A method for identifying site liquefaction

Abstract

This invention relates to the field of geological hazard identification technology and provides a method for identifying site liquefaction. The method includes: filtering collected seismic records based on surface acceleration in the horizontal direction; performing a Hilbert-Huang Transform (HHT) on the acceleration signals recorded in "time-acceleration" format to obtain the corresponding Hilbert time-frequency plot; and determining the dominant frequency time history curve and marginal spectrum based on the Hilbert time-frequency plot. Then, calculating the average dominant frequency decrease rate (MEFDr) based on the average dominant frequency (MEF) before and after the occurrence of peak ground acceleration; and determining the proportion of low frequencies (R) in the seismic record based on the marginal spectrum. L Finally, the possibility of liquefaction is determined by comprehensively considering the average dominant frequency decline rate and the proportion of low frequencies. Compared with the site liquefaction identification method in the current "Code for Seismic Design of Buildings" GB50011-2010 (2016), the identification method provided by this invention has the advantages of wide applicability, speed, and reliability.

Description

Technical Field

[0001] This invention relates to the field of geological hazard identification technology, specifically to a method for identifying site liquefaction. Background Technology

[0002] Soil liquefaction presents a critical opportunity for disaster relief and rescue efforts, posing significant safety hazards and economic losses to society. Therefore, a rapid and reliable method for identifying site liquefaction is urgently needed, which is crucial for both pre-earthquake disaster prevention planning and post-earthquake disaster relief.

[0003] Currently, commonly used methods for liquefaction assessment include field testing (standard method) and laboratory testing. However, the stress state of disturbed soil samples in laboratory tests differs significantly from that in the field, thus limiting their applicability. While my country's current "Code for Seismic Design of Buildings" GB50011-2010 (2016) can predict the likelihood of site liquefaction relatively well, it still has some problems due to a lack of samples in high-intensity seismic zones and poor sample distribution uniformity, such as limited applicability. Summary of the Invention

[0004] To address the problems existing in the prior art, the purpose of this invention is to provide a site liquefaction identification method. This method overcomes the problems of misjudgment caused by poor sample distribution uniformity during the identification of site liquefaction in existing standard methods, as well as the changes in the stress state of soil disturbance in indoor tests. It has the advantages of wide applicability, speed, and reliability.

[0005] To achieve the above objectives, the present invention provides a site liquefaction identification method, the method comprising:

[0006] S1. Based on the surface acceleration in the horizontal direction, the collected field seismic records are filtered; the seismic records are time history curves of acceleration signals recorded in the form of "time-acceleration";

[0007] S2. Perform empirical mode decomposition on the selected seismic records to obtain the intrinsic mode function components corresponding to the seismic records, and perform Hilbert spectral analysis on the obtained intrinsic mode function components to obtain the corresponding Hilbert time-frequency diagram;

[0008] S3. Obtain the dominant frequency time history curve and marginal spectrum corresponding to the earthquake record based on the Hilbert time-frequency diagram;

[0009] S4. Based on the obtained dominant frequency time history curve, obtain the average dominant frequency before and after the occurrence of the peak ground acceleration, and calculate the average dominant frequency decrease rate based on the average dominant frequency.

[0010] S5. Obtain the low-frequency proportion of the earthquake record based on the marginal spectrum;

[0011] S6. When the average dominant frequency decline rate is greater than the threshold Y1 and the low frequency proportion of the earthquake record is greater than the threshold Y2, the identified site is a liquefied site; otherwise, the site is not liquefied.

[0012] Preferably, step S1 involves screening the collected field seismic records, including the following steps:

[0013] S11: For any seismic record D in all collected field seismic records i The Arias intensity I is obtained according to equation (1). A If in the horizontal east-west or north-south direction, I A All are greater than the preset value I a Then D i Included in earthquake record set D1,

[0014]

[0015] In the formula, g is the gravitational acceleration, and a(t) is the seismic record D. i The time history of the surface acceleration signal, where T is the seismic record D i The total duration;

[0016] S12: For any earthquake record in D1 D j Based on the time history curve of its acceleration signal, determine whether its peak ground acceleration is greater than or equal to the preset value a1. If so, then set D... j Included in earthquake record set D2;

[0017] S13: For any earthquake record in D2 D k According to equation (2), the main part with 90% energy during the duration is extracted, and the extracted D is... k Included in earthquake record set D3,

[0018]

[0019] In the formula, t represents the earthquake record D. k At any time within the total duration, a(t) represents the earthquake record D. k The time history of the surface acceleration signal, where T is the seismic record D k The total duration;

[0020] D k As a result of screening earthquake records.

