A method for determining the lateral expansion coefficient of expansive soil

Abstract

The invention discloses a method for determining the lateral expansion coefficient of expansive soil. The method targets the stress and deformation characteristics of the reinforced soil deformation allowed by a flexible support structure, adopts a rectangular parallelepiped specimen, and combines humidification two-dimensional expansion test data to fit a lateral expansion force relationship and a vertical expansion strain relationship. Elastic mechanics is used to analyze the stress and deformation of the specimen, and a theoretical horizontal strain relationship and a theoretical vertical strain relationship are established. The lateral expansion force relationship, the vertical expansion strain relationship, the theoretical horizontal strain relationship, and the theoretical vertical strain relationship are combined to obtain a calculation formula for the lateral expansion coefficient of the expansive soil. The formula can be used to calculate the lateral expansion coefficient of an engineering site. The obtained lateral expansion coefficient can be used for humidification lateral deformation of the expansive soil, stress analysis, humidity stress field and displacement field calculation, and calculation of reinforcement spacing of the flexible support structure. The method has the advantages of high precision, simple operation, and easy promotion.

Description

Technical Field

[0001] The invention relates to a method for measuring the lateral expansion coefficient of expansive soil, and belongs to the technical field of geotechnical engineering. Background Art

[0002] Because expansive soils are rich in highly hydrophilic clay minerals such as montmorillonite, illite, or montmorillonite-illite mixed layers, the soil within the atmospheric influence depth exhibits significant water absorption, swelling, and softening, as well as water loss, shrinkage, and cracking. Shallow collapse and sliding failures are common during engineering excavation of cutting slopes. Therefore, as Jin Yingwei noted in "Application of Geogrid Reinforcement Structures in Treatment of Expansive Soil Cutting Slopes," in the construction of cutting slopes in areas with expansive soils, rigid support structures such as retaining walls and anti-slide piles, or flexible support structures such as geogrid reinforcement, are used to coordinate the expansion and contraction deformation of the expansive soil. Furthermore, Yang Jie et al., in "Research and Application of Anisotropic Humidification Expansion Coefficient of Expansive Soil," pointed out that the expansion coefficient is a core parameter for quantifying the expansion and contraction potential of expansive soils. It can be used to calculate expansion strain and humidity stress fields, and its value directly affects the design safety of support structures and engineering countermeasures. A Chinese invention patent application numbered CN202311495117.7 discloses a method for determining the lateral expansion coefficient of expansive soil. The patent establishes a functional relationship between the unloaded expansion rate and the initial moisture content, as well as a relationship between the vertical loaded expansion rate and the initial moisture content and vertical load, and a relationship between the lateral expansion pressure and the initial moisture content and vertical load. A mathematical model is established based on the stress conditions of the specimen, and a formula for calculating the lateral expansion coefficient is derived. However, the method for determining the lateral expansion coefficient provided in this invention is based on a rigid support structure that does not allow horizontal deformation, or deformation is minimal or even negligible. It is used to calculate the lateral expansion force of expansive soil at different depths behind the rigid structure under humidification conditions. However, the lateral expansion force under the rigid structure is inconsistent with the lateral stress state under the flexible support. Therefore, the expansion coefficient calculation method under the rigid structure is not applicable to the stress and deformation characteristics of flexible support structures, which allow deformation of reinforced soil. Therefore, there is an urgent need for a method to determine the lateral expansion coefficient of expansive soil at different depths under humidification conditions, which is suitable for geogrid-reinforced flexible support structures and can take lateral deformation into consideration, so as to provide a basis for adjusting the design parameters of geogrid-reinforced expansive soil flexible support structures, such as spacing and reinforcement length. Summary of the Invention

[0003] In order to overcome the problems existing in the prior art and considering the lateral deformation characteristics of the flexible support structure, the present invention provides a method for determining the lateral expansion coefficient of expansive soil. The lateral expansion coefficient can be obtained by a simple experiment. The specific technical solution is as follows:

