Shield-Driven Tunneling in Japan

Atsushi Koizumi

(Department of Science ＆ Engineering，Waseda University，Tokyo，Japan)

1 Introduction

The shield-driven tunneling method，whose patent was granted to M.I.Brunel in 1818 which is about 200 years ago，is still widely applied for tunneling in soft ground all over the world as an active service＇s tunneling method.The basic concept of the shield-driven tunneling method is composed of 1)providing a steel shell that is called“shield”in order to keep the stability at a cutting face of a tunnel so as to assure the safety tunneling operation;2)erecting the tunnel lining with segment pieces at the rear part inside of the shell;and 3)thrusting the shell by taking reaction force on the erected lining.Thus the key points of the shield-driven tunneling are pointed out as“How to keep the stability at the cutting face of a tunnel”，“How to erect tunnel linings with segment pieces”and“How to manage work in safety”.But according to these fundamental factors，the technology has not been altered since the first challenging of using shield-driven tunneling in the river bed of Thames River in 1825.The technological innovation had to wait until Japan appeared in full scale in the field of shield-driven tunneling.

It is known over the world that the remarkable advancement in the shield-driven tunneling method has been achieved during the high economic growth period in Japan.As for shield machine，various kinds of advanced shield such as slurry shield and earth pressure balanced one etc.were developed in order to keep the stability at the cutting face of a tunnel.As for segments of tunnel lining and safety operation by mechanization，the similar advancement was also achieved.

The author emphasizes that Japan has made continuous efforts for the shield-driven tunneling at the points of views of items 1)，2)and 3)mentioned above.Owing to the efforts，the shield-driven tunneling method is recognized as the most reliable to construct urban tunnels in soft ground.This is prevailed in the fields of sewers，water supply systems，subways，electric power grids，communication cables，traffic roads，gas pipes and regulating ponds etc..It is not an exaggeration to say Japanese shield-driven tunneling being at the top level in the world.

The author describes the background of the technological development in the shield-driven tunneling and explains the technological essences in detail in the later chapters.The main issues of this method which will be faced in the future are also mentioned in the last chapter.

2 Background of Prevalence of the Shield-Driven Tunneling in Japan

The most parts of Japan islands are occupied by many steep mountains and the areas which are available for people to live and to make business are limited to a few narrow alluvial plains.Mentioning to development and extent of towns and cities in Japan，a lot of parts of housing and business zones were apt to be constructed prior to making the plan of infrastructures.Moreover，the infrastructures were forced to be constructed in the congested conditions of traffic and business activities.

Before the shield-driven tunneling method was introduced into Japan，the open cut method or the cut and cover method was applied for tunneling in soft ground.In these methods，the depth of tunnel location is limited to shallow and a large ground subsidence to affect neighboring structures is induced in general.In addition，neither traffic nor the passengers’obstructions are avoided during the construction period.Therefore，in Japan there were many potential needs for the shield-driven tunneling method in order to reduce the public troubles in the urban areas.

The main reason that the technological advancement in the shield-driven tunneling had been expedited was due to the intensive increase in the amount of shield-driven tunneling construction in Japan.After World War II，to construct sewerage systems and sewer networks merged as one of the essential and urgent issue in Japan.Comparing with the open cut method or the cut and cover method，the shield-driven tunneling induces less surface subsidence and less traffic obstruction.This tunneling method was prevailed rapidly as the standard tunneling method for underground infrastructure of city areas in the soft ground at the middle of the 1960’s.The usage of the shield-driven tunneling method has been at peak from the 1970’s to the 1990’s in Japan.During this period，many infrastructure facilities such as subways，water supply systems，electric power tunnels，cable tunnels for the communication，gas pipe lines and under ground traffic roads etc.were constructed in urban areas introducing an innovative development and improvement on shield machines and tunnel lining as well as management system as described in lntroduction.

Japan is always subject to great earthquakes.Recently the Great East Japan Earthquake，magnitude 9.0，occurred on March 11，2011 and unforeseen tsunami disaster induced by the earthquake was received on the Pacific Ocean side of East Japan.Fortunately，no damage of the shield-driven tunnels by the seismic ground motion of this earthquake was reported.This fact proved that the shield-driven tunnels have seismic capacity and are superior to ordinary tunnels in seismic resistance because of the flexibility of segmental lining.From viewpoint of seismic capacity，the shield-driven tunneling is indispensable for construction of underground infrastructures in Japan.

