بررسی موقعیت بهینۀ دیواره های آب بند در حضور زهکش در پی ناهمگن سد بتنی

نوع مقاله: مقاله پژوهشی

نویسندگان

1 دانش‌آموخته کارشناسی ارشد سازه‌های آبی، گروه آب، دانشکده کشاورزی و منابع طبیعی، دانشگاه ارومیه، ارومیه، ایران

2 دانشیار، گروه آب، دانشکده کشاورزی و منابع طبیعی، دانشگاه ارومیه، ارومیه، ایران

چکیده

به­ منظور مطالعۀ تأثیر توأم به­ کارگیری زهکش و دیوارۀ آب­ بند در سد­های بتنی یا انحرافی و تعیین موقعیت بهینه آنها، رفتار هیدرولیکی دیواره­ های آب­ بند و زهکش در عمق­ های مختلف روی پی نا­همگن ماسه­ ای به ­صورت عددی و آزمایشگاهی بررسی شد. در مدل آزمایشگاهی ساخته شده، مقادیر نشت و فشار پیزومتری در 18 نقطه از مدل تحت چهار هد آبی مختلف برداشت گردید. بعد از واسنجی و صحت­ سنجی مدل ریاضی با داده­ های آزمایشگاهی، موقعیت بهینۀ  دیواره ­های آب­بند با استفاده از مدل عددی اجزای محدود تعیین گردید. نتایج این تحقیق به هر دو صورت آزمایشگاهی و عددی نشان داد که استفاده از زهکش در حضور دو دیوارۀ آب­بند سبب افزایش مقدار دبی نشت، کاهش چشمگیر مقدار زیر­فشار و کاهش گرادیان خروجی نسبت به حالت مبنا (بدون دیواره آب­بند و زهکش) می ­شود. همچنین، افزایش عمق زهکش سبب افزایش مقدار دبی نشت و کاهش گرادیان خروجی می­ گردد، اما تأثیر چندانی بر تغییر نیروی زیر­فشار ندارد. نتایج تحقیق همچنین نشان  داد که در صورت وجود دو دیوارۀ آب­ بند، موقعیت بهینۀ  دیوارۀ آب­ بند بالا­دست در حضور زهکش در پاشنۀ سد و دیوارۀ پایین ­دست به فاصلۀ 0.27 طول بدنه از پنجه است که در این حالت هر سه پارامتر یعنی دبی نشت، زیر­فشار و گرادیان خروجی کمترین مقدار خود را خواهند داشت.

کلیدواژه‌ها


عنوان مقاله [English]

Assessment of Optimal Location of Cutoff Wall with Drainage System Beneath Concrete Dams in Non-Homogenous Foundation

نویسندگان [English]

  • Negin Akbari 1
  • Hojjat Ahmadi 2
  • Mohammad Hemmati 2
1 Department of Water, Faculty of Agriculture and Natural Resources, Urmia University, Urmia, Iran.
2 Associate Professor, Department of Water, Faculty of Agriculture and Natural Resources, Urmia University, Urmia, Iran. Email: hojjat.a@gmail.com
چکیده [English]

Extended abstract
 
Introduction
Seepage flow through the base of hydraulic structures such as concrete dams causes different issues in these types of structures. It may cause under-scouring of foundations that are constructed in previous material like loose sands or non-cohesive soils. Moreover, seepage flow creates uplift pressure that may destroy the whole structure by overturning or sliding. Engineers have tried to reduce upward pressure created by seepage flow using different methods and techniques. Normally, applying drainage systems such as well-graded granular material, geo-materials, and artificial filters could control the moving of tiny soil particles from spaces between large particles. However, selecting optimal location of the mentioned elements to control seepage forces as well to fix bed material under the foundation of a hydraulic structure is the main challenge for engineers and designers. Nevertheless, by using appropriate filters and drainage system, moving of bed material can be controled; this method couldn’t control the harmful impacts of seepage flow singly. Since the last decades, researchers have proposed different methods to control uplift forces by using cut-off walls due to increasing seepage passway. This techniques drops the hydraulic gradient of seepage flow. Employing the relief well is another practical method to break high pour water pressure and reduce acting uplift pressure to the hydraulic structures.
Conducted studies showed that the closer the drainage systems to the toe of the dam, the lower pressure (Yaghan, 1998). Ahmadi et al, (2010) confirmed that the best location for relief well and the downstream cut-off wall is 0.28L and 0.72L from the toe, respectively in homogenous materials to reduce uplift pressure and seepage flow (L is the length of the dam foundation). The literature review revealed that almost all the research and studies in the field of seepage problem and uplift issues beneath concrete structure have been done under homogenous material, meanwhile, in nature, the base materials are in non-homogeneous conditions especially in the layered form with different permeability and properties. Therefore, in this research, we tried to study the effects of non-homogeneities of foundation materials in the seepage and uplift problem beneath concrete dams.
 
