Experimental method for simulating anti-scouring structure of deepwater cofferdam

Abstract

The invention relates to the technical field of bridge foundations, in particular to an experimental method for simulating a deepwater cofferdam anti-scouring structure, and the experimental method comprises the following steps: measuring and calculating the starting flow velocity of natural sediment; selecting a model scale, wherein the model scale comprises a model geometric scale and a model flow velocity scale; determining a model sand starting flow velocity; building a basic framework of the experimental model; selecting a model sand material; and performing model fitting checking calculation and simulating a river section scouring experiment. According to the experimental method provided by the invention, the anti-scour effect of the anti-scour structure can be evaluated, and the scale of the anti-scour structure and the economic cost are reduced on the premise of ensuring the sufficient anti-scour effect; a simulation experiment is carried out before formal construction, appropriate anti-scouring structure data are found, and actual construction is guided.

Description

Technical Field

[0001] The invention relates to the technical field of bridge foundations in deep-water high-flow-velocity river sections, and in particular to an experimental method for simulating a deep-water cofferdam anti-scour structure. Background Art

[0002] The cofferdam is a temporary water-blocking structure first invented in my country for the construction of bridge piers. It was first successfully used in the construction of the Jiujiang Yangtze River Bridge in the 1970s. With the development of the construction of large-span bridges, cofferdams have been widely used, and the volume of cofferdams has become larger and larger. The construction area is mostly located near the deep-water main channel. Therefore, the problem of riverbed scour in the construction of bridge piers using cofferdams has become more and more prominent. Especially when encountering floods during the flood season, there will be obvious local scour near the cofferdam, which will have a serious impact on the construction and stability of the bridge piers. The stable landing and safe flood crossing of the cofferdam have become the key to the entire project construction. Therefore, it is very necessary to adopt certain engineering structures to reduce the scour of the riverbed near the cofferdam.

[0003] Most of the existing technologies adopt the method of setting a sacrificial pile group or a sacrificial pile group and a bottom riprap area in front of the cofferdam as described in patent CN108442326A "A protective method for resisting local scour of bridge pier foundation by combining a sacrificial pile group and a bottom riprap area" to reduce the scour of the riverbed surface near the cofferdam. Although the above method can effectively protect the cofferdam from river scour, there is no method to make a clear assessment of the anti-scour effect and economic cost rationality of the adopted structure; the existing technology has no way to solve the anti-scour problem of the cofferdam in deep water river sections only by calculation, and blindly constructing an anti-scour structure without experimental rehearsal will pose a safety hazard. Summary of the invention

[0004] On the one hand, the present application provides an experimental method for simulating the anti-scour structure of a deep-water cofferdam, evaluates the anti-scour effect of the anti-scour structure, and reduces the scale of the anti-scour structure and the economic cost while ensuring sufficient anti-scour effect; before formal construction, a simulation experiment is carried out to find suitable anti-scour structure data to guide actual construction.

[0005] The technical solution of the present application is: an experimental method for simulating a deepwater cofferdam anti-scour structure, the simulation experiment comprising: measuring the natural sediment starting velocity; selecting a model scale, the model scale comprising a model geometry scale and a model velocity scale; determining the model sand starting velocity; building a basic framework of the experimental model; selecting the model sand material; model fitting verification and simulated river section scour experiment.

[0006] According to the present application, an experimental method for simulating a deepwater cofferdam anti-scour structure is provided, and the measured natural sediment starting flow velocity is calculated using the Sha Yuqing formula.

[0007] According to the present application, an experimental method for simulating a deepwater cofferdam anti-scour structure is provided, wherein the model geometry scale is numerically equal to the square of the model velocity scale, and the model geometry scale ranges from 1:50 to 1:150.

[0008] According to the present application, an experimental method for simulating a deepwater cofferdam anti-scour structure is provided, and the starting flow velocity of the model sand is calculated using Dou Guoren's formula.

