International Symposium on Underground Excavation and Tunnelling 2-4 February 2006, Bangkok, Thailand

Small But Important – Gaskets for Tunnel Segments Marc Schurch1

1 Basler & Hofmann Singapore Pte Ltd., Singapore ABSTRACT Most of the tunnels in Southeast Asia are TBM-driven single shell tunnels. With no inner lining applied, a proper and reliable segment lining is crucial. A single shell segment construction combined with elastomer gaskets was first time applied in Germany 30 years ago. The first gasket generation could only withstand a maximum water pressure of 3 bar. With the progressive development of shield TBMs the requirements for the gaskets were continually growing. For the Hallandsas Tunnel in Sweden, the specification asks for a gasket that can stand a test water pressure up to 45 bar. To achieve the required performance, sealing gaskets have become a sophisticated technical product – small but important! The present paper highlights the most important factors for design and application of tunnel sealing gaskets. 1. DESIGN REQUIREMENTS 1.1 Project specific requirements A tunnel has to be waterproof to ensure minimum maintenance and operating safety as well as to pro-tect the expensive electro-mechanical installations housed in the tubes. Already in planning stage, de-signer and client have to formulate the necessary requirements for their specific tunnel project. The technical solutions to achieve these watertightness requirements vary depending on project specific circumstances.

The most important parameter is the water pressure. Depending on the expected height of the wa-ter pressure in the tunnel, the specifications have to define the watertightness performance of a sealing gasket and thereby include a safety factor that takes rubber relaxation effects into account. To get a long-term post construction performance, the rubber compound needs to uphold a certain load deflec-tion force to withstand the applied water pressure even years after its application. This can only be achieved with a specific rubber mixture that further may also need to be resistant against aggressive water, oil affected soil or even fire-retardant.

It is the task of a specialized gasket manufacturer to prove that the proposed gasket meets all these requirements.

On the other hand, designer and contractor have to ensure that the segmental lining is properly designed so that a precise erection of the segmental lining can be achieved throughout the tunnel. Fo-cusing on the segmental lining, the most important parameters to achieve watertightness are: • to define the maximum allowed gap and off-set between two segments, • to limit the distortion in the radial joints (ring build tolerances) and • to limit segment tolerances for profile and groove.

1.2 Material behavior The applied rubber gasket has to fulfill high material requirements to be able to guarantee a life span of often more than 100 years. To meet all the requirements and to achieve the expected technical per-formance, the use of rubber compound EPDM (Ethylen-Probylen-Dien-Kautschuk) is favorable and it

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has replaced the formerly used Chloroprene. EPDM is the economically and technically most suitable material to withstand climate and groundwater with varying water composition and is enough stable not to react with the environment or deteriorate in contact with other materials, such as concrete, grease or injection material.

Figure 1. EPDM raw material (left) and mixture granules 1.3 Load deflection Every gasket has a characteristic elasticity. The elasticity of a gasket is defined through the load-deflection curve. This curve shows the force that is required to compress the gasket. It is called com-pressive force or reaction force.

0

10

20

30

40

0 2 4 6 8 10

Gap A (m m )

Forc

e (k

N/m

')

12

Figure 2. Load deflection curve of a sealing gasket

The load-deflection behavior depends on the compound, the shape of the gasket and also whether the two gaskets are pressed together in line or with an offset (Figure 4).

From a designer’s point of view, the following has to be kept in mind: To bridge a wide joint opening the gasket needs a higher load deflection force. To be able to withstand the water pressure on a long-term perspective, the profile manufacturer has to ensure that the profile doesn’t loose all the compression force with the years (relaxation). However, is the deflection force too high, a strong erec-tor has to be provided to properly compress the segments and with a high compression force there is a risk to get cracks in the concrete groove that leads to a water circulation underneath the gasket.

Therefore, it is a delicate challenge to design a gasket cross section that can fulfill all these re-quirements and to exclude unwanted side effects. Gasket manufacturers have realized this and found appropriate solutions (see chapter 3).

