Process for obtaining hollow structures such as conduits, silos or shelters
The present invention relates to a new means of carrying out the process for obtaining hollow structures, such as conduits, silos or shelters, and to such hollow structures. 
     To improve the structural rigidity of such a hollow structure once it has been assembled on site, it is judicious to subject it to poststress, i.e., stressing following assembly, in order to secure the various shells constituting the structure to one another more closely, improve the leak-proofing of the assembly and make it possible to do without abutments at the location of the bends. 
     SUMMARY OF THE INVENTION 
     The process according to the invention is defined in that, when a certain length of hollow structure comprised of several longitudinal sections placed end to end is assembled, these sections are arranged on longitudinal supports intended for preventing the friction of the ground from acting against the base of the structure, when the structure is subjected to a prestressing operation by means of cables or rods arranged longitudinally, with relative displacement of the sections of the hollow structure relative to the supports. 
     According to other particular features of the invention: 
     (a) The longitudinal supports consists of ballasting or anchoring blocks or of longitudinal laying beams, 
     (b) After the length of hollow structure has been subjected to stress, the longitudinal supports are fastened positively to the base of the structure, in particular as the result of the screwing of threaded members from the interior of the structured, (c) Moveable members are provided between longitudinal supports and the sections of the structure, 
     (d) The moveable members may consist of any suitable material or shape, such as round iron concrete reinforcing bars, greased metal members or plastic sliding sheets. 
     On the other hand, in some large-size conduits, where the ground is particularly unstable, it has become apparent that the joints between adjacent elements do not make it possible to give the structure sufficient flexibility in the transverse direction. 
     It was found that it was possible to increase this transverse flexibility by the use of prefabricated elements which have, in cross-section, at least one thinned portion oriented parallel to the axis of the hollow structure which gives them flexibility. 
     These elements can be made of concrete constructed with reinforcing iron bars grouped in at least one continuous layer in cross-section from one edge of the element to the other, a single continuous layer in cross-section is provided, this layer located approximately at the center of each thinned portion. The unthinned portions may or may not incorporate other reinforcing iron bars. 
     It will be seen that such elements have a certain flexibility in the transverse direction, but that their rigidity in the longitudinal direction is practically unaffected. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the invention will be revealed in the rest of the description given by the way of non-limiting example, with reference to the attached drawings in which: 
     FIG. 1 is a perspective view showing a length of the hollow structure which is placed on supports; 
     FIG. 2 is an enlarged partial view of FIG. 1: 
     FIG. 3 is a sectional and perspective view of a prefabricated concrete element of conventional type; and 
     FIG. 4 is a similar view of a prefabricated element according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates, in a perspective view, a hollow structure of large cross-section, which is designated as a whole by SC, and which is composed of a plurality of sections, such as T1, T2, T3, themselves formed from several prefabricated elements designated by 1, 2, 3, 4, 5 for the element T1. 
     To improve the rigidity and cohesion of the hollow structure and of its component sections, the following procedure according to the invention is adopted: After a certain length of hollow structure, comprised of several longitudinal sections T1, T2, T3 etc. placed end to end, has been assembled, this length of structure is subjected to prestress by means of cables C placed in channels D arranged longitudinally in the thickness of the elements, as indicated in FIG. 2. It should be noted that this stressing of the structure is considered as a prestress before it is put into operation and as a poststress after it has been installed. However, if the sections were installed directly on the ground, they could not be displaced relative to one another because of the friction exerted by the ground. 
     Consequently, to prevent the tension of the cables from being rendered ineffective by the friction of the ground, longitudinal supports are arranged under the hollow structure beforehand, as indicated at L1 and L2 in the Figure. These longitudinal supports can consist of ballasting or anchoring blocks or longitudinal beams. 
     However, according to another characteristic of the invention, to reduce even further the friction exerted between the base of the hollow structure and the longitudinal supports, sliding or rolling members are arranged between the base and the supports, as indicated at 6, 6&#39;; 7, 7&#39;; 8, 8&#39;; 9, 9&#39;. For example, simple round iron concrete reinforcing bars can be used as rolling members. It is also possible to insert greased metal members or plastic sheets. 
     The prestressing cables can also be arranged longitudinally on the outside of the hollow structure, preferably outside the ballasting or anchoring blocks and the actual structure itself. It should be noted, moreover, that the ballasting or anchoring blocks can be replaced by longitudinal laying beams. 
     After the length of structure has been subjected to prestress, the ballasting or anchoring blocks or the longitudinal beams can be fastened positively to the base of the structure and/or in the ground, particularly by means of threaded members which are screwed from the interior of the structure. For this purpose, in the exemplary embodiments there are holes 10, 11, 10&#39;, 11&#39; for the passage of the threaded members through the structure, these holes being oblong so as to match the relative displacement of the sections T1, T2, T3, etc., when they are subjected to stress. 
     In all the foregoing, the description relates to longitudinal sections, placed end to end and subjected to prestress by means of cables or rods arranged longitudinally, are assembled. The invention can also be used when large-size structures, formed from several individual elements to form one cross-section, are subjected to prestress by means of cables or rods arranged in a transverse plane. In this case, the elements forming the lower portion of the structure have horizontal supporting surfaces carried, for example, by projecting shoulders and resting on supports intended for preventing or reducing friction on the ground. 
     The iron concrete bars, if used, are then arranged with their axis directed longitudinally. 
     If greased metal members or plastic sheets are used, they can serve to make it easier to put the structure under prestress both in one direction and in the other. 
     FIG. 3 shows a prefabricated element 4 made of reinforced concrete and prepared according to the standard technique, that is to say with two layers of reinforcing iron bars 41, 42 located respectively in the vicintiy of the inner surface and the outer surface of the element. It will be noted, moreover, that the longitudinal edges of the element are flattened or rounded, to make it possible to provide a junction allowing certain pivoting in the manner of a joint with the bed plate and/or the adjacent elements. 
     FIG. 4 shows element 4 modified according to the invention possessing thinned portions 43 which give it flexibility. The reinforcing iron bars 44 are grouped in a single layer which extends approximately at the center of each thinned portion. The unthinned portions can, if appropriate, incorporate other reinforcing iron bars. 
     The invention can of course be used when the elements are made of materials other than reinforced concrete. 
     FIG. 1 shows a structure, of which the lower element 3 of the section T1 consists of an element prefabricated in one piece. According to another embodiment, the lower element comprises two prefabricated pieces in the form of a gutter, and on these are mounted lateral elements, the lower edge of which positions itself in the gutter, thus making an articulated connection. Cross members which can likewise be prefabricated elements retain the gutter-shaped pieces, at the same time ensuring their strict positioning. It becomes easier to assemble the lateral elements because it is possible for these to pivot in the grooves. 
     The grooves and crossmembers are subsequently embedded in the concrete to constitute the bed plate, thus locking the articulated connections while at the same time ensuring leak-proofing. 
     According to another procedure of the invention, there are longitudinal beams L1, L2 which are each fixed to several longitudinal elements T1, T2, T3 end to end and which secure them to one another so as to prevent their relative movement. These longitudinal beams can extend over the entire length of the conduit or can have a length corresponding to 3 or 4 elements, in order to allow minor adjustments. Each interruption in the longitudinal beams corresponds to one element end. The left and right longitudinal beams can be interrupted at different locations. These longitudinal beams can be made of concrete and can be poured on the spot, after the installation of a length of conduit corresponding to several longitudinal elements end to end. If the elements are made of metal, the longitudinal beams are preferably of the same metal and are installed, for example, by being bolted together.