Modelling long-term deterioration of lining in tunnels

Tunnel linings constructed using traditional methods typically consist of two layers. The outer layer, known as the primary or first-stage lining, is installed immediately after excavation and is designed to provide the necessary ground stability, allowing for partial stress release within the rock mass. The inner layer, or secondary lining, is usually applied at a later stage and is intended to bear the long-term ground load and, potentially, water pressure. Current design practices for simulating the load transfer between the two lining are based on semi-empirical approaches, assuming that the full load presented at the primary lining is eventually transferred directly to the secondary lining at the end of the primary lining’s service life. However, several studies have shown that in many cases, even after 30-40 years, the primary lining remains effective, with only limited ground load transferred to the secondary lining. This master thesis aims to explore the effect of the load transfer generated due to degradation of the first-stage lining, considering the different degrees of deterioration in the sprayed concrete layer and stee ribs. A series of finite element analyses is used to study this process parametrically, with the goal of identifying less conservative yet reliable design choices. The design parameter varied in this analysis include consideration of rock mass degradation, tunnel cross section geometry, rock mass quality and secondary lining composition This study is conducted to assess the sensitivity of each factor to the degradation of the primary lining and its impact on overall tunnel stability. The analysis examines these effects at 25-year intervals to identify the parameter most susceptible to influencing long-term performance.