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1.4 Relaxation Relaxation is the reduction of tension in a certain time on a deformed gasket body. This means that the sealing gasket looses its reaction force over the years. It is crucial that also the reduced compression force is still sufficient to tighten the tunnel. The majority of the relaxation occurs in the first months after installation.

The relaxation can be tested with so called accelerated aging tests. This aging test is using an ac-celerated procedure with elevated temperatures to get results within a reasonable timeframe (Figure 3). The LTA specifications for the Circle Line MRT projects in Singapore for example ask for a residual compressive stress of 65% after 120 years.

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0.1 1 10 100 1000 10000 100000 1000000 10000000

time (h)

resi

dual

str

ess

(%)

20 °C-mastercurve

Log. (kalk)

Log. (25°)

120 years

Figure 3. Mastercurve of compressive stress relaxation (aging)

As the relaxation behaviour of a sealing gasket is mainly influenced by the geometry of the pro-file, such aging tests have to be carried out independently for every profile. 2. RECOMMENDATION FOR SPECS In South East Asia, the most tenders for TBM excavated tunnels call for a single-shell segment con-struction method. Generally, a dry tunnel is expected and required for a reliable operation and mini-mum maintenance. To assure a high quality standard, specifications for tunnel segments and sealing gaskets must cover at least the following aspects:

Tunnel segment: The tolerances for the erection of the tunnel lining have to be limited, as the wa-tertightness performance of a gasket depends on gap opening and offset (Figure 4). With a gap opening of more than 6 mm and an offset of more than 10 mm the gasket performance is not opti-mum anymore. Hence, by limiting the tunnel segment tolerances, the effectiveness of the gasket can indirectly be improved.

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GAP A GAP A

Off-Set

Figure 4. Illustration of gap opening and off-set

Water pressure: The maximum water pressure further defines the design of the sealing gasket. Considering the relaxation effects of rubber, a safety factor of two is advisable to ensure that the gasket is able to withstand the design pressure in the long term. The watertightness of the gaskets under the defined water pressure shall be tested according to an official test procedure, given an un-favorable combination of maximum gap and off-set between the segments. Chapter 4 emphasizes on this topic.

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Groove design: The watertightness further depends on the groove geometry. Angle and groove depth has to be in line with the chosen gasket. To avoid spalling of concrete, it is crucial that the net volume of the rubber can be housed within the groove when the tunnel segments are fully closed (gap=0). Therefore, it is only possible to finalize the groove design, once a specific gasket has been selected. Chapter 3.2 further elaborates this issue. Load deflection: The manufacturer has to measure the compressive stress of the proposed gasket and to provide load-deflection curves to ensure that the gasket resistance does not exceed the de-sign erector force. Durability: As elaborated before, a service life of 100-120 years is standard for most of the newly built tunnels and the gasket performance shall be proven by respective aging tests. In the same time, also residual compressive stress after aging has to be proven (Figure 3).

Depending on the project requirements, these basic recommendations can be supplemented with

other specifications to achieve a high-quality standard for the watertightness of a tunnel. In Singapore, the Land Transport Authority (LTA) has laid down extremely sophisticated specifications for the new MRT-Lines. The specifications touch the above points but in addition also ask for hydrophilic features of the gasket. The groundwater contains a relatively high amount of salt on account of the insular loca-tion and the gasket supplier had to prove the aging resistance and swelling capacity in salty groundwa-ter.

To comply with the high standards, a new gasket type was developed and introduced to the mar-ket. This new gasket combines the two systems of compression gasket and hydrophilic seal into one. It is designed as a co-extruded gasket with an EPDM core and a hydro-swelling layer.

The following chapter looks into the latest development of profiles and emphasizes on the most important criteria for profile design. 3. PROFILE DESIGN As elaborated before, decisive factors for the functioning of a gasket are shape and admixture of a pro-file but also gap opening, groove design, the influences of the linking system, TBM-forces and erector capacity. These parameters make it highly difficult and complex to design a gasket, define its optimum shape and to judge its behaviour performance in a tunnel.

