# Root Cause Analysis of a Collapse in a Hydropower Tunnel
### Tunnel Design, Construction, and Support Levels  
The main power tunnel is a relatively low-pressure hydropower tunnel with a horseshoe shape and approximately 6 m in diameter. It was excavated using drill-and-blast methods with rock support provided by rock bolts, fibre-reinforced shotcrete, and, in places, lattice girders. Potentially swelling hydrothermally altered reddish tuffs (HARTs) were identified during the construction upstream of the collapse and towards the downstream end of the tunnel well beyond the collapse. For this reason, HART zones in both the upstream and the downstream ends of the tunnel (well beyond the collapse) were lined with shotcrete or composite concrete/shotcrete lining, which were designed to resist swelling pressure. Neither of these HART zones was complicit in the collapse, and the support details are not provided here; it is sufficient to say that the support levels upstream of the collapse were similar to those installed in the collapse zone. It should be noted that the authors of the RCA were not responsible for the original/pre-failure tunnel design (the type and support levels described below are provided for context but were designed by others), but is provided for completeness as part of the forensic RCA. The following references that consider the design of tunnels in swelling rock were reviewed as part of the RCA, including [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11] to name few. This was important given that significant lengths of the main tunnel were potentially in swelling rock even though these section did not suffer any major damage during the collapse. As described below, the rock types in the collapse zone had limited to no swelling pressure issues, and the failure mechanism is described in detail below. Where the rock cover was low near the powerhouse, the tunnel was also constructed with a steel pensock to prevent the hydro-jacking of the rock formation. There were no major hydro-fracturing issues related to the collapse other than the effects of transient water pressure during the cyclical collapse, and this is discussed in Section 4. It should also be noted that the authors of the RCA were not involved in collecting or analyzing the monitoring data during the construction or during the operation of the scheme prior to the collapse. The authors also did not have full access to these data, and it is therefore not possible to provide a comprehensive or meaningful contribution on trends before the collapse other than some anecdotal input that was provided post collapse. Also, given that the tunnel was only operational for approximately 1 year before the collapse, there were no long-term operational monitoring data either. During the initial excavation of the tunnel, the geologic mapping of the sidewalls, crown, and face of the tunnel was carried out throughout the tunnel and in the vicinity of the collapse as the excavation proceeded. The geotechnical quality of the rock in the tunnel was classified using the [PERSON]'s \"Q\" index [12; 13; 14], and the corresponding tunnel design rock class support was selected based on these results. The operational hydrostatic pressure in the tunnel at the collapse was estimated to be around 40 m of pressure head, and the rock cover above the tunnel is about 600 m at the collapse section. The flow volume in the tunnel before the collapse was about 50 m\({}^{3}\)/s, and a maximum velocity of about 2 m/s when the powerhouse was operating at full capacity. Q values [12; 13] assigned during construction were defined for the long-term operational condition and included a groundwater rating (Jw) to account for operational water conditions in the tunnel, and provisions for higher stress reduction factor (SRF) and/or joint alteration rating (Ja) values were included where swelling conditions were suspected. In the collapse zone, support installed included [PERSON]'s \"Q\" values [12] of between 0.1 and 4.