Optimization of the outlet portal of Heybat Sultan twin tunnels based on the value engineering methodology

Abstract

Due to uncertainties in determining the geotechnical parameters, it is inevitable to change the supporting system and excavation method during the tunnel construction projects. Such changes will in many cases lead to an increase in the foreseen time and cost. Since the beginning of the 1960s, value engineering (V.E.) was raised based on three principles of safety, cost optimization and sustainable design. Iraqi Kurdistan has had an increasing trend in design and implementation of development projects since 2008, but high safety factors considered to compensate for uncertainties in geotechnical parameters has imposed high construction costs to these projects. The design of more than 70 km of tunnels, 212 dams and more than 500 km of highways under construction in this region necessitates performing applicable case studies on the implementation of V.E. in these development projects. The Heybat Sultan twin tunnels project is being constructed as a part of the Erbil–Koya–Sulaymaniyah highway in Iraqi Kurdistan. Preliminary studies showed that the outlet portals of these tunnels had been designed with a high factor of safety. Hence, the decision was made to make changes based on V.E. principles prior to the commencement of the excavations and at the time of site mobilization. For this purpose, the most optimal method statement was selected for the outlet portal, and then the slope stability was evaluated using numerical analysis. By implementing the optimized design, the factor of safety was increased from 1.27 to 2.48 and the corresponding construction costs was decreased for 2.4 million USD compared to the original plan.