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Analysis of slope stability in soil dams using Slope/w program (case study of Mahabad's Dam).

INTRODUCTION

Continuous monitoring and determination of the slope stability rate in soil dams are the most important parameters in closely identifying the time of problems occurrence caused by seepage especially after water intake [1]. Regarding Iran is an earthquake prone region, the study of natural conditions and geology of dams in different areas has a high priority. The variety of static characteristics of the dam's body and various materials and thickness of the skeleton are effective factors in the slope stability or weakness, and if the dam is within the bounds of active fault line the importance of the study will be double [2]. With regard to the high costs of infrastructural installations in mainly mountainous regions it is necessary to provide safety before the hydraulic waves and pressure of the reservoir water do damage. Nowadays there are a lot of methods to calculate the stability rate of the dams the most important of which are computer programs. Using these methods is low-cost, economical, exact and dependable.

Hassani [3] studied the slope stability and subsidence of Ilam's dam by means of Geo Studio program and stated that from the point of view of stresses and pressures caused by seepage the soil dams with clay texture have a suitable behavior and are statically stable and they are more economical than other types of dams. Duncan [4] suggested geotechnical methods for analysis of huge soil structures. Keskin et al. [5] studied the effects of the thickness of the impermeable core and earthquake coefficient on the stability of both uppermost and down most slopes of Kizilaka's dam. Taiiari et al. [6] dynamically analyzed the slope of soil dams using the Plaxis program and stated that drifts caused by earthquakes in Kerman showed that with decrease in core slope and dam's crust the vertical drifts of the dam decreased, but affected the horizontal drifts trivially. Also, with increase in Mirabi Riley coefficient, rather big drifts would occur in the body and foundation of the dam. Rahmani et al. [7], using the SEEP/w program, studied the effects of soil texture on the seepage in soil dams with clay core in two vertical and horizontal states and announced that when the mesh making is more minute the program can analyze more points in an area and give more accurate results.

Reviewing the damage causes of the soil dams it can be concluded that illegal drainage (seepage) has a great share in their damage (Wafaeeyan, 2010).

Due to the importance of Mahabad's Dam in supplying agricultural and drinking water of the region it is necessary to analyze the stability of vertical and horizontal slopes to prevent any occurrence leading to drainage in the clay core. The program Slope/w has a special ability at doing the mentioned analysis which is the major objective of the present case study in order to calculate the confidence coefficient to protect the dam. Due to the importance of Mahabad's Dam in supplying agricultural and drinking water of the region it is necessary to analyze the stability of vertical and horizontal slopes to prevent any occurrence leading to drainage in the clay core. The program Slope/w has a special ability at doing the mentioned analysis which is the major objective of the present case study in order to calculate the confidence coefficient to protect the dam.

Reportedly, after water withdrawal in 1970 part of the dam's body subsided by 1m and repeatedly got transformed and made uncommon drifts which in worrying so that it has been repaired partly. The area of the dam is 11 [km.sup.2]. The analyzed section in fig. 1 is the section in which there is the dam's height and hydraulic level normal values in maximum condition.

Description of the soil dam's section and its characteristics and geographical location:

The dam is a soil one with a central pebble impermeable core. The core a little tends towards the upper.

This core is protected by two filter layers on both upper and lower sides. The outer part of the upper crust is made with pebbles protected by a stone layer. The inner part of the upper crust is made with river sand and clay. The dam's lower crust is made with two types of clay and mixed materials placed beside the lower filters. The dam's volume is 230 [mm.sup.2] and its height from the foundation is 47.5m. The crown is 700m in length and 8.32m in width. The width of the body in the foundation is 221.5m and the crown's level (msal) is at an altitude of 1361.5m. The vents are 6m in height. It's located in 120km to the south of Orumiyeh and in north of mahabad on Dehbokr, Koutar and Beytas Rivers. The aim of establishing the dam was supplying irrigation water for 20000 hectares of farming land and controlling the upper highlands flooding as well as supplying drinking water.

