Prediction method for local scouring of pier

Abstract

The invention relates to the technical research field of ocean engineering construction, in particular to a method for predicting local scouring of a pier, which innovatively combines a pier local scouring empirical formula which is verified by actual measurement data and is suitable for a target sea area with a two-dimensional numerical simulation technology. The selected empirical formula is fit based on a large amount of measured data, so that the calculation process is simple, convenient and rapid, and the high precision of a prediction result is ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of offshore engineering construction, and particularly relates to a method for predicting local scour of a bridge pier. BACKGROUND

[0002] At present, local scour of a bridge pier is one of the key factors threatening the safety of a bridge structure. Under the action of hydrodynamic force, a complex vortex system is generated around the bridge pier, which leads to continuous loss of riverbed sediment around the foundation, and then forms a local scour pit. With the development of scour depth, it may cause serious consequences such as decrease of bearing capacity of the bridge pier foundation, structural instability and even overall collapse. Therefore, at the design stage of a bridge engineering, scientific prediction of the local scour depth of a bridge pier has important engineering significance: on the one hand, it can provide a reliable basis for the design of a bridge foundation and optimize the structure type; on the other hand, it is helpful to develop a reasonable scour protection scheme to prevent scour damage from the source and ensure the safe and stable operation of a bridge in the whole life cycle. Accurate calculation of the local scour depth is not only a necessary link of engineering design, but also an important technical measure to ensure the durability and safety of a bridge structure.

[0003] At present, there are mainly three methods for predicting local scour of a bridge pier: empirical formula method, numerical simulation method and physical model test method.

[0004] 1. Empirical formula method: This method is based on the establishment of a prediction formula based on laboratory and field observation data, which has been included in industry specifications. Its advantages are simple calculation, high efficiency, and good accuracy under certain conditions. However, this method can only predict the local scour depth of a single group of bridge piers, and has relatively limited adaptability.

[0005] 2. Numerical simulation method: This method simulates the interaction of water flow, sediment and bridge pier structure through computational fluid dynamics (CFD), which can finely depict the complex flow field and scour process near the bridge pier, and is suitable for analysis of different working conditions. However, its calculation is time-consuming and limited by computing resources, and it can only simulate a single or a small number of bridge piers, which is difficult to directly apply to scour prediction of the whole bridge.

[0006] 3. Physical model test method: This method reproduces the bridge pier and riverbed environment in the laboratory according to the scale, and directly observes the shape and development law of the scour pit by simulating the actual flow conditions. Its advantages are intuitive and reliable results, and it is especially suitable for detailed analysis of important bridge engineering. However, the physical model test has high cost and long cycle, and is affected by the scale effect.

[0007] In view of the shortcomings of the existing methods, the present application proposes an innovative calculation method to realize accurate calculation and efficient collection of local scour of a bridge pier. This method not only can realize accurate prediction of the local scour depth, but also can pre-select bridge pier positions with low scour risk to provide a scientific basis for scheme comparison and selection in the early stage of a project. SUMMARY

[0008] The present application provides a method for predicting local scour of a bridge pier, which combines an empirical formula for local scour of a bridge pier, which is verified by measured data and suitable for a target sea area, with a two-dimensional numerical simulation technique. The selected empirical formula is fitted based on a large amount of measured data, which ensures both the simplicity and speed of the calculation process and the high accuracy of the prediction results.

[0009] The technical scheme adopted by the present application to solve the above technical problem is as follows: Step 1: selecting an empirical formula for local scour depth of a bridge pier h b =8.48k1k2B 0.326 u 0.628 h 0.193 d 50 0.167 to calculate and determine the input parameters of the calculation domain;

[0010] The empirical formula for local scour depth of a bridge pier is an expression established based on regional measured data, wherein h b is the local scour depth of a bridge pier; k1 is a basic pile plane arrangement coefficient; k2 is a basic pile vertical arrangement coefficient; B is the water-blocking width under the condition of the maximum water depth of a full tide; u is the maximum flow velocity of a full tide; h is the maximum water depth of a full tide; d 50 is the average median particle size of riverbed sediment;

[0011] Step 2: establishing a flow model, training the flow model, and simulating the flow dynamic conditions of the engineering area;

