Water crossings — the options

TUNNELS FOR WATER CROSSINGS

Water Crossings -

the Options

L. C. F. Ingerdev Abrfrao-Water in the shape of rivers, lakes, fiords, oceans and so on, sometimes seems to provide a barFieF to improving the environment andpFeventing further development. Hitherto unconsidered options may be available to cross over OF under a water barrier. Without fill knowledge of the available possibilities, planners and decision-makers are not able to make the best-informed choices. This paper initially exnmines the relative attributes ofbridges, ferries, submerged floating tunnels, immersed tunnels, soft ground tunnels and rock tunnels. Reasons for selection OF rejection of each option are discussed. The paper goes on to introduce immersed and floating tunnels in more &tail. &ecifi examples are given to convey the concepts. 0 1999 Published by Elsevier Science Ltd. All rights reserved.

F

or thousands ofyears, humankind has built bridges or taken a boat or ferry to cross water. Even the first tunnels were built more than two thousand years ago, although these were primarily for water supply. However, it was not until 1997 that the first significant vehicular tunnel was completed for horse and carriage. The stories surrounding the construction, use, and recent renovation of that tunnel beneath the River Thames in London make fascinating reading. Since then, countless bridges and tunnels of one form or another have been built.

What is meant by a “Water Crossing”? The most obvious water crossing that springs to mind is a bridge across a river. However, a water crossing might also be over a lake or a deep fjord, an estuary, a strait, or perhaps even a harbo:r. A bridge can be and often is the best solution or the cheapest solution to getting from one side to the other. If a bridge is the cheapest solution, perhaps one should consider whet,her it satisfies other criteria such as whether it is in the right place, since- bridges are more easily built in some locations than others, whether it is visually or environmentally the right solution, and so on. One can only know if it is the best solution if other appropriate options have been considered, and their relative merits compared. In some instances, water crossings need not be from one side to the other; consider the following: When a town has grown along a riverbank or a shore, relief of trafllc congestion may no longer be possible within the town. Particularly if it is a bigger town, a highway bypass on the inland side may no longer be sufIlcient. The only practical solution may be to put a relief road along the waterfront in the water. Both shores ofthe harbor in Hong Kong have areas

Present address: L. Christian Ingerslev, Parsons Brinckerhoff Quade & Douglas, One Penn Plaza, New York, NY 10119, U.S.A. Mr. Ingerslev is a member ofthe ITAWorking Group on Immersed and Floating Tunnels (W.G. 11).

Tunnelling and Underground Space Technology, Vol. 13, No. 4, pp. 357-363,1998 W367799/99/ $-see front mat&r C 1999 Published by Elsevier Science Ltd. All righta reserved. PII:SO88&7798(9S)MO77-7

where viaducts have been placed in the water parallel to the shore. This may be perfectly acceptable along industrial areas, but in residential, recreational or downtown areas, some of the options presented below may be much more appropriate. Water crossings can involve methods other than a bridge. Ferry routes, though not as many exist today as formerly, are still much in evidence, and the demand for further routes still exists. Sweden is considering a new ferry route across the Baltic direct to Germany, particularly if a fixed connection between Denmark and Germany does not materialize. Such a route could only be by ferry. If the distance were shorter, tunnel options could also be explored, such as have been used to cross the English Channel between England and France, or Japan’s Seikan Tunnel between the two major islands of Hokkaido and Honshu, built for the Shinkansen high-speed rail system. Once ferry terminals have been built, the cost of the crossing is relatively independent of the length of the crossing, since only the sailing time is affected. Unfortunately, sailings can be cancelled under storm conditions and when the water is frozen. The latter can often happen in colder regions, effectively severing the link until conditions improve.

