Method for measuring lateral expansion coefficient of expansive soil

Abstract

The invention discloses a method for measuring the lateral expansion coefficient of expansive soil, which comprises the following steps of: aiming at the stress and deformation characteristics of a flexible support structure under the condition of permitting reinforcement soil deformation, fitting a lateral expansion force relational expression and a vertical expansion strain relational expression by adopting a cuboid test piece and combining humidifying two-dimensional expansion test data, and determining the lateral expansion coefficient of the expansive soil. The stress and deformation of a test piece are analyzed by utilizing elastic mechanics, a theoretical horizontal strain relational expression and a theoretical vertical strain relational expression are established, and a lateral expansion coefficient calculation formula of expansive soil is obtained by combining a lateral expansion force relational expression, a vertical expansion strain relational expression, the theoretical horizontal strain relational expression and the theoretical vertical strain relational expression. The formula can be used for calculating the engineering field lateral expansion coefficient; the obtained lateral expansion coefficient can be used for swelling soil humidifying lateral deformation, stress analysis, humidity stress field and displacement field calculation and flexible supporting structure reinforcement spacing calculation, and has the advantages of being high in precision, easy to operate and easy to popularize.

Description

Technical Field

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

[0002] Since expansive soil is rich in strongly hydrophilic clay minerals such as montmorillonite, illite or montmorillonite-illite mixed layers, the soil within the depth affected by the atmosphere has significant characteristics of water absorption and expansion softening and water loss and shrinkage cracking. Shallow landslides often occur during the excavation of road cut slopes in engineering. Therefore, as mentioned by Jin Yingwei in "Application of Geogrid Reinforced Structure in the Treatment of Expansive Soil Road Cut Slopes", in the construction of road cut slopes in areas where expansive soil is distributed, rigid support structures such as retaining walls and anti-slide piles, or flexible support structures such as geogrid reinforced structures are used to coordinate the expansion and contraction deformation of the expansive soil mass. Yang Jie et al. pointed out in "Research and Application of Anisotropic Wetting Expansion Coefficient of Expansive Soil" that the expansion coefficient is the core parameter for quantifying the expansion and contraction potential of expansive soil, which can be used for calculating expansion strain and humidity stress field, and its value directly affects the design safety of the support structure and engineering countermeasures. The Chinese invention patent with the application number CN202311495117.7 discloses a method for measuring the lateral expansion coefficient of expansive soil, establishing a functional relationship between the unloaded swelling ratio and the initial water content, a relationship between the vertical loaded swelling ratio and the initial water content and vertical load, and a relationship between the lateral expansion pressure and the initial water content and vertical load. A mathematical model is established according to the stress condition of the specimen, and the calculation formula for the lateral expansion coefficient is derived. However, the method for measuring the lateral expansion coefficient provided by this invention is based on a rigid support structure where horizontal deformation is not allowed or the deformation is very small or even negligible, and is used to calculate the lateral expansion force of expansive soil at different depths behind the rigid structure under humidification conditions. The lateral expansion force under the rigid structure is inconsistent with the lateral stress state under the flexible support. The calculation method for the expansion coefficient under the rigid structure is not applicable to the stress and deformation characteristics of the flexible support structure that allows the deformation of the soil-reinforcement. Therefore, there is an urgent need for a method for measuring the lateral expansion coefficient of expansive soil at different depths under humidification conditions, which is applicable to the flexible support structure of geogrid reinforcement, and can consider the lateral deformation, so as to provide a basis for adjusting the design parameters such as spacing and reinforcement length of the geogrid reinforced flexible support structure for expansive soil. Summary of the Invention

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

[0004] Take soil at the engineering site and measure the maximum dry density and the optimum water 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 is the optimal 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] 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;

[0010] Establish the theoretical horizontal strain of the rectangular specimen Relationship and theoretical vertical strain Relationship: , ;

[0011] Where, is the horizontal stress component, is the vertical stress component, is the horizontal expansion coefficient, are the vertical expansion coefficients respectively;

[0012] corresponding to , corresponding to , corresponding to , corresponding to , corresponding to ; combined with the lateral expansion force relation formula, vertical expansion strain relation formula, theoretical horizontal strain relation formula and theoretical vertical strain relation formula, to obtain the lateral expansion coefficient of expansive soil Calculation formula: 。

[0013] Furthermore, the determination of the maximum dry density and the optimum moisture content of the fill soil includes the determination by the wet method heavy compaction test and the determination by the dry method heavy compaction test. The above-mentioned determination methods are used to standardize the determination of the maximum dry density and the optimum moisture content of expansive soil.

