Taizhou Yangtze River Highway Bridge is the world's first three-tower two-span suspension bridge with a span of more than 1,000 meters, and the main bridge span is 2×1080m. The bridge is built according to the standard of two-way 6-lane highway, with a design speed of 100km/h, a vehicle load grade of highway-I, and a net width of 33m. The taizhou bridge began construction in December 2007 and was completed and opened to traffic on November 25, 2012.
Since the opening and operation of the Taizhou Bridge, the operation and maintenance management unit has continued to carry out the monitoring, analysis and prediction of the erosion depth of the sinking foundation, which is of great significance to the research on the impact of the local scouring of the large-scale deepwater sinking foundation.
Sink the foundation of the well in the deepest water
Taizhou Bridge is located in the Yangtze River section of the Yangtze River Taixing Shun straight section. The Yangzhong River section, up to the Zhenyang River section of Wufeng Mountain, down to the Jiangyin Waterway GooseBitsui. The river is southeast-oriented, with a length of 91.7 km from the main stream and about 68 km from the branches. At the upper end of the river, at Wufeng Mountain, the average width of the river is about 1.3 km, and the narrowest point is about 1.1 km. Taipingzhou is 44.2 km long, the length of the continent is 31.0 km, and the widest point is 11.0 km, which is the largest Jiangxinzhou in the lower reaches of the Yangtze River. The continent divides the water flow into two streams, the left one is the main one, the river is wide and deep, and the diversion ratio has been maintained at about 90% for many years. The right side is a branch, narrow and shallow and curved. The Taipingzhou Zuofen to Tianshenggang waterway consists of two bends, of which the Hissing Horse Bend is the upper bend and the lower bend below the XiaoduiGang, and the plane form of the entire river section is of the curved and fencing type.
Taizhou Bridge has the largest longitudinal herringbone transverse gate frame steel tower in China, the steel tower is located in the Yangtze River, the foundation adopts a rounded rectangular sinking foundation, the standard section size of the sinking well is 58m×44m, the total height is 76m, of which the bottom 38m is a double-walled steel shell concrete structure, and the upper 38m is a reinforced concrete structure. The sinking depth of the entire sinking foundation is -70m, which is the deepest sinking foundation in the world. According to the geological drilling data, the soil layer of the riverbed at the sinking well of the middle tower is mainly fine and silt, and the impact resistance is poor, and the bed surface erosion is prone to occur.
Figure 1 General map of the underlying structure of the sinking well of taizhou bridge
The construction of the sinking well in the tower of Taizhou Bridge lasted 197 days from the implantation on December 1, 2007 to the sinking to the design elevation on September 1, 2008, and experienced the entire dry water period - the flood period. After the sinking well was implanted, scour monitoring was carried out in 2009 and 2010. According to the monitoring results, the actual erosion form during the construction period of the sinking well is basically consistent with the test results of the river model, and the maximum erosion depth is 20.7m, and the elevation of the deepest point of the actual erosion is -34.9m.
Monitoring and analysis of local erosion of sinking foundations
The Taizhou Bridge Area is located in the estuary section of the Yangtze River, at the confluence of runoff and currents, with complex water movement and topographic evolution. In recent years, the establishment of a number of large-scale wading projects in the upper and lower reaches of the Yangtze River and the bridge area has intensified the local adjustment of the river section, especially after the construction of the Three Gorges Dam in the upper reaches, the amount of sediment transport has been sharply reduced, and its long-term effects have yet to be further revealed. Since the opening and operation of the Taizhou Bridge, in order to accurately grasp the topographic changes in the riverbed at the bridge location and the erosion situation around the sinking well foundation in the water, relevant monitoring work has been carried out every year, and the main monitoring schemes are as follows——
Area A: (1) Measurement range - centered on the axis of the bridge, 2000m up and down, and 0m contour range measured on both sides to the shore. The measured area is about 8.4 km2. (2) Measuring scale - 1:10000. (3) Monitoring time - monitor once after the main flood season. (4) Monitoring method - single-beam monitoring.
Area B: (1) Measurement range - 500×500m in the center area of the main tower sinking well. (2) Measuring scale - 1:1000. (3) Monitoring time - monitoring once before and after the main flood season. (5) Monitoring means - multi-beam monitoring.
