A test device and method for permeable pavement coarse sand cushion infiltration characteristics

Abstract

The application discloses a water-permeable paving coarse sand cushion infiltration characteristic test device in the field of geotechnical engineering test, which comprises a test cylinder body, which is a detachable multi-section structure, is internally used for filling a test sample, is provided with a water-permeable structure at the bottom and is internally provided with a flow resistance structure; a rainwater simulation assembly arranged at the top of the test cylinder body; a data acquisition module arranged on the test cylinder body and used for collecting moisture content data at different depths in the test cylinder body; and a prediction method. The application has the beneficial effects that: the test cylinder body is arranged in a split type, adjacent cylinder bodies are connected through flanges, a flow resistance groove is arranged in the cylinder body to prevent the formation of the inner wall flow, the filling and sampling of the filler are more convenient, the influence of the inner wall flow is avoided, and the test precision is higher; the prediction method can couple the influences of gradation, compaction degree, rainfall intensity and rainfall times, quickly obtain the saturated permeability coefficient under multiple working conditions, and the prediction result is in line with the actual service state.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering testing, specifically to a test device and method for the infiltration characteristics of permeable coarse sand cushion layers. Background Technology

[0002] In the context of sponge city construction, permeable pavement, with its rainwater infiltration, retention, and absorption capabilities, has become a core infrastructure for urban flood control, groundwater recharge, and improvement of the road surface thermal environment. The coarse sand cushion layer, as a crucial functional layer connecting the upper and lower layers of permeable pavement, plays multiple roles, including permeability and water conduction, stress dispersion, and filtration and siltation prevention. Its permeability directly determines the drainage efficiency, structural load-bearing stability, and long-term service durability of the permeable pavement. Accurately detecting the permeability coefficient of the coarse sand cushion layer and clarifying the coupling law between material parameters, construction conditions, rainfall conditions, and permeability characteristics is a prerequisite for permeable pavement structural optimization, construction quality control, and operation and maintenance life assessment. Currently, the industry mainly relies on indoor infiltration tests to calibrate the permeability parameters of the coarse sand cushion layer. The structural rationality, ease of assembly, and anti-interference properties of the testing equipment directly determine the accuracy of the test data.

[0003] Currently, conventional coarse sand cushion infiltration test devices in the industry generally have imperfect structural designs, resulting in multiple technical defects such as difficulties in processing and molding, cumbersome disassembly and maintenance, and large test interference errors, making it difficult to balance test reliability and engineering operability. Existing infiltration test equipment has a high degree of overall integration, mostly adopting an integrated sealed cylinder and a porous composite base plate. The components have complex structures, and the sheet metal and machining processes are cumbersome, resulting in high processing and production costs. At the same time, the equipment assembly structure is highly coupled, making disassembly and assembly difficult. Post-test sand sample cleaning, pipeline maintenance, and component repair are extremely inconvenient, resulting in high subsequent maintenance costs. The excessive smoothness of the inner wall of the cylinder causes the test water to easily adhere to the cylinder wall, forming a side-wall flow that seeps into the sample. This deviates from the premise of the vertical one-dimensional seepage test, severely disrupting the seepage flow field and reducing the test accuracy. In addition, the integrated molding cylinder structure makes it easy to disturb the coarse sand sample during the entire process of sample filling, layer compaction, test sampling, and demolding. This damages the original pore structure of the cushion layer, resulting in a large sampling disturbance error. It is impossible to reproduce the actual compaction degree and particle size distribution conditions at the construction site, leading to distorted test data.

[0004] Therefore, we propose a test device and method for the infiltration characteristics of permeable coarse sand cushion layers. Summary of the Invention

[0005] To address the aforementioned shortcomings of existing technologies, this invention provides a test device and method for the infiltration characteristics of permeable coarse sand cushion layers.

[0006] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A test device for the infiltration characteristics of a permeable coarse sand cushion layer includes: a test cylinder, which is a detachable multi-segment structure for filling samples inside, with a permeable structure at the bottom and a flow-blocking structure on the inner wall to prevent internal flow; a rainwater simulation component, located at the top of the test cylinder, for simulating rainwater weather inside the test cylinder; and a data acquisition module, located on the test cylinder, for collecting moisture content data at different depths inside the test cylinder.