3 Advancements Achieved in Shield-Driven Tunneling Method in Japan

The author points out that the great advancements as seen now in the shield-driven tunneling method are owing to severe designs and construction conditions such as soft ground in alluvial plain，many environmental requirements and difficult site locations for tunnel construction.Characteristics of alluvial strata are commonly weak in strength and high in groundwater level，so that it is difficult to keep self-standing at the cutting face of a tunnel.Because living and working populations are large and dense in urban areas，a large scale of excavation by the cut and cover method was not allowed from the points of views of keeping traffic safety for vehicles and passengers as well as saving the total costs of constructions.

As it is seen that sewer tunnels are used to be constructed beneath narrow roads，underground infrastructures are forced to be located in very limited space and to be designed as abnormal shapes and alignments.Shield-driven tunneling is often forced to be executed in the vicinity of the existing structures on the ground and/or in the underground.This situation requires difficult shield driving，i.e.，it is essential to drive a sharp-curved tunnel or to have a careful driving with the minimum affection to the existing structures in the vicinity of tunnels driven.

It is obvious that the severe restrictions and requirements mentioned above have contributed to the well-organized and sophisticated tunneling technology in Japan.The author describes hereunder the special achievements of the advancements in shield-driven tun-neling in Japan by dividing into the following three categories.They are 1)Development of Shield Machine;2)Development of Special Shield-Driven Tunneling;and 3)Design of Segments for Tunnel Lining.

3.1 Development of Shield Machine

3.1.1 Earth Pressure Balanced Shield(EPB Shield)

In the pneumatic shield，air pressures acting at the cutting face are uniform.Therefore air pressure distribution is not coincided with either the earth pressure one and/or the underground hydraulic pressure distribution at the cutting face，so the imbalance occurs among them.In the slurry shield，the cutting face is retained by slurry，but the balance between slurry pressures and earth pressures is not kept because the density of slurry is always smaller than the one of soils of ground.Due to these facts，the pneumatic shield and the slurry one can not be applied to very soft ground.

EPB shield was developed so as to improve these weak points in both of the types of the shield.In the chamber of this shield excavated soils are mixed up with muddy water and then compressed.So that pressures induced in the chamber can be easily controlled in order to encounter the earth pressure distributions at the cutting face of a tunnel during the excavation.In addition to the above，as the mixed up soils in the chamber can be kept in fluid state，the watertight function can be perfectly mobilized and excavated soils can be smoothly discharged.Contributed by these improvements，the EPB shield has been widely prevailed in Japan.

3.1.2 Articulated Shield

In Japan there are a lot of situations along the narrow road where road alignment is curved in an acute angle or bent in the right angle.In these situations，it is usually difficult to find out the optimal locations for turning shafts of a shield machine due to insufficient road space.Under the no shaft condition，the shield machine is forced to be driven along a sharp curved road alignment and it is required to make space around the cutting face for turning the direction of the shield machine.But in case of that the space is too large than scheduled and the shield machine makes turnabout forcibly，so the shield machine moves beyond the designated alignment and the unexpected large subsidence is induced on the ground surface.During the turnabout，eccentric loads are acted on the segments.Due to the acting eccentric loads，compressive destruction is caused at the edges of segments and joints of segments suffer serious damages.

In order to avoid these troubles，the articulated shield that was able to drive a sharp curved alignment was developed.This shield body is composed of the front part and the rear one in separated condition and the two parts are connected with articulation device.By using this device，the shield machine can achieve turnabout along ultra sharp curved alignment smoothly without any troubles mentioned above.At present，there is a successful construction result at the curve of 15 meters in radius by articulated shield with two articulated systems of 5 meters outer diameter.

3.1.3 Parent and Child Shield

This shield was developed in order to drive two different diameter tunnels by one shield machine.In the case that the station of a subway cannot be constructed by the cut and cover method，it is rational that the demand is merged to construct the station by the same shield used for the running tunnel.This shield machine is composed of two different diameter cutting faces.The smaller diameter shield is stored in the larger main shield body.After the completion of driving the lager diameter tunnel，the smaller shield is thrust out and starts to excavate the smaller diameter tunnel.

3.1.4 Large Section Shield

Replying on the technological reliability of the shield-driven tunneling cultivated through the construction of sewer networks，demands to drive large cross sectional tunnels for subways，traffic roads and storage ponds of runoff water were remarkably increased in the past three decades.The shield machine of 14 meters class diameter was used for the Tokyo Bay crossing tunnel，which was the largest cross section in the world at that time.This trail opened the era of large section shield.From this moment，road tunnels of 12 to 13 meters in diameter，tunnels of 12 meters class diameter for Shinkansen(Bullet trains)with the double-track lines，tunnels built for river diversion and for emergent storage of runoff water were driven by the large section shield.Now a tunnel of 16 meters class in diameter is planed for the Outer Traffic Loop in Metropolitan.