Bridge Crossings Developments in bridge technology are providing longer and longer spans for all types of water crossings. Suspension bridge spans can now be measured in kilometers, and spans of five kilometers and more have been considered for several locations where water depths are large, such as for the Straits of Gibraltar and between some of the Indonesian islands. World record span lengths are also being broken for cable-stayed bridges, swing bridges, and floating bridge draw spans (Ford Island Bridge, Hawaii), to name but a few. There can be difficulties to providing a bridge, particularly where navigational requirements are demanding, water depths great, or soils are poor. Navigational requirements are usually onerous, often resulting in long spans and high clearances. Many ingenious methods have also

Pergamon

C&e Cod Canal. (photo by Walter Grant4 been devised to move a bridge out of the way of shipping, such as swinging the bridge sideways or upwards, or even lifting the whole bridge up out ofthe way. Where the water has been too deep, floating bridges have been used in sheltered areas, some even with draw spans that can be floated out of the way. Too often, though, opening a bridge on a busy road reduces the total amount oftraffic that it can carry, and the lost time cannot be tolerated. It is then necessary to use high and long span bridges, unless other solutions are adopted. The long approaches to such bridges may be unacceptable in a city, even when spiral ramps are used at the ends to shorten the perceived length of the bridge. Bridge crossings are also at a disadvantage in severe weather conditions, being susceptible to the effects of ice, snow and strong winds. The table included in this paper compares the relative merits in any one row of the different alternatives methods of achieving a water crossing, where 5 is the best and 1 the worst. Afigurelower than 5 in a row does not necessarily imply a poor rating, just that another method of crossing may be better. No attempt should be made to deduce relative merits of different options in one single column. Options in a row that score one less or better are most likely worth investigating. Examination of the table shows as expected that in many cases, bridges are the best solution. Immersed Tunnels Since the beginning of this century, more than one hundred immersed tunnels have been built world wide’ for road or rail crossings. Immersed tunnels, constructed as floatin structures and then buried, are constructed in two basic

types, a steel tunnel, and a concrete tunnel. Both types are usually made up of a number of tunnel elements essentially prefabricated in manageable lengths, each often about 100 m long, that are eventually joined up below water to form the final tunnel. They have temporary bulkheads across the ends of each element to allow them to float with the insides dry. Fabrication is either completed in a dry dock, or the elements are launched like a ship and then completed afloat close to their final location. In most cases, the completed tunnel elements are barely capable of staying afloat unaided. Tunnel elements can and have been towed successfully over great distances (see Fig. 1). After outfitting at their final destination, they are attached to temporary supports capable of lowering the elements into a prepared trench in the bed. The supports may typically be provided by a purpose-built catamaran or barges with winches, or by cranes. Elements are lowered and butted up to preceding elements, after which the joint between them is dewatered. The foundation can be prepared prior to lowering the elements, or it can be completed after placing the elements on temporary supports in the trench. Following this, backfilling of the trench is completed and any necessary protection added to the top of the tunnel and the fill. It is sometimes necessary to make the closing joint underwater, so that the optimum sequence of construction can be adopted. As mentioned earlier, immersed tunnels come in two types, known as steel tunnels and concrete tunnels (see Fig. 2). The choice between them depends very much on

Figure 2. Central Artery steel immersed tunnel under fabrication. Sailors)

358 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY

(photo by David

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Table 1. Comparison

of the relative merits of the different alternatives

methods of achieving a water crossing.

h-

Tunnels

Open Ai r

Attribute

Type

Mixed ground Soft ground Very deep water

1

2 5 I 2 I 5 1 5

Shallow crossing Deeper crossing Verv deep crossing

5 2 3 3 I 2 I 3

IFireand-explosions

time

5 5

Rock at high level Mixed ground Soft around

Shallow water Deep water rn

5 2

I

Overall risk (construction and operation)

:

45

5

3

1

5 5

5 4 3

1 2

4 3 3

-

;

5 4

1 4 1 5

3

4

I 4 I 5

i-3 4 4 4 4

4

1 5 1 2 5 2

Hazardous cargoes restricted I&

IShorttunnel

z

4

3 4 4 3

5

5

4

4

2

2

4 3

4 3

4

5

5

5

5

5

1 4 4 2

4 3 4 4

Operation 0%Maintenance costs

2

2

3

4

Occur probabilii Water problems and leakage Problems arising

z

;

