URUP, an abbreviation for Ultra Rapid Under-Pass, was originally intended as a method to bore tunnels passing under roadways or railways in order to technically differentiate Obayashi in the ever more competitive global and domestic tunnelling markets. Dr. Kanai questioned the necessity of access shafts designed for the purpose of launching and receiving TBMs. He set a target to reduce tunnelling costs and periods by 30 to 50 per cent compared to conventional methods which consist of shafts, tunnels and cut and cover sections, along with a 50 per cent or more reduction in carbon. In order to achieve this, he decided to eliminate both launching and receiving shafts at the same time by developing a boring system which allows operations to be undertaken with shallow cover and without causing ground disturbances.

Experimental Project
Obayashi initially carried out an experimental shaftless tunnel at their R&D Institute where a 100m long bore with cover ranging from 0.65m to 2.2m was constructed using a prototype rectangular shield equipped with an excavation width of 4.8m and height of 2.15m. Obayashi confirmed the feasibility of the system by developing a valid lining design under very shallow cover. Management of face stability and ground settlement control has been improved through accumulated knowledge and reapplied over actual projects.

During this experiment, ground settlement was successfully reduced to 10mm or less, even with cover as small as 0.65 to 2.2m. The use of hardened backfill material was also questioned around the circumferential and homogeneous portions. These were replaced with pressurised and fluidised excavated muck. This replacement was important to the stabilisation of the cutting face in the circumferential direction during TBM advancement.

Metropolitan Expressway Road Tunnel
In 2008 Obayashi was awarded to a design and build (D&B) contract for a 13.6m diameter road tunnel project using the system. The tunnel was driven through 895m of soil composed of very soft marine clay and loose sand with a cover of 0 to 25m. The maximum ground settlement observed was 5mm. The TBM started its outbound bore from the surface to a turning shaft over 550m away, it was then turned in the shaft and set on its return bore back to the surface in the opposite direction over a length of 345m.

Metropolitan Ring Road Tunnel
The second project was also a D&B for two 392m twin road tunnels with a cover of 0.6 to 4.6m. The TBM had a section 11.96m wide and 8.24m high. To accommodate the lining configuration, a hybrid-arch shape was chosen for economical design with an optimum thickness of 400 to 500mm. The geology at heading was a very stable loam soil for the upper half, and gravels and cobbles situated above the ground water table in the lower half. Therefore Obayashi selected an open type TBM with mechanical excavators. A monorail-mounted erector was newly developed for rapid assembly of the lining segments as well as for its reusability for any shaped tunnel sections (circular, rectangular or oval).

The TBM started its boring with a cover of 2.2m below a quiet residential area.

The TBM appeared at the first exit portal 400m away and then its direction was changed for the 400m return-trip. The thrust reaction of the supporting frame was rather small due to reduced thrust forces. Furthermore the distance between the twin tubes is only 200mm.

The TBM reappeared at the starting point after successfully boring under a cover of 1.2m with maximum ground settlement of only 10mm or less. The traffic above the tunnel was uninterrupted throughout the entire duration of tunnel construction work.

A single belt conveyor unit was implemented to transport the muck back through the first bore while excavating the return tunnel by enabling right angle turning belts, which simplifies conveyor belt start and stop control as well as its emergency control.

Slip road tunnel for new interchange
The third project was another D&B slip road tunnel for a new interchange composed of multiple 70m bores, each with a cover of 2.8m to 10m under a heavily congested highway. The ground settlement was only 3mm or less. The geology at heading was composed of compacted medium sand due to the existence of an old embankment.

In this project the experimental prototype TBM was re-used four times for bottom and side bores. For the top bores an open-face TBM was used due to the large amount and size obstructions encountered; such as steel sheet piles, concrete blocks and boulders.

The TBM first bored two bottom horizontal tunnels, and then two side vertical tunnels and finally two top horizontal tunnels to create space for the construction of an RC underpass lining, followed by removal of trapped soils from the tunnel and temporary steel lining segments.

Cross harbour gas pipe line tunnel
The forth project was another 1,080m D&B gas pipeline tunnel, crossing a bay with excavation diameter of 2.13m and cover of 0.8 to 23m. The ground settlement was only 5mm or less. Thanks to the omission of shafts, the project was completed five and a half months ahead of the original program. More importantly, the following work of gas pipe installation was also eased by an increased length of gas pipes and a reduced number of welds.

Essential technologies
URUP is based on the five following key elements supporting the system. These elements minimise disturbances to soils above a tunnel while maintaining high levels of productivity.

Additives
Firstly, it is important to select the optimum additive to ensure fluidity of excavated muck in relation to the particle-size distribution at the heading. Obayashi uses a variety of additives such as: mineral, surfactant (foam), water soluble polymer, and water absorbency resin depending on soil properties at heading. As additives are important in making the excavated muck plastic, the proper additive should be selected in terms of the soil and ground water level.

