A method and system for vertical compression test of photovoltaic pile foundation by rapid maintenance load method

Abstract

This invention discloses a rapid sustained load method and system for vertical compressive strength testing of photovoltaic (PV) pile foundations, belonging to the field of geotechnical engineering testing technology. Addressing the "short, small, and shallow" structural characteristics of PV piles, and utilizing the size effect of short drainage paths and rapid consolidation in shallow soil, this invention employs a minute-level rapid graded loading strategy. Each load level is maintained for a preset short time interval, and the next load level is applied without waiting for settlement stabilization. Settlement data is continuously acquired at high frequency, and the settlement-time curve is fitted using a hyperbolic or exponential model based on soil creep theory to extrapolate and calculate the equivalent stable settlement. This invention establishes a dynamic parameter correction mechanism, updating model parameters in real time based on previous load level data, and features a fully parallel safety monitoring logic. Once accelerated settlement is detected, it automatically switches to a slow-speed method mode. The core advantage of this invention lies in its output results being fully compatible with the current pile foundation testing standards (such as JGJ 106) judgment system: by directly replacing the traditional "measured stable settlement" with the extrapolated "equivalent stable settlement," a standard load-settlement curve is constructed, thus directly adopting the bearing capacity judgment criteria of the current standards without the need to establish new standards. This method, while ensuring testing accuracy, reduces the single-pile test time by more than 90%, making it particularly suitable for rapid acceptance testing of large-scale pile foundation projects such as photovoltaic power stations.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering testing technology, specifically to a method and system for static load testing of vertical compressive strength of foundation piles. Background Technology

[0002] In the construction of photovoltaic power plants, pile foundations play a crucial role in supporting photovoltaic modules. Compared with traditional engineering pile foundations such as those used in buildings and bridges, photovoltaic piles have significant structural characteristics, including small pile diameter (typically 100–300 mm), short pile length (generally 4–8 m), and shallow penetration depth (mostly 3–6 m). These structural characteristics determine that their stress mode differs from conventional pile foundations: due to the relatively small pile length and limited penetration depth, the load at the pile top is mainly transmitted through the skin friction of the shallow soil around the pile, with the end resistance accounting for a relatively small proportion. Photovoltaic projects are characterized by "huge numbers and tight schedules," with a single 100 MW project potentially requiring tens of thousands of pile foundations. Since the testing cycle is often short, extremely high demands are placed on the efficiency of pile foundation testing, requiring large-scale and rapid operations while ensuring reliable results.

[0003] 1. Existing mainstream technologies and their limitations Currently, domestic static load tests for vertical compressive strength of foundation piles mainly follow the slow-maintained load method in the "Technical Specification for Testing of Building Foundation Piles" (JGJ 106-2014). The key technical points of this method are: dividing the estimated ultimate bearing capacity into 8 to 10 levels and applying them step by step; after each level of loading, measuring the pile top settlement at time intervals of 5 min, 10 min, 15 min, 15 min, 15 min, and 30 min, and then measuring every 30 to 60 min until the settlement stabilizes before applying the next level of load. The stabilization standard is usually a settlement rate of less than 0.1 mm / h.

[0004] From a soil mechanics perspective, the theoretical basis of the slow-maintained load method lies in the fact that pile top settlement consists of two parts: instantaneous elastic deformation and creep deformation over time (i.e., consolidation and secondary consolidation). For low-permeability soil layers such as cohesive soil, the dissipation of excess pore water pressure takes a relatively long time. If the loading is too rapid, the pore water pressure will not dissipate sufficiently, resulting in an underestimation of the settlement and an artificially high bearing capacity. Therefore, the slow method ensures that the excess pore water pressure under each load level is basically dissipated by "waiting for settlement to stabilize," allowing the pile side friction and pile end resistance to be fully utilized, thereby obtaining a true load-settlement relationship.

[0005] However, this method suffers from a significant efficiency bottleneck, typically requiring 10 to 20 hours, or even longer, to complete the full loading cycle for a single pile. For the specific application scenario of photovoltaic pile foundations—characterized by a massive number of piles, relatively low individual pile bearing capacity, and tight deadlines—using the slow method could result in months of pile foundation testing for a large-scale photovoltaic project, severely impacting the overall project schedule. This "precision first, efficiency compromise" approach clashes sharply with the large-scale, fast-paced demands of photovoltaic projects.

[0006] 2. Current Status and Limitations of the Rapid Load Maintenance Method The core feature of the rapid sustained load method is that after each load level is applied, settlement is measured at short time intervals. When the settlement rate reaches a certain convergence criterion, the next load level is applied without waiting for the strict "0.1 mm / h" stability criterion.