5 2

5 4

Abilii to make repairs

5

5

2

3

3

5

Durabilii Note: l-no

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good, 2=poor, 3=0K, 4=good, 5=best

TUNNELLING ANDUNDERGROUND SPACE!C’ECHNOLAXY 389

similar to those of other tunnels. The possible length of a vehicular tunnel will be dictated by the ability to dilute sufficiently the noxious emissions. For a given size of tunnel and amount ofventilation, longer and longer tunnels are possible as the numbers of vehicles using it drop. Such a drop may be caused by a lack of demand, or by artificially metering the traffic, as is already done on some access roads to freeways. For example, concept designs made for a tunnel between Denmark and Germany call for about 9 km between ventilation buildings, perhaps not long for a trans-alpine tunnel but long for an immersed tunnel. Noxious emissions in rail tunnels are much less, so that the posFigure 3. Boston’s Ted Williams steel immersed tunnel element afloat. (photo by sible length of a rail tunnel Patricia Pidlean) can be correspondingly longer. For very long crossings where navigation is important, bridge /immersed tunnel local conditions and practice. Steel tunnels are fabricated combinations can provide a most economical solution. initially in much the same way as a ship, essentially a steel Generally, long trestle bridges extend out from the shores hull or shell within which at time of launch there is usually through relatively shallow water to man-made islands at little or no concrete. Draft in this condition is usually only which the transition between bridge and tunnel is made, a few meters. Close to the installation site and while afloat with the tunnel extending across the usually deeper navior held afloat, concrete is placed within the steelwork to gation channels. form the final pressure-resistant structure. The steel shell The Hampton Roads bridge-tunnel (1957) in Virginia and the concrete work compositely together. Ballast conwas the first immersed tunnel to be built between two mancrete is also placed to provide the necessary weight to made islands, and has since been widened from its initial prevent the structure from floating up from its final resting one lane each way configuration by constructing parallel place (see Fig. 3). bridges, widening the islands and laying a parallel tunnel. Concrete tunnels, on the other hand, are usually conThe nearby Chesapeake Bay bridge-tunnel, which was structed within a dry dock or in a dewatered casting basin completed seven years later, is over 17 miles long and has below sea level, and then floated out when complete. Freeimmersed tunnels at each of the two main shipping chanboard in this condition is usually less than half a meter. nels (see Fig. 4). Its bridges, too, are currently being Although some concrete tunnels have an outer steel plate waterproofing membrane, it does not work compositely with the reinforced or prestressed concrete structure. Both types perform the same function after installation. While it is usual for the individual steel immersed tunnel elements to cost a few percent more to construct than equivalent concrete elements, this may be more than offset by advantages to be gained from their shorter construction duration. Immersed tunnels have special advantages since they lie only a short distance below bed level. Approaches can be relatively short and the visual intrusion negligible compared with high level bridges. Tunnels can be made to suit horizontal and vertical alignments, and to match the requirements of road or rail traffic. Ventilation requirements for immersed tunnels are Figure 4. Chesapeake Bay Bridge-Tunnel. (photo by Walter Gmntz)

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widened from their initial one lane each way configuration by constructing parall.el bridges. A third bridge-tunnel, the Monitor-Merrimac, was completed in the same vicinity in 1992. Bridge-tunnels have been proposed for other long crossings, such as across the mouth of the Pearl River near Hong Kong in China; and a bridge-tunnel has been constructed for the Tokyo Bay Crossing in Japan.

