The pipe roof method has increasingly been used as a supplementary method of tunnel excavation during the recent period of rapid economic growth in Japan. The pipe roof method has developed and has been adapted from opened face shield tunnelling and auger boring. Recently, the viable range for pipe roof construction by slurry pipe jacking has expanded, up to one case of a 150m-long jacking. Because of the rigid behaviour of the lining, the results show little subsidence, making this a highly reliable process for tunnel construction.

The use of underground space directly beneath buildings, highways and railways has become a focus due to the recent emphasis on effective use of underground space, as well as the elimination of regulations relating to underground use. Therefore the pipe roof method has been attracting the attention of engineers as a supplementary construction method. In particular, since slurry shield pipe jacking has been used with the pipe roof method, use of the method has increased sharply.

This method uses mud slurry that is formed around the pipes in order to stabilise the surrounding soil. In the pipe roof method, tubing elements constructed by slurry pipe jacking are near each other longitudinally. They are installed around the tunnel exactly, and create a rigid and stable lining before the excavation of the main tunnel. Thus, 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 the presence of ground water, limitations in surface settlement and also different depths of overburden.

Technology development
Pipe roofs are constructed along with the shapes of the main tunnel prior to the tunnel excavation. Figure 1, shows a schematic view of a pipe roof.

Generally, methods of pipe roof construction can be classified into four types: horizontal boring, auger boring, cutting edge and also slurry pipe jacking. Horizontal boring is viable for variable soil conditions, with a rotating cutter bit in front of steel pipe. A horizontal bore within the range of 80 to 300mm in diameter is typically used in Japan.

The first horizontally bored pipe roof was constructed for the Atami Tunnel, a Bullet Train (Shinkansen) tunnel project in the 1960s. A congested urban environment of hotels and roads meant that ground subsidence had to be avoided. A set of 30m-long, 84mm-diameter steel pipes formed the earth retaining wall. These steel pipes, and steel supports successfully prevented any subsidence from excavation.

In 1963, the first use of the pipe roof method by auger pipe jacking was undertaken to construct a pedestrian underpass. It was used as an auxiliary procedure; pre-bored steel pipes with interlinked steel sections were jacked. In the early 1970s, the diameter of the pipes used began to increase as the system became more popular and was adopted for larger tunnels. The auger method and cutting edge method were the leading systems in the early projects.

The pipe roof method is one of the best ways to protect surrounding ground or existing infrastructure. One problem that emerged was the difficulty to maintain the proper deviation and alignment on long distance drives. However, this kind of problem could be overcome following the later adoption of slurry jacking.

The first pipe roof executed by slurry pipe jacking was in Japan in 1993. This was some 30 years after the first (horizontal bore) pipe roof. This pipe roof construction was formed of 812mm o.d. steel pipes at 130m long. The project was for construction of a railway staion under a live railway and congested road. Under these conditions, a long distance pipe roof was requested as temporary works. It had to cut through a layer of mixed soil and required highly accurate pipe jacking to keep in alignment over the long distance.

Slurry pipe jacking is the preferred method if water is expected as there is no need for grouting or injection as there is with auger boring, for example.

A siphon culvert was carried out with slurry pipe roof construction under the Karasawa River in Japan in 1995. The geology was made of a gravel and cobble layer with a high water volume. Under such severe conditions, the auger or cutting edge methods would call for chemical grout injections to the excavation face area to stabilise the ground. In addition, efficiency would be uncertain and unreliable. Therefore, the slurry method is the most accurate and safe method among them.

Generally speaking, an arrival shaft is necessary for slurry pipe jacking. These are becoming difficult to build in increasingly congested city areas and auger pipe jacking has been substituted in. However, new slurry machines have been developed for pipe roof construction, which can be pulled back to the jacking shaft through the pipe upon completion, mitigating the need for an arrival shaft.

Following the development of a retrievable slurry machine, pipe roof use has expanded in Japan and long distance pipe roofs have become popular in underpass construction. Table 1 gives a chronological history of the pipe roof method in Japan.

Improving accuracy
Horizontal boring has no steering method and therefore tolerance cannot be guaranteed. Generally, this method has a tolerance of approximately 1/200 of driven length (Ishibashi et al, 1997).

Auger boring incorporates augers that remove the soil from the face and are not suitable for wet soil conditions below the water table. Drive lengths are restricted to about 50m because the driving device is located in the launching shaft. Typically, an auger method within the range of 300 to 600mm in diameter is used in Japan. It is difficult to ensure the accuracy in particular soils such as gravel or cobble layers, and the auger machine only has a simple steering device. Therefore specific tolerances cannot be guaranteed. Generally, this method gives a tolerance of approximately 1/300 of driven length (Ishibashi et al, 1997).

With the slurry type pipe jacking method, a range of 300 to 1,200mm in diameter is typically used in Japan. The machines are equipped for remote control and steering, and are operated by a central management system to ensure the required safety and accuracy in the construction work. Generally, this method yields a tolerance of approximately 1/2,000 of driven length.

Table 2 shows tolerance of construction. It is clear that slurry pipe jacking is superior to the other methods.

If the pipes are set with greater accuracies, the gaps between each constructed pipeline can be narrower. In order to analyse the effects on surrounding soil stability of the gap between each constructed pipeline, numerical analysis is used. In the trial, a 50m wide horseshoe shaped tunnel was modelled. The tunnel had an overburden of 6.5m and a height of 50m. The pipe roof was installed and consisted of a single row of jacked steel pipes. The pipe roof row was formed in an arch shape.