[0007] Currently, the rapid load maintenance method is mainly applied to special scenarios with complex environments such as offshore wind power and water transport projects. These projects often face complex conditions such as marine environments and water erosion, and have high requirements for the efficiency of detection work. Therefore, the rapid method is adopted as an alternative.

[0008] However, existing rapid load maintenance methods still have the following limitations: (1) The loading time is still too long: the current rapid load maintenance method generally maintains each load for no less than 30 minutes. It still takes at least 3 to 5 hours to complete a single pile test. For photovoltaic projects with tens of thousands of piles, the efficiency improvement is limited.

[0009] (2) The correction method is highly empirical: it relies heavily on regional empirical coefficients for bearing capacity correction, lacks a general mathematical model based on creep theory, and its applicability to different regions and soil conditions is difficult to guarantee.

[0010] (3) Limited scope of application: Existing rapid methods are mainly developed for special engineering environments such as offshore wind power and water transport. They lack targeted research on the structural characteristics of photovoltaic piles, such as "small diameter, short pile length, and shallow burial depth", and fail to make full use of their size effect advantages.

[0011] In summary, while the existing slow-maintaining load method is accurate and reliable, it is inefficient and cannot meet the needs of large-scale testing of photovoltaic piles. On the other hand, although the existing fast-maintaining load method has been applied in some special scenarios, the loading time is still too long and the correction method is highly empirical, which also cannot meet the minute-level rapid testing requirements of photovoltaic projects.

[0012] 3. Theoretical Feasibility Analysis of Rapid Loading of Photovoltaic Piles The photovoltaic piles involved in this application have the structural characteristics of small diameter, short pile length, and shallow burial depth, which makes it possible to achieve rapid testing within minutes. Specifically, for shallowly buried small-diameter piles like photovoltaic piles, the drainage path of the soil around the pile is short, the radial drainage distance is only the pile radius, and the vertical drainage distance is the burial depth. Compared with large-diameter deep and long piles, the dissipation time of excess pore water pressure in photovoltaic piles is shortened geometrically, and the consolidation completion time is only 1 / 10 to 1 / 20 of that of conventional pile foundations. Theoretical estimates show that within a minute-level loading time, the excess pore water pressure in the shallow soil has basically dissipated, and the pile side friction is fully developed. Moreover, for shallowly buried small-diameter piles, due to the small load volume and low stress level of the soil around the pile, the proportion of creep deformation to total settlement is usually less than 20% to 30%, which is much lower than that of deep and long piles. According to creep mechanics theory, when the creep ratio is small, the settlement-time curve shows strong convergence in the early stage of loading, which conforms to the mathematical characteristics of hyperbolic or exponential models. This makes the extrapolation prediction after short-term observation highly reliable.

[0013] Furthermore, from the perspective of pile-soil interaction mechanism analysis, small-diameter piles have limited shear zone thickness on the pile side, small soil volume under load, and high stress concentration. After the soil enters the plastic state, deformation development is more regular, and its ultimate bearing capacity is mainly controlled by the instantaneous strength parameters of the soil, with little time effect. This provides favorable conditions for directly extrapolating the stable settlement under rapid loading. Moreover, photovoltaic piles are shallowly embedded, and the core stress characteristics of shallow soil are extremely low confining pressure and short drainage paths. Whether it is the dilatational property of overconsolidated soil or the rapid drainage characteristics of normally consolidated soil, its bearing capacity exhibits a significant rate effect. That is, the faster the loading rate, the more the soil can mobilize its peak strength, while slow loading may underestimate its true bearing capacity due to stress redistribution or suction attenuation.

[0014] In summary, by employing rapid loading and high-frequency sampling, the entire process of pile-soil failure under instantaneous loads can theoretically be captured more accurately.

[0015] Therefore, existing technologies urgently need to establish a rapid sustained load method that is tailored to the structural characteristics of photovoltaic piles, has a shorter loading time, and has a clear theoretical basis, while ensuring the reliability of test results. This method should not only significantly shorten the test time but also be compatible with the current national standards for bearing capacity assessment. Summary of the Invention

[0016] The purpose of this invention is to provide a rapid sustained load method and system for photovoltaic pile foundations based on time effect correction. While ensuring the accuracy of bearing capacity determination, it shortens the single pile test time to within 1 hour, and the output results can be directly used for project acceptance.