Floating Tunnels In essence, floating tunnels can be very similar to immersed tunnels, except that they are not buried in the bed and are located within the water column between the bed and the surface (see Fig. 5). They can be made heavier than water, like an immersed tunnel, and supported either on columns from below or from floating pontoons from above2. If the length is short, they could span directly from the ends in the manner of an underwater bridge. Ifthey are designed to be lighter than water, they can be held down by flexible or rigid anchors from the bed. However there are a number of significant differences from other forms of tunnel, the most important ofwhich is that they experience dynamic loading. There can also be a real risk of being hit by an errant vessel or submarine, much as a bridge structure can be, if located incorrectly. The tunnels and their support system must therefore be capable of surviving the loss of at least one of the supports, if the supports are at risk (such as if pontoons are used). Many of these “risks” are under the control of the designer who can either eliminate them, or mitigate them to an acceptable level. Floating systems themselves are not new. Well known floating bridges in the U.S.A. are the Lake Washington bridge and the Hood Canal bridge, both in Seattle. A new bridge just completed. for the U.S. Navy is the Ford Island Bridge in Hawaii. Large ships ply the waters in the vicinity of both the Hood Canal and the Ford Island bridges, making these structures much more vulnerable than a floating tunnel would be if 1oc:atedbelow the draft of such vessels. A floating tunnel is currently undergoing detailed plan-

ning for a deep fiord crossing in Norway. It is for this type of crossing that a floating tunnel is particularly suited, and at that location is the only option for the replacement of an existing ferry system. While the first such tunnel remains to be built, none of the technology is new. Even the holding down system proposed is proven technology from tension leg oil platforms. While immersed tunnel construction technology could be used to make floating tunnels, other types of construction may be used, such as incremental launching similar to bridge structures. For a floating tunnel, launching need not be on the final alignment, since it can later be easily swung into position.

Staged Cofferdam This is included here just for the sake of completeness.

Where the waterway is perhaps not in continuous use, and where soil conditions are suitable for the use of cofferdams, tunnels may be formed using this methodology. This can often be the cheapest way of forming a tunnel, identical, apart from the cofferdam, to cut-and-cover tunneling on land. Waterproofing of the tunnel can be assured. It is not unusual for river crossings to be made in this manner when the river authorities permit it, a recent example being the Hsintien River rail crossing in Taipei, Taiwan. Both immersed and staged cofferdam solutions were proposed, with the cheaper solution being accepted by the river authority.

Soft Ground Tunneling This form of tunneling is today almost exclusively done

using shield technology and tunnel boring machines (TBMs). The New Austrian Tunneling Method (NATM) is not generally suitable for use below the water table without extra special precautions being taken, such as ground freezing. Such methods are not really suitable for large scale use beneath a water crossing. With the rotary motion of TBMs, such tunnels tend be circular, although the Japanese have been particularly innovative with their multi-face machines. Though generally only used for short specialized runs, such machines can be desiened to mine virtually any ihape ofhole. Most pictures that one sees of a tunnel boring machine are only of the cutter head. Such a head is only a very small part of such a machine. Behind the head follow jacks, segment erection equipment, conveyors, control equipment, and so on, often extending some distance behind the tunnel face. Pressurized face TBMs have revolutionized the construction of tunnels in sofi saturated soils, with little need remaining for compressed air working. Although TBMs are manufactured in several countries, the Japanese have developed double and even triple faced machines for working under difficult conditions to make non-circular tunnels. Although they were each used for a few hundred meters only, these specialized machines are highly suitable for the construction of stations,

Figure 5. Some options for floating tunnels.

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TUNNEIAING ANDUNDEFUZROUND SPACETECHNOLOGY 361

the Herrenkneeht Mixshield, currently in use at 14.2-m diameter for the Elbe Tunnel in Hamburg, Germany. Another interesting feature of this particular shield is that it is able to be used barely half a diameter beneath the river bed, much more shallow than the hitherto rule of thumb that a tunnel had to be one diameter clear beneath the surface. With this form of shield, many new tunnels thought to be prohibitively expensive or impossible will be possible, such as in soft soils with cobbles and boulders, as are found in the softer deposits and moraines around New York City. The amelioration of settlement due to tunneling in this medium will be very site-dependent and will depend to a great extent on whether or not pressure grouting can be achieved. The rate of advance of such a tunnel through mixed material will naturally be slower than through uniform soft material. Depending upon expected conditions, it may also be necessary to provide access through an airlock to the face in case of encounters with extra large boulders or historical debris such as piles. In other respects, mixed ground tunneling is very similar to soft ground tunneling.

