Ventilation Technology for High Altitude Long Distance Tunnel Construction（To be continued）

2. Recommendations on ventilation and hygiene standards for high-altitude tunnel construction in China

In the plateau area, the air is thin, and the exhaust emission of tunnel construction machinery increases, and there is very little test data in this regard.
In this paper, combined with the Guanjiao railway tunnel project under construction, the exhaust emission test of diesel loaders, dump trucks and excavators was carried out in the tunnel at an altitude of 3400~3500m, and the valuable measured data was obtained as shown in Table 2.

Table 1   Construction length table of each section of line II tunnel

At high altitudes, the air is thin and the air pressure decreases. When the explosive explodes, the volume expands, the mass of CO per unit volume decreases, and the mass density of CO decreases. Compared with the maximum allowable concentration of CO in domestic and foreign regulations, in high-altitude areas, the maximum allowable concentration of CO in tunnel construction ventilation pollutants can be set at 50ppm.

Table 2   Measured value of CO generated by internal combustion engine equipment in Guanjiao tunnel

Compared with the tunnel hygiene standards of various countries, China has strict regulations on the average concentration of NO₂ 8h, while the United States, Japan, Germany and other countries have no such regulations on the 8h average concentration of NO₂ occupational exposure limits. Considering domestic and foreign regulations and health and safety requirements, the design limit of NO₂ in the tunnel is preferably 8mg/m³.

The research data shows that when the wind speed in the cave reaches 0.15m/s, the dust below 5μm can be suspended and evenly mixed with the air to move with the wind flow. The wind speed between 0.26 and 0.4m/s is the most reasonable. In addition to considering the dust removal efficiency, attention should also be paid to the comfort requirements of the construction workers, so it is recommended that the air velocity at the air supply outlet be at least 0.30m/s.

China’s construction of uniform norms, O₂ content within the tunnel project of no less than 20% by volume, in fact, the tunnel in high altitude area containing oxygen content is below 20%, even lower than 18%, within the tunnel on the plateau region of oxygen content should be taken into account according to the actual circumstances of the plateau area, should with the provisions of the low altitude area.  According to the actual climatic conditions of The Qinghai-Tibet Plateau in China, from the aspects of construction safety and labor protection, it can be stipulated that it should not be less than 18%. When it is lower than 18%, oxygen bar should be set in the cave, and individuals should carry oxygen bags.

According to the previous discussion and the calculation of the ventilation volume of the tunnel, to meet the hygiene requirements, the fresh air supply per person per minute is calculated as 4m³. According to the emission characteristics of the internal combustion engine in the plateau above, when the internal combustion engine is used, the air supply volume of 1kw should not be less than 4m³/min.

According to the characteristics of low speed of vehicles and relatively frequent movement of people during tunnel construction, the upper limit of smoke concentration can be calculated according to relevant theories. Proposed smoke concentration for tunnel construction is 12 x 10^-3/m.

Based on the above and the construction practice of Fenghuoshan Tunnel of Qinghai-Tibet Railway, the ventilation and hygiene standards for high-altitude tunnel construction are proposed as shown in Table 3.

Table 3    Table of recommended standards for ventilation and hygiene in high-altitude tunnel construction

 To be continued…