the present invention relates to a method of detecting flexion in a structure . the present invention furthermore relates to a structure equipped with such a device , for example a geotechnical , mechanical , building or civil engineering structure etc . from ep - a - 0 264 622 it is known to measure a variation in length along a measuring line able to have a length of several meters , decameters or hectometers by means of an optical wave guide which is associated with the structure all along the measuring line . as is well - known , the tensile forces applied on the optical wave guide have the effect of dampening the light pulses sent through the guide in a manner which varies according to the intensity of these forces . by measuring the attenuation of the light pulses , a measurement of the elongation of the wave guide with respect to its nominal length is obtained indirectly from a characteristic curve . ep - a - 0 264 622 also reveals the pre - stressing the wave guide in tension in order to be able to also detect a compression of the structure along the line of detection , because of the reduction in the elongation of the guide with respect to its pre - elongated state when the structure is in the rest state . it is also known from ep - a - 0 264 622 how to arrange the measuring line along a path which is particularly likely to exhibit a significant variation in length in a predetermined direction under the effect of the load applied to the structure . the variations in length of the measuring line will be interpreted in terms of flexion of the structure or of the part of the structure in question . firstly , errors are possible in certain cases , in particular when the structure does not deform in the predicted manner . in such a case the interpretation of the measured elongation will be inexact . secondly , it is not always possible to place the measuring line in the position which would be the most favourable for the application of the known detection method . the purpose of the present invention is to overcome these disadvantages by proposing a detection method and devices which make it possible to determine the orientation and amplitude of flexion with remarkable reliability even on structures of which certain parts at least are very difficultly accessible , like structures that are partially or totally buried in the ground , and / or exposed to road or rail traffic etc . an idea which is at the basis of the invention consists in associating with the structure a model for which it is easy to predict the deformation modes and the effects of these deformations on a measurement line . the deformation of the model is evaluated and conclusions are drawn therefrom regarding the deformation of the structure or the part of the structure in question . thus , according to a first aspect of the invention , the method for detecting flexion in a structure is characterised in that there is coupled in flexion with the structure a long - shaped model having in cross - section at least one dimension which is sufficiently large that under the effect of flexion , the elongations are sufficiently different at the two ends of this dimension , at least one detection line having a length at least equal to about two meters being suitably positioned with respect to the cross - section of the model and / or with respect to the positioning of the model in the structure so that the variations in length detected by means of the measuring line can be interpreted in terms of direction and amplitude of the flexion of the model . the measuring line gives results which can be interpreted for the deformation of the model and these results can , in their turn , be interpreted in order to know the deformation of the structure itself , or for the section of structure concerned itself . two main lines of development of the idea which is at the basis of the invention and more particularly of the above method are envisaged . according to a first line , the model comprises a matrix which is heterogeneous with respect to the structure and it is stressed in flexion with the structure , and the detection line , judiciously placed in the matrix , undergoes variations of length which reveal the deformation in flexion of the matrix . for implementing this first line of development , it is possible to place the matrix in a groove in material bound to the structure , for example a groove formed in a face of the structure or a grove produced by means of a gutter or casing fixed to the structure . the matrix becomes deformed in flexion with the structure , and there is at least one detection line which is associated with the matrix and makes it possible to measure its deformation in flexion , and consequently that of the structure . according to another line of development of the idea which is at the basis of the invention , the model essentially comprises two detection lines which are associated with the deformation in flexion of the structure and which are sufficiently separated from one another for this flexion to cause them to undergo different variations in elongation . by comparatively analysing the variations in length undergone by the two detection lines , indications of the flexion of the structure can be derived therefrom . it is even possible to provide at least a third detection line spaced from the other two and not coplanar with them in order to obtain indications of the flexion in two planes forming an angle between them , in particular two perpendicular planes . the two lines of development are compatible : it is possible to provide two lines of detection separated from one another or even three line of detection which are not coplanar in the matrix according to the first line of development . furthermore , the number of detection lines is not limited to three , at least one fourth line can be provided , for example for corroboration purposes . the detection lines can in practice be materialised by pre - elongated optical wave guides cords . this technology is preferred because it provides very accurate measurements of the integral of the local deformations , without drift . but many other technologies could be envisaged ( measurement of mechanical tension or of electrical resistivity of a wire , etc .) provided that they allow “ long - based ” detection . the invention relates in fact to the field of measurement known as “ long - based ”. this expression indicates that the deformation of a structure or of a part of structure is perceived on a macroscopic scale over a distance which is of the order of one meter , one decameter or one hectometer , that is to say not locally or at a particular place such as can be achieved by strain gauges or inclinometers , nor simply between two separated points , but by the cumulative picking up of local phenomena over a section considered to be critical . in this way there is obtained an average of the deformations which is representative of the overall deformation at the