SECTOR | POWER, WATER & STORAGE


2500.00 2300.00 2100.00 1900.00 1700.00 1500.00 1300.00 1100.00 900.00


Chainage Description


Support Type (I-V)


Overburden greater than 900m. HIGH STRESS CONDITION


Surge Shaft Powerhouse Tailrace Tunnel Alignment


Ch. 1783m Ch. 881m Ch. 402m 0 to 402


Good to very good rock mass, banded gneiss, some shear zones


5%; 10%; 35%; 35%; 15%:


402 to 881


Good to very good rock mass, banded gneiss, some shear zones


5%; 10%; 35%; 35%; 15%:


881 to 1783


Good to very good rock mass, banded gneiss


15%; 35%; 35%; 15%; 0


1783 to 3122 High overburden. Mostly banded gneiss


Higher percentages of Type 1-III than before, plus additional support (steel ribs, spot bolts, shot crete)


3122 to 6095


Good banded gneiss


35%; 30%; 20% 10%, 5%


feldspathic schist


25%; 30% 25% 10%; 10%


Above, figure 12: Longitudinal profile for Upper Tamor Hydropower SOURCE: SANIMA HYDRO AND ENGINEERING The projects have used Robbins double shield TBMs


as well as a double shield Herrenknecht TBM. Tunnel lining is precast concrete segmental rings. The geology along these tunnel alignments has comprised very strong (250MPa-450 MPa) and abrasive quartzites, hampering TBM progress to about 250m per month (Harding, 2010). The north TBM drive of the AMR project recently experienced a major inrush of weak and soft material that resulted in multiple fatalities and the rotation of the TBM backup, which may not be salvageable.


8 TBM TYPES AND RISK MITIGATION REQUIREMENTS 8.1 TBM Types Hydropower tunnels are generally sited in mixed and competent bedrock and face-pressurised TBMs operated in close mode are generally not required. The typical types of TBMs used for hydropower tunnels are: ● Open gripper with traditional rock support; ● 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 can warrant the use the face-pressurised or hybrid types of TBMs operated in close mode for limited sections when encountered along a long and deep hydropower tunnel. Such fault zones can comprise highly fractured and/or soft clay gouge with elevated groundwater pressures or be within unique geological formations, such as highly permeable lahar.


8.2 TBM Evaluation and Selection Criteria A comprehensive technical evaluation must be undertaken for the selection of the most appropriate


type of TBM to be used for the construction of headrace tunnels for hydropower projects given the severe impacts that may arise from the various prevailing geological and geotechnical risks. Grandori et al., (2018) and Brox (2020) present and discuss the various geotechnical and logistical aspects that should be considered as part of a TBM evaluation and selection process, including the following: ● Rock Types and Distribution; ● Geological Faults and Weak Zones; ● Geological Synclines and Anticlines/Folding; ● Durability of Rock and Final Lining Requirements; ● Squeezing Potential; ● Overstressing Potential, including Rockbursting; and, ● High Groundwater Inflows, Pressures and Temperatures.


In addition to the important geotechnical aspects there exist multiple logistical and other aspects that require a careful evaluation, including access for mobilisation, portal space availability, power availability, environmental spoil disposal requirements, and contractor experience. Brox (2021) presents a TBM selection criteria logic chart of the key technical aspects to highlight the typical selection process, as presented in Figure 14.


8.3 TBM Risk Mitigation The key risk mitigation requirements related to the use of TBMs for the construction of headrace tunnels for hydropower projects are that adequate geotechnical investigations and planning are performed, well in advance, during the early study stages of a project. While the challenges of high elevation and deep drilling are recognised for mountainous regions, alternative methods of investigations are critically important


6220 to 8713 Fair to good


Ch. 3122m Ch. 6095m Open Canal


1700.00 1500.00 1300.00 1100.00 900.00 700.00


Good to very good gneiss


30% 40%; 20%; 10%; 0


Above, figure 13: Longitudinal profile for Melamchi Phase 2 Tunnel SOURCE: ADB 26 | October 2025


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