Reinhold Gerstner and Elmar Netzer, Vorarlberger Illwerke AG, Austria
Alois Vigl, viglconsult ZT, Austria

Pressure tunnels of Alpine hydropower stations have been designed for long lifetimes and often give good service for long periods with no measures other than regular maintenance. For those pressure tunnels showing signs of damage to the lining after many years of operation, repair methods have been developed that make economic solutions possible. Reinhold Gerstner, Elmar Netzer, and Alois Vigl give an overview of damage patterns and repairs for some Alpine hydro pressure tunnels.

The generation of electricity from hydropower began about 100 years ago. Several Alpine hydropower stations have been in operation for decades, so practical experience of their long-term behaviour is available from repair and refurbishment works. Long-term behaviour becomes particularly important when a concession expires and the requirements have to be defined and fulfilled for a further period.

The pressure tunnel is normally the longest part of the headrace down to the power station. 'Pressure tunnel' here refers to a headrace or tunnel under low to medium internal pressure up to about 20 bar. The Vorarlberger Illwerke AG operates pressure tunnels in this category with an overall length of about 60km.

During the lifetime of a pressure tunnel, maintenance works are planned and their extent is restricted since they cannot, normally, be carried out under continued operation. Major refurbishment should thus be the exception rather than the rule. The frequency and predictability of maintenance work is also important, since it affects the serviceability of the plant. Inspection intervals of 10 years are typical for the diagnosis of the condition of a pressure tunnel.

Fig 1. Typical cross sections of several Alpine pressure tunnels

The operation of a pressure tunnel is closely related to the rock mass and groundwater conditions, which greatly affect the excavation of the tunnel but also continue to determine the conditions for the tunnel in operation^[1,2]. This applies particularly to the stress conditions in the rock mass, even after equilibrium has been achieved in the secondary stress state with completion of the support.

When a pressure tunnel is put into operation, the stress conditions in the lining and in the direct surroundings of the pressure tunnel are changed again. This could be described as a tertiary stress state, and is subject to more or less fluctuation depending on the mode of operation of the power station. The relationship of the minimum primary stress in the rock mass to the internal pressure of the tunnel is decisive for the design and operation of a pressure tunnel.

The groundwater conditions are also decisive, particularly the relationship of groundwater pressure to the internal pressure in the tunnel. This not only concerns the waterproofing of the tunnel but also the loading on the tunnel lining^[1,2].

The general requirements for a pressure tunnel are: long-term structural stability; adequate waterproofing; resistance of the structure, without any part of the lining or rock mass being able to enter the generation water; and low hydraulic losses. The design concept for Alpine pressure tunnels demands that these requirements be fulfilled, not by the tunnel lining alone, but by the lining as a composite with the rock mass. The cooperation of the rock mass is utilised in design and construction measures.

Lining systems
A circular profile with the smoothest possible internal surface is normally preferred in order to keep hydraulic losses as low as possible and provide a structurally favourable profile (Fig 1). The invert is normally part of the profile or is flat to improve vehicle access and with or without a water channel. In mechanised tunnel drives, prefabricated invert elements are often laid, which then become part of the final lining. Inner linings of in-situ concrete are common, and precast segments are becoming more common in conjunction with TBM excavation.

Under special rock mass and groundwater conditions, the core ring (Kernring) principle has been used for linings, with an inner ring assembled from concrete elements and an outer ring of reinforced or unreinforced in-situ concrete. Grouting is intended to prestress the core ring in most cases, although core rings without prestress are also used. In some cases, plastic membranes or a thin steel lining is used, with appropriate provision, for later grouting as a separating layer between the inner and outer lining rings.

Shotcrete linings, with or without reinforcement, are less common in the construction of pressure tunnels and are mostly used only when a different type of lining is uneconomical due to profile geometry, size or tunnel length, or when friction losses are not a decisive design criterion. Inner rings of shotcrete are mostly restricted to tunnel sections with good rock conditions and low operating stresses and relatively low and stable groundwater pressures.

In pressure tunnels built before about 1955, the inner linings were often finished with smooth render, since the production of waterproof concrete was scarcely possible given the methods, machines, and materials available at that time. Pressure tunnels constructed before, during, and shortly after World War II were affected by a lack of materials of all sorts. Some typical defects may often be due to these circumstances.

Defect patterns
In most cases, a defect cannot be assigned to one cause but has arisen from a combination of effects, which derive from either the complexity of the rock mass, the construction methods and materials, or the operating conditions.

The overloading of a pressure tunnel by rock pressure is rarely the sole cause of defects, since in most cases equilibrium in the secondary stress state has already been provided with the completion of the support and is not first achieved by the lining. Overloading from rock pressure during operation thus almost always occurs as a result of other causes, which change the rock mass properties and thus the external loading on the lining. This could be the swelling of clay minerals or the development of swelling pressure as a result of anhydrite-gypsum reaction, although these are not frequent occurrences. More frequently, particularly with tunnels on a steep gradient, are loading changes on the linings due to slope deformation.

Overloading of the lining can also be caused by weakness on the resistance side caused, for example, by a chemical conversion due to lack of compatibility between aggressive groundwater and the concrete, which has been observed in several cases.