[0017] Specifically, the first aspect of this application discloses a method for static load testing of vertical compressive strength of foundation piles, characterized in that the method includes the following steps: S1 Experimental Design Based on the design parameters of the test pile, including pile diameter, pile length, penetration depth and surrounding soil quality, its estimated ultimate bearing capacity is determined, and the number of load levels and the load values ​​of each level are set accordingly. S2 quickly loads data in tiers and acquires data in real time. According to the test designed in step S1, vertical loads are applied to the test pile in stages. The duration of each load is fixed. During the duration of the load, regardless of whether the settlement at the top of the pile reaches the stability standard, the next load is applied on time. At the same time, the settlement data at the top of the pile is continuously collected and recorded at high frequency throughout the entire duration of each load, thereby generating a high-resolution settlement-time curve under that load. Extrapolation of S3 equivalent stable settlement Based on the soil creep theory, the settlement-time curve is fitted using a preset nonlinear mathematical model, and the equivalent stable settlement under this load level is extrapolated based on the fitting results. S4 Bearing Capacity Judgment The load-settlement curves were plotted for each load level and its corresponding equivalent stable settlement, and the ultimate bearing capacity of the test piles was determined.

[0018] In a preferred embodiment, the loading levels in step S1 can be set to 8 to 10 levels, referring to the slow method, and ensuring that the maximum loading amount is not lower than the estimated ultimate bearing capacity. Since the equivalent stable settlement obtained by this invention is physically equivalent to the stability limit value measured by the slow method, no additional empirical conversion coefficient needs to be introduced when determining the ultimate bearing capacity of the test pile, thus ensuring the legal compliance of the test results.

[0019] In a preferred embodiment, the preset short time interval in step S2 is 2 to 5 minutes; In a preferred embodiment, in step S3, the sampling frequency of the high-frequency sampling mode is not less than 1 Hz; and the acquisition accuracy of the pile top settlement data is not less than 0.01 mm.

[0020] In a preferred embodiment, the nonlinear mathematical model in step S3 includes a hyperbolic model and an exponential model.

[0021] In a preferred embodiment, the expression for the hyperbolic model is as follows: in, for t Settlement at any given time To calculate the initial settlement at the moment of loading. a ,b The parameters to be fitted; further, the equivalent steady-state settlement. We obtain the following by extrapolation: In a preferred embodiment, the expression for the exponential model is as follows: in, for t Settlement at any given time To calculate the initial settlement at the moment of loading. For equivalent stable settlement, τ The time constant is used; the parameters are solved directly through curve fitting. .

[0022] In a preferred embodiment, step S3 further includes a dynamic correction process: based on the measured settlement-time data of the previous several load levels, the empirical parameters (such as those of the hyperbolic model) in the model are corrected in real time through inversion. b Value or exponential model τ The updated parameters are then used to predict the equivalent stable settlement for subsequent load levels, allowing the model to adapt to changes in site soil properties. The mathematical essence of dynamic correction is to minimize the sum of squared residuals between the measured values ​​and the model's predicted values, which can be achieved using nonlinear optimization algorithms such as the Levenberg-Marquardt algorithm.

[0023] In a preferred embodiment, step S2 further includes the following sub-steps: At least one control pile is selected at the same test site, and the slow sustained load method is used to conduct the test. The site empirical conversion relationship between the measured stable settlement value of the slow method and the equivalent stable settlement of the fast method is established, which is used to calibrate the nonlinear mathematical model or verify the reliability of the method.

[0024] In a preferred embodiment, the method further includes step S6: security monitoring and anomaly handling; During the test, the settlement rate under each load level was monitored in real time. If the settlement rate exceeded the preset threshold or showed an accelerating settlement trend, the following actions were taken immediately: (1) Pause the fast loading program and keep the current load constant; (2) Automatically switch to slow maintenance load mode to extend the maintenance time of the current load level until the settlement reaches the traditional stability standard; (3) Judgment logic: If the settlement converges after switching, the subsequent load level can choose to restore the fast loading mode or continue to use the slow mode; if the settlement still does not converge after switching, the test is terminated and the pile is judged to be unqualified.

[0025] A second aspect of the present invention discloses a rapid sustained load testing system, characterized in that the system comprises an automatic loading module, a high-frequency data acquisition module, a real-time correction and analysis module, a judgment output module, and a safety monitoring module, wherein: The automatic loading module is configured to apply graded loads to the test piles according to a preset program and control the duration of each load. The high-frequency data acquisition module is connected to the displacement sensor at the top of the pile and is configured to continuously acquire pile top settlement data at a frequency of not less than 1 Hz. The real-time correction analysis module has a built-in nonlinear mathematical model algorithm, and uses the algorithm to fit and extrapolate the equivalent stable settlement in real time based on the collected settlement data, and performs dynamic updates of the model parameters. The judgment output module is configured to draw a load-settlement curve based on the equivalent stable settlement and output the bearing capacity judgment result according to the standard rules. The safety monitoring module is configured to monitor the settlement rate in real time and send a command to the automatic loading module when an abnormal trend is detected, triggering a switch to slow loading mode or an emergency shutdown.