[0007] By designing the test cylinder into multiple sections that are detachable, it is easy to fill the internal sand and gravel filler layer by layer, and the cleaning of the sand sample after the test is also more convenient. Furthermore, the test cylinder is equipped with a flow-blocking structure to prevent the formation of internal wall flow, which can effectively avoid the formation of internal wall flow. In addition, the split test cylinder can achieve non-destructive layered sampling without cutting the soil, which can preserve the original pore structure of the filler, avoid fine particle migration and clogging, and improve the test accuracy.

[0008] Further defined, the test cylinder comprises, from top to bottom, a first cylinder, a second cylinder, a third cylinder, a fourth cylinder, and a fifth cylinder. Adjacent cylinders are detachably connected by flanges, and the connecting end faces of the cylinders are provided with sealing grooves, with sealing rings embedded in the sealing grooves.

[0009] By connecting adjacent cylinders with flanges and setting sealing grooves and sealing rings on the connecting end faces of the cylinders, the sealing effect between adjacent cylinders can be improved.

[0010] Further, the inner walls of the second, third, fourth, and fifth cylinders are provided with staggered flow-blocking grooves, which are circular, and the outer walls of the first, second, third, fourth, and fifth cylinders are provided with vertical scales.

[0011] A vertical flow-blocking groove is opened inside the test cylinder as a flow-blocking structure to prevent the formation of internal wall flow. This effectively blocks the test water and prevents it from forming internal wall flow and seeping into the sample along the cylinder wall, thus ensuring the accuracy of the test. The addition of a scale makes it easier to read the water level inside the first cylinder.

[0012] Further specifying, the permeable structure is a porous permeable plate, and the porous permeable plate and the fifth cylinder are integrally formed.

[0013] Further defining the rainwater simulation component, it includes a water supply tank, a liquid pump, and a spray structure; the water supply tank is connected to the inlet end of the liquid pump via a flexible water pipe, and the outlet end of the liquid pump is connected to the spray structure via a flexible water pipe, with the spray structure fixedly positioned directly above the first cylinder.

[0014] Further defining the spray structure, it includes a nozzle, a spray pipe, a slide, a slider, and a connector; the spray pipe is inverted L-shape, the nozzle is connected to the horizontal end of the spray pipe, the slider is wrapped around the vertical end of the spray pipe, the bottom of the vertical section of the spray pipe is closed, the slide is vertically located on the outer wall of the first cylinder, the connector is horizontally connected to the bottom circumference of the vertical section of the spray pipe and communicates with the spray pipe, the outer wall of the slide has an extension groove that does not penetrate the bottom of the slide and is used for the connector to extend out, and the outlet end of the pump is connected to the connector through a soft water pipe.

[0015] This spray structure design allows the entire assembly of the spray nozzle and spray head to be slidably secured within a slide rail via a slider. The connector and the extension slot of the slide rail extend out and connect to the outlet end of the pump, facilitating the assembly and disassembly of the nozzle and spray pipe and enabling separate storage after testing.

[0016] Further, it also includes a water distribution cap, which is conical in shape. The cone of the water distribution cap has two staggered, elongated through-holes at the rounded ends. The inner wall of the first cylinder is provided with a placement groove, the inner diameter of which matches the outer diameter of the water distribution cap, and the water distribution cap is secured in the placement groove.

[0017] This water distribution cap design further improves the uniformity of water sprayed from the nozzle into the filler, and it is easy to assemble and disassemble, and simple to manufacture.

[0018] Further specifying, the data acquisition module includes a moisture content sensor and a signal acquisition device; the moisture content sensors are vertically and evenly spaced on the second, third, fourth, and fifth cylinders, with a total of seven moisture content sensors.

