Global Journal of Engineering Sciences (GJES)
Seepage
Analysis for Soork Dam Foundation
Authored by Masoud Cheraghi Seifabad
Abstract
This
paper presents a 3-D seepage analysis case study of the Soork Dam in Iran. For
this study geotechnical information of the site was collected in two phases.
Then the condition of the ground waters characteristics and the seepage through
the foundation of the dam was modeled. In this way, numerical simulation was
attempted by employing Seep/W, and Seep 3D software. This study shows the
results of numerical simulations thus providing seepage measures.
Keywords: Joint Characteristics; Underground
Flow; Seepage; 2D and 3D Modeling
Introduction
It is
typical that the water reserved behind the dams cause the seepage and sometimes
leakage. The rate of seepage should be specifically controlled so that it would
not pass the permissible limits. Cedergreen [1] investigated the importance of
seepage in all aspects and maintained that most damages to the water facilities
can be from the seepage of underground water which are explained in two
categories:
• That
causes the soil particles move through an escape exit and cause piping or
erosion failures;
• The
damages caused by uncontrolled seepage patterns leading to saturation,
excessive uplift or excessive seepage forces. The seepage control of the dam
foundations was studied, and it was suggested that due to the discharge of the
soil particles and weakening the foundations [2], the seepage of water occurred
eventually leading to dam damage. The possibility of dam collapse due to
erosion by the seepage was studied by Meyer [3].
The
3-dimensional seepage was analyzed by using a finite element method based on variable
permeabilities [4]. The seepage of Kafrein dam and the ways to control it was
studied by Malkavi [5]. The stress-strain, and seepage of a high rock-fill dam
with clay core on the sand-gravel foundation was numerically simulated with
FLAC 3D software [6]. A study [7] of seepage monitoring of RCC dam was
conducted using entropy-based fuzzy matter-element analysis. Curtain grouting
with minimum seepage pressure resultant [8] was studied. A regional numerical
model was developed and used to evaluate the interaction between surface water
bodies and the Quaternary aquifer system in the Nile Valley [9]. The seepage
behavior of Gotvand Dam considering effects of dissolution of gypsum was
evaluated with 2D and 3D models [10]. A mathematical model was established for
coupled unsteady seepage and stress fields in the fractures network of the dam
foundation [11]. The 3-D program was developed to analyze the coupling unsteady
seepage and stress fields in the fracture rock mass (3D-coupled) used to
analyze the influence of seepage field on the dam foundation due to water level
fluctuation. The role of nuclear logging techniques and tracer tests were
discussed to delineate the path of the seepage from a damaged portion of
tailrace channel at the Bhama-Askhed Dam, Maharstra, India [12]. Penalty
Function Element-Free Method was used to study the steady seepage field in main
dam, that is, with a protective concrete slab and structure of hydro fracture
grouting cutoff wall of Dashi Bu reservoir located in Donghai County (China),
when it is normal water level for the reservoir, and then the errors between
the measured value and computed values from piezometer tubes [13] were
compared. An attempt was made to estimate the water seepage from the upper
reservoir of Azad pumped storage power plant based on the combined geotechnical
studies and geostatistical methods. In their study, using a geostatistical
method (Kriging), Lugeon values were estimated for the walls of the reservoir
[14]. Analytical methods and the results for assessing the variation in the
concentration of sulfates (and other ions) over space and time in the ground
water flowing through a soluble evaporate terrain beneath a dam [15] were
presented. Remote sensing and geographic information system (GIS) techniques
were applied to construct and integrate the hydrogeological data, inventory for
potential sources of contamination and mapping the sensitive areas, in order to
construct the protected buffer zone for the Ismailia Canal and to constrain the
development activities in all the surrounding areas of surface water supply
[16]. The aim of this present article is to analyze the seepage pattern within
the Soork Dam.
Dam Site
Soork
Dam is located southeast of Isfahan (Boroojen, central Iran). The average
rainfall in this area is 323 mm/y. The Soork Dam is an earth dam with the clay
core. The height of the dam measured from river bed is 39 m with a length of
590 m and a width of 11 m in crown.
