Construction activities

An overview of the construction activities will be presented and outlined for the landfall based on a tunnel diameter of DN2000 mm (outer diameter approx. 2,500 mm) to allow access for people.

The activities related to tunnelling are expected to include sheet piling only during daytime only and continuous tunnelling 24/7 (for avoiding being stuck in the ground) during approximately 20 weeks. The overall construction methodology for tunnelling consists of the following operations (see Figure 3-8):

1. Establish a launch shaft for the tunnel.

2. Set up necessary machinery:

a. pipe jacking station;

b. TBM including control container and power supply;

c. slurry separation plant (if relevant).

3. Deliver pre-fabricated tunnel elements to site.

4. Start tunnelling:

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a. Push TBM into soil while excavating;

b. Lower tunnel elements into launch shaft;

c. Push tunnel elements forward while excavating;

d. repeat steps b. and c.

5. Retrieve TBM off-shore, including all necessary operations to seal off the machine and excavate it from below the seabed.

Figure 3-8 Visualisation of the launch shaft for a pipe jack tunnel. Source: www.terratec.co.

The tunnel must be constructed under water pressure and the cutterhead must be accessible in case obstructions (such as boulders) are encountered. Closed-face TBMs can excavate soil and rock under high pressure from both the surrounding soil/rock and water because the pressure from the surroundings is counteracted either by (i) a slurry suspension or (ii) using the excavated soil. Two methods for excavating the soil using closed-face TBMs are optional for this method; (i) Slurry TBM and (ii) Earth Pressure Balance TBM. The preferred method depends on the

geotechnical conditions.

Slurry TBM and separation plant

The Slurry TBM (see Figure 3-9) uses a bentonite suspension (called slurry) to counteract the earth and water pressure. The slurry is used for two purposes:

1. Achieving and maintaining the desired pressure;

2. As a transport medium for the excavated soil.

The pressure is controlled by the flow of slurry to and from the excavation chamber in a closed circuit of pipes. The “loaded” slurry is pumped through the entire tunnel to a separation plant at the launch shaft, where the excavated material is separated from the slurry. The excavated material can be disposed of according to the local regulations (clean soil for e.g. filling or reclamation, contaminated soil for disposal), and the slurry can (for the most part) be reused in the excavation chamber.

Document ID: PL1-RAM-12-Z02-RA-00003-EN 19/433 Figure 3-9 Schematic of Slurry TBM. Source: Herrenknecht.de.

The use of slurry requires a separation plant to separate the excavated soil from the slurry suspension. The most basic form of separation plant is a precipitation tank but often a more advanced system with screens and centrifugal pumps or cyclones is installed, as it is more efficient.

The slurry TBM uses very few additives for the tunnelling operation. The main additive is the bentonite used for the slurry suspension, which is a clay type/mineral. In case “hard water” is used in the slurry, bi-carbonate can be added to optimize the mixing.

Further to this, the challenges with separating fine particles from the slurry has in some cases been solved by adding flocculants to the slurry, which has not been properly separated.

Some oil and grease used for lubrication of motors and pumps can also be expected.

The tunnelling works for one landfall gives rise to approximately 8,200 m³ of excavated material.

While no evidence of contaminated ground has been found, it may be reasonable to expect that around 1.5% of the soil excavated at the landfalls may be contaminated and therefore subject to treatment as such. A total slurry volume of approximately 1,000 m³ is expected for each landfall.

The slurry is prepared from a mixture of water and bentonite, of which the raw bentonite content is expected to be approximately 50 tonnes. The slurry with its content of bentonite will be

disposed of according to applicable legal requirements.

Earth Pressure Balance TBM

Earth Pressure Balance (EPB) TBM (see Figure 3-10) does not use slurry to counteract the surrounding earth and water pressure but instead uses the excavated soil. The excavated soil is extracted from the excavation chamber with a screw conveyor and by controlling the rate of extraction, the pressure in the excavation chamber can be controlled.

Figure 3-10 Schematic of EPB machine. Source: Herrenknecht.de.

The excavated soil is then transported by a muck wagon to the launch shaft, where it is picked up and emptied and the soil can be disposed of (clean soil for e.g. filling or reclamation, contaminated soil for disposal). In order to condition the soil in front of the cutterhead, it is common to use different types of additives. This is done in order to optimize the boring and aims at improving consistency and clogging behaviour.

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It is common to use additives such as cement-based materials, dispersants and foams, including polymers and other chemicals that usually require an authority permit. The dispersants, foams and polymers are used to condition the material in front of the cutterhead and most of these additives are excavated and transported to the launch shaft. The cement-based materials are used along the circumference of the final tunnel lining (pipes) to fix the position of the tunnel.

These will remain in the ground. Some oil and grease used for lubrication of motors and pumps can also be expected.

Interventions

Both EPB and Slurry TBMs are designed to work under soil and water pressure. When access to the cutterhead is necessary to inspect and replace cutting tools, the TBM is fitted with a pressure chamber at the rear end of the machine. This allows decompression for divers that have carried out inspections and replacement of cutterhead tools at the excavation front under pressure delivered by compressed air.

When inspection and maintenance of the cutterhead and tools are necessary, the tunnelling must stop, and the stability of the excavation front is maintained by an air pressure. The intervals between these interventions and their locations depends very much on the geology of the project and must be carefully planned. The duration of an intervention depends on the need for

exchanging cutting tools.

Pipeline pull-in

The pipeline will be installed pulling the pipeline ashore from a shallow water lay-barge anchored off the coast, following completion of the tunnel. The pipeline will be welded together on the lay barge, and the pipe string will be pulled ashore using a pull winch at the onshore work site.

The landfall construction for the pull-in operation includes set-up pull winch spread at the onshore work site, establishing pipeline trench until a water depth of approximately 10 m, installing pull-wire from onshore work site to offshore pick-up location and mobilization of shallow-water lay barge and shore-pull operations.