A filler grading design method for road bio-retention belt based on filler porosity

Abstract

The application discloses a filler grading design method for a road bio-retention zone based on filler porosity. The method comprises the following steps: taking the designed target porosity of the filler and the volume content of the natural soil as control indexes, and determining the amount of various medium fillers. The method is simple and practical, has small design error, can guarantee that the coarse aggregate forms a stable skeleton structure, and can guarantee that the fine aggregate and the natural soil as secondary fillers fill the voids of the skeleton structure. The obtained mixed filler structure is stable, can resist rainwater scouring, is not prone to causing the loss of fine materials, and improves the service life of the road retention zone.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of sponge city road low impact development rainwater runoff management and utilization, and particularly relates to a filler gradation design method for a road bio-retention belt based on filler porosity. BACKGROUND

[0002] The road bio-retention belt is a commonly used LID facility in the construction of a sponge city road, collects road rainwater runoff through road side orifices, thereby reducing runoff at the source, and rainwater entering the bio-retention belt is infiltrated, stored, purified, and then infiltrated to supplement the underground or is recycled through a drainage pipe.

[0003] The road bio-retention belt generally needs to process runoff much larger than its own catchment area. The permeability of the filler is an important parameter for the hydrological design of the bio-retention facility, and directly affects the storage effect of the road bio-retention belt on rainwater runoff and the water accumulation emptying time. However, the permeability of natural soil is usually poor, and the runoff is difficult to infiltrate quickly. Therefore, the filler of the road bio-retention belt is usually made by combining natural soil with various media to improve the permeability of the natural soil.

[0004] At present, the gradation design of the engineering filler in the road bio-retention belt is usually based on experience, and the amount of each medium is determined within a certain proportion range according to the type of the filler medium. This method ignores the differences in regional and climate conditions and does not consider the permeability of the bio-retention facility and the water storage capacity of the soil filler. At the same time, the particle size and chemical properties of various fillers are quite different, and a small change in the particle size or unevenness of the particle size distribution will change the performance of the filler. Therefore, the selection of the medium type and the gradation of the filler are key factors affecting the rainwater runoff treatment performance of the bio-retention belt. From the perspective of economy and construction convenience, the types of fillers applied to the road bio-retention belt are limited. Therefore, the gradation design of the filler is crucial to the function of the bio-retention belt system. SUMMARY

[0005] The purpose of the present application is to solve the problems in the background art and provide a filler gradation design method for a road bio-retention belt based on filler porosity.

[0006] The technical scheme adopted by the present application is as follows: A filler gradation design method for a road bio-retention belt based on filler porosity, comprising the following steps:

[0007] S1: selecting a medium filler for improving the natural soil in the road bio-retention belt filler, dividing the medium filler with a nominal maximum particle size greater than 9.5 mm into coarse aggregate, dividing the medium filler with a nominal maximum particle size less than or equal to 9.5 mm into fine aggregate, taking the coarse aggregate as the main skeleton, and calculating the main skeleton porosity T vc ;

[0008] S2: The volume content of natural soil in the design gradation (T) vs and the target porosity T of the filler design vm As control indicators, the value ranges for the two are 18%–28% and 20%–30%, respectively;

[0009] S3: After initially determining the values ​​of several control indicators within the range, based on the fine aggregate volume content and the natural soil volume content T vs and the target void T of the filler design vm The sum equals the porosity T of the main frame. vc Based on the principle of calculating the amount of coarse aggregate, fine aggregate, and natural soil:

[0010] S4. Based on the calculated amounts of coarse aggregate, fine aggregate, and natural soil, calculate the relevant parameter d for the particle size distribution of fillers with different gradations. 10 d 20 d 70 d 80 The saturated permeability coefficient K for fillers with different gradations s Make predictions;

[0011] S5. Based on the saturated permeability coefficient K of the packing material. s The initial selection of the packing material's design gradation is performed, and the saturated permeability coefficient K is chosen. s A packing gradation between 80 and 400 mm / h is the preferred option for the next step.

