of more than two partial elements the above stated principles of assembly are likewise applied see the left hand part of fig3 the external face of the preframe also comprises fig6 temporary reinforcements 54 and or anchorage sockets 55 with screwthreaded rods 59 and or metal plates 56 with dogs 57 and staples 60 which permits after the concrete has set and the prefabricated concrete has been deshuttered the placing of peripheral reinforcements 5 which will be attached to the temporary reinforcements 54 to the screwthreaded rods 59 or to the staples 60 this last operation is executed in the prefabrication factory or on site before the installation of the elements in order to achieve fluid tightness if required and continuity between the elements it is provided according to the invention to execute the second stage concreting 18 astride the joint 88 between two preframes with an overlap reinforcement 89 as shown in fig7 the fluid tightness possibly required between these elements is achieved by peripheral joints 160 of compressible material placed between the preframes fig2 and 7 generally speaking this type of compressible joint withstands the lateral water pressure only if the joint is compressed along the longitudinal axis of the tunnel before the second stage concreting 18 the assembly and.