space 6 and a ballastable chamber 7 and if necessary a counterweight compartment shown schematically as l the counterweights are intended to locally and temporarily compensate any tunnel weight insufficiency with respect to buoyancy the working space 6 is fluid tight peripherally and at the front in the direction of tunnel advance and it is connected to the portion of tunnel already built by a fluid tight tailskin 27 the working space is designed to accommodate personnel and everything necessary to at the least build the standard section to be built for example the standard section of tunnel is a ring made up of precast segments which are conveyed through the portion of tunnel already built to the working space from the bank or shore as required and the working space is equipped with appropriate means erector arms for example for grasping the segments and placing them so as to build an annular section these means may be similar to those used in terrestrial tunnel boring machines designed to build and erect the segments of a ring for a tunnel in a bored body of rock fig2 therefore shows merely a schematic representation of these means of construction and erection using a transverse pivoting arm 8 solidably mounted on a longitudinal support 8 u2032 and designed to grasp a segment of a section and place it in its service position to keep the figure uncluttered the stock of segments waiting to be grasped and erected is not shown it shows only one of the segments v of the ring t 7 in the cross section on fig2 in the construction.