[0012] S6. Prepare soil samples for testing according to the filler gradation scheme selected in S5. Obtain the optimum moisture content of the filler through compaction tests, prepare the filler, and determine the permeability coefficient K of each group of fillers at the specified compaction degree. c ;

[0013] S7, Permeability coefficient K at a specified compaction degree c The optimal packing gradation scheme is selected based on economic cost factors, meeting the requirements. If the permeability coefficient K of each packing group at a specified compaction degree is... c If none of the requirements are met, then adjust the porosity T of the main framework. vc Target porosity T of the packing material vm Natural soil volume content (T) vs Repeat steps S1 through S6 until the permeability coefficient K of the packing material reaches the specified compaction degree. c The requirements are met.

[0014] Main skeleton porosity T vc Calculated using the following formula:

[0015]

[0016] In the formula, ρsc ρa is the apparent density of the selected coarse aggregate, kg / m3 3 ρa is the apparent density of the selected coarse aggregate, kg / m3 tc ρa is the apparent density of the selected coarse aggregate, kg / m3 3 .

[0017] The amounts of coarse aggregate, fine aggregate and natural soil are calculated by the following formula:

[0018]

[0019] q tc + q fx + q ss = 100

[0020] q ss = T vs ρa ss

[0021] In the formula, q tc , q fx , q ss are the amounts of coarse aggregate, fine aggregate and natural soil, respectively, in percentage; ρa tc , ρa fx are the apparent density of the coarse aggregate and the combined apparent density of the fine aggregate, respectively, in kg / m3 3 ; ρa ss is the bulk density of the natural soil, in kg / m3 3 ; T vc , T vm are the percentages of the target void ratio of the main skeleton and the filler, respectively, and T vs is the percentage of the volume content of the natural soil.

[0022] The saturated permeability coefficient K s is predicted by the following formula:

[0023]

[0024] In the formula, K s is the saturated permeability coefficient, mm / min; d 10 is the effective particle size, i.e. the particle size at which the soil content is 10% of the total mass, mm; d 20 is the average void particle size, i.e. the particle size at which the soil content is 20% of the total mass, mm; d 70 is the average particle size, i.e. the particle size at which the soil content is 70% of the total mass, mm; d 80 is the average void particle size, i.e. the particle size at which the soil content is 80% of the total mass, mm; n is the porosity of the filler, which is equal to T vm / 100; γ is the kinematic viscosity coefficient of water, cm 2g is the acceleration of gravity, m / s 2 .

[0025] The apparent density of the fine aggregate is calculated by the following formula:

[0026]

[0027] In the formula, q1, q2,..., q n represent the passing percentage of each particle size filler in the fine aggregate, and the sum is 100; p1, p2,..., p n represent the apparent density of each particle size filler in the fine aggregate, kg / m 3 .

[0028] The grading composition of the fine aggregate is based on the maximum density theory, and the Taylor index n is 0.3-0.5,

[0029]

[0030] In the formula, qi is the passing percentage of each particle size filler in the synthetic fine aggregate; D is the maximum particle size in the synthetic fine filler; d i is the particle size value in the fine aggregate.

[0031] The permeability coefficient K under the specified degree of compaction c meets the requirement that the measured permeability coefficient K under the specified degree of compaction c is not less than the permeability coefficient K0 required by the engineering filler, and 80≤K c ≤400 mm / h.

[0032] The specified degree of compaction is 90%.

[0033] The filler of the bioretention belt is mixed by natural soil, coarse aggregate and fine aggregate, and can be regarded as the improvement of the permeability of natural soil by using coarse aggregate, fine aggregate and other media. The present application determines the amount of various media fillers by taking the design target porosity of the filler and the volume content of the natural soil as control indexes. Compared with the existing grading design method based on experience, the porosity of the filler designed by the present application is closer to the design target porosity of the filler, and the embedded and extruded skeleton structure of the filler effectively ensures the stability of the proportioning structure layer during use. At the same time, considering the correlation between the porosity of the retention belt filler and the permeability coefficient under a certain degree of compaction, the permeability of the designed mixed filler to the rainwater runoff of the retention belt is evaluated by means of the permeability test.

