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International Practices for Connecting One Pass Precast Segmental Tunnel Linings

Christophe Delus, Bruno JeanroyAnixter-Sofrasar, Sarreguemines, France

David R. KlugDavid R. Klug and Associates, Pittsburgh, Pennsylvania

ABSTRACT: use of one pass precast segmental tunnel linings has advanced throughout the world to the point where they are used in not only soft ground applications, but in mixed geology and hard rock tunnels. The paper will give a short history of the use of precast linings and the associated connectors used to connect the segments in the ring and ring to ring, beginning with curved steel “banana” bolts to the current industry trend of using high performance polymer plastic dowels and straight bolts with polymer plastic socket embeds.

The paper will review the tunnel lining practices being used throughout the world as various countries have developed practices and quality control requirements to meet specific geotechnical and national requirements.

HISTORICAL REMINDER

Up to 1930, TBM-driven tunnels were mainly lined using cast iron segments (Figure 1). Thereafter, pre-cast concrete segments tunnels lining started to appear, mainly in Great Britain, for small diameter tunnels (1.5 to 3.0m) driven in London clay for use as sewers.

Since that period, several hundred kilometers of generally small diameter tunnels driven in the London area have been lined with concrete segments of various shapes and types. Oftentimes they were ribbed. In other words, their shape stemmed from that of cast iron segments. It should be noted that, most of the time, these underground structures were built in very low permeability ground in which the excavated periphery offered short-term stability (London clay).

In time, British manufacturers offered a whole range of standard off-the-shelf tunnel lining seg-ments covering a wide range of diameters (1.5 to 6m internal diameters). One of the significant features of these segments was their small size and reduced weight (100 to 400 kg per segment) which resulted in a large number of ring elements for the largest diam-eter tunnels (12 segments per ring for a diameter of the order of 6.0m). The main reason for this large number of elements is that at that time the construc-tion process was not mechanized and only made by hand labor.

Since 1965, major developments in the use of concrete segments linings in Europe (Germany,

Austria, France, UK and Belgium) and Japan is note-worthy, in parallel with the development of TBMs for excavating large diameters tunnels (approximately 5.0 to 10.0m) in soft and water-bearing ground con-ditions. Specifically, mechanized erectors, larger size

Figure 1. Construction of the Tower Subway, London, 1869, using cast iron segments

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segments with very low precasting tolerances, elas-tomeric gaskets capable of guaranteeing lining water tightness even in heavily water-bearing ground and new connectors systems, have advanced this type of tunnel lining.

PURPOSES AND EVOLUTION OF THE CONNECTORS

First it is important to notice that there are two kinds of connections (Figure 2): the connections in the cir-cumferential joint, to connect one ring to the next. The other connection is in the radial joint, from one segment to another segment. Their purpose is not the same and the systems used in both joints can be dif-ferent one from each other. We have listed 5 main purposes for the assembly systems.

Purposes of Assembly Systems

Indeed, in the circumferential joint (connection ring to ring), their first purpose is to prevent any ring opening due to the gasket reaction load or to the pressure of the bentonite face slurry (in the case of a slurry TBM) when the TBM thrust cylinders are removed to install the next ring. Over time, this oper-ation evolved as in the past, segments were almost pre-stressed by the connectors, when nowadays they only keep the force applied by the erector. The prog-ress in the design of the erectors changed the philos-ophy and the parameters in how to design the assem-bly systems. Of course this feature is very important as it is linked to the sealing of the tunnel. In the radial joint, as a perfect ring is stable, their purpose is first to prevent any ovalization and to keep the ring in its original geometry when it comes out of the TBM tail shield and before the annulus back-grouting process.

One of the other main purposes of the assem-bly systems is to provide a good erection accuracy to prevent any offset between the segments and the rings. This is very important as it has significant influence on the water tightness, the more the offsets are reduced, the better the sealing gasket will work. We will see that with time, some specific systems

were developed to ensure a high accuracy in the installation for both joints ensuring stability at the ring building stage even when no load is exerted by the TBM thrust cylinders.