[0012] Step 3: verifying the flow model by using measured data of the engineering area;

[0013] Step 4: obtaining the maximum flow velocity u of a full tide and the maximum water depth h of a full tide by using the verified flow model;

[0014] Step 5: obtaining the average median particle size d 50 of sediment by using the measured value of bottom sampling near the engineering area of the proposed sea-crossing bridge before the engineering construction;

[0015] Step 6: inputting the maximum flow velocity u of a full tide, the maximum water depth h of a full tide, and the median particle size d 50 of sediment into the model constructed based on the empirical formula for local scour depth of a bridge pier, and finally obtaining the local scour depth distribution field of the bridge pier in the engineering sea area, and outputting the local scour depth distribution field data of the bridge pier to provide a basis for bridge pier location.

[0016] Further, the flow model in Step 2 is calculated by using the MIKE21 software, and the control equation of the flow model includes:

[0017]

[0018] wherein: h = eta + d; eta represents water surface height, d represents static water depth; x represents the horizontal axis coordinate, y represents the vertical axis coordinate; t is time; g is gravity acceleration; and respectively are the depth-averaged flow velocities in x and y directions; f is the Coriolis force coefficient; rho is the fluid density; rho0 is the reference density; T ij is the stress term; tau bx , tau by is the bottom stress; tau Sx , tau sy is the wind stress.

[0019] Further, the shoreline boundary in the water flow model is obtained according to satellite photos, and the terrain is determined by using multiple charts and engineering measured water depth charts, and the calculation domain is divided by using unstructured triangular meshes in the water flow model.

[0020] Further, the tidal level and flow velocity and flow direction process simulated by the water flow model are verified by using hydrological full-tide observation data, the tidal level and flow velocity and flow direction process simulated by the water flow model at the station position are compared with the measured tidal level and flow velocity and flow direction process data, and if the simulated tidal level and flow velocity and flow direction process at the station position are successfully fitted with the measured tidal level and flow velocity and flow direction process data, it is indicated that the water flow model is successfully verified.

[0021] The method has the following advantages:

[0022] 1. The method combines the bridge pier local scouring empirical formula verified by measured data and suitable for the target sea area with the two-dimensional numerical simulation technology. The selected empirical formula is fitted based on a large amount of measured data, so that the calculation process is simple and fast, and the prediction result is high in precision.

[0023] 2. Compared with the CFD numerical simulation method and the physical model test method which need complex modeling and are time-consuming and laborious, the method realizes efficient and convenient prediction process, and greatly saves the time cost and manpower investment.

[0024] 3. The traditional empirical formula and the numerical simulation method can only predict the local scouring of a single pier or a small number of piers, and the physical model test is also limited to the scouring analysis of the given bridge line position. The method realizes the leap from point-like and line-like prediction to global surface prediction for the first time, can provide comprehensive scouring field distribution data for the pre-project scheme comparison and selection, and provides a scientific basis for the layout of the cross-sea bridge and the optimization of the pier position. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the specific embodiments or prior art of the present application, the drawings needed to be used in the description of the specific embodiments or prior art will be briefly introduced as follows. Obviously, the drawings described below are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0026] Figure 1 The method flowchart of the present application.

[0027] Figure 2 The tide verification schematic of the present application Figure One .

[0028] Figure 3 The tide verification schematic of the present application Figure Two .

[0029] Figure 4 The tide verification schematic of the present application Figure Three .

[0030] Figure 5 The tidal current velocity and direction verification schematic of the present application Figure One .

[0031] Figure 6 The tidal current velocity and direction verification schematic of the present application Figure Two .

[0032] Figure 7 The tidal current velocity and direction verification schematic of the present application Figure Three . EMBODIMENT

[0033] The technical solutions of the present application will be described clearly and completely below in combination with the drawings. Obviously, the described embodiments are some embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor belong to the protection scope of the present application.

