A method for identifying earthquake liquefaction considering the terrain effect of river valley sites

Abstract

The present invention discloses a method for distinguishing earthquake liquefaction by considering the terrain effect of a river valley site. The method includes extracting the coordinate data of the river valley site based on the geological exploration data of the river valley site, then using the coordinate data of the river valley site to establish a two-dimensional river valley site model, and performing finite element analysis on the river valley site model to obtain the earthquake response analysis result of the river valley site, and finally performing earthquake liquefaction analysis on the river valley site according to the earthquake response analysis result to obtain the dynamic shear stress time history at any position, and realizing earthquake liquefaction distinction of the river valley site by comparing the site anti-liquefaction strength. The present invention can simulate the refraction, reflection and scattering of seismic waves in the terrain of the river valley site, consider the amplification or suppression effect of the spatial terrain effect of the river valley site on the seismic load in the calculation of the seismic load, improve the calculation efficiency, and can more truly reflect the spatial inconsistency of the propagation of seismic motion and the spatial radiation damping effect of the seismic wave energy compared with the seismic wave vibration input method.

Description

Technical Field

[0001] The invention belongs to the technical field of earthquake liquefaction identification, and in particular relates to an earthquake liquefaction identification method taking into account the terrain effect of a river valley site. Background Art

[0002] Soil liquefaction is one of the main sources of earthquake damage to buildings. Therefore, the seismic design specifications of countries around the world involve methods for distinguishing foundation soil liquefaction. There are two main methods for distinguishing soil liquefaction at home and abroad. The first is the empirical analysis method summarized through earthquake site investigation, that is, by investigating earthquake damage in liquefied or non-liquefied sites and summarizing empirical judgment formulas. The method adopted by my country's specifications belongs to this category. The specifications give the critical value of the standard penetration hammer number of saturated sand and silt. When the measured standard penetration hammer number of saturated sand and silt on site is less than or equal to the critical value of the standard penetration hammer number for liquefaction judgment, it is judged as liquefied soil. The second is to determine the quantitative parameters of soil liquefaction through on-site or indoor soil tests to determine whether it is liquefied. Taking the Seed simplified judgment method as an example, the seismic shear stress ratio is used as the liquefaction judgment parameter, that is, the ratio of the dynamic shear stress generated by the earthquake to the effective stress of the overlying soil at the corresponding depth. If the seismic shear stress ratio is greater than the anti-liquefaction strength of the soil, liquefaction will occur at that location.

[0003] At present, many important infrastructure projects such as large dams have been built, are under construction, or are planned to be built in the high mountain valleys in western my country. Most of these key projects are located at the junction of the Indian Plate and the Eurasian Plate, surrounded by a series of small and dense fault zones, with active plate movement and serious potential earthquake threats, belonging to high seismic intensity areas. Soil liquefaction in river valley sites is one of the main earthquake hazards, and it is a major national need to reasonably prevent and solve earthquake disasters in key projects in river valley sites. Earthquakes are three-dimensional movements, including two horizontal movements and one vertical movement. Theoretically, seismic design should consider the combination of three-dimensional seismic effects. The terrain of river valley sites usually leads to complex reflection, refraction and scattering of seismic waves, which may lead to the concentration and amplification of seismic waves, thereby intensifying seismic effects. The code stipulates that vertical seismic effects cannot be ignored in strong earthquake areas (areas with seismic fortification intensity of 8 degrees and above). There is no clear requirement for three-dimensional seismic effects in the current code, but it is recommended that the combination of three-dimensional seismic effects should also be considered in strong earthquake areas and areas with special terrain.

[0004] When judging foundation soil liquefaction, the Chinese standard method and Seed simplified method only consider the correlation with the design earthquake intensity for the earthquake action, and neither considers the impact of earthquake multi-directionality and the terrain effect of the site on the earthquake load, and is only applicable to the case where the site can be regarded as a horizontally layered site. The existing technology lacks a method for judging earthquake liquefaction that couples the terrain effect of the valley site with the site liquefaction. Summary of the invention

[0005] In order to solve the problems existing in the background technology, the purpose of the present invention is to provide a method for distinguishing earthquake liquefaction taking into account the terrain effect of the valley site.

[0006] The technical solution adopted by the present invention is as follows, comprising the following steps:

[0007] Step S1, firstly, conduct on-site survey of the target river valley site to obtain geological exploration data of the river valley site, and then extract the coordinate data of the bedrock and overburden in the river valley site according to the geological exploration data;

[0008] Step S2, using the coordinate data of the valley site, establishing a two-dimensional valley site model, and performing finite element analysis on the valley site model to obtain the seismic response analysis results of the valley site;

[0009] Step S3: perform seismic liquefaction analysis on the valley site according to the seismic response analysis results to obtain liquefaction identification results of the target valley site.