[0026] The main advantages of this invention are: (1) Efficiency breakthrough: The single pile test time has been shortened from 10 to 20 hours in the traditional slow method to within 1 hour, and the testing efficiency has been improved by more than 90%, which meets the schedule requirements for large-scale testing of photovoltaic projects.

[0027] (2) Highly targeted: It fully considers the structural characteristics of photovoltaic piles, such as "small diameter, short pile length, and shallow burial depth", and utilizes its characteristics of small creep deformation ratio and obvious size effect to make rapid loading more applicable.

[0028] (3) Sufficient theoretical basis: Based on creep theory, a hyperbola / exponential correction model is established, so that the extrapolation of the equivalent stable settlement under rapid loading has a clear physical meaning and mathematical basis.

[0029] (4) Good compatibility: By correcting the time effect, the rapid loading result is mapped to the equivalent stable settlement, and the bearing capacity judgment rules of the current specifications can be directly used without the need to formulate new judgment standards, which facilitates project acceptance.

[0030] (5) Accuracy Adaptation: The introduction of dynamic parameter inversion based on measured data enables the correction model to be continuously optimized during the loading process, thereby improving prediction accuracy and site adaptability.

[0031] (6) Safe and reliable: Real-time monitoring of settlement anomalies and automatic switching to slow method verification function to ensure the safety of the detection process.

[0032] (7) High degree of automation: The system integrates loading, data acquisition, analysis and judgment, reducing manual intervention and making it suitable for batch testing.

[0033] The specification of this application contains numerous technical features distributed across various technical solutions. Listing all possible combinations of these technical features (i.e., technical solutions) would make the specification excessively lengthy. To avoid this problem, the various technical features disclosed in the above-described invention, the various technical features disclosed in the following embodiments and examples, and the various technical features disclosed in the accompanying drawings can be freely combined to form various new technical solutions (all of which are considered to have been described in this specification), unless such a combination of technical features is technically infeasible. For example, one example discloses feature A+B+C, and another example discloses feature A+B+D+E. Features C and D are equivalent technical means that serve the same function, and technically only one needs to be used; they cannot be used simultaneously. Feature E can technically be combined with feature C. Therefore, the solution A+B+C+D should not be considered as described because it is technically infeasible, while the solution A+B+C+E should be considered as described. Attached Figure Description

[0034] Figure 1 This is a flowchart illustrating a method according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the module configuration of a rapid load maintenance method test system according to an embodiment of the present invention; Figure 3 This is a diagram showing the fitting effect of a hyperbola model according to an embodiment of the present invention, which uses a hyperbola model as a nonlinear mathematical model.

[0035] Figure 4 This is a graph showing the fitting effect of an exponential model according to an embodiment of the present invention, which uses an exponential model as a nonlinear mathematical model.

[0036] Figure 5 This is a comparison diagram of the load-settlement curves according to an embodiment of the present invention. Detailed Implementation

[0037] Through meticulous and in-depth research, the inventors of this invention have developed for the first time a rapid sustained load method for vertical compressive strength testing of photovoltaic pile foundations. Compared with existing technologies, the output results of this application are fully compatible with the current pile foundation testing standards (such as JGJ 106) judgment system: by directly replacing the traditional "measured stable settlement" with the extrapolated "equivalent stable settlement," a standard load-settlement curve is constructed, thus directly adopting the bearing capacity judgment criteria of the current standards without the need to establish new standards. While ensuring testing accuracy, the single-pile test time is shortened by more than 90%, making it particularly suitable for rapid acceptance testing of large-scale pile foundation projects such as photovoltaic power plants.

[0038] Example 1 The first embodiment of this invention is an application of the hyperbolic model, specifically applied to a photovoltaic power station. The general overview of the photovoltaic power station project is as follows: The photovoltaic power station is located in a coastal tidal flat area, where the soil layers are mainly silty clay and silty sand. The pile foundation uses prestressed concrete pipe piles with a diameter of 300 mm, a length of 6 m, and an embedment depth of 5 m. The estimated ultimate vertical compressive bearing capacity of a single pile is 250 kN. 200 engineering piles need to be tested, and the project is required to be completed within 20 days.

