Device and method for measuring settling characteristic of calcareous sand

Abstract

The invention relates to the technical field of geotechnical engineering, and particularly provides a device for measuring calcareous sand sedimentation characteristics, which comprises a mounting seat, a test assembly, a leachate supply system and a monitoring system, the testing assembly comprises a testing container and a pressurizing mechanism; the leachate supply system comprises a liquid supply box, a liquid inlet pump communicated with the liquid supply box and a waste liquid box, the liquid inlet pump is communicated with a liquid inlet of the test container through a first pipeline, and a liquid outlet of the test container is communicated with the waste liquid box through a second pipeline; and the monitoring system obtains a corrected Mesri type creep model according to the change condition of the calcareous sand sample under the action of the load and the percolate with different hydrogen ion concentrations. The invention further provides a method for measuring the settling characteristic of the calcareous sand. According to the method, the settlement rule of the calcareous sand under the coupling action of the constant load and the acidification leachate corrosion can be simulated, the creep model with the prediction capability is constructed, and the settlement characteristics of the calcareous sand under the coupling action of the load and the corrosion can be described.

Description

Technical Field

[0001] This application relates to the field of geotechnical engineering technology, and in particular to an apparatus and method for determining the settling characteristics of calcareous sand. Background Technology

[0002] Calcareous sand is a special type of soil and rock material widely distributed in tropical and subtropical marine environments, with calcium carbonate (CaCO3) as its main mineral component. Due to its ease of acquisition, calcareous sand is often used as a subgrade or backfill material in coastal landfills, airports, roads, and other engineering projects. In specific engineering applications such as landfills, the environment of calcareous sand subgrades is extremely harsh and complex. On the one hand, the subgrade needs to withstand the overburden load generated by the accumulated waste above; on the other hand, as the waste degrades, landfills generate large amounts of complex acidic leachate. In this acidic environment, calcareous sand, with its calcium carbonate framework, undergoes severe chemical corrosion, leading to particle dissolution and degradation of its mechanical properties. Under the combined effects of leachate corrosion and overburden load, calcareous sand subgrades are highly susceptible to excessive settlement and instability.

[0003] Currently, research on the mechanical properties of calcareous sand mainly focuses on its physical and mechanical behavior in freshwater or seawater environments, while research on its long-term settlement characteristics under the corrosive effect of landfill leachate is still insufficient. Furthermore, traditional consolidation apparatus or compression apparatus are mainly used to simulate the compression behavior of soil under different loads, but they lack the function of simultaneously applying chemical corrosive liquids. This makes it impossible to reflect the settlement law of calcareous sand cushion layers under the coupled effect of landfill leachate corrosion and overlying loads, resulting in a lack of reliable theoretical support when designing and assessing the safety of landfills. Summary of the Invention

[0004] The purpose of this application is to provide an apparatus and method for determining the settling characteristics of calcareous sand, in order to solve the problem that the existing technology cannot simulate the settling law of calcareous sand under the coupled effect of overlying load and acid leachate corrosion, which leads to a lack of reliable theoretical support for the design and safety assessment of landfills.

[0005] To achieve the above objectives, this application provides an apparatus for determining the settling characteristics of calcareous sand, comprising: Mounting base; The test assembly, mounted on the mounting base, includes a test container for holding a calcareous sand sample and a pressurizing mechanism connected to the test container. The pressurizing mechanism is used to apply a load to the calcareous sand sample inside the test container, so that the height of the calcareous sand sample decreases. The test assembly is provided in multiple sets. A leachate supply system includes a supply tank, an inlet pump connected to the supply tank, and a waste tank. The inlet pump is connected to the inlet of the test container via a first pipe for supplying the prepared leachate to the test container. The outlet of the test container is connected to the waste tank via a second pipe. Control valves are provided on both the first and second pipes. A monitoring system connected to the pressurization mechanism; the monitoring system is configured to obtain a modified Mesri-type creep model based on the changes in the calcareous sand sample under load and leachate with different hydrogen ion concentrations.

[0006] Optionally, the pressurizing mechanism includes a weight, a lever, a bracket, and a loading rod; the weight is connected to one end of the lever, the other end of the lever is hinged to the bracket, and the bracket is fixedly connected to the mounting base; the upper end of the loading rod is hinged to the lever via a cylindrical pin, and the lower end of the loading rod is movably disposed within the test container.

[0007] Optionally, the pressurizing mechanism further includes a linear bearing, which is sleeved on the loading rod and fixedly connected to the mounting base.

[0008] Optionally, the monitoring system includes a control unit, a displacement sensor, and a data acquisition instrument electrically connected to the displacement sensor and the control unit; the displacement sensor is connected to the loading rod and is used to measure the initial height H0 of the calcareous sand sample and to continuously measure the height H(t) of the calcareous sand sample at time t at fixed intervals Δt, and transmit the measured information to the control unit through the data acquisition instrument.

[0009] Optionally, both the first and second pipes are equipped with pH display devices.

[0010] This application also provides a method for determining the settling characteristics of calcareous sand, using the apparatus described above for determining the settling characteristics of calcareous sand, comprising the following steps: S1: Form multiple groups of calcareous sand samples with the same initial height; S2: Obtain leachate from the landfill or prepare leachate according to the main components of the landfill acidification stage, and record the hydrogen ion concentration; S3: Monitor the changes of a portion of the calcareous sand samples under the combined action of a preset load and leachate with different hydrogen ion concentrations to obtain the corrosion correction coefficient; S4. Monitor the changes of another part of the calcareous sand sample under the action of leachate with different hydrogen ion concentrations to obtain the corrosion factor; S5. Based on the corrosion correction coefficient and the corrosion factor, the corrected Mesri-type creep model is obtained.

[0011] Optionally, step S3 includes: S31: Apply a preset load to the calcareous sand sample and inject leachate into the calcareous sand sample to ensure that the calcareous sand sample is completely immersed, and record the test start time. S32: Continuously record the height of the calcareous sand sample at fixed intervals, and calculate the settlement and axial strain of the calcareous sand sample based on the initial height and the height recorded at each current moment to obtain the axial strain-time curve. S33: Repeat steps S31-S32 multiple times, and in step S31, inject leachate with different hydrogen ion concentrations into the calcareous sand sample each time to obtain axial strain-time curves under different hydrogen ion concentrations, and further obtain corrosion correction coefficients.

