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Value Filed Stress type: Total Stress Ratio (in plane) Total Stress Ratio (out-of- plane) Locked-in hor. stress (in-plane) (MPa, Comp+) Locked-in hor. stress (out-of-plane) (MPa, Comp+) Estimated for rock mass based on Hoek- Brown Failure Criterion: Geological Strength index (GSI) Constants (s) from RocLab Constants (a) (RocLab) Material Constant (mb(peak)) 0.55 0.59 0 0 79 0.096 0.5 13.22 The numerical approach of analysis compensates the completely neglected effects of stress due to the topographic and tectonic stress unlike the deterministic design approaches used in evaluation of hydraulic fracturing. The effect of topography is very dominant in the stress regime in the lower part of the unlined pressure tunnel of the project that was analyzed. This result revealed when the modes were built. Figure 8 shows the effect of the topography on the stress regime. The analysis by numerical modeling revealed a conservative result for up to hydrostatic head up to 1100 m with a factor of safety 1.3 against hydraulic fracturing. Moreover, the result of numerical analysis shows fairly good degree of correlation between the simulation results and what was actually found in the existing project unlined tunnel and shaft. Figure 9 shows the minimum principal stress (σ3) of the mode and the water pressure (γw.H) for the study point at location of chainage 11+750 (power house). Figure 8. Effect of topography is very dominant on the stress regime. Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 6 Increasing the static head on the Nye Tyin HEP will increase the water pressure inside the tunnel and consequently large rock mass cover will be required. The geological restrictions are the main challenges in the development unlined high-pressure tunnels and shafts. It is essential to understand the geologic conditions along the tunnel alignment, relative to the hydraulic forces that will be applied during operation. With respect to the geology condition at Nye Tyin Hydroelectric project, increasing head will have a challenge in related to the existing minor weakness zone on the top of the mountain plateau above the power station location even though it does not intersect the underground powerhouse as well as the tunnel (Lilleland, 2002). Figure 10 shows the result of the model considering the effect of the joint near to the power station. 3. RESULTS, COMPARISONS AND DISCUSSIONS The first and foremost requirement for a pressure tunnel or shaft is that leakages have to be avoided by providing adequate overburden which has a weight greater than the water pressure. This method was used In Norway before the rule of thumb method was introduced and in this paper, the method gives surprisingly a conservative result of the possibility to develop ultrahigh head in unlined tunnels/shafts up to static head of 1137 m with a factor of safety 1.3 for the exemplified project Nye Tyin HEP. In fact, this method does not consider side valley cover requirement for confinement, the topographic effect on stress, and the existence of considerable tectonic stresses. The Norwegian rule of thumb has been widely applied in cases for tunnels and shafts sited in valley sides. It considers both vertical cover and the steepness of the slope of the adjacent valley side. The method assumes that, the minimum in-situ principal stress should exceed the water pressure at any point to avoid the risk of hydraulic fracturing. The analysis of this method indicated a static head of more than 1427 m with a factor of safety 1.3 being feasible to develop for the Nye Tyin HEP. The method accounts for gravitational rock stresses only but in many cases considerable tectonic and residual stresses also exist. Hence the effect of required increased confinement should be disregarded where the valley is undergoing tectonic extensions. This method of criteria for confinement is still used in prefeasibility studies. The Snowy Mountains for confinement requirement was recognized during the development of the Snowy Mountains project in Australia. The side cover is found less effective compared to the vertical rock cover to secure that the tunnel/shaft has the required confinement. This method has fairly similar results as the Norwegian rule of thumb method with some more conservative result on the side cover. Maximum water pressure of 16.77 MPa can be safe against hydraulic fracturing based on this method of analysis for the Nye Tyin, which is hydrostatic head of 1677 m with 1.3- safety factor. Both methods, Snowy Mountain and Norwegian rue of thumb reinforce that the pressurized water tunnel and shaft for Nye Tyin HEP is placed deep enough laterally and vertically. Hence, neither methods are actually considering the effect of tectonic horizontal stresses and topographic conditions on the stress regime. The comparison of depth of minimum cover specified by vertical, Snowy Mountain and Norwegian rule of thumb are in good agreement with the results, while the vertical criterion is not to the safe side relative to the Figure 9. σ3 and water pressure for the study point at location of powerhouse Figure 10. σ3 considering the weakness zone and joints near to the study point at chainage 11+873 Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 7 numerical analysis. For preliminarily layout in terms of minimum requirements, it appears that the Norwegian and Snowy Mountain criteria are very useful tools. Two-dimensional standard design charts based on the use of a numerical finite element model (FEM) has given a solution to compensate the completely neglected effect of tectonic horizontal stresses and topographic conditions on the above deterministic approaches. The basic principle of the finite element method for this purpose is to find the location, which for all parts of the shaft ensure that no-where along the unlined pressure tunnel or shaft should the internal water pressure exceed the in-situ minor principal stress in the surrounding rock mass. This method is a useful tool at the feasibility stage of the project and it makes it possible to find a preliminary location of the pressure tunnels and shafts; a location that in many case turns out to be the final one. The analysis based on this method indicated that the existing unlined pressure tunnel and shaft at Nye Tyin could handle a maximum static head of 1140 m with a factor of safety 1.3, which is a conservative result with respect to the above deterministic approaches. However, The result tends to be mesh-dependent and error can arise when selecting the boundary conditions of the domain of interest. Besides, using this analysis method will lead to errors, as the in-situ topography is very simplified and idealize with regard to the topography condition used in the model. Finally, comprehensive 2-dimensional finite element calculation using Phase2 has been performed and the aim was to analyze the minimum principal rock mass stress situation in the vicinity of the pressure tunnel and to compare it with the induced water pressure inside the tunnel and shaft. The maximum static head that can be utilized for the existing unlined high-pressure tunnel and shaft at Nye Tyin hydropower project is 1097 m with 1.3 factors of safety. The analysis shows fairly good degree of correlation between the simulation results and the in-situ situation of the existing unlined tunnel. The author believes that the use of numerical analysis with profound knowledge of the geological condition