Analysis of Plastic Deformations for Tunnel Support Design in Weak Flysch Rock Mass of a Hydropower Tunnel in Central Albania

Abstract The prediction and analysis of deformations in tunnels excavated in weak rock mass have been traditionally done with the application of analytical methods. However, these methods are seldom evaluated in terms of their performance. This paper aims to evaluate the performance and limitations of the most commonly used methods for evaluating plastic deformations in tunnels excavated in weak and deformable flysch rock mass. For this purpose, a comparative analysis considering the analytical solutions of Carranza-Torres and Fairhust (Tunn Undergr Space Technol 15(2):187–213, 2000), the Hoek and Marinos (Tunnels Tunnel Int 32(11):34–51, 2000), and Panthi and Shrestha (Rock Mech Rock Eng 51:1823–1838, 2018), together with numerical analyses based on the finite-element method (FEM) was performed. The study is based on the construction of a headrace tunnel in central Albania, excavated in weak and highly deformed flysch rock mass subject to plastic deformation. The analysis results have revealed limitations in the four approaches to assess plastic deformations in weak flysch, and suggested that a better characterization of the ground behavior and a more detailed basis for the design of optimal permanent tunnel support can be achieved from the combined, hybrid use of these four approaches. As such, Ribs of Reinforced Shotcrete (RRS) were found functional as permanent tunnel support in sections of weak flysch, where the classical use of analytical approaches would likely have suggested the use of uneconomical cast concrete lining solutions. Based on the results, the study also provides a set of recommendations that can be used to assess plastic deformation and tunnel support in similar weak and complex rock masses of flysch.