[0010] The step S2 is specifically as follows:

[0011] Step S2.1, based on the on-site in-situ test, obtain the material constitutive model of the bedrock and overburden in the river valley site;

[0012] Step S2.2, according to the coordinate data and material constitutive model of the river valley site, a two-dimensional river valley site model is established using finite element software; the river valley site model includes a cross-river valley site model and a downstream valley site model, the plane of the cross-river valley site model is perpendicular to the river flow direction, and the plane of the downstream valley site model is parallel to the river flow direction;

[0013] Step S2.3, performing geostress balance on the river valley site model according to the boundary conditions of the river valley site model to obtain the geostress of the river valley site model;

[0014] Step S2.4, according to the earthquake time history of the on-site river valley site, obtain the earthquake equivalent nodal force of each node on the infinite element artificial boundary in the river valley site model, and apply the earthquake equivalent nodal force to each corresponding node of the infinite element artificial boundary;

[0015] Step S2.5: Perform finite element analysis on the river valley site model to which the node force is applied to obtain the seismic response analysis result of the river valley site model, wherein the seismic response analysis result includes the seismic acceleration time history and the vertical effective stress.

[0016] The step S3 specifically comprises the following steps:

[0017] Step S3.1, take a unit body in the cross-river / down-river valley site model, and extract the seismic acceleration time history of each node on the unit body according to the seismic response analysis results obtained in step S2;

[0018] Step S3.2, along the depth direction of the river valley site, respectively integrate the earthquake acceleration time histories in the transverse and downstream directions to obtain the dynamic shear stress time histories in the transverse and downstream directions of the unit body at any depth; then perform vector superposition on the dynamic shear stress time histories of the unit body in the transverse and downstream directions at the same position to obtain the final dynamic shear stress time histories at the position; take the ratio of the final dynamic shear stress time history to the vertical effective stress as the dynamic shear stress ratio time history at the position, and extract the maximum dynamic shear stress ratio from the dynamic shear stress ratio time history curves at each position as the target seismic shear stress ratio at the position;

[0019] Step S3.3: Compare the target earthquake shear stress ratio with the site anti-liquefaction strength to obtain the liquefaction identification results of the target river valley site at various locations.

[0020] The specific steps of step S2.1 are:

[0021] Step S2.1.1, conduct in-situ tests on the bedrock and the overburden respectively to obtain the shear wave velocity of the bedrock and the mechanical parameters of the overburden soil, wherein the mechanical parameters of the overburden soil include the shear wave velocity V s , Undrained shear strength S u and the variation curve of shear modulus with shear strain;

[0022] Step S2.1.2, importing the shear wave velocity of the bedrock mass obtained from the on-site in-situ test into the elastic constitutive model of the bedrock to obtain model parameters of the constitutive model of the bedrock material;

[0023] Step S2.1.3, using the Mises yield criterion and the associated flow law, the mechanical parameters of the covering layer soil are imported into the elastic-plastic constitutive model of the covering layer to obtain the model parameters of the constitutive model of the covering layer material; the elastic-plastic constitutive model parameters of the covering layer include the initial kinematic hardening modulus C, the initial yield stress σ0 and the dimensionless constant η that characterizes the hardening speed.

[0024] The establishment of the valley site model in step S2.2 includes the following steps:

[0025] Step S2.2.1, first construct a bedrock-cover layer two-dimensional model according to the coordinate data of the bedrock and the cover layer, then extend the edge of the bedrock-cover layer two-dimensional model outward to form an artificial boundary, and use the bedrock-cover layer two-dimensional model with the artificial boundary (3) as the finite element model of the valley site;

[0026] Step S2.2..2, respectively setting the material properties of the bedrock and overburden in the finite element model of the valley site according to the material constitutive models of the bedrock and overburden obtained in step S2.1;

[0027] Step S2.2.3, setting static and dynamic analysis steps, including static analysis step and dynamic analysis step;

[0028] Step S2.2.4, meshing the two-dimensional river valley site finite element model to obtain a plurality of mesh units;

[0029] Step S2.2.5, change the unit type of the artificial boundary from the initial finite element to the infinite element, so that the initial artificial boundary is converted into an infinite element artificial boundary, and at the same time, the finite element model of the valley site is converted into a finite element-infinite element coupling model of the valley site, and the infinite element artificial boundary is used as the boundary condition of the valley site model.