[0019] A prediction method for predicting the saturated permeability coefficient in the aforementioned permeable pavement coarse sand cushion layer infiltration characteristic test device includes the following steps: S1. Test Preparation: Sand was layered into the second, third, fourth, and fifth cylinders according to the target compaction degree. The cylinders were then connected as a single unit. The pump was adjusted to set the flow rate according to the required rainfall intensity. Once the water flow stabilized, the test was initiated. After the test began, several continuous observation periods with fixed durations were set. The infiltration rate within each observation period was calculated using the soil infiltration rate calculation method. ; , This represents the increase in water infiltration into the soil at the sampling points within the current observation period compared to the sampling points in the previous observation period. The time interval between the sampling points in this observation period and the sampling points in the previous observation period. The cross-sectional area of ​​the test cylinder is... The number for the observation period; S2. Determine a stable infiltration state: If a continuous infiltration state exists... If the relative fluctuation range of the infiltration rate and water content sensor readings within a given observation period is less than or equal to 5%, then the continuous period is considered to be... If the infiltration reaches a stable state within one observation cycle, then the infiltration has not reached a stable state. Take the continuous state that has reached a steady state The arithmetic mean of the infiltration rates corresponding to each observation period is taken as the stable infiltration rate under this operating condition. At the same time, take the continuous The arithmetic mean of the surface water height of the first cylinder corresponding to each observation period is taken as the stable surface water height. ; S3. Calculate the seepage hydraulic gradient: based on the height of the sample filled inside the test cylinder. With stable surface water height Calculate the hydraulic gradient : ; S4. Based on the steady infiltration rate and hydraulic gradient Calculate the unsaturated permeability coefficient ; S5. Calculation of saturated permeability coefficient based on the Irmay model: The measured saturation is calculated using the data collected by the moisture content sensor. ,when At that time, effective saturation Combined with the unsaturated permeability coefficient Calculate the saturated permeability coefficient : , The power exponent of the Irmay model is used to obtain the saturated permeability coefficient for each observation period. ; S6. Based on the existing Kozeny-Carman equations: ; in, Porosity The effective particle size of the medium, For comprehensive parameters, , For fluid specific weight, For fluid dynamic viscosity, It is a shape constant; in, Since the particle size is uniform, it is impossible to characterize the fine-particle filling effect; therefore, an introduction of... and This represents the filling effect of fine particles on the pores of the framework. , These are all particle size specifications, which can be adjusted according to the coarse sand gradation. For particles in Screen throughput based on aperture size For particles in The screen throughput of the aperture size is obtained as follows: ; Substituting into the existing Kozeny-Carman equation, we obtain the saturated permeability equation considering the effect of gradation flow rate: ; ; in, To absorb the proportionality constant The new coefficients after , All are undetermined experience indices; S7. Construction of the saturated permeability coefficient model under static physical structure: based on the maximum dry density of the soil sample. Particle density of soil sample Calculate constants : and using constants and the compaction degree of the sample Calculate porosity : Substituting the porosity into the saturated permeability equation that considers the influence of gradation flow rate, we obtain a model for predicting the saturated permeability coefficient under static physical structure: ; S8. Construction of the saturated permeability coefficient model under rainfall conditions: based on rainfall intensity and empirical power function decay term Number of rainfalls introduced Cumulative power function decay term Define rainfall attenuation factor for: ; in, The rainfall intensity attenuation index, The attenuation index of rainfall frequency and These are the parameters to be fitted; The rainfall attenuation factor is substituted into the saturated permeability coefficient prediction model under static physical structure, and the overall constants are combined into... The saturated permeability coefficient model under rainfall conditions is obtained as follows: ; ; in, , , , , , These are all parameters to be fitted to the model; S9. Using the designed gradation, compaction degree, rainfall intensity, and rainfall frequency as independent variables, calculate the saturated permeability coefficient corresponding to each working condition. With the dependent variable as the dependent variable, construct a set of fitted data, input several sets of fitted data into the saturated permeability coefficient model under rainfall conditions, fit the model to be fitted parameters, and output the fitted saturated permeability coefficient model under rainfall conditions. S10. The maximum dry density of the soil sample used in engineering applications. Particle density of soil sample Compaction degree Rainfall intensity Number of rainfalls Particles in Screen throughput of aperture and particles in Screen throughput of aperture Input the fitted saturated permeability coefficient model under rainfall conditions, and output the predicted saturated permeability coefficient.

