A method for analyzing the buoyancy reliability of rafts considering time-varying groundwater and extreme events.

Abstract

This invention, entitled "A Method for Reliability Analysis of Raft Foundation Uplift Considering Time-Varying Groundwater and Extreme Events," belongs to the technical field of geotechnical engineering and underground structure safety evaluation. The technical problem it aims to solve is the inability to comprehensively consider, or a combination thereof, the time-varying characteristics of groundwater level, the impact of extreme events, and the random degradation characteristics of uplift prevention measures when analyzing the reliability of raft foundation uplift resistance. The key technical solution includes the following steps: S1, establishing limit state models, including limit state models with and without anchor bolts; S2, using MATLAB software to obtain time-varying diagrams of groundwater head simulation, cumulative failure probability of uplift resistance with anchor bolts, parameter sensitivity ranking diagrams with and without anchor bolts; S3, analyzing the results obtained in step S2.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering and underground structure safety evaluation technology, and in particular to a method for analyzing the buoyancy reliability of raft foundations considering time-varying groundwater and extreme events. Background Technology

[0002] Raft foundations are widely used in underground structural engineering due to their good integrity and high load-bearing capacity. However, in areas with high groundwater levels or significant time-varying groundwater levels, raft foundations often bear the buoyancy of groundwater. When the buoyancy exceeds the self-weight of the raft, the weight of the overburden, and the load transmitted by the superstructure, the structure may experience buoyancy failure problems such as overall floating, bottom slab cracking, or local peeling, seriously affecting structural safety.

[0003] In current engineering design, the buoyancy verification of raft foundations usually adopts a deterministic method, that is, under a certain unfavorable working condition, the design value of the groundwater level is taken and compared with the raft foundation's buoyancy resistance to judge safety. However, this method has the following shortcomings: (1) The groundwater level has obvious seasonality, randomness and long-term evolution characteristics, and a single design water level is difficult to reflect the real working condition; (2) Extreme rainfall, sudden recharge and other events may cause the groundwater level to rise sharply in the short term, which is difficult to consider in traditional methods; (3) Measures such as anti-buoyancy anchors have construction randomness and bearing capacity degradation problems during long-term service, which are usually not considered in existing calculation methods; (4) It is impossible to quantify the failure probability and key influencing factors of the structure throughout its entire life cycle. Summary of the Invention

[0004] The purpose of this invention is to provide:

[0005] A method for analyzing the buoyancy reliability of raft foundations considering time-varying groundwater levels and extreme events, and related technologies, are proposed to address the technical issues, or combinations thereof, in the analysis of the buoyancy reliability of raft foundations, which cannot comprehensively consider the time-varying characteristics of groundwater levels, the impact of extreme events, and the random degradation characteristics of buoyancy-resistant measures.

[0006] Terminology Explanation: Unless otherwise defined, all technical terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which the subject matter pertains. Unless otherwise stated, all patents, patent inventions, and publications cited throughout this document are incorporated herein by reference in their entirety. Where multiple definitions exist for terms in this document, the definitions provided in this chapter shall prevail.

[0007] It should be understood that the above brief description and the following detailed description are exemplary and for illustrative purposes only, and do not limit the subject matter of the invention in any way. In this invention, the singular is used in conjunction with the plural unless otherwise specifically stated. It should also be noted that, unless otherwise stated, the use of “or” or “or” means “and / or”. Furthermore, the use of the term “comprising” and other forms such as “including,” “containing,” and “contains” are not limiting.

[0008] The definition of the standard terminology can be found in the reference "Theory, Key Technologies and Engineering Applications of Controllable Stiffness Pile Raft Foundation".

[0009] Unless otherwise stated, conventional methods within the scope of the art, such as "overall buoyancy verification", shall be used.

[0010] Unless otherwise defined, the use of various commercially available products as described herein employs standard techniques. These techniques and methods can generally be implemented according to conventional methods well-known in the art, based on the descriptions in the numerous general and more specific documents cited and discussed in this specification.

[0011] The terms “optional / arbitrary” or “optionally / arbitrarily” mean that the event or situation described below may or may not occur, including both the occurrence and non-occurrence of the event or situation.

