HYDRO - TUNNEL LINING: TECHNICAL NOTE | SECTOR


water sensitive rock conditions. The costs and repair durations of the most recent collapses have each surpassed more than US$120 million and 36 months, which represent significant additional project costs and lost project revenue. The saturation of the surrounding bedrock occurs


due to the cracking of the shotcrete lining, particularly for irregularly shaped drill and blast profiles that are associated with blast damage, as a result of the internal hydraulic pressure, which often can exceed 5-10 bar, which represents a very significant pressure. The bedrock saturation results in the reduction of


strength of water sensitive conditions, such as clays associated with fault and fracture zones, as well as with degradable conditions of also water sensitive vein-filled mineralogy such as laumontite. This postulated root cause is strongly supported by


and is considered to be verified by many of the recent 12 cases; varying degrees of damage occurred to other areas of the shotcrete linings, including severe cracking and rupturing (without collapses), which represent precursor failure levels before collapse that were confirmed to be associated with lower levels of water-sensitive adverse conditions in these locations. The root cause of the latter group of deferred collapses (40% after five years) in the database are also considered to be design errors of inadequate support and final linings; these were at unidentified localised or discrete weak zones among the majority of good quality conditions (majority of projects in Norway and Sweden), which is analogous to trying to identify a needle in a haystack and remains a challenge for the hydropower industry. The root cause of the recent collapses – due to


design errors of inadequate support and/or final linings due to key decisions made during construction – are considered to be a result of the limitations of both detailed visual inspections and the implementation of simplistic geotechnical design standards that are commonly used for the description and characterisation of the encountered conditions during construction. Key contributing factors of the design errors for the


increased near-annual rate of collapses are considered to be attributed to the following: ● Project delivery methods of Design-Build or Engineer, Procure, Construction (EPC) for fast- track project execution to meet Power Purchase Agreement (PPA) 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 a) the dismissal of low strength and durability values recognised as outlying results, and b) 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 in the construction stage and assuming equivalent long- term durability and performance to secondary, cast- in-place concrete linings; and, misunderstanding that shotcrete linings are of high permeability and deformability, especially for irregularly shaped drill and blast tunnel profiles.


ROUTINE INSPECTIONS USING ROVS Given the continued and even increasing practice, particularly for Design-Build or EPC project delivery methods for major hydropower projects, of adopting aggressive designs that mainly rely on shotcrete for the final linings in long tunnels to minimise the total project costs, it is strongly recommended there are routine inspections using remotely operated vehicles (ROVs); the frequency of inspections should be subject to the method of past, current, and future hydraulic operations (Brox, 2020 and 2022). Figure 3 presents a high-resolution graphic


generated of the results of a ROV inspection, using 3D multi-beam sonars, of a 22km-long headrace tunnel that identified a 35m-long section of ongoing distress of the concrete lining after 13 years of hydraulic operations; the section was responsible for the dislodgement of pieces of concrete lining over a three-year period that were transported 13km along the headrace and into the powerhouse and posed risks to power generation. These results were immediately recognised as a compression type of rupture of the concrete lining, which occurred at a location of very weak geotechnical conditions that required additional invert support during construction as documented in the as-built information.


September 2026 | 31


Frequency and Cumulative Frequency chart of historical collapses.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45