[0021] It should be noted that S11-S13 are merely preferred solutions for implementing the seismic record screening step. Those skilled in the art can also screen suitable seismic records from numerous seismic records through other methods or steps, which will not be listed here. Furthermore, as long as the screened seismic records can support subsequent steps, the screening mechanism is reasonable.

[0022] Preferably, step S3, which involves obtaining the dominant frequency time history curve corresponding to the seismic record based on the Hilbert time-frequency diagram, includes: obtaining the dominant frequency time history curve corresponding to the seismic record according to equation (3):

[0023]

[0024] In the formula, H(ω, t) is the Hilbert spectrum corresponding to the seismic record, and ω(t) is the dominant frequency time history corresponding to the seismic record.

[0025] Preferably, in S4, the average dominant frequency before and after the occurrence of the peak ground acceleration is obtained based on the obtained dominant frequency time history curve, as shown in equation (4):

[0026]

[0027] In the formula, t b and t e t corresponds to the start and end times of the 90% energy duration corresponding to the earthquake record, respectively. p The peak ground acceleration (MDA) corresponding to this earthquake record occurs at the time when Δh is the time interval between adjacent acceleration signals in the time history. MEF b and MEF e These are the mean dominant frequencies before and after the occurrence of peak ground acceleration within 90% of the energy duration corresponding to this earthquake record;

[0028] Then, the mean dominant frequency decrease rate corresponding to this earthquake record is calculated according to equation (5).

[0029]

[0030] MEFDr is the mean dominant frequency decay rate corresponding to this seismic record.

[0031] Preferably, the step in S3 to obtain the low-frequency proportion corresponding to the seismic record based on the marginal spectrum includes: calculating the low-frequency proportion R of the marginal spectrum corresponding to the seismic record according to equation (6). L :

[0032]

[0033] In the formula, h(ω) is the Hilbert marginal spectrum corresponding to the earthquake record, ω1 is the liquefaction characteristic threshold, and ω2 is the full frequency component threshold, with a value of 20Hz.

[0034] This technical solution utilizes surface acceleration data from seismic records for site liquefaction identification. It leverages the existing low-cost, high-density network of seismic stations built across various geographical and geological environments, employing their vast data collection for site liquefaction identification. This makes the solution widely applicable: wherever a seismic station exists, seismic data can be used for site liquefaction identification. Furthermore, for a specific site, identification based on seismic data from the localized seismic station allows for more accurate and faster identification, yielding more reliable results. Attached Figure Description

[0035] Figure 1 This is a flowchart of the site liquefaction identification method of the present invention.

[0036] Figure 2 This is a soil profile of the Wildlife liquefaction array.

[0037] Figure 3 This is a schematic diagram showing the duration of 90% energy retention in one embodiment of the present invention.

[0038] Figure 4 This is a Hilbert spectrum processed by empirical mode decomposition in one embodiment of the present invention.

[0039] Figure 5 This is a superior frequency time history curve of an embodiment of the present invention.

[0040] Figure 6 This is a marginal spectrum according to an embodiment of the present invention. Detailed Implementation

[0041] Example 1

[0042] According to an embodiment of the invention, a method for identifying site liquefaction is provided, such as... Figure 1 As shown, the method includes:

[0043] S1. Collect the time histories of surface acceleration monitored by the post-earthquake strong-motion monitoring network. Based on the Arias intensity described in equation (1), perform an initial screening of the horizontal earthquake records and select those with Arias intensities greater than the preset value I in the east-west and north-south earthquake records. a The earthquake records are obtained, and the records are screened a second time by the peak ground acceleration (PGA) to select earthquake records with a PGA of not less than the preset value a1. The effective time period of the earthquake records is extracted by the 90% energy duration described in formula (2).