[0039] In this embodiment, three piles were selected as control piles. A slow, sustained load method was first used to obtain a standard load-settlement curve for reference. The remaining 197 piles were then tested using the method of this invention for rapid testing.

[0040] The implementation process of the method of the present invention is as follows: S1 Experimental Design Based on the design parameters of the test pile, including pile diameter, pile length, penetration depth, and surrounding soil conditions, its estimated ultimate bearing capacity was determined, and the number of load levels and the load values ​​for each level were set accordingly. In this embodiment, based on the estimated ultimate bearing capacity, the loading levels were set to 9 levels: 50 kN, 75 kN, 100 kN, 125 kN, 150 kN, 175 kN, 200 kN, 225 kN, and 250 kN (the maximum load is 1.0 times the estimated bearing capacity). The duration of each load level was 3 minutes. This load grading takes into account the small diameter of the photovoltaic pile, and the load increment is relatively uniform, which facilitates subsequent curve analysis.

[0041] S2 quickly loads data in tiers and acquires data in real time. According to the test designed in step S1, vertical loads were applied to the test pile in stages, with a fixed maintenance time for each load stage. During this maintenance time, regardless of whether the pile top settlement reached the stability standard, the next load stage was applied on time. Simultaneously, pile top settlement data was continuously acquired and recorded at high frequency throughout the entire maintenance period of each load stage, thereby generating a high-resolution settlement-time curve for that load stage. In this embodiment, settlement data was acquired at a frequency of 2 Hz after each loading stage. Taking the 6th load stage (175 kN) as an example, the measured settlement-time data of a certain test pile are shown in Table 1. Table 1. Measured Settlement-Time Data of Test Piles under Level 6 Load (175 kN) Extrapolation of S3 equivalent stable settlement Based on the soil creep theory, the settlement-time curve is fitted using a preset nonlinear mathematical model, and the equivalent stable settlement under this load level is extrapolated based on the fitting results. This embodiment uses a hyperbolic model as a nonlinear mathematical model to fit the above data. The fitting process is as follows: First, the hyperbola model Transform into linear form: Pick =2.50 mm (settlement at time 0), calculate at each time point value: Table 2 Calculated value right t and Performing linear regression, we obtain: Right now a =4.3, b =0.228.

[0042] Substituting into the hyperbola model, the fitted curve is obtained as follows: The equivalent steady-state settlement is calculated as follows: The above fitting process and results are as follows: Figure 3 As shown in the figure, the dots represent measured data points, the solid line represents the hyperbolic model fitting curve, and the goodness of fit is... R 2 =0.998, indicating that the model matches the measured data well.

[0043] In this embodiment, after rapid testing, the pile was retested using the slow sustained load method. The measured stable settlement under level 6 load (175kN) was 7.0 mm. The relative error between the extrapolation result of the hyperbolic model of this invention and the measured value of the slow method is: The error is within 5%, which meets the engineering accuracy requirements.

[0044] S4 Bearing Capacity Judgment Load-settlement curves were plotted for each load level and its corresponding equivalent steady-state settlement, and the ultimate bearing capacity of the test pile was determined. Specifically, in this embodiment, according to the "Technical Specification for Testing of Building Foundation Piles" (JGJ 106-2014), for a gradually changing Qs curve, the following parameters were taken: s The load corresponding to 40 mm is the ultimate bearing capacity. This is determined by interpolation from the fitted curve. s =40 mm corresponds to a load of 238 kN, which means the ultimate vertical compressive bearing capacity of the pile is 238 kN.

[0045] Compared with the results of the slow-speed method for the control pile (235 kN), the error is 1.3%, which meets the engineering accuracy requirements (generally, an error within ±10% is allowed). A comparison of the Qs curve obtained by the method of this invention with the measured Qs curve of the slow-speed method is shown below. Figure 5 As shown in the figure, the two curves generally match, verifying the accuracy of the method of the present invention.

[0046] The rapid single-pile test in this embodiment took approximately 32 minutes (including loading preparation, data acquisition, and real-time analysis). The total testing time for 197 piles was approximately 105 hours, averaging 0.53 hours per pile. If the traditional slow sustained load method were used, with an average single-pile test time of 12 hours, 197 piles would require 2364 hours. The method of this invention improves efficiency by approximately 22.5 times, successfully meeting the 20-day project deadline.

