Deterioration of weak rocks over time and its effect on designing tunnel support systems

The long-term behavior of rock mass plays a vital role in the overall stability of underground structures. One possible long-term scenario is the degradation of the rock mass, especially in unstable clay-bearing rocks susceptible to water ingress. Lack of efficient countermeasures in extreme cases could significantly affect the physical and mechanical properties of rocks and alter the engineering behavior to some extent. In this research, we describe the case of the Golab access tunnel where it is subject to failure in a specific section containing meta-shales. The situation was considered to have occurred as a consequence of long-term deteriorating phenomenon after a 5-year period. To characterize the rock mass, point load tests were conducted on the rock samples in failure zones and values were compared with the ones obtained through primary geotechnical studies. Slake durability index (SDI) was also used to assess the vulnerability of rocks to changes in moisture content and was correlated to point load values. Additionally, GSI was measured in the failed regions, and the decreasing trend was presented in accordance with time. By taking advantage of the engineering characteristics, ground reaction curves in short-term and long-term conditions were obtained. It was deduced that the temporary support system applied to the tunnel in this section cannot provide a safe margin against long-term strains imposed by the ground. Therefore, another support system was suggested and applied to the unstable sections of tunnel to prevent further hazards.