[0044] S2. Based on the definition of empirical mode decomposition, the horizontal seismic records after initial screening are decomposed to obtain the corresponding intrinsic mode function components, and then Hilbert spectral analysis is performed on them to obtain the Hilbert time-frequency diagram. In this field, this method of combining empirical mode decomposition and Hilbert spectral analysis is a common data processing method, known as the "Hilbert-Huang Transform" (HHT).

[0045] S3. According to the definition of dominant frequency in equation (3), the dominant frequency time history curve corresponding to the Hilbert spectrum is obtained, and the marginal spectrum corresponding to the seismic record is obtained by analyzing the Hilbert spectrum.

[0046] S4. Using the time corresponding to the peak ground acceleration as the boundary, the average dominant frequency before and after the occurrence of the peak ground acceleration is obtained by the definition in Equation (4), and based on this, the average dominant frequency decrease rate MEFDr within 90% energy duration of the seismic signal is obtained by the definition in Equation (5).

[0047] S5. Based on the Hilbert marginal spectrum, the low-frequency richness ratio R of the seismic signal is obtained through the definition described in equation (6). L .

[0048] S6, based on MEFDr and R L The boundary values ​​of two parameters between liquefied and non-liquefied sites are used to identify whether a site is liquefied or not.

[0049] As a preferred option: In S1, the peak ground acceleration threshold used for secondary screening is 0.09g. That is, for records with a peak ground acceleration less than 0.09g, it is considered that the site where the record was obtained has not liquefied; otherwise, it is considered that liquefaction may have occurred.

[0050] As a preferred option, the superior frequency time history curve in S3 should be the time-frequency curve corresponding to 90% energy duration.

[0051] As a preferred option, the MEFDr boundary value between liquefied and non-liquefied sites in S4 should be 0.5, meaning that surface seismic records with MEFDr greater than 0.5 may indicate liquefaction; conversely, if the MEFDr value is less than 0.5, the recorded site is considered to be non-liquefied.

[0052] As a preferred option: liquefied site and non-liquefied site R in S5 L The cutoff value is set to 0.55, that is, for R L Earthquake records with a magnitude greater than 0.55 indicate the possibility of liquefaction; conversely, records with a magnitude less than 0.55 are considered to indicate that the site from which the records were obtained is not liquefied.

[0053] As a preferred embodiment: MEFDr and R described in S6L The two parameters are combined to form a composite criterion for identifying whether a site is liquefied. Specifically, when MEFDr is greater than 0.5 and R... L If the value is greater than 0.55, the site where the record was made can be considered to have liquefied; otherwise, it can be considered that the site where the record was made has not liquefied.

[0054] It should be noted that, whether the above methods utilize peak ground acceleration (GFCA) for secondary screening of earthquake records or combine MEFDr and R... L Identifying whether a site is liquefied is merely a selective value used in the embodiments of this invention; in reality, the corresponding boundary values ​​are not fixed. Under different geological conditions and climatic environments, those skilled in the art possess the corresponding judgment ability and can adjust the magnitude of the boundary value based on data collected by other equipment or practical experience to adapt to the complex and ever-changing actual environment and calibrate the accuracy of site liquefaction identification. Of course, this adjustment of the boundary value is not an independent technical solution separate from the technical solution of this invention.

[0055] Through the above embodiments, it can be seen that the present invention has the following beneficial effects: the technical solution proposed by the present invention provides a new perspective on the liquefaction process of seismic sites, and the data required by the present invention comes from the currently widely distributed seismic station monitoring network, which can effectively utilize the information of the seismic station monitoring network to provide a new method for identifying liquefaction of seismic sites.