MATERIALS AND METHODS

In the present study the slope stability of the soil dams was analyzed using Slope/w program having the following features:

1. Determining stability confidence coefficient of slippery, circle and wedge slopes.

2. Determining confidence coefficient for uneven surfaces.

3. Definition of porous water pressure in three forms; water pressure of landscape with water pseumetric level, definite water pressure in points, water pressure of extra pores as a fraction of total water pressure (ru.[gamma].h.u=[DELTA]). ([gamma]= total soil density, h= depth of intended point, ru= extra pore pressure coefficient).

4. Calculating stability of dam's slope in two states of static loading and static-like or dynamic loading (earthquake occurrence).

5. Analysis of 2-dimensional slope stability by method of parts.

In using this program it's hypothesized that gable slip occurs when the power from weight dominates the symmetrical power of from soil cutting resistance. Thus, the cut stresses along the most resistant slip surface should be calculated and compared with soil cut (section) resistance. Confidence coefficient is defined as mobilized medium cut stress ratio in slip surface on the average created cut stresses along slip surface.

RESULTS AND DISCUSION

A. Stability analysis:

In order to compare the results of analysis and slope stability we used the method of limit balance with circular break of Morgan Strin Price as described in Table.

According to the achieved confidence coefficients under dam loading conditions and passage axis of critical upper and lower slip surfaces in constant seepage conditions passes through friction materials. The confidence coefficient was beyond the limit value and supplies stability at a high level. Gentle slope of the crust and short width of the core have lead to high confidence coefficient of critical upper and lower slip surfaces.

B. Analysis of water movement in soil using the Seep program:

Regarding the soil is semi-saturated and this condition influences the soil permeability, seepage analysis and water movement in semi-saturated soils lead to untrue results. Thus, a formula was developed in which permeability changed linearly and it was described as fixed out of this formula. This formula was so that in pressure of 0[K.sub.sat] and in -100 bar the permeability was 100/[k.sub.sat]. The water discharge (debi) was 6.1 x [10.sup.-4] [m.sup.3]/s. Potential lines, flow lines and water frantic line was as the Fig. 1.

C. Based on the effects of design parameters for stability it could be concluded that the angle of inner friction and adhesion coefficient have a direct relationship with the confidence coefficient while the specific weight (weight density) of the materials has an inverse relationship with stability. Also, the inner friction angle of the materials had the greatest effect on the slope stability.

ARTICLE INFO

Article history:

Received 12 October 2013

Received in revised form 18

December 2013

Accepted 29 December 2013

Available online 25 February 2014

REFERENCES

[1] Ildoromi, A., 2012. Analysis and Modeling of Landslide Surface with Geometrical Hydrology and Slope Stability: Case Study Ekbatan Watershed Basin, Hamedan, Iran, Adv. Environ. Biol. 6(3): 920-924.

[2] Arekhi, A., A. Darvishi Bolourani, A. Shabani, H. Fathizad, S. Ahamdyasbchin, 2012. Mapping Soil Erosion and Sediment Yield Susceptibility using RUSLE, Remote Sensing and GIS (Case study: Cham Gardalan Watershed, Iran), Adv. Environ. Biol. 6(1): 109-124.

[3] Hasani, H., J. Mamizadeh, 2011. Analysis of slope stability in soil dams using Geo-Slope program. The Proceedings of 4th. Conference of Source water management in Iran, Tehran Amirkabir Industrials University, 11 pages.

[4] Duncan, I.M., S.G. Wright, 1996. Slop stability during 16 Rapid, Drawdown Proceeding of R Bolten seed Memorial symposium, Vol. 2.

[5] Keskin, S.N., U.S. Cavus, H. Yildinm, 2004. Slope stability of earth fills dams: A case study of Kizilca dam, Proceedings of the ICE--Geotechnical Engineering, 157(1): 3-7.

[6] Taiiari, O.M., S. Mohammadabadi, Y. Mosavizadeh, R. Mahmodi Lirzdrdi, 2011. Dynamic -Analysis of slope stability in soil dams using Plaxis program, The Proceedings of 5th National Conference of Source water and soil management, Kerman, Iran, 12 pages.

