In mountain tunnel construction, the water that springs up from the tunnel face and construction water that is generated during tunnel excavation often becomes muddy and is discharged when it is mixed with fine-grained soil from drilling, piled-up, and transporting the waste, cement from concrete placement and chemical injection, grouting materials, or oil leaking from various underground work machinery. The volume and quality of this muddy water vary depending on the size and length of the tunnel, the excavation method, the geology, the amount of underground spring water, etc. Tunnel construction takes a relatively long time, so discharging muddy water directly into rivers, lakes, agricultural waterways, etc. will cause water pollution and cause damage to the living environment and the natural environment.
Generally, the environmental issues that arise with turbid water during tunnel construction are suspended solids (SS), pH (hydrogen ion concentration), and oil content. Depending on the construction project, regulatory values for treated water may include biochemical oxygen demand (BOD), chemical oxygen demand (COD), and chromium content. Spring water containing heavy metals may also be a problem. When excavating areas where industrial wastewater has flowed into the ground or where ground containing heavy metals is being excavated, separate consideration must be given to how to deal with these issues.
When planning wastewater treatment, it is necessary to set the expected amount of wastewater and understand the wastewater standards.
The raw water used in tunnel construction is divided into two categories: underground spring water and construction water. There is often a large difference between the predicted amount of underground spring water and the actual amount, making it difficult to accurately predict the amount of raw water at the planning stage. For this reason, a method is adopted in which minimal turbid water treatment equipment is installed at the start of tunnel excavation, and additional equipment is added as the amount of spring water increases as excavation progresses. Also, if the amount of spring water increases, and there is no problem with the components of the spring water itself, a method known as clear water separation is often adopted, in which clear water and turbid water are separated, and the clear water that does not contain fine soil or cement is directly discharged through a separate system.
Before construction begins, the source of polluted water, its presence and extent, and related laws and regulations, as well as regulatory values, must be investigated, and appropriate purification treatment must be carried out before the water is discharged. In addition, the amount and quality of the discharged water must be measured and reported in accordance with regulations such as ordinances.
This paper will discuss the points to note and measures to be taken when dealing with SS, pH and oil content, which are generally considered to be sources of water pollution.
[SS (Suspended Solids)]
The particle size composition of soil particles contained in wastewater during tunnel construction varies depending on the type and properties of the rock, the excavation method, and the drainage method, but fine particles such as silt and clay often make up a large proportion. The settling rate of these particles is very slow and takes a long time to settle, so physical and chemical treatments are applied to aggregate the particles and form flocs (=large particles generated by flocculation, which are fluffy aggregates of suspended matter) to accelerate settling. There are two types of flocculants used for floc formation: inorganic flocculants and polymer flocculants. The inorganic flocculant mainly used for treating turbid water in tunnel construction is PAC (polyaluminum chloride). Polymer flocculants are sold by many manufacturers under their own product names, and there are many types of products, but anionic ones are commonly used. These inorganic and polymer flocculants are often used in combination in the hope of a synergistic effect.
[pH (Potential of Hydrogen)]
The pH of spring water generated during tunnel construction is generally around 6.5 to 7.5, but it often rises to 9 to 13 due to the inclusion of sprayed concrete, chemical grout, etc. Depending on the rock and soil type of the surrounding ground, the spring water itself may be strongly acidic or strongly alkaline. When discharging into public water bodies, the pH must be adjusted to within the range of discharge standards. pH adjustment can be performed before or after sedimentation separation. The former is increasingly used because coagulation treatment can be performed within the optimal pH range, but in the latter case, turbid water contains many substances that inhibit the neutralization reaction, resulting in a slightly larger amount of neutralizing agent being used compared to post-neutralization. Carbon dioxide is generally used for alkaline wastewater, but dilute sulfuric acid may be used temporarily in cases where the pH value is particularly high. Slaked lime or caustic soda are used for acidic wastewater.
[Oil (n-hexane extractable substances)]
When oils and greases leaking from tunnel boring machinery (rock drills, boulder excavators, locomotives, rails, spoil transport vehicles, etc.) mix with groundwater, their concentration can reach 10-15 ppm. The main problem with oil is the generation of unpleasant odors and the transfer of odor to seafood. Even low concentrations can cause odor, so removal should be considered as much as possible. If oil is mixed into the raw water, polyethylene-based cloth-like absorbents are used in the raw water tank.
(Reference: "From Investigation and Design to Construction of Mountain Tunneling Methods," Japanese Geotechnical Society)
Environmental assessment
(environmental impact assessment)
Overview of the Japan-Korea Tunnel
