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 drainage water during tunnel construction varies depending on the type and properties of rock, the excavation method, and the drainage method, but fine particles such as silt and clay often account for a high proportion. Because these particles have a very low settling velocity and require a long time to settle, physical and chemical processes are used to aggregate the particles and form flocs (large particles formed by coagulation, forming a fluffy aggregate of suspended matter) to speed up settling. Flocculants used to form flocs include inorganic and polymeric flocculants. The inorganic flocculant primarily used for treating turbid water in tunnel construction is PAC (polyaluminum chloride). Polymeric flocculants are sold by many manufacturers under their own trade names, and while there are many different types, anionic ones are commonly used. These inorganic and polymeric flocculants are often used in combination for their synergistic effects.
[pH (Potential of Hydrogen)]
The pH of spring water generated during tunnel construction is generally around 6.5 to 7.5, but the addition of sprayed concrete and chemical grout often pushes the pH to 9 to 13. Depending on the rock and soil quality of the ground, the spring water itself may be highly acidic or alkaline. When discharging into public waterways, the pH must be adjusted to within the wastewater standards. pH adjustment can be performed before or after sedimentation. The former is increasingly used because it allows coagulation treatment to be performed within the optimal pH range. However, the latter requires a slightly larger amount of neutralizing agent than post-neutralization, as the turbid water contains many substances that inhibit the neutralization reaction. Carbon dioxide is commonly used for alkaline wastewater, but dilute sulfuric acid may be temporarily used when the pH is particularly high. For acidic wastewater, slaked lime or caustic soda are used.
[Oil (normal hexane extractables)]
When oil leaking from tunnel boring machines (rock drills, rock excavators, locomotives, rails, muck transporters) gets mixed into spring water, the concentration can reach 10-15 ppm. The main problem with oil is that it generates an unpleasant odor and can affect seafood, even at low concentrations, so it is important to consider removing it as much as possible. When oil gets mixed into the raw water, polyethylene cloth absorbent materials are used in the raw water tank.
(Reference: "From Survey and Design to Construction of Mountain Tunnel Construction Methods," Geotechnical Society of Japan)
Environmental assessment
(environmental impact assessment)
Overview of the Japan-Korea Tunnel