[0012] Optionally, step S4 includes: S41: Inject leachate into the calcareous sand sample to ensure that the calcareous sand sample is completely immersed, and set multiple corrosion ages; S42: Unconfined compressive strength test is performed on uncorroded calcareous sand samples to measure the initial strength of the calcareous sand samples; at each corrosion age, a group of calcareous sand samples is taken out to perform unconfined compressive strength test and the strength value is measured. S43: Calculate the strength retention rate at each corrosion age as an experimental observation of the corrosion factor; S44: Based on each corrosion age and each experimental observation, a nonlinear least squares fit is performed on the function model of the corrosion factor to obtain the intensity degradation coefficient and degradation rate coefficient. S45: Repeat steps S41-S44 multiple times. In step S41, acidic percolate with different hydrogen ion concentrations is injected into the calcareous sand sample each time to obtain multiple sets of strength degradation coefficients and degradation rate coefficients under different hydrogen ion concentrations. Corrosion factors are obtained based on the multiple sets of strength degradation coefficients and degradation rate coefficients.

[0013] Optionally, the ambient temperature is kept constant during the experiment.

[0014] According to the above-mentioned apparatus and method for determining the settling characteristics of calcareous sand, this application has at least the following beneficial effects: This application, by setting up a test component, a monitoring system, and a leachate supply system, including a test container and a pressurization mechanism, can simulate the settling behavior of calcareous sand under the coupled effect of overlying load and acidic leachate corrosion, and can accurately measure the settling amount of calcareous sand under this coupled effect. At the same time, by analyzing and fitting the experimental data, a modified Mesri creep model with predictive capabilities is constructed, which can simulate the settling characteristics of calcareous sand under the coupled effect of load and corrosion, providing reliable theoretical support for the design and safety assessment of landfills.

[0015] Other features and advantages of this application will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing this application. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the device for measuring the settling characteristics of calcareous sand according to an embodiment of this application; Figure 2 yes Figure 1 AA cross-section view; Figure 3 yes Figure 1 Enlarged view of point B in the middle; Figure 4 This is a schematic diagram of the pressurization mechanism in an embodiment of this application.

[0017] Figure label: 1. Mounting base; 2. Test assembly; 21. Test container; 22. Pressurization mechanism; 221. Weight; 222. Lever; 223. Bracket; 224. Loading rod; 225. Cylindrical pin; 226. Linear bearing; 3. Monitoring system; 31. Displacement sensor; 32. Data acquisition instrument; 4. Leachate supply system; 41. Supply tank; 42. Inlet pump; 43. Waste tank; 44. First pipeline; 45. Second pipeline; 46. pH display. Detailed Implementation

[0018] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0019] In the description of this application, it should be understood that the terms "center", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0020] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0021] See Figures 1-4 As shown, a preferred embodiment of this application provides an apparatus for determining the settling characteristics of calcareous sand, including a mounting base 1, a testing component 2, a monitoring system 3, and a leachate supply system 4. The testing component 2 is mounted on the mounting base 1 and includes a testing container 21 for holding a calcareous sand sample and a pressurizing mechanism 22 connected to the testing container 21. Specifically, the testing container 21 is cylindrical and made of transparent glass to facilitate observation of changes in the sample height. The pressurizing mechanism 22 applies and maintains a constant load on the calcareous sand sample inside the testing container 21, causing the calcareous sand sample to settle. Multiple sets of the testing component 2 are provided, allowing for the application of loads to some calcareous sand samples while leaving others unloaded as needed for the experiment.

[0022] The leachate supply system 4 includes a supply tank 41, an inlet pump 42 connected to the supply tank 41, and a waste tank 43. The inlet pump 42 is connected to the inlet of the test container 21 via a first pipe 44, and is used to deliver the prepared leachate to the test container 21. The outlet of the test container 21 is connected to the waste tank 43 via a second pipe 45. Simultaneously with the injection of leachate, the test container 21 also discharges leachate through the outlet, achieving dynamic flow of the leachate. This ensures that the hydrogen ion concentration of the leachate in the test container 21 remains stable, preventing the hydrogen ion concentration from continuously decreasing as the chemical reaction proceeds. Preferably, both the first pipe 44 and the second pipe 45 are equipped with control valves (not shown in the figure), which are used to control the flow rate of the leachate.

[0023] Monitoring system 3 is connected to pressurizing mechanism 22. Monitoring system 3 is configured to obtain a modified Mesri-type creep model based on the changes in the calcareous sand sample under load and leachate with different hydrogen ion concentrations. Monitoring system 3 can measure and record test data in real time, and can automatically analyze and process the test data to obtain the relevant parameters of the modified Mesri-type creep model.

[0024] Based on the above technical solution, by setting up test component 2, monitoring system 3 and leachate supply system 4, test component 2 includes test container 21 and pressurization mechanism 22, which can simulate the settling law of calcareous sand under the coupled effect of overlying load and acidified leachate corrosion. At the same time, the test data is analyzed and fitted by monitoring system 3 to construct a modified Mesri creep model with predictive ability, providing reliable theoretical support for the design and safety assessment of landfills.

[0025] In some preferred embodiments of this application, such as Figure 2 , Figure 3 and Figure 4 As shown, the pressurizing mechanism 22 includes a weight 221, a lever 222, a bracket 223, and a loading rod 224. The weight 221 is mounted on one end of the lever 222, providing a load to the calcareous sand sample through the lever 222 and the loading rod 224. The load applied to the calcareous sand sample can be easily adjusted by increasing or decreasing the weight of the weight. The other end of the lever 222 is hinged to the bracket 223, which is fixedly connected to the mounting base 1. Therefore, the lever 222 can rotate about the axis hinged to the bracket 223. The upper end of the loading rod 224 is hinged to the lever 222 via a cylindrical pin 225. The lever 222 has an elongated connecting hole 2221 that engages with the cylindrical pin 225. Under the gravity of the weight 221, the rotation of the lever 222 can drive the loading rod 224 to move downwards in a straight line, providing a constant load to the calcareous sand sample. Preferably, a pressure block 2241 is provided at the lower end of the loading rod 224. The pressure block 2241 is detachably connected to the loading rod 224 for easy assembly. The pressure block 2241 is movable up and down inside the test container 21 and applies a load to the calcareous sand sample. The pressure mechanism 22 adopts a lever structure, which is simple in structure and low in cost.