The linking systems purpose is not limited to the construction stage. For example, in water con-veyance projects, when the tunnel is put under inter-nal hydrostatic pressure by the water, they have to prevent any deformations or openings of the ring. In this specific case, their purpose is still to keep the gasket compressed, as in the case of a primary lin-ing, the gasket must have double action capabilities to prevent the ground water from coming inside the tunnel and preventing any polluted water to migrate in the ground outside of the lining. In some seismic areas, the connectors can also have specific features to allow the lining to deform with the earthquake and avoid any breaking of the lining.

In general, circumferential assembly systems are regularly spaced around the ring. Their number varies from one project to another depending on:

• The force to be balanced (reaction load of the gasket)

• The desired possibilities for relative rotation of a ring with respect to the last one installed (universal ring)

The number of connectors in the radial joint may vary from one to three elements (combination of different type of connectors) depending on the length of the ring and the type of connector used. In standard international ring design, the connectors are only designed for the purpose in the construction stage (except in specific cases, like described previ-ously for the water conveyance or seismic areas or in North America), therefore it is now standard in con-tinental Europe that the bolts are removed when the TBM is about 150 meters away and all the grouting operations have been completed. If the connectors are designed for a permanent use, it is important that they are designed with material able to provide the same durability as that of the structure itself.

Evolution of Assembly Systems

In 1869, the first subway was built in London, using cast iron segments. These segments were linked using standard straight bolts and nuts. The first con-crete segments were produced in the ’30s using the same design (with hollow and ribbed). The connec-tion systems and design of the segments evolved from this start.

Straight Bolts

This system was the first to be used with concrete segments, there was no significant innovation from

Figure 2. Segment joints

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the cast iron segments. The main issue is safety as the result of the significant number of bolts. Indeed numerous human operations are needed below the erected segment to connect the lining. Due to the nar-row concrete thickness next to the bearing surface a specific reinforcement must also be considered.

Straight Bolts with Steel Plates (Figure 3)

This system, which is still used in Japan or Korea, is nearly the same as the previous described. In this case, the load bearing surface is decomposed between the concrete and the steel plates. It solves the prob-lem of the reinforcement but does not improve the

safety during the installation process. Furthermore using steel plates may induce some durability issues (corrosion).

Curved Bolts (Figure 4)

The curved bolts were used from the beginning of the ’50s and it is still a connection method which is mainly used in Asia. In using this technology and compared to the two previous, the number of pockets is not reduced but their size and volume can be reduced. The installation of these bolts is not easy as the threaded bolt end sections must remain straight and therefore the tolerances of assembly are very large. This may induce some steps and lips. Furthermore during tightening operation the curved bolts tends to straighten and it may induce local stresses below the bolt towards the intrados of the segment, for this reason the reinforcement needs to be strengthened at this location.

Straight Bolts with Sockets (Figure 5)

This was a major evolution in the design of seg-ments, for the first time, sockets were embedded in the segments. This change provided a reduction of the number of pockets by two in each segment. The installation is also safer for the worker as it can be made below a segment already installed. The main advantage is that the force exerted by the bolting system can be defined thanks to the relation between the torque and the tensile strength (according to the Norm NF E25-030, Figure 6). This calculation helps to define the adequate linking systems and there-fore saves money in not over-sizing the connection. This has also some influence on the reinforcement design and on the global behavior of the segment as the smaller the connection will be the more concrete

Figure 3. Detail on steel plate pocket

Figure 4. Detail on curved bolt assembly

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there will be to provide a better resistance and a larger cover for the reinforcement.

The next step in this evolution is the change of the material to produce the sockets. In the early ’80s, plastic sockets were used for the first time. These sockets were not designed for the tunnel construc-tion. In France or Germany plastic sockets from the railway industry, designed for the concrete sleep-ers, were used. The sockets were produced out of High Density Polyethylene (HDPE) and the bolts used were coach screw bolts with a sharp thread. This socket from the railway industry offered many advantages, such as a good flexibility and a suffi-cient pull-out resistance for small transportation projects (metro). But on the other hand, the flex-ibility of the socket or the ability for the bolt to be easily installed was also a disadvantage from the engineer’s point of view. In fact, if the bolt was not properly aligned, the bolt could have cut its own thread in the socket and in this case the maximum pull-out resistance could not be reached. Properties of the plastic were also studied and it was showed that the HDPE is not the proper material for a tun-nel bolting system. In fact, the HDPE socket under a load stage creeps and therefore does not offer any safety to keep the gasket compressed. This creep leads to a release of the pressure on the gasket once

the TBM rams are removed and before the installa-tion of the next segment.