[0009] According to the present application, an experimental method for simulating a deepwater cofferdam anti-scour structure is provided, wherein the basic framework of the experimental model meets the following conditions at the same time:

[0010] The actual flow velocity of the simulated river section is greater than or equal to the starting flow velocity of the model sand;

[0011] The model water flow type is turbulent flow, and the model Reynolds number is greater than 2000;

[0012] The water depth of the model is greater than 1.5 cm to eliminate the influence of the surface tension of the model;

[0013] The underwater repose angle of the model sand is equal to that of the prototype sand, so that the geometric shapes of the bridge pier scour pits in the simulated river section and the actual river section are similar;

[0014] The compression ratio of the pier B / b is greater than 8, where: b is the width of the pier, B is the width of the water channel, and the minimum width of the pier is not less than 0.03m.

[0015] According to the present application, an experimental method for simulating the anti-scour structure of a deepwater cofferdam is provided. The method for selecting the model sand material is as follows: a plurality of candidate model sand materials are preliminarily screened through the experience and basic common sense of technical personnel in the industry, and after the starting flow velocity of the candidate model sand is obtained, the ratio of the starting flow velocity of each candidate model sand to the starting flow velocity of natural sediment is compared with the model flow velocity scale. If the difference between the two is within 5% of the model flow velocity scale, it meets the simulation experiment requirements.

[0016] According to the present application, an experimental method for simulating a deepwater cofferdam anti-scour structure is provided, and the model fitting verification includes: calculating whether the ratio of the model sand starting flow velocity to the natural sand starting flow velocity at different water depths is consistent with the selected model flow velocity scale, and the difference between the two is considered to be consistent if it is within 5% of the model flow velocity scale.

[0017] According to the present application, an experimental method for simulating the anti-scour structure of a deep-water cofferdam is provided. The simulated river section scour is tested according to the principle of the controlled variable method. Multiple control experiments are carried out on cofferdams without anti-scour structures and cofferdams with different anti-scour structures to verify the anti-scour effects of different anti-scour structures.

[0018] According to the present application, an experimental method for simulating the anti-scour structure of a deep-water cofferdam is provided. The control experiment is to compare an experimental group with the same anti-scour structure of a cofferdam but different characteristic data with a control group of a cofferdam without an anti-scour structure, so as to compare the anti-scour effects of the same anti-scour structure of a cofferdam under different characteristic data.

[0019] On the other hand, the present application also provides a method for testing the effectiveness of an anti-scour structure of an already constructed deep-water cofferdam, which is a method for testing the effectiveness of an anti-scour structure of an already constructed deep-water cofferdam through any of the simulation experiments described above.

[0020] The advantages of this application are:

[0021] 1. The experimental method of this application can evaluate the anti-scour effect of the anti-scour structure, reduce the scale of the anti-scour structure and reduce the economic cost under the premise of ensuring sufficient anti-scour effect; conduct simulation experiments before formal construction to find appropriate anti-scour structure data to guide actual construction;

[0022] 2. Considering factors such as adhesion and porosity caused by film water contact, the Sha Yuqing formula used in this application is used to calculate the starting flow velocity of natural sediment. The Sha Yuqing formula can simply and effectively calculate the starting flow velocity of natural sediment;

[0023] 3. The geometric scale of the model in this application is between 1:50 and 1:150. If the scale of the model is too large, the cost of the simulation experiment will be too high, and if the scale of the model is too small, the accuracy of the simulation experiment will be greatly reduced;

[0024] 4. Considering that the molecular attraction between the surface of the model sand and the binding water has an impact on the binding force, and the model sand is relatively light, it is more reliable to use Dou Guoren's formula to calculate the starting flow rate of the model sand;

[0025] 5. The fitting degree of the basic framework of the experimental model is improved by controlling the flow velocity, Reynolds number and water depth of the simulated river section, the underwater repose angle of the model sand, and the compression ratio parameters of the bridge pier;

[0026] 6. This application calculates and compares the ratio of the starting velocity of various candidate model sand materials to the starting velocity of natural sediment, and controls the difference between the ratio and the model velocity scale within 5% of the model velocity scale, thereby ensuring the reliability and fit of the experimental model;