[0026] Of course, the pressurizing mechanism 22 can also be a pneumatic pressurizing mechanism or a hydraulic pressurizing mechanism, and there is no limitation here.

[0027] In some preferred embodiments of this application, such as Figure 2 , Figure 3 and Figure 4 As shown, the pressurizing mechanism 22 also includes a linear bearing 226, which is sleeved on the loading rod 224 and fixedly connected to the mounting base 1. The linear bearing 226 ensures smoother movement of the loading rod 224, prevents deviation or wobbling, and improves test accuracy.

[0028] In some preferred embodiments of this application, such as Figure 1 and Figure 3As shown, the monitoring system 3 includes a control unit (not shown), a displacement sensor 31, and a data acquisition unit 32 electrically connected to the displacement sensor 31 and the control unit. The displacement sensor 31 is connected to the loading rod 224 and is used to measure the initial height H0 of the calcareous sand sample and continuously record the height H(t) of the calcareous sand sample at time t at fixed intervals Δt. The measured data is then transmitted to the control unit via the data acquisition unit 32. Specifically, a connecting arm 227 is fixedly installed on the loading rod 224. The connecting arm 227 abuts against the detection end of the displacement sensor 31. When the pressurizing mechanism 22 applies a load to the calcareous sand sample, the calcareous sand sample in the test container 21 descends, causing the loading rod 224 to descend as well. When the loading rod 224 moves the connecting arm 227 downward in a straight line, the displacement sensor 31 can detect the height of the calcareous sand sample in real time and transmit the measured information to the control unit via the data acquisition unit 32 for analysis and calculation.

[0029] In some preferred embodiments of this application, both the first pipe 44 and the second pipe 45 are equipped with a pH display 46 to display the hydrogen ion concentration of the leachate in real time, ensuring the accuracy of the test.

[0030] A preferred embodiment of the present invention also discloses a method for determining the settling characteristics of calcareous sand using the above-described apparatus for determining the settling characteristics of calcareous sand, comprising the following steps: S1: Take the dried calcareous sand, weigh and mix it according to the designed dry density and moisture content, and fill the test containers with the calcareous sand in several layers. Compact each layer to the predetermined height and level the sand surface, ultimately forming a final initial height of... Calcium sand sample.

[0031] S2: Obtain leachate from the landfill or prepare leachate according to the main components of the landfill acidification stage, and record the hydrogen ion concentration.

[0032] The main components of the acidification stage of landfills include ammonia nitrogen and Cl. - SO4 2- and Na + .

[0033] S3: Monitor the changes of a portion of the calcareous sand samples under the combined effects of load and leachate with different hydrogen ion concentrations (μ) to obtain the corrosion correction coefficient. .

[0034] S4: Monitor the changes in another portion of the calcareous sand samples under the action of leachate with different hydrogen ion concentrations (μ) to obtain the corrosion factor. This portion of the calcareous sand sample was not subjected to any load; only leachate was injected.

[0035] S5: According to , Thus, the modified Mesri-type creep model was obtained.

[0036] In some preferred embodiments of this application, step S1 includes: Take calcareous sand dried to constant weight and determine the design dry density based on the actual working conditions of the project. and optimum moisture content Calculate the required dry mass of calcium sand and the amount of water to be added based on the effective volume of the test container. Mix the mixture evenly to the predetermined moisture content using a spray method, and seal and let it stand for 24 hours to achieve moisture balance.

[0037] Calcareous sand was filled into the test container in 4-6 layers, with each layer being 1 / 4 to 1 / 6 of the total sample height. An automatic compactor was used to compact the sand to the predetermined height using a predetermined compaction force. After each layer was compacted, the sand surface was leveled with a ruler to ensure tight bonding between layers and uniform sample density. The final sample reached an initial height of... Columnar calcareous sand sample.

[0038] After the filling is completed, samples are taken at three points: the top, middle, and bottom of the sample, and the dry density is measured at each point. Moisture content and soil particle size This ensures that the physical properties of the sample meet the design requirements.

[0039] In some preferred embodiments of this application, step S2 includes: Simulated leachate was prepared based on typical components of landfill acidification stages, with the concentration range of each ion determined by referring to industry measured data. Analytical grade reagents were dissolved in deionized water for preparation. The initial pH value of the leachate was measured using a pH meter and then calculated using the formula... Calculate hydrogen ion concentration Determination was performed using ion chromatography. , The plasma concentration was recorded to ensure that the chemical composition of the leachate was consistent with that of the actual landfill acidification stage, and all initial chemical parameters were recorded.

[0040] In some preferred embodiments of this application, step S3 includes: S31: Apply a preset load to the calcareous sand sample Then, leachate was injected into the calcareous sand sample to ensure that the sample was completely immersed, and the start time of the test was recorded. ; After confirming that the test container was sealed by the sealing components and that the internal calcareous sand sample was prepared according to the preset specifications, the preset load was applied using the pressurization mechanism. The load is uniformly applied to the top surface of the calcareous sand sample, maintaining a stable load until the preset stress level is reached. Permeate of the corresponding hydrogen ion concentration is then slowly injected from bottom to top via the inlet pump of the permeate supply system, avoiding disturbance to the sample structure during injection. The injection continues until the permeate level is above the predetermined height above the top surface of the sample to ensure complete immersion. After re-checking the cavity for leaks, the start time of the test is recorded. .

[0041] S32: Monitored by system 3 at fixed intervals Continuously record the time of calcareous sand samples The altitude at that time and according to , Calculate the sedimentation of the calcareous sand sample. With axial strain Obtain axial strain -time Curve; Time Refers to the start time Initially, after n intervals... The time after that, where n is a natural number starting from 1.

