560 is formed by a tube 561, preferably made of steel. There are formed in the lateral wall thereof a plurality of slots 562 which are elongate in the longitudinal direction, parallel with the axis of the tube 561. The slots 562 are regularly distributed circumferentially on the lateral wall of the tube 561 and constitute a predetermined weakening of the resistance to peak load of the tube 561.

When the centring 10 is arranged in the definitive configuration thereof with the structural elements connected to each other and covered by sprayed precoating concrete, the compensation devices 516 are in a configuration of minimal compression. If the rocky wall of the excavation S subjects the centring 10 to a load, the one or more compensation devices 516 react by becoming compressed, resiliently and/or plastically depending on the formation and the material used.

In the example of the compensation device 516' of Figure 29, the compressible block 530 is illustrated in a configuration of minimal compression, in which the telescopic group 531 is in an extended configuration. When the load on the structural elements connected to the ends of the compensation device 516' is increased, the tubular member 532 and the pressing member 534 compress between them the compressible block 530 which becomes resiliently shorter, thereby allowing the telescopic group 531 to shorten, reducing the extension of the centring 10 in the direction of the longitudinal extent thereof.

In the example of the compensation device 516 of Figure 30, the tube 561 is illustrated in a configuration of minimal compression, in which the telescopic group 551 is in an extended configuration. When a given compression value on the ends of the tube 561 is exceeded, the portions of lateral wall between the slots 562 collapse, completely or partially, bringing about a shortening of the tube 561 and therefore of the telescopic group 551 which can reach the configuration of minimal extension, in which the edge 558 of the first tubular member 552 moves into abutment against the flange 556 of the second tubular member 554.

The centrings described above, whether of the type illustrated in Figure 1, in Figure 21, in Figure 25 or in Figure 28, are transported to the site of the excavation S in the configuration in which the structural elements are folded in order to take up a compact configuration, the extent of which in the plane which contains the structural elements is less than the extent of the section of the excavation S. If the centring does not have any base structural element, as in the example of Figure 21, it can be transported to the site of the excavation S together with the centring 210, or provided after the positioning and definitive installation of the centring 210. In the case of the centring 10 of Figure 1, the transport and the installation are carried out with the second end portion 142 of the base structural element 14 which is remote from the second end portion 112 of the lateral structural element 11.

The support elements of the centring 161, 162 are preferably raised. The transport is therefore carried out readily and the elements which constitute the centring are already provided for correct positioning in the definitive configuration .

When the centring is in the position provided inside the excavation S, the central structural element is raised so that the other structural elements rotate at the articulated joints in order to be brought into an open configuration, as illustrated, for example, in Figure 2. With the central structural element arranged in the region of the upper wall of the excavation, the lateral structural elements adjacent thereto are beside the lateral wall of the excavation. The base structural element, whether it is in one piece as in the example of Figure 1 or in two portions as in the example of Figure 25, is lowered until being arranged in the region of the base of the excavation.

The lowering of the base structural element 14 is facilitated if it is slidingly secured to the lateral structural element, as in the example of Figure 1. In this position, it is possible to connect the second end of the base structural element 14 thereof to the second end of the lateral structural element 11 so as to stabilize the overall structural continuity of the centring. The lowering of the base structural element 14 can be facilitated by urging the lateral structural elements of the centring away from each other. Conversely, where necessary by urging the base structural element 14 downwards, it is possible to apply a widening thrust to the lateral structural elements. Similarly, the connection of the two portions 14a, 14b of the base structural element 14 of the example of Figure 25 or the insertion of the base element 214 between the lower ends of the two lateral structural elements in the example of Figure 21 also brings about a widening thrust thereon.

At the end of the relative rotation, the structural elements are fixed in the installation position by means of the joint members mentioned above. The construction of the snap-fit joint members ensures that the structural elements remain securely in the intended position desired for the complete installation without any risks that one may close on the other. The support elements are arranged laterally and low down with respect to the lateral walls of the excavation and are extended or rotated until they touch the ground. The installation of the centring is therefore carried out rapidly and substantially automatically.

Each centring is then connected to an adjacent centring by means of the connection members called "chains", two examples of which have been described with reference to Figures 19 and 20. The connection members are constructed so as to preferably allow automatic connection of one centring to another .

If there are provided one or more of the monitoring and control devices 416 described above by way of example with reference to Figures 26 and 27, when the centring 10 is arranged in the definitive configuration thereof, with the structural elements connected to each other, and covered by sprayed precoating concrete, the monitoring and control devices 416 allow checks on the support conditions of the excavation S and a reaction to any deformations of the rocky wall which might subject the centring 10 to an excessive load. Taking as an example the hydraulic thrustors described above, a pressurized fluid is first supplied, such as water, to the chamber 444 of each thrustor via the individual hydraulic circuit 450. The actuation of the hydraulic thrustors brings about the circumferential extension of the centring 10 which will thereby press against the wall of the excavation S. When the pressure of the fluid has reached a predetermined level, the supply of the fluid to the hydraulic thrustors is interrupted.