[0027] 7. This application also uses the control variable method to measure whether the ratio of the starting flow velocity of natural sand and simulated sand under different water depths is consistent with the model flow velocity scale, which can effectively verify the fitting degree of the experimental model;

[0028] 8. This application conducts multiple groups of control tests on different anti-scour structures, verifies the anti-scour effects of different anti-scour structures, and can well screen out efficient anti-scour structures;

[0029] 9. This application further conducted multiple groups of control tests to study the anti-scour effect of the anti-scour structure under different characteristic data, which is conducive to studying the anti-scour effect of the anti-scour structure under different characteristic data;

[0030] 10. This application can also verify the anti-scour effect of the anti-scour structure in actual engineering through simulation experiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 :Workflow diagram of the simulation experiment of this application. DETAILED DESCRIPTION

[0032] The embodiments of the present invention are described in detail below, wherein the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0033] In the description of the present invention, it should be understood that the terms "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside" and "outside" etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0034] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0035] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] The present application relates to an experimental method for simulating a deepwater cofferdam anti-scour structure, which is mainly used to evaluate the anti-scour effect of the deepwater cofferdam anti-scour structure, reduce economic costs and guide actual construction while ensuring sufficient anti-scour effect.

[0037] Specifically, Figure 1As shown, the experimental method of the present application includes: measuring the starting flow velocity of natural sediment; selecting a model scale, wherein the model scale includes a model geometry scale and a model velocity scale; determining the starting flow velocity of the model sand; building a basic framework of the experimental model; selecting the model sand material; model fitting verification and simulated river section scouring experiment.

[0038] In some embodiments, the natural sediment starting velocity calculation method of the present application is optimized, and the factors such as the adhesion force and porosity caused by the film water contact are considered, and the natural sediment starting velocity is calculated using the Sha Yuqing formula:

[0039]

[0040] Where V0 is the starting velocity (m / s), R is the hydraulic radius (m), ε is the porosity, and d is the particle size (mm).

[0041] In some embodiments, the model geometry scale and model flow velocity scale of the present application are optimized, and the model geometry scale is numerically equal to the square of the model flow velocity scale. The model geometry scale ranges from 1:50 to 1:150. If the model scale is too large, the cost of the simulation experiment will be too high, and if the model scale is too small, the accuracy of the simulation experiment will be greatly reduced.

[0042] In some embodiments, the basic framework of the experimental model of the present application is optimized, and the experimental model meets the following conditions:

[0043] Flow velocity: The actual flow velocity of the simulated river section is greater than or equal to the starting flow velocity of the model sand. When the actual flow velocity is less than the starting flow velocity of the model sand, the model sand does not have the scouring ability and the simulation experiment loses its meaning;

[0044] Reynolds number: The model water flow type is turbulent flow, and the model Reynolds number is greater than 2000. The turbulent flow is more consistent with the water flow conditions of the actual river section, which improves the reliability of the experimental model;

[0045] Water depth: The water depth of the model is greater than 1.5 cm to eliminate the influence of the surface tension of the model;

[0046] Underwater repose angle: The underwater repose angle of the model sand is equal to that of the prototype sand, so that the geometric shapes of the bridge pier scour pits in the simulated river section and the actual river section are similar, improving the fitting degree of the experimental model;

[0047] Pier compression ratio: The pier compression ratio B / b is greater than 8 (b is the pier width, B is the water channel width), and the minimum pier width is not less than 0.03m.