The technical key point of the large section shield is to strengthen the cutter spokes，the cutter face plate and bulkheads in the chamber in order to endure high earth pressures as well as high hydraulic pressures.This matter was solved by using high strength steel sup-plied by Japanese steel industries.

3.1.5 Multi-circular Cutting Face Shield

For traffic roads and subways，double-track tunnels are constructed in general.Space of a double track tunnel automatically becomes twice as a single-track one.In case that the located space is too narrow to drive the double-track tunnel，reduction of tunnel section is required in order to fit the given site space.The multi-circular cutting face shield was developed by aiming at reduction of tunnel sections.The shape of the multi-cutter face is double overlapped circular like a binoculars and the overlapped area is corresponding to the required reduction of the tunnel sections.As for the stabilization system at the cutting face，there are two types of the shields.The one is the slurry shield and the other one is the EPB shield.Now the triple-track tunnels have been constructed，too.

As a variation of this type of shield，a double ream type was developed.This can make turnabout from the horizontal direction into a perpendicular direction because of the two cutting faces.In this type of shield，the shield machine is separated into two independent shields during construction and after the separation two tunnels can be driven simultaneously.

3.1.6 Non-circular Cutting Face Shield

The cross section of a tunnel driven by a shield machine is generally circular because tunneling excavation at the cutter face is executed by rotating a set of cutter spokes.Many tunneling engineers have demanded to excavate non-circular cross section of a tunnel in order to minimize the cross section.The minimization of the tunnel cross section renders desirable merits which are that the shield can be driven within the limited underground space and the dead space in a tunnel can be decreased.Especially a large cross section has large dead space at the crown and the invert parts in a tunnel.In addition to these，the minimization of the cross section is able to reduce the amount of excavated soils.

In order to minimize the cross section，an oval section or a rectangular one is selected as the target shape.Moreover，a complex rotational system is introduced to excavate at the cutter face instead of a single rotation system.Those are to build in the parallel linkage function on cutter spokes，to combine special copy cutters，swing cutters and planet cutters，and to use boom cutters etc.These are all the result of develop-

ment of the non-circular cutting face shield.In category of the rectangular cross section，the horizontal double ream tunnels can be driven in Japan by the systems mentioned above.

3.2 Development of Special Shield-Driven Tunneling

3.2.1 Long Distance Shield Tunneling

It is known that the construction cost of the shaft increases rapidly as the depth of the tunnel becomes deeper.Therefore，it is necessary and rational for cost saving to reduce the number of the shaft and to drive a tunnel in a long distance with only one shield machine.When it is a long distance，ground conditions often tend to vary.This means that the excavating system and the stabilizing system at the cutting face should be managed properly in order to correspond variations of the subsoil conditions.

In order to achieve the long distance tunneling，two kinds of technology were developed.The one is to switch the shield machine type from slurry type to EPB one alternately corresponding to ground conditions.The other one is to change worn out cutter bits on the spokes from inside of the chamber by using mechanical systems.This method still continues to be improved and developed at present.

3.2.2 High-Speed Tunneling

As tunneling distance becomes longer，it is required to speed up tunneling for aiming at reduction of the construction period.It was conducted to shorten cycle-time for assembling segments of tunnel lining and to execute underground shield docking.As for shortening cycle-time for assembling segment pieces，development and improvement were conducted on the shapes and strength of segment pieces，joint structure for assembling segment pieces and erector to catch and assembly segment pieces.

3.2.3 Underground Docking

In order to shorten the period of shield driving，the technology was developed，that is two shield machines launch simultaneously from two shafts located at the both ends of the tunnel and the two shield machines connect at the middle point of the tunnel.This technology is called underground docking.Depending on this technology，tunneling period could be almost the half as the original case.For successful achievement in underground docking，it is very necessary to conduct the precise control on shield posture and shield forwarding direction，and an auxiliary construction such as ground freezing or mechanical supporting are very necessary.In case of Tokyo Bay crossing tunnel called Aqua Line，ground freezing was applied on the large scale and the underground docking was successfully achieved.

3.2.4 Launch and Arrival of Shield on the Ground or in Shallow Overburden

When the shield launch and shield arrival can be executed on the ground or in a shallow overburden condition，a shaft can be omitted or shortened，so the construction cost for the shaft can be reduced.Japan has many disadvantages for these operations in terms of the ground stability due to soft ground and high groundwater level conditions.Researches and developments have been conducted to overcome the disadvantages for a long time.

Recently，the first successful result was reported about a road tunnel project that the shield machine started from the ground and made turnabout in the reached shaft，and returned to the starting ground.The key point is a countermeasure by increasing self-weight of the tunnel by providing pre-cast road slabs in order to against buoyancy caused by groundwater.