[0030] The step S2.3 is specifically as follows:

[0031] Step S2.3.1, firstly, the unit type of the artificial boundary in step S2.2 is changed from infinite element to initial finite element, then based on elastic analysis, the boundary conditions of the river valley site model are adopted to apply gravity to the river valley site finite element model to balance the ground stress of the finite element model, and the model ground stress obtained after ground stress balance is used as the initial ground stress;

[0032] Step S2.3.2: Based on plastic analysis, the initial geostress obtained in step S2.3.1 is input as prestress into the finite element-infinite element coupling model of the valley site using the geostress balance odb import method. Gravity is then applied to the finite element area in the finite element-infinite element coupling model to balance the geostress of the coupling model, and the model geostress obtained after the geostress balance is used as the final geostress.

[0033] The method for obtaining the equivalent nodal force of the earthquake motion at each node on the infinite element artificial boundary in step S2.4 is specifically as follows:

[0034] The equivalent nodal force of the bottom node in the infinite element artificial boundary is obtained according to the following formula:

[0035]

[0036]

[0037] in, It represents the nodal force on the bottom node l in the x-axis direction at time t; It represents the nodal force on the bottom node l in the y-axis direction at time t; t represents time;

[0038] The equivalent nodal force of the side nodes in the infinite element artificial boundary is obtained according to the following formula:

[0039]

[0040]

[0041]

[0042]

[0043] in, It represents the nodal force on the left side node l in the x-axis direction at time t; It represents the nodal force on the left side node l in the y-axis direction at time t; It represents the nodal force on the right side node l in the x-axis direction at time t; A represents the nodal force on the right side node l in the y-axis direction at time t; l is the influence range of node l on the boundary; is the earthquake velocity history; C lT and C lN They represent the tangential and normal damping coefficients of the infinite element artificial boundary respectively; Δt1 and Δt2 represent the time delays of the incident SV wave at node l and the reflected SV wave at the bottom edge respectively.

[0044] In step S3.3, the seismic shear stress is compared with the site anti-liquefaction strength to identify seismic liquefaction. Specifically, the ratio of the site anti-liquefaction strength to the seismic shear stress is used as the soil anti-liquefaction safety factor Fs of the valley site, and seismic liquefaction is identified based on the soil anti-liquefaction safety factor Fs:

[0045] If the soil anti-liquefaction safety factor Fs is less than 1, it indicates that earthquake liquefaction will occur at that location;

[0046] If the soil anti-liquefaction safety factor Fs is not less than 1, it means that earthquake liquefaction will not occur at that location.

[0047] The method of the present invention is based on ABAQUS finite element software, adopts infinite element artificial boundary to simulate the radiation damping of semi-infinite foundation, realizes the non-consistent input (or consistent input) of earthquake motion in the form of equivalent node force, completes the nonlinear seismic response analysis of two-dimensional river valley site, and extracts the earthquake acceleration of the model along the river and across the river that can reflect the terrain effect of the river valley site. Based on Newton's second law, the earthquake acceleration is integrated along the depth direction to obtain the dynamic shear stress time history at any position, the dynamic shear stress along and across the river is vector-superimposed and compared with the vertical effective stress at the position, the maximum value of the shear stress ratio at each position is taken as the earthquake shear stress ratio, and compared with the anti-liquefaction strength of the site soil, so as to realize the earthquake liquefaction judgment of the river valley site, and can provide reference for the site selection of the proposed project, the design of the site anti-liquefaction scheme, etc.

[0048] The vertical incidence of seismic waves is simulated in the transverse and downstream models respectively, and then the seismic response acceleration in the downstream and transverse models is extracted. Based on Newton's second law, the seismic acceleration is integrated along the depth direction to obtain the dynamic shear stress time history. The dynamic shear stress time history in the downstream and transverse directions is vector-superimposed to obtain the seismic shear stress under the combination of horizontal two-way seismic action at any position; the oblique incident seismic wave is input into the transverse model to simulate the combination of vertical and horizontal seismic action, and then the seismic shear stress time history generated under the combination of three-way seismic action is vector-superimposed with the seismic shear stress under the vertical incident seismic wave in the downstream model. The maximum shear stress ratio at each position is compared with the vertical effective stress of the soil at that position as the seismic shear stress ratio, and compared with the anti-liquefaction strength of the corresponding soil, which can realize the seismic liquefaction judgment of the valley site and provide reference for the site selection of the proposed project and the design of the site anti-liquefaction plan.

[0049] Beneficial effects of the present invention:

[0050] 1. The method of the present invention can simulate the refraction, reflection and scattering of seismic waves in the terrain of the river valley site through the nonlinear seismic response analysis of the two-dimensional refined model in the transverse and along-river directions, and consider the amplification or suppression effect of the spatial terrain effect of the river valley site on the seismic load in the calculation of seismic load. The calculation results are consistent with the three-dimensional numerical simulation, and the calculation efficiency is greatly improved.