[0020] The beneficial effects of this invention are as follows: by setting the test cylinder to be split and connecting adjacent cylinders through flanges, and setting a flow-blocking groove inside the cylinder to prevent the formation of internal wall flow, it is more conducive to the filling of the packing and sampling, avoids the influence of internal wall flow, and the test accuracy is higher. Attached Figure Description

[0021] Figure 1 This is a simplified structural diagram of the present invention from a frontal view. Figure 2 for Figure 1 An enlarged schematic diagram of part A in the middle; Figure 3 This is a top view of the water distribution cap; Figure 4 This is a schematic diagram of the internal structure of the liquid spraying structure; Figure 5 This is a schematic diagram of the internal structure of the flange; Figure 6 This is a graph showing the variation of infiltration rate over time under the influence of rainfall frequency. Figure 7 This is a comparison chart of measured and predicted values ​​of saturated permeability coefficient under different operating conditions.

[0022] The symbols for each component are as follows: Test cylinder 1, first cylinder 11, second cylinder 12, third cylinder 13, fourth cylinder 14, fifth cylinder 15, porous permeable plate 16, flange 17, sealing ring 18, flow obstruction groove 19, scale 110, placement groove 111, rainwater simulation component 2, water supply tank 21, liquid pump 22, spray structure 23, nozzle 231, spray pipe 232, slide 233, slider 234, connector 235, extension groove 236, data acquisition module 3, moisture content sensor 31, signal acquisition device 32, water distribution cap 4. Detailed Implementation

[0023] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0024] Example: like Figures 1-7 As shown, a test device for the infiltration characteristics of a permeable coarse sand cushion layer includes a test cylinder 1, a rainwater simulation component 2, a data acquisition module 3, and a water distribution cap 4. The test cylinder 1 is used to fill the sample. The bottom of the test cylinder 1 is provided with a water-permeable structure, and the inner wall is provided with a flow-blocking structure to prevent the formation of internal flow. The test cylinder 1 includes a first cylinder 11, a second cylinder 12, a third cylinder 13, a fourth cylinder 14, and a fifth cylinder 15 from top to bottom. Adjacent cylinders are detachably connected by flanges 17. The connecting end face of the cylinder is provided with a sealing groove, and a sealing ring 18 is embedded in the sealing groove. The inner walls of the second cylinder 12, the third cylinder 13, the fourth cylinder 14, and the fifth cylinder 15 are all provided with staggered flow-blocking grooves 19. The flow-blocking grooves 19 are circular. The outer walls of the first cylinder 11, the second cylinder 12, the third cylinder 13, the fourth cylinder 14, and the fifth cylinder 15 are provided with vertical scales 110. The water-permeable structure is a porous water-permeable plate 16, which is integrally formed with the fifth cylinder 15. The rain simulation component 2 is used to simulate rainy weather inside the test cylinder 1. The rain simulation component 2 includes a water supply tank 21, a liquid pump 22, and a spray structure 23. The water supply tank 21 is connected to the inlet end of the liquid pump 22 through a soft water pipe, and the outlet end of the liquid pump 22 is connected to the spray structure 23 through a soft water pipe. The spray structure 23 is fixedly located directly above the first cylinder 11. The spray structure 23 includes a nozzle 231, a spray pipe 232, a slide 233, a slider 234, and a connector 235. The spray pipe 232 is inverted L-shape. The nozzle 231 is connected to the horizontal end of the spray pipe 232. The slider 234 is covered by the vertical end of the spray pipe 232. The bottom end of the vertical section of the spray pipe 232 is closed. The slide 233 is vertically arranged on the outer wall of the first cylinder 11. The connector 235 is horizontally connected to the bottom circumference of the vertical section of the spray pipe 232 and communicates with the spray pipe 232. The outer wall of the slide 233 has an extension groove 236 that does not penetrate the bottom of the slide 233 and is used for the connector 235 to extend. The outlet end of the pump 22 is connected to the connector 235 through a soft water pipe. The data acquisition module 3 is used to collect moisture content data at different depths inside the test cylinder 1. The data acquisition module 3 includes a moisture content sensor 31 and a signal acquisition device 32. The moisture content sensor 31 is vertically and evenly spaced on the second cylinder 12, the third cylinder 13, the fourth cylinder 14 and the fifth cylinder 15. There are seven moisture content sensors 31. The probes of the moisture content sensors 31 are sealed to the cylinder wall of the test cylinder 1. The water distribution cap 4 is conical in shape. The cone of the water distribution cap 4 has two staggered, elongated through-holes at the round ends. The inner wall of the first cylinder 11 is provided with a placement groove 111. The inner diameter of the placement groove 111 matches the outer diameter of the water distribution cap 4, and the water distribution cap 4 is secured in the placement groove 111.

