A numerical analysis method for ecological slope of expansive soil

Abstract

The present invention discloses a numerical analysis method for an expansive soil ecological slope, comprising the following steps: using finite element software to establish a slope model, defining material parameters, assembling, establishing analysis steps, applying boundary conditions and loads, meshing, submitting calculations, and analyzing results. Field surveys are conducted to obtain slope characteristics and root morphological parameters; the cohesion increment of the shallow soil of the slope is calculated in layers and the material parameters are changed to reflect the reinforcement effect of the root system, and anchor rods are added to reflect the anchoring effect of the main taproot system; the thermal expansion coefficient is inverted from the experimental results of the swelling force of the rooted soil, and the humidity field is simulated by the temperature field. The present invention can establish a finite element numerical model of an expansive soil ecological slope that conforms to reality, which not only takes into account the soil consolidation effect of the root system, but also reflects the expansion and contraction characteristics of the expansive soil; the analysis results can provide a reference for the ecological protection of the expansive soil slope, and further in-depth research under other working conditions can be carried out on the basis of this model.

Description

Technical Field

[0001] The invention relates to the technical field of numerical analysis of expansive soil ecological slopes, and in particular to a finite element analysis method for numerically simulating expansive soil ecological slopes. Background Art

[0002] Expansive soil is a widely distributed type of engineering disaster soil. Common treatment methods include physical and chemical improvement, non-expansive soil replacement, etc. However, these methods have defects such as large masonry and material consumption, and in the long run they do not conform to the concept of green development. Therefore, it is of great value to use ecological protection for the treatment of expansive soil slopes.

[0003] Ecological protection methods include laying turf, artificial transplanting, grass seed spraying, and eco-bag greening. The above-ground part of the plant covering the soil layer can reduce the scouring of the surface soil of the slope by rainwater and control the change of surface moisture. The fine roots can play a reinforcing role and enhance the shear strength of the soil. The thick roots are similar to anchor rods and play an anchoring role. The wind and the deadweight of the vegetation may have an adverse effect on the stability of the slope. Therefore, the ecological greening of the expansive soil slope is mainly based on the combination of grass and shrubs. Herbs with shallow roots and close to the ground can effectively slow down the scouring of precipitation and shallow reinforcement. Shrubs with deep roots and tall plants play a role in deep anchoring and retaining soil moisture because of their strong and developed taproots. Landslides on expansive soil slopes are mostly shallow landslides. The ecological protection form of grass and shrubs can not only play the mechanical role of shallow reinforcement and deep anchoring, but also control the development of cracks in the surface soil of the slope, and the roots can retain soil moisture, thereby inhibiting the expansion and contraction of the expansive soil. The combination of grass and shrubs has been widely used in the ecological protection of expansive soil slopes.

[0004] The stability analysis of engineering slopes is a major topic in the field of geotechnical engineering. A large number of studies have been conducted by predecessors, but methods for calculating the stability of expansive soil ecological slopes under the effect of root consolidation and slope protection are relatively rare. In the existing analysis of the role of vegetation roots in consolidating soil and protecting slopes, for example, a patent application with publication number CN111581853A discloses a slope ecological control stability analysis method, which quantifies the mechanical effects of plant root reinforcement and anchoring through experiments, and uses the strip method to calculate the slope stability. For example, the invention patent with patent number 201810602863.4 provides a prediction method for calculating the shear strength of slope soil enhanced by plant roots as it changes with depth through on-site shear tests. The above two methods are suitable for actual engineering because of their simple operation, but they lack consideration of the swelling and shrinking characteristics of expansive soil, and it is difficult to obtain the dynamic change trend of slope deformation and stress, which limits their scope of use. Summary of the invention

[0005] The purpose of the present invention is to provide a numerical analysis method for expansive soil ecological slopes, which not only takes into account the influence of the swelling and shrinking characteristics of the expansive soil itself, but also quantifies the shallow reinforcement effect of the herbaceous vegetation root system and the deep anchoring effect of the main taproot system of the shrub.

[0006] The finite element analysis method used in the present invention can take into account the expansion and contraction characteristics of the expansive soil itself and the quantitative root consolidation effect, thereby obtaining more accurate slope stress and deformation calculation results, providing a certain reference for the practice of ecological protection engineering. The numerical analysis method of the expansive soil ecological slope formed by the present invention can also provide support for further consideration of complex working conditions on the expansive soil slope.

