The main objective of this study is to investigate the effect of hydropower plant operation on the long-term stability of unlined pressure tunnels of hydropower plants in Norway. The authors analyzed the past production data of some hydropower plants to find out the number of starts/stops and the frequency and magnitude of load changes. The study demonstrates that an average of 200–400 start/stop events are occurring per turbine per year for the analyzed period, with an increasing trend. Currently, 150–200 large load changes per turbine smaller than 50 MW are occurring every year, and this is expected to increase by 30–45% between 2025 and 2040 for one of the studied power plants. Most importantly, the monitored pressure transients and pore pressure response in the rock mass during real-time operation at Roskrepp power plant are presented. A new method is proposed to calculate and quantify the hydraulic impact (HI) of pressure transients on rock joints and the effect of duration of shutdown/opening, which is found to be the most dominant parameter affecting the magnitude. The results show that faster shutdown sequences cause unnecessary stress in rock mass surrounding pressure tunnel. The hydraulic impact (HI) can be more than 10 times higher when the shutdown duration is reduced by 50 percent. The study recommends that duration of normal shutdowns/openings in hydropower plants should be slower so that hydraulic impacts on the rock joints are reduced and cyclic hydraulic fatigue is delayed, prolonging the lifetime of unlined pressure tunnels and shafts.
Load changes in hydropower plants result in significant pressure transients and unsteady flow in the waterway. It has been observed that instances of block falls in tunnels have increased in unlined pressure tunnels subjected to frequent load changes. To examine this problem, field instrumentation was conducted in the 3.5 km long unlined headrace tunnel of 50 MW Roskrepp hydropower plant in southern Norway. This article describes the methodology of instrumentation, presents the observations and findings. The monitoring clearly demonstrates that frequent load changes have a considerable effect in the rock mass consisting of system of joints. The observations show that pressure transients can travel deep into the rock mass irrespective of their time period. Moreover, pressure transients with longer time periods, i.e. mass oscillations, are seen to induce a higher hydraulic gradient between the rock mass and the tunnel itself. A delayed response from the rock mass is observed during pressure transients, which is the main cause of development of hydraulic gradient and additional pore pressure acting on the rock blocks. Hence, it is evident that the cumulative impact of small but frequent pressure gradients is significant and is responsible for increased instances of block falls over a long period of operation of the unlined tunnels of hydropower plants with frequent start-stop sequences. The overall impact is governed by pore pressure response of the jointed rock mass which depends on the conditions of joint geometry and joint wall properties.
Frequent pressure transients are identified as the cause of block failures in many unlined hydropower tunnels. The primary design objective of such tunnels is to prevent hydraulic jacking at design static pressure and mass oscillation but neglects the effect of short transients, i.e., water hammer. The issue has not been studied from the perspective of hydro-mechanical interactions due to frequent pore pressure changes in the rock mass. This article mainly focuses on the effect of pressure transients at different static heads, or different effective normal stresses across the joints and the effect of time period of pressure transient. Further, the change in such behaviour due to different mechanical properties of rock joints, such as stiffness, friction angle and dilation, is investigated. Numerical simulations of observed pore pressure response in the rock mass during a pressure transient are carried out using distinct element code 3DEC. The results show that relative joint deformation due to short pressure transients are the highest when joint normal stresses are 1.5–2.5 times higher than static water pressure in the tunnel and thus the vulnerability to weakening of such joints by hydraulic fatigue is higher. Further, results show that water hammers can travel up to 4 m into the rock mass even in stiff joint conditions and sufficiently high normal stresses. Results further indicate that the hydraulic impact due to water hammer is smaller as compared to mass oscillation. It is concluded that water hammers, wherever applicable along the waterway, can still contribute to hydraulic fatigue of rock joints in addition to the effect of mass oscillation and cannot be neglected when pressure transients occur frequently. Tunnel filling/dewatering and mass oscillations cause macroscopic joint displacements or block movements over long-term operation which is the major cause of block falls in unlined pressure tunnels.
Optimization of rock support is a key factor for successful use of underground space for hydropower development in the Himalaya. Therefore, finding innovative, optimum and economic solution will be the only way to guarantee such optimization. A main issue is to determine the extent of hydraulic fracturing and assess the water leakage possibility during the operation of such tunnels. The leaked water not only causes economic loss but also may severely affect the stability of tunnel, valley side slopes and the environment.The use of fully concrete/steel lined pressure tunnels against hydraulic fracturing in the rock mass is a costly alternative. Hence, it is advantageous to explore possibilities of minimizing the length of the concrete or steel lining in high pressure tunnels and shafts. A proper assessment of hydraulic fracturing of the rock mass plays an important role in this endeavor. This paper evaluates whether or not hydraulic fracturing (splitting) will occur at the 4,746m long shotcrete-lined high pressure headrace tunnel of 456 MW Upper Tamakoshi Hydroelectric Project (UTKHEP). The Upper Tamakoshi HEP is a high head project (gross head 822m) and the proposed shotcrete lined high pressure headrace tunnel will experience maximum hydrostatic pressure head of 40 bar (400m water column) at normal plant operation. To check the possibility of hydraulic fracturing, both deterministic and two dimensional numerical modeling techniques have been used. In addition, the paper also highlights the importance and challenges to be faced while estimating representative input variables needed for both deterministic and numerical modeling.
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