[0051] 2. The present invention is based on ABAQUS finite element software, adopts infinite element artificial boundary to simulate the radiation damping of semi-infinite foundation, realizes the input of earthquake motion fluctuation in the form of equivalent nodal force, and can well simulate the seismic wave propagation characteristics that cannot be ignored in long and large structures. Compared with the earthquake wave vibration input method, it can more realistically reflect the spatial inconsistency of earthquake motion propagation and the spatial radiation damping effect of earthquake wave energy.

[0052] 3. The present invention can reflect the different combination effects of three-way earthquake action. By simulating the vertical incidence of seismic waves in the transverse and downstream models, the seismic shear stress time history generated under the combination of three-way earthquake action can be obtained. By comparing with the anti-liquefaction strength of the corresponding soil, the seismic liquefaction of the river valley site can be identified, which can provide reference for the site selection of the proposed project and the design of the site anti-liquefaction plan. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 It is a schematic diagram of the process of the present invention;

[0054] Figure 2 This is a three-dimensional schematic diagram of the valley site;

[0055] Figure 3 This is the cross-river model grid division diagram for the valley site;

[0056] Figure 4 The grid division diagram of the river valley site along the river;

[0057] In the figure: 1-bedrock; 2-cover layer; 3-infinite element artificial boundary. DETAILED DESCRIPTION

[0058] The present invention is described in detail below in conjunction with specific implementation cases. The following implementation cases will help those skilled in the art to further understand the present invention, but will not limit the present invention in any form.

[0059] The method of the present invention comprises the following steps: Figure 1 As shown:

[0060] Step S1: First, conduct an on-site survey of the target valley site to obtain geological exploration data of the valley site, and then use AutoCAD to extract the two-dimensional coordinate data of the bedrock and overburden in the valley site based on the geological exploration data; at the same time, based on the on-site seismic safety assessment data of the target valley site, determine the seismic parameters / seismic time history of the on-site bedrock boundary;

[0061] Step S2, using the coordinate data of the valley site, establishing a two-dimensional valley site model, and performing finite element analysis on the valley site model to obtain the seismic response analysis results of the valley site;

[0062] Step S3: perform seismic liquefaction analysis on the valley site according to the seismic response analysis results to obtain liquefaction identification results of the target valley site.

[0063] In the specific implementation, step S2 is specifically as follows:

[0064] Step S2.1: Based on the on-site in-situ test, the material constitutive model of the bedrock 1 and the overburden 2 in the valley site is obtained, such as Figure 3 and Figure 4 As shown, the overburden layer adopts the elastoplastic constitutive model, and the bedrock adopts the elastic constitutive model;

[0065] Step S2.2, according to the coordinate data and material constitutive model of the valley site, a two-dimensional valley site model is established using ABAQUS finite element software; the valley site model includes a cross-river valley site model and a downstream valley site model, the plane of the cross-river valley site model is perpendicular to the river flow direction, and the plane of the downstream valley site model is parallel to the river flow direction; wherein the coordinate data of the valley site includes the coordinate data of the bedrock and the overburden;

[0066] Step S2.3, according to the boundary conditions of the river valley site model, the ground stress balance is performed on the cross-river / along-river river valley site model to obtain the ground stress of the river valley site model;

[0067] Step S2.4, according to the earthquake time history of the on-site river valley site, obtain the earthquake equivalent nodal force of each node on the infinite element artificial boundary in the river valley site model, and apply the earthquake equivalent nodal force to each corresponding node of the infinite element artificial boundary through the earthquake non-consistent input or consistent input;

[0068] Step S2.5, based on the model geostress, perform finite element analysis on the cross-river / along-river valley site model with applied node force to obtain the seismic response analysis results of the cross-river / along-river valley site model, which include seismic acceleration time history and vertical effective stress.

[0069] Step S3 includes the following steps:

[0070] Step S3.1, take a unit body in the cross-river / down-river valley site model, and extract the seismic acceleration time history of each node on the unit body according to the seismic response analysis results obtained in step S2;

[0071] Step S3.2, along the depth direction of the valley site, respectively integrate the seismic acceleration time histories of the unit bodies in the valley site models in the transverse and downstream directions to obtain the dynamic shear stress time histories of the unit bodies in the transverse and downstream directions at any depth; then perform vector superposition of the dynamic shear stress time histories of the unit bodies in the transverse and downstream directions at the same position to obtain the final dynamic shear stress time histories at the position; take the ratio of the final dynamic shear stress time history to the vertical effective stress as the dynamic shear stress ratio time history at the position, and extract the maximum dynamic shear stress ratio from the dynamic shear stress ratio time history curves at each position as the target seismic shear stress ratio at the position;

[0072] Step S3.3: Compare the target earthquake shear stress ratio with the given site anti-liquefaction strength to obtain the liquefaction identification results of the target river valley site at various locations.