[0004] Take soil from the project site to measure the maximum dry density and optimum moisture content of the expansive soil;

[0005] According to the maximum dry density and the optimum moisture content, standard triaxial specimens and rectangular specimens were made. The initial moisture content of the rectangular specimen was For the best moisture content;

[0006] Conduct unsaturated soil triaxial tests on standard triaxial specimens to establish the elastic modulus Relationship and Poisson's ratio Relationship: , ;

[0007] Where: is the moisture content, 、 、 、 is the fitting parameter;

[0008] Conduct a two-dimensional expansion test on a rectangular specimen to establish the lateral expansion force Relationship and vertical expansion strain Relationship:

[0009] , ;

[0010] Where: is the lateral strain. According to the geogrid type and its elastic stage elongation, multiple lateral strains are determined. ; The overburden pressure is determined based on the atmospheric influence depth at the project site. ; The moisture content increment , is the final moisture content, is the initial moisture content; 、 、 、 、 、 、 、 is the fitting parameter;

[0011] Establish the theoretical horizontal strain of the rectangular specimen Relationship and theoretical vertical strain Relationship: , ;

[0012] Where, is the horizontal stress component, is the vertical stress component, is the horizontal expansion coefficient, are the vertical expansion coefficients respectively;

[0013] correspond , correspond , correspond , correspond , correspond Combined lateral expansion force Relationship, vertical expansion strain Relationship, theoretical horizontal strain Relationship and theoretical vertical strain The lateral expansion coefficient of expansive soil is obtained by the relationship Calculation formula:

[0014] .

[0015] Furthermore, the determination of the maximum dry density and optimum moisture content of the fill includes a wet heavy compaction test and a dry heavy compaction test. The above determination methods are used to perform standardized determination of the maximum dry density and optimum moisture content of expansive soil.

[0016] Furthermore, the compaction degree of the rectangular specimen is set to [93%, 100%) to comply with the Highway Subgrade Design Code (JTGD30-2015) which stipulates that the compaction degree of the lower embankment of a highway must reach 93% or above.

[0017] Furthermore, the method for preparing the rectangular parallelepiped specimen includes a layered static pressing method, which can prevent the prepared specimen from being affected by lateral pressure caused by overlying loads.

[0018] Furthermore, the geogrid type includes a fiberglass grid, and the elastic stage elongation of the fiberglass grid is [0.5%, 1.5%) to ensure that the fiberglass geogrid flexible support structure is in elastic deformation rather than plastic deformation to ensure horizontal deformability.

[0019] Furthermore, the geogrid type includes steel-plastic grid, and the elastic stage elongation of the steel-plastic grid is [0.8%, 2%) to ensure that the geogrid flexible support structure made of steel-plastic material is in elastic deformation rather than plastic deformation to ensure horizontal deformability.

[0020] Furthermore, the geogrid type includes HDPE grid, and the elastic stage elongation of the HDPE grid is (1%, 3%) to ensure that the geogrid flexible support structure made of HDPE material is in elastic deformation rather than plastic deformation to ensure horizontal deformability.

[0021] Furthermore, the geogrid type includes polyester grid, and the elastic stage elongation of the polyester grid is (2%, 4%) to ensure that the polyester geogrid flexible support structure is in elastic deformation rather than plastic deformation to ensure horizontal deformability.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The method for determining the lateral expansion coefficient of expansive soil provided by the present invention takes into account the anisotropic characteristics of the expansive soil in the flexible support structure based on the stress and deformation characteristics of the flexible support structure. This overcomes the drawback of traditional loaded expansion tests that cannot determine the lateral expansion coefficient of soil with horizontal allowable deformation. The method is suitable for calculating the lateral expansion coefficient of expansive soil at different depths within geogrid-reinforced flexible supports. Substituting the resulting lateral expansion coefficient calculation formula into field data, the lateral expansion coefficient of the engineering site can be calculated. The resulting lateral expansion coefficient can be used for humidification lateral deformation of expansive soil, stress analysis, calculation of humidity stress and displacement fields, and calculation of reinforcement spacing of flexible support structures. The method has the advantages of high accuracy, simple operation, and ease of promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a schematic diagram of the structure of a two-dimensional dilatometer;

[0025] Figure 2 It is a schematic diagram of the flexible support structure and the atmospheric influence depth.

