The 10.6 km long Sabzkuh water conveyance tunnel is currently under construction in the High Zagros structural zone, located in western
Iran, through conventional  (in  section  T1) and mechanized (in section T2)  methods. The section T1 of  the  tunnel  driven through alluvial
geological units with a great  complexity in an area of highly potential for  sliding  and water-bearing  fault  which  has  made  the  excavation
operation encounter many challenges. After excavating 35 m of the tunnel  using  conventional  methods, geological problems and the inappropriate method of the tunnel excavation resulted in the tunnel collapse and failure in the tunnel inlet portal. After the incidence of the collapse, the Design and Construction Process (DCP) of tunnels were revised by using an Analysis of Controlled Deformations in Rocks and Soils  (ADECO-RS) in conjunction with  the Sequential Excavation Method (SEM). The ground improvement in the  portal and tunnel excavation continued in five phases including survey, diagnosis, therapy, operation, and monitoring. The application of this approach into construction of Part T1 of the tunnel increased the safety and  minimized risks and costs, despite geological complexities and hazards.
Moreover, the excavation process with appropriate daily advance rate of 1 to 1.5 meters was achieved according to a predicted schedule.