[0025] In this application, the liquid pump 22 has a built-in controller, and the data acquisition module 3 can be connected to a computer for data export.

[0026] By designing the test cylinder 1 into multiple detachable sections, it facilitates the layered filling of internal sand and gravel filler, and makes post-test sand sample cleaning more convenient. A flow-blocking structure is incorporated inside the test cylinder 1 to prevent internal wall flow, effectively avoiding its formation. Furthermore, the modular design of the test cylinder 1 allows for non-destructive layered sampling without soil cutting, preserving the original pore structure of the filler, preventing fine particle migration and clogging, and improving test accuracy. Adjacent cylinders are connected via flange 17, with sealing grooves and sealing rings 18 on the connecting end faces to enhance the sealing effect between adjacent cylinders. Vertical flow-blocking grooves 19 are vertically formed inside the test cylinder 1 as a flow-blocking structure to prevent internal wall flow. The design effectively restricts the flow of test water, preventing it from forming an inner wall flow and seeping into the sample along the cylinder wall, thus ensuring the accuracy of the test. The scale 110 makes it easier to read the water level inside the first cylinder 11. The spray structure 23 connects the spray pipe 232 and the nozzle 231, which are slidably locked in the slide rail 233 by the slider 234. The connector 235 and the extension groove 236 of the slide rail 233 extend out and connect to the outlet end of the pump 22, facilitating the disassembly and assembly of the nozzle 231 and the spray pipe 232, and making it easy to store them separately after the test. The water distribution cap 4 further improves the uniformity of the water sprayed from the nozzle 231 into the packing material, and is easy to disassemble and assemble, and simple to manufacture.

