Applicability of Unlined/Shotcrete Lined Pressure Tunnels for Hydropower Projects in the Himalaya

Abstract

Nowadays, unlined or shotcrete lined pressure tunnels and shafts are used in hydropower projects worldwide. The prime requirements for these tunnels and shafts are that they should be economically attractive and should be able to operate without any significant problems in the long run. The concepts and design principles behind these conduits developed from the Norwegian planning, design, construction and operational experiences have been crucial for their successful implementation. However, by virtue of the different topographical, geological and tectonic environment of the Himalaya than that of the Scandinavia, the implementation of unlined pressure tunnels in the Himalaya has been emerging as a challenging issue. As a matter of fact, it was realized that a clear gap exists between the success of the unlined pressure tunnel concept in Norway and the challenge of its implementation in the Himalaya. To fulfill this gap, this PhD research project was formulated to study the possibility of implementing the unlined or shotcrete lined pressure tunnels in the Himalaya.

First of all, the economical attractiveness of the shotcrete lined pressure tunnel of the Himalayan hydropower projects was evaluated. Since the tunnel roughness is one of the decisive parameters for cost effectiveness, a methodology was developed to estimate the roughness of shotcrete lined tunnel based on the study of two tunnel cases from Nepal Himalaya. The tunnel cases were taken from Modi Khola Hydroelectric Project (MKHP) and Chilime Hydroelectric Project (CHP). It was found that the shotcrete lined tunnels are one of the economically attractive solutions in the waterway system of hydropower projects. In addition, the roughness of shotcrete lined tunnel of Upper Tamakoshi Hydroelectric Project (UTHP), which is also located in Nepal and is under construction, was predicted by using the developed methodology. More importantly, the shotcrete lined tunnels in all cases were provided with the concrete lining in the invert.

The PhD work further reviewed the Norwegian design principles for unlined pressure tunnel and their applicability in different topographical, geological and tectonic environments. In doing so, ten Norwegian hydropower projects including both failure and successful cases of unlined pressure shafts and tunnels were studied in detail. The review process revealed that the hydrostatic head gives water pressure to the rock mass surrounding the tunnel periphery. In an unlined tunnel, the confining pressure from the rock mass should be able to counteract the water pressure for the safety of unlined tunnel against hydraulic jacking. The attempt of all design criteria is then to define the confining pressure as accurate as possible. The Norwegian confinement criteria use both vertical and lateral rock covers to estimate the confining pressure. On the other hand, the magnitude of minimum principal stress in the rock mass is considered as a limiting confining pressure to counteract the water pressure. This criterion came out as a stress criterion and is the state-of-the-are design principle for unlined pressure shaft and tunnel. However, some discrepancies were noticed between the confining pressures given by these different criteria.

The Norwegian design concepts and criteria were then applied to the UTHP. The fact is that the pressure tunnel of the UTHP was designed as a shotcrete lined tunnel with concrete lining in the invert. This tunnel is different from the one which is normally fully unlined in Norwegian Hydropower projects. However, same design criteria as for the unlined pressure tunnel were used in the shotcrete lined pressure tunnel as well by virtue of the permeable nature of shotcrete lining. The extensive assessments carried at the UTHP concluded that the good quality rock mass with tight joints is suitable for unlined or shotcrete lined tunnel provided that the stress requirement is fulfilled. However, the presence of weakness zones, local shear bands, unfavorable jointing, and destressed area makes the use of unlined or shotcrete lined tunnel more challenging. Even though the Norwegian confinement criteria show headrace tunnel alignment is safe for unlined tunnel concept at the UTHP, the detailed rock engineering assessment, stress state analysis, fluid flow and leakage analyses indicates that some critical locations along the headrace tunnel alignment are vulnerable for the unlined or shotcrete lined tunnel concept. More importantly, the weakness zone considerably attenuates the in-situ stress state. In addition, the open joints and the joints filled with silt and clay having low stiffness are vulnerable for hydraulic jacking and water leakages even the stress conditions are fulfilled. Considering these facts, this thesis finally argues that there is a need for the modification of the Norwegian confinement criteria in order to successfully apply in the Himalayan rock mass conditions. This is mainly due to the presence of complex topography, geology and tectonic environment of this region.