[0034] The beneficial effects of the present application are:

[0035] 1. The method is simple and practical, the design error is small, the coarse aggregate can form a stable skeleton structure, the fine aggregate and the natural soil are used as the secondary filler to fill the voids of the skeleton structure, the obtained mixed filler structure is stable, resistant to rainwater erosion, and the loss of fine materials is avoided, and the service life of the road detention zone is improved.

[0036] 2. The relationship between the permeability and the void volume of the road detention zone filler is combined, the filler grading design is carried out based on the pore volume, the void volume parameter of the mixture is controlled, the unreasonable filler design is avoided, the pore blockage is avoided, the permeability coefficient is rapidly attenuated, the design filler permeability is ensured to meet the engineering requirements, and the rainwater runoff entering the detention zone is effectively collected and infiltrated.

[0037] 3. The influence of the biological detention zone filler medium type and particle size distribution, the compaction degree and the like on the design target void ratio and the permeability coefficient of the mixed filler is comprehensively considered, the calculation formula comprehensively considers the influence factors, the analysis is rigorous, the disadvantages of the high experience of the existing design method are overcome, and the theoretical basis is provided for the filler grading design. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 is a flow chart of the present application;

[0039] Figure 2 is a schematic diagram of the void structure of the filler main skeleton;

[0040] Figure 3 is a schematic diagram of the grading curve of the designed filler;

[0041] Figure 4 is a schematic diagram of the compaction curve of the designed filler; DETAILED DESCRIPTION

[0042] The present application will be further described in detail below in combination with the drawings and specific embodiments, so that the present application can be clearly understood.

[0043] As shown in Figure 1 , the present application comprises the following steps:

[0044] S1: selecting the medium filler of the natural soil used for improving the road biological detention zone filler, dividing the medium filler with a nominal maximum particle size greater than 9.5 mm into coarse aggregate, dividing the medium filler with a nominal maximum particle size less than or equal to 9.5 mm into fine aggregate, taking the coarse aggregate as the main skeleton, and calculating the main skeleton porosity T vc ;

[0045] S2: taking the volume content T vs of the natural soil in the design grading and the filler design target void ratio T vm as control indexes, and the value ranges of the two are 18% to 28% and 20% to 30%, respectively;

[0046] S3: After the values of the control indexes are preliminarily determined in the value range, the amounts of the coarse aggregate, the fine aggregate and the natural soil are calculated based on the principle that the volume content of the fine aggregate, the volume content of the natural soil T vs and the design target void ratio of the filler T vm sum up to the main skeleton void ratio T vc .

[0047] S4: According to the calculated amounts of the coarse aggregate, the fine aggregate and the natural soil, the filler particle size distribution related parameters d 10 , d 20 , d 70 , d 80 are calculated for different gradations of the filler, and the saturated permeability coefficients K s of the fillers with different gradations are predicted.

[0048] S5: The design gradation of the filler is preliminarily selected according to the saturated permeability coefficients K s , and the filler gradation with the saturated permeability coefficient K s between 80-400mm / h is selected as the next preferred scheme.

[0049] S6: The soil samples for the test are prepared according to the filler gradation scheme selected in S5, the optimal water content of the filler is obtained by the compaction test, the filler is prepared, and the permeability coefficients K c of each group of fillers under the specified degree of compaction are measured.

[0050] S7: The filler gradation scheme whose permeability coefficient K c under the specified degree of compaction meets the requirements is selected as the best gradation based on the economic cost factor, if the permeability coefficients K c of each group of fillers under the specified degree of compaction do not meet the requirements, the main skeleton void ratio T vc , the design target void ratio of the filler T vm , the volume content of the natural soil T vs are adjusted, and S1-S6 are repeated until the permeability coefficient K c of the filler under the specified degree of compaction meets the requirements.

[0051] The main skeleton void ratio T vc is calculated by the following formula:

[0052]

[0053] In the formula, p sc is the selected coarse aggregate bulk density, kg / m 3 ; p tc is the selected coarse aggregate apparent density, kg / m 3 .

