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 boring surveys (see photo) are selected from those deemed necessary for creating cross-sectional diagrams of the geological layers. This is because the angle at which the surface layers penetrate underground is unknown, and there may be faults that are not visible on the surface. A fault is a point where the geological layers are broken due to crustal movements such as earthquakes. The location of faults cannot be determined by surface surveys alone. The biggest problems in tunnel construction are water and faults. Boring is performed to investigate their locations.
Ocean surveys cannot be conducted by walking as on land. Therefore, sonar surveys are performed first. A seismic source is suspended behind the survey vessel, and a vibration wave receiver is floated further behind it. Vibration waves from explosions in the ocean are reflected back by the seabed and the geological layers beneath the seabed. This allows for the investigation of the subsurface structure, just as on land. Typical sound sources include electrical signals and methods that eject pressurized gas (water guns). The signals reflected at each boundary are recorded, and by matching them to past patterns, the approximate structure can be determined.
Simultaneously, dredge is performed. This involves lowering an iron cylinder, approximately 50 cm in diameter and 3 m long, to the seabed and dragging it with a ship to collect samples of surface rock. This is done in the same sea area as the sonar survey. In the Tsushima Strait, it was performed at approximately 500 locations in total. Furthermore, in areas with suspicious locations or faults, ocean boring is conducted. A boring survey (all-core boring) involves drilling through the ground with a hollow pipe and extracting a sample of the geological layer at that location. This sample is called a "core." While drilling for oil wells or hot springs aims to reach the target oil reservoir or hot spring, in geological surveys, the goal is to collect cores along the way, so careful drilling is necessary.
The end of the rotating pipe is fitted with a blade containing industrial diamonds. By drilling 500m, you can obtain a core sample of up to 500m. Typically, the core is pulled up every 3m of drilling. The core has an average diameter of 7.5cm, and a thick pipe is used at the beginning, with thinner pipes used as you go further down, so the core also gradually becomes 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.
Therefore, the boring machines installed on board the ships are designed to withstand vertical movement. They are equipped with powerful springs to absorb the vertical motion. Off the coast of Tsushima, in water 150m deep, they drilled to a depth of 500m below the seabed over 40 days. This is about the same number of days as drilling on land.
Once the data is collected and geological maps are created, the tunnel route is selected based on them. Technically, the topography, geology, and construction methods are the issues. Faults and flooding make construction difficult and increase construction costs, so the route is selected to drill through the most stable geological layers possible. In reality, however, administrative conditions are also taken into consideration.
Geological and
construction survey
Overview of the Japan-Korea Tunnel