[0026] Description of reference numerals:

[0027] 001 geogrid, 002 expansive soil;

[0028] 1 base, 2 water inlet hole, 3 water tank, 4 upper cover plate, 5 permeable stone, 6 loading plate, 7 force plate, 8 displacement benchmark, 9 limit block, 10 lateral displacement screw, 11 pressure sensor, 12 vertical dial indicator, 13 lateral dial indicator. DETAILED DESCRIPTION

[0029] In order to describe the technical solution of the present invention more clearly and completely, the present invention is further described in detail below through specific examples. Unless otherwise defined, all technical and scientific terms used herein have the same meanings as those commonly understood by technicians in the technical field of the present invention.

[0030] The present invention provides a method for measuring the lateral expansion coefficient of expansive soil, comprising the following steps:

[0031] S1. Soil is collected at the construction site in accordance with the provisions of the "Highway Geotechnical Test Code" (JTG3430-2020). The expansive soil samples collected on site are subjected to a wet heavy compaction test or a dry heavy compaction test as specified in the "Soil Test Method Standard" (GB / T 50123) to obtain the maximum dry density and optimal moisture content of the expansive soil filled with the flexible support structure. Based on the requirement that the compaction degree of the lower embankment of the expressway must reach 93% or above as specified in the Highway Subgrade Design Code (JTGD30-2015), the dry density of the rectangular specimen is determined. ; Using the layered static pressure method (see Long Mingxu. Stability analysis method and application of reinforced expansive soil slope considering the influence of lateral expansion_Long Mingxu[D]. Changsha University of Science and Technology, 2021) to make multiple sets of rectangular specimens, and according to the "Standard for Geotechnical Test Methods" (GB / T 50123-2019), make multiple sets of standard triaxial parallel specimens and place them in a moisturizing box for moisture retention. The initial moisture content of the rectangular specimens is The optimal moisture content is 54.77mm long × 54.77mm wide × 20mm high; multiple groups of standard triaxial parallel specimens have different moisture contents. ;

[0032] S2. Carry out triaxial test on the triaxial specimen in S1 according to the provisions of the Highway Geotechnical Test Code (JTG3430-2020) to establish the elastic modulus and moisture content The relationship between Poisson's ratio and water content The relationship between:

[0033] ,

[0034] ;

[0035] in, 、 、 、 is the fitting parameter;

[0036] S3. Consult the data or use dynamic penetration test to determine the local atmospheric influence depth. According to the atmospheric influence depth at the project site and the deadweight of the soil at different depths within the flexible support structure, determine multiple overburden pressures. ;

[0037] S4. Select different types of geogrids based on the deformation characteristics of the roadbed. Determine multiple lateral strains based on the type of geogrid and its elastic stage elongation. The elastic stage elongation refers to the recoverable elongation deformation of the geogrid from the beginning of stress to the reaching of its proportional limit or yield point during the stretching process. The specific value needs to be determined through a standard wide strip tensile test and refer to the product technical data sheets provided by each manufacturer. At the same time, it must meet the requirements of relevant design specifications. The selection should be based on the specific needs of the project for deformation control. The lateral strain is determined by the actual elastic stage elongation of the specific type, model and manufacturer of the geogrid used. , lateral strain The elastic phase elongation of each type of geogrid is within the range, so that the flexible support structure is in elastic deformation rather than plastic deformation to ensure horizontal deformability. Types of geogrids include fiberglass grid, steel-plastic grid, HDPE (High Density Polyethylene) grid, and polyester grid. Among them, the elastic phase elongation of fiberglass grid is [0.5%, 1.5%); the elastic phase elongation of steel-plastic grid is [0.8%, 2%); the elastic phase elongation of HDPE grid is (1%, 3%); the elastic phase elongation of polyester grid is (2%, 4%).

