SECTOR | HYDRO - TUNNEL LINING: TECHNICAL NOTE
In addition, project developers should include, and/or
project insurers should mandate, that ROV inspections are performed when shotcrete-lined hydropower tunnels are adopted as part of the project design given the near annual frequency of collapses that has occurred in the industry. The recognition of such related risks with shotcrete-
lined hydropower tunnels is considered to be consistent with the new inherent defects insurance (Keime, 2025) to be offered by project insurers to address the gap between construction and operations. These inspections should utilise ROVs to avoid
High-resolution ROV 3D graphic of identified damage of concrete lining.
The ROV inspection allowed for the confirmation
of the localised section of distress in the headrace to justify the dewatering of the tunnel for a physical inspection and, subsequently, for the careful planning and successful execution of the repairs of the localised area (Brox et al, 2023). It is strongly suggested that project developers should
require warranty periods of a minimum of two years, and possibly extended to five years, for new hydropower projects where shotcrete has been used for any portion of the final lining of the headrace tunnel since it represents a major component of the project.
REFERENCES
● Hansson, M. (2020). Energiforsk Report 660, Inspection of water-filled rock tunnels (in Swedish).
● Nordstrom, E. (2021). Energiforsk Report 730, Inland Waterway Management Strategy (in Swedish).
● Palmström, A. (2003). ‘Slides and collapses in Norwegian water tunnels – a maintenance problem?’ Conference on Vassdragsteknisk forum, arranged by Norwegian Electricity Industry Association, Oslo, 6 p. (in Norwegian).
● Hakansson, W. (2013). ‘Durability of power plant tunnels: A study on aging phenomena and degradation processes’, MSc Thesis, Engineering Geology, LTH, Lund University (in Swedish).
● Brox, D.R. (2017). Practical Guide to Rock Tunneling. Tayor and Francis. Pp. 248. ● Brox, D. (2018). ‘Hydropower Tunnel Failures – Risks and Causes’. London Engineering Group Conference, Chesham, England. 18-19 October 2018.
● Brox, D. (2020). ‘Hydroelectric Tunnel Inspections – Recommendations for Industry Practice’. Hydropower & Dams, Issue 5, 2020.
● Brox, D. (2022). ‘Hydroelectric Tunnel Inspections – Recommendations for Industry Practice’. Hydro 2022, Presentation update.
● Brox, D., Wangdi. S., & Namgyal, D. (2023). ‘Correlation of ROV Observations with Actual Damage in the Tala Headrace Tunnel’, World Tunnel Congress (WTC 2023), Athens, Greece.
● Brox, D. (2024). Key Principles for the Planning, Design, Construction, Operations, and Inspection of Hydropower Tunnels: 2024 Update from Lessons Learned in the Industry.
● Rosin, S. (2005). ‘Geotechnical Risk Assessment and Management for Maintenance of Water Conveyance Tunnels in Southeastern Australia’. Presented at the Australian Geotechnical Society–Australian Underground Construction and Tunnelling Association Mini- Symposium: Geotechnical Aspects of Tunnelling for Infrastructure Projects, 2005.
● Keime, J. (2025). ‘Inherent Defects Insurance is a win-win for insurers, governments and society’, SwissRe Engineering Article.
https://ow.ly/mTyH30sX0XE.
dewatering or emptying for a physical inspection that can cause damage if performed too rapidly, whereby pore pressures in the surrounding rock are not dissipated adequately and can cause the dislodgement of blocks, small-scale collapses and even full-scale total blockages.
RISK EVALUATION OF NEW AND AGEING HYDROPOWER TUNNELS Ageing hydropower tunnels are certainly at risk of increasing distress and the need for repairs but their risk status is often overlooked versus dam and powerhouse structures. Hydropower asset risk assessments for major plants
with headrace tunnels should carefully evaluate the overall distribution of the type of final lining (unlined/ shotcrete/concrete) and the mode of hydraulic operations (peaking/non-peaking). And, the risk assessment method of Rosin (2005) is considered to represent a reliable means to identify the residual risks associated with adverse conditions to provide an overall risk profile for a hydropower tunnel. In addition, the overall historical performance, in terms of identified concerns from previous inspections and/or completed repairs, should be thoroughly reviewed in order to present a reliable risk profile for the consideration of the need for future inspections given the outage requirements for inspections and their economic impact on electricity generation. This risk-based approach is different to that of
Nordstrom (2021) and suggests inspections of greater frequency, based on these industry-justified criteria (Brox, 2021) that is irrespective of access since ROVs are now capable of inspections of long distances from a single launch site up to 18km to date. ROV inspections offer an attractive and technologically-proven alternative to capture reliable information. Finally, many aged hydropower tunnels that were
constructed in fair to good quality bedrock conditions with justified limited linings have performed satisfactorily for decades without any need for repairs. However, these tunnel performances should not be simply assumed to be expected in other project regions, and, in particular, in geologically young regions of volcanic bedrock, where more adverse and water sensitive geotechnical conditions may be present. Hydropower tunnels are being increasingly designed and constructed in more unfavourable conditions within such high-risk site conditions; whereby more conservative design approaches should be adopted to avoid the recent track record of near-annual collapses.
32 | September 2026
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