[0007] To achieve the above object, the present invention provides a numerical analysis method for expansive soil ecological slope, comprising the following steps:

[0008] S1. Using finite element software Abaqus to construct a slope model and a shrub main taproot system model, wherein the shrub main taproot system in the shrub main taproot system model is equivalent to an anchor rod; that is, the shrub main taproot system model established in this step is an anchor rod model;

[0009] S2. Assign physical and mechanical parameters to the surface soil of the slope and the main taproot system of the shrubs in the model, and assign values ​​to the layers of the surface soil that can be affected by the vegetation roots;

[0010] S3, constructing the model assembly and analysis steps, and embedding the main taproot system of the shrub into the slope model;

[0011] S4. Implement boundary conditions and loads, and mesh division, and add temperature fields to equalize the expansion and contraction of the slope caused by the change in moisture content of the expansive soil;

[0012] S5. Submit calculation and result analysis: Obtain the slope stability coefficient and slope stress and deformation, analyze the slope failure trend by cumulative plastic strain and stress, and identify weak areas; then dynamically adjust the spatial distribution position and density of vegetation roots on the slope to provide a vegetation greening plan that can enhance the stability of expansive soil slopes.

[0013] Furthermore, in step S1, the morphological parameters of the expansive soil ecological slope and the vegetation root system can be obtained by field survey sampling; before constructing the model, the following steps are also included:

[0014] S1.1. Conduct on-site survey of the expansive soil ecological slope to obtain slope surface information, which includes basic dimensional characteristics such as slope height and slope gradient;

[0015] S1.2. Conduct a sampling survey on the growth of existing vegetation on the expansive soil ecological slope, and sample the roots of the dominant shrubs and herbaceous plants in the typical section of the expansive soil ecological slope using the full excavation method to obtain the morphological characteristic parameters of the vegetation root distribution; the root distribution morphological characteristic parameters include the root distribution range and the quantitative distribution of roots of different diameters along the depth direction.

[0016] Furthermore, in step S1.1, the on-site investigation of the expansive soil ecological slope also includes: first sampling the slope soil, and then conducting geotechnical tests on the soil samples obtained in the laboratory to obtain multiple physical and mechanical indicators including soil specific gravity, liquid limit, plastic limit, swelling force, internal friction angle, cohesion, free swelling rate, and the relationship between swelling rate and moisture content.

[0017] Furthermore, in step S2, the physical and mechanical parameters of the soil within the root growth range of the slope can be obtained through a pull-out test and a simplified calculation formula for the added value of the shear strength of the soil by the roots, which specifically includes the following steps:

[0018] S2.1. Conduct pull-out tests on the roots of selected shrubs and herbaceous plants to obtain the maximum pull-out force and elastic modulus of the roots, and the tensile strength T a Fit the relationship between the root diameter D to obtain the empirical formula of the power function:

[0019] T a =αD -β (1);

[0020] In formula (1), α and β are both fitting empirical parameters. The root systems of the selected shrubs and herbaceous plants mentioned above refer to the root systems of the shrubs and herbaceous plants selected in the typical cross section.

[0021] S2.2. The slope soil is divided into four depth ranges from the surface to the inside, namely 0-10 cm, 10-20 cm, 20-30 cm and 30-40 cm, and Wu's theory is used to calculate the enhancement effect of various vegetation roots on cohesion in each depth range:

[0022]

[0023] In formula (2), Δc i Added value of root soil cohesion; t iR A is the shear strength of the root system of the i-th vegetation at the R-th diameter level; si / A R It is the ratio of the cross-sectional area of ​​the root system at the Rth diameter level to the total shear area, also known as the root area ratio RAR iR ; It is used to measure the shear strength of roots in slope soil at different angles, which can be taken as a constant of 1.2. Vegetation roots are divided into four diameter classes according to 0-1mm, 1-2mm, 2-5mm and above 5mm. The enhancing effect of various vegetation roots on cohesion, i.e. the influence of various vegetation roots at different depths on cohesion, includes roots of herbaceous plants and shrubs.

[0024] Furthermore, in step S3, the main taproot system of the shrub is added to the finite element model by embedding anchor rods, the size of the anchor rods is determined by the size of the main root of the shrub root system, and the elastic modulus of the anchor rods is determined by a pull-out test. Specifically, the following steps are included:

[0025] S3.1. Assemble the main taproot system of the shrubs, which is equivalent to anchor rods, to the slope model in an embedded manner;

[0026] S3.2, establish a geostress balance analysis step, in which gravity load is applied to the entire model and geostress balance is performed;

[0027] S3.3. Establish the first static equilibrium analysis step for applying external loads. In this analysis step, other loads acting on the slope are determined according to different working conditions, and the initial temperature within the depth of sudden atmospheric influence is defined as equivalent to the initial humidity.

[0028] S3.4. Establish the second static equilibrium analysis step of moisture content change. In this analysis step, the temperature in the depth range of rapid atmospheric influence is changed to achieve an equivalent humidity field, so as to realize the influence of expansion deformation of expansive soil on the slope under humidity change.