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Nonetheless, there are some facts and factors that make a gasket design predictable. First off all, a good rubber quality is essential. Then – according to the individual project requirements – a techni-cally suitable profile has to be designed (Figure 6). Once a new gasket is designed, prototypes are pro-duced (Figure 5) and its behaviour is tested in respective compressive stress and watertightness tests. This procedure is an interactive process and a lot of experience is necessary as the exact rubber behav-ior can not be calculated but only tested.

Figure 5. Extrusion machine Figure 6. Profile design

Standard gaskets are mono-extrusion gaskets (Figure 7) with only one rubber extruder forming the required gasket shape. Recent development and particularly the client’s need to achieve higher per-formance has changed this and the latest gasket generation is designed as co-extruded profiles, consist-ing of a swelling mix and EPDM mixture (Figure 8). In a so-called co-extrusion process, the two rub-ber materials are simultaneously extruded and jointly vulcanized. Such a type of co-extruded gaskets with hydrophilic layer is used among other projects in Singapore for the Circle Line Projects.

Figure 7. EPDM gasket Figure 8. Co-extruded gaskets (EPDM body with hydroswelling layer)

In this connection, the EPDM takes over the function of directional orientation of the swelling pressure at the core of the seal. Therefore a proper gasket skeleton is compelling.

Another application for co-extruded profiles has been developed to cater for a smooth installation of the key stone. To reduce the friction between keystone and counter-keystones during the installa-tion, a co-extruded slip plane is applied as a top layer of the gasket. With a moistened hydro-swelling layer the equal effect can be achieved. a. Frame corner Experiences gained from former tunnel construction have shown that leakage is most commonly oc-curring in corner areas of gasket frames, e.g. at T-joints of the tunnel segments. This leads to the con-clusion that the corner design needs a major focus in the design. A main reason for this phenomenon lies in the manufacturing process of the corners. If the pre-cut gasket strips are just vulcanized together to form a frame, there is proportionally more rubber in the corner area than on the segment sides (Fig-ure 9). This leads to the fact that the corners are stiffer than the regular profile along the sides of the tunnel segment. These stiffer areas hinder a proper load distribution and lead to potential leakage, es-pecially when – as it happens in the tunnel - two such stiff corners are compressed onto each other.

To avoid this uneven load distribution, the goal for gasket manufacturing has to be to produce a frame with equivalent material behavior all around, e.g. to guarantee a constant load deflection force along the whole segment frame. This can be achieved by volume balanced corner vulcanization (Fig-ure 10).

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Figure 9. Monoblock corner Figure 10. Volume balanced corner

This process technology for producing vulcanized corners with internally located cavities was further developed in the last years. Beside leakage prevention, such ‘soft corners’ also efficiently avoid concrete spalling at corner areas. b. Concrete Spalling As mentioned in chapter 2, a proper groove design can avoid concrete spalling problems. To achieve that, the net profile volume (seen in a cross section) should be some percentages smaller than the groove cross section. In that way, even in case of very high forces applied onto the segment joints (e.g. TBM jacks) there is enough space for the gasket to ‘fit’ into the groove.

But, practical experiences in tunnels with segments with narrow grooves and high TBM jack forces have shown that the concrete segment corners show cracks or concrete spalling even when the above criteria of volume ratio is fulfilled.

There is a wide spread attempt to explain this observation. This theory bases on the fact that EPDM rubber is an incompressible material. When compressed, the applied force R will strongly de-form the gasket profile and close the hollow body. When reaching its full compression (no voids), the applied force R will be evenly spread onto the groove flanks (p) and the groove bottom – analogue to hydrostatic pressure. Basing on this, one comes to realize that the summarized spalling force P at the flanks is directly depending on the groove depth (Figure 11). At the same time, this increase in lateral force impact can only partly be compensated by the slightly longer shear area (a≈b). This leads to the conclusion that the danger of spalling increases with a deeper gasket groove.