[0042] Let the height of the sample at time t be... Settlement Defined as: ; Axial strain for: S33: Repeat steps S31-S32 multiple times, and in step S31, inject different hydrogen ion concentrations into the calcareous sand sample each time. Percolate with pH values ​​of 2, 4, 6, etc., was used to obtain multiple groups of different hydrogen ion concentrations. Axial strain under -time The curves were fitted to the parameters of the Mesri-type creep model under different hydrogen ion concentrations to further obtain the corrosion correction coefficient. .

[0043] Specifically, monitoring system 3 obtains - The curve was analyzed to separate the instantaneous strain. The creep strain was obtained. The process of separating instantaneous strain includes: Once loading is complete, immediately start the monitoring system to continuously collect strain data until the loading stabilizes. The strain value at time t is taken as the instantaneous strain. Based on the creep characteristics of soil and rock materials, the rapid elastic deformation and particle rearrangement deformation of calcareous sand under constant load are usually completed within 1 minute, with subsequent deformation mainly consisting of time-dependent creep. Therefore, [the following is a possible interpretation:] [The text then mentions selecting a time-dependent creep value, but the context is unclear.] As a separation point between instantaneous strain and creep strain, it can effectively eliminate the interference of rapid physical deformation on creep analysis, which is in line with the data processing principles of creep test in the "Standard for Geotechnical Testing Methods".

[0044] For any time creep strain .

[0045] In a corrosive environment, this embodiment uses the following modified Mesri-type creep model: ; in, For creep strain; This is the creep amplitude coefficient; This is a reference time (usually 1 minute). The corrected creep index; The basic creep index of uncorroded materials can be obtained through routine testing. This is a corrosion correction factor, which is related to the hydrogen ion concentration.

[0046] By fitting the creep strain to a power function and taking the natural logarithm of both sides of the equation, we obtain: Linear regression using the least squares method can be used to obtain the result. Then calculate using the following formula : After obtaining multiple sets of hydrogen ion concentrations Related corrosion correction factor Then, data fitting is performed to determine the corrosion correction coefficient. The specific form.

[0047] In some preferred embodiments of this application, step S4 includes: S41: Inject leachate into the calcareous sand sample to ensure complete immersion, and set multiple corrosion ages. ; Place the prepared calcareous sand sample in the test container, ensuring that the sample is placed flat and without gaps between it and the inner wall of the container. Slowly inject acidified permeate with the corresponding hydrogen ion concentration from bottom to top through the inlet pump of the permeate supply system. Control the flow rate during the injection process to avoid disturbing the sample particle structure until the permeate level is higher than the top surface of the sample by a predetermined height (e.g., 2 cm), ensuring that the sample is completely immersed in the permeate.

[0048] Check the sealing condition of the container components to ensure there is no leakage of leachate and maintain a static corrosion environment. At the same time, keep the test environment temperature constant at 20±2℃ (meeting the temperature requirements of the geotechnical test standard) to avoid temperature fluctuations affecting the corrosion reaction rate. According to the corrosion evolution monitoring requirements of the test design, preset multiple continuous and gradient-distributed corrosion ages such as 7 days, 14 days, 28 days, 60 days, and 90 days. Assign an independent sample group to each age to ensure that continuous corrosion degradation data can be obtained from the strength test at different time points.

[0049] S42: Unconfined compressive strength (UCS) test is performed on uncorroded calcareous sand samples to determine the initial strength of the calcareous sand samples. Reaching each predetermined age At that time, a set of calcareous sand samples were taken out for unconfined compressive strength (UCS) tests, and the strength values ​​were measured. Among them, the unconfined compressive strength (UCS) test is a routine test in the field of geotechnical engineering. For ease of understanding, a brief explanation is provided here: The experiment employed a computer-controlled electronic universal testing machine. The specimens were prepared as standard cylinders with dimensions identical to those used in the corrosion test. Before the test, both uncorroded and prematurely corroded specimens were placed in a constant temperature and humidity chamber for 24 hours to equilibrate and prevent moisture content fluctuations from affecting strength. Axial displacement control was used during loading, with a loading rate of 1 mm / min (compliant with GB / T50123-2019). Load-displacement curves were acquired in real time. Loading was stopped when the load reached its peak and then decreased to 80% of its peak value, or when the axial strain reached 20%. The maximum destructive load was recorded. The formula for calculating unconfined compressive strength is: ,in This represents the cross-sectional area of ​​the sample.

[0050] S43: Calculate the strength retention rate at each predetermined age using the following formula, as an experimental observation of the corrosion factor. ; S44: According to and The intensity degradation coefficient is obtained by performing nonlinear least squares fitting on the functional model of the corrosion factor. and degradation rate coefficient .

[0051] In this embodiment, the corrosion factor The following function model is used for calculation: This function is a classic saturated exponential decay function, which can well describe how performance starts from an initial value of 1, decays over time, and gradually approaches a stable value. The process.

[0052] Specifically, in terms of time As the independent variable, with As the dependent variable, the corrosion factor The intensity degradation coefficient is obtained by performing nonlinear least squares fitting on the function model. and degradation rate coefficient ; S45: Repeat steps S41-S44, injecting different hydrogen ion concentrations into the calcareous sand sample each time in step S41. Acidified percolate (e.g., pH=2, 4, 6, etc.) yields different hydrogen ion concentrations. Multiple sets of strength degradation coefficients and degradation rate coefficient And based on multiple sets of strength degradation coefficients and degradation rate coefficient Obtaining parametric formulas through data fitting and This allows us to determine the corrosion factor. The specific form.

[0053] In some preferred embodiments of this application, step S5 includes: Determine the corrosion correction coefficients separately and corrosive agents After obtaining the specific form, the modified Mesri-type creep model can be obtained as follows: in, In time and stress level Total axial strain below; This refers to the undrained shear strength. This is the initial tangent modulus of the material; The stress level represents the relative magnitude of the load. For destruction ratio; For reference time, it is usually taken as 1 minute; The hydrogen ion concentration of the acidified leachate; This is the corrosion correction factor; It is a corrosive agent; It represents the basic creep index for uncorroded materials.

