the tunnel under the seine is made up of five box beams, between 35 and 44 meters long, pre-assembled on the quai de javel (now the quai andré-citroën). this tunnel was dug using the method of wooden planks on top of a foundation made of chalk and shale. it contains a lining of iron positioned under a brick roof. the construction of the tunnel on the rive droite was more delicate because of its less solid alluvium, thus three additional beams were required which were assembled at mirabeau. the presence of a railroad line running from invalides to versailles (line c of the rer) made work particularly difficult, and a final beam was buried under the tunnel. work began in august 1907 but was not finished until 1913, also delayed by the 1910 great flood of paris.