place considered to be critical . according to a second aspect of the invention , the flexion detector device is characterised in that it comprises at least one line for detecting length variations , having a length of at least about two meters , closely bonded with a matrix intended to be coupled in flexion with the structure whose flexion is to be determined , the detection line being off - centred with respect to the cross - section of the matrix . according to a third aspect of the invention , the flexion detector device is characterised in that it comprises at least two length variation detection lines , having a length of at least about two meters , and disposed parallel with and closely bonded to a structure whose flexion is to be detected , and means of comparatively analysing the length variations undergone by the two detection lines . according to a fourth aspect of the invention , the latter also relates to a structure such as a geotechnical , civil engineering , building , mechanical and similar structure equipped with a device according to the second or third aspect . other features and advantages of the invention will furthermore emerge from the following description relating to non - limitative examples . fig1 is a diagrammatic perspective view , with a cross - section in two perpendicular planes , of an example of implementation of the invention to provide safety in the drilling of a tunnel under an existing railway ; fig2 is a diagrammatic view in elevation of the detector device shown in fig1 ; fig3 is a diagrammatic view in perspective with cross - section and tear - away illustrating the method according to the invention at the stage of positioning the model ; fig4 and 5 are two cross - selectional views relating to two variants of the device shown in fig1 and 2 ; fig6 is a view in cross - section of another embodiment of the device ; fig7 and 8 are two cross - sectional and perspective views of two other implementations of the invention ; fig9 is a view in longitudinal cross - section illustrating the implementation of the invention in the ground ahead of the working face of a tunnel in the process of being bored ; and fig1 is a view in transverse cross - section of the model used in the implementation shown in fig9 at a larger scale . in the example shown in fig1 a masonry bed 1 supports two parallel rail tracks 2 . the boring of a tunnel 3 orientated perpendicular to the rail tracks 2 is envisaged through the ground 4 which supports the bed 1 . the problem arises of knowing if and how the bed 1 will flex . a simple length measurement carried out on the upper surface of the bed 1 presents a risk of leading to errors of interpretation and furthermore the lower face of the bed 1 is totally inaccessible . according to the invention , there is hollowed into the upper face 6 , and therefore the accessible face of the bed 1 , a groove 7 having a rectangular or square profile for example of side length 30 cm . the groove 7 is oriented parallel with a vertical plane in which the curvature due to the main flexion expected will be observable . in the illustrated case , this direction is parallel with the rails 2 . then , as more precisely shown in fig3 there is placed in the bottom of the groove 7 a first detection line 8 extending all along the groove 7 and parallel with the latter . there is then placed in the groove 7 , at regular intervals , supports 9 and then a second detection line 11 which the supports 9 maintain substantially in the centre of the opening of the groove 7 , and therefore at a substantially maximum vertical distance from the first detection line 8 . the detection line 11 is also straight and parallel with the longitudinal direction of the groove 7 , and extends all along the latter , whose length can be several meters . the two detection lines 8 and 11 are preferably optical wave guide cords , such as described in ep - a - 0 264 622 . they are capable of detecting both a decrease and an increase in their length with respect to their initial pre - elongated state . there is then cast into the groove 7 a mass of substance 12 forming a matrix which on the one hand adheres to the walls of the groove 7 and on the other hand traps the detection lines 8 and 11 by adhering to them in order to form with the latter a model which will in service undergo flexions which are indicative of the flexion of the bed 1 . due to the separation el between the two measuring lines 8 and 11 in the plane of the cross - section of the groove 7 perpendicular to the axis 13 of the flexion to be detected , this flexion will cause different variations in length in the detection lines 8 and 11 . as shown in fig2 each detection line 8 and 11 , including an optical fibre as an optical wave guide , is associated at one of its ends with a light pulse transmitter 14 and , at its other end , with a respective receiver 16 detecting the damping of the pulses in the line 8 or 11 which is associated with it . calculating means 17 calculate the flexion of the model by analysis of the difference between the length variations of the two detection lines 8 and 11 . those skilled in the art know how to carry out such analysis based on the fact that for a given length of the two lines and a given separation e 1 between the two lines , each difference in elongation corresponds to a radius of curvature and consequently to a specific deflection of the model . in fig2 a negative deflection ( an upward bulging of the model ) has been shown in dotted and dashed line ; and in long dashed line there has been shown a positive deflection ( a downward flexion of the model ). according to the sense of the difference between the elongations of the two detection lines , the calculating means 17 can distinguish and indicate the sense of the flexion , from among the two senses which have just been described . taking account of the close bond between the model and the structure , that is to say the bed 1 , for which it is desired to know the deformation under flexion , the indications given by the means 17 with regard to the model can be transposed to represent the structure . even though the invention is mainly applicable to determining deflections , it is noted that the mean elongation along detection lines is also easy to calculate by taking the mean of the elongation of the two detection lines 8 and 11 . for the mass constituting the matrix , a material is chosen which is more easily deformable than the structure itself , in order that the model may follow the deformations of the structure without significantly participating in the strength of the structure and without exhibiting the risk of becoming detached from the structure when the latter is under load . a casting of cement , a resin , a rubber or an elastomer are suitable . it is more economical for the chosen material to be a filling of cement . in a way which is itself known by those skilled in the art , the filling will be formulated such that it is