Iran Water and Power Resources Development Company was entrusted 1983
with the design of Siah Bisheh pumped storage scheme. The waterways of
the plant are now under construction (Fig. 1) (Moshanir
Consultant Engineer, 2002). It is located in the northern part of the
Alborz Mountain, at a distance of 80 km from the Caspian Sea.
|| Layout of Siah Bisheh pumped storage project
The creation of underground openings results in significant changes in
stresses in the rock mass. In order to assess the stability of such openings
and slops it is necessary that the stresses and deformations in the rock
mass parameters, in the analysis, rather than using their approximate
or average values (Hoek et al., 1991). However, limitation in modeling
is expected due to the difficulties associated with obtaining realistic
input data. In the recent past, numerical simulation is being preferred
over the modeling where in the rock mass properties are established either
with the help of empirical methods, based on hundreds of case histories,
or rational approaches, based on mainly laboratory or in situ testing
(Giraud et al., 1993). The evaluation of stress distribution, around
the underground openings, is important for designing a proper support
system. In addition to this, nonlinear constitutive behavior of the rock
mass also is considered. In such a situation, finite element analysis
is found to be quite efficient in handling such complexities (Bathe et
al., 1980). A critical review of the available literature, on stability
analysis of underground openings, indicates that mostly analysis of underground
openings are based on the rock mass parameters which are generally assumed
or are representative of the rock mass. However, sufficient information
on 3D finite element nonlinear analysis, of such shafts in conjunction
with headrace tunnels in the rock slope is not available in the literature.
As such, for an efficient design of underground openings realistic behavior
of materials and an appropriate model for the analysis must be adopted.
With this in view, relative suitability of 3D analysis has been investigated
for power intake shafts and headrace tunnels in the rock slope of Sih
Bisheh Pumped Storage dam by adopting in situ and laboratory rock
The pumped storage plant is situated in layers of the Jurassic Shemshak
formation. The Garmrudbar thrust fault separates the Jurassic Formation
from the Triassic one (Alavi, 1996).
Height of power intake shafts is 50 m with 9.3 m diameters. Spacing of
the shafts is 25 m (center to center). The shafts joint to two headrace
tunnels with 7.5 m diameters (Farab and Tablieh Geotechnical Company,
2003). These underground spaces are located at vicinity of a rock slope
in the Shemshak Formation. The formation consists of shaly, slightly sandy
siltstone, sandstone and thin layered limestone.
The whole formation is folded and forms the southern flank of an anticline.
The folding process caused a shearing of incompetent layers such as thin
layers of beds but also between siltstones with different content of fines
like clay or fine sand.
This study provides a brief explanation on geological conditions of the
power intake portal area based on the latest observations and findings
from the recent excavations for the access road and also presents the
results of stability analysis of the inlet portal, intake gate shafts
and the transition zone nearby the shafts in the headrace tunnels. This
study also aims to determine rock support system required for stabilizing
the excavations at inlet portal, intake gate shafts and the headrace tunnels
nearby the shafts and slope.
GEOLOGY OF THE SITE
Geological condition of power intake area is shown in the geological
map drawing in Fig. 2 (Kayson Company, 2005).
The power intake portal area is located in the Shemshak formation composed
of mainly siltstone and sandstone with coal and or shale intercalations.
Structurally, the power intake area is situated on south flank of a syncline
(Fig. 2). The siltstone beds are slightly to moderately
weathered, laminated, grey to dark grey, soft to moderately hard. Sandstone
beds are slightly weathered, medium to coarse grained, grey and hard.
The thickness of beds varies between 0.2-2.5 m. Generally there are three
joint sets which are limited to the bedding plane in most cases. There
is Fe-oxide on all joint surfaces.
||Geological map of site of power intake shafts (Kayson
||Orientations and Characteristics of major discontinuities
in power intake area
The bedding and major joints orientations and their characteristics have
obtained from field surveying and are shown in Table 1.
GEOMECHANICAL PARAMETERS OF THE ROCK MASS
Orientations and characteristics of the discontinuities have been defined
based on the field measurements. Shear strength parameters of the discontinuities
are obtained based on the conditions of the joint surfaces, type of rock
materials and using the earlier experience in this field (Hassani et
The shear strength parameters for the discontinuities have been selected
based on earlier experience, the existing lithology (intercalations of
coal, shale and sandstone) and the disturbances. A cohesion value of zero
has been assumed due to the potential for filling material in the discontinuities
being washed out by drainage water in the long term and friction angle
of bedding planes and joint sets are measured to and respectively, as
shown in Table 2.
Geomechanical classification of various types of rock masses in Shemshak
formation have been performed using GSI classification system (Hoek, 2001).
The required data obtained from field observations and joint survey data
collected from direct measurements on rock exposures and laboratory tests.
Estimate of rock mass parameters has been done using RocData. This information
is shown in Table 3. As shown in this table, only one
rock mass type has been identified in the power intake and gate shafts
area, from the lithology and degree of weathering point of view, which
is slightly to moderately weathered, moderately disturbed siltstone, with
intercalation of coal, shale and sandstone (Flysch Type C, GSI = 38).
Dilation angle for rock mass is calculated by below formula (Arshadnejad
et al., 2006):
||Orientations and shear strength parameters of discontinuities
||Rock mass characteristics and parameters at power intake
portal and gate shafts (Kayson company, 2005)
Discontinuity analysis in underground spaces is very important. Because
of it occurs individually displacement around the underground space (Goodman,
1989). Therefore it must be separately analysis, before numerical modeling
(Brady and Brown, 1993).
