GHOST TUNNEL BENEATH MOTORWAY.

The interesting detail of the tunnel structure was the connection between the diaphragm walls and the tunnel roof slab. The structural concrete of the diaphragm walls was only poured to a level of about 1. 8 m below the existing surface, while the top portion in the trench was made up with lean concrete. At some later stage the area of the tunnel roof was excavated, and appropriate blinding was placed to the exact level of the underside of the roof slab. At the same time the lean concrete topping was removed and the structural concrete and the protruding bars of the diaphragm walls were prepared for the subsequent roof concreting stage. Ventilation problems are discussed.