A process for the construction of a segment of an open-air work by prefabricated structural members of reinforcing concrete, includes the steps of arranging first and second prefabricated structural members (18, 19) including an upright portion (22) and at least a cover portion (26), and the phase of installing the two structural members (18, 19) by resting the free ends (20) of the respective upright portions (22) on respective foundation portions (12), in such a manner that the two structural members (18, 19) are arranged symmetrically opposed. Each structural member (18, 19) has a cavity in correspondence of the respective cover portion (26), the cavities of the two structural members (18, 19) constituting a continuous channel (34) in order to allow to form a beam superimposed to the two members (18, 19) and fastened to them, by executing a casting (36) in said continuous channel (34) during the work.