For this analysis was used Unwedge 3.0 software. All of the wedges are
stable. Bolt spacing in the shafts is 22 m with 5 m length and it is 1.5
1.5 m with 4 m length in the tunnels. Shotcrete thickness in the shafts
and the tunnels are 10 and 15 cm, respectively. Table 4
and 5 show that the results of underground wedge on
the shafts and tunnels after installation of the supports.
|| Stereonets of joint sets (Dip and Dip Direction) in
the shafts and tunnels
|| Wedges in the shafts and tunnels
||Results of underground wedge analysis in the water intake
shafts with Unwedge 3.0 software
||Results of underground wedge analysis in the water intake
tunnels with Unwedge 3.0 software
3 shows the Stereonets of joint sets in the shafts and tunnels and Fig 4 shows the wedges in the shafts and tunnels.
NUMERICAL MODELING OF THE POWER INTAKE SHAFTS AND HEAD RACE TUNNELS
For the power intake portal, two sets of analysis were carried out. Firstly,
the overall stability of the rock slope with out the tunnels and the shafts
was analyzed in 2 dimensions. When there is a transition zone in the rock
mass 3D modeling is the best technique for numerical modeling (Roth, 2001;
Brady and Brown, 1993; Goodman, 1989). After modeling, results show that
this method (2D modeling) is wrong, when some underground spaces are nearby
to the slope. Because of some parts of model show infinite displacement
that is not true. Modeling of the shafts and tunnels in separately, obtained
the same results. For these modelings were used Phase 2 and slide softwares.
After that was used ABAQUS software for 3D modeling of all structures
with together. In the 3D model were used 21,078 triangle elements. This
type of element is suitable for complex geometry (Goodman, 1976), like
topography in mountain. Type loading selected gravitational model and
whole side of the model fixed by 3 degree of displacements and 3 degree
of rotations. Scale of the model is 1:1.
Discontinuities in the rock masses normally reduce the strength properties.
These characteristics can be model by a continuous media, based on reduced
strength of geomechanical parameters (Hoek, 2000). Therefore the method
was used for 3D numerical modeling by ABAQUS software.
Time of the running was 1843 min and computer`s CPU was Pentium 4, 3.0
GHz with 384 MB. Figure 5 shows the numerical model
with all of structures that was made by ABAQUS soft ware.
For the stability analysis were made 6 models with different loadings.
In the modeling was used reaction of some supports system (Hoek, 1999),
like shotcrete and rock bolt. Anchor bolts installed by pre-tension force
with 70% of bearing capacity of the bolts (Hoek, 2000).
Overall stability analysis of both the power intake slope and the gate
shafts were performed in two conditions, static and pseudo static conditions.
In this case a seismic coefficient 0.35 g was considered for horizontal
PGA. In model number 5 and 6, was considered dynamic loading (pseudo static).
No water pressure was considered in none of the stability analysis of
the gate shafts, because they are above the underground water table. Figure
6 and 7 show that the structures after modeling.
Power intake slope is located between top of the tunnels and the shafts.
For stability analysis of this location was used analytical methods (Ortigao
and Sayao, 2004; Wyllie Mah, 2004) and numerical modeling, together. Results
of modeling shows a minimum safety factor for unsupported slope and supported
slope with static and pseudo static conditions was taken as 0.94, 1.57
and 1.34, respectively.
Results of overall stability analysis (minimum safety factor) of power
intake shafts, head race tunnels and rock slope are shown in Table
6. As shown in Table 6, the results are satisfactory
and in general the power intake portal is stable in both static and pseudo
static conditions. Figure 8 shows that displacements
around the tunnels and the shafts. Maximum displacement occurs between
the two shafts and the value is 8.93 mm.
Figure 9 shows distribution of displacements on the
structures into the rock mass. Results of modeling and stability analysis
of all the structures for design of supports are summarized in Table
7. Maximum displacement occurs between transition zone and tunnel
portals. It is located on the roof of right tunnel and its value is 7.52
mm. The displacement is small and acceptable (Sakurai, 1997). Figure
10 shows this location and its displacement.
||3D numerical model of power intake shafts and head race
tunnels at vicinity of a rock slope (ABAQUS)
||Stress distribution in vertical section of the shafts
and tunnels at vicinity of rock slope
||Stress distribution (sigma 1) in the rock mass with
shafts and tunnels at vicinity of rock slope
||Displacements around the tunnels and the shafts
||Displacements in the walls of the shafts, tunnels and
rock slope in meter
||Displacements on the top of the roof of right tunnel
||Minimum safety factors in whole of 3D model in static
and pseudo static conditions
|| Characteristics of the supports in shafts, tunnels
and rock slope
Power intake shafts and head race tunnels in Siah Bishe dam have a complex
geometry. In site investigation was found 3 joint sets in the rock mass. Consequently,
first of all the structure analyzed for finding underground wedges. After that
overall and local stability of the power intake shafts, the headrace tunnels
and slope were analyzed by two conditions (static and pseudo static). The results show that in the complex geometry, 2D modeling is wrong.
Especially, when there is a transverse shaft to a tunnel. In this situation
3D modeling is necessary. The 3D modeling show that maximum displacement
occurs on the roof of tunnels between transition zone and tunnel portal
(about 7 mm). Consequently, pattern of bolting in the location and around
of transition zone, will be closer than far away points in the tunnels
and the shafts. This reduction of bolt spacing is about 30% (11 m). Moreover
thickness of shotcrete increases about 30% (200 mm), in the tunnels. Pre-tensioning
force for rock anchor in the tunnels is about 70% of bearing capacity
(13 tons). These Patterns of rock bolts and shotcrete supply the stability
of wedges in the shafts, tunnels and slope in static and pseudo static