For shallow water crossings, civil engineering can provide either bridges or tunnels. The feasibility of either, and the construction method chosen, primarily depends on the parameters of distance, water depth and sea state.

Technically, the immersed tunnel method ought to be considered when the profile of the sea bed or river bed is suitable for dredging or upfill. A water depth of 20m to perhaps beyond 40m is perfectly acceptable. Immersed tunnels can be advantageous over the bored option as they can be constructed with a variety of cross sections tailored to the actual project requirements. The tunnel structures can be closely adjusted to the gauges for railways, roads, service ducts, ventilation ducts or galleries, and several such tubes can be combined into a single monolithic tunnel structure with an alignment that can also be chosen practically at random.

Long bored tunnels can require considerable construction time, especially without intermediate access. Immersed tunnels can be assembled with considerable speed, provided production of the elements is sufficiently fast. Crossings of narrow water stretches are much shorter as immersed tunnels, as their shallow depth permits portals to be located closer to the waterside banks, thereby reducing costs. Land occupation for mobilisation sites can be minimal at the location of immersion, if tunnel elements are fabricated remotely. Heavy road traffic can also be avoided or at least considerably reduced.

The immersed tunnel construction method is also feasible in most ground conditions, including soft materials and can be flexible enough to follow longitudinally unequal settlement. If necessary, piled foundations can be used. The construction method is generally fully suitable for earthquake zones.

Although it is not obvious that some or all of the advantages mentioned are applicable in every single case of a fixed link, the above aspects ought to be considered carefully early on to determine the most feasible, usable and economic solution.

Disadvantages

Immersed tubes do have disadvantages including the embankment’s need to be at temporary disposal and the water depth must not exceed practical limits. The tunnel alignment, after shaping of the sea bed or bed of the watercourse by dredging or upfill, must not be discontinuous or too steep. Poor visibility, resulting from adverse weather conditions during the immersion phase might impede surveying, delaying placement of elements, whilst strong water movement or ice flows can hamper transportation and immersion of elements.

Possibly the most important consideration is the disruption to ship traffic on a watercourse during the immersion phase which can pose economic restrictions on the area served by the river.

Construction method

Immersed tunnel construction follows several relatively straightforward techniques. Firstly a trench must be dredged in the water course bed. Excavations at each shore terminus of the trench will hold the land and portal elements of the tunnel. During construction, one of these pits can double as a fabrication yard for the elements and in this case needs to be shut off against the open water by, for instance, a cofferdam.

A fabrication yard, casting basin, dry dock or ship-lift platform is then required to construct the actual tunnel elements. As transport costs for floating elements are not usually critical, the construction yard may be situated some distance from the final tunnel.

Once cast, the elements are sealed at either end by temporary bulkheads to make them float during transport to the location of the crossing. The elements are also fitted with temporary ballast tanks, to serve during immersion. Snorkel-like temporary steel shafts provide means of access to each element and serve as points of reference for surveying during floating and lowering to the bottom of the dredged trench.

Once a floating element has been towed to its destination, flooding of the temporary ballast tanks permits the lowering and placement of the element.

During settling on its preliminary support at the bottom of the trench, the element is horizontally docked to touch its neighbouring, previously placed element along the front of their common joint, the joint having been fitted with rubber profile seals. As soon as these seals are just tight enough, the water between the temporary bulkheads of both adjoining elements is evacuated activating the static hydraulic pressure on the bulkhead at the far end of the new element, ultimately causing the joint to close and the gasket of the joint to seal. Using the natural hydrostatic forces to make an almost instant watertight connection between elements with a weight of perhaps tens of thousands of tons, is not only an ingenious engineering trick, but is also one of the basic reasons for the surprising success of the immersed tunnel construction method.

After an element has been settled and locked, the space underneath the element must be backfilled with sand or gravel by pumping a mixture of water and material through openings in the bottom slab of each element (so-called sand-flow system; this system does not require gantries). The material/water mix can be jetted underneath the tunnel through pipes inserted sideways from outside the tunnel (sand jetting system). Alternatively the foundation can consist of a gravel bed that requires no further backfill.

The entire trench is than backfilled and the structure covered to prevent physical damage. Backfill material must comply with undertow and suction caused by ships, usually using layers of stones.

Experience has shown a number of pitfalls that can occur during construction, but once identified, can easily be avoided: siltation and sedimentation of the trench needs to be minimised to prevent future settlements. As long as backfill is incomplete, shipping movements can cause adverse hydraulic loads on the newly placed tunnel elements.

Structural design

Reinforced concrete and prestressed concrete, as well as composite steel/concrete, are the principal construction technologies for immersed tunnels. Design rules vary with national regulations and specifications imposed by the client. In addition, immersed tunnels require further consideration to particular loads, specific to this kind of structure and its construction.

During transportation and immersion, the elements are required to float. Geometry of elements must be designed accordingly. The temporary ballast tanks permit control weight balance. It is desirable to adjust the permanent weight of the floating structure in such a way that ballasting necessary for immersion is minimal. Variations in water density (varying salinity) and density of construction materials need to be considered in detail. Hydraulic suction caused by ships passing over during the installation phase might be critical.