[0034] Embodiment 1:

[0035] The application process of the method of the present application in the practical engineering will be described systematically below by taking the proposed Daishan-Yangshan cross-sea bridge project in Hangzhou Bay as a typical case. The method innovatively combines the local scour empirical formula of the bridge pier with the large-scale shallow two-dimensional numerical simulation technology to realize the accurate prediction of the local scour of the bridge pier foundation of the cross-sea bridge, as shown in Figures 1 to 7 The prediction method of the local scour of the bridge pier provided by the present application specifically includes the following steps:

[0036] Step 1: determining the input parameters of the calculation domain

[0037] This study takes the proposed Daishan-Yangshan cross-sea bridge in Hangzhou Bay as the research object. Considering the complex hydrogeological conditions of the project area, an empirical formula for calculating the local scour depth of bridge piers, established by Han Haiqian et al. based on long-term measured data from the Hangzhou Bay Bridge, was specifically selected for calculation. This formula fully considers the unique strong tidal dynamic environment and complex seabed characteristics of Hangzhou Bay, demonstrating significant regional applicability and engineering reliability. Its basic expression is: h b =8.48k1k2B 0.326 u 0.62 8 h 0.193 d 50 0.167 In the formula, h b denoted as k1, where k1 is the local scour depth of the pier; k2 is the foundation pile planar layout coefficient (k1 = 1.0 for strip piles, k1 = 0.862 for quincunx piles); k3 is the foundation pile vertical layout coefficient (k2 = 1.0 for straight piles, k2 = 1.176 for inclined piles); B is the water-blocking width under the maximum water depth at full tide; u is the maximum flow velocity at full tide; h is the maximum water depth at full tide; d is the maximum water depth at full tide. 50 The median particle size of the riverbed sediment is denoted as .

[0038] Among them, the planar layout coefficient of the foundation piles is selected as quincunx k1 = 0.862; the vertical layout coefficient of the foundation piles is selected as inclined pile k2 = 1.176; and the design water-blocking width B is 4.0.

[0039] Step 2: Training and Validation of the Flow Model

[0040] Step 2-1: Model Selection: The flow model calculation software used in the numerical simulation mainly adopts the internationally recognized MIKE21 / FM module. The governing equations of this model are as follows:

[0041]

[0042]

[0043] Where: h = η + d; η represents the water surface height, d represents the still water depth; x represents the horizontal axis coordinate, y represents the vertical axis coordinate; t is time; g is the acceleration due to gravity; and ρ is the depth-averaged flow velocity along the x and y directions, respectively; f is the Coriolis force coefficient; ρ is the fluid density; ρ0 is the reference density; T ij For stress term; τ bx , τ by For bottom stress; τ Sx , τ sy This is wind stress.

[0044] Step 2-2: Determination of the calculation domain: The proposed sea-crossing bridge is located in the Hangzhou Bay estuary, with intersecting routes, complex shoreline boundaries, and beach groove topography and dynamic conditions. To ensure the accuracy of the numerical simulation, a large-scale tidal current model covering the entire Hangzhou Bay and its adjacent sea areas must be established. Therefore, the calculation range of the flow model is determined to be centered on the study area, with the northern side to Lvsi and the southern side to Haimen. The shoreline boundary in the model is obtained based on the latest satellite images, and the topography is determined using multiple charts and the latest measured depth chart near the project.

[0045] Step 2-3: Grid division: The unstructured triangular grid is used to divide the calculation domain in the flow model.

[0046] Step 2-4: Open boundary determination

[0047] During the tidal current numerical simulation calculation process, the setting of the open boundary condition needs to be handled. The initial tidal level of the open boundary is generated by the high-precision astronomical tide prediction module integrated in the model, and then the measured tidal level data in the project area and the model calculation tidal level data are iteratively calculated and optimized to optimize the open sea tidal level.

[0048] Step 2-5: Model verification

[0049] To verify the accuracy of the flow model calculation results, the 2017 hydrological full-tide observation data are selected to verify the tidal level and flow velocity and direction process simulated by the flow model (the tidal level and flow velocity and direction process at the model simulation station position are compared with the measured data. If the simulated tidal level and flow velocity and direction process at the model simulation station position are successfully fitted with the measured tidal level and flow velocity and direction process data, it indicates that the flow model verification is successful).