R

p

p p

R

p

p p

pp

P1 < P2

a

b

P1

P2

Figure 11. Hydrostatic Distribution of impact force R in a gasket groove

With regard to concrete spalling, a flat profile design is more favorable than a stocky profile re-quiring a deeper groove.

The real impact of the superimposed factors (e.g. shear, torsion, jack pressure) can only roughly be determined. And a successful application without spalling also depends on the experience of the in-volved parties, especially on the skills of the worker guiding the TBM-erector.

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4. WATER TIGHTNESS TEST a. In general Until a few years ago, all gasket manufacturers had their own testing procedure. Comparison of the re-sults of these gasket tests was misleading and in some cases incorrect.

That’s why the leading gasket suppliers in Europe agreed on a standard test procedure that is now state-of-the-art and established in the DIN-specification (Germany). In comparison with former test methods, the standardized testing equipment allows to simulate the delicate corner areas as they appear in the real tunnel. b. Preparation of Test rack The three-piece steel test rack replicates a T-joint with the 90° gasket frame corners that exist in the real tunnel. The steel pieces with the interior test gasket are tensed up with the help of screw bolts. The off-set is adjusted by moving the steel plates counterpart (Figure 12). The test rack is then closed and fed with water that is in accordance with the actual groundwater composition of the project. The de-sign water pressure is applied in steps of 1 bar (Figure 13).

Figure 12. Open test rig with gasket Figure 13. Water pressure test equipment 1.5 Test procedure Water tightness tests shall normally be carried out in research laboratories. Some of the more techni-cally oriented gasket manufacturers test their new profiles in their own R&D department and on-site tests can be avoided.

To test the efficiency of the gasket profile against leakage in a comprehensive way, it is necessary to run the watertightness test with different gaps and off-sets. For every off-set setting (0 – 20 mm) the test has to run through a range of different gaps. For every gap, the water pressure is built up in steps of 1 bar and is hold there for 5 minutes. In that manner, every setting has to be tested until the profile shows leakage. The recording of all “failure points” leads to the required ‘Watertightness-Gap’ dia-gram (Figure 14).

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Gap A [mm]

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offset 0offset 5offset 10offset 15W

ater

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]

Figure 14. Typical water tightness curve

The tests are normally carried out at room temperature (23 ± 2 °C). For project specific tests it has again to be highlighted that: • the maximum gap between the faces of the test rig pieces has to be greater than the theoretical

value calculated from the joint surfaces plus the maximum thickness of any packing. • The test pressure has to be twice the hydrostatic pressure (safety factor: 2) to consider relaxation

effects and to guarantee the long-term performance of the gasket.

Tests with co-extruded gaskets do not influence the test results in a short term, as the hydro-swelling top layer of a co-extruded gasket takes longer than 24 hours to react. About 50% of the swell-ing occur within 7 days, nearly 100% of swelling occurs within 30 days. To measure the positive in-fluence of hydro-swelling layers long-term watertightness tests have to be carried out.

In reality, a proper application on site is even more important than test results because only a careful and professionally applied gasket can guarantee the tested gasket performance. 5. MOUNTING AND INSTALLATION PROCESS

The gaskets are delivered on site as prefabricated frames. The frame sizes are tailor-made and stay in accordance with the segment design. Once the tunnel segment design is known, gasket manufacturers will calculate and define the necessary pretension to guarantee a tight fitting of the gasket on the seg-ments. To prevent water circulation between groove and gasket, the gasket frame is glued into the groove of the segments.

A safe application requires a clean groove surface. Before mounting the gasket, it further has to be checked whether there are cracks or chipping damages in the segment groove. Such damages have to be repaired as otherwise they could lead to unwanted and uncontrollable waterways.

All gasket suppliers demand to apply their gaskets with an adhesive into the segment groove. Firstly, all four corners are pressed into the groove and aligned. It is important, that they are not dis-torted. As the gasket frames have a pretension, i.e. they are smaller than the segment groove length. The purpose of the pretension is to reach a tight fit in the groove.

