| Monitoring & inspection Root cause of hydropower


tunnel collapses The root causes of the more recent early collapses occurring since 2009 have been evaluated in detail by Brox (2024), based on post-collapse forensic investigations undertaken on behalf of insurers and project stakeholders. Analysis of 12 cases since 2009 concluded that the likely root cause was design errors involving inadequate initial tunnel support and final linings, arising from a failure to recognise the additional loads imposed on tunnel linings following saturation of surrounding water-sensitive rock conditions. The consequences have been substantial. Repair costs for the most recent collapses have each exceeded $120 million, with repair programmes extending beyond 36 months, resulting in significant additional project costs and losses in revenue. Saturation of the surrounding bedrock can occur when the shotcrete lining cracks under internal hydraulic pressures that may exceed 5–10 bar. This is particularly relevant for irregular drill-and-blast profiles, where blast damage may further increase susceptibility to cracking. Such pressures are significant and can allow water to penetrate the surrounding rock mass.Bedrock saturation can subsequently reduce the strength of water-sensitive materials, including clays associated with faults and fracture zones, as well as degradable, water-sensitive vein-filling minerals such as laumontite. This proposed root cause is strongly supported by evidence from many of the 12 recent cases examined. In these cases, varying degrees of damage were observed elsewhere in the shotcrete linings, including severe cracking and rupturing without collapse. These features are considered precursor stages of failure and were associated with lower levels of adverse water-sensitive conditions than those identified at the collapse locations. The root cause of this latter group of deferred


collapses, 40% of which occurred more than five years after construction, is also considered to be design errors involving inadequate support and final linings at unidentified localised or discrete weak zones within otherwise predominantly good-quality rock conditions. This was the case for the majority of projects in Norway and Sweden. Identifying these isolated weaknesses has been likened to finding a needle in a haystack and remains a significant challenge for the hydropower industry. The root cause of these recent collapses is considered to be design errors involving inadequate initial support and final linings, arising from key decisions made during construction. These errors reflect the limitations of both detailed visual inspections and the simplified geotechnical design standards commonly used to describe and characterise ground conditions encountered during construction. The key factors considered to have contributed to these design errors, and consequently to the increased near-annual rate of collapses, include the following:: ● Project delivery methods of design-build (EPC) for fast-track project execution to meet power purchase agreement deadlines to avoid penalties and/or investor funding deadlines;


● Project sites increasingly located in adverse geotechnical conditions despite attractive hydrology and associated energy generation profile;


● EPC design and construction executed with inadequate time for detailed geotechnical investigations and testing to fully appreciate and understand the behaviour of water sensitive


(saturated) geological formations and mineralogy;


● Limited geotechnical investigations and testing assuming optimistic conditions with the dismissal of low strength and durability values recognized as outlying results and an incomplete understanding of saturation-induced strength reduction and behaviour of the surrounding rock conditions during hydraulic operations;


● One-pass construction approaches whereby the decisions for final support and linings are made rapidly, and often by young professionals, during excavation advance including for TBMs, and;


● The acceptance of final shotcrete linings by many designers as a cost-savings alternative assuming equivalent long-term durability and performance to concrete linings with the misunderstanding that shotcrete linings are of high permeability and deformability especially for irregularly-shaped drill and blast tunnel profiles.


Routine unwatered inspections using


remote operated vehicles (ROVs) Given the continued, and in some cases increasing, use of aggressive design approaches for major hydropower projects, particularly those delivered under design-build or EPC contracts, there is a growing reliance on shotcrete as the final lining for long hydropower tunnels to minimise overall project costs. Against this background, routine inspections of water-filled tunnels using remotely operated vehicles (ROVs) are strongly recommended, with inspection frequency determined by past, current and anticipated hydraulic operating conditions (Brox, 2020; 2022). Figure 3 presents a high-resolution three-dimensional (3D) model generated during an ROV inspection using 3D multibeam sonar in a 22km-long headrace tunnel. The inspection identified a 35m-long section of distressed concrete lining after 13 years of hydraulic operation. Over a three-year period, pieces of concrete had become dislodged from this section and were transported approximately 13km along the headrace tunnel into the powerhouse, posing a risk to power generation. The inspection results indicated a compression-type rupture of the concrete lining at a location characterised by very weak geotechnical conditions. As-built records confirmed that additional invert support had been required


www.waterpowermagazine.com | September 2026 | 25


Figure 2: Frequency and Cumulative Frequency Chart of Historical Collapses.


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