SECTOR | HYDRO - TUNNEL LINING: TECHNICAL NOTE
ROUTINE INSPECTION OF SHOTCRETE-LINED HYDRO TUNNELS
Asset management presents a need for routine inspections of shotcrete-lined hydropower tunnels, says consultant Dean Brox in this Technical Note.
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.
Historical collapses of hydropower tunnels with shotcrete linings.
HYDROPOWER TUNNEL COLLAPSES DATABASE UPDATE The largest database of hydropower collapses originates from Palmström (2003) who questions whether several historical collapses in Norwegian hydropower tunnels are a result of poor maintenance. The causes of such historical collapses has been postulated mainly due to three mechanisms as: 1) the presence of swelling clays; 2) sliding rock in a weakness zone; and, 3) variations in the hydrostatic pressure inside the tunnel; and also concluded that most of the collapses occurred shortly after being filled for operation (Hakansson, 2013).
A graphical presentation of the historical hydropower
tunnels in terms of operating years prior to failure was first realised by Brox (2017 – 38 cases). The increasing frequency of collapses of recently completed projects, that became well recognised by the international insurance industry in 2018, were presented and explained by Brox (2018) as being due to design errors and not construction quality. Updates of the graphical presentation of the collapse cases were presented by Brox (2020 – 48 cases) with further additional cases by Brox (2022 – 53 cases). It is particularly noted that the majority of the collapses
occurred in areas of final shotcrete linings. A further update has now been made with a total
of 60 cases as presented in Figure 1, with Figure 2 as the frequency and cumulative frequency graphic that importantly reveals that about 30% of the collapses occurred within the first year of operations (including during commissioning), with about 60% of the collapses occurred within five years of power generation operations beginning, and about 80% of the collapses occurred within 15 years of the start operations. These findings clearly indicate that a significant portion of collapses are therefore not related to slow progressive mechanisms of failure postulated by others but rather a much earlier mechanism and root cause related to design and/or construction. Figure 1 suggests that there may be two different
main types of failures responsible for the collapses: 1) short-term due to an early degradation mechanism with increased loading; and, 2) long-term due to slow progressive failure such as erosion/scour. These failures appear to have occurred, in the noted cases presented by Palmström (2003), to be associated with relatively short and discrete weakness zones.
ROOT CAUSE OF HYDROPOWER TUNNEL COLLAPSES The root cause of the most recently occurred early collapses since 2009 has been evaluated in detail by Brox (2024) as part of post-collapse forensic investigations on behalf of insurers and stakeholders. The postulated root cause has been concluded, based on 12 cases since 2009, to be the result of design errors of inadequate initial tunnel support and final linings due to the non- recognition of additional loading conditions on a tunnel lining that are due to the saturation of the surrounding
30 | September 2026
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