susceptible to micro - cracks . in this way it is ensured that the model has little mechanical strength of its own and consequently follows the movements of the structure well . for greater reliability it is also possible to place in the groove 7 , before casting the matrix in it , screens 18 which divide up the length of the groove 7 in a substantially fluid - tight manner . the matrix will therefore be subdivided into segments between which the screens 18 function like artificial cracks in the matrix . the screens 18 are made of a material , such as polytetrafluoroethylene , chosen such that at least one of their faces is non - adherent to the mass constituting the matrix . the embodiments shown in fig4 to 6 are only described where they differ with respect to the one shown in fig1 to 3 . in the example shown in fig4 the two detection lines 8 and 11 have been shifted towards one of the lateral walls of the groove 7 and a third detection line 19 has been placed in the vicinity of the angle between the bottom of the groove and the opposite lateral wall of the groove 7 . there is therefore , between the first and third detection lines 8 and 19 , a significant distance e 2 , approximately equal to e 1 but oriented perpendicular to e 1 . the detection lines 11 and 19 are thus situated on two perpendicular planes intersecting along the detection line 8 . by analysis of the difference between the length variation undergone by the detection lines 8 and 19 respectively , there is also obtained information on a possible component of deformation of the model by flexion about a vertical axis 21 . the invention thus makes it possible to determine , in orientation and amplitude , the complex flexions producing a buckling of the observed structure . there can even be provided a fourth detection line 22 , for example in the vicinity of the fourth longitudinal edge of the groove 7 , for the purpose of corroboration of the measurements and calculations taken and carried out on the basis of the other three lines . in the example shown in fig5 there is , on the contrary , only one detection line 8 , placed in a distinctly off - centred position in the matrix 12 , for example , as shown , in the same position as the detection line 8 shown in fig3 that is to say in the vicinity of the bottom of the groove 7 . this implementation can be envisaged when the direction of the flexion is known in advance and all that is necessary is to determine its amplitude . the amount of off - centring is chosen such that there is a distance between the detection line 8 and the zone called “ neutral fibre ”, of the matrix 12 , which will undergo neither extension nor compression under the effect of the expected flexion . instead of sinking a groove in the structure whose flexion it to be determined , it is possible to fix to the structure a member forming a gutter in which the mass 12 forming the matrix will be cast . this possibility is illustrated in fig6 in the case in which the structure 1 is a support wall in which a bulging is feared which can be observed in a horizontal plane . for this purpose , there is fixed against the apparent face 23 of the wall a profile 24 defining a groove 7 in conjunction with the face 23 of the wall . the two detection lines are this time disposed at a distance from one another in a same horizontal plane and they are as before trapped in the matrix 12 which is cast in the groove 7 and bonded with the walls defining the latter . in the examples of fig7 and 8 , the structure 31 whose flexion is to be determined is a body of elongated shape , namely a beam in fig7 and a pile or pillar in fig8 . the model consists of several detection lines 28 which are oriented parallel with the length of the structure 31 , suitably separated from one another and parallel with one another . to achieve this the lines 28 have been embedded at the time of manufacture of the structure 31 in the mass of the material constituting the structure 31 , at least a portion of this mass being able to be considered as constituting at the same time a matrix of the model , joining the two , or respectively , the three detection lines firmly with each other and with the structure 31 . in the example shown in fig8 the three detection lines 28 are placed at 120 ° from one another about the axis of the structure . this angularly symmetrical arrangement is favourable for avoiding to preselect certain flexion axes with respect to which the detection will be more accurate , to the detriment of the other axes , especially when it is a matter of a vertical structure like a pile or pillar which has a risk , a priori , of flexing in any direction whatsoever . on the contrary , in the beam shown in fig7 there are only two detection lines 28 , situated on either side of the axis 13 of the expected flexion . the detection lines 28 will , in both examples , undergo different variations in length when the structure 31 is subjected to flexion . in the example shown in fig9 and 10 , the model is shaped like a bar 33 which is driven into the ground or land 26 in order to couple it intimately with the latter at least as regards flexion . the body 32 of the bar 33 constitutes the matrix and has ( see fig1 ) a single off - centred detection line embedded therein , or two detection lines spaced from one another or even , as shown in fig1 , three detection lines 38 separated from one another in two different planes . the analysis of the differences between the length variations of the detection lines , or respectively the analysis of the length variations of the single detection line when it is known that the direction of flexion can be assumed , enable the direction and amplitude of the flexion of the model to be known , and consequently that of the land . in such an embodiment , it is convenient that the transmission and reception of light waves takes place at a same end of each detection line . fig9 and 10 illustrate this embodiment more particularly when applied to the prediction of collapses or of subsidence during the boring of a tunnel 27 . the bar 33 is driven into the working face of the tunnel in the process of being bored . the invention is not of course limited to the embodiments described and illustrated . the invention is not restricted to the detection of flexion in massive structures , it is also applicable to mechanical constructions . the preferred field of application of the invention is that of structures or parts of structures which are difficultly accessible because the invention then often constitutes the only technology which can be applied . but the invention can also compete favourably , because of its accuracy and its low cost , with other technologies in cases where the latter can be implemented physically . the invention is applicable to levelling measurements ( measurement of variations in level along an axis ) and to differential subsidence measurements , which are equivalent to levelling measurements along several axes .