In the afternoon it was time to visit the Tokyo Electric Power Company (TEPCO). We were welcomed at a construction site between several high voltage pylons. With all fuzziness on construction sites, we had to take off our shoes and wear slippers in the construction shack. After Luuk told who we are and where we’re from, and all people from TASCO were introduced, the talk about a new tunnel for electric cables in the southeast of Tokyo could start.
The old tunnels between the Koto Substation and Eitabashi Substation are 51 year old with paintings instead of concrete to prevent ductile iron shields from corroding. The tunnels are full of cables (both high and low voltage), which makes it impossible to renovate them from the inside. It was therefore chosen to build a new tunnel for the high voltage cables. After these cables are in use, the old tunnel will be renovated and continues service for low voltage cables.
The new tunnel will run 6.2 kilometres before it connects to an underground man hole on 10.8 metres depth. The tunnels follows the street grid. There are other tunnels for sewage, water supply, telecom cables, electricity cables and subways crossing the route. This causes, in combination with the soil compositions, that the tunnel is built with a maximum depth of 53 metres.
There is inflammable gas inside the ground where the tunnel is built. This safety risk, together with the long distance and large depth make that it is explosion proof inside the shield machine and evacuation drills are conducted twice every year.
The tunnel has steep curves in both horizontal and vertical direction. Next to the tunnel itself, A departure shaft and two ventilation shafts are built. These structures are built with the open caisson method.
First, earth retaining walls are set. After the setting of reaction-use ground anchors, primary excavation is performed and mounding and a plate bearing block is installed. An repeating phase consists of setting the scaffolding, reinforcing bars and forms, casting concrete and pushing-in the lot and excavating. See an explanation video on https://www.youtube.com/watch?v=yPFrdOsTyI0.
After a presentation, we visit the site where the last lot was prepared for pushing-in. Noticeable was the high attention for safety. An large wall with instructions and information was present at the entrance of the site and a walking route was well separated from the working area.
The visit was finished with a nice group photo, with a special made plate in front of us.