In the numerical models, the following assumptions were used:
1) two-dimensional plane-strain problem
2) the failure criterion of Mohr-Coulomb was selected
3) all the materials used in modelling were assumed to be the plastic material.

In the first stage of analysis the gap between each pipe of the roof and its effect on ground surface settlement was considered. This analysis was carried out with a gap set at a distance of 0.6m, 0.4m, 0.25m and 0.1m. Figure 2 (above) shows the steps of the analyses in this case. The gap between the pipe roofs and the tunnel crown in this step was 0.5m. The results show that the concentration of stress around the pipes reduces when the gap narrows. Surface settlement graphs showed an increase in settlement when the gap between each pipe roof widened. When the density of the pipe roofs around the tunnel crown is higher, stress distribution in the surrounding soil is better than when at a low density.

In the second stage of analysis, the gap between the pipe roof and the tunnel crown was examined. To vary the distance between the pipes and the tunnel crown, the pipe roof arch diameter was increased.The number of pipes in the arch remained fixed at 15, therefore as the arch diameter increased, the distance between the pipe roof and the tunnel crown increased, and the gap between each pipe increased (Shimida, Sato, Sasaoka, Matsui, 2010). The results show that an increased distance between the tunnel crown and the pipe roof leads to a greater settlement. A distance of 0.5m creates a settlement 1.4 times greater than a 0.1m gap.

The accuracy of pipe roof construction has a large impact on the stability of surrounding soil.

Soil stability

To compare the effects on soil stability of each jacking method, a three-dimensional numerical analysis was implemented.

For the cutting edge jacking method, the extremity of the cutting face is open. This allows direct access to the cutting face for manual excavation. Therefore, it is supposed that this method has an unsupported space of 1m in front of the cutting face for the analysis model. An EPB pipe jacking machine is a closed-face TBM. The excavated soil and material is transported from the machine chamber by a screw auger or screw conveyor. Pressure in the machine chamber is controlled by the rate of passage of excavated material through the balanced screw auger or valves on the screw conveyor. Therefore the analysis modelling of the earth pressure pipe jacking method considered that only extremity of machine has a pressure. In this study, its pressure was set at an earth pressure plus 20kPa for cutting face pressure.

The slurry pipe jacking machine is also a closed-face TBM. The excavated material is transported from the face suspended in slurry. Slurry pressure and forward thrust maintain the stability in the face.

Therefore the analytical model of slurry pipe jacking is assumed that a space of 1m in front of a machine and the circumference of the machine have a pressure. Its pressure was set at an earth pressure plus 20kPa. The earth pressure adopted for the model is set at the centre of the machine (Sasaoka, 2003).

Results show the largest settlement is caused by the cutting edge jacking method, the only open face method modelled. Second is EPB pipe jacking, and the smallest settlement is with slurry pipe jacking. It is considered that EPB has no pressure on the tail end of a machine, while the slurry machine is pressurised on all faces. The results show that slurry pipe jacking can prevent settlement by pressurising the cutting face and tail skin.

Tohoku Shinkansen pipe roof
The pipe roof method has a strong track record in Japan. In the Ichikawa tunnel on the Tohoku Shinkansen Line, it was necessary for the contractor to execute the supporting construction of a pipe roof to protect a highway from tunnel works. The earth cover was approximately 3m. The shallow earth cover meant a pipe roof was the preferred support method. Slurry pipe jacking was chosen because the 91m distance to cross the highway was too great for other jacking methods (Iigima et al, 2006). The pipe roof was installed and consisted of a single row of steel pipes jacked at 0.9m spacing in the crown of the tunnel top heading.

The roof was an arch shape. The outer diameter of the installed pipe was 0.8m. The distance between the pipe roof and the tunnel crown was 0.1m. Distance between the pipes was 0.1m. The number of the pipes was 17.

Due to the highway, careful work was demanded. Pipe roof works took about four months. Though the earth cover was 3m, the effect of the pipe roof meant the tunnel was able to be excavated with stability. The work was completed laying a new tunnel without damage to highway safety. The surface settlement of the highway after excavation was about 30 – 50mm.

Pipe roofing progress
As said previously, slurry pipe jacked roofs have been around in Japan for some years. In recent years long-distance pipe roof underpasses have been carried out. There is an increasing requirement for construction work to have no influence on surface traffic near heavily congested intersections. For the tunnel under the Chioharaguchi intersection, the pipe roof constructed is 150m long, longer thanany other in Japan (Shimada et al, 2009). Soon, a pipe roof constructed on the project will be approximately 200m long, on an underground ramp junction and through-lane.

The technology for pipe roofs is being advanced day by day and is being paid a great deal of attention as an advanced technology, as well as being adopted by many engineers.

This paper describes chronological development of the pipe roof method in Japan and the effect it can have on the stability of surrounding soil.

It was clear that the importance of accuracy in pipe setting and selection of pipe jacking method for pipe roof jacking is an important factor for effective ground stabilisation. Slurry pipe roof jacking was superior in the maintenance of ground stability. It also allows for various underground constructions.

Currently, there are many examples of this pipe jacking method in the world’s urbanised areas. It is expected that this technology will grow in popularity around the world in coming years.


Figure 1, Schematic view of the pipe roof method Table 1, a history of pipe roof construction in Japan Figure 2, steps of the first analysis Figure 3, numerical models were developed for different jacking methods Figure 4, cross section of tunnel and pipe roof Figure 5, slurry pipe roof execution accuracy