# Root Cause Analysis of a Collapse in a Hydropower Tunnel
## 5 Conclusions  
This paper provides a distillation of the work performed for the root cause analysis (RCA) of the collapse in a hydroelectric tunnel. The RCA and the lessons learnt do not represent a laboratory-based research project in a controlled environment. The RCA was undertaken in real time to help expedite the design of the by-pass tunnel and make the scheme become operational again. The RCA work was undertaken to establish an understanding of the reasons for the collapse and why it was not foreseen prior to or during construction. The lessons learned were integrated into the design of the by-pass tunnel, which passed through the same FDZ rocks. Nevertheless, the analysis and results presented in this paper represent a state-of-the art distillation and advancements in the knowledge related to durability problems due to the presence of laumontite in volcanic rocks and the potential effects on tunnelling projects. The conclusion from the RCA presented in this paper shows that there was no clear evidence of a simple or single failure mechanism. A complex and unusual interrelationship of factors converged in this zone, and this resulted in a failure of unusual magnitude and nature despite the ground support installed during construction. The coupling effect of each factor's contribution is quantified implicitly in the finite element numerical modellingundertaken to back-analyze the initiation of the collapse, i.e., strength reduction in the rocks in the FDZ due to the desiccation of laumontite followed by minor swelling as a result of the rehydration of laumontite once the tunnel was operational. The RCA data used to calibrate the strength parameters are described in detail in this paper. The coupled effect of the in situ stress field is also implicitly considered in the modelling, with the input data coming from in situ stress testing post collapse (no stress measurements were taken during construction). The coupled effect of the transient water pressure changes during the several episodes of partial and final collapse is not explicitly included in the modelling primarily because there are no data; rather, this effect is assumed based on evidence from the observation of cyclical bedding deposits in the debris pile downstream of the collapse. The paper is mainly based on post-accident investigations, tests, and modelling. The collapse occurred a year after the project became operation and therefore lacks long-term monitoring data during the tunnel operation. The authors also did not have access to operational monitoring data; it is sufficient to say that some anecdotal evidence was provided, and there were drops in pressure and sediment noted at the powerhouse a few days before the scheme was shut down. It is therefore not possible to fully analyze the development trends of various influencing factors over time before the accident. This, however, does not negate the RCA or the lessons learnt. The RCA and the design process for the by-pass tunnel (not described in this paper) indicated that to have halted the interaction of all of the factors described above, it would have required a 0.5 m permanent concrete liner in the FDZ. The support designed in the collapse zone was insufficient for the unforeseen conditions encountered, but as the saying goes, hindsight is perfect, and the recognition of the interaction of the above factors was not feasible based on the information gathered from the routine tunnel mapping and the testing (following industry standard of care) undertaken during tunnel construction. The lessons from Lesotho's experience and the confidential projects presented in this paper show that the presence of elevated laumontite can be used as an indicator, but this alone does not provide an explicit design method that fits all circumstances. The degradation process needs to be understood and halted, and further research in this regard is required.