TBM risk assessment for hydropower projects

TBMs are increasingly being used for the construction of long and deep hydropower tunnels given the well-recognised higher rates of production advance that allow for early completion and the economic benefits for an early project start-up. Hydropower tunnels, which are constructed for the conveyance of water to generate electricity are the most important component of a major hydropower project and are also the highest risk component of a project, particularly for long and deep tunnels, since they are subjected to geotechnical uncertainties during construction and access challenges during operations for maintenance and repairs.  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 all relevant information assessed in terms of applicable risks. Appropriate information is required from geotechnical investigations to perform a suitable TBM risk assessment. 

Geotechnical and construction risks

The most important aspects to be considered for the risk assessment and application of a TBM for the construction of a hydropower tunnel are the geological, geotechnical, and hydrogeological conditions. The various rock units and types to be anticipated, especially the total number and conditions associated with geological faults - including their expected locations, are of critical importance to understand. The durability of the rock units is also important to appreciate in case of non-durable conditions that warrant the use of a tunnel lining to prevent excessive erosion during operations. 

The potential for squeezing at the intersection of geological faults or within low strength rock units as well as the potential of overstressing including rock bursting should be fully evaluated to identify such possible risks that can have a significant impact on TBM productivity and worker safety. Figure 1 presents the damage that occurred to a TBM after a rockburst at the Neelum Jhelum project in Pakistan.

High groundwater pressures more than 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 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, the minimum TBM size is about 6m which may result in having to oversize the hydropower tunnel which may have economic consequences. 

Logistical considerations

Many long and deep hydropower tunnels for the use of TBMs are aligned parallel to and sited within valleys with very steep side slopes that may be unstable, particularly during the rainy seasons, and therefore where intermediate access is either not possible or associated with high risks. 

The cost of constructing access roads to intermediate access adits along such steep slopes are often very high since this work typically involves significant blasting and end-hauling. Lastly, there exists a large market of used TBMs that can be refurbished for further use to reduce total project costs. A comprehensive technical evaluation should also be performed to confirm the minimum requirements for the application of a used TBM. In some cases, it may be necessary for the TBM refurbishment to include an increase in installed power or larger size cutters due to stronger rock conditions, or the modification of a shield component to mitigate construction risks. 

Types and applicability

Open Gripper TBM 

Open gripper TBMs have been to date the most common type of TBM selected for the risk of overstressing. However, while open gripper TBMs pose the lowest risk for becoming trapped due to squeezing conditions, they are 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 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, and also offer very high rates of production greater than 30m/day in good quality rock conditions.

If the risk of overstressing is limited in both its degree and extent along an entire tunnel alignment it is considered appropriate to select an open gripper TBM as long mitigation measures are included during construction. 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 as shown in Figure 2. 

Single shield TBM 

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 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 to 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 ahead of the cutterhead is required and possibly for cutter changes unless backloading 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 installation of the pre-cast concrete segmental lining.

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 whereby a pre-cast concrete segmental lining is installed due to concerns of rock durability and/or for improved hydraulics. 

Double shield TBMs offer enhanced production typically as much as 30m/day in comparison to single shield TBMs. They also offer the increased protection of workers by preventing the exposure to open ground when pre-cast concrete segmental linings are installed but also when such linings are not required and traditional rock support is used. However, the extended length of a double shield TBM is not considered to drastically reduce the risk of overstressing with the exposure of workers when a pre-cast 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 to 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 pressurization if required. 

Pre-cast segmental lining

TBM tunnel excavation in conjunction with pre-cast concrete segmental linings have been used for the construction of hydropower tunnels since 1995. Many of these pre-cast tunnel linings comprised hexagonal segments that were non-bolted and non-gasketed. The performance of these hydropower tunnels and their respective lining systems is unknown but believed to be acceptable as there are no known or disclosed failures or major problems reported in the industry to date. 

Risk assessment

A comprehensive risk assessment including a risk workshop should be performed as part of the overall evaluation of the applicability of the use of a TBM and the most appropriate type of TBM to be used. The following aspects should be considered: 

• Squeezing.
• Overstressing.
• Main rock units - strength/quality/abrasivity/alteration.
• Geological Faults/Shears (major).
• Geological Faults/Shears (moderate and minor).
• Geological Folds (Synclines/Anticlines).
• Geological Contacts.
• Rock durability - final lining requirements.
• Groundwater - high pressure and depressurization.
• Groundwater - adverse chemistry (corrosion potential).

It is important to recognise that not all risks can be eliminated for a project for the consideration of the use of a TBM for hydropower tunnel construction. TBMs are robust machines for which technological advances have been ongoing to reduce risks for their application to continue to be successful in the industry. 

* Dean Brox Consulting Ltd, Vancouver, Canada. Email: drbrox@telus.net