Evaluation of karst development impact on water transfer through the Hezar Masjed tunnel in the context of Mashhad's water crisis

The Hezar Masjed water transfer tunnel is under construction in the mountainous areas of Hezar Masjed, with an approximate length of ۸۷۶۳ meters, in order to provide drinking water to the city of Mashhad from the northeastern border water sources. On its way, the tunnel passes through various formations, including limestone units at the end of the excavation path. Therefore, the current study focused on the zoning of surface karst development in two watersheds in the receiver of the tunnel (Chahchaheh and Kashafrood Mani) using criteria such as height, precipitation, density of structural elements, vegetation cover, distance from the spring, and topographic slope. The results showed higher karst development of the limestone units of the tunnel route than its southeast and northwest limestone areas. However, supplementary studies determined that the zoning of surface karst development cannot be a good representation of what is happening in the tunnel level. Field studies of different units of Mozduran formation as the most important karst formation in the region showed that only part of this formation is susceptible to high karst development and other parts are in lower levels in this respect despite the interlayers of marl. The low discharge of springs around the tunnel and the absence of karst features were another proof of the poor development of karst in the region. All this shows that there is an urgent need for subsurface studies such as borehole drilling, lithology knowledge and Lugeon tests to assess karst risks in water transmission tunnels.