In a later stage, at the beginning of the 90s, new bolting systems were designed for the tunnel indus-try. The thread design was different and developed with non cutting threads. The sockets were produced out of polyamide that is stronger than HDPE and is not subject to creep. This new design provided the ability to install a bolt with slight misalignments (up to ±10°) and also to make sure that the theo-retical designed pull-out resistance was reached and maintained.

Figure 5. Detail on straight bolt with socket

Norm NF E25-030 (French Standard)T = F(0.16P+µ(0.583D2+rm))T: Torque appliedF: Tensile force exerted by the bolting system P: pitch of the bolt thread µ: mean friction coefficient under the bolt head and in

the bolt thread D2: diameter on the flank thread rm: mean radius of the bearing surface under the bolt

head

Figure 6. Norm E25-030, defining the torque to be applied depending on the required tensile strength

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Dowels

This kind of connection can only be installed in the circumferential joint, for the ring to ring connection. It is a great evolution in the connectors, as there is no human intervention below the segment and fur-thermore there is no pocket in the concrete, which has many advantages among them the durability of the connection and a smooth concrete surface which is a key criteria for water conveyance one-pass lin-ing project. The use of the dowel in the tunnel con-struction started in the first half of the 20th century where some mined tunnels in Switzerland used

dowels made of wood. The same kind of dowel was still used 50 years later on a non gasketed project in Munich, Germany (Hofoldinger Stollen). The devel-opment of dowels mainly started in the beginning of the 90s. Plastic dowels were developed and used for the first time in Italy on the “Passante Ferroviaro” project in Milano, Italy.

The first type of dowels were friction dowels, there was no embed receiving socket, only a recessed hole in the concrete segments; the dowel was pushed in the hole by the TBM rams and thanks to its geom-etry or material, it provided a sufficient pull-out resistance to keep the gasket compressed. Because there is no embedment, and the dowel during its installation induces radial stresses in the concrete, it is important to carefully design the reinforcement around the reservation. To achieve higher pull-out resistance, several types of dowels were developed by using different combinations of material (steel and plastic) and/or geometry. Nowadays, most of the dowels are made of polymer plastics and are made of two different components for the dowel and sock-ets. With this new design of locked in dowels, higher pull-out resistances are reached and less stresses are transmitted to the concrete during the installation because of the improved socket design.

Alignment Dowels (Figure 8)

For large diameter tunnels, where the technical requirements (pull-out and shear resistance) of the dowels are not sufficient for dowels but where the

Figure 7. Segment installation with dowel system on circumferential joint

Figure 8. Segment equipped with alignment dowels in both joints

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steps and lips could be a main issue because for example of the ground water pressure: a combina-tion of bolts and alignment dowels can be used. This combined solution offers a higher pull-out resis-tance thanks to the steel bolting system and a good alignment with higher shear resistance thanks to the plastic alignment dowel. This solution also presents specific interests in Europe where bolts are mainly removed. In this case, the alignment dowels, which always remain in place provides a final shear resis-tance between the rings but also between the seg-ments, as the guiding rod can be considered as an alignment dowel to be installed in the radial joint. The alignment dowels do not comprise any sock-ets as the pull-out resistance is not an issue. In this case, their only function is for guidance and shear resistance. The dowels are installed in a recess in the concrete segment in the circumferential joint and the guiding rods are glued in a groove in the radial joint.

