The standards of ventilation system for bi-directional tunnel have not been established now. For this reason, with regard to the bi-directional tunnel below 1km, some problems have been appeared in ventilation capacity designing and in determiningwhether the mechanical ventilation system is needed or not for each case. In this study, we examine the characteristics of natural ventilations, analyze ongoing ventilation design casesfor bi-directional tunnels and classifythose cases into two groups.

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... This study is carried out about the capability of using natural ventilating system by calculation of reasonable ventilation capacity in bi-directional tunnel and review of relationship between natural wind speed (Vr*) and required speed(Vreq). This paper aims at providing a basis data for bi-directional tunnel ventilation design standards.