[0048] In some embodiments, the calculation method of the starting flow rate of the model sand of the present application is optimized. In this embodiment, since the model sand is relatively light, in order to eliminate the influence of the molecular attraction between the surface of the model sand and the binding water on the bonding force, the starting flow rate of the model sand is calculated using the Dou Guoren formula:

[0049]

[0050] Where u c ——Starting velocity (cm / s), k'——take 0.32, h——water depth (cm), Δ——bed roughness height (cm), d'——particle size parameter (cm), d * ——Take 1cm, ρ s ——Sand density (g / cm 3 ), ρ——water density (g / cm 3 ), g——gravitational acceleration, d——particle size (cm), γ0——bed sediment dry density (t / m 3 ), γ 0* ——Stable dry bulk density of sediment particles (t / m 3 ), ε0——comprehensive bonding strength parameter (cm 3 / s 2 ), δ——film water thickness parameter (cm);

[0051] In some embodiments, the method for selecting model sand materials is optimized. In this embodiment, a variety of candidate model sand materials are preliminarily screened through the basic experience and common sense of technical personnel in the industry. After the starting flow velocity of the candidate model sand is obtained, the ratio of the starting flow velocity of each candidate model sand to the starting flow velocity of natural sediment is compared with the model flow velocity scale. If the difference between the two is within 5% of the model flow velocity scale, it meets the simulation experiment requirements.

[0052] Specifically, take a Yangtze River highway-railway bridge as an example. The Yangtze River highway-railway bridge connects Tongyang County, Tongling City and Guichi District, Chizhou City. It is a double-deck cable-stayed bridge with a main span of 812m. The main tower foundation is equipped with 77 piles with a diameter of 2.8m, and the designed pile length is 117.5m; the abutment is rectangular, with a plane outer contour of 75.6m×47.6m, a thickness of 7.0m, and a top elevation of -13m. The rectangular steel cofferdam used to build the abutment has a plane size of 80.6m×52.6m and a wall thickness of 2.2m. The bridge pier is located at the inlet section of the Datong waterway in the lower reaches of the Yangtze River. The surface water is abundant. The flood season is from May to October every year. The highest water level is 15.40m and the lowest water level is 1.25m. The water level varies greatly within the year. Considering the riverbed scouring during the flood season, the water depth of the bridge pier exceeds 35m, and the average maximum flow rate during the flood season is 2.5m / s. Combined with the average flow velocity of the cross section measured on site at the pier, the maximum flow velocity at the bridge pier during the cofferdam construction phase was 1.7m / s.

[0053] The model test was conducted in a 42m long and 5m wide flume, with a 6m long and 5m wide sediment moving bed section, a 0.6m thick sand layer, and a cofferdam model in the center of the test section. The normal model was selected for the test, with a model geometry scale of 1:100 and a sediment starting velocity scale of 1:10.

[0054] According to Sha Yuqing's formula, when the water depth of the construction section is 15m, the starting velocity of natural sediment is 66cm / s; Dou Guoren's formula is used to calculate the median particle size d of the model sand. 50 =0.39mm, particle density γ S =1.15t / m 3 When wood powder is added, the starting velocity is 6.6 cm / s, and the ratio of the starting velocity of natural sediment is 1:10. This ratio is equal to the model velocity scale, which meets the requirement that the difference between the simulation experiment ratio and the scale is within 5% of the model velocity scale.

[0055] In some embodiments, the model fitting verification is optimized. In this embodiment, the starting flow velocity of natural sand and the starting flow velocity of model sand are measured under various water depths. The model fitting is considered to be successful when the difference between the ratio of the starting flow velocity under various water depths and the model flow velocity scale is within 5% of the model flow velocity scale.

[0056] Specifically, taking a Yangtze River highway-railway bridge as an example, the fitting verification results are shown in Table 1:

[0057] Table 1 Sediment start-up velocity scale

[0058]

[0059] It can be seen from Table 1 that the difference between the ratio of the starting flow velocity at three different water depths and the model velocity scale is less than 5% of the model velocity scale, and the model fitting is successful.

[0060] In some embodiments, the simulated river section scour experiment is optimized, and the experiment is carried out according to the principle of controlled variable method. Control experiments are carried out on cofferdams without anti-scour structures and cofferdams with different anti-scour structures for multiple times to verify the anti-scour effects of different anti-scour structures.