SELECTION OF THE LINK SYSTEM

First, it is essential to state that there is no unique design for a segmental lining. On most tunnel proj-ects the design is based on the experience and skills acquired by Consulting Engineers and Contractors on previous projects, applicable design codes and accepted practices. For this reason the purpose of this section is only to review the key factors enter-ing into the selection of the connector system. The main technical features of the connector (pull-out and shear resistance) must be evaluated on a project specific basis. We make the following comparisons considering that all the systems match the project requirements. The three main types of connectors are compared: curved bolts, straight bolts (with or w/o alignment dowel) and dowels.

Guiding Function

How the connector will facilitate the erection of the segment rings by holding the different elements in place? The curved bolt does not offer any guid-ance, as the bolts are only inserted when the segment is fully positioned and in place. It is the same for straight bolt with socket; however the plastic socket offers much more flexibility than the curved bolt. The dowels are self-adjusting to the proper position of the segment in the ring while being pushed into position.

Time of Assembly

Curved bolts are typically installed in the pocket with a hammer which can lead to a damage bolt thread and therefore the installation can be very time consuming. The straight bolting systems offers more flexibility and therefore less time for the instal-lation, if they are combined with alignment dowel

the assembling is even faster but still requires human intervention. The dowels systems need less time for the assembling as there is no operation required to secure the circumferential joint connection.

Flexibility

The connection system must allow enough flexibility for tolerances in segment design and to the segment during the erection process. The curved bolt does not offer any flexibility, the segments have to be prop-erly aligned otherwise the installation is not pos-sible. Furthermore this kind of connection is 100% metallic and if it comes in contact with concrete in the guide hole, cracks may appear. The straight bolt-ing system offers a little bit more flexibility, as the strong plastic socket is manufactured of polyamide and can absorb some misalignment (±10°) thus the stresses are transmitted to the concrete. Plastic dow-els cater for maximum flexibility during erection and are self-adjusting.

Durability

The curved bolt is twice more likely to corrode than a straight bolting system, as the bolt head and nut are exposed, when only the head is exposed for the straight bolting system. Dowel systems are pro-tected and not subject to corrosion as they are fully embedded in the middle of the concrete segments. Currently most of the dowels are 100% plastic made and are therefore not subject to corrosion and offer a long design life.

Safety

The safety of the worker can also be one key cri-terion for the choice of the connector. Bolting sys-tems require a worker to go down and insert the steel screw during the erection of the ring. With curved bolts, the worker needs to be beneath the moving and already installed segment. In using dowels, workers can assemble the rings with the help of the erector and of the TBM rams.

Cost Benefit Analysis

The decision on what connector to be used should not be based solely on the base cost of the connec-tor delivered to the precaster. Many different criteria must be analyzed prior to specifying or purchasing a connector such as service life required, is the con-nector going to be in a corrosive environment, will a dowel connector system provide multiple benefits (i.e., alignment mechanism during installation, faster ring installation time, connector of segments, elimi-nation of post installation filling of bolt pockets), will the connector provide adequate tensile strength with a proper safety factor to keep the gaskets compressed

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during the ring installation process, does the connec-tor system provide adequate shear and safety factor to keep the segments aligned in the tunnel geology to be encountered. Dowel connector systems may not be applicable in meeting all project requirements.

FUTURE DEVELOPMENTS

In the past 10 years, the use of dowels expanded in all international tunnel markets. It has been used on various projects, covering a range of diameter from 3.0 m up to 7.0 m ID, mainly for water conveyance projects due to strong market requirements and also on transportation projects (metro). At this stage the larger diameter projects, are still built more tradition-ally, using straight bolts with plastic sockets, but the use of alignment dowels (in combination with bolts)

becomes more accepted. In the future, in order to be able to automate the ring installation, the use of dowel connections on large diameter projects will be needed. Manufacturers will have to look into new dowel designs and materials in order to achieve higher pull-out and shear resistance corresponding to these projects requirements

BIBLIOGRAPHY

AFTES Recommendations 1999http://www.subbrit.org.uk/sb-sites/sites/t/tower_

subway/index.shtmlDevelopment of Dowelled Connectors for Segmental

Linings, Davorin Kolic, Harald Wagner and Alfred Schulter, Felsbau 6/2000