Methodology
To study the effects of drainage and cutoff wall application in concrete or diversion dams and to determine their optimum location simultaneously, the hydraulic behavior of cutoff wall with installed drainage system in different depth at the non-homogenous sandy foundation has been
studied experimentally and numerically. At the prepared experimental setup seepage rate and piezometric pressure under 4 different hydraulic head have been recorded using by 18 mounted piezometers. To study the various condition of non-homogeneities of foundation material as the ratio of permeability of the upper layer to the lower seep/w, a module of Geostudio as a finite element base mathematical model has been used. Calibration and validation of mathematical model have been conducted based on observed experimental data by modifying the permeability of each layer to reach the same seepage flow rate in both model and experimental setup. The optimal location of cutoff walls also has been determined numerically. To study the under scouring phenomenon beneath the dam, critical seepage gradient has been checked under different heads of water at upstream and downstream of the dam.
 
Results and Discussion
The results showed that employing a drainage system within two cutoff wall led to increase in seepage flow and at the same time a significant reduction in uplift force and exit gradient in compare to the ordinary state (without drainage well). As well, increasing the depth of drainage installation makes the growth of seepage flow and dropping in the exit gradient but it does not have any effect on the uplift pressure. Also based on achieved results, in case of implementation of two cutoff walls with drainage, the optimum location for the upper cutoff wall would be exactly at the heel and the down cutoff should be installed at the distance of 0.73 of lengths of foundation from the heel. In current condition, all three parameters as seepage flow, uplift pressure, and exit gradient take in the minimum magnitude.
 
Conclusion
Assessment of the achieved results about the effects of non-homogenous material of the foundation of the dam in uplift pressure acting to the dam proved that layered strata didn’t have significant effects on the pressure distribution and its magnitude. However, analyzing of flow-networks showed that the equipotential lines lean horizontally in the case of the ratio of permeability of the upper layer to the lower is smaller than one. In fact, in such cases, seepage streams lines tend to pass through the lower layer.
 
Acknowledgment
The authors would like to express special gratitude to the vice-chancellor of research and technology of Urmia University for provhding the opportunity and facilities to conduct this research.
 

کلیدواژه‌ها [English]

  • Cutoff wall
  • Exit gradient
  • Seepage
  • Uplift force
Abolpour, B. (1994). Methods of estimating seepage, uplift pressure and exit gradient in hydraulic structures using a computer model (M. Sc. Thesis) Shiraz University, Iran. (in Persian)
 
Ahmadi, H., Salimi, H., & Hemmati, M. (2018). Analysis of seepage through earth dams by using finite difference numerical method and extended pressure technique. Irrigation and Drainage Structures Engineering Research, 19(72), 1-14. (in Persian)
 
Ahmadi, H., Ghahremannejad, M., & Rezaverdinejad, V. (2011). Assessment of drainage systems and seepage control walls on uplift force in diversion dams. 3rd Irrigation and Drainage Network Management National Conference. Feb. 20. Faculty of Water Engineering. University of Chamran. Ahvaz, Iran. (in Persian)
 
Amiri-Tokaldani, A. (1990). Investigation of pressure on action under large dams (M. Sc. Thesis) Tehran University, Iran. (in Persian)
 
Anon. (2007). Seepage Modeling with SEEP/W. An Engineering Methodology. GeoSlope International Ltd., Calgary, Alberta, Canada.
 
Azar, E., Sedghi-Asl., M., & Parvizi, M. (2014). Numerical modeling of seepage flow behavior from permeable alluvial foundations. Journal of Applied Research in Irrigation and Drainage Structures Engineering. 16(65), 85-100 (in Persian)
 
Bligh, W. G. (1910). Dams, barrages and weirs on porous foundations. Engineering News, 64(26), 708-710.
 