[0061] Specifically, the anti-scour structures adopted by the experimental group are to set a row of sacrificial piles in front of the cofferdam and to set a riprap protection area in front of the cofferdam. According to the experimental results, both can effectively prevent the cofferdam from scour.

[0062] In some embodiments, the above-mentioned control test is optimized. This embodiment compares the experimental group using the anti-scour structure cofferdam with front row sacrificial piles of different lengths with the control group without the anti-scour structure cofferdam, so as to compare the anti-scour effect of the front row sacrificial piles of different lengths.

[0063] Specifically, this embodiment is carried out under the condition of an experimental water flow velocity of 2.5cm / s. The deepest scour pit of the control group without an anti-scour structure cofferdam is located at the corner of the cofferdam, reaching -14.1cm, and the range of the local scour pit of -13cm is 23cm*26cm; the lengths of the front row sacrificial piles used in the experimental group are 78cm and 98cm respectively. When the front row sacrificial piles with a length of 78cm are used, the deepest scour pit is located outside the sacrificial piles, reaching -17.2m, the deepest at the corner of the cofferdam is -13.9cm, and the range of the local scour pit of -13cm is 10cm*10cm; when the front row sacrificial piles with a length of 98cm are used, the deepest scour pit is located outside the sacrificial piles, reaching -17.6m, the deepest at the corner of the cofferdam is -8.8cm, and the range of the local scour pit of -7cm is 20cm*20cm. During actual construction, the length of the front row sacrificial piles can be selected as appropriate according to the actual project budget and local flood control standards.

[0064] Furthermore, this embodiment also compares an experimental group of riprap protection area anti-scour structure cofferdams with different protection areas with a control group of cofferdams without anti-scour structure to compare the anti-scour effects of riprap protection areas with different protection areas.

[0065] Specifically, this embodiment is also carried out under the condition of experimental water flow velocity of 2.5cm / s. The deepest scour pit of the control group without anti-scour structure cofferdam is located at the corner of the cofferdam, reaching -14.1cm, and the range of the -13cm local scour pit is 23cm*26cm; the riprap protection area used by the experimental group is 1900cm 2 and 3000cm 2 , the area is 1900cm 2 When the riprap protection area is formed, the outer terrain within the protection area is eroded, but the amplitude is not large, between 1cm and 2cm; the area is 3000cm 2 When the riprap protection area is built, the protection effect is excellent and the terrain within the protection range remains basically unchanged. During actual construction, the area of ​​the riprap protection area can be selected according to the actual project budget and local flood control standards.

[0066] During the actual construction, the basic situation of the river section where the bridge is to be built is first obtained, including collecting the river flow profile of the river section where the bridge is to be built, investigating the water and sediment conditions of the river section where the bridge is to be built, checking the local flood control standards and planning, and surveying the engineering geological conditions of the river section where the bridge is to be built;

[0067] Then design a simulation experiment: calculate the starting velocity of natural sediment by Sha Yuqing formula, determine the model geometry scale and model velocity scale of the experimental model, determine the required model sand starting velocity (the product of natural sediment starting velocity and model velocity scale), build the basic framework of the experimental model by limiting the velocity, Reynolds number, water depth, underwater angle of repose, and pier compression ratio, select a variety of candidate model sand materials based on the basic common sense of technical personnel, and then calculate the starting velocity of the candidate model sand by Dou Guoren formula, select the model sand material that meets the conditions, and measure whether the ratio of the starting velocity of natural sand to the starting velocity of model sand at different water depths is consistent with the selected model velocity scale to verify the fitting degree of the experimental model;

[0068] Conduct simulated river scour experiments: including comparing the experimental group using anti-scour structure cofferdams with front-row sacrificial piles of different lengths with the control group without anti-scour structure cofferdams, and comparing the experimental group using anti-scour structure cofferdams with riprap protection areas of different protection areas with the control group without anti-scour structure cofferdams;

[0069] Study and analyze simulation results: summarize the experimental results and draw them into charts, including the relationship diagram between the characteristic data of the cofferdam anti-scour structure and the anti-scour effect, and the relationship diagram between the cost of the cofferdam anti-scour structure and the anti-scour effect, so as to obtain the optimal solution data of the cofferdam anti-scour structure suitable for the actual project, and select the anti-scour structure according to the actual project budget;

[0070] Finally, the experimental results are used to guide the construction of actual deepwater cofferdam anti-scour structure.

