Analysis of slope stability in soil dams using Slope/w program (case study of Mahabad's Dam).
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 . 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 . 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  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  suggested geotechnical methods for analysis of huge soil structures. Keskin et al.  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.  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. , 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.
Received 12 October 2013
Received in revised form 18
Accepted 29 December 2013
Available online 25 February 2014
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 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.
 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.
 Duncan, I.M., S.G. Wright, 1996. Slop stability during 16 Rapid, Drawdown Proceeding of R Bolten seed Memorial symposium, Vol. 2.
 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.
 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.
 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.
 Gahraman, B., 2009. The monitoring of Mahabads Dam foundation and Embankment settlement in operation period, Msc. Thesis 255 Pages.
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(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.email@example.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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|Author:||Pourazar, J.; Ghaderi, J.; Yousefzadeh, M.|
|Publication:||Advances in Environmental Biology|
|Date:||Jan 1, 2014|
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