[0054] The amounts of coarse aggregate, fine aggregate and natural soil are calculated by the following formula:

[0055]

[0056] q tc +q fx +q ss =100

[0057] q ss =T vs ρ ss

[0058] In the formula, q tc , q fx , q ss are the amounts of coarse aggregate, fine aggregate and natural soil, respectively, in percentage; ρ tc , ρ fx are the apparent density of coarse aggregate and the synthetic apparent density of fine aggregate, respectively, kg / m 3 ; ρ ss is the bulk density of natural soil, kg / m 3 ; T vc , T vm are the percentage of the void ratio of the main skeleton and the design target void ratio of the filler, respectively, T vs is the percentage of the volume content of natural soil.

[0059] The saturated permeability coefficient K s is predicted by the following formula:

[0060]

[0061] In the formula, K s is the saturated permeability coefficient, mm / min; d 10 is the effective particle size, i.e. the particle size at which the soil content of less than this particle size accounts for 10% of the total mass, mm; d 20 is the average void particle size, i.e. the particle size at which the soil content of less than this particle size accounts for 20% of the total mass, mm; d 70 is the average particle size, i.e. the particle size at which the soil content of less than this particle size accounts for 70% of the total mass, mm; d 80 is the average void particle size, i.e. the particle size at which the soil content of less than this particle size accounts for 80% of the total mass, mm; n is the porosity of the filler, which is equal to T vm / 100; γ is the kinematic viscosity coefficient of water, cm 2 / s; g is the acceleration of gravity, m / s 2 .

[0062] The synthetic apparent density of fine aggregate is calculated by the following formula:

[0063]

[0064] In the formula, q1, q2,..., q n represents the passing percentage of each particle size filler in the fine aggregate, and the sum is 100; p1, p2,..., p n represents the apparent density of each particle size filler in the fine aggregate, kg / m 3 .

[0065] The grading composition of the fine aggregate is based on the maximum density theory, and the taylor index n = 0.3-0.5,

[0066]

[0067] In the formula, qi is the passing percentage of each particle size filler in the synthetic fine aggregate; D is the maximum particle size in the synthetic fine filler; d i is the particle size value in the fine aggregate.

[0068] The permeability coefficient K under the specified degree of compaction c The measured permeability coefficient K under the specified degree of compaction meets the requirements c The measured permeability coefficient K under the specified degree of compaction meets the requirements c The measured permeability coefficient K under the specified degree of compaction meets the requirements

[0069] Embodiment:

[0070] Referring to Figure 1 , the process of designing the grading of the mixed filler using a sponge city road bioretention belt filler grading design method based on the void ratio of the filler is described in further detail:

[0071] It is assumed that the permeability coefficient of the mixed filler in the road bioretention belt in the embodiment is at least 280 mm / h to meet the rainwater runoff treatment requirements. The type and ratio of each filler medium in the mixed filler are determined according to the following steps:

[0072] The natural soil in the embodiment is loess. First, the permeability performance and engineering economy requirements of the mixed filler are considered, and the preferred improved medium filler is cobble, three-grade quartz sand, and blast furnace slag. The density indicators of the selected materials are measured to obtain the basic material characteristics of each medium as shown in Table 1:

[0073] Table 1 Basic material characteristics of preferred medium

[0074]

[0075] Figure 2The figure is a schematic diagram of the main skeleton gap structure of the mixed filler in this embodiment. According to the particle size distribution of the medium and the principle of embedding and extruding the main skeleton, pebbles are used as coarse aggregates to form the main skeleton, and three-grade quartz sand, blast furnace slag and undisturbed loess are used as fine aggregates to fill the gaps formed by the main skeleton.

[0076] The porosity T of the main skeleton formed by pebbles is calculated according to the formula vc :

[0077]

[0078] The pebbles with a discontinuous particle size of 4.75-9.5 mm as the main skeleton have a void ratio of 56.3% in a loose state of coarse aggregate, which is between 40% and 60%, meeting the requirements.