[0071] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. The present invention may be subject to various changes and improvements without departing from the spirit and scope of the present invention. These changes and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. An experimental method for simulating a deepwater cofferdam anti-scour structure, characterized in that: The experimental method includes: measuring the natural sediment starting velocity; selecting a model scale, wherein the model scale includes a model geometry scale and a model velocity scale; determining the model sand starting velocity; building a basic framework of the experimental model; selecting model sand materials; model fitting verification and simulated river section scouring experiment.

2. An experimental method for simulating a deepwater cofferdam anti-scour structure as claimed in claim 1, characterized in that: The calculation of the natural sediment starting velocity is performed using the Sha Yuqing formula.

3. The experimental method for simulating the anti-scour structure of a deepwater cofferdam as claimed in claim 1, characterized in that: The model geometry scale is numerically equal to the square of the model velocity scale, and the model geometry scale ranges from 1:50 to 1:

150.

4. An experimental method for simulating a deepwater cofferdam anti-scour structure according to claim 1, characterized in that: The starting velocity of the model sand is calculated using Dou Guoren's formula.

5. The experimental method for simulating the anti-scour structure of a deepwater cofferdam as claimed in claim 1, characterized in that: The basic framework of the experimental model meets the following conditions at the same time: The actual flow velocity of the simulated river section is greater than or equal to the starting flow velocity of the model sand; The model water flow type is turbulent flow, and the model Reynolds number is greater than 2000; The water depth of the model is greater than 1.5 cm to eliminate the influence of the surface tension of the model; The underwater repose angle of the model sand is equal to that of the prototype sand, so that the geometric shapes of the bridge pier scour pits in the simulated river section and the actual river section are similar; The compression ratio of the pier B / b is greater than 8, where: b is the width of the pier, B is the width of the water channel, and the minimum width of the pier is not less than 0.03m.

6. The experimental method for simulating the anti-scour structure of a deepwater cofferdam as claimed in claim 1, characterized in that: The method for selecting the model sand material is as follows: after preliminarily screening a plurality of candidate model sand materials and obtaining the starting flow velocity of the candidate model sand, the ratio of the starting flow velocity of each candidate model sand to the starting flow velocity of natural sediment is compared with the model flow velocity scale. The model sand material is determined if the difference between the two is within 5% of the model flow velocity scale.

7. The experimental method for simulating a deepwater cofferdam anti-scour structure according to claim 1, characterized in that: The model fitting verification includes: measuring whether the ratio of the model sand starting flow velocity to the natural sand starting flow velocity at different water depths is consistent with the selected model flow velocity scale, and the model fitting is correct if the difference between the two is within 5% of the model flow velocity scale.

8. The experimental method for simulating a deepwater cofferdam anti-scour structure according to claim 1, characterized in that: The simulated river scour is tested according to the principle of the controlled variable method, and multiple control experiments are carried out on cofferdams without anti-scour structures and cofferdams with different anti-scour structures to verify the anti-scour effects of different anti-scour structures.

9. An experimental method for simulating a deepwater cofferdam anti-scour structure as claimed in claim 8, characterized in that: The control experiment is to compare the experimental group with the same cofferdam anti-scour structure but different characteristic data with the control group of cofferdam without anti-scour structure, so as to compare the anti-scour effect of the same cofferdam anti-scour structure under different characteristic data.

10. A method for testing the effect of a built deepwater cofferdam anti-scour structure, characterized in that: The invention relates to a method for testing the effect of the anti-scour structure of a built deep-water cofferdam by means of a simulation experiment as described in any one of claims 1 to 9.

Images