TBMs are increasingly being used for the construction of long and deep hydropower tunnels given their well-recognised higher rates of production advance that allow for early completion and the economic benefits of an early project start-up.

Hydropower tunnels constructed for the conveyance of water to generate electricity are the most important component of a major hydropower project. But they are also the highest risk component, particularly for long and deep tunnels, since they are subjected to geotechnical uncertainties during construction, as well as access challenges during maintenance and repair operations. Hence, the TBM design and construction methodology for long and deep hydropower tunnels should be carefully evaluated during the early stages of a project and relevant information assessed in terms of applicable risks.

Previous design and construction practice from local and international projects in similar geological conditions should be reviewed in detail and the relevant lessons learned. Selecting the most appropriate type of TBM to be used should be mainly based on a comprehensive technical evaluation of all geotechnical and construction risks. Secondary aspects including project economics, site access, and availability of used TBMs should also be considered to confirm that the TBM is practical and affordable in relation to local labour costs associated with drill and blast excavation.

The primary geotechnical aspects to be considered include the potentials for squeezing and overstressing, including rock bursting, and the strength, quality and abrasivity of the main rock units along the tunnel alignment. Other primary geotechnical aspects anticipated to be intersected include the number and nature of major geological features including faults and shear zones, moderate and minor geological faults and shear zones, geological folds as synclines and anticlines, geological contacts between different rock units, the durability of the rock units, anticipated groundwater pressures and storage, and the final tunnel lining requirements necessary for long-term uninterrupted hydropower generation operations.

Figure 1 shows the double-shield TBM used at the Xe Pian Xe Namnoy hydropower project in Laos which allowed onepass installation of precast concrete segmental lining for the prevailing non-durable rock conditions along the 5.7m-diameter, 11.5km headrace tunnel (Terratec, 2019).

Geotechnical and Construction Risks

Rock types and distribution

High-risk rock types and conditions affecting TBMs include young volcanic bedrock or tuffs, breccias, agglomerates, lahar, mudstones (clay, shales and siltstones), karstic and disturbed or highly-fractured limestones, as well as mining areas with mineralisation that commonly include highly altered and associated weak rock conditions.

A longitudinal geotechnical characterisation profile drawing should be prepared that allows the project designer and client to fully appreciate the distribution of all the relevant geotechnical conditions along the alignment; this is necessary to understand the assessment of the most appropriate type of TBM. From this, the different risk levels of anticipated geotechnical conditions should be summarised to highlight the high-risk conditions.

Geological faults and weak zones

The most misunderstood aspect of hydropower tunnels relates to the oscillating internal operating pressures and their impact on geological faults and weak zones. Geological fault zones and associated highly disturbed conditions represent the greatest risk for TBMs. High-risk geological regions include most of the major mountain ranges of the Andes, Himalayas, Apennines, Alps, as well as the regions of tectonic plate margins including East Asia, western South America, and Central Asia that are generally associated with highly disturbed rock conditions.

A detailed geological map should be prepared as part of any tunnel design to identify the locations of all field-confirmed and inferred geological faults.

Geological synclines and anticlines

Geological synclines and anticlines are important zones for TBM tunnelling as they are typically associated with high horizontal insitu stresses contributing to overstressing and/or squeezing. Highly fractured conditions also contribute to instability at the tunnel face, as do hydraulic connections to overlying aquifers allowing for major groundwater inflows.

The identification and characterisation of geological synclines and anticlines is also critically important as part of a comprehensive structural geological assessment of the entire tunnel alignment.

Durability of rock and final lining requirements

The short and long-term durability of rock along a proposed hydropower tunnel alignment is of paramount importance for safe long-term uninterrupted operations.

Petrological and durability testing of rock should be performed during the early design stages to confirm mineral constituents and possibly to correlate and evaluate rapid or short-term deterioration and the susceptibility of scour during operations. Simple soak tests, and more detailed durability testing such as using ethylene-glycol should be carried out. Early deterioration of drill cores during site investigations should also be documented with photographs.

Non-durable volcanic bedrock including tuffs that are present throughout several regions of Central and South America are subject to rapid deterioration upon exposure to natural humidity from ventilation during construction. Similarly, some types of basalt bedrock, namely amygdaloidal, that contain smectite minerals and other relevant swelling-type minerals, including laumontite, are known to deteriorate upon exposure to natural humidity.

