Optimization and engineering geological evaluation of headrace tunnel system of Akavreta HPP in Georgia

Current thesis is about designing and evaluating Akavreta HPP headrace tunnel system in Georgia. Akavreta HPP is 20 MW hydropower project located in Adjara region. There are three dams, which collect water from three different rivers and transfer discharge through five free flow channels and two headrace tunnels to forebay, were it is directed to the powerhouse with surface penstock. The project was purposed by Georgian Hydro Power LLC (GHP). First, the tunnel stability challenges, and stability assessment methods, which are relevant to be used for the stability assessment of headrace tunnel system of the project were reviewed. Initial design was evaluated and major deficiencies were highlighted. After deciding that the current alignment is not safe and reliable, because of very small overburden in major weakness zones, alternative one was introduced. In alternative alignment the headrace tunnel N1 size increases from 2150 m to 2750 m, but it eliminates the 820 m of free flow channel with 820 m of the road, it also highly increases the overburden at major weakness zones. Empirical, semianalytical and numerical analysis were carried out. With empirical and semi-analytical calculations, it was found that in weak rock type, which is tuff-breccias, there would be small squeezing. For support system design, the Q system was used, which is one of the most widely used rock mass classification system. Further, elastic and plastic analysis were carried out for numerical modeling to find headrace tunnel deformation and to find optimal tunnel support system. After getting results, the conclusion and recommendations were given.