[0079] According to the requirements of plant growth and permeability, three groups of design target void ratios T vm and the volume content of natural soil T vs are preliminarily determined within a reasonable range. Different mixed filler gradations are respectively gradation A (T vm = 25%, T vs = 28%), gradation B (T vm = 28%, T vs = 20%) and gradation C (T vm = 25%, T vs = 24%).

[0080] The fine aggregate in this example is a plurality of filler media with a particle size range of 0.16-4.75 mm formed by three-grade quartz sand and blast furnace slag. The synthetic apparent density of the fine aggregate is calculated according to the formula, and the mass proportion of the synthetic sand is designed according to the Terzaghi formula based on the maximum density theory, with a Terzaghi coefficient of 0.5. The mass proportion of each particle size filler of the fine aggregate and the synthetic apparent density are as follows in Table 2:

[0081] Table 2 Mass proportion of each particle size filler of the fine aggregate and synthetic apparent density

[0082]

[0083] According to the void ratio of the main skeleton, the volume content of natural soil and the design target void ratio of the mixed filler, the amount of coarse aggregate and fine aggregate and the mass proportion of each component are determined. Figure 3 The cumulative gradation curves of the three groups of mixed fillers with gradations A, B and C.

[0084] Based on the void ratio of the designed filler and the particle size distribution of the filler, the saturated permeability coefficient K s of the three groups of fillers with different gradations is predicted. The different filler gradation characteristic parameters and the predicted permeability coefficients are listed in Table 3:

[0085] Table 3 Different filler grading characteristic parameters and predicted permeability coefficients

[0086]

[0087] According to the preliminary selection of the grading according to the predicted permeability coefficient, the condition 80≤K s ≤400mm / h is met.

[0088] The soil sample for the test is prepared according to the composition and ratio of the mixed filler of the grading C, and the optimal water content of the mixed filler obtained by the actual test is re-prepared, and the actual void ratio of the mixed filler is 23.4% under the compaction degree of 90%, which is close to the design target void ratio, and the difference is about 6%. The T variable head permeability test in the Highway Geotechnical Test Specification (JTG 3430-2020) is used to further determine the permeability coefficient K c of the mixed filler under the corresponding compaction degree. The compaction curve of the filler is shown in Figure 4 , the optimal water content thereof is 8.7%, and the permeability coefficient thereof is K c =291.4mm / h, which is greater than the required permeability coefficient K0=280mm / h, and still meets the requirements; and no phenomenon of loss of fine materials with water flow scouring occurs in the test process, which indicates that the designed mixed material has sufficient structural stability.

[0089] For this embodiment, the filler with the grading A cannot meet the requirements due to the too small design target void ratio and too much loess content and too small permeability coefficient. The filler with the grading B cannot meet the requirements due to the too large design void ratio and too little loess content, which leads to too large permeability coefficient and reduces the water retention of the filler layer of the retention zone and is not conducive to the growth of plants. Therefore, after comprehensive comparison, the filler with the grading B has moderate permeability coefficient and meets the design requirements, and can be used as the required grading of the project.

[0090] It can be found by comparing the measured void ratio with the design void ratio that the measured void ratio of the filler designed by the grading design method based on the void ratio is close to the target void ratio, and through the permeability coefficient test, it can be found that the mixed filler designed according to this method can meet the hydrological permeability performance requirements while ensuring the void ratio by adjusting the ratio of each medium.

[0091] The contents not described in detail in the specification belong to the prior art known to those skilled in the art.

