The 1st subsea tunnel project in Korea, Boryung tunnel which is located in western Korea, South Chungcheong province between the famous beach Daecheon, Boryung city and Wonsan Island as a part of the local road No.77 along the western coast. Large shielded TBM will be supplied more than 10m for the construction of the Boryung subsea tunnel which will form a connection for motor vehicles between Boryung city and Wonsan island over a distance of 7km twin tunnels.

These powerful High-power mobile tunneling machines will run beneath the Daecheon strait at depth of up to 80m and pressures of up 8 bar. Each tunnel stretches for a length of 6.9km, with inside space for double deck. The upper deck will accommodate 1 lane roadway, and lower deck will provide space for service and safety installations. As the Daecheon strait is one of the busiest shipping routes including ship parking lot, it is important to make sure that the construction works do not impair water borne traffic, and environment protection problems by artificial bridge construction with expected claims from the local fisher groups.

For these reason, the mechanized shield TBM tunnelling procedure was chosen over the submerged twin tube method, a bridge, or NATM conventional system for the busy and foggy area in Daecheon beech. With a diameter 10m, this is the largest TBM machine in Korea market. Delivering reinforced concrete lining segments to the two tunnels and transporting the excavated material make for a great activity in Deacheon side dumping area. Each tunnel ring consists of segments which are weighing up to 15 ton that are transported from the mobile segment plants only 0.5km away using special vehicles. Subsea tunnels pass beneath the sea where the geology is hidden by water. Normally, they are more affected by geological uncertainties than other tunnel projects because of limited geological information by the large amount of sea water.

Abnormally, the main rock types below the Daecheon strait are divided into 2types. Sedimental rock type is composed of the schists, sandstones, and shale which were found in the Boryung side to the center of the strait, and the volcanic rock type is found at the Wonsan Island side to the center of the strait with granite, rock veins along the tunnel alignment. From the detail ground investigation, five fault zones are found along the road tunnel alignment. Lots of seepage flows are expected during the tunnel excavation because of many fracture zones, maybe the special requirement to settle the water contents from the mucking materials to keep the delivery regulation by the environmental laws. Highly weathered rock zones are developed at the starting point of the Boryung side, therefore, it is required to pre-reinforce this zone to allow the TBM machine easy to pass. Finally, it is estimated that this tunneling zones are composed of soft ground to design the suitable machine type as a dual type composite high-power TBM to accommodate the both soft and hard rock bodies.

This paper summarizes how these ambitional challenging projects were planned to overcome the high risks below the seas. The contents of this paper are included into the field investigation methods, mechanized tunnel design and excavation experiences around the world.

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