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Urban railways have been widely constructed due to high economic and operational efficiency compared with other traffic systems. In the other hands, urban railways on the grounds cause severe traffic congestion as well as noise and vibrations. In addition, fire accidents in underground tunnels causes many causalities due to suffocation. To solve the drawbacks around world, many researches in various fields have been conducted to utilize underground spaces, e.g., shallow underground tunnels. Natural ventilation systems (NVS) have been widely used for exhausting smoke flows in shallow underground tunnels due to convenience in installation, maintenance and low cost. When the plugholing phenomena occurs in NVS, the actual smoke ventilation rate becomes smaller than the design value due to fresh air inflows below smoke layer. The phenomena correlate relative intensity ratio between horizontal flow and vertical flow immediately below the shaft. Therefore, tunnel geometrics can affect the plug-holing occurrence even fire size is equivalent. In this study, we experimentally investigated the effect of area ratio on the plug-holing phenomena in shallow underground tunnels. The area ratio is defined as the cross-sectional area ratio of shaft to tunnel. To change the area ratio, the only shaft size was changed in the same model tunnel in 1/20 reduced scale. Experiments were carried out for four area ratios (AR) of 0.054, 0.071, 0.09 and 0.11. Also, fire size was fixed to 11.49 kW. As the results, the plug-holing occurred in case of AR = 0.071, 0.09 and 0.11. As the area ratio increases, vertical flow through the shaft becomes relatively stronger than the ceiling jet flow in tunnel due to an increase in the exhaust rate of smoke, it causes that the fresh air below the smoke layer can be entrained into the shaft directly. Consequently, the potential for the occurrence of plug-holing increases as the area ratio increases.

EXPERIMENTAL STUDY ON THE EFFECT OF CROSS-SECTIONAL AREA RATIO BETWEEN TUNNEL AND SHAFT ON PLUG-HOLING PHENOMENA IN SHALLOW UNDERGROUND TUNNELS