[0047] To verify the reliability of the method of the present invention, eight test piles were randomly selected and retested using the slow method after the rapid test was completed and a rest period was observed. The bearing capacity results are shown in Table 3. Table 3 Comparison of Detection Results between the Fast and Slow Methods The average absolute error was 2.1%, and the maximum error was 3.4%, both within the allowable range for engineering applications, demonstrating the reliability of the method of this invention.

[0048] Furthermore, this embodiment also discloses a rapid sustained load test system applicable to the above method, characterized in that the system includes an automatic loading module, a high-frequency data acquisition module, a real-time correction and analysis module, a judgment output module, and a safety monitoring module, wherein: The automatic loading module is configured to apply graded loads to the test piles according to a preset program and control the duration of each load. The high-frequency data acquisition module is connected to the displacement sensor at the top of the pile and is configured to continuously acquire pile top settlement data at a frequency of not less than 1 Hz. The real-time correction analysis module has a built-in nonlinear mathematical model algorithm, and uses the algorithm to fit and extrapolate the equivalent stable settlement in real time based on the collected settlement data, and performs dynamic updates of the model parameters. The judgment output module is configured to draw a load-settlement curve based on the equivalent stable settlement and output the bearing capacity judgment result according to the standard rules. The safety monitoring module is configured to monitor the settlement rate in real time and send a command to the automatic loading module when an abnormal trend is detected, triggering a switch to slow loading mode or an emergency shutdown.

[0049] Example 2 The second embodiment of this invention further verifies the applicability of the exponential model in the method of this invention and compares the performance of the two models under different load levels. A second test pile (300 mm diameter, 6 m length, 5 m depth) was selected at the same site as in Embodiment 1 for a rapid sustained load test. The estimated ultimate bearing capacity of this pile was also 250 kN, and the loading stages were the same as in Embodiment 1 (9 stages, 50–250 kN), with each load stage maintained for 3 minutes. The above method was repeated while using the exponential model as the nonlinear mathematical model. The specific process is as follows: Taking the 8th load (225 kN, close to the estimated ultimate bearing capacity) as an example, the creep deformation ratio under this load level is significantly larger than that under the 6th load level, making it more suitable for verifying the fitting effect of the exponential model. Settlement data collected at a frequency of 2 Hz during the instant of loading and the subsequent 180 seconds are shown in Table 4. Table 4. Measured Settlement-Time Data of Test Piles under Level 8 Load (225 kN) An exponential model is used to perform a nonlinear fit on the above data. The model expression is as follows: in, For loading the instantaneous initial settlement ( t When =0), the data is directly taken from the measured data. =6.20 mm; For equivalent stable settlement, is the time constant, and the other two are the parameters to be fitted.

[0050] By fitting the data in Table 4 using a nonlinear least squares method (such as the Levenberg-Marquardt algorithm), we obtain: =22.5 mm, τ =55.0 s Goodness of fit R 2 =0.996, indicating that the exponential model is in high agreement with the measured data.

[0051] The above fitting process and results are as follows: Figure 4 As shown in the figure, the dots represent measured data points, the solid line represents the fitting curve of the exponential model, and the goodness of fit is... R 2 =0.996, the model and measured data agree well. The equivalent steady-state settlement can be directly obtained from the fitting results: mm After rapid testing, the pile was retested using the slow sustained load method. The measured stable settlement under level 8 load (225 kN) was 23.5 mm. The relative error between the extrapolation result of the exponential model of this invention and the measured value of the slow method is: The error is within 5%, which meets the engineering accuracy requirements.

[0052] To compare the applicability of the two models, a hyperbolic model was simultaneously applied to fit the same set of data. (The text then repeats the steps.) =6.20 mm, calculate at each time point The values ​​are then used to perform linear regression to obtain the parameters. a =8.2, b =0.063. The extrapolated equivalent steady-state settlement of the hyperbolic model is: Compared with the measured value of the slow method (23.5 mm), the error is 9.0%, which is higher than the 4.3% of the exponential model.

[0053] The above comparison shows that at high load levels (225 kN) where creep accounts for a large proportion, the exponential model has a better fitting accuracy than the hyperbolic model, demonstrating the advantage of the exponential model in describing the asymptotic characteristics of creep settlement. However, at medium load levels (175 kN, as shown in Example 1) where creep accounts for a smaller proportion, the hyperbolic model already achieves good results. Therefore, the method of this invention can flexibly select either the hyperbolic model or the exponential model based on the load level and soil creep characteristics, or simultaneously use both models for comparative verification at key load levels to further improve prediction reliability.

