Precast concrete segmental linings have been increasingly used globally for the design and construction of long hydropower tunnels. Used in conjunction with TBM excavation since the 1970’s, they continue to be recognized in the hydropower sector as the most cost-effective and low-risk approach for safe, future hydraulic operations without undue maintenance.
Key advantages offered by precast concrete segmental linings include:
? Improved hydraulics;
? Durability against abrasion;
? Simple pre-design and construction implementation without decision-making;
? High-quality control of fabrication;
? Rapid installation for overall high productivity and shorter construction durations;
? Greater cost certainty;
? Confident long-term stability for safe and uninterrupted future hydraulic operations for the full range of tunnel sizes generally varying from 3m to 10m, and
? Improved worker safety.
Risks associated with their use are limited since they have become a well-established approach for the design and construction of tunnels for civil infrastructure.
However, since 2009, several hydropower tunnels have experienced serious failures, some with multiple collapses, both soon after commissioning as well as after several years of operation (Brox, 2019). While some of these tunnels were long and the collapses occurred at geological faults – and some with ongoing deterioration problems and repairs – these tunnels could have been constructed using precast linings which would have eliminated the risk of any collapses and ongoing maintenance requirements.
In addition, there are multiple regions globally which have non-durable geological conditions but which also have great potential for hydropower development. Precast tunnel linings therefore provide a viable solution to allow long tunnels to be constructed economically in these regions. Figure 1 shows the completed precast concrete segmental lining for the 10m-diameter, 11.8km headrace tunnel at the Kargi hydropower project located around 90km south of Turkey’s Black Sea coast.
TUNNEL LININGS FOR HYDROPOWER TUNNELS
Concrete tunnel linings are commonly used for sections or entire lengths of hydropower tunnels where adverse geotechnical conditions are anticipated, including in multiple geological fault zones and/or the presence of non-durable bedrock. Such linings can be designed and constructed for any method of tunnel excavation; hydropower tunnels are typically constructed using either traditional drill and blast, or TBM methods.
TBMs are particularly suited for long tunnels where intermediate access adits are not possible and/or where the impacts of blasting to local communities and/or the environment need to be eliminated.
Precast concrete (PC) segmental tunnel linings represent a one-pass design and construction solution and are commonly used for tunnels in both rural and urban areas, such as for subway/metro, wastewater, drinking water, road/traffic and utilities. PC tunnel linings are well-established for civil infrastructure and this has provided the confidence and successes for applications in hydropower tunnels since the 1970s.
PRECAST CONCRETE TUNNEL LININGS
Precast lining components and ring geometry
Precast linings are used exclusively and concurrently with tunnel excavation by TBM. Typically, a series of curved, prefabricated concrete segments are installed to form a circular-shaped ring that includes a key segment as the final piece. Each individual segment is vacuum lifted into the excavated space inside the TBM shield.
For some types of precast linings, the segments may be interconnected with dowels. The final stage of installation comprises grout injections into the annulus between the tunnel lining and the excavated tunnel profile. A full and quality fill of the annulus is critical for the success of precast linings in preventing groundwater or internal water flows behind the lining that can result in erosion and instability, particularly when non-durable rock conditions exist.
The precast concrete segments are formed in specially-dimensioned moulds according to the size of the tunnel. The concrete used is typically dense, durable and impermeable with a high early-strength design mix with strict quality control.
Precast linings are designed for multiple loading conditions encountered during handling and transport, TBM thrusting/jacking, grouting, ground and groundwater conditions, and bursting and distortion – all of which ultimately define the final design thickness for fabrication and installation. Multiple design standards and codes have been compiled for the design of precast linings (ITA 2016/2018 and ACI, 2020).
To meet design load conditions, precast tunnel linings typically include a steel rebar reinforcement cage. However, recent practice, including for seismic regions, now adopts the use of steel (or polypropylene) fibres as an alternative reinforcement method to bring increased durability, lower corrosion, simpler manufacturing and lower costs (Hurt, 2017).
Precast lining types
There exist multiple types of precast lining named after their respective segment geometry. This includes rectangular, trapezoidal, rhomboidal, expanded and hexagonal (honeycomb).
Rhomboidal and trapezoidal types appear to be most adopted, at least for civil infrastructure, whereas rectangular and expanded types represent early design approaches and are no longer extensively used (Harding and Hurt, 2019). Trapezoidal and hexagonal types appear to have been most widely used to date for hydropower tunnels due to their greater simplicity and lower costs. The hexagonal type lining was first developed in 1972 in conjunction with the introduction of the double-shield TBM (Vigl, 2000).
Some types of precast lining include gaskets fitted along each of a segment’s edges to limit and prevent leakage into the tunnel. These are mainly used for civil infrastructure but are also gradually being included for some hydropower tunnels.
Where highly variable geotechnical conditions are anticipated, precast linings are sometimes designed with variable amounts of reinforcement, such as in the early 1990s for the Delivery Tunnel North (Lesotho Highlands Project) as an alternative design to the original tender (Richards et al. 1994). In addition, early precast linings’ practice for hydropower tunnels included one-way pressure relief valves. These are designed to prevent the build-up of high groundwater pressures and minimise the risk of damage, since many hydropower tunnels are located at great depths where high groundwater pressures may be present. This practice appears to be rarely used today as the reliability and long-term durability of such valves has often been questioned.