[0038] S5, adopt Figure 1 The two-dimensional expansion instrument shown in the figure performs multiple humidification two-dimensional expansion tests on a rectangular specimen. The operating steps of the humidification two-dimensional expansion test are as follows:

[0039] S51. Place the rectangular specimen in the specimen box above the base 1 of the two-dimensional dilatometer. Apply a thin layer of vaseline to the sides and bottom of the specimen box. Place a cut filter paper slightly smaller than the plane size of the specimen at the bottom of the specimen box. After placing the rectangular specimen in the specimen box, rotate the lateral displacement screw 10 to abut one side of the limit block 9 until the pressure sensor 11 reads 1-3 kPa, indicating that the force plate 7 on the other side of the limit block 9 is in full contact with the specimen. Install the upper cover plate 4, loading plate 6, vertical dial indicator 12, and lateral dial indicator 13.

[0040] S52, overburden pressure determined by S3 , a load is applied once on the upper part of the specimen, that is, the overburden pressure is determined When the displacement reading of the vertical dial indicator 12 does not exceed 0.01 mm per hour, the specimen is considered to be deformed stably and the reading of the pressure sensor 11 is recorded;

[0041] S53, lateral strain determined according to S4 Determine the lateral displacement, rotate the lateral displacement screw 10 until the reading of the lateral dial indicator 13 on the displacement mark 8 reaches the determined lateral displacement, set the vertical dial indicator 12 reading to zero, and then inject pure water into the water tank 3, and always keep the water level 5mm above the top surface of the specimen. The pure water in the water tank 3 penetrates into the rectangular specimen through the water inlet hole 2 and the permeable stone 5. After immersion, record the vertical dial indicator 12 reading every two hours. When the displacement reading per hour does not exceed 0.01mm, record the vertical dial indicator 12 reading and the pressure sensor 11 reading. The difference between the readings of the pressure sensor 11 in step S52 and step S53 is the lateral expansion force The difference between the initial and final readings of the vertical dial gauge 12 is the vertical expansion strain ;

[0042] S54, drain the water, release the load, remove the specimen, wipe the ring wall and other floating water; weigh and dry the rectangular specimen to obtain the final moisture content of the rectangular specimen , and calculate the moisture content increment : ;

[0043] S56, repeat S51 to S54, and perform multiple overburden pressure tests on the remaining rectangular specimens. and multiple lateral strains Wet two-dimensional expansion test;

[0044] S6. Analyze the test results obtained in S5 and establish the lateral expansion force and lateral strain and overburden pressure , moisture content increment The vertical expansion strain is established by the relationship and overburden pressure , lateral expansion force The relationship:

[0045] ,

[0046] ;

[0047] S7, since the expansion deformation test uses a rectangular specimen, its stress and strain can be solved using a rectangular coordinate system; assuming that the soil is an elastic material, considering that the expansion coefficient of the expansive soil sample in the horizontal and vertical directions is anisotropic, the physical equation of plane strain can be used to represent the horizontal strain component and the vertical strain component caused by moisture absorption, that is, the theoretical horizontal strain of the rectangular specimen is Relationship and theoretical vertical strain Relationship:

[0048] ,

[0049] ;

[0050] Where, is the horizontal stress component, is the vertical stress component, is the horizontal expansion coefficient, are the vertical expansion coefficients respectively; the lateral strains due to the humidification two-dimensional expansion test Theoretical horizontal strain Correspondingly, the vertical expansion strain of the humidification two-dimensional expansion test and theoretical vertical strain Correspondingly, the lateral expansion force of the humidification two-dimensional expansion test and horizontal stress component Correspondingly, the overburden pressure of the humidification two-dimensional expansion test and vertical stress components Correspondingly, the horizontal expansion coefficient Lateral expansion coefficient Combined with lateral expansion force Relationship, vertical expansion strain Relationship, theoretical horizontal strain Relationship and theoretical vertical strain The lateral expansion coefficient of expansive soil is obtained by the relationship Calculation formula:

[0051] .