[0029] Furthermore, in step S4, the temperature field is added to be equivalent to the humidity field, so as to reflect the expansion and contraction characteristics of the expansive soil; specifically, the following steps are included:

[0030] S4.1. Establishing boundary conditions of the slope model: completely constraining the bottom surface of the slope model, and establishing constraints along the extension direction on each side of the slope model;

[0031] S4.2, applying load: adding gravity in the geostress equilibrium analysis step to simulate geostress equilibrium, and applying loads of different magnitudes according to different working conditions in the first static equilibrium analysis step;

[0032] S4.3, constructing a temperature field: in the first static equilibrium analysis step, the soil at the depth of rapid atmospheric influence is given an initial temperature to simulate the humidity field, and in the second static equilibrium analysis step, the temperature is changed to achieve the expansion and contraction deformation and force effect of the shallow expansive soil due to the change in humidity;

[0033] S4.4. Model grid division: Select an appropriate grid division method based on the morphological characteristics of the established slope model and the main taproot system model of the shrub (ie, the anchor model).

[0034] Furthermore, in step S4.3, the temperature field of the expansive soil slope is defined in each analysis step, which specifically includes the following steps:

[0035] S4.31. Conduct swelling force tests on remolded soil with different root content to obtain swelling characteristic index of rooted soil; the root content of remolded soil is determined based on the distribution density of roots at shallow depths obtained from on-site investigation;

[0036] S4.32, obtaining the linear expansion coefficient by performing a ring cutter expansion test in the Abaqus finite element software; that is, using the Abaqus finite element software to establish a ring cutter model of equal size, constraining the deformation of the bottom and side surfaces of the ring cutter model in Abaqus, applying a vertical load equal to the maximum expansion force in the root-containing soil expansion characteristic index obtained in the step S4.31 to the surface of the ring cutter model, and setting the starting and ending temperatures, and then changing the temperature expansion coefficient, the linear expansion coefficient is obtained under the control condition that the ring cutter sample does not produce expansion deformation;

[0037] S4.33. Assign the linear expansion coefficient obtained in the step S4.32 to the material parameters of the soil, and assign the initial temperature and the ending temperature equivalent to the ring knife test in the first static analysis step and the second static analysis step respectively. The temperature field application area is limited to the depth of rapid influence of the atmosphere.

[0038] Furthermore, the depth of rapid atmospheric impact of the expansive soil may be determined according to the temperature, evaporation and rainfall of the area where the expansive soil ecological slope is located, and may also be determined from 1.35 to 2.25 m.

[0039] Furthermore, there are five types of main taproot systems of shrubs, namely, type I, type H, type V, type R, and type VH, which are specifically determined according to the investigation of the morphology of the main taproot systems of shrubs in the on-site sampling in step S1.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] The present invention can provide a practical, accurate and easy-to-promote calculation method for the stability analysis of expansive soil ecological slopes. On the one hand, the method investigates the distribution of the root morphology of the surface soil, quantifies the increment of the cohesion of the surface soil with a simplified formula to express the shallow reinforcement effect mainly played by the roots of herbaceous plants; establishes anchor rods to simulate the anchoring effect of the main taproot of shrubs; and simulates the expansion and contraction deformation and force caused by the change of the moisture content of the expansive soil by constructing an equivalent humidity field of the temperature field, reflecting the characteristics of the expansive soil.

[0042] On the other hand, compared with existing stability calculation methods such as the strip method, the present invention adopts the finite element analysis method to avoid tedious theoretical calculations, and the calculation results are more accurate and reliable. The weak points can be found according to the displacement stress cloud map in the calculation results to guide engineering practice. Moreover, this method can be used as a basis for in-depth development according to other complex working conditions, which has certain promotion value.

[0043] In addition to the above-described purposes, features and advantages, the present invention has other purposes, features and advantages. The present invention will be further described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The accompanying drawings are used to provide a further understanding of the embodiments of the present invention and constitute a part of the specification. Together with the following specific implementations, they are used to explain the embodiments of the present invention, but do not constitute a limitation on the embodiments of the present invention. In the accompanying drawings:

[0045] Figure 1 It is a flow chart of the expansive soil ecological slope stability analysis method of the preferred embodiment of the present invention;

[0046] Figure 2 It is a schematic diagram of improving the shear strength of soil by reinforcing the root system in a preferred embodiment of the present invention;

[0047] Figure 3 The distribution of the increase in shear strength caused by the vegetation roots on the slope along the depth in the preferred embodiment of the present invention;

[0048] Figure 4 A schematic diagram of the swelling force of expansive soil under different root contents in a preferred embodiment of the present invention;

[0049] Figure 5 It is the vertical displacement field of the ring cutter model constructed in the preferred embodiment of the present invention after the expansion force is balanced;

[0050] Figure 6 A vertical stress cloud diagram of the model constructed in the preferred embodiment of the present invention;

[0051] Figure 7 A vertical displacement cloud diagram of the model constructed in the preferred embodiment of the present invention;

[0052] Figure 8 It is a plastic strain cloud diagram of the model constructed in the preferred embodiment of the present invention.