PRECAST LININGS FOR HYDROPOWER TUNNELS
Early tunnel projects pre-2010 and today
The first noted use of a precast lining for a hydropower tunnel was in 1972 for the 4.3m-diameter, 4.1km Orichella tunnel in southern Italy. Contractor Seli needed a TBM solution that would protect workers in fractured granitic ground and provide a rapid rate of advance, while simultaneously lining the tunnel. This challenge inspired the advent of the double-shield TBM by Robbins which has gone on to be used in conjunction with precast linings for many hydropower tunnels globally.
Based on a database of N=72 projects of nearly 925km, a total of about 50% of projects (460km) were constructed using precast linings prior to 2010 but with a noted increase starting in 1992 as presented in the histogram of Figure 5 (Brox, 2021).
PRECAST LINING THICKNESSES IN USE TO DATE
A key advantage of precast linings is that they can be designed and constructed to be appreciably thinner than poured in situ concrete linings for drill and blast tunnels. This advantage is due both to the superior stability of circular geometry versus the typical D-shape of drill and blast, as well as the TBM technique’s near total elimination of excess excavation. For example, for the 5.8m ID, 3.3km El Alto hydropower tunnel in Panama, a 500mm cast-in-place lining was originally designed but was replaced with a 350mm precast lining; this resulted in major savings of 9% excavation costs and 46% concrete costs.
The relationship between tunnel diameter and lining thickness for the database of projects is presented in Figure 5. This illustrates that most tunnels with a diameter between 3.5m and 6m have a lining thickness of 250mm, and that tunnels with a diameter between 6m and 8m have a lining thickness of 300mm. Both of these are significantly thinner than their typical cast in situ equivalents.
From the database of N=54 hydropower projects, 54% had hexagonal, 30% had trapezoidal, and 16% had rectangular-type precast linings.
Key aspects of precast linings in use to date
Precast tunnel linings have been used since the early 1970s for more than 600km of tunnels in variable ground conditions. Key aspects associated with some of the projects are shown in Table 1.
Upcoming hydropower projects where precast linings have been designed and/or are under construction include for tunnels at Polihali, Lesotho (5mØ, 38km); Majes II, Peru (5.5mØ, 16km); Snowy II, Australia (11mØ, 32km); Jangi Thopan, India (9mØ, 12km) and Nenskra, Georgia (4mØ, 37km).
During the construction of over 70km of tunnels at the Alto Maipo project in Central Chile, precast linings were adopted as a design change for a 12km stretch (in place of the final shotcrete linings of the original design) using open gripper TBMs. This came about after the contractor and client recognized the challenges of adverse and non-durable geology.
Fabrication requirements
Fabricating precast linings requires significant effort and working space due to the size of the individual segments. While preferable, it may not always be possible to produce the segments close to the project site, particularly for many hydropower projects which are located in mountainous regions.
Segment size will determine the space required for production and will include preparing and inserting reinforcement cages into the moulds (for rebar option), pouring the concrete, setting and stripping the moulds, and curing. Most production facilities have a carousel system to optimize and maximize output.
Transport of linings into tunnels
Hydropower tunnels generally vary in diameter from 3.5m –10m. During construction, transporting precast linings into the tunnel generally depends on both the tunnel size, the preferred method of TBM set-up and the associated logistics, such as mucking. For tunnels less than 6m diameter where locomotive and wagons are used for muck removal, precast segments are typically transported by rail-based transport wagons as employed at the 4.3m diameter, 9.6km-long Kovanlik Tunnel, Turkey. However, for tunnel diameters greater than 6m diameter, it is common to transport the much larger and thicker precast segments by specially-made rubber-tired vehicles.
Installation risks
While the fabrication and installation of precast linings are now based on well-established procedures, there remain some risks for overall good quality installation. This includes from overhead handling, misalignment of segments, and damage to segments during erection and installation. Typically, the damage will be of limited extent; small defects can be repaired, otherwise replacement will be necessary.
The construction of a TBM-excavated hydropower tunnel should not be assumed to be free of risk, especially in adverse geotechnical conditions. For example, at the Tapovan project in India, significant schedule delays were experienced in 2009 when the TBM became entrapped for several years when it intersected a geological fault under deep cover of 1,100m. Similarly, in 2017, a TBM became entrapped for about 18 months at the Mtkvari project in Georgia, including collapses to the surface, before being liberated and able to complete the headrace tunnel.
A further noted challenge for the use of precast linings occurs when very high external groundwater pressures are present, such as those during the construction of the 4.5m-diameter, 5km Pando tunnel in Panama, which saw fully-saturated lahar bedrock causing problems for the advance of the EPB TBM (Grandori, 2013). Difficulties were also encountered during the construction of the 4.4m diameter, 14kmlong Uma Oya Tunnel in Sri Lanka, where extreme groundwater inflows through the precast lining were experienced prior to annulus grouting.