[0027] A prediction method for predicting the saturated permeability coefficient in the aforementioned permeable pavement coarse sand cushion layer infiltration characteristic test device includes the following steps: S1. Test Preparation: Sand was layered into the second cylinder 12, third cylinder 13, fourth cylinder 14, and fifth cylinder 15 according to the target compaction degree. The cylinders were then connected as a whole. The pump 22 was adjusted to set the flow rate according to the required rainfall intensity. After the water flow stabilized, the test was started. After the test started, several continuous and fixed-duration observation periods were set. Samples were taken once in each period, and the infiltration rate in each observation period was calculated according to the soil infiltration rate calculation method. ; , This represents the increase in water infiltration into the soil at the sampling points within the current observation period compared to the sampling points in the previous observation period. The time interval between the sampling points in this observation period and the sampling points in the previous observation period. Let be the cross-sectional area of ​​test cylinder 1. The number for the observation period; S2. Determine a stable infiltration state: If a continuous infiltration state exists... If the relative fluctuation range of the infiltration rate and water content values ​​collected by sensor 31 within a certain observation period is less than or equal to 5%, then the continuous period is determined to be... If the infiltration reaches a stable state within one observation cycle, then the infiltration has not reached a stable state. Take the continuous state that has reached a steady state The arithmetic mean of the infiltration rates corresponding to each observation period is taken as the stable infiltration rate under this operating condition. At the same time, take the continuous The arithmetic mean of the surface water height of the first cylinder 11 corresponding to each observation period is taken as the stable surface water height. ; S3. Calculate the seepage hydraulic gradient: based on the height of the sample filled inside the test cylinder 1. With stable surface water height Calculate the hydraulic gradient : ; S4. Based on the steady infiltration rate and hydraulic gradient Calculate the unsaturated permeability coefficient ; S5. Calculation of saturated permeability coefficient based on the Irmay model: In this experiment, the residual moisture content of the coarse sand was <2%. The measured saturation was calculated using the data collected by the moisture content sensor 31. ,when At that time, effective saturation Combined with the unsaturated permeability coefficient Calculate the saturated permeability coefficient : , The power exponent of the Irmay model. Using 3.0, the saturated permeability coefficient for each observation period is obtained. ; S6. Based on the existing Kozeny-Carman equations: ; in, Porosity The effective particle size of the medium, For comprehensive parameters, , For fluid specific weight, For fluid dynamic viscosity, It is a shape constant; in, Since the particle size is uniform, it is impossible to characterize the fine-particle filling effect; therefore, an introduction of... and This represents the filling effect of fine particles on the pores of the framework. , These are all particle size specifications, which can be adjusted according to the coarse sand gradation. For particles in Screen throughput based on aperture size For particles in The screen throughput of the aperture size is obtained as follows: ; Substituting into the existing Kozeny-Carman equation, we obtain the saturated permeability equation considering the effect of gradation flow rate: ; ; in, To absorb the proportionality constant The new coefficients after , All are undetermined experience indices; S7. Construction of the saturated permeability coefficient model under static physical structure: based on the maximum dry density of the soil sample. Particle density of soil sample Calculate constants : and using constants and the compaction degree of the sample Calculate porosity : Substituting the porosity into the saturated permeability equation that considers the influence of gradation flow rate, we obtain a model for predicting the saturated permeability coefficient under static physical structure: ; S8. Construction of the saturated permeability coefficient model under rainfall conditions: based on rainfall intensity and empirical power function decay term Number of rainfalls introduced Cumulative power function decay term Define rainfall attenuation factor for: ; in, The rainfall intensity attenuation index, The attenuation index of rainfall frequency and These are the parameters to be fitted; The rainfall attenuation factor is substituted into the saturated permeability coefficient prediction model under static physical structure, and the overall constants are combined into... The saturated permeability coefficient model under rainfall conditions is obtained as follows: ; ; in, , , , , , These are all parameters to be fitted to the model; S9. Using the designed gradation, compaction degree, rainfall intensity, and rainfall frequency as independent variables, calculate the saturated permeability coefficient corresponding to each working condition. With the dependent variable as the dependent variable, construct a set of fitted data, input several sets of fitted data into the saturated permeability coefficient model under rainfall conditions, fit the model to be fitted parameters, and output the fitted saturated permeability coefficient model under rainfall conditions. S10. The maximum dry density of the soil sample used in engineering applications. Particle density of soil sample Compaction degree Rainfall intensity Number of rainfalls Particles in Screen throughput of aperture and particles in Screen throughput of aperture Input the fitted saturated permeability coefficient model under rainfall conditions, and output the predicted saturated permeability coefficient.

[0028] Table 1 shows the test conditions for Examples J-1~J-5, Y-1~Y-3, Q-1~Q-3, and C-1~C-3. Table 2 shows the calculated saturated permeability coefficients for Examples J-1~J-5, Y-1~Y-3, Q-1~Q-3, and C-1~C-3.

[0029] Table 1 Test conditions

[0030] Table 2 Calculation results of saturated permeability coefficient

Claims

1. A test device for the infiltration characteristics of a permeable coarse sand cushion layer, characterized in that, include: The test cylinder (1) is a detachable multi-section structure, and the interior is used to fill the sample. The bottom of the test cylinder (1) is provided with a water-permeable structure, and the inner wall is provided with a flow-blocking structure to prevent the flow from forming inside the wall. Rainfall simulation component (2) is installed on the top of the test cylinder (1) to simulate rainy weather inside the test cylinder (1); The data acquisition module (3) is installed on the test cylinder (1) to collect water content data at different depths inside the test cylinder (1).