3.2.5 Tunneling without Shaft

If one shield machine can dig a vertical tunnel up to the designated depth and switch to dig a horizontal tunnel，shafts for start and arrival are not needed.By using Multi-circular cutting face shield，this plan was achieved.The technological key point is to install two cutting faces inside of one shield machine.The one is for vertical excavation and the other one is for horizontal direction.

3.3 Design of Segments for Tunnel Lining

3.3.1 Segments and Joints

Up to now，many types of segments have been developed.The segment types are classified by the material used，such as reinforced concrete(RC)，steel，ductile iron and hybrid of steel plate and concrete slab.Steel segments are used for small to medium sized diameter tunnels，RC segments are for medium to large sized diameter and ductile iron segments or hybrid segments are mainly used in condition of eccentric loads or high intensity acting loads.

The typical trait of Japanese segments can be considered as joints of the segment pieces.The types of joints are roughly classified into three types as bolting joints，knuckle joints and interlocking joints.The bolting joints are the most conventional，the knuckle joints are easy to be set up and the interlocking joints are structurally sophisticated.The types of joints are selected according to the required strengths，segment assembling systems and workers’skill.Until now，more than 50 kinds of joint have been developed and are put to practical use.

3.3.2 Secondary Linings

Secondary linings in the shield-driven tunnel are constructed for meandering correction，waterproof，fireproof，surface finishing for primary lining and reserved strength of the tunnel lining.In sewer tunnels，surface finishing on the interior of the tunnels is essential to get the proper roughness coefficient and secondary linings are necessary.In cases of subways and power cables，however，secondary linings are apt to be omitted because shield driving can be executed with high accuracy and high quality water-seal is developed.

The author disapproved that secondary linings can be removed.Though no water leakage could be found in the tunnels with secondary linings constructed 40 years ago，a lot of lining’s deteriorations and water leaks have been noticed in the tunnel without secondary lining within 20 years after the construction.The water leakage can cause the corrosion of the steel material，concrete neutralization and deterioration of the natural ground conditions around the tunnel.In terms of shield tunnel’s durability，waterproof is one of the most important problems，so secondary linings should be strictly discussed considering its function as waterproof.

4 Further Tasks as Closing Word

The author has worked for the university and been related to the shield tunneling method in nearly four decades as a researcher，an educator，and an adviser.The majority of the shield-driven tunnels were constructed from the 1970’s to the 1990’s that was called the high economic growth period in Japan.The author’s four decades mostly overlapped this period.Therefore the contents described in the former chapters are mainly based on the experiences which have been seen or done by the author himself.Though a lot of advancements and innovations were achieved up to the present in Japan，the further tasks are still remained to be solved.They are described hereunder.

4.1 Technology

As for the shield-driven tunneling，one of the im-portant problems is to construct the junctions of traffic road tunnel for diverging vehicles.For diverging，ON/OFF lamp tunnels should be provided to parallel to the main tunnel in a long distance.The key point of construction of the junctions is to extend the transition zone connecting from the main tunnel to the branch ones.At the transition zone，a large cross sectional extension is required，of which shape is arbitrarily altered in longitudinal direction of the tunnel.The present construction method for this part is not sufficient in technology level due to applying with complicated procedures.Therefore，more improvement is required.The author points out this as the first issue among the further tasks.

Today，precise numerical analyses are available for structural stability evaluation of the tunnels.But evaluation on acting loads to the tunnels is still carried out by the conventional method and was not made any new progress in the past decades.The author points out as the second issue that research on the real acting loads should be conducted more with site in situ observation.

4.2 Management

In Japan，a huge amount of infrastructures are stocked.Stock management such as repairing，reinforcing and renewal has become very essential for existing shield-driven tunnels.For this purpose，the overall skim which is composed of the field investigation and tests on the linings，the analyses for safety evaluation of the tunnels，and the planning of maintenance including finance is required to be established.

4.3 Engineering Education and Engineers’Training

Now tunneling engineers must be in charge of the software services such as life-cycle management and asset management as well as hardware engineering in tunnel construction.Corresponding to these needs，tunnel engineers should learn knowledge about the humanities social science including economy，finance，system and financial management etc.plus natural science and civil engineering.The author proposes herein to conduct，so-called，the new engineering education including all the fields mentioned above.

The number of the skilled engineers has begun to decrease recently as the population decrease in Japan.Serious shortage of skillful engineers will occur in the near future.This tendency could aggravate the delay of the technology progress.In order to avoid this situation，the author stresses as the closing word in this article that the site education should be enhanced promptly for young engineers while the skillful and experienced engineers still exist at the jobsite.