[0054] Example 3 In this embodiment, a safety monitoring module is set up during the experiment, which calculates the settlement rate in real time. v This embodiment sets a threshold for normal conditions. v <0.4 mm / min, warning threshold 0.4≤ v <0.5 mm / min, the abnormal threshold is v ≥0.5 mm / min or Δ v / Δ t >0 (accelerated settlement).

[0055] When an abnormal situation is detected, the system immediately performs the following operations: (1) Stop pressurizing and keep the current load constant.

[0056] (2) Switch the control logic to “slow mode” to extend the current load maintenance time until the settlement rate meets the traditional stability standard of <0.1mm / h.

[0057] (3) Decision branches: ① If the settlement converges (reaches stability) after an extended period of time, record the true stable value and allow the operator to choose to resume fast mode or continue slow mode for subsequent load levels.

[0058] ② If the settlement continues to fail to converge (e.g., if it remains unstable after more than 2 hours), the system will automatically unload and terminate the test, and the pile will be deemed unqualified.

[0059] This logic ensures the safety of the testing process under extreme geological conditions and avoids pile foundation damage caused by blind and rapid loading.

[0060] During normal testing, if only the warning threshold is triggered, the system will issue an audible and visual alarm and encrypt the sampling frequency to 5 Hz, but will not interrupt the loading process to enhance monitoring.

[0061] Example 4 To further optimize the loading parameters, this embodiment selected five photovoltaic piles for comparative tests at different sustaining times (2 min, 3 min, 4 min, 5 min, and 10 min). Using the slow-speed method results as a benchmark, the average error for each sustaining time was statistically analyzed. Table 5. Statistical table of results from comparative experiments with different maintenance times. Considering both accuracy and efficiency, this invention preferably uses a maintenance time of 3-5 minutes, which can control the time for a single pile to within 1 hour while ensuring an error of <5%. This embodiment uses a maintenance time of 3 minutes, achieving a good balance between accuracy and efficiency.

[0062] Example 5 To verify the advantages of small-diameter piles in rapid loading, this embodiment selected another control pile (simulating a conventional pile foundation) with a diameter of 500 mm and a length of 15 m in the substation area of ​​the site and conducted the same rapid method test. The estimated ultimate bearing capacity of this pile was 1200 kN, and the loading levels were 10 (from 120 kN to 1200 kN), with each level held for 3 minutes.

[0063] At a load level of 720 kN (approximately 0.6 times the ultimate bearing capacity), the equivalent settlement extrapolated using the rapid method is 12.3 mm, while the measured stable settlement using the slow method is 14.8 mm, with an error of 16.9%. In contrast, the average error for photovoltaic piles (300 mm diameter) under similar load ratios is only 4.9%. This comparison verifies that the method of this invention has better applicability to photovoltaic piles; that is, the "size effect advantage" of small-diameter piles under rapid loading makes their creep characteristics easier to extrapolate, resulting in higher prediction accuracy. Further analysis suggests that large-diameter piles have longer drainage paths and larger volumes of loaded soil, leading to more complete creep deformation development. Therefore, the extrapolation under rapid loading becomes more difficult, verifying the specificity and effectiveness of the method of this invention for photovoltaic piles.

[0064] Summarize Based on the above five embodiments, the following conclusions can be drawn: (1) Method effectiveness: The rapid sustained load method based on creep theory proposed in this invention can control the error of single pile bearing capacity determination within 5% by extrapolating the equivalent stable settlement through hyperbolic or exponential models under 3-minute loading conditions.

[0065] (2) Model selectivity: The hyperbolic model is suitable for medium load levels, while the exponential model performs better at high load levels. In practical applications, the model can be flexibly selected according to the load level, or the two models can be compared and verified at key load levels.

[0066] (3) Advantages of dynamic correction: Based on the measured data of the previous few levels, the model parameters can be inverted in real time, which can effectively track the changes in soil creep characteristics and improve the accuracy of subsequent predictions.

[0067] (4) Significantly improved efficiency: The test time for a single pile is reduced from 10 to 20 hours in the slow method to about 0.5 hours, and the testing efficiency is improved by more than 20 times, which fully meets the schedule requirements for large-scale testing of photovoltaic piles.

[0068] (5) Theoretical verification: The measured data confirmed the theoretical analysis of photovoltaic piles, such as "short drainage path, small creep ratio, and size effect advantage", which provided a solid theoretical basis for the method of the present invention.

[0069] It should be noted that in the invention documents of this patent, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element. In the invention documents of this patent, if it refers to performing an action according to an element, it means performing the action at least according to that element, including two cases: performing the action only according to that element, and performing the action according to that element and other elements. Expressions such as "multiple," "repeatedly," and "various" include two, two times, two kinds, and more than two, more than two times, and more than two kinds.

