With a population of almost 7,000 inhabitants per square kilometer, Hong Kong is one of the most densely populated areas on earth – and the population is growing. A consequence of the necessary continuous improvement of the infrastructure is that the chronic shortage of space also applies underground. It was no different with the 1.4-kilometer-long "Kai Tak Transfer Scheme" sewage tunnel, where the engineers had to think "around corners".
For the first time, a tunnel boring machine of the Mixshield type was used in Hong Kong's challenging and heterogeneous geology. The Mixshield S-199, equipped with a hard rock cutterhead, started out from an extremely tight shaft with short a short launch cavern in September 2002. At the start of tunnelling, it therefore had to be supplied through "umbilical cords", bridging the gap between the tunnel boring machine and the backup system.
The next challenge was waiting at 973 meters, where the tunnel alignment abruptly changed direction through 90 degrees. The supply hoses and cables were once again called into operation as the machine was split into segments and turned. Breakthrough was celebrated on February 26, 2004 with a successfull Mixshield premiere.