[0073] The coordinate data of bedrock and overburden are extracted in step S1.1 through AutoCAD, the earthquake acceleration time history of each node on the unit body is extracted in step S3.1 through Python compilation, the inconsistent input or consistent input of earthquake motion in the form of equivalent nodal force in step S2.4 and the acquisition of dynamic shear stress ratio time history along the depth direction in step 3.2 are respectively implemented through Matlab compilation.

[0074] The specific steps of step S2.1 are:

[0075] Step S2.1.1, conduct in-situ tests on the bedrock and the overburden respectively to obtain the shear wave velocity of the bedrock and the mechanical parameters of the overburden soil. The mechanical parameters of the overburden soil include the shear wave velocity V s , Undrained shear strength S uThe variation curve of shear modulus with shear strain (i.e. G / G max -γ curve), G / G max -G / G in the γ curve max represents the ratio of soil shear modulus to maximum shear modulus, γ represents soil shear strain;

[0076] Step S2.1.2, selecting an elastic constitutive model corresponding to the bedrock, importing the shear wave velocity of the bedrock mass obtained from the on-site in-situ test into the elastic constitutive model of the bedrock, and obtaining model parameters of the constitutive model of the bedrock material;

[0077] Step S2.1.3, select the elastic-plastic constitutive model corresponding to the covering layer, use the Mises yield criterion and the associated flow law, import the mechanical parameters of the covering layer soil into the elastic-plastic constitutive model of the covering layer, and obtain the model parameters of the constitutive model of the covering layer material; the elastic-plastic constitutive model parameters of the covering layer include the initial kinematic hardening modulus C, the initial yield stress σ0 and the dimensionless constant η that characterizes the hardening speed.

[0078] In the specific implementation, the elastic-plastic constitutive model parameters of the cover layer are obtained according to the following formula:

[0079]

[0080]

[0081]

[0082] Where ρ is soil density; υ is Poisson's ratio; λ is an intermediate parameter, which is expressed by G / G max -γ curve calibration.

[0083] The establishment of the cross-river / down-river valley site model in step S2.2 includes the following steps:

[0084] Step S2.2.1, firstly construct a two-dimensional model of bedrock-cover layer in the cross-river direction / along-river direction according to the coordinate data of the bedrock and the overburden in the finite element software, then extend both sides and the lower edge of the two-dimensional model of bedrock-cover layer in the cross-river direction / along-river direction outward to form an artificial boundary 3, and use the two-dimensional model of bedrock-cover layer in the cross-river direction / along-river direction with the artificial boundary (3) as the finite element model of the cross-river direction / along-river direction valley site;

[0085] Step S2.2..2, according to the material constitutive models of the bedrock and the overburden obtained in step S2.1, respectively set the material properties of the bedrock and the overburden in the finite element model of the cross-river / down-river valley site. In the specific implementation, the material properties of the artificial boundary (3) are consistent with the material properties of the material in contact with it, that is, the material constitutive model of the bedrock is used to set the material properties of the part of the artificial boundary (3) in contact with the bedrock, and the material constitutive model of the overburden is used to set the material properties of the part of the artificial boundary (3) in contact with the overburden;

[0086] Step S2.2.3, setting static and dynamic analysis steps, including static analysis step and dynamic analysis step;

[0087] Step S2.2.4, meshing the two-dimensional cross-river / along-river valley site finite element model to obtain a plurality of mesh units;

[0088] Step S2.2.5, modify the unit type of the artificial boundary 3 from the initial finite element to the infinite element, so that the initial artificial boundary 3 is converted into an infinite element artificial boundary, and at the same time, the finite element model of the cross-river / down-river valley site is converted into a finite element-infinite element coupling model of the cross-river / down-river valley site, and the infinite element artificial boundary is used as the boundary condition of the valley site model, and the boundary conditions of the model include displacement boundary, velocity boundary, and stress boundary; the valley site model includes the valley site finite element model and the valley site finite element-infinite element coupling model;

[0089] Among them, steps S2.2.1 to S2.2.4 are for constructing and obtaining the initial model, and step S2.2.5 is the model optimization step, which is used for subsequent calculation of infinite element artificial boundary parameters, seismic response analysis, etc.

