Monitoring & inspection |
Figure 3: High-resolution ROV 3D Graphic of Identified Damage of Concrete Lining.
at this location during construction. The ROV inspection enabled the affected section of the headrace tunnel to be accurately identified, providing justification for dewatering and a subsequent physical inspection. This information also supported the detailed planning and successful execution of repairs to the localised area (Brox et al., 2023). It is strongly recommended that project developers
require warranty periods of at least two years, and potentially up to five years, for new hydropower projects where shotcrete forms any part of the final lining of a headrace tunnel, given the critical role of these tunnels within the overall project. In addition, developers should incorporate routine inspections of water-filled tunnels into project requirements, while insurers should consider making such inspections mandatory where shotcrete- lined hydropower tunnels form part of the design. This is particularly important given the near-annual occurrence of tunnel collapses recorded across the industry. Recognition of the risks associated with shotcrete-lined hydropower tunnels is also considered consistent with the emerging use of inherent defects insurance (Keime, 2025), intended to address the coverage gap between construction and operation. Where possible, these inspections should be undertaken using ROVs without dewatering the tunnel. This avoids the risks associated with rapid dewatering for physical inspection, which may not allow sufficient time for pore pressures in the surrounding rock mass to dissipate. Under such conditions, elevated pore pressures can contribute to block dislodgement, localised collapses and, in more severe cases, complete tunnel blockages.
Risk evaluation of new and aging
hydropower tunnels Ageing hydropower tunnels are increasingly susceptible to deterioration and the need for repair, yet their risk profile is often given less attention than that of dams and powerhouse structures. Asset risk assessments for major hydropower plants incorporating headrace tunnels should therefore carefully consider the distribution and type of final lining, whether unlined, shotcrete or concrete, together with the hydraulic operating regime, including whether the plant operates in peaking or non-peaking mode. The risk assessment methodology developed by Rosin (2005) is considered a reliable means of identifying residual risks associated with adverse conditions and establishing an overall risk profile for a hydropower tunnel. This assessment should be supported by a thorough review of the tunnel’s historical performance, including concerns identified during previous inspections and details of completed repairs. Together, this information can provide a more robust basis for determining the need
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and frequency of future inspections, particularly given the outage requirements and associated economic impacts on power generation. This risk-based approach differs from that proposed by Nordstrom (2021), instead supporting more frequent inspections based on industry-derived risk criteria (Brox, 2021). Access is becoming less of a constraint, as ROV technology can now inspect long sections of tunnel from a single launch location, with distances of up to 18km achieved to date. ROV inspections therefore provide a practical and technologically proven alternative for obtaining reliable information on the condition of hydropower tunnels. 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 unfavorable 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.
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, Master of Science 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. October 18-19.
Brox, D. 2020. Hydroelectric Tunnel Inspections – Recommendations for Industry Practice, Hydropower & Dams, Issue 5.
Brox, D. 2022. Hydroelectric Tunnel Inspections – Recommendations for Industry Practice, HYDRO2022 Presentation update.
Brox, D., Wangdi. S., and Namgyal, D. 2023. Correlation of ROV Observations with Actual Damage in the Tala Headrace Tunnel, World Tunnel Congress, 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.
Keime, J. 2025. Inherent Defects Insurance is a win- win for insurers, governments and society, SwissRe Engineering Article.
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