[0014] Furthermore, the compaction degree of the cuboid specimen is taken as [93%, 100%) to meet the requirement that the compaction degree of the subgrade of the expressway lower embankment specified in the Highway Subgrade Design Code (JTGD30 - 2015) must reach 93% or above.

[0015] Furthermore, the manufacturing method of the cuboid specimen includes the layered static pressing method. The layered static pressing method can avoid the influence of the lateral pressure caused by the overlying load on the prepared specimen.

[0016] Furthermore, the type of geogrid includes glass fiber grid, and the elongation rate at the elastic stage of the glass fiber grid is [0.5%, 1.5%) to ensure that the flexible support structure of the glass fiber geogrid is in elastic deformation rather than plastic deformation to ensure the deformability in the horizontal direction.

[0017] Furthermore, the type of geogrid includes steel - plastic grid, and the elongation rate at the elastic stage of the steel - plastic grid is [0.8%, 2%) to ensure that the flexible support structure of the steel - plastic geogrid is in elastic deformation rather than plastic deformation to ensure the deformability in the horizontal direction.

[0018] Furthermore, the type of geogrid includes HDPE grid, and the elongation rate at the elastic stage of the HDPE grid is (1%, 3%) to ensure that the flexible support structure of the HDPE geogrid is in elastic deformation rather than plastic deformation to ensure the deformability in the horizontal direction.

[0019] Furthermore, the type of geogrid includes polyester geogrid, and the elongation rate of the polyester geogrid in the elastic stage is (2%, 4%) to ensure that the flexible support structure made of polyester geogrid is in elastic deformation rather than plastic deformation, so as to ensure the deformability in the horizontal direction.

[0020] The present invention has the following beneficial effects compared with the prior art:

[0021] The method for measuring the lateral expansion coefficient of expansive soil provided by the present invention considers the anisotropic characteristics of the expansive soil mass in the flexible support structure according to the stress and deformation characteristics of the flexible support structure, overcomes the drawback that the traditional loaded expansion test cannot obtain the lateral expansion coefficient of the soil mass with allowable deformation in the horizontal direction, and is applicable to the calculation of the lateral expansion coefficient of expansive soil at different depths inside the geogrid-reinforced flexible support. After substituting the obtained lateral expansion coefficient calculation formula into the on-site data, the lateral expansion coefficient of the engineering site can be calculated; the obtained lateral expansion coefficient can be used for the calculation of the lateral deformation, stress analysis, humidity stress field and displacement field of the expansive soil during humidification, as well as the calculation of the reinforcement spacing of the flexible support structure, and has the advantages of high accuracy, simple operation and easy popularization. Description of the Drawings

[0022] Figure 1 is a schematic structural diagram of a two-dimensional dilatometer;

[0023] Figure 2 is a schematic diagram of the flexible support structure and the depth of the atmospheric influence.

[0024] Description of the Reference Numerals:

[0025] 001 Geogrid, 002 Expansive soil;

[0026] 1 Base, 2 Water inlet hole, 3 Water tank, 4 Upper cover plate, 5 Permeable stone, 6 Loading plate, 7 Stress plate, 8 Displacement rod, 9 Limit block, 10 Lateral displacement screw rod, 11 Pressure sensor, 12 Vertical dial gauge, 13 Lateral dial gauge. Detailed Embodiments

[0027] In order to describe the technical solution of the present invention more clearly and completely, the following further details the present invention through specific embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs.