[0012] The term "raft foundation" as used in this article refers to a type of integral foundation that transfers the building load to the ground through a single reinforced concrete slab.

[0013] The term "buoyancy resistance" as used in this article refers to a series of structural design and engineering measures taken to prevent underground structures from floating, heaving, or being damaged due to the buoyancy of groundwater.

[0014] This invention provides a method for analyzing the buoyancy reliability of rafts that takes into account the time-varying nature of groundwater and extreme events.

[0015] Includes the following steps: S1. Establish limit state models, including limit state models for anchor bolt conditions and limit state models for non-anchor bolt conditions. The limit state model for the anchor bolt working condition is expressed as follows: ; The limit state model for the anchorless working condition is expressed as follows: ; ; In the above formula, Indicates the limiting state. This indicates that there is no anchor bolt to resist downward force. This indicates that there is downward resistance from the anchor bolt. Indicates the density of water. Indicates the area of ​​the raft foundation; Indicates the initial water head. Indicates the annual cycle fluctuation range. Indicates phase, Indicates a long-term linear trend. Represents the stochastic correlation fluctuation term. Indicates water level changes during extreme events; S2. Using MATLAB software, based on the limit state models for the anchored and unanchored conditions obtained in step S1, and through... The calculation model was used to obtain the time-varying diagram of groundwater head simulation, and then the time-varying diagram of cumulative failure probability of anti-buoyancy under anchor bolt condition, the parameter sensitivity ranking diagram under anchor bolt condition, the time-varying diagram of cumulative failure probability of anti-buoyancy under non-anchor bolt condition, and the parameter sensitivity ranking diagram under non-anchor bolt condition were obtained. S3. Analyze the time-varying diagram of cumulative failure probability of anti-buoyancy under anchor bolt conditions, the parameter sensitivity ranking diagram under anchor bolt conditions, the time-varying diagram of cumulative failure probability of anti-buoyancy under non-anchor bolt conditions, the parameter sensitivity ranking diagram under non-anchor bolt conditions, and the time-varying diagram of groundwater head simulation.

[0016] Among them, the downward resistance without anchor bolts Specifically, it is calculated using the following formula: ; In the above formula, represents the self-weight of the raft, This indicates the effective downward gravity of the soil covering the upper part of the raft. This indicates the weight of the superstructure. This indicates an additional load.

[0017] Among them, the raft foundation's self-weight Specifically, it is calculated using the following formula: ; In the above formula, Indicates the specific weight of the raft foundation. This indicates the thickness of the raft slab.

[0018] Among them, the effective downward gravity of the soil covering the upper part of the raft. Specifically, it is calculated using the following formula: ; ; ; In the above formula, Indicates saturation height. Indicates the depth of soil cover. Indicates the unit weight of the saturated soil cover; Indicates the unsaturated height. This indicates the dry weight of the soil covering.

[0019] Among them, the self-weight of the superstructure Specifically, it is calculated using the following formula: ; In the above formula, This represents the average vertical load transmitted from the superstructure to the raft slab.

[0020] Among them, additional load Specifically, it is calculated using the following formula: ; In the above formula, This indicates an additional uniformly distributed load on the ground.

[0021] in, , The results are obtained by calculating using the following formulas: ; ; In the above formula, This represents the autocorrelation coefficient across months. Indicates the noise amplitude at each step; Indicates the time when the i-th extreme event begins. hour, ,when hour, ; This represents the total number of extreme events. This indicates the magnitude of the water level rise during each extreme event. This represents the attenuation coefficient.

[0022] Among them, there is the downward resistance of the anchor bolt. Specifically, it is calculated using the following formula: ; In the above formula, This indicates the total pull-out resistance of the anchor system; Indicates the number of effective anchor bolts. This indicates the current bearing capacity of a single anchor bolt.

[0023] in, , Specifically, it can be derived from the following formula: ; ; In the above formula, This represents the initial static bearing capacity of a single anchor bolt. This indicates the rate of decrease in the bearing capacity of a single anchor bolt. Indicates the number of anchor bolts designed. This indicates the success rate of installing a single anchor bolt. This represents the Bernoulli distribution.