The pressure in the hydraulic thrustors is monitored constantly, where applicable using electronic systems. If the wall of the excavation is subjected to such deformations as to press on the centring 10, the pressure value measured by the monometer 452 increases. Before this value reaches a critical limit, it is possible to intervene in the hydraulic thrustors of the centring in order to restore the correct and secure support conditions of the excavation S in order to prevent the centring from becoming deformed to the point of risking collapse. In fact, it is possible to open the valve 454 in order to discharge a small amount of fluid from the chamber 444. In this manner, the thrustor shortens until it serves to reduce the pressure applied to the centring by the wall of the excavation S. In practice, the radius of the centring is slightly modified and is adapted to the restriction of the section of the excavation. In this manner, the function of safely supporting the centring 10 is restored.

The centring can be arranged so as to be controlled remotely. To this end, it is possible to use electronic pressure sensors which are connected - for example, via wires or wirelessly - to a local electronic control unit. The local electronic control unit can transmit a message to a remote node, such as, for example, but in a non-limiting manner, a server, a cluster of servers, a cloud service, so as to have immediate control of all the centrings positioned in a remote manner.

Naturally, it is possible to have variants with respect to those described above without this modifying the inventive concept claimed. For example, in some configurations of the centring, depending on the geological formation of the excavation, the lateral support members might not be necessary, that is to say, the feet of the centring, or it might be necessary to use only one rather than both of them.

The main inventive concept of the invention is not dependent on the type of profile of the cross-section used for the structural elements. Although profiles with an open cross- section have been mentioned above as being preferable, it is not excluded that the invention may be used with tubular profiles with which to completely or partially construct the overall profile of the centring and/or one or more of the structural elements thereof. In general terms, it is in any case possible to use different profiles for different portions of the centring, the possibility of also using different profiles for portions of structural elements not being excluded, with open and/or closed/tubular profiles which are separated from each other, for example, by the resilient joints or compensation devices which are described above.

Naturally, the principle of the invention remaining the same, the forms of embodiment and details of construction may be varied widely with respect to those described and illustrated without thereby departing from the scope of the present invention.

Claims

1. A centring for supporting and consolidating an excavation, comprising a plurality of movable structural elements (11,

12, 13, 14) which are connected to each other in such a manner that the centring can move from a preassembled configuration, at least partially folded before the installation, to a definitive installation configuration, in which the structural elements are locked with respect to each other in a mutual position which generally defines a centring which is at least formed in an arched manner.

2. A centring according to claim 1, wherein the structural elements are locked with structural continuity with respect to each other in a mutual position which generally defines a centring with the closed geometric profile, comprising the centring formed with a closed arched profile at the bottom by a structural element which acts as a strut or inverted arch.

3. A centring according to claim 1 or 2, wherein the structural elements are mounted so as to be articulated to each other by means of hinges.

4. A centring according to any one of the preceding claims, wherein a structural element which acts as a strut or inverted arch has an end which is mounted slidingly on another different structural element of the centring.

5. A centring according to any one of claims 1 to 3, wherein a structural element which acts as a strut or inverted arch has two end connectors for connection in seats which are constructed in two opposite lateral structural elements of the centring.

6. A centring according to any one of the preceding claims, wherein a structural element which acts as a strut or inverted arch comprises two portions which are articulated to two lateral structural elements, respectively, a joint member being provided in order to join the two portions in the definitive installation configuration of the centring.

7. A centring according to any one of the preceding claims, comprising at least three structural elements or at least four structural elements or at least five structural elements.

8. A centring for supporting and consolidating an excavation, comprising at least two elongate structural elements (11, 12, 13, 14) which are joined consecutively in the direction of the extent thereof in order to define at least one portion of a support arch of the wall of the excavation (S), the centring (10^,f) further comprising at least one compensation device (90, 516) for the thrust applied by the walls of the excavation, the compensation device (516) being movable between an extended configuration thereof and a reduced configuration thereof in the direction of the extent of the structural elements, at least one compressible member (530, 560) being provided in the compensation device in order to offer resistance to the movement of the compensation device (516) from the extended configuration to the reduced configuration .

9. A centring according to any one of the preceding claims, comprising at least one compensation device (90, 516) for the thrust applied by the walls of the excavation (S), which is interposed between two sections of the same structural element or between two contiguous structural elements.

10. A centring according to claim 8 or claim 9, wherein the compensation device (516) comprises a telescopic group (531, 551), in which the at least one compressible member (530,

560) is received.

11. A centring according to any one of claims 8 to 10, wherein the compressible member is a resiliently compressible member, the resistance of which to the movement of the compensation device (516) from the extended configuration to the reduced configuration is proportional to the extent of the compression thereof.

12. A centring according to claim 11, wherein the compressible member is a prismatic or cylindrical block (530) which is resiliently compressible and which is made from a resilient material, preferably a technical polymer.