The requirements for a hydropower tunnel’s final lining where non-durable rock is present warrant a careful evaluation given the economic impact of final linings. The use of shotcrete for final linings in hydropower tunnels for nondurable rock and at the intersection of major geological faults is not considered to represent a prudent design. Saturation of shotcrete occurs over time and in combination with oscillating internal operating pressures. This results in the instability of prevailing weak zones, introducing large additional loads that shotcrete linings cannot support.

Accordingly, only full profile circular concrete linings are deemed to provide adequate support capacity for such weak zones and should be adopted for design with good quality construction for long term safe operations to prevent collapses.

Squeezing potential

Squeezing of ground typically occurs at the intersection of weak zones such as geological faults but also within low-strength rock formations. Squeezing is one of the highest risks for the use of TBMs and influences the most appropriate type of TBM to be considered. Therefore, a careful technical assessment of potential squeezing should be carried out to identify all zones and locations where squeezing may occur. Hoek and Marinos (2000) present an assessment for potential squeezing.

Overstressing potential including rock bursting

TBMs do not offer the effect of destressing at the tunnel face as with drill and blast methods but rather concentrate insitu stresses at the face and in the immediate L1 area behind the cutterhead.

Furthermore, elevated insitu stresses, namely, high horizontal insitu stresses can be expected to be present for project locations which are in close proximity to the margins of tectonic plates. The World Stress Map can be referenced to identify project locations of potential elevated in-situ stresses.

Overstressing in deep tunnels can have a serious impact on worker safety, as well as cause damage to TBMs. Clients and project designers have a fundamental responsibility to thoroughly evaluate the potential for overstressing for any deep tunnel and to fully disclose all relevant information in the Geotechnical Baseline Report (GBR) in order that bidders can review the information and perform a self-selection of the most appropriate type of TBM. This should include the presentation of representative laboratory testing of rock strength and in-situ stress testing results. Brox (2017a) presents a simplified method of analysis for the prediction of overstressing, including rock bursting that has been validated for over 30 case projects where different levels of overstressing were observed; Brox (2019) discusses the impacts that can occur to TBMs due to overstressing.

High groundwater pressures and storage

High groundwater pressures and storage may be associated with certain geological conditions in mountainous regions and other unique rock formations. Groundwater levels and any fluctuations, as well as in-situ rock mass permeability, should be confirmed as part of early design studies to confirm the expected maximum groundwater pressures and inflows that may be faced during TBM tunnel construction. Rock formations with elevated in-situ rock mass permeabilities (and possible hydraulic interconnections) at depth should also be evaluated for the potential for drawdown of the regional or local groundwater table; in some project areas, this may represent a serious environmental or social impact that may be unacceptable, even over the short duration of a project.

Adequate geotechnical investigations

Adequate geotechnical investigations should be performed along a tunnel alignment in order to provide acceptable information for the technical assessment of TBM applicability. Brox (2017b) presents the different types and methods of geotechnical investigations along with recommendations for underground hydropower projects in order to reduce the risks which may be realised during construction.

If inadequate or limited geotechnical information is available, then it is very difficult to assess and evaluate the applicability of a TBM for any given project site.

Inadequate or limited geotechnical investigations have been responsible for significant cost and schedule overruns on major hydropower projects, including the use of inappropriate TBM types (Hoek and Palmieri, 1998). Hydropower developers are therefore strongly encouraged to allocate adequate budgets and schedules to enable geotechnical investigations during the project designs.

Very limited geotechnical investigations were completed for the Alto Maipo hydropower in Chile which included over 70km of tunnels. This resulted in the change from an open gripper TBM to shielded TBM with pre-cast concrete segmental lining due to risks associated with overstressing at depth and nondurable rock conditions that were not originally recognised as part of the initial project design. While the completion of geotechnical site investigations including deep drilling is recognised as being difficult and costly in mountainous regions, project developers are encouraged to allocate extended durations and possibly seasons to perform such fieldwork.