[0054] This model can be used to describe the settling characteristics of calcareous sand under different leachate concentrations and different loads at different time scales.

[0055] The modified Mesri-type creep model overcomes the limitations of traditional geotechnical creep models that only consider mechanical loads or analyze chemical corrosion in isolation. By coupling the corrosion correction coefficient and the corrosion factor as two parameters, it constructs a three-in-one quantitative system of "chemical corrosion-mechanical load-time evolution," which can accurately capture the full-dimensional settlement characteristics of calcareous sand in the complex environment of landfills. Its implementation process is not a simple parameter superposition, but based on the physicochemical mechanism of calcareous sand from micro-particle deterioration to macro-sedimentation, it dynamically transforms the corrosion effect into the correction of mechanical parameters, and then incorporates the stress-strain-time relationship of the classical creep model, ultimately achieving a systematic description of the settlement law under different leachate concentrations, different load conditions, and different time scales.

[0056] The implementation path is explained in detail below from four dimensions: model coupling logic, concentration response mechanism, load synergy effect, and cross-timescale quantization.

[0057] 1) Model coupling logic: The service environment of calcareous sand in landfills exhibits significant coupling characteristics: the subbase is subjected to both constant pressure (mechanical load) from the overlying landfill and long-term immersion in acidic leachate (chemical corrosion). These two effects are not independent but rather form a synergistic effect where corrosion weakens the material's constitutive properties, and load amplification performance deteriorates, leading to deformation. The modified Mesri-type creep model, through precise definitions of corrosion correction coefficients and corrosion factors, achieves a quantitative description of this coupling effect. Its core logic is reflected in the dynamic correlation at three levels.

[0058] Firstly, there is the coupling of strength and stiffness due to corrosion degradation. The corrosion factor dynamically corrects the undrained shear strength and initial tangential modulus of calcareous sand according to a saturation exponential decay law. The main component of calcareous sand, calcium carbonate, reacts with hydrogen ions in the leachate, leading to the failure of the cementitious material on the particle surface and particle breakage. Its inherent strength and stiffness continuously decrease with prolonged corrosion time, and the higher the leachate concentration, the greater and faster the decrease. The model uses this parameter to transform the chemical degradation of microscopic particles into dynamic changes in macroscopic mechanical parameters. For example, under high-concentration leachate conditions, the strength retention rate of calcareous sand after long-term corrosion is only 20%–30% of the initial value, while under low-concentration conditions, it can still retain more than 80% of its strength. This difference is precisely quantified through the concentration-response law of the corrosion factor.

[0059] Secondly, there is the effective amplification coupling of stress levels. Stress level is a core parameter of the Mesri model, representing the ratio of the actual applied load to the material's failure strength. Because corrosion factors reduce the failure strength of calcareous sand, even with a constant external load, the relative load (effective stress level) experienced by the material continuously increases with the corrosion process. This amplification effect is the key reason why the settlement of calcareous sand under coupled action is significantly greater than that under a single load. For example, under conditions with an initial stress level of 0.5, under high-concentration corrosion, as strength degrades, the effective stress level may rise above 0.8, approaching the material's critical failure value. At this point, creep deformation will intensify dramatically. This closely matches the actual phenomenon in landfill engineering where the initial settlement of the subgrade is gradual, but the subsequent settlement is abrupt due to leachate accumulation and corrosion.

[0060] Finally, the corrosion-accelerated coupling of creep rate is discussed. The corrosion correction coefficient, by adjusting the creep exponent, quantifies the accelerating effect of the acidic environment on the creep rate of calcareous sand. In the uncorroded state, the creep exponent of calcareous sand reflects the time-dependent deformation rate under pure mechanical load; however, in the corrosive environment, the interparticle clamping force weakens due to surface dissolution, increasing the probability of particle breakage and leading to a significant increase in creep rate. The model uses this parameter to transform the microscopic mechanism of corrosion-induced reduction in particle slip resistance and intensified breakage into the slope change of the macroscopic creep curve. The creep rate under high-concentration leachate can reach 1.8–2.0 times that of the uncorroded state, and at low concentrations, it increases by 10%–20%, achieving a quantitative description of the difference in creep rate under different corrosion intensities.

[0061] It should be noted that the corrosion factor provides the "mechanical basis" for settlement deformation (strength degradation leads to an increase in effective stress), the corrosion correction coefficient provides the "rate guarantee" for deformation development (increased creep index leads to accelerated deformation), and the mechanical load acts as an "amplifier" of the coupling effect; the higher the stress level, the more significant the settlement increment caused by both. This logical design enables the model to realistically reproduce the service characteristics of calcareous sand in landfills, laying a theoretical foundation for the description of settlement under multiple working conditions.

[0062] 2) Leachate concentration response mechanism: Leachate concentration (characterized primarily by hydrogen ion concentration) is a key factor controlling the degree of corrosion in calcareous sand, and its impact on settling characteristics is quantified through the concentration response law of model parameters. The model, fitted with extensive experimental data, establishes a functional relationship between the corrosion correction coefficient and corrosion factor and hydrogen ion concentration. This allows for precise description of settling characteristics at different concentrations through parameter adjustments, based on a four-level mapping relationship of "concentration-microscopic degradation-macroscopic parameters-settling curve".

[0063] In a low-concentration leachate environment (corresponding to pH 5-6), the hydrogen ion concentration is low, and the corrosion of calcareous sand mainly manifests as slight dissolution of weak points on the particle surface, while the overall morphology and cementation of the particles remain largely intact. At this time, the strength degradation coefficient of the corrosion factor is only 0.1-0.2, meaning that the strength can still retain 80%-90% of its initial value after long-term corrosion; the degradation rate coefficient of the corrosion factor is small, and it takes 180-360 days for the strength to reach a stable state. The corresponding corrosion correction coefficient is 0.1-0.2, and the creep index is only 10%-20% higher than the uncorroded state. Reflected in the macroscopic settling curve, the settling of calcareous sand in a low-concentration environment exhibits a short rapid deformation phase and a low steady-state creep rate. The rapid deformation phase lasts only 1-3 days, followed by rapid entry into steady-state creep, and the long-term (1 year) settling amount only increases by 5%-15% compared to the uncorroded state. This pattern is highly consistent with the results of microscopic experiments, namely that low-concentration acid only causes slight erosion on the particle surface, the interlocking effect between particles is not significantly weakened, creep is still mainly due to particle rearrangement, and the deformation increment is limited.