[0070] All documents mentioned in this invention are considered to be incorporated integrally into the disclosure of this invention so that they can serve as the basis for modifications if necessary. Furthermore, it should be understood that the above are merely preferred embodiments of this specification and are not intended to limit the scope of protection of this specification. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of one or more embodiments of this specification should be included within the scope of protection of one or more embodiments of this specification.

Claims

1. A static load test method for vertical compressive strength of photovoltaic pile foundations, characterized in that, The method includes the following steps: S1 Experimental Design Based on the design parameters of the test pile, including pile diameter, pile length, penetration depth and surrounding soil quality, its estimated ultimate bearing capacity is determined, and the number of load levels and the load values ​​of each level are set accordingly. S2 quickly loads data in tiers and acquires data in real time. According to the test designed in step S1, vertical loads are applied to the test pile in stages. The duration of each load is fixed. During the duration of the load, regardless of whether the settlement at the top of the pile reaches the stability standard, the next load is applied on time. At the same time, the settlement data at the top of the pile is continuously collected and recorded at high frequency throughout the entire duration of each load, thereby generating a high-resolution settlement-time curve under that load. Extrapolation of S3 equivalent stable settlement Based on the soil creep theory, the settlement-time curve is fitted using a preset nonlinear mathematical model, and the equivalent stable settlement under this load level is extrapolated based on the fitting results. S4 Bearing Capacity Judgment The load-settlement curves were plotted for each load level and its corresponding equivalent stable settlement, and the ultimate bearing capacity of the test piles was determined.

2. The method according to claim 1, characterized in that, The loading levels in step S1 can be set to 8 to 10 levels with reference to the slow method, and the maximum loading amount should not be lower than the estimated ultimate bearing capacity.

3. The method according to claim 2, characterized in that, The preset short time interval in step S2 is 2 to 5 minutes.

4. The method according to claim 1, characterized in that, In step S3, the sampling frequency of the high-frequency sampling mode is not less than 1 Hz; the acquisition accuracy of the pile top settlement data is not less than 0.01 mm.

5. The method according to claim 1, characterized in that, The nonlinear mathematical models in step S3 include: hyperbolic model and exponential model.

6. The method according to claim 5, characterized in that, The expression for the hyperbolic model is as follows: in, for t Settlement at any given time To calculate the initial settlement at the moment of loading. a , b The parameters to be fitted; further, the equivalent steady-state settlement. We obtain the following by extrapolation: 。 7. The method according to claim 2, characterized in that, The expression for the exponential model is as follows: in, Let be the settlement at time t. To calculate the initial settlement at the moment of loading. For equivalent stable settlement, τ The time constant is used; the parameters are solved directly through curve fitting. .

8. The method according to claim 1, characterized in that, Step S3 further includes a dynamic correction process: based on the measured settlement-time data of the previous load levels, the empirical parameters in the nonlinear mathematical model are corrected in real time using the least squares method; the updated parameters are used as prior values ​​for predicting the equivalent stable settlement of subsequent load levels, so as to realize the adaptive adjustment of the model to changes in site soil properties.

9. The method according to claim 1, characterized in that, Step S2 further includes the following sub-steps: At least one control pile is selected at the same test site, and the slow sustained load method is used to conduct the test. The site empirical conversion relationship between the measured stable settlement value of the slow method and the equivalent stable settlement of the fast method is established, which is used to calibrate the nonlinear mathematical model or verify the reliability of the method.

10. A rapid sustained load testing system applicable to the method described in any one of claims 1-9, characterized in that, The system includes an automatic loading module, a high-frequency data acquisition module, a real-time correction and analysis module, a judgment and output module, and a security monitoring module, wherein: The automatic loading module is configured to apply graded loads to the test piles according to a preset program and control the duration of each load. The high-frequency data acquisition module is connected to the displacement sensor at the top of the pile and is configured to continuously acquire pile top settlement data at a frequency of not less than 1 Hz. The real-time correction analysis module incorporates the nonlinear mathematical model algorithm described in any one of claims 1-9, and uses the algorithm to fit and extrapolate the equivalent stable settlement amount in real time based on the collected settlement data, and performs dynamic updates of the model parameters. The judgment output module is configured to draw a load-settlement curve based on the equivalent stable settlement and output the bearing capacity judgment result according to the standard rules. The safety monitoring module is configured to monitor the settlement rate in real time and send a command to the automatic loading module when an abnormal trend is detected, triggering a switch to slow loading mode or an emergency shutdown.

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