[0024] In step S3, the time-varying graph of the cumulative failure probability of anti-buoyancy under the anchor bolt condition is analyzed to obtain the cumulative failure probability under the simulation time under the anchor bolt condition; the time-varying graph of the cumulative failure probability of anti-buoyancy under the non-anchor bolt condition is analyzed to obtain the cumulative failure probability under the simulation time under the non-anchor bolt condition; the sensitivity ranking graph of parameters under the anchor bolt condition is analyzed to obtain the negative and positive correlation parameters under the anchor bolt condition; and the sensitivity ranking graph of parameters under the non-anchor bolt condition is analyzed to obtain the negative and positive correlation parameters under the non-anchor bolt condition. The present invention has at least the following beneficial effects: This invention can more realistically reflect the long-term evolution, seasonal fluctuations and impact of extreme events of groundwater level, quantify the probability of buoyancy failure of raft foundations throughout their entire life cycle, evaluate the buoyancy safety of both anchored and anchored conditions, identify key parameters affecting buoyancy safety, provide a basis for design optimization, and is applicable to the buoyancy design and risk assessment of underground structures in complex groundwater environments. Attached Figure Description

[0025] Figure 1 This is a flowchart of the method of the present invention.

[0026] Figure 2 This is a schematic diagram of the physical model of the raft plate of the present invention.

[0027] Figure 3 This is a time-varying comparison chart of the cumulative failure probability of anti-buoyancy under anchor bolt conditions and without anchor bolt conditions in this invention.

[0028] Figure 4 This invention includes a sensitivity ranking chart of anchor bolt working condition parameters.

[0029] Figure 5 This is a sensitivity ranking diagram of the parameters under the anchorless working condition of the present invention.

[0030] Figure 6 This is a time-varying diagram of groundwater head simulation according to the present invention.

[0031] Figure 7 This is a schematic diagram of the anti-buoyancy calculation conditions for the atrium area of ​​an office building according to an embodiment of the present invention.

[0032] Figure 8 This is a schematic diagram of the anti-buoyancy anchor rod according to an embodiment of the present invention.

[0033] Figure 9 This is a cumulative failure probability diagram of the anti-buoyancy of the office building according to an embodiment of the present invention. Detailed Implementation

[0034] The following non-limiting embodiments are intended to enable those skilled in the art to gain a more comprehensive understanding of the present invention, but do not limit the invention in any way. The following content is merely an exemplary description of the scope of protection claimed by the present invention, and those skilled in the art can make various changes and modifications to the present invention based on the disclosed content, and such changes should also fall within the scope of protection claimed by the present invention.

[0035] The present invention will be further described below by way of specific embodiments. Unless otherwise specified, all instruments, devices, equipment, reagents, products, etc., used in the embodiments of the present invention are obtained through conventional commercial means.

[0036] like Figure 1 As shown, this invention provides a method for analyzing the buoyancy reliability of raft foundations considering time-varying groundwater and extreme events, comprising the following steps: S1. Based on the physical model of the raft and the anti-buoyancy force analysis, establish the limit state model, which includes the limit state model with anchor bolts and the limit state model without anchor bolts.

[0037] like Figure 2 The physical model of the raft foundation shown depicts a structure where the foundation is typically located at the bottom of an underground structure (such as a basement, equipment room, or water tank). When the groundwater level rises, the groundwater exerts an upward buoyancy force on the foundation's bottom surface. If this buoyancy force exceeds the structure's self-weight and the downward pressure generated by the superstructure load, the structure may experience overall uplift (buoyancy failure) or partial stripping. A simplified diagram of the raft foundation's buoyancy stress analysis clearly illustrates the forces acting on the raft and their relationships.

[0038] The objective of constructing the limit state function is to compare the upward and downward resistance caused by the time-varying groundwater level over the next T years on a simulated raft slab (area A), and to statistically calculate the cumulative failure probability, specifically expressed as: ; ; In the above formula, Indicates the limiting state. Indicates downward resistance. Indicates upward buoyancy; when At that time, it was determined that the raft was damaged. This indicates that there is no anchor bolt to resist downward force. This indicates that there is downward resistance from the anchor bolt.