Claims

1. A method for designing a filler gradation of a road bio-retention strip based on filler void ratio, characterized in that: The method comprises the following steps: S1: Select the medium filler for improving the natural soil in the road bio-retention zone filler, divide the medium filler with a nominal maximum particle size greater than 9.5 mm into coarse aggregate, divide the medium filler with a nominal maximum particle size less than or equal to 9.5 mm into fine aggregate, take the coarse aggregate as the main skeleton, and calculate the main skeleton porosity T vc ; S2: the volume content T of natural soil in the design gradation vs and the design target porosity T of the filler vm as control indexes, and the value ranges of the two are 18%~28% and 20%~30% respectively; S3: After the value range of several control indicators is preliminarily determined, the amounts of coarse aggregate, fine aggregate and natural soil are calculated based on the principle that the sum of the volume content of fine aggregate, the volume content of natural soil T vs and the design target void of filler T vm equals the main skeleton void ratio T vc ; S4. According to the calculated amounts of coarse aggregate, fine aggregate and natural soil, the particle size distribution related parameters d of different gradations of fillers are calculated 10 20 70 80 The saturated permeability coefficients K of different gradations of fillers are predicted s ​​​​ S5. Selecting the saturated permeability coefficient K of the filler s The design gradation of the filler is preliminarily selected by selecting the saturated permeability coefficient K s The filler gradation between 80-400 mm / h is selected as the next preferred scheme S6, according to the selected filler grading scheme in S5, prepare the soil sample for the test, to the optimum moisture content of the filler obtained by the compaction test, prepare the filler, and determine the permeability coefficient K of each group of fillers at the specified degree of compaction c ; S7, the permeability coefficient K of the filler at the specified compaction degree c The filler grading scheme that meets the requirements is selected as the best one based on economic cost factors, and the permeability coefficient K of each group of fillers at the specified compaction degree is determined c If none of the above meets the requirements, adjust the main skeleton porosity T vc , the design target porosity T of the filler vm , the volume content T of the natural soil vs , repeat S1-S6 until the permeability coefficient K of the filler at the specified compaction degree c meets the requirements; Main skeleton porosity T vc Calculated by the following formula: wherein p is the bulk density of the selected coarse aggregate, kg / m3 sc 3 ;​ p tc ρs is the apparent density of the selected coarse aggregate, kg / m3 3 ; The amount of coarse aggregate, fine aggregate and natural soil is calculated by the following formula: q tc +q fx +q ss = 100 q ss = T vs p ss wherein q tc , q fx , q ss are the percentage of the amount of coarse aggregate, fine aggregate, and natural soil, respectively; p tc , p fx are the apparent density of the coarse aggregate and the combined apparent density of the fine aggregate, respectively, kg / m 3 ; p ss Natural soil bulk density, kg / m 3 ; T vc , T vm are the percentage of the target void fraction of the primary skeleton and of the filler, respectively, T vs is the percentage of the volume content of the natural soil. Saturation permeability K s The prediction formula is as follows: wherein K s is the saturated permeability coefficient, mm / min; d 10 is the effective grain diameter, i.e. the grain diameter below which 10% of the mass of the soil is contained, mm; d 20 is the average void grain diameter, i.e. the grain diameter below which 20% of the mass of the soil is contained, mm; d 70 is the average grain diameter, i.e. the grain diameter below which 70% of the mass of the soil is contained, mm; d 80 is the average void grain diameter, i.e. the grain diameter below which 80% of the mass of the soil is contained, mm; n is the porosity of the filler, which is equal to T vm / 100; γ is the dynamic viscosity of water, cm 2 / s; g is the acceleration of gravity, m / s 2 ; The apparent density of the fine aggregate is calculated by the following formula: In the formula, q1, q2,..., q n represents the passing percentage of each particle size filler in the fine aggregate, and the sum is 100; p1, p2,..., p n represents the apparent density of each particle size filler in the fine aggregate, kg / m 3 ; The grading composition of the fine aggregate is based on the maximum density theory, and the Taylor index n = 0.3-0.5, wherein: wherein: qi is the passing percentage of each particle size filler in the synthetic fine aggregate; D is the maximum particle size in the synthetic fine filler; d i is the value of each particle size in the fine aggregate; Permeability coefficient K at the specified degree of compaction c Satisfies the requirement that the permeability coefficient K at the specified degree of compaction be measured c Not less than the permeability coefficient K0 required to be reached by the filler of the engineering requirement, and 80≤K c ≤400 mm / h; The specified compaction degree is 90%.

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