[0052] In practical applications, lateral strain Determined by the elongation of the geogrid, the overburden pressure Determined by the depth of soil excavation. 、 The vertical strain is obtained from the triaxial test of unsaturated soil. , lateral expansion force and moisture content increment The lateral expansion coefficient of the flexible support structure in a specific project is measured by the humidification two-dimensional expansion test. Calculation formula is used to calculate the result.

[0053] Example

[0054] Take the geogrid reinforced expansive soil flexible support structure of the expansive soil slope of a highway as an example. Figure 2As shown in Figure 1, the geogrid-reinforced flexible support structure for an expansive soil slope is as follows: the bottom layer of the support structure is a permeable foundation layer, under which a seepage ditch is excavated; a permeable layer is located between the support structure and the excavation surface, and the permeable layer and the permeable foundation layer are integrated; geogrid 001 reinforcement bars are laid layer by layer throughout the support structure, and the bars of each layer of geogrid 001 exceed the width of the support structure, wrap back to the upper layer, and connect to the bars of the upper layer. Each layer of reinforcement is backfilled and compacted with the excavated expansive soil 002. The slope of the expansive soil slope is 1:1.5, the local atmospheric influence depth is 2.5m, and the geogrid is made of HDPE. Its fill moisture content is controlled according to the optimal moisture content of 20.5% obtained by the wet heavy compaction test, the compaction degree is 93%, and the dry density is 1.65×10 3 kg / m 3 30 rectangular specimens with the initial moisture content of 20.5% and the size of 54.77mm long × 54.77mm wide × 20mm high were made by static pressure method from the expansive soil filler taken from the site, as well as the moisture content of The results are as follows: 18%, 20.5%, 22%, 24% and 26%, and 30 standard triaxial specimens with a size of Φ50mm×100mm.

[0055] The unsaturated soil triaxial test was carried out on the standard triaxial specimen, and the elastic modulus was obtained by fitting , Poisson's ratio and moisture content The relationship between them is:

[0056] ;

[0057] ;

[0058] Since the local atmospheric influence depth is 2.5m, the four overburden pressures are determined. The pressures are 7.5 kPa, 12.5 kPa, 25 kPa, and 50 kPa, respectively. Using HDPE grating produced by a certain manufacturer, whose elastic elongation is 2.8%, five lateral strains were determined: 0%, 0.5%, 1.0%, 1.5%, and 2.0%. A humidified two-dimensional expansion test was conducted on the rectangular specimens, and the test results are shown in Table 1.

[0059] The lateral expansion force is determined by the data in Table 1. and vertical expansion strain The relationship is fitted to obtain:

[0060] ;

[0061] ;

[0062] Increase the moisture content , lateral strain , overburden pressure , elastic modulus after fitting , Poisson's ratio , lateral expansion force Substitute the lateral expansion coefficient of expansive soil into Calculation formula: The lateral expansion coefficient of the HDPE geogrid at different depths within the atmospheric influence depth range is obtained. The specific data are shown in Table 2.

