Geological layers are formed when sediment is deposited and solidified under pressure over many years, so generally, older layers are harder and more stable. The layers just below the seabed are often newly deposited sand that has not yet turned into rock. The geological layers around the Japanese archipelago are relatively recent in their formation and are complex due to active volcanic and seismic activity.
Based on the geological surveys conducted so far, the following conclusions can be drawn:
① As a prerequisite for route selection, the route specifications must be determined. Vehicles that can run through tunnels include automobiles, trains, and maglev trains, but the route specifications will differ depending on the case.
② There are concerns about the ergonomic limitations of drivers when driving automobiles through extremely long tunnels. Also, maglev trains have only just entered the testing phase, and it is thought that it will take a considerable amount of time before they are put into practical use.
Therefore, at this stage, we have decided to assume the most realistic Shinkansen (bullet train) tunnel and use its route specifications.
Specifically,
① the maximum gradient is 20/1000 (a drop of 20m over 1000m), and
② the minimum curve radius is 5000m—
this can be easily adapted even if it is replaced with a road tunnel.
The biggest problem in selecting a route for the Japan-Korea tunnel is the presence of a large fault with a deep drop in the western channel of the Tsushima Strait, and the unconsolidated, young sedimentary layers accumulated there. To deal with this geological formation, two main route cases are considered.
The first case is to run the tunnel through the bedrock beneath the unconsolidated sedimentary layers. In this case, construction is considered relatively safe, but it will pass through considerable depth, so the total length of the tunnel will be correspondingly long. In terms of construction, it can be excavated using the mountain tunneling method combined with rock mass watertightness injection, which has been proven in the Seikan Tunnel. The second case is to run the tunnel through the unconsolidated layer, and the shield tunneling method will be used. In this case, the tunnel depth will be shallower, so the total length will be correspondingly shorter. However, shield tunneling under high water pressure exceeding 150m in depth has no precedent, and there are many technical challenges that need to be solved.
In any case, it is thought that the section from Kyushu to Iki can be excavated using the mountain tunneling method that excavates through bedrock. The problem lies in how to pass through the newer sedimentary layers in the eastern and western channels of the Tsushima Strait, and the excavation method will differ depending on the properties of these layers.
Based on the geological conditions described above, the tunnel routes shown above have been proposed. This is a geological plan from Kyushu to Geoje Island in South Korea. Incidentally, Route C is a route that goes directly to Busan, but this has the drawback of being extremely long in total length. Here, I would like to explain the geological overview limited to the route that goes to Geoje Island.
First, in the Kyushu region, there are Tertiary layers of the Karatsu coalfield, and the surface layer is covered with basaltic lava, and intrusive rocks can also be seen. Magnetic surveys of the Iki Channel have revealed that there is a lot of igneous rock intrusion. Iki consists of the Tertiary Iki Group, which is also covered with basaltic lava.
In the eastern channel between Iki and Tsushima, there is a reef called Shichirigasone in the middle, and the presence of igneous rocks is expected around it. The general geological composition is the Katsumoto Group on the Iki side and the Taishu Group on the Tsushima side, but there are some newer sedimentary layers in between, so if you try to avoid them, the tunnel will have to be dug quite deep.
Most of Tsushima is the Taishu Group, with a large granite mass in the south, and the area around it has been transformed into hornfels. Hornfels is the process by which rocks are altered by heat. Granite is hot, so the surrounding strata it comes into contact with are also subjected to heat. Hornfels rocks are extremely hard. Offshore in
the western channel of Tsushima, a large fault runs parallel to the coast, and to the west of it, the bedrock drops deeply, and newer sedimentary layers are deposited on top of it. These newer sedimentary layers have been continuously deposited on the seabed like falling snow from the Pliocene epoch of the Tertiary period, more than 2 million years ago, or even earlier, until the present day. They are still in the process of diagenesis and have not yet become solid rock. Therefore, it is thought to contain a large amount of water and be extremely soft.
To pass through here, two methods are considered: excavating through the sedimentary layer using the shield tunneling method, or excavating the bedrock below using mountain tunneling methods. The deepest point of the West Channel is 150m deep, and if the tunnel passes through the bedrock, the depth will be 550-600m. This bedrock is thought to become shallower at an angle of approximately 4 degrees toward Korea.
Based on this geological overview, the problems expected during construction can be summarized as follows.
First, in the Kyushu region, there are places where granite has weathered into sandy lumps, and there is a risk that it will collapse if it contains groundwater. Igneous rocks are distributed on the seabed of the Iki Channel and East Channel, and there is a possibility of sudden seepage of water.
In Iki and Tsushima, water resources are limited, and they rely on groundwater for domestic water, agriculture, and fishing. Therefore, consideration must be given to minimizing the impact on groundwater use. Furthermore, how to deal with the unconsolidated, newly formed sedimentary layers beneath the seabed of the West Channel is arguably the biggest problem affecting the entire tunnel project.
The geological surveys conducted so far have been general and qualitative, so the next stage will require specific and quantitative surveys that will be deeply involved in the consideration of design, budget, and construction methods. Therefore, the challenges for the future can be summarized into the following four points:
① First, strengthen the elucidation of the engineering properties of the geology along the entire route, both on land and underwater. ②
To address groundwater issues in the land areas of Kyushu, Iki, and Tsushima, understand the hydrogeological conditions in each region.
③ Understand and evaluate the geological and engineering properties of the newly formed sedimentary layers in the East Channel and West Channel.
④ Understand and evaluate the distribution and structure of the geology on the seabed, as well as their engineering properties. It should be noted that
while surface seismic surveys have been the mainstream method for seabed geological surveys so far, this alone is insufficient to grasp the physical properties of the geology, so in the future, we plan to implement surface seismic surveys in addition to seismic surveys.
Regarding the geology of the land area, we will continue stratigraphic correlation of each area to clarify the geological structure between Kyushu, Iki, and Tsushima. At the same time, we will continue hydrogeological surveys. Furthermore, we plan to conduct geotechnical engineering studies corresponding to the construction methods for both the land and seabed areas.
Standardizing the survey data is also a major challenge; it is necessary to unify the survey standards and organize the data from surveys conducted so far on land and at sea. In addition, there is the issue of standardizing surveys between Japan and South Korea. The leveling datum is located in Nihonbashi, Tokyo, in Japan, and in Incheon, South Korea, so it is necessary to accurately understand this relationship. For this reason, it will be necessary to continue to promote exchanges between Japan and South Korea.
Geological and
construction survey
Overview of the Japan-Korea Tunnel
