Time-related effects on primary support system in an unlined tunnel

This Master’s Degree thesis provides the return experience of the rock mass and supports evolution, within a tunnel of small diameter and high overburden. It is an exploratory excavation, realized between 2013 and 2017 for a length of 7km in severe micro climatic conditions. The tunnel, named La Maddalena 1, was realized for the characterization of the Ambin Massif, which is to be crossed by the base tunnel of the TELT project, developed between Italy and France. The tunnel was excavated adopting a mechanized approach with a rock TBM, after which the structure has been maintained partially inactive until the present day. The lining then remained exposed to the aggressive micro-climatic and stress conditions for several years. The TELT project prescribes a rehabilitation of the existing tunnel to be used as service access for the base tunnel, both during the construction and exercise phases of the project. Considering the facts above, it emerged the will to define the evolution of the rock mass and the supports, occurred during the time of exposure. The methodology adopted saw the research and reorganization of the vast amount of data obtained during the exploratory excavation. A survey campaign then took place in the tunnel, for the collection of actualized parameters and characteristics on supports and rock mass. Last, it saw the elaboration and analysis of the data. The evidence and conclusions obtained allowed to present a complete overview on how the rock mass and supports characteristics evolved in time (i.e. strength, deformations, presence of water inflows, variation of the induced micro-climatic conditions). The elaboration of the data allowed to quantify the severe degradation surveyed, both in qualitative and quantitative terms. In future such information can then be a valuable input for the design of remedial works, necessary to guarantee the safety requirements for the tunnel during its design life. In the final section of this thesis work, some comments are reported on the effectiveness and durability of a lining in time, based on the conclusions obtained from the analyses. It is discussed how the primary lining installed in the La Maddalena 1 tunnel, satisfies the common characteristics adopted for exploratory purposes. But it cannot guarantee the structural safety and durability that a final lining would have. This concept is remarked upon the unique conditions posed by this project, whose prolonged construction stages cause a longer temporary phase, to which the primary lining is exposed. Considering these developments, the primary support system installed should have been conceived with the characteristics and requirements of a final lining.