2. The permeable pavement coarse sand cushion layer infiltration characteristic test device according to claim 1, characterized in that, The test cylinder (1) includes, from top to bottom, a first cylinder (11), a second cylinder (12), a third cylinder (13), a fourth cylinder (14), and a fifth cylinder (15). Adjacent cylinders are detachably connected by flanges (17). A sealing groove is provided on the connecting end face of the cylinder, and a sealing ring (18) is embedded in the sealing groove.

3. The permeable paving coarse sand cushion layer infiltration characteristic test device according to claim 2, characterized in that, The inner walls of the second cylinder (12), the third cylinder (13), the fourth cylinder (14) and the fifth cylinder (15) are provided with staggered flow-blocking grooves (19), which are circular. The outer walls of the first cylinder (11), the second cylinder (12), the third cylinder (13), the fourth cylinder (14) and the fifth cylinder (15) are provided with vertical scales (110).

4. The permeable paving coarse sand cushion layer infiltration characteristic test device according to claim 3, characterized in that, The permeable structure is a porous permeable plate (16), and the porous permeable plate (16) and the fifth cylinder (15) are integrally formed.

5. The permeable pavement coarse sand cushion layer infiltration characteristic test device according to claim 4, characterized in that, The rainwater simulation component (2) includes a water supply tank (21), a liquid pump (22), and a spray structure (23). The water supply tank (21) is connected to the inlet end of the liquid pump (22) through a soft water pipe, and the outlet end of the liquid pump (22) is connected to the spray structure (23) through a soft water pipe. The spray structure (23) is fixedly located directly above the first cylinder (11).

6. The permeable pavement coarse sand cushion layer infiltration characteristic test device according to claim 5, characterized in that, The spray structure (23) includes a nozzle (231), a spray pipe (232), a slide rail (233), a slider (234), and a connector (235); the spray pipe (232) is inverted L-shape, the nozzle (231) is connected to the horizontal end of the spray pipe (232), the slider (234) covers the vertical end of the spray pipe (232), the bottom end of the vertical section of the spray pipe (232) is closed, and the slide rail (233) is connected to the horizontal end of the spray pipe (232). The connector (235) is vertically mounted on the outer wall of the first cylinder (11). The connector (235) is horizontally connected to the bottom circumference of the vertical section of the nozzle (232) and communicates with the nozzle (232). The outer wall of the slide (233) is provided with an extension groove (236) that does not penetrate the bottom of the slide (233) and is used for the connector (235) to extend. The liquid outlet of the pump (22) is connected to the connector (235) through a soft water pipe.

7. The permeable pavement coarse sand cushion layer infiltration characteristic test device according to claim 6, characterized in that, It also includes a water distribution cap (4), which is conical in shape. The water distribution cap (4) has two staggered, elongated through-holes on its conical body. The inner wall of the first cylinder (11) is provided with a placement groove (111). The inner diameter of the placement groove (111) matches the outer diameter of the water distribution cap (4). The water distribution cap (4) is mounted on the placement groove (111).

8. The permeable paving coarse sand cushion layer infiltration characteristic test device according to claim 7, characterized in that, The data acquisition module (3) includes a moisture content sensor (31) and a signal acquisition device (32); the moisture content sensor (31) is vertically and evenly spaced on the second cylinder (12), the third cylinder (13), the fourth cylinder (14) and the fifth cylinder (15), and there are seven moisture content sensors (31).

