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Geological survey for tunnel construction

Before following the progress of the geological survey in the Japan-Korea tunnel, I will explain the overall picture of what the geological survey for tunnel excavation does.In general, what you do is collect existing material.Data from surveys conducted by the government are stored in the Geospatial Information Authority of Japan.There are also materials that the university has.The collected materials will be used to create a schematic geological map (plan and cross section) of the area where the tunnel will be dug.Based on the geological map, surface reconnaissance (see photo), that is, actually visiting the area and investigating the surface and sediments.

 


* State of surface reconnaissance

If you make a plan view of the geology, you can automatically make a cross section, but that is only a tentative one.In order to make it more accurate, we will carry out "boring survey" and "elastic wave exploration".The data obtained by them is tabulated for each layer.If you apply it to the data pattern of thousands of tunnels that have been done in Japan so far, you will find out what kind of stratum structure it has and what is the appropriate "how to dig a tunnel" for that stratum. ..JR, Japan Railway Construction Public Corporation, Japan Highway Public Corporation, Ministry of Construction, electric power companies, etc. have the pattern, and each has its own pattern based on its own data.The tunnel is completely "empirical engineering", and if a survey value is obtained, the applicable construction method will be determined empirically.

 

The geological survey until the tunnel route is decided is called "overview".At first, it is called geophysical exploration because it is investigated by a physical method so as to cover a wide area, and elastic wave exploration and electrical exploration are often used.Geophysical exploration is a surface survey, and boring is a point survey.In elastic wave exploration, a vibration wave is created by an explosion of explosives at one point in the stratum to be investigated, and when it hits the boundary of the stratum off the ground and bounces off, it is captured by another receiver, and the wave is detected. It explores the actual condition of the stratum by speed and pattern.Dynamite is often used as the epicenter of elastic waves.Elastic waves are proportional to the hardness of the formation, and the harder the formation, the faster the waves propagate and the smaller the amplitude.On the contrary, when the stratum is soft, the amplitude becomes large and the wave propagates slowly.

 
The vibration wave is reflected from the surface where the hardness of the stratum is different.Even within the same stratum, the hardness of the stratum differs depending on the formation process.If the hardness is the same in different strata, there is no reflection from the boundary surface.If there is a fault, it will naturally be reflected from it.The "hydrological survey" is to investigate the movement of water in the stratum.As geophysical exploration, a technique called electrical exploration is used.When one point on the surface of the earth is set to the positive pole and the other is set to the negative pole, and electricity is passed through the ground, the waveform of the current changes depending on the water in the ground.It tells you where the water is and where the groundwater veins are.When digging a tunnel in a mountain, there is often pooled water in the ground, which can lead to flood accidents.Therefore, the entire mountain is subjected to electrical exploration to check for water.Electrodes will be erected on the entire mountain like a needle mouse.When it finds out where the pool is likely to be, this time it is confirmed by boring.

 


* Boring survey on land

Select the location for the boring survey (see photo) from the locations considered necessary for creating a cross section of the stratum.There are cases where it is not known at what angle the strata that are exposed on the surface of the earth are in the ground, and there are faults that are not exposed on the surface of the earth.A fault is a place where the stratum is cut due to crustal movements such as earthquakes.The location of the fault cannot be determined from the surface reconnaissance alone.Water and faults are the biggest problems in tunnel construction.Do bowling to find out where it is.

Surveys in the ocean cannot be surveyed by walking like on the ground.Therefore, the sound wave exploration is performed first.The epicenter is hung behind the research vessel, and the vibration wave receiver is floated behind it.Oscillating waves from an explosion in the sea are reflected back to the seafloor and the strata below the seafloor.By doing so, it is possible to examine the underground structure in the same way as on land.A typical sound source is an electrical signal or a method of ejecting pressure gas (water gun).Bounced signals are recorded at each boundary and applied to past patterns to get an approximate structure.
 
Dressing is done in parallel with that.In this method, an iron cylinder with a diameter of about XNUMX cm and a length of about XNUMX m is lowered to the seabed and dragged by a ship to collect a sample of rock on the surface.Do this for the same area as the sonic exploration.In the Tsushima Strait, it was carried out at about XNUMX places in total.In addition, ocean boring is performed in places where something is wrong or there is a fault.Boring survey (all-core boring) is to dig underground with a hollow pipe and take out a sample of the stratum at that location as it is.This sample is called the "core".In the case of excavation of oil wells and hot springs, it is sufficient to hit the target oil reservoir or hot spring, but in the case of geological survey, the purpose is to collect the core of the process, so it is necessary to dig in carefully.
 
The tip of the rotating pipe has a blade with an industrial diamond attached.So if you dig XNUMXm, you can get a core of up to XNUMXm.Normally, the core is pulled up to dig XNUMXm.The average diameter of the core is XNUMX cm, and a thick pipe is used at first, and the pipe becomes thinner as it goes, so the core also becomes thinner.


* Maritime boring
 


* Bowling core loaded on board

In the case of the ocean, a research vessel dedicated to bowling (see photo) will be used.At that time, one Tokai salvage ship was owned by Japan, but it was abandoned after a survey of the Japan-Korea tunnel in the Tsushima Strait.At sea, four wires are extended from the research vessel, and a huge weight called an anchor ring is buried in the seabed to eliminate the movement of the vessel in front, back, left and right.When I was bowling in the waters on the Korean side off Tsushima, the ocean current was so fast that one wire broke.The most difficult thing is to deal with vertical shaking.
 

Therefore, the boring machine itself installed on the ship is made to withstand vertical movement.It has a strong spring and absorbs vertical movement.At a depth of XNUMXm off the coast of Tsushima, we dug up to XNUMXm below the seabed over XNUMX days.This number of days is about the same as on land.
 
Once the data is available and a geological map is available, the tunnel route will be selected based on the data.Technically, topography, geology and construction methods are issues.If there are faults or floods, construction will be difficult and the construction cost will increase accordingly, so the route should be selected so as to dig as stable a stratum as possible.However, in reality, in addition to that, administrative conditions are also taken into consideration.

  • Geology of the Japan-Korea Tunnel
    Construction survey

  • Overview of the Japan-Korea Tunnel

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