Sub Aqueous Tunnels

10 Subaqueous tunnels

The following possibilities are available for the crossing of water ways1: Ferries: These are slow and have a reduced transport capacity when compared with bridges and tunnels (e.g. the Channel tunnel has shortened the drive from Paris to London from 6 hours to 2 hours and 40 minutes). In addition they are dangerous to other ships and are affected by bad weather.2 Stationary or mobile bridges: The following types can be considered: • floating pontons with movable sections in the centre to allow the passage of ships • mobile bridges. Opening and closing of the bridges is time consuming and hinders the traffic. High bridges: These must be sufficiently high to allow the passage of ships: 10 m for inland navigation, 25 m for coastal navigation, 70 m for open sea navigation. With a longitudinal slope of 4% for roads and 1.25% for rail, long ramps may be necessary. High bridges may affect the scenery and traffic may be impaired by bad weather (e.g. hurricanes). Subaqueous tunnels: They can be up to 50% more expensive than bridges but they help to avoid the disadvantages cited above. The following variants are available: • anchored floating tunnels • subaqueous bridges erected on piers

1

M. Kretschmer und E. Fliegner: Unterwassertunnel, Ernst und Sohn, 1987 In 1954 a storm caused 5 ferryboats to sink in Japan, 1,430 persons died. Thereupon, the 54 km long Seikan tunnel was built (from 1971 to 1988), that connects the islands Honshu and Hokkaido. 2

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•

cut and cover below water level, either within a sheet- or bored pile wall, or constructed with towed and lowered parts, or built as caissons or mined. Mined tunnels have covers between 30 and 100 m and are, therefore, deeper than tunnels constructed with cut and cover. Therefore, they need longer ramps. Navigation requires a free water depth between 50% (for inland navigation) and 10% (for open sea navigation) of the clearance above sea level.

10.1 Towing and lowering method Pre-fabricated blocks (Fig. 10.1, 10.2) with lengths between 40 and 160 m are towed and subsequently lowered to their final position. To render them floatable, their front parts are temporarily bulkheaded off. They are designed to withstand loads between 10 und 80 kN/m2 due to wracks as well as an impact of anchor of 1,300 kN distributed over a surface of 1.5×3 m. A pressure drop of 7.5 bar due to passage of ships should also be taken into account. Tunnel elements of concrete are waterproofed by means of steel sheets, bitumen or synthetic membranes. Water-tight concrete is increasingly used. If steel is used, corrosion should be taken into account. If the water contains dissolved carbonate, a protective layer is created which inhibits corrosion. Otherwise losses of 0.01 to 0.02 mm steel annually have to be taken into account due to corrosion. The so-called cathodic corrosion protection is based on a galvanic voltage applied between the steel to be protected and an anode made of zinc, magnesium or aluminium. The stability of the floating tunnel elements has to be checked according to the rules of ship-building. A resonance of the tunnel element and of the water swapping within the ballast tanks should be avoided. The subsoil investigation is based on sampling from boreholes and sounding. This can be carried out from ships, if the water depths do not exceed 15 m. For depths up to 25 m the investigations can be operated from leg jack-up drilling rigs. Remote controlled vehicles that are capable of sampling and sounding are also applicable on the sea ground. The depth of the basement must comply with the requirements of navigation, i.e. a minimum free water depth of 15 m must be allowed for. Taking into account a tunnel height of 8 m and the corresponding cover, depths of ca 30 m must be dredged. The surface of the dredged channel should be controlled e.g. with ultrasonic sounding. Before touching the ground the lowered element causes the water to escape from the gap. The accelerated water flow may induce flutes. Therefore, sandy ground should be protected with a gravel cover.

8.545

Glacial deposits Copenhagen limestone

Northern motorway tube

Southern motorway tube

Escape and service galleries

75 6.57

6.605

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0. 95

9.75

0.