Fluidity
Secondly, it is important to monitor and maintain the fluidity of excavated muck in the pressure chamber in order to keep the cutting face stable. Although the state of fluidity is invisible, a remote sensing and data back analysis technology was invented which provides computerised visual output. In order to monitor the plastic flow continuously, rods with rotary vanes were installed in the pressure chamber. The rod rotates in the chamber as the shield advances, and the rotation torque is measured. The measured torque is immediately back-analysed, enabling real-time visualisation of the pressure chamber fluidity. It enables assessment of the area and degree of not-flowing solid muck in the chamber.

Based on visual information, the necessary volume of the additive will be injected into the most critical areas of the chamber from both the bulkhead and the cutting spoke to improve the fluidity. Additionally, a rotary agitator and fixed agitator rod are installed at the center of the TBM and at the back of cutting spokes and bulk head for optimised agitating. The most appropriate chamber configuration can be designed using this system, thus minimizing obstructions from lack of muck fluidity.

Face Pressure
Thirdly, another prerequisite is control and maintenance of face balancing pressure at both the front and around the circumference of the TBM.

Earth pressure theory will help to determine a provisional control pressure for face stability during initial advancement. In theory, the lateral pressure is normally separated into a hydrostatic pressure portion and an effective earth pressure portion. For face stability, earth pressure at rest should be incorporated in the face control pressure to minimise disturbance to soils above tunnel. In addition, it is always recommended to provide at least two trial advancement sections whereby soil deformations, movements, earth pressures and pore pressures are monitored to verify or modify the provisional control pressure.

The optimum balancing pressure in the chamber shall be specified by carefully modifying surplus pressure. In the experimental case as documented by Obayashi, a surface settlement of some 6mm at an initial stage was improved to only 3mm by slightly increasing the surplus pressure.

The TBM operator usually pays attention to pressures at the screw conveyor tip-toe for a smooth discharge of excavated muck in order to maintain a higher rate of production.

This often results in a fall of pressure control at the crown followed by ground subsidence once the TBM’s tail has passed. In an adequate operational situation, the actual face pressure at the crown level to the invert level is slightly higher than control pressure line, resulting in minimum ground settlement. This is Obayashi’s standard for EPB tunnel boring.

Backfill
Fourthly, in order to prevent ground settlement it is important to concurrently backfill circumferential void while boring. The technique consists of backfilling, by replacing the pressured fluid muck filling circumferential void by a watertight homogeneous self-hardened material as the TBM advances. Cementbentonite is normally used with a coagulant mixed in just before injection which hardens afterwards, forming a watertight layer.

Dr. Kanai prefers injection from lining segment to injection from the TBM tail plate, as long as it is injected simultaneously and concurrently with the advancement of the shield.

From experience gained on the 12m shield tunnel projects, no difference was observed between both types of backfilling method, resulting in a minimal ground settlement of 3 to 5mm throughout the 2,000m bore in sandy soils. Another noticeable finding is that a 20 to 40mm layer of consolidated muck proves a valid replacement of backfill material for fluid excavated muck when used in conjunction with a proper backfill material and with a proper backfill method.

From previous experience of slurry shields, back in 1984, where the same material and injection method were used, the ground settlement was also observed to be only 3 to 5mm.

In this project, an interesting observation was made whereby a 7 to 8mm mud cake of bentonite and sawdust mixed in slurry appeared in between hardened backfill and surrounding soil. It is assumed that the circumferential and homogeneous portions are replaced by self-hardened backfill materials from pressurised slurry for face stability during TBM advancement.

Rigid Joint and Water tightness
Additional attention should be paid to prevent deformation of assembled segmental lining and consolidation of surrounding soils due to water leakage through the lining. Obayashi has developed a bending rigidity controllable joint system for lining segments. While enabling a one pass assembly for a higher productivity it also includes technology ensuring watertightness of both segment bodies and segment joints.

Rigidity Controllable Joint
The bending rigidity controllable joint system for lining segments (called ‘horizontal cotter joint’) was invented by Obayashi. Where the soil around the tunnel is soft and the ground settlements is a critical issue, pre-stress forces can be introduced in the joint automatically during segment assembly work, resulting in minimum deformation of the lining.

The joint mechanism is characterised by high and equal rigidity both inward and outward bending moments which enable an economic design of joint bending rigidity control. Additionally, the joint system offers higher durability, increased water tightness due to pre-stress forces applied to a sealing material, and a smooth inner surface without recesses such as bolt boxes.

Therefore it enables sewer tunnels and flood reservoir tunnels to be designed without a secondary lining even though they undergo extreme internal water pressure. Furthermore, the joint consists of a one pass assembly system which enables rapid erection. This assembly system does not require bolt fixing works; therefore it reduces assembly cost and improves safety significantly.

HF-SFRC Lining Segment Steel fibre reinforced precast segment with highly fluid concrete were developed with simplified steel rebar caging in order to produce a watertight and durable lining. The highly fluid SFRC lining segment excels in tensile resistance as well as in compressive resistance, and also excels in bending/shear resistance and ductility.

