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FIELD OF THE INVENTION 
   The invention generally concerns road equipment and its installation. 
   More precisely, according to a first of its aspects, the invention concerns road equipment which, after having been put into service, is intended to cover a ditch and which has first and second main beams and a cover that has first and second parallel and opposite edges, the first and second main beams having respective first and second rabbets, respective first and second upright edges bordering the respective rabbets on the outside, and first and second horizontal flanges bordering the respective first and second upright edges on the outside, the main beams and the cover, in service, assuming definitive relative positions in which the cover is applied, by its first and second opposite edges, on the respective rabbets of the main beams arranged parallel to one another, and the main beams, in service, being buried in a layer of covering material extending over the flanges until its height hides the upright edges. 
   BACKGROUND 
   Road equipment of this type, although widely used for numerous years, poses considerable installation problems. 
   In effect, after having erected the parallel walls that delimit the ditch, it is necessary first to install side boards at regular intervals on the facing edges of the walls. 
   Then, quick-setting concrete pads are poured on each of the walls at the site of these side boards, so as to be able to level the upper surface of the walls. 
   After drying, the main beams are placed on the pads, and the cover is placed the main beams. 
   Each wall is then raised by another layer of concrete, so as to bury the base of the main beams. 
   A final layer of concrete or a layer of asphalt is then poured around the main beams, outside of the cover. 
   After drying, the cover is withdrawn, and the side boards and their holding elements are extracted. 
   Under these conditions, the installation of the known road elements is time-consuming and expensive. 
   SUMMARY OF THE INVENTION 
   The invention, which is situated in this context, aims to propose a road element whose installation, on the contrary, is rapid and simple. 
   For this purpose, the apparatus of the invention, includes main beams having respective first and second lower webs extending below the respective first and second rabbets, and which are at least drilled with respective first and second holes. This apparatus, before having been put in service, has an installation configuration in which it has at least a first bar passing through the respective first and second holes, which is attached to the first and second main beams and maintains the latter and a cover in fixed positions, and has at least three level-adjustment screws screwed into the flanges of the main beams and defining, for this apparatus configuration, a support polygon. 
   Preferably, this equipment has, in an installation configuration, a second bar, distant from the first bar, which passes through third and fourth holes respectively drilled in the first and second lower webs, which is attached to the first and second main beams, and which maintains the latter and the cover in fixed relative positions. 
   Each bar has, for example, first and second ends by which it is respectively attached to the first and second flanges by respective first and second attachment screws. 
   The flange and the rabbet of each main beam can be coplanar. 
   The road equipment of the invention can have a first pair of level-adjustment screws screwed in respective ends of the first flange, and a second pair of level-adjustment screws screwed in respective ends of the second flange. 
   For each attachment or adjustment screw engaged in a hole of a flange, a threaded sleeve can be inserted between the screw and the hole that receives it. 
   For each bar, this equipment can also include a holding bolt engaged in an opening of the cover and in a hole in this bar. 
   The equipment of the invention can moreover have carriers straddling the upright edge of each main beam and inserted between this main beam and the cover. 
   According to a second aspect, the invention relates to a process for installation of the apparatus over a cavity, in particular, a ditch delimited between first and second parallel walls. The process includes putting down the first and second main beams of the apparatus in an installation configuration on the first and second walls, the lower webs being inserted with slight play between the walls, manipulating the leveling screws to immobilize the cover at a predetermined level with respect to the walls, and depositing, on both sides of the main beams, a layer of covering material which covers the walls up to the lower webs, burying the leveling screws, and covering the flanges of the main beams until the material hides the upright edges. 
   This installation process itself advantageously includes, possibly after setting of the layer of covering material, detaching the cover from each bar and sawing off each bar in the vicinity of each lower web. 

   
     BRIEF DESCRIPTION OF DRAWINGS FIGURES 
     Other characteristics and advantages of the invention will emerge clearly from the description given for it in the following, in an indicative and nonlimiting way with reference to the appended drawings in which: 
       FIG. 1  is a view in partial cross section of a known device, represented after installation; 
       FIG. 2  is a plan view of the known device illustrated in  FIG. 1 ; 
       FIG. 3  is a plan view, on a larger scale than that of  FIG. 2 , of a device according to the invention; 
       FIG. 4  is a view in vertical and transverse section of the device illustrated in  FIG. 3 , taken along the line defined by arrows IV—IV of  FIG. 3 ; 
       FIG. 5  is a view in vertical and transverse section of the device illustrated in  FIG. 3 , observed exclusively in the plane passing through arrows IV—IV of  FIG. 3 , the dotted lines pointing to background elements that are hidden by visible homologous elements to which the solid lines point; and 
       FIG. 6  is a view in vertical and transverse section of the device illustrated in  FIG. 3 , observed exclusively in the plane passing through arrows VI—VI of  FIG. 3 , the dotted lines pointing to background elements which are hidden by visible homologous elements to which the solid lines point. 
   