[0053] Fig. 9 The main taproot system model diagrams of shrubs available for selection in the preferred embodiment of the present invention are five types, namely, type I, type H, type V, type R, and type VH. DETAILED DESCRIPTION

[0054] The embodiments of the present invention are described in detail below with reference to the accompanying drawings, but the present invention can be implemented in many different ways as defined and covered by the claims.

[0055] See also Figure 1 This embodiment provides a numerical analysis method for an expansive soil ecological slope. The basic steps are to input the parameters obtained from the test into the Abaqus finite element software for numerical simulation. The surface cohesion of the ecological slope is assigned values ​​according to the stratification. The main taproot system of the shrub is replaced by an embedded anchor rod, and the expansive soil expansion force is considered in the temperature field. The stability coefficient of the expansive soil ecological slope is calculated according to the strength reduction method. Specifically, the following steps are included:

[0056] S1. Use finite element software Abaqus to build a slope model and a shrub main taproot system model. The shrub main taproot system in the shrub main taproot system model is equivalent to an anchor rod; that is, the shrub main taproot system model established in this step is an anchor rod model. Before building the model, the following steps are also included:

[0057] S1.1. Conduct on-site survey of the expansive soil ecological slope to obtain slope surface information, including slope height and slope gradient.

[0058] S1.2. Sampling survey of the existing vegetation growth on the expansive soil ecological slope. The root systems of the dominant shrubs and herbaceous plants in the typical sections of the expansive soil ecological slope were sampled by the full excavation method to obtain the root distribution morphological characteristic parameters; the root distribution morphological characteristic parameters include the root distribution range and the number distribution of roots of different diameters along the depth direction. In addition, basic geotechnical tests were carried out to obtain the physical and mechanical parameters of the test soil samples. The location distribution of various shrubs that dominate this slope was determined based on the results of the field survey; the anchor diameter and depth used in the model to replace the main taproot system of the shrubs were equivalently replaced by the taproot diameter and depth of the excavated shrubs.

[0059] Furthermore, the number distribution of the excavated roots at each root diameter level was counted at the burial depth of 10 cm, 20 cm, 30 cm and 40 cm respectively, and the root diameter level was classified into four categories: 0-1 mm, 1-2 mm, 2-5 mm, and above 5 mm.

[0060] Furthermore, when sampling the slopes on site, representative locations with lush vegetation should be selected for root-containing soil sampling. When the root system is fully excavated, tools such as picks, screwdrivers, tweezers, and brushes are required to gradually excavate along the root branches, and be careful when excavating the terminal roots until all the roots are excavated. When reading the diameter and number of the root system, if conditions permit, the WinRHIZO instrument can be used to scan and read the root system information. If this instrument is missing, a self-made rectangular grid can also be used to manually read the root number distribution.

[0061] Furthermore, when sampling soil, first remove the vegetation on the upper part of the expansive soil ecological slope, then remove the topsoil of several centimeters, and then press the sampling ring with a height of 40mm and a diameter of 61.8mm into the soil for sampling. Below the surface soil, sample at a vertical distance of 5cm. When sampling, pay attention to compacting the ring, and use a scraper to scrape both sides of the ring flat. After the soil sample is taken out, wrap it with plastic wrap as soon as possible to prevent moisture loss, and send it to the laboratory in time for relevant analysis such as soil natural density and specific gravity.

[0062] In this embodiment, a slope of expansive soil in Dangyang, Hubei Province is taken as an example. The slope is 5m high and the slope ratio is 1:2. The slope model is constructed with these parameters. The dominant shrub growing on the slope is oleander, and the herbaceous plants are mainly bermudagrass. The root distribution characteristics of oleander growing on the slope are investigated, and the root classification is shown in Table 1.

[0063] Table 1 Root system classification

[0064]

[0065] The spatial distribution of anchor rods in the slope model is determined based on the spatial distribution of well-growing shrubs on the slope. The actual spacing of anchor rods in the model is taken as 1m. The length of anchor rods is set to 0.6m based on the morphological distribution of shrub roots obtained from excavation, which is the range within which the anchoring effect can be exerted.

[0066] Furthermore, various geotechnical tests including specific gravity test, water content test, liquid limit test, free expansion rate test, expansion force and expansion rate test, and triaxial test were carried out on the soil samples obtained in the laboratory. Multiple physical and mechanical indicators including specific gravity, liquid limit, expansion force, internal friction angle, cohesion, free expansion rate, and the relationship between expansion rate and water content were tested. The soil density measured in the test was 2.054g / cm 3 , cohesion is 37.9 kPa, internal friction angle is 32.2°, elastic modulus is 20 MPa, and Poisson's ratio is 0.35.

[0067] S2. Construct material attribute tables and carry out the work of assigning physical and mechanical attributes of materials. Assign physical and mechanical parameters to the surface soil of the slope and the main taproot system of the shrubs in the model, and assign values ​​to the surface soil layers that can be affected by the vegetation roots.