Hard Rock Tunneling

Figure 6. Hitachi-Zosen shield tunneling machine used for the Trans Tokyo Bay Tunnel. (photo by Hitachi-Zosen) etc., under very exacting conditions requiring maximum control of settlement, such as under existing buildings. Where ground conditions permit, the space around the installed tunnel segments can be injected with controlled amounts of grout and reduce significantly the amounts of settlement due to the tunneling. One of the largest tunnels recently completed was the very successful 10 km Tokyo Bay Tunnel at 14.16 m diameter (see Fig. 6). This is large enough for a threelane highway, or in the case off okyo, for a twolane highway with a full emergency shoulder. When soils are uniform, very high rates of production can be achieved, leading to great economy.

While the traditional drill-and-blast tunnel still has its place, in many cases shield tunneling can provide a really excellent solution (see Fig. 7, for an example of such tunnel construction). TBMs are particularly suited to single- or double-track rail tunnels, since little surplus space is generated. For two-lane highways, the surplus space above and below the roadways in a circular tunnel can generally be matched to ventilation needs, but for a three-lane highway, this space may become excessive. TBMs for highway construction are therefore sometimes used for pilot tunnels, out from which the main tunnel can be constructed by benching and slashing. Highway tunnels in rock appear less common than rail tunnels, since highways can often negotiate steeper grades and so avoid tunnels. Machines designed for rock tunneling are very much set up for a single type of rock likely to be encountered, Significant variations in rock strength can cause problems in that the teeth used on the cutter may not be appropriate.

Mixed Ground Tunneling Until recently, this form of tunnel was extremely costly to carry out, since excavation nearly always had to be by hand. Becent advances in shield technology have produced TBMs capable ofhandling some rock, boulders and cobbles as well as more uniform soft materials. A typical example is

A

Figure 7. Pioneer Square Station, Seattle, Washington Pidlean)

362 TUNNELLING ANDUNDERGROUND SPACETECHNOLOGY

4

A.). (photo by Patricia

Volume 13, Number 4,1998

Conclusion In the preceding Iparagraphs, brief summaries of many forms of water crossing have been given. Extensive literature is available not only for bridges, but also for tunnels3. Furthermore, a table has been provided to list the relative merits of these type,6 to handle specific attributes of such a crossing. With these as a starting point, it may be possible to consider a much wider range of choices for making a water crossing without necessarily being an expert in them all. 13yselecting and listing the attributes of a crossing, the figures given may enable an initial qualitative selection of types for further, more detailed study. Immersed and floating tunnels are also described, forms of tunnel which are less widely known but which can often provide a much cheaper solution and which produce much less visual impact. With tools such as this available, perhaps a better understanding of th.ose options for a water crossing that

Volume 13, Number 4,1998

ought to be considered in an initial screening might be better understood. Another such tool is The First Road Tunnel’, and in course of preparation is Immersed Tunnels, A Better Way to Cross Water?, to be issued by the Immersed and Floating Tunnels Working Group of ITA at the ITA 1999 World Tunnel Congress.

References l State-of-the-Art Report, second edition, International Tunnelling

Association Immersed and Floating Tunnels Working Group, Pergamon, 1997 * State-of-the-Art Report, first edition, International Tunnelling Association Immersed and Floating Tunnels Working Group, Pergamon, 1993, and also the second edition referenced earlier. aTunnel Engineering Handbook, first editionvanNoetrand Reinhold 1982, and second edition. 4Le Premier Tunnel Fbutier / The First Road Tunnel, 1995 PIARC Committee on Road Tunnels.

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