[0090] Step S2.3 is specifically as follows:

[0091] Step S2.3.1, first change the unit type of artificial boundary 3 in step S2.2 from infinite element to initial finite element, then based on elastic analysis, adopt the boundary conditions of the valley site model, apply gravity to the cross-river / down-river valley site finite element model to balance the ground stress of the finite element model, and use the model ground stress obtained after ground stress balance as the initial ground stress; at the same time, obtain the vertical displacement of the finite element model, and when the vertical displacement of the model is less than 10 -4 m, indicating that the modeling is reasonable; otherwise, return to step S2.2 to re-model;

[0092] Step S2.3.2: Based on plastic analysis, the initial geostress obtained in step S2.3.1 is input as prestress into the finite element-infinite element coupling model of the cross-river / down-river valley site using the geostress balance odb import method. Gravity is then applied to the finite element area in the finite element-infinite element coupling model to balance the geostress of the coupling model. The model geostress obtained after the geostress balance is used as the final geostress corresponding to the cross-river / down-river valley site model.

[0093] The method for obtaining the equivalent nodal force of the earthquake motion at each node on the infinite element artificial boundary in step S2.4 is as follows:

[0094] In the specific implementation, according to the earthquake time history of the valley site, the earthquake equivalent nodal force can be applied to each corresponding node of the infinite element artificial boundary through the earthquake non-consistent input or consistent input, so that the force condition of the valley site model is closer to the actual situation; when the earthquake consistent input is adopted, the SV wave is vertically incident on the infinite element artificial boundary, and the earthquake equivalent nodal force calculation formula under consistent input is as follows:

[0095] The equivalent nodal force of the bottom node in the infinite element artificial boundary is obtained according to the following formula:

[0096]

[0097]

[0098] in, It represents the nodal force on the bottom node l in the x-axis direction at time t; It represents the node force on the bottom node l in the y-axis direction at time t; t represents time; in the valley site model, the width direction is defined as the x-axis and the height direction is defined as the y-axis;

[0099] The equivalent nodal force of the side nodes in the infinite element artificial boundary is obtained according to the following formula:

[0100]

[0101]

[0102]

[0103]

[0104] in, It represents the nodal force on the left side node l in the x-axis direction at time t; It represents the nodal force on the left side node l in the y-axis direction at time t; It represents the nodal force on the right side node l in the x-axis direction at time t; A represents the nodal force on the right side node l in the y-axis direction at time t; l is the influence range of node l on the boundary; is the earthquake velocity history; C lT and C lN represent the tangential and normal damping coefficients of the infinite element artificial boundary, respectively, where C lT =ρV s , C lN =ρV p , V p is the compression wave speed; Δt1 and Δt2 represent the time delays of the incident SV wave at node l and the reflected SV wave at the bottom edge, respectively.

[0105] The superscript of the equivalent nodal force indicates the direction of the external normal of the finite element boundary interface where the node is located, and the subscript indicates the node number and component direction. The negative sign represents the direction opposite to the coordinate axis, and the positive direction of the x-axis is from left to right. Figure 3 The figure shows the cross-river model of the river valley site, where the x-axis is perpendicular to the river flow direction; Figure 4 The figure shows the river-going model of a valley site, where the x-axis is parallel to the river flow direction and the y-axis is the depth direction.

[0106] In step S3.3, the earthquake shear stress and the site liquefaction resistance strength are compared to identify earthquake liquefaction. Specifically, first, the ratio of the site liquefaction resistance strength to the earthquake shear stress ratio is used as the soil liquefaction resistance safety factor Fs of the valley site, and earthquake liquefaction is identified based on the soil liquefaction resistance safety factor Fs:

[0107] If the soil anti-liquefaction safety factor Fs is less than 1, it indicates that earthquake liquefaction will occur at that location;

[0108] If the soil anti-liquefaction safety factor Fs is not less than 1, it means that earthquake liquefaction will not occur at that location.

[0109] In order to verify the accuracy of the present invention, a three-dimensional river valley site model was used as a control group for comparison:

[0110] It is generally believed that the three-dimensional refined dynamic time history analysis method is a relatively accurate analysis method, but it has great disadvantages, such as the algorithm is too complicated and the model calculation and analysis is time-consuming. It is limited in application in practice and is usually used as an evaluation standard for other methods. Figure 2In the model of the method of the present invention, the results obtained by the three-dimensional model calculation are used as a benchmark, and it is found that the earthquake acceleration calculated by the earthquake liquefaction discrimination method considering the valley site terrain effect provided by the present invention is very close to the three-dimensional model result. Since the operation efficiency of the present invention is higher than that of the three-dimensional model, it has good engineering applicability.