[0064] In a high-concentration leachate environment (corresponding to pH 2-3), the hydrogen ion concentration is extremely high, causing calcareous sand particles to dissolve violently. The calcium carbonate crystal structure becomes loose and fragmented, and the cementing effect completely fails. Scanning electron microscopy observation shows that the surface porosity of the particles increases by 23.9%-41.7% compared to the uncorroded state, and coarse particles larger than 5 mm in diameter can break down under relatively low stress. Reflected in the model parameters, the strength degradation coefficient of the corrosion factor reaches 0.7-0.8, and the long-term strength retention rate is only 20%-30%; the degradation rate coefficient increases significantly, and the strength degradation stabilization time is only 60-90 days. The corrosion correction coefficient is 0.7-1.0, and the creep index increases by 70%-100% compared to the uncorroded state. The macroscopic settlement curves exhibit characteristics of a long rapid deformation phase, high steady-state creep rate, and large long-term settlement. The rapid deformation phase lasts 15-20 days, the steady-state creep rate is nearly twice that of the uncorroded state, and the long-term settlement is 80%-120% higher than that of the uncorroded state. Under some high-stress conditions, there is even a trend of prolonged steady-state creep phase. The essence of this phenomenon is that the high concentration of acid reduces the particle breakage threshold. The fine particles generated by the breakage continue to migrate and fill the pores under load, inducing continuous deformation. The model effectively quantifies the intensified deformation process of "corrosion-breakage-migration-recompression" through high-intensity parameter correction.

[0065] By understanding the parameter response patterns under this concentration gradient, the model achieves a precise mapping of "different leachate concentrations → different corrosion levels → different parameter combinations → different settling characteristics." Whether it is slight settling at low concentrations, moderate settling at medium concentrations, or significant settling at high concentrations, it can be quantitatively described through the corresponding parameter combinations, providing a reliable tool for predicting settling at different operational stages of landfills (where leachate concentration changes over time).

[0066] 3) Load synergy effect: The settling characteristics of calcareous sand are not solely determined by leachate concentration, but rather by the combined effects of concentration and load. The modified Mesri-type creep model, through the coordinated response of parameters and stress levels, achieves a quantitative description of the complex relationship between load-amplified concentration effects and concentration-regulated load effects. Crucially, the influence of corrosion parameters on settlement varies significantly under different stress levels. By accurately capturing these differences, the model reconstructs the settlement patterns under multiple operating conditions.

[0067] Under low stress levels (corresponding to stress levels of 0.2–0.4), the external load is relatively small, and the deformation of calcareous sand particles is mainly characterized by slippage and rearrangement, with little particle breakage. At this level, even with higher leachate concentrations, the increase in settlement caused by corrosion parameters is relatively limited. For example, in a low-concentration leachate environment, as the stress level increases from 0.2 to 0.4, the long-term settlement increases from 12 mm to 20 mm, an increase of only 8 mm; while in a high-concentration leachate environment, with the same increase in stress level, the long-term settlement increases from 22 mm to 38 mm, an increase of 16 mm, twice that of the low-concentration environment. The essence of this difference is that under low stress, the intergranular embedment force can still partially resist the slippage risk caused by corrosion, while high-concentration corrosion weakens the embedment force, significantly amplifying the driving effect of the load. The model accurately captures this amplification effect through dynamic correction of the effective stress level.

[0068] At medium stress levels (corresponding to stress levels 0.4–0.6), the load approaches the critical slip stress of calcareous sand. Corrosion-induced particle degradation significantly reduces slip resistance, making the synergistic effect of concentration and load most prominent. In the model, the degradation of the corrosion factor leads to a rapid increase in the effective stress level, while the corrosion correction coefficient accelerates the creep rate. Both factors contribute to a dual sensitivity of settlement to concentration and load. For example, in a medium-concentration leachate environment, the long-term settlement is 28 mm at stress level 0.4, increasing to 45 mm at stress level 0.6, an increase of 17 mm. In a high-concentration leachate environment, the long-term settlement is 38 mm at stress level 0.4, increasing to 65 mm at stress level 0.6, an increase of 27 mm. At this point, the coupling effect of high concentration and high load results in settlement far exceeding the sum of the effects of a single factor.

[0069] Under high stress levels (corresponding to stress levels of 0.6–0.8), when the load approaches or exceeds the particle breakage threshold of calcareous sand, the presence of corrosion accelerates particle breakage, generating a large number of fine particles that fill the pores, leading to significant compressive deformation. In the model, the degradation of the corrosion factor under high stress levels causes the effective stress level to approach or exceed 1.0. At this point, the term representing instantaneous elastoplastic strain in the model increases sharply, and the high corrosion correction coefficient causes creep strain to accumulate rapidly. For example, in a high-concentration leachate environment, the long-term settlement is 65 mm at stress level 0.6, increasing to 92 mm at stress level 0.8, an increase of 27 mm. Furthermore, the rapid deformation stage in the settlement curve is significantly prolonged, and the steady-state creep rate is nearly three times that at low stress levels. The physical essence of this phenomenon is that under high stress, corrosion causes the particle breakage stress threshold to drop from a higher value in the absence of corrosion to a lower level. A large number of particles undergo angular breakage, overall breakage, or grinding. The fine particles generated by this breakage fill the pores between particles, resulting in continuous compaction of the soil structure, which macroscopically manifests as a sharp increase in settlement. The model accurately quantifies the coupling law of "high stress-high corrosion-high breakage-high settlement" through the amplification effect of effective stress level and the acceleration effect of creep index.