[0039] (1) In calculating the upward buoyancy When the base water pressure is simply equivalent to hydrostatic pressure as an upward force uniformly distributed over the base area, it is expressed as: ; In the above formula, Indicates the density of water. This indicates the calculated head height relative to the bottom of the raft. This indicates the area of ​​the raft deck.

[0040] The following formula can be used to calculate: ; ; ; In the above formula, The initial head, equivalent to the bottom plate of a raft, follows a normal distribution. Negative values ​​are allowed (indicating the water level is below the bottom of the raft), and then... deal with; Indicates the annual cycle fluctuation range; Indicates phase, This indicates the annual periodic change in water level (including the rainy season). It represents a long-term linear trend, indicating a slow upward trend (e.g., sea level rise or long-term groundwater recharge) or a downward trend. The values ​​are usually small (e.g., ±0.001–0.01 m / year) and can be fitted based on historical observations. Represents the stochastic correlation fluctuation term; This indicates water level changes during extreme events.

[0041] Groundwater changes exhibit inertial characteristics, and stochastic correlation fluctuations can capture both randomness and continuity. This represents the autocorrelation coefficient across months, set to be greater than 0 and less than 1. This indicates the noise amplitude at each step.

[0042] This represents the start time of the i-th extreme event, which falls randomly in any month. hour, ,when hour, ; This represents the total number of extreme events, which is randomly generated by the computer based on a Poisson process. This indicates the magnitude of the water level rise during each extreme event. This represents the attenuation coefficient, used to describe the rate at which groundwater levels recede after an extreme event ends.

[0043] Final decision This indicates that the water level cannot be lower than the bottom of the raft when calculating buoyancy.

[0044] (2) Downward resistance without anchor bolts Represented as: ; In the above formula, represents the self-weight of the raft, This indicates the effective downward gravity of the soil covering the upper part of the raft. This indicates the weight of the superstructure. Additional loads refer to gravity loads other than the building's own weight that act on the ground or foundation.

[0045] , , , They are represented as follows: 1) ; In the above formula, Indicates the specific weight of the raft foundation. This indicates the thickness of the raft slab.

[0046] 2) ; ; ; In the above formula, This indicates the saturation height, which is the thickness of the portion of the overburden above the raft that is submerged by groundwater. Indicates the soil cover depth, the total thickness of the soil cover from the bottom of the raft slab to the ground surface; Indicates the unit weight of the saturated soil cover; Indicates the unsaturated height, which is the thickness of the soil cover above the bottom of the raft that is not submerged by water; This indicates the dry weight of the soil covering.

[0047] 3) ; In the above formula, This represents the average vertical load transmitted from the superstructure to the raft slab.

[0048] 4) ; In the above formula, This indicates an additional uniformly distributed load on the ground, such as soil piles, vehicles, construction materials, etc.

[0049] Will , , , Substitute the calculation formula into From the calculation formula, we get The final model representation: .

[0050] (4) There is downward resistance from the anchor bolt. The calculation method is as follows: Since anti-buoyancy anchors increase pull-out resistance by transferring the pull-out force of the raft foundation to the underlying soil or anchoring structure, the key failure modes are defined as follows: 1. Anchor pull-out failure: The bond force or frictional resistance between the anchor and the soil reaches its limit, and the anchor is pulled out or slips; 2. Anchor member breakage: The anchor itself is not strong enough, and the member breaks; 3. Overall anchor body instability: Local soil failure or anchor body instability leads to overall failure; 4. Anchor corrosion and deterioration: The single anchor bearing capacity decreases over time; 5. Installation defects: Construction is not carried out according to the design, resulting in some anchors becoming ineffective (installation success rate <1%). The overall pull-out resistance of the anchor system is defined as the number of effective anchors multiplied by the current single anchor bearing capacity (decaying over time); expressed as: ; ; ; ; In the above formula, This indicates the total pull-out resistance of the anchor system; This represents the number of effective anchor bolts, which follows a Bernoulli distribution. This indicates the current bearing capacity of a single anchor bolt (which decreases over time). This represents the initial static bearing capacity of a single anchor bolt. This indicates the rate of decrease in the bearing capacity of a single anchor bolt. Indicates the number of anchor bolts designed. This indicates the success rate of installing a single anchor bolt. This represents the Bernoulli distribution.