[0111] Compared with the three-dimensional seismic response analysis, the two-dimensional river valley site nonlinear seismic response analysis of the present invention greatly reduces the calculation cost. The results show that the calculation results of this model are in good agreement with the calculation results of the three-dimensional model, and can accurately reflect the dynamic response of the soil in the river valley site, which can provide a reference for the site selection of the proposed project and the design of the site anti-liquefaction plan.

[0112] The present invention simulates the vertical incidence of seismic waves in the transverse and downstream models respectively, then extracts the seismic response acceleration in the downstream and transverse models, integrates the seismic acceleration along the depth direction based on Newton's second law, obtains the dynamic shear stress time history, and vector superposition is performed on the dynamic shear stress time history in the downstream and transverse directions, so as to obtain the seismic shear stress under the combination of horizontal two-direction seismic action at any position; inputs the oblique incident seismic wave in the transverse model, which can simulate the combination of vertical and horizontal seismic action, and then vector superposition is performed with the seismic shear stress under the vertical incident seismic wave in the downstream model, so as to obtain the seismic shear stress time history generated under the combination of three-direction seismic action. The maximum value of the shear stress ratio at each position is compared with the vertical effective stress of the soil at the position as the seismic shear stress ratio, and compared with the anti-liquefaction strength of the corresponding soil, so as to realize the seismic liquefaction discrimination of the valley site, and provide reference for the site selection of the proposed project, the design of the site anti-liquefaction scheme, etc.

[0113] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for identifying earthquake liquefaction taking into account the terrain effect of a river valley site, characterized in that: The following steps are involved: Step S1, firstly, conduct on-site survey of the target river valley site to obtain geological exploration data of the river valley site, and then extract the coordinate data of the bedrock and overburden in the river valley site according to the geological exploration data; Step S2, using the coordinate data of the valley site, establishing a two-dimensional valley site model, and performing finite element analysis on the valley site model to obtain the seismic response analysis results of the valley site; Step S3, performing seismic liquefaction analysis on the valley site according to the seismic response analysis results, and obtaining liquefaction identification results of the target valley site; The step S2 is specifically as follows: Step S2.1, based on the on-site in-situ test, obtain the material constitutive model of the bedrock (1) and the overburden (2) in the river valley site; Step S2.2, according to the coordinate data and material constitutive model of the river valley site, a two-dimensional river valley site model is established using finite element software; the river valley site model includes a cross-river valley site model and a downstream valley site model, the plane of the cross-river valley site model is perpendicular to the river flow direction, and the plane of the downstream valley site model is parallel to the river flow direction; Step S2.3, performing geostress balance on the river valley site model according to the boundary conditions of the river valley site model to obtain the geostress of the river valley site model; Step S2.4, according to the earthquake time history of the on-site river valley site, obtain the earthquake equivalent nodal force of each node on the infinite element artificial boundary in the river valley site model, and apply the earthquake equivalent nodal force to each corresponding node of the infinite element artificial boundary; Step S2.5: Perform finite element analysis on the valley site model to which the node force is applied to obtain the seismic response analysis result of the valley site model, wherein the seismic response analysis result includes the seismic acceleration time history and the vertical effective stress.

2. The earthquake liquefaction identification method considering the valley site topography effect according to claim 1 is characterized by: The step S3 specifically comprises the following steps: Step S3.1, take a unit body in the cross-river / down-river valley site model, and extract the seismic acceleration time history of each node on the unit body according to the seismic response analysis results obtained in step S2; Step S3.2, along the depth direction of the river valley site, respectively integrate the earthquake acceleration time histories in the transverse and downstream directions to obtain the dynamic shear stress time histories in the transverse and downstream directions of the unit body at any depth; then perform vector superposition on the dynamic shear stress time histories of the unit body in the transverse and downstream directions at the same position to obtain the final dynamic shear stress time histories at the position; take the ratio of the final dynamic shear stress time history to the vertical effective stress as the dynamic shear stress ratio time history at the position, and extract the maximum dynamic shear stress ratio from the dynamic shear stress ratio time history curves at each position as the target seismic shear stress ratio at the position; Step S3.3: Compare the target earthquake shear stress ratio with the site anti-liquefaction strength to obtain the liquefaction identification results of the target river valley site at various locations.