Chahar B. R. (2004). Determination of length of a horizontal drain in homogeneous earth dams. Journal of Irrigation and Drainage Engineering, ASCE, 130(6), 530-536.
 
Daghestani, T. (2018). A study on optimum layout of drainage gallery for concrete dams (M. Sc. Thesis) Ankara University, Turkey.
 
Heidarzadeh, M., Mirghasemi, A. A., & Niroomand, H. (2015). Construction of relief wells under artesian flow conditions at dam toes: engineering experiences from Karkheh earth dam, Iran. International Journal of Civil Engineering, 13(1), 73-80.
 
Obead, I. H., Al-Baghdadi, H. M., & Hamad, R.(2014). Reducing the impact of uplift pressures on the base of a concrete dam by configuration of drainage holes (hypothetical case study). Civil and Environmental Research, 6(1), 120-131.
 
Khosla, A. N., Bose, N. K., & McKenzie, E. T. (1936). Design of Weirs on Permeable Foundations.  Central Board of Irrigation India Pub.
 
Liang, Y., Zeng, C., Wang, J. J., Liu, M. W., Yeh, T. C. J., & Zha, Y. Y. (2017a). Constant gradient erosion apparatus for appraisal of piping behavior in upward seepage flow. Geotechnical Testing Journal, 40(4), 630-642.
 
Liang, Y., Yeh, T. C. J., Wang, J., Liu, M., Zha, Y., & Hao, Y. (2017b). An auto-adaptive moving mesh method for the numerical simulation of piping erosion. Computers and Geotechnics, 82, 237-248.
 
Lane, E. W. (1935). Security from under seepage masonary dams on earth foundations. Transaction of the ASCE, 100, 1235-1272.
 
Melondy, B., Abrishami, J., & Akhtari, A. (2007). Assessment of deep drainage behavior in control of uplift under concrete dams by solving of 3D seepage equations. The 6th Conference of Iranian Hydraulics. Nov. 4. Shahrkord University, Iran. (In Persian)
 
Qiao, X., Shu, L., Honglong, Z., Duopeng, W., Youzhi, J., & Xiu, X. (2018). The design of research experiment about cohesive soil piping failure under the action of unsteady head. IOP Conference Series: Materials Science and Engineering. 382(5), 052046.
 
Poureskandar, S., Farsadizadeh, D., & Hosseinzadeh-Dalir, A. (2012). Experimental and numerical effect of grout curtain on seepage through layered base of dams. The 11th conference of Iranian Hydraulics. Nov. 6. Urmia University, Iran. (in Persian)
 
Sayadzadeh, F., & Zomorodian, M. A. (2007). Seepage analysis in layered soil using by finite element method. The 6th Conference of Iranian Hydraulics. Nov. 4. Shahrkord University, Iran. (in Persian)
 
Sedghi-Asl, M., Rahimi, H., & Khaleghi., H. (2005). Effect of vertical cutoff wall optimal position on seepage reduction and flow rate under water structures using numerical model. The 5th Conference of Iranian Hydraulics. Nov. 12. Kerman University, Iran. (in Persian)
 
Uday, A. M., & Hassan, H. M. (2016). Optimal location of drainage gallery under gravity dam by using finite element method. International Journal of Multidisciplinary Research and Modern Education (IJMRME). 2(1), 611-622.
 
Yaghini, A. (1998). The optimum location of drainages under concrete dams (M. Sc. Thesis) Ferdowsi University of Mashhad, Iran. (in Persian)
 
Yifeng, C., Chuangbing, Z., & Hong, Z. (2008). A numerical solution to seepage problems with complex drainage systems. Computers and Geotechnics, 35(3), 383-393.
 
Yousefi, M., Parvizi, M., & Sedghi-Asl, M. (2016). Laboratory investigation the effects of sheet pile on seepage control and sand ‎boiling through alluvial foundation of hydraulic structures. AmirKabir Journal of Science and Research Civil and Environmental Engineering. 48(3), 121-123.
 
Wang, L., Liu, Z., & Zhang, Y. T. (1992). Analysis of seepage field near a drainage-holes curtain. Journal of Hydraulic Engineering, 12(4), 15-20.