Building an Underground Tunnel to Protect Town from Flooding

SCENE

01

Installing Shaft by the Open Caisson Method

The shaft was to be used afterwards as a manhole, so the open caisson method was used, with a reinforced concrete cylinder 12m in diameter forced down hydraulically from the ground surface. The work required digging out and removing wet mud from the site, and there were fears that the wet mud would splash onto adjacent tennis courts, and so a 2m net was installed on top of a temporary 3m fence to prevent this.

SCENE

02

Disassembled Transportation and Reassembly of Tunneling Shield

The tunneling shield used for boring underground was cylindrical in shape, with an outer diameter of 4,530mm. It was 7,300mm long and 140ton in weight. As it could not be transported fully assembled, it was moved to the site in separate parts, consisting largely of the cutter head that digs out the tunnel, the drive assembly that moves the cutter head, a screw conveyor that shifts the displaced soil, a shield jack needed for forward progress and the front and rear that form the outer shell of the machine. These parts were lowered in turn down the vertical shaft using a 160ton rough terrain crane and were then assembled.

SCENE

03

Preventing Working Face Collapse as Tunneling Starts

When implementing the shield tunneling method, breaking wall as shield tunneling is launched is one of the most risky processes. Usually, the ground in which the machine is set to move is improved over a large area, as a method to prevent collapse of the working face (mud slurry) when breaking wall. But in this case the SEW method was employed, embedding FFU (rigid urethane resin + glass fiber) material that the machine cuts through into the side wall. By using the SEW method face collapse was prevented and also the scale of ground improvement could be reduced, thus shortening the work schedule.

Breaking wall is the preparatory task of creating an aperture in the side wall of the shaft for the tunneling shield to start operation.

SCENE

04

Moving Materials On and Off Site with Consideration for Local Traffic

The starting point for the tunneling shield was set up in one corner of a city park. The road facing it carried heavy traffic and care was needed controlling works vehicles because of access restrictions to the road from the site.
The vehicles accessing the site consisted of three large trailers for carrying reinforced concrete segments, one 10ton lorry to transport cement, 50 10ton dump trucks for removing the displaced soil as well as many other vehicles carrying goods and materials. A nearby location was reserved as a waiting zone so that these vehicles did not obstruct other traffic, and a contact system was set up. To keep displaced soil from spilling during transit, enclosed dump trucks were used. As a result, the work was completed without accident or any complaint.