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Filed Stress type: 
Total Stress Ratio (in plane) 
Total Stress Ratio (out-of- plane) 
Locked-in hor. stress (in-plane) (MPa, 
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)) 
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. 
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. 
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 
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. 
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 
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