Figure 2 Schematic diagram of the bridge monitoring range
According to the results of the erosion monitoring of the riverbed topography of taizhou bridge and the foundation of the sinking well in the water over the years, the main analysis is as follows:
1. Riverbed changes in the bridge area. The contour lines of the main bridge area of Taizhou Bridge -5 and -10m are basically unchanged and relatively stable, and the contour lines of 20m and -30m are slightly moved down, and the main pier sinking well has a local scouring pit due to the erosion effect of water flow. From the perspective of the change in the operation period, the -20m and -30m contour lines are generally slightly washed, and the -30m scour pit around the foundation of the sinking well of the middle tower is gradually increased during the construction period, and the change is slightly increased during the operation period, and the change is small.
Bridge area -20m contour change situation
Bridge area -30m contour change situation
Figure 3 Changes of contour lines in the main bridge area of Taizhou Bridge -20m and -30m
2. Bridge axis cross-section change. The statistics of the axis change of the Taizhou Bridge are shown in Figure 4. According to the monitoring results, the riverbed washed out within about 230m on both sides of the sinking shaft foundation of the middle tower was obvious, and the other areas were slightly washed out in general, and the amplitude was small. Since entering the operation period, the riverbed on both sides of the sinking well is still in a slight erosion state, and the erosion range on both sides has reached about 5 times the width of the sinking well, and the erosion around the sinking well has little impact on the stability of the riverbed section of the bridge axis.
Fig. 4 Bridge axis cross-section change
3. Wash the deepest point change. According to the multi-beam topographic measurement results around the foundation of the tower sinking well during the construction period and operation period of the Taizhou Bridge, the statistics of the maximum local erosion depth around the sinking well are shown in Table 1. Based on the monitoring results during the construction period and the operation period, it can be seen that the maximum erosion depth of the sinking shaft foundation is closely related to the peak flow of Chase Station. In 2008, the initial sinking of the sinking well increased with the hydrodynamic force, and the erosion depth increased accordingly, and the maximum flow rate was 4 to 9.9 m at 120003/s, 15000m3/s, and 30000m3/s, respectively. After entering the main flood period, the maximum thrust depth developed rapidly, and the flow level in the first flood period in 2008 was about 45000m3/s, and the maximum erosion depth reached 18.6m; in 2010, the flow level increased to 65000m3/s, and the maximum depth of impact was 20.7m; the maximum flood peak flow level in 2010-2015 did not exceed 65000m3/s, and the maximum local erosion depth changed between 20.7-22.2m. In 2016, the maximum peak flow level reached 70700m3/s, and the maximum depth of impact increased to 23.7m; from 2016 to 2019, the flow level did not break through, and the maximum erosion depth varied between 21.0-23.7m.
Figure 5 shows the trajectory map of the deepest point washed over the years. It can be seen that the deepest point of erosion around the foundation of the sinking well basically swings left and right around the swimming side of the central line of the bridge. The deepest point of scouring is no more than 25m away from the edge of the sinking shaft foundation.
Figure 5 Erosion of the deepest point of the trajectory line chart of the year
Local scour depth prediction
Since the three gorges reservoir hub began to store water in June 2003, and the xiangjiaba and Xiluodu large hydropower hubs in the upper reaches of the Yangtze River were put into operation in 2012 and 2013, the sediment conditions under the dam of the Three Gorges Reservoir have changed greatly. The monitoring of the characteristics of incoming water and sand under new water and sand conditions is of great significance to the study of the impact of local erosion of the sinking foundation in the water of Taizhou Bridge, and also provides a basis for the erosion prediction of the sinking foundation of Taizhou Bridge under new water and sand conditions.
Datong Hydrological Station is 320km away from Taizhou Bridge, and its flow and sediment characteristics basically represent the characteristics of water and sand in the lower reaches of the Yangtze River. According to the monitoring data, during the construction of the Taizhou Bridge, the total amount of sand transported by Chase Station showed a decreasing trend; after entering the operation period, the total amount of sand from Chase Station was further reduced, with a reduction of 20%. At the same time, the data show that the total amount of runoff over the years has decreased slightly, and the distribution of runoff during the year tends to be uniform.