[7] Rahmani, S., H. Hasani, S. Bamdadnia, 2011. The evaluation of mesh making in Seep/w software and its effect on leakage calculate on soil dams with clay core in two vertical and horizontal states, The Proceedings of 1th. National Conference of development and civil, http/www.civilca.com.

[8] Gahraman, B., 2009. The monitoring of Mahabads Dam foundation and Embankment settlement in operation period, Msc. Thesis 255 Pages.

[9] Wafaeeyan, M., 2010. The soil Dams. Jahaddaneshgahi publication. Second edition, Esfahan.

(1) Pourazar, J., (2) Ghaderi, J., (3) Yousefzadeh, M.

(1) M.Sc in Department of Water-Civil Engineering, Mahabad Branch, Islamic Azad University, Mahabad, Iran.

(2,3) Department of Civil Engineering, Mahabad Branch, Islamic Azad University, Mahabad, Iran.

Corresponding Author: Pourazar J., Master of Water-Civil Engineering Department, Mahabad Branch, Islamic Azad University, Mahabad, Iran. E-mail: J.sazeh@yahoo.com

Table 1: Relative percentage of different causes
of dam's destruction.

Destruction by water overflow         30%
Illegal drainage and soil washing     25%
Separation of the mountainsides       15%
Wash of tunnel sides                  13%
Wash of impermeable upper cover       5%
Other phenomena such as earthquake    7%
Unknown causes                        5%
Total                                 100%

Table 2: Geotechnical parameters which affect stability
analysis.

Area       Area       Type of      Wet      Saturation
number               Building    Specific    Specific
                                  weight      weight

2        The upper    pebbles       20          22
          crust of
          pebbles
5        The upper     CM-GC        19          21
         crust of
           sandy
6          Clay         Non         16          18
                       organic
                        clay
7        The down      SC-CL        19         20.5
           crust
8        The lower     Silt         19          20
         platform      gravel

Area       Area        Dry      Porosity   Infiltration    Optimum
number               Specific                K(m/day)     percentage
                      weight                               humidity
                                                              %W

2        The upper     19.6        --          350            --
          crust of
          pebbles
5        The upper     18.6       0.57         0.75          13.5
         crust of
           sandy
6          Clay       17.11       0.47         0.00         17-23
                                               008

7        The down       17        0.55         0.00           12
           crust                               008
8        The lower      17        0.55         0.05           14
         platform

Area       Area          Strength Properties of Soil
number

                     CD State                UU State

                     Angle of     Sticky     Angle of
                     friction      (KN/      friction
                                [m.sup.2])

2        The upper      45          0           45
          crust of
          pebbles
5        The upper      27          0           35
         crust of
           sandy
6          Clay         25         210          20

7        The down       28          70          26
           crust
8        The lower      25          0           23
         platform

Area       Area        Strength Properties
number                      of Soil

                             UU State

                       Sticky       Sticky
                        (KN/         (KN/
                     [m.sup.2])   [m.sup.2])

2        The upper       0            0
          crust of
          pebbles
5        The upper       8            8
         crust of
           sandy
6          Clay          7            80

7        The down        10           60
           crust
8        The lower       10           15
         platform

Area       Area      Elastic   Coefficient
number               modulus     Poisson

2        The upper    72000        0.2
          crust of
          pebbles
5        The upper    50000       0.15
         crust of
           sandy
6          Clay        --          0.4

7        The down     5200-       0.35
           crust      6400
8        The lower    13000       0.25
         platform

Table 3: comparing the results of analysis and slope
stability.

Analysis stage     Gable    Coefficient    Circular
                           of confidence   break in
                                           balance
                                            method

Constant seepage   Upper        1.5         1.639
  from the tank    Lower        1.5          1.68
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Article Details
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Author:Pourazar, J.; Ghaderi, J.; Yousefzadeh, M.
Publication:Advances in Environmental Biology
Article Type:Report
Geographic Code:7IRAN
Date:Jan 1, 2014
Words:2006
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