Engineering geology and geotechnical characteristic of the Soork
Dam
On the
dam axis, the valley is an asymmetrical U shape with wide spans. The dips of
the walls on the left and right abutments are about 25 and 6 degrees
respectively. The rock outcrops of the area are generally of early Cretaceous
limestone and shaly limestone. In the dam area, there is Quaternary alluvium
with considerable thickness (about 100 m). At the deep areas of the valley, the
coarse sediments are replaced by fine sediments of the surface area. Figures 1
and 2 illustrate the area geology of the dam site and the geological section
respectively.
Geomechanical characteristics
of Soork Dam
The rocks in the area consist
of limestone with some abundant cracks, joints, and some small cavities on the
surface which are possibly karstic cavities in depth. Two faults with the
strikes N5E, N15W and the vertical dips cause the crush of the rock layers.
According to the discontinuities, the joints of this area divide into two
zones. Each zone is examined separately from another.
The Zone P1
This zone consists of light grey
limestone, full of white crystalline calcite veins. The abundance of calcific
veins causes color change in rocks. The abundant joints in limestone create
long blocks. The water flow and karst effects are observed on the joint
surfaces and outcrops. This section is studied linear arrangement based on the
spatial positions. The direction is vertical to main joint groups (S1, S2).
However, in order to study the other groups (S3, S4),
another scan line vertical to the first direction was chosen.
Joint Set S1
This joint set is the most
abundant and important. About 67% of the total data in this research is related
to this joint. According to the information, the joint set is characterized as
N23W/65NE and the average of vertical distance between the joints is 1.8 m.
Most of the joints have openings which filled with clay, silt, and partly sand
to gravel. There is no water leakage, or flow of water in any of the joints.
Figure 3 illustrates all the joint sets of zones P1. Joint set S1 is more than
20 meters in 34 % of them has a length of 3-10 meters in 40 % of the cases.
Joint set S1 is rough, and planar in 58% and 10mm opening and defined in 38 %,
referred to as a very open one.
Joint Set S2
This joint set is at a lower
grade with reference to the abundance and persistence which includes 17 % of
the total joints. The characteristic of the joint set S2 is N10E/35NW and the
vertical distance between the joints is 4.5m. 35% of this joint set is 3-10m
long and 29% has a length of over 20 m. Over 50% of the joints are rough,
planar and 36% are rough and undulating.
Joint Sets S3, S4
Compared with the first two
sets, these joint sets are less important. They actually cross the others. Most
joints are filled with transparent, white crystalline calcite and at the
outcrop have openings filled with clay and silt. The representative plane
characteristics of these two joint sets are as follows:
Joint Set S3:
N90E/30SE
Joint set S4:
N50E/80NW
The average spacing of the
joint sets S3 and
S4 are
1.5-2 m and 2-2.5 m respectively.
The Zone P2
Lithologically this zone
consists of a small outcrop of light grey limestone with brown to dark red
chert nodules. The limestone is folded. The characteristics of the
discontinuities in this zone are investigated linear arrangement in the
direction of N38E. Figure 4 shows the counter of discontinuities. Accordingly,
the joint sets S1 and S2 are defined from the discontinuity point-of-view. The
characteristics of these planes are:
S5: N72W/52SW
S6: N12W/48NE
The average spacings of joint
sets S5and S6 are
1-1.5m and 2m respectively.
Seepage Flow Analysis
Ten boreholes were drilled
during the 1st stage of the studies. According to the technical exploration
specifications and geotechnical studies, the water level of all boreholes was
achieved fortnightly. The trend of the underground water level changes shows
the alternative periods of dry and dump conditions of the underground water at
dam axis. Such a water table at the dam site has been modeled. As shown in
Figure 5, the direction of the underground water involves a mild deep slope
towards the middle of the valley and the water level of the dam axis passes
borehole 10.
Water flow monitoring
The movement of water in the
cavities and different joints in the rock differed during water pressure tests.
This depends on the type of joint or cavity. Thus, the water flow behaved
differently. In most cases the turbulent type of the water flow was observed.
Normally in these cases the permeabilities are high and there is need for
grouting. In these areas, the joints are very open and there is lack of
filling.
