in the drawings , f shows the excavation bottom and t the pipeline being formed . as appears from fig1 a , the excavation is of trapezoidal section , provided at at least one side with a service path p . the pipeline is formed by successively connecting pipe lengths together , head - against - head , by means of circumferentially continuous bead welds . the work schedule comprises checking operations concerning the bead welds , as well as a continual taping for sheathing the pipeline with a protective material . according to the invention , all operations pertaining to the work schedule are carried out on the excavation axis . to this end the pipeline is formed by successively introducing the additional pipe lengths into the excavation and connecting each of them with the completed pipeline end . the method in question is carried out using operating trolleys generically indicated by uo , travelling to and fro in the excavation , having also the function of movable supports for the pipeline end . together with the operating trolleys cooperate service vehicles um , traversing on the service path p . the driving units consist essentially in power generators , connected with the operating trolleys , with the aim of providing the trolleys with the necessary power required for their traversing , as well as for the operations or a part of the operations being carried out by the operating trolleys on the pipeline being formed . the service vehicles are advantageously provided with a motorized pump set , feeding the operating trolleys with pressurized hydraulic fluid , the operating units trolleys provided with hydraulic motors for their movement , as well as with operative auxiliary equipment . with reference to fig2 each service vehicle comprises a frame 10 and a driving seat 11 , a unit consisting in a thermic motor and a hydraulic pump , as referred to by 12 , as well as means for controlling the delivery of the pressurized fluid . the frame is installed on a tracked chassis which is self - propelled ( see fig3 ). the chassis is provided with hydrostatic traction means . the above said frame 10 can be mounted on the longitudinal center of the tracks , or laterally in the case that the service vehicle is also used for transporting pipe lengths , as shown in fig5 and 6 . fig4 is a front view of an operating trolley . it consists of a tracked chassis , including a frame 20 provided with hydraulic traction motors 21 , fed by a corresponding driving unit . the vehicle is provided with rollers 22 acting as a saddle for supporting , guiding and holding a pipeline length t ; said rollers 22 being advantageously supported by the ends of the stems on the pistons of hydraulic jacks 23 . according to the kind of work to be carried out , the operating trolleys are provided with the appropriate equipment . accordingly , as an instance , fig4 shows equipment consisting of electro - welders 24 and boom 25 for supporting the welding pliers . further equipment may consist of a sheathing machine as well as of conventional equipment for checking the welded beads . referring again to fig1 a diagram concerning the optional employment of the operating trolleys according to the invention will now be described . starting from the free end of the pipeline , on the right - hand side of the drawing , it can be seen that a pipe length t 0 is approached , inside the excavation , to the pipeline end , supported by an operating unit uo 1 , equipped with electro - welders 24 . to the rear of the joining section g a second operating unit uo 2 is placed , also equipped with electro - welders . the aforesaid trolleys are advantageously provided with four electro - welders , so that eight bead welds can be made according to a conventional method . the operating trolley uo 2 has also the aim of supporting the pipeline end and , due to the presence of the supporting rollers 22 and of the jacks 23 cooperating therewith , it can move forwards under the pipeline , varying both the height and inclination of the same , in order to exactly line up both pipe ends at the connecting section g . proceeding along the pipeline to the left , with reference to the drawing , to the rear of the operating trolley uo 2 , further operating trolleys uo 3 and uo 4 are arranged in order , one of them being provided with instruments for checking the welded seams , the other being provided with equipment for sheathing the pipeline with protection taping n . further operating trolleys follow , respectively uo 5 and uo 6 , hereafter termed &# 34 ; pipelayers &# 34 ;, having the function of supporting the pipeline and facilitating its laying down into the excavation , according to a progressively decreasing inclination . the last unit uo 6 is provided with a device 30 , of the scoop elevator type , for removing rubble out of the excavation bottom , before laying the pipeline thereon . accordingly , the pipeline is progressively formed by joining on to its end additional lengths such as t 0 - t 1 etc . and correspondingly advance the operating trolleys u 0 along the excavation bottom . for transporting the additional pipe lengths to the site , a service vehicle umt is employed , purposely equipped to that object . fig5 and 6 are schematic , detailed views of the structure of said service vehicle , consisting in a self - propelled tracked frame operated by a hydrostatic transmission and carrying a pump - generator set 12 on one side . at the opposite side a pair of grippers 40 are provided for grasping each additional pipe length . together with said grippers cooperates an opening jack 41 and a tilting jack 42 . the latter , by tilting the grippers around a fulcrum 43 , permits grasping of the pipe length and the laying down thereof , the length having been transported in a lifted position at the side of the motor pump 12 . the ends of the pipe length are machined while they are in said lifted position , in particular they are chamfered to facilitate connecting them together with welded seams . accordingly , the vehicle is equipped with spindle heads 44 - 45 , hydraulically operated by the motor pump 12 . the spindle heads are engaged with the respective ends of the pipe length , so that the chamfering operation can be carried out during transport . accordingly , the additional pipe lengths are unloaded at the site , ready for their immediate assembling . the transport service vehicle travels on the service path p , along the excavation and will be stopped in correspondence with the completed pipeline end . for laying each length into the excavation , a chute 50 is advantageously employed ( fig7 ), the end of which can be hooked at one side onto an operating trolley uo 6 stationed on the excavation bottom . the chute 50 is provided with slides 51 which are hydraulically operated for guiding and holding each pipe length so that , as soon as released by the grippers 40 , it is lowered on to the above - said stationary operating trolley . the latter may consist in a carriage , as shown in the drawing but preferably forms an operating unit equipped with electrowelding sets , in order to execute the connection of the additional pipe lengths to the completed pipeline end . it is also possible directly to articulate , by means of a pivot 52 , the chute 50 on to the operating trolley , the operating trolley then assuming the function of a carriage for unloading several additional lengths . fig7 shows such a modification , the carriage with the chute aforesaid being referred to by uos . a jack 53 is provided for producing oscillation of the chute and bringing the free end thereof into contact with the transport vehicle umt . the machinery as described above is completed by auxiliary service vehicles uma , as shown in fig8 and 8a . the vehicles in question are intended to travel to and fro on the side service path p , and are purposely equipped with a work cabin 60 supported by the end of an articulated arm 61 , subjected to the action of controlling hydraulic jacks so as to be able to make universal displacements in the vertical as well as in the lateral directions . the work cabin is provided with control levers for carrying out the aforesaid displacements , as desired . the auxiliary service vehicles will be used for corresponding inspection and / or finishing operations ; for instance , auxiliary welding operations by means of electrowelding sets 24 , permitting aso checking of the automatic operations as carried out by the operating trolleys on the excavation bottom , in a position of absolute safety for the operator . from the above description it will be clear that the method and apparatus according to the invention , besides permitting ( on account of the minimal bending to which the pipeline is subjected ) ready laying of pipelines of big diameter , also permit the operations in question to be carried out in conditions of absolute safety , the presence of workers in the excavation being excluded , each operating trolley being remote - controlled by the operator from the corresponding service vehicle stationed on the service path beside the excavation . furthermore , since the operating trolleys are not equipped with thermic engines or the like , their operations inside the excavation are possible under extreme conditions , especially in the presence of water , sand and the like , this making it possible greatly to simplify the preparation of the excavation by reducing the preventive tamping , drainage and reinforcement operations , with a noticeable reduction in costs . it will be understood that the principles of the invention , the performance details and the ways of carrying out the invention can widely vary in relation to the above description and the accompanying drawings , which are given by way of non - limitative example , without departing from the scope of the invention as defined in the appended claims .