[0028] The present invention provides a method for measuring the lateral expansion coefficient of expansive soil, including the following steps:

[0029] 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. ;

[0030] 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: , ;

[0031] in, 、 、 、 is the fitting parameter;

[0032] 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. ;

[0033] 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 elongation at the elastic stage refers to the recoverable elongation deformation that occurs during the tensile process of the geogrid from the start of loading until it reaches its proportional limit or yield point. The specific value needs to be determined through the standard wide-strip tensile test, referring to the product technical data sheets provided by each manufacturer, and meeting the requirements of relevant design specifications. When selecting, it should be determined by the actual elongation at the elastic stage of the specific type, model, and manufacturer of the geogrid used according to the specific requirements of the project for deformation control, and the lateral strain , the lateral strain is within the range of the elongation at the elastic stage of different types of geogrids, so that the flexible support structure is in elastic deformation rather than plastic deformation to ensure deformability in the horizontal direction. The types of geogrids include fiberglass grids, steel-plastic grids, HDPE (High Density Polyethylene) grids, and polyester grids; among them, the elongation at the elastic stage of fiberglass grids is [0.5%, 1.5%); the elongation at the elastic stage of steel-plastic grids is [0.8%, 2%); the elongation at the elastic stage of HDPE grids is (1%, 3%); the elongation at the elastic stage of polyester grids is (2%, 4%);

[0034] S5. Adopt a two-dimensional dilatometer as Figure 1 shown to conduct multiple wetting two-dimensional swelling tests on cuboid specimens. The operating steps of the wetting two-dimensional swelling test are as follows:

[0035] S51. Place the cuboid specimen in the specimen box above the base 1 of the two-dimensional dilatometer. Apply a thin layer of vaseline on both sides and the bottom of the specimen box. Place the cut filter paper at the bottom of the specimen box, with a size slightly smaller than the plane size of the specimen; after placing the cuboid specimen in the specimen box, rotate the lateral displacement screw rod 10 to abut against one side of the limit block 9 until the reading of the pressure sensor 11 is 1 - 3 kPa, which can be regarded as the force plate 7 on the other side of the limit block 9 being in full contact with the specimen. Then place the upper cover plate 4, loading plate 6, vertical dial gauge 12, and lateral dial gauge 13;

[0036] S52. According to the overburden pressure determined in S3 , apply the load once on the upper part of the specimen, that is, the determined overburden pressure . When the displacement reading of the vertical dial gauge 12 per hour does not exceed 0.01 mm, it is considered that the specimen deformation is stable, and record the reading of the pressure sensor 11;

[0037] S53. According to the lateral strain determined in S4 Determine the lateral displacement. Rotate the lateral displacement screw rod 10 until the reading of the lateral dial gauge 13 on the displacement benchmark 8 reaches the determined lateral displacement. Set the reading of the vertical dial gauge 12 to zero. Then, pour pure water into the water tank 3 and always keep the water surface 5 mm above the top surface of the specimen. The pure water in the water tank 3 infiltrates into the cuboid specimen through the water inlet hole 2 and the permeable stone 5. After immersion, record the reading of the vertical dial gauge 12 every two hours. When the displacement reading per hour does not exceed 0.01 mm, record the reading of the vertical dial gauge 12 and the reading of the pressure sensor 11. The difference between the readings of the pressure sensor 11 in step S52 and step S53 is the lateral swelling force The difference between the initial and final readings of the vertical dial gauge 12 is the vertical swelling strain ;

[0038] S54. Drain the water, relieve the load, take out the specimen, and dry the ring wall and other floating water; weigh and dry the cuboid specimen to obtain the final moisture content of the cuboid specimen and calculate the moisture content increment : ;

[0039] S56. Repeat S51 to S54 for the remaining cuboid specimens to conduct humidification two-dimensional swelling tests with multiple overburden pressures and multiple lateral strains ;

[0040] S6. Analyze the test results obtained in S5, and establish the relationship between the lateral swelling force and the lateral strain , the overburden pressure , and the moisture content increment . Establish the relationship between the vertical swelling strain and the overburden pressure , and the lateral swelling force : , ;