   DETAILED DESCRIPTION 
   The invention relates to road apparatus which, after having been put in service, is intended to cover ditch K ( FIG. 1 ), this equipment having two main beams  1  and  2 , and cover  3  placed on the main beams by its parallel and opposite edges  31  and  32 . 
   Each of main beams  1  and  2  ( FIGS. 1 ,  4  and  5 ) has a rabbet such as  11  and  22 , an upright edge such as  13  and  24  bordering the rabbet on the outside, and a horizontal flange such as  15  and  26  bordering the corresponding upright edge,  13  and  24 , on the outside. 
   In a known manner, and as shown in  FIG. 1 , main beams  1 ,  2  and cover  3  adopt, in service, definitive relative positions in which cover  3  is applied, by its opposite edges  31  and  32 , on respective rabbets  11  and  22  of main beams  1  and  2 , arranged parallel to one another. 
   Furthermore, in service, main beams  1  and  2  are buried in layer C of a covering material, such as concrete or asphalt, which extends over flanges  15  and  26  until its height hides respective upright edges  13  and  24  of main beams  1  and  2 . 
   According to the invention, main beams  1  and  2  have respective lower webs  17  and  28  that extend below respective rabbets  11  and  22  and that are drilled with respective holes such as  171 ,  173 ,  282  and  284  ( FIG. 5 ). 
   Furthermore, the equipment according to the invention, before it is put in service, adopts an installation configuration that is represented in  FIGS. 3 to 6 , and in which it has a bar such as  41 , or two bars spaced apart from one another such as  41  and  42 . 
   Each bar passes through respective holes, namely  171  and  282  for bar  41  ( FIG. 4 ), and  173  and  284  for bar  42 , is attached to main beams  1  and  2 , and maintains these main beams and cover  3  in fixed relative positions. 
   Finally, as shown in  FIGS. 3 and 6 , this equipment has three or four level-adjustment screws, such as  51  to  54 , which are screwed in respective flanges  15  and  26  of main beams  1  and  2  and which define, for the equipment in definitive configuration, a support polygon. 
   For example, each of bars  41  and  42  has respective ends, such as  411 ,  412  for bar  41 , and  421 ,  422  for bar  42 , by which this bar is attached to flanges  15  and  26  by respective attachment screws. 
   Bar  41  is thus attached to flanges  15  and  26  by attachment screws  61  and  62 , while bar  42  is attached to flanges  15  and  26  by attachment screws  71  and  72 . 
   In order to facilitate attachment of each bar, it can be advisable, for each main beam and as shown in  FIGS. 4 to 6 , to make the flange such as  15  or  26  and the rabbet such as  11  or  22  coplanar. 
   In practice, this equipment preferably has two pairs of level-adjustment screws per cover  3  ( FIGS. 3 and 6 ). 
   Screws  51  and  52  of the first pair of adjustment screws are screwed in respective ends  151  and  152  of flange  15  of the first main beam, and screws  53  and  54  of the second pair of adjustment screws are screwed in respective ends  261  and  262  of flange  26  of the second main beam. 
   In order to avoid having to thread flanges  15  and  26 , it can be advantageous, as shown in  FIGS. 5 and 6 , to provide, for each screw such as  61 ,  62 ,  71  and  72 , threaded sleeve  8  that is inserted between the screw and hole  150  or  260  of the flange which receives it. 
   For each of bars  41  and  42  ( FIG. 5 ), this equipment has a holding bolt, such as  91  and  92 , which is engaged in an opening such as  301  and  302  of cover  3 , as well as a hole such as  410  and  420  made in this bar. 
   In order to dampen impacts during transport, the equipment of the invention can have carriers ( FIG. 3 ), such as  131 ,  132 ,  241  and  242 , straddling the upright edge of each main beam and inserted between this main beam and cover  3 . 
   The equipment which is described can be installed very quickly and easily over a cavity delimited between parallel walls M 1  and M 2  ( FIG. 4 ), for example, over ditch K. 
   The installation first includes an operation which consists of putting down main beams  1  and  2  of this equipment, in installation configuration, on respective walls M 1  and M 2 . 
   The distance between the walls and the width of the cover are such that in the course of this operation, lower webs  17  and  28  are inserted with slight play between walls M 1  and M 2 . 
   Leveling screws  51  to  54  are then individually screwed in or out or in order to fix cover  3  at a predetermined level with respect to walls M 1  and M 2 . 
   After this operation, the equipment is completely stable on walls M 1  and M 2  and is fixed in such a way that cover  3  is horizontal or inclined according to a desired slope, and is at the desired height. 
   A layer of covering material, such as concrete or asphalt, can then be deposited on both sides of main beams  1  and  2 , this material covering walls M 1  and M 2  up to lower webs  17  and  28 , which block it and prevent it from falling into cavity K. 
   The material thus deposited buries leveling screws  51  to  54 , and covers flanges  15  and  26  of main beams  1  and  2  until its height hides upright edges  13  and  24 . 
   After drying of the layer of covering material, it is possible to detach cover  3  from bars  41  and  42  by unscrewing screws  91  and  92 , and to saw off bars  41  and  42  in the vicinity of lower webs  17  and  28  of main beams  1  and  2 .

Summary:
An apparatus for covering a ditch includes a cover and girders having rabbets with projecting edges and flat-lying edges, the cover being placed on the rabbets. The apparatus has, before it is set up, a configuration for installation including one or several bars passing through lower webs of the girders and fixed to the girders and to the cover to maintain their final configuration, and level adjusting screws in edges of the girders defining a polygon providing stable support.