[0063] Table 1 Two-dimensional expansion test results

[0064]

[0065] Table 2 Calculation results of lateral expansion coefficients at different depths of flexible support structures

[0066]

[0067] Currently, the reinforcement length of geogrids in flexible support is typically determined by dividing the local atmospheric influence depth by the slope, then multiplying it by a safety factor. The reinforcement length is typically 3.5-4.5m, and the reinforcement spacing is typically 0.5m. This method does not consider the stress on the geogrid or the expansion characteristics of expansive soils in different regions. Regardless of whether the soil is strong, medium, or weak, the reinforcement length and spacing are the same. This design value is overly risky in areas with strong expansive soils, leading to consequences such as expansive soil cutting landslides. However, the design value is conservative in areas with weak expansive soils, resulting in material waste. The method for determining the lateral expansion coefficient of expansive soil provided by the present invention can be used to calculate the lateral expansion coefficient of expansive soil in the engineering field, and the support structure is designed based on the lateral expansion coefficient. As the lateral expansion coefficient increases, the support structure is adjusted accordingly, such as: increasing the tensile strength of the grid itself, adjusting the spacing of the reinforcement layer, the anchor length, the thickness of the buffer layer or the density of the drainage pipe, etc. The constructed flexible support structure can maintain the stability of the slope, avoid landslides, and reduce subsequent maintenance costs.

[0068] The embodiments of the present invention are described above in conjunction with the accompanying drawings. The above specific embodiments are only illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the present invention and the claims. These all fall within the scope of protection of the present invention.

Claims

1. A method for determining the lateral expansion coefficient of expansive soil, comprising the following steps: Take soil from the project site to measure the maximum dry density and optimum moisture content of the expansive soil; According to the maximum dry density and the optimum moisture content, standard triaxial specimens and rectangular specimens are made, among which, Initial moisture content of rectangular specimen is the optimal moisture content; Conduct unsaturated soil triaxial tests on standard triaxial specimens to establish the elastic modulus Relationship and Poisson's ratio Relationship: , ; Where: is the moisture content, 、 、 、 is the fitting parameter; Conduct a two-dimensional expansion test on a rectangular specimen to establish the lateral expansion force Relationship and vertical expansion strain Relationship: ， ； Where: is the lateral strain. According to the geogrid type and its elastic stage elongation, multiple lateral strains are determined. ; The overburden pressure is determined based on the atmospheric influence depth at the project site. ; The moisture content increment , is the final moisture content, is the initial moisture content; 、 、 、 、 、 、 、 is the fitting parameter; Establish the theoretical horizontal strain of the rectangular specimen Relationship and theoretical vertical strain Relationship: ， ； Where, is the horizontal stress component, is the vertical stress component, is the horizontal expansion coefficient, are the vertical expansion coefficients respectively; correspond , correspond , correspond , correspond , correspond Combined lateral expansion force Relationship, vertical expansion strain Relationship, theoretical horizontal strain Relationship and theoretical vertical strain The lateral expansion coefficient of expansive soil is obtained by the relationship Calculation formula: 。 2. The method for determining the lateral expansion coefficient of expansive soil according to claim 1, wherein: The determination of the maximum dry density and the optimum moisture content of the fill includes a wet heavy compaction test and a dry heavy compaction test.

3. The method for determining the lateral expansion coefficient of expansive soil according to claim 1, wherein: The compaction degree of the rectangular specimen is [93%, 100%).

4. The method for determining the lateral expansion coefficient of expansive soil according to claim 1, wherein: The method for manufacturing the rectangular parallelepiped specimen includes a layered static pressing method.

5. The method for determining the lateral expansion coefficient of expansive soil according to claim 1, wherein: The geogrid type includes a glass fiber grid, and the elastic stage elongation of the glass fiber grid is [0.5%, 1.5%).

6. The method for determining the lateral expansion coefficient of expansive soil according to claim 1, wherein: The geogrid type includes steel-plastic grid, and the elastic stage elongation of the steel-plastic grid is [0.8%, 2%).

7. The method for determining the lateral expansion coefficient of expansive soil according to claim 1, wherein: The geogrid type includes HDPE grid, and the elastic stage elongation of the HDPE grid is (1%, 3%).

8. The method for determining the lateral expansion coefficient of expansive soil according to claim 1, wherein: The geogrid type includes polyester grid, and the elastic stage elongation of the polyester grid is (2%, 4%).

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