[0041] S7. Since the swelling deformation test uses a cuboid specimen, its stress and strain can be solved using a rectangular coordinate system; assuming that the soil is an elastic material and considering that the swelling coefficients of the swelling soil sample are anisotropic in the horizontal and vertical directions, the physical equation of plane strain can be used to characterize the strain components in the horizontal and vertical directions caused by moisture absorption, that is, the theoretical horizontal strain relationship and the theoretical vertical strain relationship: , ;

[0042] In the formula, is the horizontal stress component, is the vertical stress component, is the horizontal expansion coefficient, are the vertical expansion coefficients respectively; since the lateral strain in the two-dimensional wetting expansion test corresponds to the theoretical horizontal strain , the vertical expansion strain in the two-dimensional wetting expansion test corresponds to the theoretical vertical strain , the lateral expansion force in the two-dimensional wetting expansion test corresponds to the horizontal stress component , the overburden pressure in the two-dimensional wetting expansion test corresponds to the vertical stress component , the horizontal expansion coefficient is the lateral expansion coefficient . Combining the lateral expansion force relationship, the vertical expansion strain relationship, the theoretical horizontal strain relationship and the theoretical vertical strain relationship, the calculation formula for the lateral expansion coefficient of the expansive soil is obtained: .

[0043] In practical applications, the lateral strain is determined by the elongation rate of the geogrid, the overburden pressure is determined according to the soil sampling depth, , are obtained from the unsaturated triaxial test, the vertical strain , the lateral expansion force and the moisture content increment are measured by the two-dimensional wetting expansion test. Thus, in a specific project, the lateral expansion coefficient in the flexible support structure is calculated through the above-mentioned calculation formula for the lateral expansion coefficient <C .

[0044] Example

[0045] Taking the geogrid-reinforced expansive soil flexible support structure of an expressway expansive soil slope as an example, as Figure 2As shown in the figure, 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, and a drainage ditch is excavated under the permeable foundation layer; there is a permeable layer between the support structure and the excavation surface, and the permeable layer is integrated with the permeable foundation layer; geogrid 001 reinforcement materials are laid layer by layer in the entire support structure, and the reinforcement materials of each layer of geogrid 001 exceed the width of the support structure, wrap around to the upper layer, and are connected to the reinforcement materials of the upper layer. Each layer of reinforcement materials is backfilled and compacted with the excavated expansive soil 002. The slope ratio of the expansive soil slope is 1:1.5, the local atmospheric influence depth is 2.5 m, the geogrid is made of HDPE material, and its filling moisture content is controlled according to the optimal moisture content of 20.5% obtained from the wet method heavy compaction test, the compaction degree is 93%, and the dry density is 1.65×10 3 kg / m 3 . 30 cuboid specimens with an initial moisture content of 20.5% and dimensions of 54.77 mm in length × 54.77 mm in width × 20 mm in height are made from the expansive soil filler taken from the site by the static pressure method, and the moisture contents are 18%, 20.5%, 22%, 24%, 26%, and 30 standard triaxial specimens with dimensions of Φ50 mm × 100 mm are made.

[0046] Unconsolidated-undrained triaxial tests are carried out on the standard triaxial specimens, and the elastic modulus , Poisson's ratio and the moisture content are fitted to obtain the relationship as follows: ; ;

[0047] Since the local atmospheric influence depth is 2.5 m, 4 overburden pressures are determined to be: 7.5 kPa, 12.5 kPa, 25 kPa, 50 kPa respectively; an HDPE grille produced by a certain manufacturer is used, and its elongation rate in the elastic stage is 2.8%, and 5 lateral strains are determined to be: 0%, 0.5%, 1.0%, 1.5%, 2.0%. Two-dimensional swelling tests with humidity increase are carried out on the cuboid specimens, and the test results are shown in Table 1.

[0048] Based on the data in Table 1, the relationship between the lateral swelling force and the vertical swelling strain is fitted to obtain: ; ;

[0049] The moisture content increment , lateral strain , overburden pressure , and the fitted elastic modulus , Poisson's ratio , Lateral swelling force Substitute the lateral swelling coefficient of the expansive soil into the calculation formula: to obtain the lateral swelling coefficients of the HDPE geogrid at different depths within the depth of atmospheric influence. The specific data are shown in Table 2.

