Rock engineering evaluation of existing and planned Salang Tunnels with high overburden through Hindukush Mountains, Afghanistan

The Kabul – Mazar Highway connects Kabul, the capital of Afghanistan, to northern provinces of the country, and further extends into Central Asian countries through the Hindukush mountains. The Salang Tunnel, which is 2,600 m long, was designed by Soviet engineers in 1958 and constructed using conventional techniques in 1964. During its construction, the tunnel ranked the distinction of being the highest-altitude tunnel at 3,400 meters. The socioeconomic importance of the tunnel is exceptionally high, as the Hindukush Mountain range to the east and south of the country is crossed by it. Since 1964, there has been a significant increase in traffic volume. Due to the narrowness of the existing tunnel, the Ministry of Public Works (MOPW) of Afghanistan has planned the construction of new twin-tube tunnels. Although there have been rehabilitation and enlargement efforts for the existing tunnel, none of these renovation works have been proven effective, and challenges persist. In 2008, the MOPW initiated a pre-feasibility study for new tunnels, which was finalized in 2012, presenting various options. Subsequently, in early 2018, the MOPW commenced feasibility and detailed studies, leading to the proposal of new twin-tube tunnels, which are designated as the planned tunnels in this paper. In this study, the available geotechnical and environmental data are used to evaluate the rock engineering aspects of the tunnels, including an assessment of the in-situ stress state. A particular emphasis is placed on the reassessment and design of the support system for both the existing and planned tunnels according to RMQR and another system, respectively. The evaluation of the response and stability of the tunnels is concluded, and the implications are subsequently discussed.