Temporary construction loads

Elements are not uniformly loaded during floating, towing and immersion with longer elements receiving higher bending moments. Sea transportation may expose elements to long waves and to correspondingly increased forces, construction joints have to be secured during transportation.

In the final state, uplift forces at the maximum expected density of water and perhaps hydraulic uplift caused by tidal lag need to be compensated by permanent stabilising loads. Such loads include the theoretical weight of the final structure including protective or ballast concrete and any later non-removable structural installations inside and outside. Backfill surcharge, friction and mechanical equipment must be excluded from this balance. A safety factor, compliant with regulations, will have to be applied.

Regarding structural analysis and design, elements for immersed tunnels do not differ too much from other concrete or composite structures and the conventional rules for those are fully adequate. Particular attention ought to be given to stress resulting from unequal settlements and accidental loads. Immersed tunnels in earthquake zones are perfectly feasible but may call for more relevant design principles. With watertightness being essential for life cycle performance of the final tunnel, crack control will be a major concern in planning and design of concrete elements. Modern standards for concrete design, such as the German DIN 1045, incorporate appropriate measures for crack control.

Civil engineering contractors sometimes submit advantageous proposals for high performance concrete structures, based on their company’s long term experience and research. Particularly interesting are any developments which are suitable to eliminate or reduce the number of shrinkage joints. Construction joints between bottom slabs and outer walls, poured in separate cycles, specifically require control of thermal shrinkage. Measures applicable have been successfully derived from experience with water basins (e.g. cooling of concrete for sewage treatment basins).

Regarding joints, four categories will have to be applied: shrinkage joints (to deal with longitudinal movements); immersion joints (to connect elements to each other); terminal joints (to connect tunnel to terminal/portal structures); and closure joints. For more than half a century, numerous variants have been developed especially for immersion joints and many have proved to work perfectly under various conditions. Transfer of shear and longitudinal forces, watertightness and relative simplicity of application are the predominant achievements. Closure joints are necessary where the tunnel is built in more than one lot. Joints for immersed tunnels merit a more thorough study far beyond the scope of this present report.

Economic aspects

Only if technical feasibility has been confirmed for a project does the immersed tunnel enter into competition with its natural alternative, the bored tunnel. The various geological, geographical, infrastructural, ecological and even political circumstances of the individual fixed crossing project will affect the economic viability of either construction method. For this reason, cost figures are difficult to state unless a particular project is defined and its conditions are determined.

When major fixed links require inhomogeneous boundary conditions, projects may still be engineered as a combination of immersed and bored tunnel sections (or bridges and immersed tunnels), yielding the overall most economic solution.

However, for all applications it will be necessary to undertake economic comparison studies in order to determine the ideal construction scheme. Such studies should not only quantify pure construction costs, but also possible economic and contractual advantages in terms of reduced geotechnical risk, construction time, space utilisation, life cycle costs, quality control and finance parameters. Where ground risks and contractual responsibilities are to be delegated away from the owner and towards the contractor, the immersed tunnel is basically fair on all parties involved. The same applies to projects where a short construction time affects economic viability.

An approximate cost breakdown for a hypothetical tunnel can only be obtained from statistical considerations and may further vary with circumstances. A hypothetical 11m diameter twin bore motorway tunnel constructed by slurry TBM would have costs of 10% for risk, 5-10% for separation, 13-18% for excavation, 25-30% for segments and 37-42% for TBM and plant. A corresponding immersed tunnel would have a spread of 2% for risk, 15-20% for floatation and immersion, 10-20% for the casting yard, 30-40% for element fabrication and 20-30% for dredging and backfilling. Emergency escape ducts to connect both tubes, necessary in accordance with modern standards, have not been included in the estimation. The corresponding immersed tunnel would be built with all four lanes incorporated in one tunnel, with a cross section tailored to the gauges. Costs for interconnecting passages between both tubes are reduced to the costs of fire escape doors. Apart from reduced risk allowances, this example yields speculative cost savings in the area of 1/3 for construction costs, if the immersed tunnel method can be applied.

Past projects equally seem to demonstrate that, if the method is feasible, an immersed tunnel is economically desirable. Only when technical obstacles prevent the application of the immersed tunnel method, bored tunnels are very welcome fallback solutions, regardless of construction costs.

Future relevance

The immersion method, where applicable, can permit rapid completion of subaqueous tunnels at reduced risks and costs. The method is a straightforward application of advanced civil engineering disciplines, combined with a century of expertise. The working group on Immersed and Floating Tunnels of the International Tunnelling Association (ITA), acts as an international platform of immersed tunnel technology. It focuses its attention on the advancement of the technology and future applications (www.ita-aites.org).

In many cases a short construction time is essential for the feasibility of a tunnelling project. Long tunnels are probably quicker to complete if the immersed tunnel method can be applied, especially if sections are constructed simultaneously. Several tubes can be constructed in a single operation, thus enhancing the efficiency of an entire project. Feasibility can be verified well before major financial risks are taken and risks assessed prior to commencement of works.

In a world of growing needs for enhanced infrastructure under tight economic and financial conditions, there is increasing demand for the technically established and safe solutions provided in many cases by the immersed tube.

Related Files
diagram showing the construction process, as used on the now famous Øresund fixed link immersed tube between Sweden and Denmark