KEY ADVANTAGES OF PRECAST LININGS
Simple design and implementation
Design requirements for precast linings are fairly straightforward: the majority of design loadings are well known, with limited key assumptions for ground and groundwater conditions. A much under-appreciated benefit of precast linings is their implementation during construction which, for most projects based on a single-ring design, does not require any decision making by site staff in relation to the encountered geotechnical conditions. Any such risk is completely eradicated from the design and construction process to the benefit of the designer, contractor and client. But for traditionally-supported hydropower tunnels based on partial shotcrete and/or concrete linings, this typically constitutes the greatest risk during construction: a number of these projects have experienced recent collapses during early operations due to incorrect decision making over the type and extent of the final linings during construction (Brox, 2019).
Quality control of fabrication
The fabrication of precast linings requires exacting quality control given the precision installation that is ultimately required. Quality control during fabrication includes the use of precision moulds without distorting curvature; checking minimum concrete cover to reinforcement; checking the quality of concrete through cube-strength testing; trimming concrete within moulds, and checking temperature and minimum curing times.
Rapid installation and high tunnel progress
Perhaps the most important benefit of precast linings for hydropower tunnels is the very high rate of progress that can and has been achieved. For most projects with precast linings, achieved progress rates are significantly higher than those for some long tunnels constructed using open gripper TBMs followed by final cast-in-place concrete or shotcrete linings. This is because precast linings allow a one-pass design and construction solution for earlier completion. Table 2 presents a selection of maximum monthly rates of rapid installation achieved on a few hydropower tunnels.
Long-term stability against deterioration and collapse
A further key benefit of precast linings for hydropower is the long-term stability of the tunnel; the limitation of abrasion (Madan, 2019); and eliminating deteriorating non-durable geotechnical conditions and potentially associated collapses, as seen on other tunnels. As previously noted, the complete filling of the annulus between precast lining and excavated tunnel profile is of critical importance to prevent possible erosion behind the lining which can lead to instability and collapse. However, well established quality-control procedures can usually confirm that total backfill grouting has been completed and will achieve the requirement of a fully-integrated lining system that can foster safe, long-term hydraulic operations.
Possible elimination of environmental impacts
Constructing hydropower tunnels in environmentally-sensitive areas using gasketed precast linings can significantly decrease and possibly eliminate some environment impacts. Gaskets can prevent the lowering of groundwater which can result from major inflows within the project area and along the tunnel corridor; these occurred during the construction of the 14km tunnel on the Uma Oya project in Sri Lanka, where significant settlement and associated damage occurred to residential buildings on the surface due to tunneling at depths of over 300m using only partial precast linings.
SUCCESS OF PRECAST LININGS – HYDROPOWER
Of the nearly 925km of global hydropower tunnels excavated since the 1970s by TBM using precast segmental linings, there have been no reported serious concerns or problems to have impacted hydraulic operations at any project.
In 2016, after 13 years of operations, the 32km Mohale tunnel of the Lesotho Highlands Water Project with its hexagonal precast lining was manually inspected as part of a planned outage; it was initially suspected that voids had formed behind the lining. However, this was subsequently confirmed to be incorrect following the completion of a subsequent dewatered inspection in 2019 that included a ground penetrating radar survey of the entire tunnel (Molapo and Lees, 2018). The initial concern of voids was warranted based on the historical knowledge of the early deterioration of the basalt bedrock in the project area. The tunnel had not previously experienced any hydraulic concerns. This example is considered to confirm the acceptability and success of hexagonal precast linings for hydropower tunnels.
A further noted example is at Turkey’s Kargi tunnel, where the 9m diameter, 12km-long tunnel which included 8km of precast lining and 4km of drill and blast with shotcrete lining saw problems occurring after less than one year of hydraulic operations, with suspected collapses. Upon inspection, a total of three collapses of more than 1,000m3 were identified and all associated with the drill and blast section. The 8km precast lining section experienced limited problems involving the cracking of 11 rings (a stretch of approximately 15m) which were removed and replaced with traditional tunnel support; core drilling was performed and included the minor additional annulus grouting of 18 rings (27m).
CONCLUSIONS
Over the past 50 years, the advantages and successes of precast concrete segmental linings in conjunction with TBM excavation for long hydropower tunnels continues to be recognized as the most cost-effective and low-risk approach for safe future hydraulic operations without undue maintenance. Key benefits of precast concrete segmental linings include:
? Improved hydraulics;
? Durability against abrasion;
? Simple pre-design and construction without the inherent risk of decision-making for ground support;
? High-quality control of fabrication;
? Rapid installation for high productivity and shorter construction durations;
? Significant schedule and cost savings with greater cost certainty;
? Confident long-term stability for safe and uninterrupted future hydraulic operations, and
? Improved worker safety due to protection provided by the TBM shield.
Good quality control of annulus grouting to prevent void formation behind the lining and the potential resulting instability is one of the most important aspects for successful installation of precast linings for hydropower tunnels. While this paper addresses hydropower tunnels that incorporate and benefit from precast tunnel linings, it should be recognized that they have also been used for numerous other types of hydraulic tunnels, including for potable water and wastewater projects.
