Pipe roof constructed by slurry pipe jacking has been established as a non-disruptive pipe line repair technique. This method utilises mud slurry that is formed around the pipes in order to stabilise the surrounding soil.

Slurry pipe jacking

Mud slurry and lubricant are injected into the face and void between the concrete pipe and the soil respectively. Slurry fills the soil voids and the soil stabilises due to the resulting mud slurry around the pipes. In Japan, a slurry type pipe jacking method within the range of 300–1,000 mm in diameter is used.

The aim of using slurry pipe jacking with the pipe roof method is to extend tunnelling operation into urban areas in the most demanding conditions, such as non-cohesive soils, working in presence of ground water, limitations in surface settlement and different overburden depth. It is proposed that this be achieved through the use of the microtunnelling technique to build drifts, forming the lining of large tunnels.

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This article describes the application of a pipe jacking system on the pipe roof method, analysed using the finite element method (FEM) analysis program. Different positions of the pipe rooms were used in this study in order to examine the stability of the excavation and natural ground for the pipe roof method.

Pipe roof technology in Japan

Pipe roof refers to a roof matching the shape of the tunnel that is formed prior to tunnel excavation by installing steel pipes. Figure 1 shows a schematic view of the pipe roof method.

Auger boring incorporates augers that remove the soil from the bore and are not suitable for wet soil conditions below water level. The driving device is located in the launching shaft, which means the drive length is restricted. Drive length is about 50 m. In Japan 300–600 mm diameter is typically used. Photo 3 shows an auger method machine.

It is difficult to ensure accuracy for some soil types because the cutter, or auger, rotates in one direction. Specific tolerance cannot be guaranteed as the auger method has a simple steering direction. Generally, this method has a tolerance of approximately 1/300 of driven length [2].

Methods of pipe roof construction can be classified into three types; horizontal boring; auger; slurry type pipe jacking. Recently there has been an increase in the slurry jacking in pipe roof construction.

The horizontal boring method is used for variable soil conditions when installing pipes, with a rotating cutter bit in front of steel pipe. In Japan, the diameter range is 100–300 mm.

In the 1960s, the first example of pipe roof method, using horizontal boring method, was executed in the construction of the Atami Tunnel for the bullet train, Shinkansen. The steel pipe successfully controlled and prevented any subsidence from the excavation. A congested urban environment of hotels and roads meant that ground subsidence had to be avoided; this consideration lead to the decision to install the pipe roof.

Photo 1 and 2 show the view of the first example of pipe roof method [1].

Tolerance cannot be guaranteed with the horizontal boring method as there is no control device for directional steering. Generally, this method has a tolerance of approximately 1/200 of driven length [2].

Conclusions

Pipe roof method was superior in the maintenance of ground stability, and in Japan there are many results by this pipe jacking method, but there are few studies and theoretical examination. The study of pipe roof construction method needs to be studied further.