ارزيابي آزمايشگاهي تاثير برداشت شن و ماسه از بستر رودخانه ها بر الگوي آب شستگي گروه پايه هاي پل

The experimental study of the river sand and gravel mining on the scouring pattern around pier group

برداشت شن و ماسه، بخصوص از مكان هايي با پتانسيل كمتر، پارامترهاي رسوب و جريان اطراف پايه هاي پل را تحت تاثير قرار داده و با ايجاد تلاطم و افزايش بار رسوبي در جريان منجر به تاثيرات منفي بر ميزان و وسعت آبشستگي حول پايه هاي پل مي گردد. در تحقيق حاضر، تاثير برداشت مصالح رودخانه اي و پارامترهاي هيدروليكي و رسوبي، بر آبشستگي گروه پايه پل، بررسي گرديد. بدين منظور 22 آزمايش در دو دانه بندي مختلف A (قطر متوسط ذرات mm0/78) و B (قطر متوسط ذرات mm0/78)، براي دو حالت بستر با گودال و بدون گودال مورد بررسي قرار گرفت. دو گروه پايه با سه پايه متوالي در راستاي جريان در بالادست و پايين دست بستر متحركي به طول 4/25 متر، درون كانالي به طول 13 متر و عرض 1/2 متر قرار گرفته است. تاثير برداشت مصالح بر ميزان آبشستگي در هر دو قسمت بالادست و پايين دست گروه پايه هاي مذكور در شرايط جريان زير بحراني (محدوده عدد فرود 0/1-0/5) مورد بررسي قرار گرفت. نتايج نشان داد كه در اعداد فرود 0/5 براي هر دو مدل A و B، وسعت در پايه اول در بستر بالادست داراي بيشترين مقدار و در اعداد فرود 0/25 نيز براي هر دو مدل A و B پايه چهارم كه در بستر پايين دست واقع است، بيشترين وسعت آبشستگي را به خود اختصاص مي دهد. بررسي پايين دست گروه پايه ها براي برداشت شن و ماسه، نشان داد كه گروه پايه ها به برداشت مصالح از پايين دست خود حساس تر هستند. چراكه با برداشت مصالح از بالادست گروه پايه ، عمق آبشستگي كاهش و برداشت از پايين دست اين گروه پايه، عمق آبشستگي را افزايش مي دهد.

Sand mining, especially from places with lower potential, impacts on the hydraulically and sedimentary properties around the bridge piers. The creation of the turbulence causes to the negative effects on the scouring depth and width around the piers. In the present study, the consequences of the mining material, the hydraulical, and sedimentary parameters on the scouring patterns of the piers group were investigated. In order to investigate the scouring of a bridge pier group, 22 experiments were carried out in a rectangular canal with a dimensions of 13 m length, 1.2 m width and 0.8 m depth. The experimental facility is housed at the Hydraulic Lab of Maragheh University. Two false glass floors were installed upstream and downstream at a distance of 4.25 m relative to each other and with a height of 22 cm in the middle of the canal. Sandy movable bed with a height of 22 cm was placed between the aprons. Two pier groups were located upstream and downstream of the bed a specified distance from the aprons. The pier groups with the same arrangement (three consecutive piers in the direction of flow and at the center of the canal's width) were located with center-to-center distances of 21 cm. To eliminate the effect of the canal wall on local scour, the ratio of the pier center-canal wall distance to the pier diameter was greater than 6.25 (Raudkivi and Ettema (1983)). Consequently, piers of diameter of 9 cm were used. To prevent the formation of a ripple, the average diameter of the bed particles should be greater than 0.7 mm and the ratio of the pier diameter to the average particle should exceed 20-25 (Raudkivi and Ettema (1983)). Therefore, the experiments were tested in two different beds, grading ꞌAꞌ with =0.78 mm and grading ꞌBꞌ with =1.7 mm. In order to investigate the effect of the mining pit hole on scouring rate, the mining was done between the upstream and downstream pier groups. The results showed that granulation with coarse particles (B) had lower scour depth than substrate with fine grain size (A). So that the maximum scour depth for B aggregate for Froud number equal to 0.5 and 0.25, was respectively, 14.14 and 47.58 percent less than A. By mining of bed from the downstream and upstream of the group piers, the scouring depth has been increased and reduces respectively. In the Froude number of 0.5 with the mining of materials from the upstream of the groups in the grain size A, the scour depth was 12 to 10.9 cm. However, taking off the bottom of the bases in the same Froude and grading number increased the maximum scour depth from 15 to 15.6 cm. Also, the maximum scour depth in a discharge of 15 liters per second was less for B, compared with A, for gravel A, in Froud number of 0.25 and 0.5 respectively, 44.87% at the fourth base and 61.86% at the first pier, less observation. Although the ratio values in the Froude number of 0.25 for grain size A in the downstream bed is approximately 4 times the grain size of B. Also, the lowest ratio in the Froude number of 0.25 is for the B model with the non-pit hole mining material, while the grain size A with the downstream bed is one of the most scouring models in this Froude number. The phenomenon of scouring in addition to depth dimension, can be studied along the transverse and longitudinal direction. Scouring area is another dimension of the scouring phenomenon that can be affected by parameters. Therefore, the extent of scour area changes in two parts of the scour pattern and bed topography were investigated. In this research, nonlinear regression method is used to predict scour depth around bridges. The relationship between experimental data and empirical of other researchers has been verified using experimental data of this study. In the present study, the scour area around the pier group, along the flow direction for two bed and pit hole was investigated. The effect of this impression on the pattern and depth of scouring showed that the mining of materials from the upstream of the pier groups led to a decrease and this withdrawal from the downstream, increased the depth and extent of scouring. Therefore, it was observed that the pier groups were more sensitive to the mining of materials from their downstream, and it would be better to take this impression upstream of the pier group. Increasing the number of Froude from 0.25 to 0.5 has increased the depth of scouring around the pier group. This increase is higher in larger discharges. Increasing the Froude number in mining of material mode will affect the downstream bed more than the no-pit. So, increasing the Froud number in the bed with the pit hole compared with the non-pit hole bed increases the scour maximum for A, 33% and for B, 73.5%.