[0068] See also Figure 2 In step S2, the physical and mechanical parameters of the soil within the growth range of the roots of various types of vegetation on the slope are assigned, that is, the shear strength of the soil within the range that can be influenced by the roots of the ecological slope is regarded as the sum of the cohesion of the soil itself and the cohesion enhancement effect of the roots of various types of vegetation.

[0069] The root strength and the added value of the root system on the soil shear strength can be obtained through pull-out tests and corresponding simplified calculation formulas. The specific steps include:

[0070] S2.1. Conduct pull-out tests on the roots of selected shrubs and herbaceous plants to obtain the maximum pull-out force and elastic modulus of the roots, and the tensile strength T a Fit the relationship between the root diameter D to obtain the empirical formula of the power function:

[0071] T a =αD -β (1)

[0072] In formula (1), α and β are fitting empirical parameters.

[0073] S2.2. Divide the slope soil into four depth ranges from the surface to the inside, i.e. 0-10 cm, 10-20 cm, 20-30 cm and 30-40 cm, and apply Wu's theory to calculate the enhancement effect of various plant roots on cohesion in each depth range:

[0074]

[0075] In formula (2), Δc i The difference between the cohesion test value of the rooted soil and the cohesion test value of the bare soil; t iR is the shear strength of the root system of the i-th plant at the R-th diameter level, A si / A R It is the ratio of the cross-sectional area of ​​the root system at the Rth diameter level to the total shear area, also known as the root area ratio RAR iR ; It is used to measure the shear strength of roots in slope soil at different angles, which can be taken as a constant of 1.2. Plant roots are divided into four diameter classes according to 0-1mm, 1-2mm, 2-5mm and above 5mm. The enhancing effect of various plant roots on cohesion is the influence of various plant roots at different depths on cohesion. Various plant roots include roots of herbs and shrubs.

[0076] Continuing with the above-mentioned expansive soil slope in Dangyang, Hubei as an example, the root quantity distribution result along the depth direction obtained in step S1 is substituted into step S2 for calculation to obtain the increase in cohesion under the root system soil consolidation slope protection. The calculation result is as follows Figure 3 shown.

[0077] Furthermore, if indoor and outdoor test conditions are available, the remolded soil samples can be prepared according to the root density obtained by sampling at 10cm, 20cm, 30cm and 40cm below the plant, and the shear strength parameters can be obtained by using a four-way direct shear apparatus to conduct an undrained quick shear test in the laboratory. The vertical load applied to the direct shear apparatus can be 100kPa, 200kPa, 300kPa and 400kPa. The added value of the root-fixing soil cohesion within this depth range can be expressed as the difference between the root-containing soil cohesion test value and the bare soil cohesion test value:

[0078] Δc i =c2-c1(3)

[0079] Among them, c2 is the cohesion test value of rooted soil, and c1 is the cohesion test value of bare soil.

[0080] S3, model assembly and analysis step construction: add the main taproot system of shrubs to the slope model by embedding anchor rods. The size of the anchor rods is determined by the taproot size of the shrub root system obtained from the on-site survey, and the elastic modulus of the anchor rods is determined by the pull-out test. The steps include:

[0081] S3.1. Assemble the main taproot system of the shrubs, which is equivalent to anchor rods, onto the slope model, and add the anchor rods in an embedded manner.

[0082] See also Fig. 9 In this embodiment, the main taproot system of the shrub equivalent to the anchor rod has five types, namely, type I, type H, type V, type R, and type VH. The type is determined according to the investigation of the main taproot system morphology of the shrub in the on-site sampling in step S1.

[0083] S3.2. Establish a geostress balance analysis step, in which gravity load is applied to the entire model and geostress balance is performed.

[0084] S3.3. Establish the first static equilibrium analysis step for applying external loads. In this analysis step, other loads acting on the slope are applied according to different working conditions, and the initial temperature within the depth of rapid atmospheric influence is defined to be equivalent to the initial humidity.

[0085] S3.4. Establish the second static equilibrium analysis step of moisture content change. In this analysis step, the temperature in the depth range of rapid atmospheric influence is changed to achieve an equivalent humidity field, so as to realize the influence of expansion deformation of expansive soil on the slope under humidity change.

[0086] S4, implement boundary conditions and loads, mesh division; add temperature field to equivalent humidity field, mesh division, so as to reflect the expansion and contraction characteristics of expansive soil. This step specifically includes:

[0087] S4.1. Establish boundary conditions for the slope model: fully constrain the bottom surface of the slope model, and establish constraints along the extension direction on each side of the slope model.

[0088] S4.2. Apply load: Add gravity in the geostress equilibrium analysis step to simulate geostress equilibrium, and apply loads of different sizes according to different working conditions in the first static equilibrium analysis step.

