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Abstract
The complex topography, geology, and tectonic environment prevailing in the Himalaya are the main challenges while applying the unlined or shotcrete lined pressure tunnel concept for hydropower projects. In addition, rock masses in the Himalayan region are influenced by faulting, folding, schistosity, and jointing to a varying degree representing geological complexity. Similarly, frequent occurrence of large scale earthquakes changes in situ stress dynamics. In spite of these challenges, an unlined/shotcrete lined pressure tunnel is being constructed at the headrace tunnel system of Upper Tamakoshi Hydroelectric Project (UTHP) in the Nepal Himalaya. This article studies the unlined or shotcrete lined pressure tunnel in terms of the topographical conditions, in situ stress state, and overall rock engineering aspects. First, the pressure tunnel alignment is assessed using the Norwegian confinement criteria. Second, the minimum principal stress state is assessed using numerical simulation by validating measured in situ stress conditions. Finally, a comprehensive assessment on the rock engineering aspects of the headrace tunnel is carried out. It has been found that if there exist good quality rock mass with tight joints, it is possible to apply the unlined/shotcrete lined pressure tunnel system in the Himalaya provided that the stress requirement is fulfilled. It was also found that the Norwegian confinement criteria are too optimistic for direct use in the design of high pressure headrace tunnel alignment. The detailed rock engineering assessment and stress state analysis indicated that there are some critical locations along the Upper Tamakoshi headrace tunnel alignment. This is specially the case for an about 700 m downstream stretch of the headrace tunnel from where there is a risk of hydraulic jacking, which may possibly lead to excessive water leakage during power plant operation.
Highlights
In an unlined pressure tunnel/shaft of a hydropower scheme, water gives pressure to the rock mass around the tunnel periphery equal to the pressure given by the vertical water column measured from ‘head water level’ (HWL) to the point of consideration
The qualitative assessment of the rock engineering aspects indicates that the tunnel stretches of the NEW HRT alignment where rock mass falls under the quality class of fracture rock mass (Q value between 0.1 and 4) and the rock mass of the weakness/shear zones are potentially vulnerable to hydraulic jacking and water leakage during operation
On the other hand, simulated in situ stress magnitude and the rock mass quality prevailing at the downstream end of the headrace tunnel show that this stretch of the tunnel is vulnerable against hydraulic jacking
Summary
In an unlined pressure tunnel/shaft of a hydropower scheme, water gives pressure to the rock mass around the tunnel periphery equal to the pressure given by the vertical water column measured from ‘head water level’ (HWL) to the point of consideration. The ground is not always flat; rather it is characterized by typical slope topography in the areas where hydropower plants are located This condition imposed more challenges in reality when a failure occurred in an unlined pressure tunnel at Askara project in 1970 where the tunnel was initially designed by using the criteria defined by Eq 1. The overcoring stress results indicated that the ‘OLD HRT’ alignment proposed in 2008 as unlined/shotcrete lined tunnel was safe for implementation, which was originally designed using the Norwegian confinement criteria. The assumption on this pre-construction phase design was that the minimum principal stress would most likely be sufficient to safeguard the
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