[0056] Compared to traditional liquefaction identification methods, the identification method proposed in this invention is applicable to site liquefaction identification of any site type (soft soil non-liquefiable site, hard soil non-liquefiable site, and liquefiable site) under seismic action. It has the advantages of being fast, reliable, and widely applicable. It can reduce the workload of post-earthquake field investigation to a certain extent, and can provide a reference for seismic damage assessment of liquefiable sites and engineering disaster prevention and mitigation. It is of great significance for the formulation of disaster prevention plans before earthquakes and for emergency rescue and relief after earthquakes.

[0057] Example 2

[0058] This embodiment will identify the site liquefaction situation after a specific earthquake event, based on Embodiment 1.

[0059] During the 1987 Superstition Hills earthquake in the United States, the Wildlife Liquefaction Array in Southern California recorded information about the earthquake. The Wildlife Liquefaction Array recorded surface and downhole seismic data and pore water pressure. Post-earthquake investigations revealed significant liquefaction damage in the area, including sand boils, ground fissures, and lateral displacement of liquefied soil.

[0060] Soil profile of Wildlife liquefaction Array as shown in the image Figure 2 As shown.

[0061] This embodiment will analyze the seismic acceleration time histories monitored by the downhole strong motion meter (SM1) and the surface strong motion meter (SM2) according to the site liquefaction identification steps described in the above-mentioned site liquefaction identification method, and identify whether the two seismic records are liquefied.

[0062] According to the above steps, the maximum intensity of the earthquake records in the east-west and north-south directions of the surface and underground is selected by the Arias intensity in formula (1) and used for analysis. The peak ground acceleration of the selected surface and underground horizontal earthquake records is 0.205g and 0.172g, respectively, both of which are greater than 0.09g, which meets the site liquefaction requirements. The next step of analysis will be carried out according to the subsequent steps.

[0063] According to equation (2), the signal result of the corresponding time period intercepted with 90% energy duration is as follows: Figure 3 As shown;

[0064] Empirical mode decomposition and Hilbert spectral analysis were used to process the data respectively. Figure 3 The earthquake record shown yielded the Hilbert spectrum corresponding to 90% of the energy duration, as follows: Figure 4 As shown;

[0065] According to equation (3), the dominant frequency time history corresponding to the Hilbert spectrum within 90% energy duration is obtained as follows: Figure 5 As shown;

[0066] After obtaining the Hilbert spectrum, the mean dominant frequencies before and after the occurrence of the surface peak ground acceleration during the 90% energy duration of the surface and downhole seismic records were calculated according to Equation (4), and the MEFDr was calculated to be 0.9426 and 0.7567 respectively according to Equation (5).

[0067] according to Figure 4 The Hilbert spectrum can be used to obtain the corresponding Hilbert marginal spectrum, as follows: Figure 6 As shown, and according to the definition of low-frequency proportion described in equation (6), the R values ​​corresponding to surface and downhole seismic records can be calculated respectively. L The values ​​are 0.6018 and 0.5114, respectively.

[0068] Overall, the average dominant frequency decrease rate within 90% energy duration of the surface (SM2) seismic record is 0.9426, and the low frequency proportion is 0.6018, which meets the liquefaction conditions described in the above steps, i.e., the surface seismic record has liquefied, which is consistent with the results of the on-site seismic damage investigation report.

[0069] The mean dominant frequency decay rate (MEFDr) over 90% of the energy duration of the downhole (SM1) seismic record is 0.7567, and the low-frequency proportion is 0.5114, which meets the liquefaction conditions described in the above steps, i.e., the surface seismic record does not liquefy, which is consistent with the results of the on-site seismic damage investigation report.

[0070] As can be seen from the two embodiments above, site liquefaction identification based on surface acceleration in seismic records can fully utilize the existing low-cost, high-density seismic station network built in various geographical and geological environments. The use of the large amount of data recorded by these stations for site liquefaction identification makes this technical solution widely applicable; that is, as long as a seismic station exists, seismic data recorded by the station can be used for site liquefaction identification. Furthermore, for a specific site, site identification based on the seismic data recorded by the local seismic station, due to the localization of the recorded data, can complete the identification more accurately and quickly, obtaining more reliable identification results.