[0089] S4.3. Constructing the temperature field: In the first static equilibrium analysis step, the soil at the depth of rapid atmospheric influence is given an initial temperature to simulate the humidity field, and in the second static equilibrium analysis step, the temperature is changed to realize the expansion and contraction deformation and force effect of the shallow expansive soil due to the change of humidity.

[0090] S4.4. Model mesh division: CPE4 units are used for soil mesh units, and T2D2 units are used for anchor mesh units. Meshing can be done by the sweep method.

[0091] Wherein, in step S4.3, the temperature field of the expansive soil slope is defined in each analysis step, including the following steps:

[0092] S4.31. Conduct swelling force and swelling rate tests on remolded soil to obtain the relationship between swelling force and swelling rate and moisture content. According to the root mass density and root coefficient density obtained by field sampling, a remolded soil ring sample with a diameter of 61.8 mm and a height of 20 mm was made and an indoor no-load swelling force test was carried out to obtain the swelling force. The acquisition of swelling force parameters is conducive to the study of swelling characteristics below. The relationship between the swelling force of the expansive soil in the area at the optimal moisture content and the root content of the oleander root system was obtained as follows: Figure 4 shown.

[0093] Specifically, the expansion force test can be carried out by indoor test methods, configuring a ring knife sample with the optimal moisture content, and adding roots at a ratio of 0.5-2% of the total mass. After the ring knife sample is made, it is placed in the consolidation instrument for testing. For the same type of test, ensure that more than 3 samples are used, and the average value can be taken as the expansion force test result.

[0094] S4.32, define the moisture expansion coefficient based on the indoor root-containing soil expansion characteristic test data, and obtain the thermal expansion coefficient by constructing a ring knife model for parameter inversion. Construct the temperature field within the range of the atmospheric sharp influence depth.

[0095] Regarding the introduction of the temperature field, the thermal expansion coefficient can be obtained by transforming the relationship between expansion strain and temperature change in thermodynamics as follows:

[0096] α T =α w Δw / ΔT (4)

[0097] In formula (4), α T is the thermal expansion coefficient, α w is the humidity expansion coefficient, Δw is the moisture content change rate, and ΔT is the temperature change. The thermal expansion coefficient required to define the temperature field can be inferred by this formula. The thermal expansion coefficient can be obtained by obtaining the vertical deformation of the ring knife sample and the initial and final mass moisture content through the indoor no-load expansion test.

[0098] Specifically, a ring cutter model was established, and the temperature field was constructed with the above parameters. The vertical stress at the top of the ring cutter was obtained by limiting the deformation of the top layer and the lateral deformation. The temperature difference before and after the temperature field in the model was taken as 10°C. The initial thermal expansion coefficient was calculated from the vertical expansion rate of the ring cutter sample obtained from the no-load expansion rate test and the increment of moisture content before and after the test to be 2×10 -2 , and use this as the starting thermal expansion coefficient for the ring knife sample expansion force test. Compare the vertical stress at this time with the expansion force measured by the expansion force test, and continuously change the value of the expansion coefficient to make the vertical stress close to the expansion force. The thermal expansion coefficient obtained at this time can be input into the model as the final result, which can truly reflect the properties of expansion and contraction deformation of expansive soil. Figure 5 As shown in the ring knife test, the thermal expansion coefficient can be taken as 9.0×10 -5 .

[0099] S4.33, assign the linear expansion coefficient obtained in the previous step to the material parameters of the soil, and assign an initial temperature of 18°C ​​and an end temperature of 28°C, which are equivalent to the ring knife test, in the first static analysis step and the second static analysis step respectively. The temperature field application area is limited to the depth of rapid influence of the atmosphere.

[0100] Specifically, the expansion field can be optimized according to the actual change of moisture content along the depth. Within the depth of rapid atmospheric influence, the different changes in moisture content of soil layers at different depths will result in different expansion forces. The soil layer can be further refined based on the measured data, and different temperature changes can be given to the temperature fields constructed at different depths to simulate actual working conditions.

[0101] S5. Submit calculation and result analysis: Obtain the slope stability coefficient and slope stress and deformation, determine the trend of slope failure by accumulating plastic strain and deformation, and identify weak areas. Then provide a vegetation greening plan that can improve the stability of the expansive soil slope.

[0102] Furthermore, the shear strength index c and To obtain the slope safety factor F s .

[0103] Define the reduction factor:

[0104]

[0105] When a plastic penetration surface or damage occurs in the slope, the reduction factor at this time can be taken as the stability coefficient. Figure 6 , Figure 7 , Figure 8 The vertical stress, displacement and plastic strain of the established slope model are shown respectively.

[0106] Furthermore, the numerical simulation can be performed using the Abaqus finite element software not limited to that used in the present invention. Other numerical software such as the general finite element analysis software Plaxis and FLAC based on the finite difference method can also be used in the numerical simulation of root-reinforced slopes.

