THE PRE-METRO LINK TO THE LEFT BANK OF THE RIVER SCHELDT BY MEANS OF A BENTONITE SHIELD --TUNNELLING 88. PAPERS PRESENTED AT THE 5TH INTERNATIONAL SYMPOSIUM, TUNNELLING 88, ORGAINZED BY THE INSTITUTION OF MINING AND METALLURGY, WITH THE COOPERATION OF THE BRITISH TUNNELLING SOCIETY, THE INSTITUTION OF MINING ENGINEERS AND TRRL, LONDON, APRIL 18-21, 1988

The decision to use a bentonite shield for the driving of the tunnels under the river scheldt was a result of eight years of successful experience with the shield method in antwerp. However, for the present project preliminary soil investigations, carried out in 1978, showed a geological profile different from that previously encountered. In addition to the sandy subsoil, the shield used for the construction of the scheldt tunnels had to cope with the boom clay layers and the presence of large loafshaped carbonate concretions, called septaria. This was the main reason for the consideration of a new concept and the adaptation of the shield. The cutting head was substantially modified in order to reduce the clay adhesion and to facilitate the crushing of the septaria. In addition five breast plates can support the face when driving is at a standstill. Shield equipment is provided with an installation which allows automatic recording at regular intervals of the most important data during the tunnelling works. After a 16 week initial tunnelling period at low progress with a prototype shield most of the difficulties were overcome. Subsequently progress was considerably increased. Shield passage under the quay wall section and under historically valuable buildings was carried out without substantial settlement. The improved working techniques and the increased slurry injection pressures have contributed to excellent results.(a) for the covering abstract of the symposium see IRRD 812521.