Geological strata are formed by deposits of earth and sand that harden under pressure over many years, so in general, older strata are harder and more stable.
The strata beneath the ocean floor are often newly deposited sand that has not yet turned into rock. The geological formations around the Japanese archipelago are relatively new, and are highly volcanic and seismic, making them even more complex. Based on the geological surveys conducted so far, the following conclusions can be drawn.
① As a prerequisite for route selection, route standards must be determined.
Vehicles that can run inside tunnels include automobiles, trains, and linear motor cars, but route standards differ depending on each case. ②There are concerns about the ergonomic limitations of the driver when driving a car through a super long tunnel.
Furthermore, the linear motor car has just entered the testing stage, and it seems that it will still take some time before it can be put into practical use. Therefore, at this stage, we have decided to assume the most realistic Shinkansen tunnel and use that route standard.
① the maximum slope is 20/1000 (20m drop at 1000m),
② the minimum curve radius is 5000m -
this can be fully adapted even if it is replaced with a road tunnel.
The biggest problem in selecting the route for the Japan-Korea tunnel is the presence of a large fault with a deep dip in the Tsushima Strait West Channel and the presence of unconsolidated young sedimentary layers there. .
In order to correspond to this stratum, the following two cases can be considered for the route. The first case involves passing through rock beneath an unconsolidated sedimentary layer.
In this case, the construction work is considered to be relatively safe, but the tunnel will have to pass through a considerable depth, making the total length of the tunnel that much longer. The tunnel can be excavated using the mountain tunnel construction method that combines rock injection with water stoppage, which was experienced in the Seikan Tunnel. The second case is to pass through an unconsolidated layer, and the shield method will be used. In this case, the depth of the tunnel becomes shallower, so the total length becomes shorter accordingly. However, there is no precedent for shield construction under high water pressure at a depth of more than 150 meters, and there are many technical issues that need to be resolved. In any case, it seems possible to excavate the area from Kyushu to Iki using the mountain tunnel method of digging through the bedrock.
The problem is how to pass through the younger sedimentary layers that exist in the East and West Channels of the Tsushima Strait, and there are two types of excavation methods depending on the properties of the layers. Based on the above geological conditions, the tunnel route shown above is currently being proposed.
This is a geological map from Kyushu to Geoje Island in South Korea. By the way, Route C is the route that takes you directly to Busan, but it has the disadvantage of being very long in total. Here, I would like to explain the general geological situation limited to the route that leads to landing on Geoje Island. First, in the Kyushu region, there is the Tertiary layer of the Karatsu Coalfield, the surface layer of which is covered with basalt lava, and intrusive rocks can also be seen.
As a result of magnetic survey, it was found that Iki Channel has many intrusions of igneous rock. Iki is comprised of the Tertiary Iki Formation Group, which is also covered with basalt lava. In the Higashi 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 geology is the Katsumoto Group on the Iki side and the Taishu Group on the Tsushima side, but there are some younger sedimentary layers in between, so if you try to avoid that, the tunnel will have to be dug quite deep. It turns out. Most of Tsushima is made up of the Tsushu Group, with a large granite rock in the south, and the surrounding area has become hornfels.
Hornfelsization is the alteration of rocks by the action of heat. Since granite has heat, the surrounding strata that it comes in contact with are affected by heat. Hornfelsized rocks are extremely hard. Off the coast of Tsushima's Nishi Channel, a large fault runs parallel to the coast, and on the west side the bedrock has fallen deeply, on which new sedimentary layers have been deposited.
This new sedimentary layer has been accumulating on the ocean floor like falling snow since the Pliocene era, more than 2 million years ago, or even earlier. It is still in the process of diagenesis and has not yet completely turned into rock. Therefore, it is thought that it contains a large amount of water and is extremely soft. There are two ways to pass through this area: use the shield method to dig through the sedimentary layer, or use the mountain method to excavate the bedrock underneath.
The deepest point of the Nishi Channel is 150 meters deep, and if the tunnel is built through bedrock, it would be 550 to 600 meters deep. This bedrock is thought to be shallowing toward South Korea at an elevation angle of about 4 degrees. Based on this geological overview, problems expected during construction can be summarized as follows.
First of all, in the Kyushu region, there are places where granite has weathered to resemble blocks of sand, and there is a risk that it will collapse more easily if groundwater is mixed in.
Igneous rocks are distributed on the seabed of Iki Channel and Higashi Channel, and there is a possibility that sudden spring water may occur there. Iki and Tsushima have limited water resources and rely on groundwater for daily life, agriculture, fishing, and other purposes.
Therefore, consideration must be given to minimizing the impact on groundwater use. Furthermore, how to deal with the unconsolidated young sedimentary layer beneath the Nishi Channel seabed can be said to be the biggest issue concerning the entire tunnel plan. The geological surveys we have conducted so far have been general and qualitative, so the next stage will require concrete, quantitative surveys that are deeply involved in considering design, budget, and construction methods. .
Therefore, future challenges can be summarized into the following four areas. ① First, strengthen the elucidation of the geological and engineering properties along the entire route, both on land and under the sea.
②Understand the hydrogeological situation in each region in order to respond to groundwater problems in the land areas of Kyushu, Iki, and Tsushima.
③To understand and evaluate the geological and engineering properties of the younger sedimentary layers in the East and West Channels.
④ Understand and evaluate the geological distribution and structure of the seabed, as well as their engineering properties.
Up to now, marine acoustic wave surveys have been the mainstay of seabed geological surveys, but since it is difficult to grasp the physical properties of the geology with just that, we plan to conduct marine seismic surveys in addition to that.
Regarding the geology of the land area, we will continue to compare the stratigraphy of each area in order to elucidate the geological structure between Kyushu, Iki, and Tsushima.
At the same time, hydrogeological surveys will continue. Furthermore, we plan to conduct geotechnical studies corresponding to construction methods for both land and seabed areas. Organizing surveying is also a major issue, and it is necessary to unify surveying standards and organize data for the surveys that have been conducted on land and at sea. There is also the issue of unifying surveying between Japan and South Korea. Since the reference point is in Nihonbashi, Tokyo in Japan and Incheon in South Korea, it is important to accurately understand this interrelationship. Therefore, it will be necessary to continue promoting exchanges between Japan and South Korea.
construction survey of Japan-Korea tunnel
Overview of the Japan-Korea tunnel