3. The earthquake liquefaction identification method considering the valley site topography effect according to claim 1 is characterized by: The specific steps of step S2.1 are: Step S2.1.1, conduct in-situ tests on the bedrock and the overburden respectively to obtain the shear wave velocity of the bedrock and the mechanical parameters of the overburden soil, wherein the mechanical parameters of the overburden soil include the shear wave velocity V s , Undrained shear strength S u and the variation curve of shear modulus with shear strain; Step S2.1.2, importing the shear wave velocity of the bedrock mass obtained from the on-site in-situ test into the elastic constitutive model of the bedrock to obtain model parameters of the constitutive model of the bedrock material; Step S2.1.3, using the Mises yield criterion and the associated flow law, the mechanical parameters of the covering layer soil are imported into the elastic-plastic constitutive model of the covering layer to obtain the model parameters of the constitutive model of the covering layer material; the elastic-plastic constitutive model parameters of the covering layer include the initial kinematic hardening modulus C, the initial yield stress σ0 and the dimensionless constant η that characterizes the hardening speed.

4. The earthquake liquefaction identification method considering the valley site topography effect according to claim 1 is characterized by: The establishment of the valley site model in step S2.2 includes the following steps: Step S2.2.1, first construct a bedrock-cover layer two-dimensional model according to the coordinate data of the bedrock and the cover layer, then extend the edge of the bedrock-cover layer two-dimensional model outward to form an artificial boundary (3), and use the bedrock-cover layer two-dimensional model with the artificial boundary (3) as the finite element model of the valley site; Step S2.2..2, respectively setting the material properties of the bedrock and overburden in the finite element model of the valley site according to the material constitutive models of the bedrock and overburden obtained in step S2.1; Step S2.2.3, setting static and dynamic analysis steps, including static analysis step and dynamic analysis step; Step S2.2.4, meshing the two-dimensional river valley site finite element model to obtain a plurality of mesh units; Step S2.2.5, change the unit type of the artificial boundary (3) from the initial finite element to the infinite element, so that the initial artificial boundary (3) is converted into an infinite element artificial boundary, and at the same time, the finite element model of the valley site is converted into a finite element-infinite element coupling model of the valley site, and the infinite element artificial boundary is used as the boundary condition of the valley site model.

5. The earthquake liquefaction identification method considering the valley site topography effect according to claim 1 is characterized by: The step S2.3 is specifically as follows: Step S2.3.1, firstly, the unit type of the artificial boundary (3) in step S2.2 is changed from infinite element to initial finite element, then based on elastic analysis, the boundary conditions of the river valley site model are adopted to apply gravity to the river valley site finite element model to balance the ground stress of the finite element model, and the model ground stress obtained after the ground stress balance is used as the initial ground stress; Step S2.3.2: Based on plastic analysis, the initial geostress obtained in step S2.3.1 is input as prestress into the finite element-infinite element coupling model of the valley site using the geostress balance odb import method. Gravity is then applied to the finite element area in the finite element-infinite element coupling model to balance the geostress of the coupling model, and the model geostress obtained after the geostress balance is used as the final geostress.

6. The earthquake liquefaction identification method considering the valley site topography effect according to claim 1, characterized in that: The method for obtaining the equivalent nodal force of the earthquake motion at each node on the infinite element artificial boundary in step S2.4 is specifically as follows: The equivalent nodal force of the bottom node in the infinite element artificial boundary is obtained according to the following formula: in, It represents the nodal force on the bottom node l in the x-axis direction at time t; It represents the nodal force on the bottom node l in the y-axis direction at time t; t represents time; The equivalent nodal force of the side nodes in the infinite element artificial boundary is obtained according to the following formula: in, It represents the nodal force on the left side node l in the x-axis direction at time t; It represents the nodal force on the left side node l in the y-axis direction at time t; It represents the nodal force on the right side node l in the x-axis direction at time t; A represents the nodal force on the right side node l in the y-axis direction at time t; l is the influence range of node l on the boundary; is the ground motion velocity history; C lT and C lN They represent the tangential and normal damping coefficients of the infinite element artificial boundary respectively; Δt1 and Δt2 represent the time delays of the incident SV wave at node l and the reflected SV wave at the bottom edge respectively.

7. The earthquake liquefaction identification method considering the valley site topography effect according to claim 2 is characterized by: In step S3.3, the seismic shear stress is compared with the site anti-liquefaction strength to identify seismic liquefaction. Specifically, the ratio of the site anti-liquefaction strength to the seismic shear stress is used as the soil anti-liquefaction safety factor Fs of the valley site, and seismic liquefaction is identified based on the soil anti-liquefaction safety factor Fs: If the soil anti-liquefaction safety factor Fs is less than 1, it indicates that earthquake liquefaction will occur at that location; If the soil anti-liquefaction safety factor Fs is not less than 1, it means that earthquake liquefaction will not occur at that location.

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