1. Effect of sand transport rate change on local scouring depth. The effect of the change of sand transport rate on the local scouring depth of the sinking well foundation in the water was studied from two aspects - one is the impact of the change of the sand transport rate on the local scouring of the sinking shaft foundation when the erosion of the authorities has occurred; the other is the local scouring depth of the sinking shaft foundation under different sand transport rates when the erosion of the authorities has not occurred. The test control conditions are shown in Table 2. When the erosion of the local department has occurred, the experimental results of the test on the effect of the change of sand transport rate on the local scouring of the foundation pier in the water are shown in Table 3. When the local erosion of the local part of the authority is not occurring, the local erosion depth of the foundation pier in the water under different sand transfer rates is shown in Table 4.
The experimental results of the relationship between the sand transport rate and the local scouring depth show that the change of the sand transport rate does not affect the local scouring depth when the local scouring of the foundation in the bridge water has already occurred; when the foundation in the bridge water is just washed, the larger the sand transport rate, the smaller the local scouring depth, and the impact of the sand transport rate on the local scouring depth of the foundation in the water is about 4%. Therefore, in view of the fact that the local scouring of the water foundation of the Taizhou Bridge has already occurred, the change of the upstream sand has little impact on the local scouring amplitude of the underwater foundation. Judging from the test results, the local scouring depth of the foundation in the water is mainly affected by the dynamic conditions of the water flow.
2. The effect of the near flow rate in front of the pier on the local erosion depth. From the construction period to the end of 2019, the maximum flow rate of the sinking shaft of the tower was 70500m3/s in July 2016, and the maximum local erosion depth of the sinking foundation was 23.1m. According to the results of the local scouring test during the construction period and the on-site monitoring data during the operation period, in the flow range of 12000-70500m3/s, the local scouring depth of the sinking shaft foundation of the tower is shown in Table 5.
According to the results of the local scouring verification test during the operation period of the foundation in the water of the large bridge, the local normal tank test studies the local scouring depth of the foundation pier in the water with high reliability. To this end, when the foundation of the sinking well of the tower encounters dynamic conditions above 70500m3/s flow rate, the tank expansion test is carried out, and the dynamic conditions and results of the test water flow are shown in Table 6.
Figure 6 Erosion pattern diagram of the expanded test of sinking well foundation
According to the results of the local scouring test during the construction period of the sunken shaft foundation of the Taizhou Bridge, the results of the local scouring verification test during the operation period of the sinking foundation, and the experimental results of the expansion test of the sinking foundation, the maximum local scouring depth of the sunken shaft of the Taizhou Bridge can reach 41m when it encounters a flow rate of 91000m3/s. The local erosion depth under different flow rates of the sinking shaft foundation of Taizhou Bridge is shown in Table 7. According to Table 7, the local scouring depth of the sinking foundation in the water is fitted to the near flow rate of the pier forward, and the relationship is obtained as shown in Figure 7.
Figure 7 Relationship between the local erosion depth and the near flow rate of the sinking shaft of the tower under new water and sand conditions
During the flood season in 2020, multiple rounds of heavy rainfall occurred in the Yangtze River Basin, and the upstream continued to flood, resulting in a basin flood in the Yangtze River in July 2020, and the water level of the Chase Hydrological Station has continued to rise rapidly since the beginning of July, reaching a maximum water level of 16.24m and a maximum peak flow of 84700m3/s by July 12. According to the post-flood monitoring results in 2020, the maximum depth of impact around the sinking foundation of Taizhou Bridge increased to 30.8m, which is in good agreement with the above the theoretical calculation of the fitting curve.
The Taipingzhou Fenfen River section where the Taizhou Bridge is located has been stable over the years, and the bridge location area and the upstream and downstream rivers are conducive to the safe operation of the bridge. In recent years, with the establishment of a number of large-scale wading projects above, downstream and bridge areas, the conditions of water flow and sediment have changed. According to the analysis of monitoring results and erosion test studies over the years, the local erosion depth of the sinking shaft foundation in the Taizhou Bridge is mainly affected by the dynamic conditions of the water flow, and the relationship between the local erosion depth of the sinking shaft foundation and the near flow velocity of the pier forward can be better predicted by fitting the local erosion depth of the sinking shaft foundation, while the change of the upstream sand has little impact on the local erosion depth of the sinking shaft foundation.
This article is published / Bridge Maintenance and Operation Magazine, Issue 4, No. 16, 2021
Author / Kim Sai On
Author Affilications / Jiangsu Taizhou Bridge Co., Ltd