[0051] S2. Using MATLAB software, based on the limit state models for the anchored and unanchored conditions obtained in step S1, obtain the cumulative failure probability for the anchored and unanchored conditions. The computational model was used to obtain simulation results of groundwater changes, and the sensitivity of statistical parameters was analyzed and ranked. Specifically, the following steps were included: S21. Define the simulation period, time step, and sample size. Set the statistical characteristics of random parameters such as raft foundation, overburden, load, groundwater level, extreme events, and anchor bolts.

[0052] S22. Generate N random variables for samples, perform big data sampling on material and load parameters, groundwater level model parameters, anchor system model parameters, etc., and calculate the number of effective anchors.

[0053] S23. Model calculation to simulate groundwater level changes; calculate buoyancy, overburden weight, raft slab self-weight, superstructure load, and total anchor resistance; determine the buoyancy failure conditions with and without anchor systems.

[0054] S24. Statistical results, through... Obtain the time-varying diagram of groundwater head simulation during the simulation process (e.g.) Figure 6 (as shown); then, for the case with anchor bolts, the total number of simulations was counted. Number of times, A positive value indicates anti-buoyancy failure, while a negative value indicates anti-buoyancy effectiveness. Dividing the number of failures by the total number of simulations yields the failure probability under the anchor bolt condition, generating a time-varying graph of the cumulative anti-buoyancy failure probability under the anchor bolt condition (e.g., ...). Figure 3 (as shown) and a sensitivity ranking chart of anchor bolt working condition parameters (as shown) Figure 4 (As shown); For the anchorless condition, the number of failures in the total number of simulations is counted, and the number of failures is divided by the total number of simulations to obtain the failure probability under the anchorless condition, forming a time-varying graph of the cumulative failure probability of the anchorless condition (as shown). Figure 3 (as shown) and the sensitivity ranking diagram of parameters in the anchorless working condition (as shown) Figure 5 (As shown).

[0055] The sensitivity ranking of parameters under both anchor bolt and non-anchor bolt conditions was obtained using existing methods, the specific methods of which will not be elaborated here.

[0056] The parameter settings for the simulation in this invention are shown in Table 1.

[0057] Table 1

[0058] S3. Analyze the time-varying graphs of cumulative failure probability of anti-buoyancy under anchor bolt conditions, anti-buoyancy under unanchor bolt conditions, parameter sensitivity ranking graphs under anchor bolt conditions, parameter sensitivity ranking graphs under unanchor bolt conditions, and groundwater head simulation time-varying graphs. For the time-varying graphs of cumulative failure probability of anti-buoyancy under anchor bolt conditions and unanchor bolt conditions, obtain the cumulative failure probability under the corresponding conditions. For the parameter sensitivity ranking graphs under unanchor bolt conditions, obtain the negative and positive correlation parameters under this condition. For the parameter sensitivity ranking graphs under anchor bolt conditions, obtain the negative and positive correlation parameters under this condition.

[0059] like Figure 3 As shown, in the case without anchor bolts, the cumulative failure probability at the end of 50 years reaches 0.12; in the case with anchor bolts, the cumulative failure probability at the end of 50 years is about 0.07, a relative reduction of 41%; anti-buoyancy anchor bolts can significantly reduce the probability of anti-buoyancy failure and are an efficient anti-buoyancy measure.

[0060] like Figure 4 As shown, the negatively correlated parameters (reducing the risk of failure) under anchor bolt conditions are: , These are core control parameters; positively correlated parameters (increase the risk of failure): , The impact is significant; when anchor bolts are present, priority should be given to ensuring the number of effective anchor bolts and controlling the long-term decay rate of the anchor bolts.