[0070] Conversely, under the same load conditions, the regulating effect of leachate concentration on settlement also increases with increasing load level. At low stress levels, the difference in long-term settlement between low and high leachate concentrations is small, only about 10 mm; at medium stress levels, this difference widens to 17-20 mm; and at high stress levels, the difference further increases to 25-30 mm. This pattern indicates that the higher the load, the more significant the effect of concentration on settlement. The model, through the coordinated response of parameters and stress levels, accurately describes the intensity relationship of this load-regulated concentration effect, providing a quantitative basis for rationally controlling the subgrade load based on leachate concentration or optimizing anti-corrosion measures based on load levels in engineering design.

[0071] 4) Quantization across time scales: The settling process of calcareous sand exhibits a significant time dependence, with different dominant factors at different time scales. Short-term settling is dominated by instantaneous settlement, medium-term by decelerating creep and the early stages of steady-state creep, and long-term by the later stages of steady-state creep. The modified Mesri-type creep model achieves accurate quantification of settlement characteristics at different time scales through the time response of its parameters. Its key lies in the high degree of fit between the model structure and the time evolution law of calcareous sand.

[0072] Short-term settlement (0–1 day) is dominated by instantaneous settlement, primarily caused by elastic deformation and rapid rearrangement of particles, accounting for 26%–70% of total settlement. The model describes this stage through an instantaneous strain separation mechanism: after loading, the monitoring system rapidly acquires strain data within one minute, using the strain value one minute after loading stabilizes as the instantaneous strain. This setting is based on the creep characteristics of soil and rock materials, namely, the rapid elastic deformation and particle rearrangement of calcareous sand under constant load are typically completed within one minute, with subsequent deformation primarily driven by time-dependent creep.

[0073] It should be noted that instantaneous settling is not entirely unaffected by corrosion. Low-concentration corrosion increases particle surface roughness but decreases embedment strength, while high-concentration corrosion causes premature slight breakage of particles. Both factors increase instantaneous settling. In the model, this effect is reflected in the initial degradation of the corrosion factor in the short term: at the initial loading stage, the corrosion factor has an initial value of 1, and the instantaneous strain is determined by the parameters of the uncorroded state; however, as leachate rapidly penetrates (within 1 day), the corrosion factor begins to degrade slightly, decreasing to 0.95~0.98 at low concentrations and to 0.8~0.9 at high concentrations, resulting in a slight increase in the effective stress level and an increase in instantaneous settling of 3%~10% compared to the uncorroded state.

[0074] The intermediate timescale (1–90 days) is the main development stage of creep settlement, including the deceleration creep stage (1–30 days) and the initial steady-state creep stage (30–90 days), accounting for 20%–50% of the total settlement. The model captures the deformation patterns of this stage through a two-parameter time response synergy: In the deceleration creep stage, the creep rate decreases rapidly over time, mainly caused by interparticle slippage and rearrangement. The presence of a corrosion correction coefficient in the model reduces the rate of decrease in creep rate under high-concentration corrosion, thus prolonging the deceleration creep stage. This is mainly manifested in the deceleration creep stage lasting 10 days under uncorroded conditions, extending to 15 days under low-concentration corrosion, and extending to 20 days under high-concentration corrosion. This is because corrosion weakens the interparticle clamping force, prolonging the duration of particle slippage and rearrangement. The model, through a high-value correction of the creep exponent, makes the power-law creep strain growth rate faster in the initial stage, thereby quantifying the prolonged effect of the deceleration creep stage. After entering the initial stage of steady-state creep, the creep rate gradually stabilizes, mainly involving particle breakage and fine particle migration. The continuous degradation of the corrosion factor in the model leads to a continuous increase in the effective stress level, while the corrosion correction coefficient keeps the creep rate stable. The synergy between the two causes the settlement to continue to increase but the rate tends to be constant.

[0075] In the long-term (90–365 days and above), the late stage of steady-state creep predominates, with settlement accounting for 10%–25% of the total settlement. At this stage, corrosion gradually stabilizes, and creep deformation is mainly caused by continuous particle breakage and fine particle filling of pores. In the model, the corrosion factor basically reaches a stable value after 90 days, stabilizing at 0.8–0.9 at low concentrations, 0.5–0.7 at medium concentrations, and 0.2–0.4 at high concentrations. The effective stress level no longer increases significantly, and the increase in settlement is mainly dominated by creep strain. Since the corrosion correction coefficient remains constant, the creep strain increases at a stable rate, which is determined by the corrected creep exponent. This means that the higher the concentration, the faster the growth rate and the greater the long-term settlement.

[0076] It should be noted that the model's long-term quantification capability is not limited to one year; it can also predict settlement characteristics over longer timescales (5-10 years) through data extrapolation. After fitting parameters with one year of experimental data, the cumulative settlement after five years can be extrapolated based on the model's time evolution, providing crucial evidence for assessing the service life of calcareous sand cushion layers in landfills.

[0077] By quantitatively describing the short-term, medium-term, and long-term time scales, the modified Mesri-type creep model achieves full coverage of the calcareous sand settlement cycle. Whether it is the instantaneous deformation at the initial loading stage or the long-term steady-state creep, it can accurately capture the time response law of the parameters, thus solving the technical bottleneck of traditional models that can only predict a single stage in the short or long term.

[0078] In summary, the modified Mesri-type creep model constructs a complete description system of calcareous sand settlement characteristics through two-parameter coupling, concentration response, load synergy, and cross-timescale quantification. Its implementation mainly involves transforming the microscopic corrosion mechanism into a dynamic correction of macroscopic mechanical parameters, and then achieving a high degree of adaptation between the model structure and actual physicochemical processes, ultimately realizing multi-condition, full-cycle settlement quantification. The successful application of this model not only solves the technical bottleneck of traditional methods in simultaneously considering the coupling effects of corrosion and load, but also provides key technical support for foundation stability assessment and disaster risk prevention in projects such as landfills, possessing significant theoretical and engineering application value.

[0079] In some embodiments of this application, the ambient temperature is kept constant during the test of the method, so that the rate at which the leachate corrodes the calcareous sand sample remains stable and the test accuracy is not affected by changes in ambient temperature.