Seepage of alluvium materials
To achieve permeability of
granular sediments in the field, the Laufran permeability test was conducted in
all of the boreholes in the field. The permeability test was conducted using
both methods ‘constant head’, and ‘falling head’. The results differed from
10-7 to 10-2cm/s. Because of the variation in permeability, it is important to
note the following:
• The boreholes BH3 & BH9
were drilled and tested around the bed river in fine materials. The average
permeability in borehole 3 was 10-4cm/s and the maximum permeability change
from 10-4 to 10-3cm/s. The maximum permeability in borehole 9 changed from 10-3
to 10-2cm/s. Therefore, it is reasonable to choose 10- 3cm/s for the horizontal
permeability of this section. Generally, the vertical permeability is less than
this and should be taken into consideration in the calculations.
• In the middle section of the
valley, the fine alluvium sediments mixed with the course were located to depth
of 100m. The boreholes BH2, BH7 were drilled and tested in this area.
Unfortunately, the drilling quality of BH2 was not satisfactory and therefore
most of the materials were washed and destroyed. For this reason, BH7 borehole
was drilled and Laufran permeability test was performed repeatedly to a depth
of 70m. The permeability results in BH7 borehole changed from 10-5 to 10-3cm/s.
There was no special arrangement for this dispersion. Hence, the upper limit,
i.e. 10-3cm/ s was chosen for the horizontal permeability of this area.
• On the right-hand side,
boreholes BH6, BH10 were drilled and tested. The permeability test of BH10 was
approximately 10-3 cm/s. This situation was approved for the whole length of
BH6. Figure 6 shows the permeability of different sections at the bed rock.
Two-Dimensional seepage analysis
To estimate the seepage water
flow, based on the model of alluvium permeabilities with it, different values
Ky/Kx (0.1,0.5,1), the amount of flow rate was calculated with Seep/W software
[17]. Figure 7 shows the two-dimensional model of section A: the flow rate and
flowlines for different Ky/Kx are also illustrated in Figures 8-10. The results
of seepage in section A (alluvium foundation) are presented in Table 2. Tables
3 and 4 illustrate the results of seepage analysis for foundation in sections B
and C. According to Tables 2,3 and 4 the total amount of flow rate in alluvium
foundation can be achieved. In Table 5, the results of alluvium foundation of
two-dimensional seepage are shown. Based on a 25-million cubic water reservoir
volume, the water loss was shown to be 5.4% in this analysis.
Three-Dimensional seepage analysis
In this part, the report of
seepage analysis for different Ky/ Kx is presented. For this purpose, the seep
3D software [17] was used. Figure 11 shows a three-dimensional model and
Figures 12 to 14 show the counters for the seepage flow rate. The results of
three-dimensional seepage analysis are shown in Table 6. As far as the
25-million cubic meter water reservoir volume is concerned, the water loss here
is shown to be 5.4% in this analysis.
omparison of 2 and
3-Dimensional analysis
Analysis shows that most
seepage occurs in section A and the least in section C. The results of 2D and
3D analyses are different. The reason for this is that since the flows are
vertical to the transverse sections, they are calculated in 3D analysis. Therefore,
the flow rates are calculated more accurately. The differences in the results
of 2D, 3D analyses are presented in Table 7.
Conclusion
• The Soork Dam site is an
asymmetrical U shape with relatively wide spans. The dips of the walls on the
left and right abutments are about 25 and 6 respectively.
• The discontinuities of the
left abutment dam site are divided into two zones: P1, P2. P1 include 4 joint
sets and P2 two joint sets.
• The quality of rock in wide
areas on the left abutment is relatively low. This affects the joint sets and
cracks.
• The materials around the
river bed on the left end of the valley are generally fine sand, silt, and
clay.
• The direction of the
underground water is from the abutment sides with a mild dip slope towards the
middle of the valley.
• The water loss in 2D and 3D
is 5.4% and 6.5% respectively.
• The 2D and 3D analyses of
seepage are different. Because the flows are vertical to transverse sections in
3D, the flow rate is calculated more accurately.
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