9. A prediction method for predicting the saturated permeability coefficient in the permeable pavement coarse sand cushion layer infiltration characteristic test device as described in claim 8, characterized in that, Includes the following steps: S1. Preparation for the test: Sand was layered in the second cylinder (12), third cylinder (13), fourth cylinder (14) and fifth cylinder (15) according to the target compaction degree. The cylinders were then connected as a whole. The pump (22) was adjusted and the flow rate was set according to the required rainfall intensity. After the water flow stabilized, the test was started. After the test was started, several continuous and fixed-duration observation periods were set. The infiltration rate in each observation period was calculated according to the soil infiltration rate calculation method. ; , This represents the increase in water infiltration into the soil at the sampling points within the current observation period compared to the sampling points in the previous observation period. The time interval between the sampling points in this observation period and the sampling points in the previous observation period. The cross-sectional area of ​​the test cylinder (1) is... The number for the observation period; S2. Determine a stable infiltration state: If a continuous infiltration state exists... If the relative fluctuation range of the infiltration rate and water content sensor (31) values ​​collected within a certain observation period is less than or equal to 5%, then the continuous period is determined to be less than or equal to 5%. If the infiltration reaches a stable state within one observation cycle, then the infiltration has not reached a stable state. Take the continuous state that has reached a steady state The arithmetic mean of the infiltration rates corresponding to each observation period is taken as the stable infiltration rate under this operating condition. At the same time, take the continuous The arithmetic mean of the surface water height of the first cylinder (11) corresponding to each observation period is taken as the stable surface water height. ; S3. Calculate the seepage hydraulic gradient: based on the height of the sample filled inside the test cylinder (1). With stable surface water height Calculate the hydraulic gradient : ； S4. Based on the steady infiltration rate and hydraulic gradient Calculate the unsaturated permeability coefficient ; S5. Calculation of saturated permeability coefficient based on the Irmay model: The measured saturation is obtained by collecting data from the water content sensor (31). ,when At that time, effective saturation Combined with the unsaturated permeability coefficient Calculate the saturated permeability coefficient : , The power exponent of the Irmay model is used to obtain the saturated permeability coefficient for each observation period. ; S6. Based on the existing Kozeny-Carman equations: ； in, Porosity The effective particle size of the medium, For comprehensive parameters, , For fluid specific weight, For fluid dynamic viscosity, It is a shape constant; in, Since the particle size is uniform, it is impossible to characterize the fine-particle filling effect; therefore, an introduction of... and This represents the filling effect of fine particles on the pores of the framework. , These are all particle size specifications, which can be adjusted according to the coarse sand gradation. For particles in Screen throughput based on aperture size For particles in The screen throughput of the aperture size is obtained as follows: ； Substituting into the existing Kozeny-Carman equation, we obtain the saturated permeability equation considering the effect of gradation flow rate: ； ； in, To absorb the proportionality constant The new coefficients after , All are undetermined experience indices; S7. Construction of the saturated permeability coefficient model under static physical structure: based on the maximum dry density of the soil sample. Particle density of soil sample Calculate constants : and using constants and the compaction degree of the sample Calculate porosity : Substituting the porosity into the saturated permeability equation that considers the influence of gradation flow rate, we obtain a model for predicting the saturated permeability coefficient under static physical structure: ； S8. Construction of the saturated permeability coefficient model under rainfall conditions: based on rainfall intensity and empirical power function decay term Number of rainfalls introduced Cumulative power function decay term Define rainfall attenuation factor for: ； in, The rainfall intensity attenuation index, The attenuation index of rainfall frequency and These are the parameters to be fitted; The rainfall attenuation factor is substituted into the saturated permeability coefficient prediction model under static physical structure, and the overall constants are combined into... The saturated permeability coefficient model under rainfall conditions is obtained as follows: ； ； in, , , , , , These are all parameters to be fitted to the model; S9. Using the designed gradation, compaction degree, rainfall intensity, and rainfall frequency as independent variables, calculate the saturated permeability coefficient corresponding to each working condition. With the dependent variable as the dependent variable, construct a set of fitted data, input several sets of fitted data into the saturated permeability coefficient model under rainfall conditions, fit the model to be fitted parameters, and output the fitted saturated permeability coefficient model under rainfall conditions. S10. The maximum dry density of the soil sample used in engineering applications. Particle density of soil sample Compaction degree Rainfall intensity Number of rainfalls Particles in Screen throughput of aperture and particles in Screen throughput of aperture Input the fitted saturated permeability coefficient model under rainfall conditions, and output the predicted saturated permeability coefficient.

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