13. A centring according to claim 11, wherein the compressible member includes at least one spring (93, 95).

14. A centring according to claim 13, comprising a first spring (93) and a second spring (95), the second spring (95) serving to reach a predetermined compression which is applied to the first spring (93) in order to add together the resilient thrust of both the springs (93, 95) so as to support a greater compression thrust by the walls of the excavation (S) on the centring (10).

15. A centring according to any one of claims 8 to 10, wherein the compressible member is a plastically compressible or collapsible member which acts counter to the movement of the compensation device (516) from the extended configuration to the reduced configuration until exceeding a threshold compression value, beyond which it becomes plastically deformed or collapses, offering a minimal resistance to the movement of the compensation device (516).

16. A centring according to claim 15, wherein the compressible member is a tube which is arranged in the compensation device (16) so as to be subjected to an axial compression load.

17. A centring according to claim 16, wherein the tube (561) comprises a plurality of slots (562) which are elongate in the longitudinal direction in a manner parallel with the axis of the tube (561) and which constitute a predetermined weakening of the peak load resistance of the tube (561).

18. A supporting centring according to claim 17, wherein the slots (562) are regularly distributed circumferentially on the lateral wall of the tube (561).

19. A centring for supporting and consolidating an excavation, comprising at least two elongate structural elements (11, 12, 13, 14) which are joined consecutively in the direction of the extent thereof in order to define at least one portion of a support arch of the wall of the excavation (S), the centring (10) further comprising at least one control device (416) for the thrust which is applied by the walls of the excavation, the control device (416) being connected to at least one pressure sensor (452) which is configured to measure a pressure value which is representative of the force applied by the structural elements to the monitoring and control device (416) following the thrust which is applied by the wall of the excavation to the arch portion.

20. A centring according to any one of the preceding claims, comprising at least one control device (416) for the thrust which is applied by the walls of the excavation (S), which control device (416) is interposed between two sections of the same structural element or between two contiguous structural elements.

21. A centring according to claim 19 or claim 20, wherein the monitoring and control device (416) comprises a hydraulic thrustor which is connected to a source of pressurized fluid and which is configured to act counter in an adjustable manner, by means of a hydraulic pressure which is measured by the pressure sensor (452), to the force applied by the elements of the structural elements to the control device (416).

22. A centring according to claim 21, wherein the hydraulic thrustor comprises a piston (430) which is connected to an end of one of the elongate structural elements (13) or a portion thereof and which is slidable in a fluid-tight manner inside a liner (430) which is connected to an end of another elongate structural element (12) or another portion of one of the elongate structural elements (13).

23. A centring according to any one of claims 19 to 22, comprising at least two control devices (416), preferably four or an even number of control devices, which are arranged symmetrically with respect to a vertical plane of symmetry (P) of the centring (10).

24. A centring according to any one of claims 19 to 23, comprising a connection joint (415) between at least two elongate structural elements (12, 13), the control device (416) being directly connected to the connection joint (415), at one side, and directly to the end of one of the two elongate structural elements (13), at the other side.

25. A method for installing a centring according to any one of the preceding claims inside an excavation (S), comprising the steps of:

- providing a plurality of structural elements (11, 12, 13,

14) of a centring (10);

- movably connecting the structural elements to each other in order to obtain a preassembled centring;

- folding at least partially the centring before the installation thereof;

- transporting the centring which is at least partially folded inside the excavation;

- installing the centring in a definitive installation configuration, locking the structural elements with structural continuity with respect to each other in a mutual position which generally defines an arch-like centring.

26. A method according to claim 25, comprising the final step of locking the structural elements with structural continuity with respect to each other in a mutual position which generally defines a centring with a closed-geometry profile.

27. A method according to claim 25 or claim 26, wherein the structural elements are locked with respect to each other in an irreversible manner.

28. A method for controlling the thrust which is applied by the wall of an excavation to a centring according to any one of claims 19 to 24, comprising the steps of:

- defining a threshold pressure value; - monitoring the pressure value measured by the at least one pressure sensor (452) which is connected to the control device (416);

- activating a control signal if the pressure value measured exceeds the threshold pressure value.

29. A method for controlling the thrust which is applied by the wall of an excavation to a centring according to claim 21 or claim 22, comprising the steps of:

- defining a minimum threshold pressure value and a maximum threshold pressure value;

- installing the centring behind the wall of the excavation;

- monitoring continuously or at predetermined intervals the pressure value measured by the at least one pressure sensor (452) which is connected to the control device (416);

- supplying a pressurized fluid to the hydraulic thrustor of the control device (416) if the measured pressure value is less than the minimum threshold pressure value;

- activating the discharge of fluid from the hydraulic thrustor if the measured pressure value exceeds the maximum threshold pressure value.

30. A method according to claim 28 or claim 29, wherein the pressure value is transmitted to a remote node for remote control of the thrust which is applied by the wall of an excavation to one or more centrings.