[0061] like Figure 5 As shown, in the case of no anchor bolts, the negatively correlated parameters (reducing the risk of failure) are: , The most significant impact comes from parameters such as [specific parameters]; positively correlated parameters (increase the risk of failure): , It is the main source of risk; when there are no anchor bolts, it is necessary to focus on controlling parameters such as the upper load and soil weight.

[0062] like Figure 6 As shown, the analysis results of the groundwater head simulation time-varying diagram are as follows: the groundwater level of 10 random samples over 50 years exhibits the characteristics of "high-frequency small-amplitude fluctuations + occasional extreme peaks", which highly restores the actual changes in groundwater level; the large and random fluctuations in groundwater level are the core cause of raft buoyancy failure.

[0063] The present invention has the following advantages: (1) It can more realistically reflect the long-term evolution, seasonal fluctuations and impact of extreme events of groundwater level; (2) It can quantify the probability of buoyancy failure of raft foundation during the whole life cycle; (3) It can simultaneously evaluate the buoyancy safety of anchorless and anchored working conditions; (4) It can identify key parameters affecting buoyancy safety and provide a basis for design optimization; (5) It is applicable to the buoyancy design and risk assessment of underground structures in complex groundwater environments.

[0064] Example The No. 9 office building on plot 2-002A of collective commercial construction land in Xihongmen Town, Daxing District, Beijing, has a raft foundation with a raft slab thickness of 0.4m and a soil cover thickness of 5.8m above the raft slab. The dry unit weight of the soil above the raft foundation is... saturated unit weight of soil The initial water head is 4.35m. Due to the hollow superstructure, the load transferred to the raft foundation in the atrium area is relatively low. After performing anti-buoyancy calculations based on the atrium area's anti-buoyancy calculation diagram, it was found that the actual water head in this area is close to the failure head. Without additional anti-buoyancy measures, there is a significant risk of anti-buoyancy failure after future groundwater level rises. It was decided to use anti-buoyancy anchors to reduce this risk. Specific details on the anchor construction are shown in the anti-buoyancy anchor construction diagram. After calculating the anchor bearing capacity, the initial static bearing capacity of a single anchor is 100KN. The initial anchor spacing is 1.9m. Taking a 100㎡ raft foundation as the study unit, 29 anchors are designed. Figure 7 A schematic diagram of the anti-buoyancy calculation conditions for the atrium area of ​​the office building, as shown below. Figure 8 The schematic diagram of the anti-buoyancy anchor structure is shown.

[0065] Using the research method of this invention, a parameter setting table is filled out according to the actual situation of this project. Relevant parameters are modified in the MATLAB code, and after running the code, the cumulative failure probability diagram of the Daxing project office building's anti-buoyancy system can be obtained (e.g., ...). Figure 9 (As shown). Analysis of the results shows that the cumulative failure probability at the end of 50 years was 0.305 for the unanchored condition and 0.0936 for the anchored condition. The installation of anchors reduced the failure probability by 69.31%, proving that anti-buoyancy anchors are a very effective measure to reduce the probability of anti-buoyancy failure.

[0066] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A method for analyzing the buoyancy reliability of raft decks considering time-varying groundwater and extreme events, characterized in that, Includes the following steps: S1. Establish limit state models, including limit state models for anchor bolt conditions and limit state models for non-anchor bolt conditions. The limit state model for the anchor bolt working condition is expressed as follows: ； The limit state model for the anchorless working condition is expressed as follows: ； ； In the above formula, Indicates the limiting state. This indicates that there is no anchor bolt to resist downward force. This indicates that there is downward resistance from the anchor bolt. Indicates the density of water. Indicates the area of ​​the raft foundation; Indicates the initial water head. Indicates the annual cycle fluctuation range. Indicates phase, Indicates a long-term linear trend. Represents the stochastic correlation fluctuation term. Indicates water level changes during extreme events; S2. Using MATLAB software, based on the limit state models for the anchored and unanchored conditions obtained in step S1, and through... The calculation model was used to obtain the time-varying diagram of groundwater head simulation, and then the time-varying diagram of cumulative failure probability of anti-buoyancy under anchor bolt condition, the parameter sensitivity ranking diagram under anchor bolt condition, the time-varying diagram of cumulative failure probability of anti-buoyancy under non-anchor bolt condition, and the parameter sensitivity ranking diagram under non-anchor bolt condition were obtained. S3. Analyze the time-varying diagram of cumulative failure probability of anti-buoyancy under anchor bolt conditions, the parameter sensitivity ranking diagram under anchor bolt conditions, the time-varying diagram of cumulative failure probability of anti-buoyancy under non-anchor bolt conditions, the parameter sensitivity ranking diagram under non-anchor bolt conditions, and the time-varying diagram of groundwater head simulation.