[0071] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. The present invention can be modified and varied appropriately. Any modifications, equivalent substitutions, improvements, etc., made within the scope of the specifications and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for identifying site liquefaction, characterized in that, The method includes: S1. Based on the surface acceleration in the horizontal direction, the collected seismic records from the field are screened; the seismic records are time history curves of acceleration signals recorded in the form of "time-acceleration"; S2. Perform empirical mode decomposition on the selected seismic records to obtain the intrinsic mode function components corresponding to the seismic records, and perform Hilbert spectral analysis on the obtained intrinsic mode function components to obtain the corresponding Hilbert time-frequency diagram; S3. Obtain the dominant frequency time history curve and marginal spectrum corresponding to the earthquake record based on the Hilbert time-frequency diagram; S4. Based on the obtained dominant frequency time history curve, obtain the average dominant frequency before and after the occurrence of the peak ground acceleration, and calculate the average dominant frequency decrease rate based on the average dominant frequency. S5. Obtain the low-frequency proportion of the earthquake record based on the marginal spectrum; S6. When the average dominant frequency decline rate is greater than the threshold Y1 and the low frequency proportion of the earthquake record is greater than the threshold Y2, the identified site may be a liquefiable site; otherwise, the site will not be liquefied.

2. The site liquefaction identification method as described in claim 1, characterized in that, The process of filtering the collected on-site earthquake records includes the following steps: S11: For any seismic record D in all collected field seismic records i The Arias intensity I is obtained according to equation (1). A If in the horizontal east-west or north-south direction, I A All are greater than the preset value I a Then D i Included in earthquake record set D1, （1）； In the formula, g is the gravitational acceleration, and a(t) is the seismic record D. i The time history of the surface acceleration signal, where T is the seismic record D i The total duration; S12: For any earthquake record in D1 D j Based on the time history curve of its acceleration signal, determine whether its peak ground acceleration is greater than or equal to the preset value a1. If so, then set D... j Included in earthquake record set D2; S13: For any earthquake record in D2 D k According to equation (2), the main part with an energy ratio of 90% during the duration is extracted, and the extracted D is... k Included in earthquake record set D3, （2）； In the formula, t represents the earthquake record D. k At any time within the total duration, a(t) represents the earthquake record D. k The time history of the surface acceleration signal, where T is the seismic record D k The total duration; D k As a result of screening earthquake records.

3. A site liquefaction identification method as described in claim 1 or 2, characterized in that, The process of obtaining the dominant frequency time history curve from the Hilbert time-frequency diagram includes the following steps: obtaining the dominant frequency time history curve corresponding to the seismic record according to equation (3): （3）； In the formula, H(ω, t) is the Hilbert spectrum corresponding to the seismic record, and ω(t) is the dominant frequency time history corresponding to the seismic record.

4. The site liquefaction identification method as described in claim 3, characterized in that, The average dominant frequency before and after the occurrence of the peak ground acceleration is obtained from the obtained dominant frequency time history curve, as shown in equation (4): （4）； In the formula, t b and t e The time t corresponds to the start and end times of the 90% energy duration of the earthquake record. p Δh represents the time when the peak ground acceleration (GFCA) corresponding to this earthquake record occurs, and Δh represents the time interval between adjacent acceleration signals in the time history of this earthquake record. MEF b and MEF e ω(t) represents the mean dominant frequency before and after the occurrence of peak ground acceleration within 90% of the energy duration corresponding to this earthquake record; ω(t) is the dominant frequency time history corresponding to this earthquake record. Then, the mean dominant frequency decrease rate corresponding to this earthquake record is calculated according to equation (5). （5）； In the formula, MEFDr is the mean dominant frequency decay rate corresponding to the earthquake record.

5. The site liquefaction identification method as described in claim 3, characterized in that, The method for obtaining the low-frequency proportion of the seismic record based on the marginal spectrum includes the following steps: calculating the low-frequency proportion R of the marginal spectrum corresponding to the seismic record according to equation (6). L , （6）； In the formula, h(ω) is the Hilbert marginal spectrum corresponding to the earthquake record, ω1 is the liquefaction characteristic threshold, and ω2 is the full frequency component threshold, with a value of 20Hz.

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