[0050] Table 1 Results of two-dimensional swelling test

[0051] Table 2 Calculation results of lateral swelling coefficients at different depths of flexible support structures

[0052] Currently, the reinforcement length of the geogrid in flexible support is usually determined by dividing the local depth of atmospheric influence by the slope ratio and then multiplying by a safety factor. The commonly used value of the reinforcement length is 3.5 - 4.5 m, and the reinforcement spacing is generally taken as 0.5 m. The above method does not consider the stress condition of the geogrid, nor does it consider the swelling characteristics of expansive soils in different regions. Whether it is strong expansive soil, medium expansive soil or weak expansive soil, the values of the reinforcement length and spacing are the same. The design value in the area of strong expansive soil is too risky, resulting in consequences such as the landslide of expansive soil cut slopes, while the design value in the area of weak expansive soil is too conservative, causing material waste. By using the method for measuring the lateral swelling coefficient of expansive soil provided by the present invention, the lateral swelling coefficient of the expansive soil in the engineering field can be calculated. Based on this lateral swelling coefficient, the support structure is designed. As the lateral swelling 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 anchorage length, the thickness of the buffer layer or the density of the drain pipes. The constructed flexible support structure can maintain the stability of the slope, avoid landslides and reduce the later maintenance cost.

[0053] The embodiments of the present invention have been described above with reference to the accompanying drawings. The above specific embodiments are illustrative rather than restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many forms without departing from the purpose of the present invention and the scope protected by the claims. All of these belong to the protection scope of the present invention.

Claims

1. A method for measuring the lateral expansion coefficient of expansive soil, comprising the following steps: Taking soil at the engineering site and measuring the maximum dry density and the optimum moisture content of the expansive soil; According to the maximum dry density and the optimum moisture content, standard triaxial specimens and cuboid specimens are made, where Initial moisture content of rectangular specimen For the best moisture content; Perform unsaturated soil triaxial tests on standard triaxial specimens to establish the elastic modulus relationship and the Poisson's ratio relationship: , ; Where: is the moisture content, , , , are fitting parameters; Perform a two-dimensional swelling test on a cuboid specimen to establish the lateral swelling force relationship and the vertical swelling strain relationship: ， ； Wherein: is the lateral strain, and multiple lateral strains are determined according to the type of geogrid and its elongation rate in the elastic stage ; is the overburden pressure, and multiple overburden pressures are determined according to the atmospheric influence depth at the engineering site ; is the moisture content increment , is the final moisture content, is the initial moisture content; , , , , , , , are fitting parameters; Establish the theoretical horizontal strain relationship and the theoretical vertical strain relationship: ， ； In the formula, 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 measurement of the maximum dry density and the optimum moisture content of the filled soil includes the measurement by wet method heavy compaction test and the measurement by dry method heavy compaction test.

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

4. The method for measuring the lateral expansion coefficient of expansive soil according to claim 1, characterized in that The manufacturing method of the cuboid specimen includes the layered static pressing method.

5. The method for determining the lateral expansion coefficient of expansive soil according to claim 1, wherein: The types of geogrids include fiberglass geogrids, and the elongation rate in the elastic stage of the fiberglass geogrids is [0.5%, 1.5%).

6. The method for determining the lateral expansion coefficient of expansive soil according to claim 1, wherein: The types of geogrids include steel-plastic geogrids, and the elongation rate in the elastic stage of the steel-plastic geogrids is [0.8%, 2%).

7. The method for determining the lateral expansion coefficient of expansive soil according to claim 1, wherein: The types of geogrids include HDPE geogrids, and the elongation rate in the elastic stage of the HDPE geogrids is (1%, 3%).

8. The method for measuring the lateral expansion coefficient of expansive soil according to claim 1, wherein, The types of geogrids include polyester geogrids, and the elongation rate in the elastic stage of the polyester geogrids is (2%, 4%).

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