[0107] Furthermore, the slope model can be modified according to the actual working conditions, such as considering the influence of surrounding loads and rainfall.

[0108] Furthermore, when ecological protection is performed on the expansive soil slope, it is combined with some methods and principles of expansive soil regulation, such as anti-seepage and moisture retention, replacement and improvement, etc. Some specific embodiments are given below: When ecological protection is performed on the expansive soil slope, in addition to conventional vegetation greening, it should also be noted that it is combined with some methods of expansive soil slope regulation and protection. Several preferred embodiments are given below:

[0109] (1) Ecological protection of expansive soil slopes should be combined with anti-seepage and moisture retention. Waterproof geotextiles can be covered on the surface, and drainage ditches can be excavated to promote drainage. In addition to the construction of the support structure, attention should also be paid to the coordinated construction of drainage ditches, intercepting ditches and other drainage projects. Sand well filter layers can be built as needed and PVC drainage pipes can be pre-buried in the slope to promote drainage. At the same time, the surface of the expansive soil should be compacted as much as possible to reduce water infiltration. The top platform of the slope can be compacted by a roller, and the slope surface can be compacted manually or with a tamping machine according to site conditions. As the vegetation on the retaining structure grows and develops, its root system gradually grows and grows, and the purpose of controlling expansion and contraction deformation is achieved by absorbing water in the soil.

[0110] (2) Combine ecological protection of expansive soil slopes with flexible treatment of swelling. For example, EPS retaining wall support, gabion mesh, geogrid and other flexible reinforcement support can be used behind the retaining structure. If the expansive soil produces swelling and shrinkage deformation, the expansion force and expansion potential will be significantly reduced as the deformation is released due to the support of the flexible structure. Combined with ecological protection, the effect of comprehensive treatment is achieved.

[0111] (3) Combine ecological protection of expansive soil slopes with replacement and improvement. For example, when constructing expansive soil projects, the surface expansive soil can be replaced according to the on-site conditions. The surface expansive soil is replaced with 40 cm thick arable soil with considerable nutrients and suitable for vegetation growth. This can not only overcome the high expansion and contraction defects of expansive soil, but also facilitate vegetation restoration and layout. Regarding engineering improvements of expansive soil, common methods include physical or chemical improvements, such as lime improvement and chemical additive improvement. Taking surface concrete protection as an example, the ratio of the surface base material can be selected from ecological base materials, and implemented according to the ratio of loam: cement: humus: medium and fine sand: seeds = 160kg: 10kg: 15kg: 10kg: 10kg.

[0112] (4) Ecological protection of slopes can also be achieved by using ecological belts and vegetation bags. When selecting slope protection plants, try to select native plants that meet local climatic conditions and have well-developed root systems. In slope ecological protection, shrubs that are not too tall are often mixed with herbs. Several suitable grass and shrub mixed varieties are recommended: Ligustrum lucidum and Cynodon dactylon, Oleander and Zoysia japonica, Amorpha fruticosa and Tall Fescue. Shrubs are mainly transplanted, and herbaceous plants can be planted by sowing grass seeds.

[0113] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A numerical analysis method for expansive soil ecological slope, characterized in that: The following steps are involved: S1. Using finite element software Abaqus to construct a slope model and a shrub main taproot system model, wherein the shrub main taproot system in the shrub main taproot system model is equivalent to an anchor rod; S2. Assign physical and mechanical parameters to the surface soil of the slope and the main taproot system of the shrubs in the model, and assign values ​​to the surface soil that can be affected by the vegetation root system in layers; assign physical and mechanical parameters to the soil within the growth range of various vegetation root systems on the slope, that is, the shear strength of the soil within the range that can be affected by the ecological slope root system is regarded as the sum of the cohesion of the soil itself and the cohesion enhancement effect of various vegetation root systems; The root strength and the added value of the root system on the soil shear strength can be obtained through pull-out tests and corresponding simplified calculation formulas. The specific steps include: S2.

1. Conduct pull-out tests on the roots of selected shrubs and herbaceous plants to obtain the maximum pull-out force and elastic modulus of the vegetation roots, and the tensile strength T a Fit the relationship between the root diameter D to obtain the empirical formula of the power function: T a =αD -β (1) In formula (1), α and β are fitting empirical parameters; S2.

2. The slope soil is divided into four depth ranges from the surface to the inside, namely 0-10 cm, 10-20 cm, 20-30 cm and 30-40 cm, and Wu's theory is used to calculate the enhancement effect of various vegetation roots on cohesion in each depth range: In formula (2), Δc i Added value of root soil cohesion; t iR A is the shear strength of the root system of the i-th vegetation at the R-th diameter level; si / A R It is the ratio of the cross-sectional area of ​​the root system at the Rth diameter level to the total shear area, also known as the root area ratio RAR iR ; It is used to measure the shear strength of roots in slope soil at different angles, which can be taken as a constant of 1.