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Figure 15. Mounting of a gasket frame Figure 16. Segments ready to lift down the shaft

After mounting the gasket, the tunnel segment can be stored outdoor. EPDM is not susceptible to normal weather influences such as sun and rain. But in case of co-extruded profiles with hydrophilic layer, the gasket should be covered with a plastic to avoid swelling before installation of the segments. Most contractors pile segments for a whole ring and protect this set with a plastic cover from possible rain.

It is a valid rule for the whole construction industry that most system failures are caused due to mistakes during installation. The proper mounting is only guaranteed with well-instructed workers and alert site supervisions. 6. FIRE SAFETY ASPECTS The stepped-up occurrence of serious and even fatal fire accidents in traffic tunnels in recent years has led to an increasing focus on safety aspects in tunnels. The extent of the damage caused to the concrete in the case of fire is mainly on the inner side of the tunnel segment, the side nearer to the source of the fire.

A sealing gasket is normally located on the outer side of the segment. It is therefore not exposed to open flames but questions might arise concerning its heat exposure and resistance in the case of a fire.

Segment thickness [m]

Tem

pera

ture

[C]

Radiation

Figure 17. Heat processing in the segment concrete in the case of a goods train in fire

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Figure 17 shows that the gasket is exposed to a heat of only about max. 140 °C, what is not enough to catch fire for EPDM. As long as the gasket is not exposed to open fire, it will not scorch. A complete failure of the gaskets does not occur before major damages to the concrete structure is done (e.g. reinforcement failure, thermal expansion, chemical conversion and concrete spalling due to steam).

EPDM is flammable, but unlike chloroprene, EPDM rubber does not release toxic gases when burning. Up to a temperature of approx. 130 °C EPDM is stable. It will only ignite for a temperature of more than 170 °C.

Based on the findings shown in Figure 17 it is very unlikely that the sealing gasket will catch fire during a fire in a tunnel. With a tunnel segment thickness of more than 25 cm and a positioning of the gasket on the outer side of the segment there is enough concrete to stop the heat radiation and to pre-vent an ignition of the rubber.

Even in case of a very thin shell (d=25 cm) or a gasket position on a more interior side, the re-tarding effect of the concrete mass is sufficient to allow passengers to escape. 7. CONCLUSION Through an early and careful planning a watertight TBM-tunnel can be guaranteed and satisfying long-term behavior can be achieved. A careful selection of the material with a professionally designed profile and a respectively adjusted groove allows achieving a high performance of sealing gaskets.

Today, where tunnelling techniques can limit the maximum offset to 15 mm, it has become pos-sible to achieve durable water tightness of up to 10 bar with a standard EPDM-profile. For higher re-quirements and higher water pressure the application of co-extruded gaskets with hydrophilic layer is recommended.

The co-extrusion technology also allows dealing with project specific needs, as for instance the integration of sliding liners for key stones or the integration of an oil-resistant protection layers around the gasket-core.

Summarized, the prerequisite to achieve satisfactory sealing results is to elaborate a system solu-tion involving segment-, gasket- and groove design. REFERENCES Krebs, Christian 1989. Einführung in die Elastomertechnik. Maag Technic AG. Switzerland: Düben-

dorf. Tan, Hong Choon 2002. Test Report Ref.: F2141/RS. SETSCO Services Pte Ltd. Singapore. Prof. Dr rer. Nat. Mang, Thomas 2001. Stellungnahme zur Altersbeständigkeit. Germany. Bodmer, Martin; Dr. Grabe, Werner; Joos, Martin 2003. Seals for Tunnels in Singapore.

Tunnel (02/2003). Germany. Dr. Schreyer, Jörg. Abdichtungen von einschaligen Tübbingauskleidungen mit Dichtungsprofilen.

STUVAtec GmbH. Germany: Köln. Lea, Phillip 2003. The bursting point – Arrowhead Tunnels. Tunnels & Tunnelling International (May

2003): 25-28. England: London.

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