[0050] The verification curve is shown in Figures 4 to 5 (where the circles represent the measured data, and the solid line represents the verification results). It can be seen that the calculated tidal level and flow velocity and direction at each station are close to the measured values in the continuous change process, and can be used to simulate the full-tide maximum flow velocity u and full-tide maximum water depth h data.

[0051] Step 3: After the flow model is verified, the first data of the full-tide maximum flow velocity u and the full-tide maximum water depth h in the sea area near the project area can be directly output.

[0052] Step 4: According to the measured values of the bottom sediment sampling near the proposed sea-crossing bridge project area before the construction, the reference data of the average median particle size d 50 of the sediment are obtained.

[0053] Step 5: Data collection and local scour calculation

[0054] The full-tide maximum flow velocity u and the full-tide maximum water depth h obtained before are combined with the reference data d 50The local scouring depth distribution field of the bridge pier in the engineering sea area is finally obtained by introducing the local scouring calculation flow model based on the formula of Han Haiqian. It can be obviously seen from the figure that the local scouring situation of different positions in the research area after the bridge pier is arranged at the position. The design can select the position with smaller predicted local scouring depth as the basis for the selection of the bridge line and the bridge pier position of the cross-sea bridge according to the distribution diagram.

[0055] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for predicting localized scour of bridge piers, characterized in that, Includes the following steps: Step 1: Select the empirical formula h for the local scour depth of the bridge pier. b =8.48k1k2B 0.326 u 0.628 h 0.193 d 50 0.167 Perform calculations to determine the input parameters of the computation domain; The empirical formula for the local scour depth of the bridge pier is an expression established based on regional measured data, where h b denoted as , where k1 is the local scour depth of the bridge pier; k2 is the foundation pile planar layout coefficient; k2 is the foundation pile vertical layout coefficient; B is the water-blocking width under the maximum water depth at full tide; u is the maximum flow velocity at full tide; h is the maximum water depth at full tide; d 50 The median particle size of the riverbed sediment; Step 2: Establish a water flow model, train the water flow model, and simulate the water flow dynamic conditions in the engineering area; Step 3: Verify the water flow model using measured data from the engineering area; Step 4: Obtain the maximum tidal velocity u and the maximum tidal depth h using the validated flow model; Step 5: Based on the measured values ​​of the bottom sediment samples taken near the proposed cross-sea bridge construction area before the project construction, obtain the average median particle size d of the sediment in the reference data. 50 ; Step 6: Calculate the maximum tidal velocity u, the maximum tidal depth h, and the median sediment particle size d. 50 By inputting the data into a model constructed based on an empirical formula for the local scour depth of bridge piers, the distribution field of the local scour depth of bridge piers in the engineering sea area is finally obtained, and the data of the local scour depth distribution field of bridge piers is output to provide a basis for the selection of bridge pier locations.

2. The method for predicting localized scour of bridge piers according to claim 1, characterized in that: The flow model in step 2 is calculated using MIKE21 software, and the governing equations of this flow model include: Where: h = η + d; η represents the water surface height, d represents the still water depth; x represents the horizontal axis coordinate, y represents the vertical axis coordinate; t is time; g is the acceleration due to gravity; and ρ is the depth-averaged flow velocity along the x and y directions, respectively; f is the Coriolis force coefficient; ρ is the fluid density; ρ0 is the reference density; T ij For stress term; τ bx , τ by For bottom stress; τ Sx , τ sy This is wind stress.

3. The method for predicting localized scour of bridge piers according to claim 1, characterized in that: The shoreline boundary in the flow model is obtained from satellite imagery, and the topography is determined using multiple nautical charts and engineering-measured water depth maps. The computational domain is divided using an unstructured triangular mesh in the flow model.

4. The method for predicting localized scour of bridge piers according to claim 1, characterized in that: In step 3, hydrological full-tidal observation data are used to verify the tidal level, velocity and direction processes simulated by the flow model. The simulated tidal level, velocity and direction processes at the station locations are compared with the measured tidal level, velocity and direction process data. If the simulated tidal level, velocity and direction processes at the station locations and the measured tidal level, velocity and direction process data fit successfully with the measured tidal level, velocity and direction process data, it indicates that the flow model has been successfully verified.

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