Tunnels need to be secured both during construction and during use. The expansion of highways in our country and the prolongation of highways and double roads increase the number of tunnels and viaducts.

Meanwhile, metro systems are the most effective solution to solve the transportation problem in crowded cities. Today, the most common form of public transportation systems, the metro, is used all over the world. Since the beginning of the twentieth century, metro constructions, which are the definitive solution of urban transportation, have gained great speed in our country in recent years. Metro systems consist of long and interconnected tunnels that open underground.

Ensuring security in tunnels has become important in many ways:

• Prevention of accidents
• Protection against fire and explosion
• Preventing health risks and occupational diseases caused by hazardous substances for employees

It is necessary to take correct, timely and reliable measures against these risks and minimize their consequences before these risks arise.

What are the risk gases in tunnels?

One of the dangers encountered during tunnel construction is the sudden leakage of gas, or the gases that pose a vital risk to employees by accumulating over time. In order to make accurate and reliable measurements, it is necessary to know which flammable and toxic gas is emitted. In general, the gases exiting the veins between the soil layers are methane at the ratio 90-95, carbon dioxide or hydrogen at the ratio 2-4.

During both construction and use of these tunnels, vehicle engines release harmful chemicals such as carbon monoxide, carbon dioxide, formaldehyde, ammonia, nitrogen oxide and sulfur dioxide. In addition, the amount of oxygen in the tunnels should be kept under control. Oxygen deficiency increases the risk of suffocation and the excess increases the risk of fire.

Briefly, the gases that create risks in the tunnels are:

• Methane: It is a colorless and odorless gas that is lighter than air. This flammable gas can explode when combined with air.
• Carbon monoxide: Flammable, toxic, odorless and colorless gas with the same density as air. When inhaled, the cells in the body become deoxygenated and begin to die.
• Carbon dioxide: It is a colorless and odorless gas that is heavier than air. When inhaled, it causes loss of consciousness and destroys oxygen in the cells and causes death due to lack of oxygen.
• Hydrogen sulfide: It is a toxic, colorless and smelly, flammable gas heavier than air. It causes headache, dizziness, nausea, low blood pressure and loss of direction. Excessive inhalation suppresses the sense of smell and causes sudden death.
• Nitrogen dioxide: Heavier than air, poisonous, slightly smelling, brown-like gas. It is not flammable but facilitates the combustion of other chemicals.
• Sulfur dioxide: Non-flammable, toxic, colorless and bitter smelling gas. It usually irritates the eyes, burns the throat, coughs and shows signs of asthma.
• Ammonia: It is a poisonous and colorless gas that is heavier than air, smelling bitter. Causes burns in eyes and skin. Combined with air, it becomes a flammable gas.
• Formaldehyde: A colorless and pungent gas. It could explode. When inhaled causes breathing difficulties, difficulty swallowing, chest pain and fluid accumulation in the lungs.

Gas Measurement Works in Tunnels

Gas measurement works in tunnels are mainly carried out in three stages:

• First, the amount of oxygen in the tunnel is measured. Because the correct operation of other gas control means depends on the presence of oxygen.
• Flammable gases in the tunnel are then detected. Explosion or fire hazard of gases is much more dangerous than exposure to toxic gases.
• Finally, in the third stage, measurements of toxic gases in the tunnel are made.

Inadequate ventilation systems in tunnels can be caused by biological activities in the environment, degradation of natural substances or leaks. These may cause the release and accumulation of flammable and toxic gases. In this way, the air concentration required for respiration falls below the vital values ​​and poses a risk to the life of the employees. Assuming that some toxic gases are colorless and odorless, their presence cannot be easily understood and the risk increases.

In this respect, it is necessary to determine the gases in the air by using the correct measurement tools in order to ensure occupational safety in the tunnels and to protect the health of the employees. These measurements, tests and analyzes are carried out in authorized testing and inspection laboratories, in accordance with the legal regulations in force and in accordance with the standards published by local and foreign organizations.