<p>There is an opening (9) in the immersion plate (4) which can be closed by a shutter plate (14) to form a bulkhead between the excavation chamber (5) and the ring chamber (8). The closing edge of the shutter (14) has an attached seal which comprises a pair of hollow tubular (18) parallel inserts (17), and there is an opening at the bottom channel between these two inserts. There is another seal between the immersion plate and the shutter following the line (30) of the opening (9), and this seal is a hollow tube. ADVANTAGE - Maintenance and repair work in the ring chamber can be undertaken without risk.</p>
