نوع مقاله : مقاله پژوهشی

نویسندگان

1 دانشیار گروه مهندسی آب، دانشکده کشاورزی، وابسته پژوهشی، گروه مهندسی آب و محیط زیست، پژوهشکده حوضه آبی دریای خزر، دانشگاه گیلان، رشت، ایران

2 گروه مهندسی آب، دانشکده علوم کشاورزی، دانشگاه گیلان

3 استادیار گروه مهندسی آب، دانشگاه گیلان

چکیده

یکی از روش‌های کاهش آب‌شستگی اطراف پایه‌های پل، استفاده از آستانه در مجاورت پایه و پی سازه است. در تحقیق حاضر تأثیر کارگذاری آستانۀ بالادست بر تغییرات توسعۀ زمانی و عمق حداکثر آب‌شستگی در اطراف گروه‌پایه کج، برای تراز‌های مختلف کارگذاری سرشمع، ضخامت‌های سرشمع، آرایش شمع‌ها و قطر آنها بررسی شد. پایۀ پل مورد بررسی از دو پایۀ مستطیلی شکل با ابعاد 5/2 در 5/3 سانتی‌متر تشکیل شد که با زاویۀ 28 درجه  روی سرشمعی با ضخامت‌های 3 و 5 سانتی‌متر و ابعاد 10 در 16 سانتی‌متر نصب گردید. سرشمع مذکور روی شمع‌هایی با آرایش 2×2 و 3×2 و قطر شمع 2 و 3 سانتی‌متر قرار داده شد. آزمایش‌ها در عمق نسبی جریان(y/D) برابر 42/6، رقوم‌ نسبی کارگذاری سرشمع (Z/D) برابر صفر، 1، 2، 3، و 4، قطرهای نسبی شمع (dp/D)برابر 57/0 و 85/0 ، و در حالت بدون آستانه و در حضور آستانۀ بالادست در شرایط آب زلال اجراشد. مقایسۀ نتایج حاکی از آن است که استقرار آستانۀ بالادست در ترازکارگذاری سرشمعبرابر صفر، 1، 2، 3، و 4،عمق حداکثر آب‌شستگی را به طور میانگین به ترتیب   5/24 ، 29 ، 5/32 ، 35 ، و 8/36 درصد کاهش می­دهد. همچنین، با احداث آستانۀ بالادست، طول چالۀ آب‌شستگی به‌طور متوسط 26 درصد کاهش می­یابد ولی متوسط عرض چالۀ آب‌شستگی بین 2 تا 3 برابر بیشتر خواهدشد.

کلیدواژه‌ها

عنوان مقاله [English]

Experimental Investigation of Effect of Upstream Sill on Reduction of the Local Scour Hole around Complex Piers

نویسندگان [English]

  • Mahdi Esmaeili varaki 1
  • Negar tavazo 2
  • Amir malekpour 3

1 Associate Professor, Department of Water Engineering, Department of Water and Environmental Engineering, Caspian Sea Basin research Center, University of Guilan, Rasht, Iran

2 2- M.Sc. student, Dept. of Water Engineering, University of Guilan, Rasht

3 Department of water Engineering, University of Guilan, Rasht, Iran

چکیده [English]