[0080] This application provides a preferred embodiment of an apparatus and method for determining the settling characteristics of calcareous sand, which, compared with the prior art: This application, by setting up a test component 2, a monitoring system 3, and a leachate supply system 4, can simulate the settling behavior of calcareous sand under the coupled effect of constant load and acidic leachate corrosion, and can accurately measure the time-varying settling amount of calcareous sand under this coupled effect. At the same time, by analyzing and fitting the experimental data, a modified Mesri creep model with predictive capabilities can be constructed, which can describe the settling characteristics of calcareous sand under the coupled effect of load and corrosion, providing reliable theoretical support for the design and safety assessment of landfills.

[0081] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and substitutions can be made without departing from the technical principles of this application, and these improvements and substitutions should also be considered within the scope of protection of this application.

Claims

1. An apparatus for determining the settling characteristics of calcareous sand, characterized in that, include: Mounting base (1); The test assembly (2) is mounted on the mounting base (1) and includes a test container (21) for holding a calcareous sand sample and a pressurizing mechanism (22) connected to the test container (21). The pressurizing mechanism (22) is used to apply a load to the calcareous sand sample in the test container (21) so that the height of the calcareous sand sample decreases. The test assembly (2) is provided with multiple sets. The leachate supply system (4) includes a supply tank (41), an inlet pump (42) connected to the supply tank (41), and a waste tank (43). The inlet pump (42) is connected to the inlet of the test container (21) through a first pipe (44) for supplying the prepared leachate to the test container (21). The outlet of the test container (21) is connected to the waste tank (43) through a second pipe (45). Control valves are provided on both the first pipe (44) and the second pipe (45). The monitoring system (3) is connected to the pressurizing mechanism (22); the monitoring system (3) is configured to obtain a modified Mesri-type creep model based on the changes of the calcareous sand sample under load and leachate with different hydrogen ion concentrations.

2. The apparatus for determining the settling characteristics of calcareous sand as described in claim 1, characterized in that, The pressurizing mechanism (22) includes a weight (221), a lever (222), a bracket (223), and a loading rod (224); the weight (221) is connected to one end of the lever (222), the other end of the lever (222) is hinged to the bracket (223), and the bracket (223) is fixedly connected to the mounting base (1); the upper end of the loading rod (224) is hinged to the lever (222) by a cylindrical pin (225), and the lower end of the loading rod (224) is movable up and down inside the test container (21).

3. The apparatus for determining the settling characteristics of calcareous sand as described in claim 2, characterized in that, The pressurizing mechanism (22) also includes a linear bearing (226), which is sleeved on the loading rod (224) and fixedly connected to the mounting base (1).

4. The apparatus for determining the settling characteristics of calcareous sand as described in claim 2, characterized in that, The monitoring system (3) includes a control unit, a displacement sensor (31), and a data acquisition instrument (32) electrically connected to the displacement sensor (31) and the control unit. The displacement sensor (31) is connected to the loading rod (224) and is used to measure the initial height H0 of the calcareous sand sample and to continuously measure the height H(t) of the calcareous sand sample at time t at fixed intervals Δt. The measured information is transmitted to the control unit through the data acquisition instrument (32).

5. The apparatus for determining the settling characteristics of calcareous sand as described in claim 1, characterized in that, Both the first pipe (44) and the second pipe (45) are equipped with pH display devices (46).

6. A method for determining the settling characteristics of calcareous sand, using the apparatus for determining the settling characteristics of calcareous sand as described in any one of claims 1-5, characterized in that, Includes the following steps: S1: Form multiple groups of calcareous sand samples with the same initial height; S2: Obtain leachate from the landfill or prepare leachate according to the main components of the landfill acidification stage, and record the hydrogen ion concentration; S3: Monitor the changes of a portion of the calcareous sand samples under the combined action of a preset load and leachate with different hydrogen ion concentrations to obtain the corrosion correction coefficient; S4. Monitor the changes of another part of the calcareous sand sample under the action of leachate with different hydrogen ion concentrations to obtain the corrosion factor; S5. Based on the corrosion correction coefficient and the corrosion factor, the corrected Mesri-type creep model is obtained.

7. The method for determining the settling characteristics of calcareous sand according to claim 6, characterized in that, Step S3 includes: S31: Apply a preset load to the calcareous sand sample and inject leachate into the calcareous sand sample to ensure that the calcareous sand sample is completely immersed, and record the test start time. S32: Continuously record the height of the calcareous sand sample at fixed intervals, and calculate the settlement and axial strain of the calcareous sand sample based on the initial height and the height recorded at each current moment to obtain the axial strain-time curve. S33: Repeat steps S31-S32 multiple times, and in step S31, inject leachate with different hydrogen ion concentrations into the calcareous sand sample each time to obtain axial strain-time curves under different hydrogen ion concentrations, and further obtain corrosion correction coefficients.

8. The method for determining the settling characteristics of calcareous sand according to claim 6, characterized in that, Step S4 includes: S41: Inject leachate into the calcareous sand sample to ensure that the calcareous sand sample is completely immersed, and set multiple corrosion ages; S42: Unconfined compressive strength test is performed on uncorroded calcareous sand samples to measure the initial strength of the calcareous sand samples; at each corrosion age, a group of calcareous sand samples is taken out to perform unconfined compressive strength test and the strength value is measured. S43: Calculate the strength retention rate at each corrosion age as an experimental observation of the corrosion factor; S44: Based on each corrosion age and each experimental observation, a nonlinear least squares fit is performed on the function model of the corrosion factor to obtain the intensity degradation coefficient and degradation rate coefficient. S45: Repeat steps S41-S44 multiple times. In step S41, acidic percolate with different hydrogen ion concentrations is injected into the calcareous sand sample each time to obtain multiple sets of strength degradation coefficients and degradation rate coefficients under different hydrogen ion concentrations. Corrosion factors are obtained based on the multiple sets of strength degradation coefficients and degradation rate coefficients.

9. The method for determining the settling characteristics of calcareous sand according to any one of claims 6-8, characterized in that, The ambient temperature is kept constant during the experimental process of the method described.

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