2. The raft buoyancy reliability analysis method considering time-varying groundwater and extreme events according to claim 1, characterized in that, Downward resistance without anchor bolts Specifically, it is calculated using the following formula: ； In the above formula, represents the self-weight of the raft, This indicates the effective downward gravity of the soil covering the upper part of the raft. This indicates the weight of the superstructure. This indicates an additional load.

3. The raft buoyancy reliability analysis method considering time-varying groundwater and extreme events according to claim 2, characterized in that, Raft's own weight Specifically, it is calculated using the following formula: ； In the above formula, Indicates the specific weight of the raft foundation. This indicates the thickness of the raft slab.

4. The raft buoyancy reliability analysis method considering time-varying groundwater and extreme events according to claim 2, characterized in that, Effective downward gravity of the soil covering the upper part of the raft Specifically, it is calculated using the following formula: ； ； ； In the above formula, Indicates saturation height. Indicates the depth of soil cover. Indicates the unit weight of the saturated soil cover; Indicates the unsaturated height. This indicates the dry weight of the soil covering.

5. The raft buoyancy reliability analysis method considering time-varying groundwater and extreme events according to claim 2, characterized in that, Superstructure self-weight Specifically, it is calculated using the following formula: ； In the above formula, This represents the average vertical load transmitted from the superstructure to the raft slab.

6. The raft buoyancy reliability analysis method considering time-varying groundwater and extreme events according to claim 2, characterized in that, Additional load Specifically, it is calculated using the following formula: ； In the above formula, This indicates an additional uniformly distributed load on the ground.

7. The raft buoyancy reliability analysis method considering time-varying groundwater and extreme events according to claim 1, characterized in that, , The results are obtained by calculating using the following formulas: ； ； In the above formula, This represents the autocorrelation coefficient across months. Indicates the noise amplitude at each step; Indicates the time when the i-th extreme event begins. hour, ,when hour, ; This represents the total number of extreme events. This indicates the magnitude of the water level rise during each extreme event. This represents the attenuation coefficient.

8. The raft buoyancy reliability analysis method considering time-varying groundwater and extreme events according to claim 1, characterized in that, Downward resistance of anchor bolts Specifically, it is calculated using the following formula: ； In the above formula, This indicates the total pull-out resistance of the anchor system; Indicates the number of effective anchor bolts. This indicates the current bearing capacity of a single anchor bolt.

9. The raft buoyancy reliability analysis method considering time-varying groundwater and extreme events according to claim 1, characterized in that, , Specifically, it can be derived from the following formula: ； ； In the above formula, This represents the initial static bearing capacity of a single anchor bolt. This indicates the rate of decrease in the bearing capacity of a single anchor bolt. Indicates the number of anchor bolts designed. This indicates the success rate of installing a single anchor bolt. This represents the Bernoulli distribution.

10. The raft buoyancy reliability analysis method considering time-varying groundwater and extreme events according to claim 1, characterized in that, In step S3, the time-varying diagram of the cumulative failure probability of the anti-buoyancy under the condition of anchor bolts is analyzed to obtain the cumulative failure probability under the simulation time under the condition of anchor bolts. The time-varying graph of cumulative failure probability of anti-buoyancy under the condition of no anchor bolts was analyzed to obtain the cumulative failure probability under the simulation time under the condition of no anchor bolts. The sensitivity ranking charts of parameters under the anchor bolt condition were analyzed to obtain the negative and positive correlation parameters under the anchor bolt condition. The sensitivity ranking charts of parameters under the non-anchor bolt condition were also analyzed to obtain the negative and positive correlation parameters under the non-anchor bolt condition.

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