Bridges are one of the main components of the transportation. One of the main causes of bridges failure, especially during flood times, is the development of scour depth near and below the pier and its foundation. Therefore, protecting the pier from scouring is an important issue in the safety of bridges at design stage .
In general, scouring can be divided into three types: general scour, scouring due to the contraction of the river section and local scouring such as around bridges and abutments .
The flow pattern around the bridge pier include downflow, horseshoe vortices, and wake vortices. By increasing the local shear stress in the vicinity of the bridge piers, these vortices cause erosion in front of the piers and gradually extend to the sides of those. The separation of the flow at the sides of the pier creates so-called wake vortices, which is unstable and shed alternatively from each side of the pier. These vortices act as little tornadoes lifting the sediment particles from the bed. As a result of these processes, a scour hole is formed around the pier and gradually developed.
Regarding to the importance of scour around the bridge piers, many studies have been conducted on understanding of governing process and estimation of the scour depth around bridge pier. Although many literature have been documented for single pier but due to geotechnical and economical reasons, multiple-pile bridge piers and complex piers have become popular in bridge design and have attracted interest of researchers.
Due to the importance of reduction of scour around the bridges, different countermeasures have been introduced and evaluated in the literature. In general, the countermeasures against pier scour are broadly classified into two categories: (1) flow-altering countermeasures, and (2) bed-armoring countermeasures.One of the devices to reduce scour depth around the bridges piers is an installation of bed sill in vicinity of the foundation.
In this research, the effect of the upstream sill on time development and maximum scour depth around inclined bridge pier group was investigated for different pile cap thicknesses, top level installation of pile caps, array and diameter of piles. The bridge pier consist of two rectangular piers with 2.5 and 3.5 cm dimensions which mounted at an angle of 28 degrees on a pile cap with 10 cm width, 16 cm length, 3 and 5cm thickness which placed on an array of 2×2 and 2×3 piles with different diameters. The experiments were performed for relative flow depth (y/D) 6.42, relative pile cap levels (Z/D) 0, 1, 2, 3, 4, relative pile diameter levels (dp/D) 0.57 and 0.85 in clear water conditions. A total of 72 runs was performed for hydraulic and geometric conditions.
Comparison of the results indicated that at the level of Z/D =0, by installation of sill at upstream of pile cap, the maximum scour depth decreases 24.5 percent. At the top installation of pile cap of Z/D =1, the maximum scour depth decreases 29 percent of sill installation location at upstream of pile cap. At the level of Z/D =2, by installation of sill at upstream of pile cap, the maximum scour depth decreases 32.5 percent. At the top installation of pile cap of Z/D =3, the maximum scour depth decreases 35 percent of sill installation location at upstream of pile cap. At the level of Z/D=4, by installation of sill at upstream of pile cap, the maximum scour depth decreases 36.8 percent. Comparison of results indicated that installation of upstream sill length of scour hole decreased 26 percent approximately. However, width of scour hole increased 2-3 times of corresponding scour width of non-sill conditions.

کلیدواژه‌ها [English]