A lack of cooperation from the rock mass, as a cause of overloading of the lining under internal pressure, is most likely to occur if it has been underestimated in the design, in which case the damage would occur typically at the start of the operation phase. Such damage is generally not to be expected after long service unless caused by other effects, such as changed operational conditions.

A groundwater table that lies considerably below the level of internal pressure in the tunnel and that causes water losses can lead to defects over time because the actual damage in such cases results from the water loss, which can lead to changes in the rock mass. A groundwater table that is considerably higher than the level of internal pressure acts as an external load on the lining, and can also lead to damage after a long period of operation if the tunnel is dewatered when the groundwater table is high or when the lining is semi-permeable and the permissible rate of lowering the internal pressure is exceeded.

The interactive effects of service water with the groundwater due to a changing relationship of internal and external pressure are considered in the design of many pressure tunnels and are permissible as long as the pressure conditions are generally acceptable and the rock mass is stable against erosion. If the rock mass, or parts of it, such as joint fillings, are susceptible to erosion, material washing out of the rock mass is a relatively common event that can directly or indirectly lead to damage.

For all of these reasons, the recording and evaluation of visible signs of groundwater conditions are of great significance during the inspection of pressure tunnels. A comparison with former images to determine any changes is particularly valuable. These images can refer to individual water ingress locations or the distribution of water ingress in the tunnel.

Additionally, recording the balance between the operating flow and the groundwater table for the entire tunnel is an effective method of monitoring the communication of service water with groundwater as a whole. This is achieved during an operational shutdown, with closed valves, by measuring the internal pressure and its development over a defined period of time. Changes are then detected by repeating the measurements over longer periods of time.

In the illustration of the water balance of the Lünersee pressure tunnel (Fig 2), it should be noted that the apparent positive correlation between internal pressure and water balance is a result of the fact that a high level in the Lünersee reservoir mostly occurs at times of year with high groundwater tables. The difference between the internal pressure and the groundwater table, exclusively, determines the water balance. In the Walgau pressure tunnel (Fig 3), on the other hand, there are only slight fluctuations of internal pressure.

Fig 4. Detailed cross sections of Lünersee and Vermunt pressure tunnels

Tunnel repairs
From the defects discovered in pressure tunnels and their various combinations, a manageable catalog of proven repair measures has been developed.

In order to repair concrete, high-quality repair mortar based on cement or synthetic resin, with or without fibre reinforcement, can be used effectively and comply with the requirements for adhesion properties, strength, durability, and workability. Where necessary, repair edges have to be cut back and groundwater inflows have to be drained in addition to the obligatory removal of defective locations and preparation of the substrate. Experience with such repair mortars is consistently positive given careful application, at least for the limited time it has been in use.

As long as the rock mass is integrated as part of the lining system, grouting through drilled holes can achieve most repairs at a reasonable cost. This applies to subsequent consolidation of the rock mass and improvement of its properties, waterproofing or reducing the permeability of the rock mass, closing cracks in the lining, and achieving a certain prestressing effect. Grouting can be carried out, depending on the suitability for injection and purpose, with normal cements, very fine binders, or chemical grouts. One essential advantage of drilling and grouting is that the essential purposes of consolidation, waterproofing, and prestressing can be achieved by one and the same measure^[3]. A further advantage is its adaptability, both for systematic grouting and for specific special treatments.

Fig 5. Longitudinal and cross sections of Rotenberg pressure tunnel

Under high groundwater pressures, it is often necessary to relieve the lining of external pressure. In repair work, this could entail the repair and maintenance of existing relief equipment such as relief drillings, drain hoses, or pressure relief valves. As a precautionary measure, this could mean the intentional provision of additional relief holes. One special case is non-return valves of various construction types, which mostly have a limited working life and have to be appropriately and regularly maintained or replaced.

In case of major or systematic damage to the lining, relining may be required, either for a part of the cross section or the full cross section of the tunnel. At the Lünersee and Vermunt pressure tunnels, systematic damage to the invert occurred and relining of the invert around approximately one quarter of the perimeter proved successful using thin reinforced concrete precast segments. The essential advantage of this lies in high-quality precasting under optimal conditions and the ability to plan the installation with certain targets^[4]. The relining of a tunnel is normally so extensive, however, that a comparison of risk and cost against those of a full replacement operation is justified.

1. Innerhofer, G., Vigl, A., Gerstner, R.: Pressure tunnels and mountain participation. Rock Engineering 25 (2007),
    No. 5, pp. 19-31.
2. Vigl, A., Gerstner, H.: Considerations of the crack water table and its fluctuation in pressure tunnel design and
    construction. Geomechanics and Tunnelling 3 (2010), No.5, pp. 442-454.
3. Vigl, A., Gerstner, R.: Grouting in pressure tunnel construction. Geomechanics and Tunnelling 2 (2009), No.5, pp.
    439-446.
4. Pürer, E., Vigl, A.: Rehabilitation of old water tunnels with precast technology using the Lünersee and Vermunt
     pressure tunnels as examples. EUROCK 2004 & 53rd Geomechanics Colloquium, Salzburg, pp. 339-344.

Reproduced with permission from Wilhelm Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG. Copyright Geomechanics and Tunnelling, 2013, Vol 6 No 5, Long-term behaviour of pressure tunnels, pp407-421, by Dipl.-Ing. Reinhold Gerstner, Dipl.-Ing. Elmar Netzer, Dipl.-Ing. Dr. Alois Vigl

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