Hydro TBM risk assessment and selection Mar 2020

Long and deep tunnels are important components of major hydropower projects and are inherently the highest risk component since they present geotechnical uncertainties and access challenges. The design and construction for long and deep hydropower tunnels to be excavated by TBM should be evaluated carefully during the early stages of a project and all relevant information assessed in terms of applicable risks.

Double shield TBM used for the 5.7m diameter x 11.5km long Xe Pian Xe Namnoy headrace in Laos

Precedent design and construction practice from international projects in similar geological conditions, as well as from local projects, should be reviewed in detail. The assessment and selection of a TBM should be based primarily 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 within the overall assessment. The primary geotechnical aspects to be considered include:

• the potential for squeezing;
• potential for overstressing, including rockbursting;
• the strength, quality and abrasivity of the main rock units along the entire tunnel alignment;
• the number and nature of major geological faults and shear zones anticipated to be intersected;
• the number and nature of moderate and minor geological faults and shear zones anticipated to be intersected;
• the number and nature of geological folds as synclines and anticlines anticipated to be intersected;
• the number and nature of geological contacts between different rock units anticipated to be intersected;
• the durability of the rock units;
• anticipated groundwater pressures and storage; and
• the final tunnel lining requirements necessary for long-term uninterrupted hydropower generation.

Geotechnical and construction risks

Geological fault zones and associated conditions represent the greatest risk for TBMs. The identification and characterisation of geological faults and weak zones is therefore critically important, and a detailed geological map should be prepared as part of any tunnel design to present the locations of all field-confirmed and inferred geological faults.

Safety risk and TBM damage from rockbursting

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 TBMs and influences the most appropriate type of TBM to be considered. A careful technical assessment should identify all zones and locations where squeezing may occur.

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

High risk rock types and conditions for TBMs include young volcanic bedrock of 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 enables the project designer and client to fully appreciate the distribution of all the relevant geotechnical conditions along the entire tunnel alignment.

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 of a project to confirm mineral constituents and possibly to correlate and evaluate rapid or short-term deterioration and the susceptibility of scour during operations.

Single shield TBM used on the 8km long upstream section of the T2 tunnel of the Kemano project in Canada

TBMs do not offer the effect of de-stressing at the tunnel face, as with drill and blast methods, but rather concentrate in-situ stresses at the face and in the immediate L1 area behind the cutterhead. Furthermore, elevated in-situ stresses, namely high horizontal in situ-stresses, can be expected to be present for project locations in close proximity to plate tectonic margins. Overstressing in deep tunnels can have a serious impact on worker safety, as well as causing damage to TBMs. Clients and project designers have a fundamental responsibility to thoroughly evaluate the potential for overstressing on any deep tunnel and to fully disclose all relevant information in the Geotechnical Baseline Report (GBR).

High groundwater pressure 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 realized during TBM tunnel construction.

Adequate geotechnical investigations should be performed along a tunnel alignment in order to provide acceptable information for the technical assessment of TBM applicability. If inadequate or limited geotechnical information is available, then it is very difficult to undertake an assessment of the applicability a TBM. Inadequate or limited geotechnical investigations and the use of inappropriate types of TBMs have been responsible for significant cost and schedule overruns on major hydropower projects. Hydropower developers are strongly encouraged to allocate adequate budgets and schedules for geotechnical investigations during the project designs.

Types of TBM and applicability

Hydropower tunnels are generally situated in mixed and competent bedrock, and pressurised closed-mode TBMs are generally not required. The typical types of TBM used for hydropower projects are: open gripper; single shield with precast concrete segmental lining; double shield with traditional rock support and double shield with precast concrete segmental lining.

Small diameter open gripper TBM

It should be noted however that conditions associated with geological faults comprising highly fractured or soft clay gouge with elevated groundwater pressures or within specific geological formations warrant the use the pressurised or hybrid TBMs operated in close mode for limited sections.

Open gripper TBMs are most commonly selected for the risk of overstressing. While they pose the lowest risk for becoming trapped due to squeezing conditions, they have elevated levels of risk since workers are exposed within the forward L1 area for the installation of ground support. If the risk of overstressing is limited, it is considered appropriate to select an open gripper TBM, as long as mitigation measures are included during construction. A particular advance for the hydropower industry is the use of small open gripper TBMs of less than 2.2m diameter in good quality rock conditions for tunnels of less than 3km.

Single shield TBMs are typically used for deep and 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 of workers against overstressing as they are most commonly used in conjunction with precast concrete segmental linings, which prevent exposure to open ground. The impact of overstressing to single shield TBMs is associated with cuttability, whereby overstressed rock fragments can cause jamming within the cutterhead and lead to a reduction in production.

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 requirement for precast concrete segmental linings. Double shield TBMs offer enhanced production of as much as 30m/day in comparison to the 15m/day of single shield TBMs,and also offer increased protection of workers. The impact of overstressing to double shield TBMs is similar to that for single shield TBMs.

Double shield TBM used on the 10km long downstream section of the Los Condores project in Chile

Practical constraints

High groundwater pressure, in excess of 6-7 bars which may be associated with the prevailing geological formations along a tunnel alignment, will limit the size of small diameter TBMs. For example, high groundwater pressure requires a manlock to access the TBM cutterhead for maintenance and the changing of cutters, but in order to have adequate internal space to include a manlock with a shielded TBM, an overall larger TBM size and diameter is required. Hydropower developers and designers should consider this constraint for the use of a TBM if such adverse groundwater conditions are anticipated.

Very high groundwater pressures were realised during the TBM drives at the Pando hydropower project in Panama (11 bar) and at the Lake Mead No 3 Intake in the USA (15 bar). 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. The TBM at Lake Mead with a diameter of 7.2m, included a manlock and successfully completed the 4.8km intake tunnel.

Conclusions

The following key conclusions are presented regarding TBM risk assessment and selection for hydropower tunnels:

• The use of TBMs for the construction of long and deep tunnels for hydropower is expected to continue.
• The assessment and selection of the most appropriate type of TBM to be used should be based mainly on a comprehensive technical evaluation of all geotechnical and construction risks with consideration of secondary aspects including project economics, site access, and availability of used TBMs.
• A comprehensive evaluation of all geotechnical information should be performed, and a longitudinal profile prepared to fully present all the anticipated conditions along the entire alignment.
• Formal qualitative and quantitative risk assessments should be discussed at workshops with input from industry experts to identify all risks for each type of possible TBM to be considered.
• The most appropriate type of TBM to be selected should be based on its proven capacity to be successful through most of the anticipated ground conditions, as it is recognised that there is no perfect type of TBM to handle all ground risks.
• While TBM production does vary with the type of TBM, it is not significant enough to warrant adopting a higher-risk type of TBM for a project;TBM production should not be part of the selection criteria.
• Clients and project designers should refrain from specifying the type of TBM to be used for deep tunnels or accept the liability of the performance and safety aspects of the type of TBM.