  • Complex piers
  • pile array
  • Scour
  • Sill
  • Top-level installation of pile cap
Ataie-Ashtiani, B. & Beheshti, A. A. (2006). Experimental investigation of clear-water local scour at pile groups. ASCE, J.Hydraul. Eng, 132(10), 1100-1104.
Ataie-Ashtiani, B., Baratian-Ghorghi, Z. & Beheshti, A. A. (2010). Experimental investigation of clear-water local scour of compound piers. ASCE, J. Hydraul. Eng, 136(6), 343-351.
Amini, A., Melville, B., Ali, T. M. & Ghazali, A. H. (2012). Clear-water local scour around pile groups in shallow-water flow.ASCE, J. Hydraul. Eng, 138(2), 177-185.
Breusers, N.H.C. & Raudkivi, A.J. (1991). Hydraulic structure design manual: scouring. IAHR design manual, Vol. 2, pp: 152, Balkema, Rotterdam, Netherlands.
Esmaeili Varaki, M. & Saadati Pacheh Kenari, S. (2016). Experimental investigation of effect of installation of group piers on foundation on scour depth around bridge piers. Journal of water and soil science, 24(3), 27-39.
Esmaeili Varaki, M., Mousapour, S. & HatamJafari, M. (2014). Experimental investigation of the effect of geometric and hydraulic conditions on  Scour Around Inclined Bridge Piers Group. Iranian Water Research Journal, 13, 141-151.
Esmaeili Varaki, M., Radice, A., Hossini, S.S. & Fazl Ola, R. (2019). Local scour at a complex pier with inclined columns footed on capped piles: effect of the pile arrangement and of the cap thickness and elevation. ISH Journal of Hydraulic Engineering, https://doi.org/10.1080/09715010.2019.1702109.
Ferraro, D., Tafarojnoruz, A., Gaudio, R. & Cardoso, A. (2013). Effects of pile cap thickness on the maximum scour depth at a complex pier. ASCE, J. Hydraul. Eng., 139(5), 482-491.
Grimaldi, C., Gaudio, R., Calomino, F. & Cardoso, A. (2009). Control of scour at bridge piers by a downstream bed sill. ASCE, J. Hydraul. Eng., 135(1), 13–21.
Lagasse, P. F., Clopper, P. E. & Zevenbergen, L. W. (2009). Impacts of debris on bridge pier scour. 33rd IAHR Congress. Vancouver, 3967–3974.
Melville, B.W. & Chiew, Y.M. (1999). Time scale for local scour at bridge piers, ASCE, J. Hydraul. Eng, 125 (1), 59-65.
Melville, B. W. & Sutherland, A. J. (1988). Design method for local Scour at bridge piers. ASCE, J. Hydraul. Eng, 114(10), 1210-1226.
Moreno, M., Maia, R. & Couto, L. (2015). Effects of relative column width and pile-cap elevation on local scour depth around complex piers. ASCE, J. Hydraul. Eng., 10.1061/(ASCE)HY.1943-7900.0001080, 04015051.
Moreno, M., Maia, R., Couto, L. & Cardoso, A. H. (2016). Subtraction approach to experimentally assess the contribution of the complex pier components to the local scour depth. ASCE, J. Hydraul. Eng, 10.1061/(ASCE) HY.1943-7900.0001270, 06016030.
Pagliara, S., Carnacina, I. & Cigni, F. (2010). Sills and gabions as countermeasures at bridge pier in presence of debris accumulations. Journal of Hydraulic Research, 48(6), 764-774.
Raudkivi A.J. & Ettema, R. (1983). Clear-water scour at cylindrical piers. ASCE, J. Hydraul. Eng, 109(3), 339-350.
Richardson, E.V. & Davis, S. R. (2001). Evaluating scour at bridges. Hydraulic Engineering Circular No. 18 (HEC-18). Rep. No. FHWA NHI 01-001, Federal Highway Administration, Washington, D.C.
Saadati Pachekenari1, S.S., Esmaeili Varaki, M. & Fazl Ola, R. (2014). Experimental investigation of effect of sill location on local scour around inclined bridge piers group. Journal of water and soil science, 28(2), 406-419.
Salim, M. & Jones, J. S. (1996). Scour around exposed pile foundations. North American Water and Environment Congress 96. Anaheim. Proceedings of the American Society of Civil Engineers. CAUS: 335-346.
Sumer, B. M., Bundgaard, K. & Fredsoe, J. (2005). Global and local scour at pile group. International Journal of Offshore and Polar Engineering, 15(3), 204-209.
Tafarojnoruz, A., Gaudio. R. & Calomino, F. (2012). Evaluation of flow-altering countermeasures against bridge pier Scour. ASCE, J. Hydraul. Eng. 138(3), 297-305.
Wang, H., Tang, H., Liu, Q. & Wang, Y. (2016). Local scouring around twin bridge piers in open-channel flows. ASCE, J. Hydraul. Eng. 10.1061/ (ASCE) HY.1943-7900.0001154, 06016008.
Zarrati, A., Gholami, H. & Mashahir, M. B. (2004). Application of collar to control scouring around rectangular bridge piers, ASCE, J. Hydraul. Eng, 42)1(, 97-103.
Zarrati, A., Nazariha, M. & Mashahir, M. B. (2006). Reduction of local scour in the vicinity of bridge pier groups using collars and riprap. ASCE, J. Hydraul. Eng, 132(2), 154-162.
Zhao, G. & Sheppard, D. M. (1998). The effect of flow skew angle on sediment scour near pile groups. Compilation of Conf. Scour Papers, ASCE, Reston, VA, 377-391.