Before following the progress of the geological survey for the Japan-Korea Tunnel, let's take a look at the overall picture of what geological surveys for tunnel excavation involve. Generally, what is done is to collect existing data. Data from surveys conducted by the government is stored at organizations such as the Geospatial Information Authority of Japan. There is also data held by universities. Using the collected data, a general geological map (plan view and cross section) of the area where the tunnel will be excavated is created. Based on this geological map, a surface survey (see photo) is conducted, which involves actually visiting the area in question and investigating the surface and sediments.
*Surface survey
Creating a geological plan view automatically creates a cross-section, but this is only provisional. To make it more accurate, drilling surveys and elastic wave explorations are conducted. The data obtained from these are tabulated for each layer. Applying this to the data patterns of the thousands of tunnels that have been built in Japan to date reveals the type of geological structure and the appropriate tunnel digging method for that layer. These patterns are held by JR, the Japan Railway Construction Public Corporation, the Japan Highway Public Corporation, the Ministry of Construction, electric power companies, and others, each of whom has their own patterns based on their own data. Tunnels are entirely "empirical engineering," and once certain values are obtained from the survey, the appropriate construction method is determined empirically.
The geological survey conducted before deciding on the tunnel route is called a "reconnaissance survey." It begins with a wide-area survey using physical methods, so it's called geophysical exploration, with elastic wave and electrical exploration being the most commonly used. Geophysical exploration is a surface survey, while drilling is a point survey. Elastic wave exploration involves creating vibrational waves at a single point in the geological strata to be investigated, such as by exploding explosives. These waves bounce back after hitting a geological boundary offshore, and are captured by a receiver at another point. The actual state of the strata is determined by the speed and pattern of the waves. Dynamite is often used as a seismic source for elastic waves. Elastic waves are proportional to the hardness of the strata; the harder the strata, the faster the waves travel and the smaller their amplitude. Conversely, the softer the strata, the greater the amplitude and the slower the waves travel.
Vibration waves are reflected from surfaces with different hardness in the strata. Even within the same stratum, the hardness of the strata varies depending on the process of their creation. If different strata have the same hardness, there will be no reflection from their boundaries. If there is a fault, it will naturally be reflected from there as well. Hydrological surveys are conducted to investigate the movement of water within strata. A physical survey method called electrical exploration is used. If one point on the surface of the earth is made a positive pole and the other a negative pole and electricity is passed through the ground, the waveform of the current will change depending on the water underground. This allows us to determine where water is accumulating and whether there are underground water veins. When digging a tunnel through a mountain, there is often underground pools of water, which can lead to flooding accidents. For this reason, electrical exploration is conducted over the entire mountain to check for the presence of water. Electrodes are placed all over the mountain in a pinwheel-like pattern. Once areas where pools of water are likely to exist are identified, they can then be confirmed by drilling.
*Drilling survey on land
The locations for the drilling survey (see photo) are selected from those deemed necessary to create a cross-section of the geological strata. There are cases where the angle at which the strata that are visible on the surface extend underground is unknown, and there are also cases where faults that are not visible on the surface. A fault is a place where the strata are cut by crustal movement such as an earthquake. The location of the fault cannot be determined by surface surveys alone. The biggest problems with tunnel construction are water and faults. Drilling is carried out to determine their location.
Marine surveys cannot be conducted by walking, as they are on land. Therefore, sonic exploration is first carried out. A seismic source is hung from the back of the survey vessel, and a vibration wave receiver is floated further behind it. Vibration waves from underwater explosions are reflected and returned by the seafloor and the strata below. This allows for the investigation of the underground structure, just as on land. Typical sound sources include electrical signals and a water gun, which sprays pressurized gas. Signals bouncing off each boundary are recorded, and by matching them with past patterns, a rough structure can be determined.
Dredging, carried out in parallel with this, involves lowering a steel tube approximately 50 cm in diameter and 3 m in length to the seafloor and dragging it by ship to collect surface rock samples. This is done in the same ocean area as sonic exploration. A total of approximately 500 such experiments have been conducted in the Tsushima Strait. Furthermore, marine drilling is conducted in areas where there are any obvious abnormalities or faults. Drilling surveys (all-core drilling) involve digging a hollow pipe into the ground and extracting intact samples of the underlying strata. These samples are called "cores." When drilling for oil wells or hot springs, the goal is simply to hit the target oil reservoir or hot spring, but in geological surveys, the goal is to extract cores during this process, so careful digging is required.
At the end of the rotating pipe is a blade coated with industrial diamonds. So, if you dig 500m, you can extract a maximum of 500m of core. Usually, the core is pulled up by digging 3m. The average diameter of the core is 7.5cm, and a thick pipe is used at first, and the pipe gets thinner as you go further, so the core also gets gradually thinner.
When drilling at sea, a research vessel specifically designed for drilling (see photo) is used. At the time, Tokai Salvage owned one in Japan, but it was scrapped after investigating the Japan-Korea Tunnel in the Tsushima Strait. At sea, four wires are stretched from the research vessel and huge weights called anchor rings are buried on the seabed to prevent the vessel from moving forward, backward, or sideways. When drilling was carried out in the waters on the Korean side off the coast of Tsushima, the ocean currents were so fast that one of the wires snapped. The most difficult part is dealing with the up and down motion.
For this reason, the drilling machine installed on the ship itself is designed to withstand vertical movement. It is equipped with powerful springs to absorb the vertical movement. Off the coast of Tsushima, at a water depth of 150m, it took 40 days to dig down to 500m below the seabed, which is the same amount of time as on land. Once all
the data is collected and geological maps and other documents are created, the tunnel route is selected based on that. Technically, the issues are the topography, geology, and construction methods. Faults and flooding make construction more difficult and therefore increase construction costs, so the route is selected to dig through as stable a layer of earth as possible. In reality, however, administrative conditions also come into consideration.
construction survey of the Japan-Korea tunnel
Overview of the Japan-Korea Tunnel