The stress distribution effect by steel fibres reduces crack width and provides higher durability. In addition, the steel fibres are effective in reducing the spalling of the concrete lining, therefore it provides an improved performance in terms of serviceability.

Watertight tunnel lining
In Japan, hydrophilic polyurethane and hydroscopic chloroprene are used as typical seals in segment joints. These are water reacting and expanding materials for reliable and durable watertightness.

Obayashi has been studying a long term sealing behavior for both sealing materials for the last 13 years and intend to continue until the testing apparatus breaks. The test will highlight the differences in water reacting performance such as water reacting mechanism, water absorption speed, and water stopping mechanism.

The polyurethane type absorbs water if exposed to ingress and discharges water when it is dried, therefore it is reversible. The chloroprene type also absorbs water but it does not discharge water.

The durability of both materials is equal. These materials would damage segments if the swelling pressure is high, therefore attention should be paid to sealing design.

Damage Prevention to Lining Segment
The lining segments would be damaged by thrust jacks during excavation. The centroid eccentricity of the thrust jack induces a radial expansion force which is the major cause of lining damage. Obayashi has invented the friction cut plates which are attached to the jack spreaders.

The twin plates decrease the ring radical expansion force to the lining and dramatically reduce cracks in the lining.

Application of the joint system
The joint system was first applied to a large bore flood diversion tunnel with an external lining diameter of 11.8m under a hydrostatic pressure of 0.6Mpa.

The tunnel lining has to bear external earth and hydrostatic pressure constantly, as well as internal water pressure during flood diversion. The tunnel, without a secondly lining, achieved a long-desired watertightness even under hydrostatic pressure of as high as 0.6Mpa.

After the project, this joint system was applied to a lot of projects such as Road Tunnels, Utility Tunnels, Reservoir and Sewer Tunnels, and Metro and Railway Tunnels as well as URUP tunnels with various hydrostatic pressure.

Watertightness in all of the tunnels is quite well maintained and quite well controlled.

Dr. Kanai concluded his presentation with an emphasis on the importance of persistently believing in oneself. We have the ability to make new ideas into practical technology.

Questions from the floor
When you were driving your shield, did you use any bentonite injection to reduce friction?
We do not inject bentonite to reduce friction. It is only used to improve the fluidity of the excavated muck. We injected bentonite only at the front of the machine, not around the machine. But it is important to maintain the circumferential stability.

Are there any difficulties with steering rectangular shaped TBMs?
No. Not in our experience, we are always able to check that there are no problems. It can be applied to rectangular shape as well as circular shaped TBMs. Even if the machine is rolling, we can control it by manipulating the direction of the twin cutter rotation. If the pitching or rolling is of concern, I recommend adding extra weight to the erector to balance the forces. In addition the friction cut plates on jack spreader are effective for controlling rolling.

If you have a fire in your tunnel, how would you repair the damage to the important seals around the concrete segments, especially if they are subjected to hydro static pressure?
Generally, we use fire resistant panels or sprayed fire resistant materials that can be shot creating layer up to 50mm thick. Obayashi has developed a fire-resistant lining by mixing 0.2 per cent polypropylene in order to avoid explosive spalling of concrete. Based on the fire test results of the joint in the fireresistant segments, the temperature at the depth where the sealing material is embedded does not rise beyond the limit that compromises its integrity.

How many grouting points you have in the lining?
Each ring has two injection points. In Japan, many engineers prefer to inject from the tail plate, but I recommend injecting through the lining. It is because injection from the tail plate requires many pipes on the plate. It causes the deformation of the tail plate if the pressure is too high or constructing a tunnel of curved alignment. Furthermore, the deformation of the tail plate may cause lining segment damage.

What kind of design life does the sealing system allow for – what is the watertighness in the mentioned flood tunnels without a secondary lining?
There are two sealing systems, one is a gasket type and the other is a hydrophilic type. The hydrophilic type is more preferable for design as it allows for ground deformation due to seismic action thanks to its flexibility and water absorbing system. From experience gained from the Tokyo Trans Bay Highway Tunnel project where the rigidity controllable joint system was not used, only a small quantity of water leakages were observed. From experience of underground Aqueduct Tunnel project where the rigidity controllable joint system was adopted with same sealing material and under same hydrostatic pressure of 6bar, no water leakages were observed. The joint system is quite effective for watertightness. The design life of this system is 100 years.

How do you consider the risks when trying out the new technology?
We have not faced any problem in this technology to this date. If the ground settlement exceeds 5~10mm, we will repair roads and rail tracks by re-paving or infilling with ballast. During the development period, we had other mitigation measures such as side cutter and multiple boring systems. However they were quickly dismissed because we found the TBM can excavate without inducing settlement as long as fluidity is maintained amongst other criteria. Therefore I think the risk is technically the same level as another tunnelling method. In addition, Obayashi spends around GBp 100M (uSD 149M) per year on research and development and there are plans to further increase in the future.