Secondary Considerations

Many long and deep hydropower tunnels using TBMs are aligned parallel to and sited within valleys with very steep side slopes that may be unstable, particularly during the rainy seasons; this makes intermediate access either not possible or else accompanied by high risks. The cost of building access roads to intermediate access adits along such steep slopes is often very high since it typically involves significant blasting and end-hauling. In addition, the removal of spoil from tunnels from such access adits can be dangerous with fully-loaded haul trucks descending at steep grades to spoil sites located within the lower elevations of the valleys. In some countries, the cost of underground labour with the use of existing drill and blast equipment is relatively low. In comparison, the costs associated with a TBM in such cases, particularly for a remote, difficult to access site may be cost prohibitive.

Last, there is a large market for refurbished TBMs which can reduce project costs. A comprehensive technical evaluation should also be performed to confirm the minimum requirements for using a refurbished TBM. In some cases, it may be necessary for the refurbished TBM to have higher installed power or larger cutters due to stronger rock conditions, or the modification of a shield component to mitigate construction risks.

Types of TBM and Applicability

General

Hydropower tunnels are generally sited in mixed and competent bedrock therefore face-pressurised TBMs operated in closed mode are generally not required. The typical types of TBMs used for hydropower tunnels are:

  • Open gripper
  • Single shield with precast concrete segmental lining
  • Double shield with traditional rock support, and
  • Double shield with precast concrete segmental lining.

However, it should be noted that geotechnical conditions often associated with geological faults comprising highly fractured and/or soft clay gouge with elevated groundwater pressures or within unique geological formations such as highly permeable lahar warrant the use of face-pressurised or hybrid-type TBMs. These are operated in closed mode for such limited sections when encountered along a long and deep hydropower tunnel.

Open gripper TBM

To date, open gripper TBMs have been the most common type selected where there is a risk of overstressing. However, while they pose the lowest risk of becoming trapped in squeezing conditions, they are also the highest risk TBM against elevated levels of overstressing, since workers are exposed within the forward L1 area for the installation of ground support. Open gripper TBMs allow for close access to the area behind the cutterhead for the early installation of ground support or possible de-stress blasting as was successfully used for the Olmos TBM tunnel in Peru; they also offer very high rates of production (30m+/day) in good quality rock conditions.

If the risk of overstressing is limited in both degree and extent along the tunnel alignment, it is considered appropriate to select an open gripper TBM as long as mitigation measures are included. A unique advance for the hydropower industry is the use of small diameter (less than 2.2m) open gripper TBMs in good quality rock conditions for the construction of relatively short tunnels less than 3km (Log, 2019).

Single shield TBM

Single-shield TBMs are typically used for deep, long tunnels (in bedrock) where there is the anticipated risk of squeezing conditions at the intersection of several geological faults.

Single-shield TBMs offer increased protection for workers against the risk of overstressing as they are most commonly used in conjunction with the installation of pre-cast concrete segmental linings that prevent all exposure to open ground. Single-shield TBMs can be operated in open or closed mode with face pressurisation if required.

The impact of overstressing on single shield TBMs is associated with cuttability whereby overstressed rock fragments can cause jamming within the cutterhead and lead to a significant reduction in production. Further risks are associated with cutter inspections where access to the area ahead of the cutterhead is required and possibly for cutter changes, unless back-loading capabilities are included in the TBM design.

Single-shield TBMs are only capable of limited production, albeit still reasonable, typically up to 15m/day, since the TBM cannot advance during the installation of the pre-cast concrete segmental lining.

Figure 2 shows a single shield TBM with precast concrete segmental lining for use on the 8km upstream section of the T2 Tunnel at the Kemano hydropower project, Canada. The geology there comprised poor-quality rock, including multiple geological faults (Herrenknecht, 2019).

Double shield TBM

Double-shield TBMs are typically used for deep and long tunnels (in bedrock) where there is a very limited risk of squeezing conditions, overall limited poor quality ground conditions, and where there is the design requirement for a lined tunnel where a precast concrete segmental lining is installed due to concerns for rock durability and/or for improved hydraulics.

Compared to single-shield TBMs, double-shield TBMs offer enhanced production, typically as much as 30m/ day, and also offer increased protection for workers. However, the extended length of a double-shield TBM is not considered to drastically reduce the risk of overstressing, with workers exposed when a precast lining is not used, as the effects of overstressing have been known to have occurred well behind the face of an advancing tunnel. The impact of overstressing on double-shield TBMs is similar to that for single-shield TBMs with respect to jamming, and cutter inspections and changes. Double-shield TBMs can be operated in open or closed mode with face pressurisation if required.

