Monitoring & inspection |
Managing risks in hydropower tunnels
Dean Brox says an updated review of historical tunnel collapses highlights the need for routine inspections and risk-based asset management of shotcrete-lined hydropower tunnels
THIS ARTICLE IS INTENDED to attempt to explain the perceived ambiguity presented in some recent technical articles and reports between the postulated failure mechanisms or types of collapses in shotcrete lined hydropower tunnels as slow progressive failures also described as cyclic fatigue, versus the postulated root cause for such collapses based on detailed forensic investigations with particular reference to the reported conclusions implied by Hansson (2020) and Nordstrom (2021). These reports reference past collapses and the importance of routine technical inspections of hydropower tunnels which is fully supported given their critical function as part of a total hydropower asset.
Hydropower tunnel collapses database
update The largest database of hydropower tunnel collapses was compiled by Palmstrom (2003), who questioned whether several historical failures in Norwegian hydropower tunnels could be attributed to inadequate maintenance. The causes of these historical collapses have generally been associated with three principal mechanisms: the presence of swelling clays, sliding of rock within weakness zones, and variations in hydrostatic pressure within the tunnel. Hakansson (2013) also concluded that most collapses occurred relatively soon after the tunnels were filled and brought into operation. Brox (2017) subsequently produced the first graphical analysis of historical hydropower tunnel collapses based on the number of operating years before failure, covering
38 cases. By 2018, the increasing frequency of collapses involving recently completed projects had become recognised by the international insurance industry. Brox (2018) attributed this trend primarily to design errors rather than construction quality. The graphical analysis was subsequently updated by Brox (2020) to include 48 cases, with a further update by Brox (2022) expanding the database to 53 cases. Notably, the majority of the recorded collapses occurred in sections incorporating final shotcrete linings. The database has now been further updated to include a total of 60 cases (Figure 1). The frequency and cumulative frequency data shown in Figure 2 reveal that around 30% of collapses occurred within the first year of operation, including during commissioning. Around 60% occurred within the first five years, while approximately 80% took place within 15 years of the start of operations. These findings indicate that a significant proportion of collapses are unlikely to be associated with the slow, progressive failure mechanisms proposed in previous studies. Instead, the relatively early occurrence of many failures points towards mechanisms and root causes associated with design and/or construction. Figure 1 also suggests that the collapses may broadly fall into two categories: short-term failures associated with early degradation mechanisms and increased loading, and longer-term failures resulting from progressive processes such as erosion and scour. The latter appear to include cases documented by Palmstrom (2003), which were associated with relatively short and discrete weakness zones.
Figure 1: Historical collapses of hydropower tunnels with shotcrete linings
24 | September 2026 |
www.waterpowermagazine.com
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