2

0. 75

9.75

0. 5 0. 5

0. 8 0. 95

10.1 Towing and lowering method

2.225

Protection rockfill

Northern Southern railway railway tube tube

Backfill

Gravel bed foundation

All dimensions in metres

Fig. 10.1. Cross section of the Øresund tunnel3

Fig. 10.2. Pre-fabricated tunnel blocks for Øresund tunnel4

Due to buoyancy the bottom contact pressure is low (usually between 5 and 10 kN/m2 ). A safety factor of 1.1 with respect to buoyancy has to be assured. The following types of bedding can be applied: Bedding upon gravel: The gravel bed has to be bulldozed. Underfilled basement: The tunnel element is deposed upon temporary footings. The remaining space is subsequently filled with hydrauliking material (mixture of sand and water). The sand bed obtained is rather loose and, thus, prone to settlement and liquefaction. A remedy is to add cement to the hydrauliking material. Pile foundation is to be applied in case of weak underground.

3

Busby, J., Marshall, C., Design and construction of the Øresund tunnel, Proceedings of the ICE, Civil Engineering 138, November 2000, 157-166 4 www.thornvig.dk

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10.2 Caissons Submerged tunnels can be constructed as arrays of caissons. A caisson is a box driven downwards by removing the included soil and, thus, evoking a series of controlled punchings. The removal of soil can be done either in open caissons or in closed chambers filled with pressurized air5 (so-called pneumatic caissons, Fig. 10.3).

Fig. 10.3. Pneumatic caisson, lock (right)

Fig. 10.4. Work chamber of a pneumatic caisson (Metro Amsterdam)

5 H. Lingenfelser: Senkk¨ asten, Grundbau-Taschenbuch, 4. Auflage, Teil 3, Ernst und Sohn, Berlin 1992

10.2 Caissons

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Fig. 10.5. Caisson for Metro Amsterdam: mould for concrete

The working chamber of pneumatic caissons is filled with concrete after arrival at the final depth. Pneumatic caissons have a series of advantages: • • • • •

The penetrated soil can be inspected during lowering and obstacles can easily be removed. The groundwater is not disturbed No vibrations are produced Loading the space above the working chamber with soil or water renders lowering always possible. The caisson can be built elsewhere (e.g. on an artificial island) and towed to its final position.

An offset of 3 - 10 cm width is provided 3 m above the cutting edge (Fig. 10.6). The resulting gap is filled (or grouted) with bentonite slurry to reduce wall friction (Fig. 10.6). Rough external walls cause an increased wall friction. The inner wall of the cutting edge should be neither too flat (to enable access to the soil) nor too steep (to avoid a too deep penetration into the soil). The tip of the cutting edge is reinforced with a steel shoe appropriately anchored within the concrete. The clearance within the working chamber should be ca 2.0 - 2.5 m. The ceiling of the working chamber should have a thickness of at least 60 cm to support the loads exerted by the pressurized air and the ballast. An appropriately formed sand heap can serve as formwork for the inner wall (Fig. 10.5). The excavation within the working chamber can be done with the help of water jets. Muck can be pumped away (Fig. 10.4). By means of a lowering plan it should be assessed that the ballast load is sufficient to punch the caisson (Fig. 10.7). On this, the driving forces G+B are balanced to the resisting forces P +R+V for every intermediate state (Fig. 10.7). Herein are P: Air pressure resultant = pl · A, where pl is the air pressure and A the loaded surface.

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peripheral trench peripheral gap

Fig. 10.6. Cutting edge and wall gap filled with bentonite suspension

Fig. 10.7. Lowering plan

R: Wall friction resulting from the horizontal earth pressure6 multiplied with tan δ. δ is the wall friction angle and can be estimated to 23 ϕ. Adjacent to the bentonite lubrication is δ = 5◦ , or a wall shear stress of 5 - 10 kN/m2 can be assumed. V: Vertical component of the edge force.7 V is obtained from the limit load pg , the distribution of which is assumed as shown in Fig. 10.8. pg can be estimated as follows: 1.2 - 1.6 MN/m2 (gravel) 0.9 - 1.3 MN/m2 (sand) .

6

although not theoretically justified, usually active earth pressure is assumed. P. Arz, H.G. Schmidt, J. Seitz, S. Semprich: Grundbau, Abschnitt 5: Senkk¨ asten. In: Beton-Kalender 1994, Ernst und Sohn, Berlin 7

10.2 Caissons

Fig. 10.8. Assumed distribution of stress at the cutting edge

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