Site Selection of Hezar-Masjed mountain water transmission tunnel using by the Analytic Hierarchy Process, and Shannon Entropy

Introduction: In recent years, the digging of water transmission tunnels has faced various risks such as squeezing potential, hydrogeological risks, environmental effects, gas risks, and sensitivity analysis of construction costs, which has multiplied the importance of the correct site locating of these structures. However, selecting the tunnel route using modern methods of weighting the effective criteria and zoning in the geographic information system has been neglected.

Materials and Methods: In the current study, the site selection of the Hezar-Masjed water transmission tunnel using the Analytic Hierarchy Process, and Shannon Entropy as a part of the water transfer project from the Hezar-Masjed mountains to Mashhad city, has been discussed. Accordingly, five components including social hazards, structural geology, hydrogeology, topography, and economic criteria have been considered. In the first step, the two criteria of distance from villages and distance from water sources, regardless of their discharge, were considered as criteria involved with the social dimension. Based on this, it is suggested the tunnel excavation at distances far from these two criteria. The second criterion of studies is assigned to geology and tectonics. Accordingly, due to the high-risk potential of tunnel excavation, the construction of these structures in a high density of faults is not recommended. Hydrogeological studies as the third criterion influencing factors, water inflow into the tunnel, or influencing factors, drying of surrounding water resources from tunnel excavation are important. Therefore, the catchment area of springs in the study area is mentioned as a hydrogeological criterion. In terms of topography, the topographic map of the study area was used to obtain the tunnel overburden thickness map. In this criterion, excavation in less overburden thickness provides more optimal conditions. Finally, the tunnel excavation near the water delivery point, Mashhad - Charmshahr Refinery No. 3, the economic criterion, was considered. Therefore, a criterion called the distance from the tunnel exit portal to the water delivery point has been proposed as the economic criterion of the project. The thematic map of stated criteria was prepared and classified in the ArcMap environment. Scoring the classes in each criterion was done using The Analytic Hierarchy Process. Finally, the weighting of the effective criteria in tunnel site selection was done by the Shannon's entropy method. The prioritization of the effective criteria on the site selection of the Hezar-Masjed water transfer tunnel shows that the distance from the springs is more important than other criteria. So, although the distance from the village is not considered an important factor compared to other criteria, the existence of the spring will complicate the situation due to its importance for the livelihood of the residents, and this means that the distance or proximity to the village cannot be an independent factor for the tunnel route. In the second criterion, the distance from the fault has special importance due to its direct effect on the stability of the structure. Other criteria are in intermediate importance conditions. Finally, by combining the prepared maps, the zoning of suitable areas for tunnel excavation, as well as the three main priorities of the proposed tunnel axis via engineering judgment, have been presented.

Results and Discussion: The results show that the northwestern part of the study area is not suitable for drilling and it will cause risks. Therefore, the excavation priority is assigned to the middle and eastern part of the study area. More precisely, drilling priority is assigned to the middle areas because the points are closer to the water delivery site, closer to Mashhad city in the west of the study area. The results show that the best option to transfer water from the northern limb to the southern limb of the Hezar-Masjed mountains is to build an 8732-meter-long tunnel around Chenarsukhteh village, the central part of the study area, and a route closer to the water delivery point. It should be noted that after the site selection of the tunnel route and before its excavation, it is necessary to carry out comprehensive studies of hydrogeology, engineering geology, and geology through field visits, drilling boreholes, and related tests along the route.

Conclusion: This study shows that the use of multi-criteria approaches and advanced zoning technologies can help to improve the decision-making process in large infrastructure projects,.