TBM Precast Segmental Lining Approach

TBM tunnel excavation using precast concrete segmental linings have been used for hydropower tunnels since 1995. These precast linings often comprised hexagonal segments that were non-bolted and non-gasketed. The performance of such hydropower tunnels and their lining systems is unknown but is believed to be acceptable as there are no known or disclosed failures or major problems reported to date.

However, there are recognised construction risks with precast segmental linings. This includes incomplete filling of the annulus due to overstressing and/or groundwater inflows preventing 100% filling, thereby allowing further groundwater inflows; washout and the deterioration of non-durable rock or dissolution of weak minerals and the formation of voids. Such conditions may lead to instabilities such as misalignment of lining rings during construction, or tunnel collapse during operations. Quality control of annulus grouting with 100% confirmation of filling is therefore critical during construction. However, significant advances have been made using two component grout systems injected from within the tail shield and controlled with the TBM advance to provide a good level of confidence in achieving the full annulus filling.

The use of precast concrete segmental linings as the final lining for hydropower tunnels, particularly in non-durable rock formations, is considered to represent a prudent design approach for a few key reasons, namely:

  • They remove the decision-making process associated with the installation of traditional rock support and the risks associated with the under- or inappropriate support of weak zones and future collapses.
  • Contractual disputes associated with the impact of unanticipated additional rock support and the impact on TBM production are avoided, and
  • They provide a more durable, high-support- capacity final tunnel lining to prevent instabilities manifesting behind the lining which may in some cases lead to a collapse.

Practical Constraints

High groundwater pressures in excess of 6-7 bars that may be associated with the prevailing geological formations along a tunnel alignment will limit the size of small diameter TBMs. High groundwater pressures require a man-lock in order to access the TBM cutterhead for maintenance and the changing of cutters. In order to have adequate internal space to include a man-lock with a shielded TBM, an overall larger TBM size and diameter is required. Typically, the minimum size TBM for a man-lock is about 6m, however this may change in the future. Hydropower developers and designers should therefore importantly consider this project constraint for the use of a TBM for tunnel construction if such adverse groundwater conditions may be present at the project site.

Such conditions may therefore require the oversizing of the pressure tunnel which may also then impact the overall economics of the project as slightly higher construction costs will be associated with a larger tunnel size.

Very high groundwater pressures were experienced during the TBM tunnel construction at the Pando hydropower project in Panama (11 bars) and at the Lake Mead No 3 Intake in the USA (15 bars). The TBM diameter for the Pando project was limited by the project client to 4m and so could not include a manlock and was unable to complete the tunnel under the extreme conditions (Brox, 2018). However, the TBM at Lake Mead had a diameter of 7.2m and included a man-lock and successfully completed the 4.8km intake tunnel. Grandori et al. (2018) present a summary of the conditions to consider and details of TBM design requirements for the selection of the type of TBM for mountainous tunnels based on extensive industry experience of difficult TBM tunnel construction.

TBM Risk Assessment Example

A comprehensive risk assessment including a risk workshop should be performed as part of the overall evaluation of the applicability of using a TBM and the most appropriate type of TBM to be used.

Both qualitative and quantitative risk assessments should be performed by incorporating both cost and schedule consequences, and realistic probabilities of their respective occurrences; these should be undertaken by well-experienced experts to derive realistic and practical total project risks from which key decisions can be made.

A qualitative risk assessment should comprise the recognition of the key hazards impacting the use of a TBM on a long and deep tunnel, the applicable project input, assigned risk ratings, and the designated applicability for each type of TBM. The key hazards impacting the use of a TBM are:

  • Squeezing
  • Overstressing
  • Main rock units – strength/quality/abrasivity/alteration
  • Geological faults/shears (major)
  • Geological faults/shears (moderate and minor)
  • Geological olds (synclines/anticlines)
  • Geological contacts
  • Rock durability – final lining requirements
  • Groundwater – high pressure and depressurisation, and
  • Groundwater – adverse chemistry (corrosion potential).

Conclusions

The use of TBMs is expected to continue to be adopted for the construction of long and deep tunnels for hydropower. Assessment and selection of the most appropriate type should be mainly based on a comprehensive technical evaluation of all geotechnical and construction risks with consideration of the secondary aspects of project economics, site access and availability of used TBMs.