2. Vegetation roots are divided into four diameter classes: 0-1mm, 1-2mm, 2-5mm and above 5mm. The strengthening effect of various vegetation roots on cohesion, i.e. the influence of various vegetation roots at different depths on cohesion, including herbaceous plants and shrubs. S3, assembling the model and constructing the analysis step, adding the main taproot system of the shrubs to the slope model; specifically comprising the following steps: S3.

1. Assemble the main taproot system of the shrubs to the slope model in an embedded manner; S3.2, establish the geostress balance analysis step, apply gravity load to the entire model and perform geostress balance; S3.

3. Establish the first static equilibrium analysis step of applying external loads, other loads acting on the slope according to different working conditions, and define the initial temperature within the depth of sudden atmospheric influence to be equivalent to the initial humidity; S3.4, establish the second static equilibrium analysis step of moisture content change, and realize the influence of expansion deformation of expansive soil on slope under humidity change by changing the temperature within the depth range of rapid atmospheric influence to equivalent humidity field; S4. Implement boundary conditions and loads, and mesh division, and add temperature fields to equalize the expansion and contraction of the slope caused by the change in moisture content of the expansive soil; S5. Submit calculation and result analysis: Obtain the slope stability coefficient and slope stress and deformation, analyze the slope failure trend based on the accumulated plastic strain and stress, and identify the weak areas.

2. The numerical analysis method according to claim 1, characterized in that: In the step S1, before constructing the model, the following steps are also included: S1.

1. Conduct on-site survey of the expansive soil ecological slope to obtain slope surface information, including slope height and slope gradient; S1.

2. Conduct a sampling survey on the growth of existing vegetation on the expansive soil ecological slope, and sample the roots of shrubs and herbaceous plants that dominate the typical sections of the expansive soil ecological slope using the full excavation method to obtain root distribution morphological characteristic parameters; the root distribution morphological characteristic parameters include the root distribution range and the number distribution of roots of different diameters along the depth direction.

3. The numerical analysis method according to claim 2, characterized in that: In the step S1.1, the on-site investigation of the expansive soil ecological slope also includes: first sampling the slope soil, and then conducting geotechnical tests on the soil samples obtained in the laboratory to obtain multiple physical and mechanical indicators including soil specific gravity, liquid limit, plastic limit, swelling force, internal friction angle, cohesion, free swelling rate, and the relationship between swelling rate and moisture content.

4. The numerical analysis method according to claim 1, characterized in that: The step S4 comprises the following steps: S4.

1. Establishing boundary conditions of the slope model: completely constraining the bottom surface of the slope model, and establishing constraints along the extension direction on each side of the slope model; S4.2, applying load: adding gravity in the geostress equilibrium analysis step to simulate geostress equilibrium, and applying loads of different magnitudes according to different working conditions in the first static equilibrium analysis step; S4.3, constructing a temperature field: in the first static equilibrium analysis step, the soil at the depth of rapid atmospheric influence is given an initial temperature to simulate the humidity field, and in the second static equilibrium analysis step, the temperature is changed to achieve the expansion and contraction deformation and force effect of the shallow expansive soil due to the change in humidity; S4.

4. Model grid division: Select an appropriate grid division method based on the morphological characteristics of the established slope model and the main taproot system model of the shrub.

5. The numerical analysis method according to claim 4, characterized in that: The step S4.3 comprises the following steps: S4.

31. Conduct swelling force tests on remolded soil with different root content to obtain swelling characteristic index of rooted soil; the root content of remolded soil is determined based on the distribution density of roots at shallow depths obtained from on-site investigation; S4.32, obtaining the linear expansion coefficient by performing a ring cutter expansion test in the Abaqus finite element software; that is, using the Abaqus finite element software to establish a ring cutter model of equal size, constraining the deformation of the bottom and side surfaces of the ring cutter model in Abaqus, applying a vertical load equal to the maximum expansion force in the root-containing soil expansion characteristic index obtained in the step S4.31 to the surface of the ring cutter model, and setting the starting and ending temperatures, and then changing the temperature expansion coefficient, the linear expansion coefficient is obtained under the control condition that the ring cutter sample does not produce expansion deformation; S4.

33. Assign the linear expansion coefficient obtained in the step S4.32 to the material parameters of the soil, and assign the initial temperature and the ending temperature equivalent to the ring knife test in the first static analysis step and the second static analysis step respectively. The temperature field application area is limited to the depth of rapid influence of the atmosphere.

6. The numerical analysis method according to claim 5, characterized in that: The depth of rapid atmospheric impact of the expansive soil is determined according to the temperature, evaporation and rainfall of the area where the expansive soil ecological slope is located, or is determined from 1.35 to 2.25 m.

7. The numerical analysis method according to claim 1, characterized in that: The main taproot system of the shrub in step S3.1 is one or more of type I, type H, type V, type R and type VH.

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