CT scanning image-based cohesive soil microstructure feature quantity determination method

Abstract

The invention discloses a cohesive soil microscomic structure characteristic quantity determination method based on a CT scanning image, and belongs to the technical field of geotechnical engineering.The cohesive soil microscomic structure characteristic quantity determination method comprises the steps that a CT workstation is used for scanning a cohesive soil sample, and the CT scanning image is obtained; defining a soil particle corresponding to each pixel of the CT image and corresponding pore water / gas as an infinitesimal; the microstructure state and change of the cohesive soil sample are studied through statistical analysis of the structural characteristics of each infinitesimal element; drawing and analyzing a ratio cumulative distribution curve of the number of pixels, smaller than a certain gray level, of the CT scanning image; by comparing pixel number proportion changes under different test conditions, the influence of different gray threshold values on the pixel number proportion changes is analyzed, and a proper gray threshold value is selected; after a gray threshold value is determined, a structure state parameter omega of the cohesive soil is defined; the cohesive soil microscomic constitutive feature quantity determination method based on the CT scanning image is beneficial to the development of a cohesive soil macro-microscomic constitutive model.

Description

Technical Field

[0001] The invention relates to the technical field of geotechnical engineering, and in particular to a method for determining microstructure characteristic quantities of clay soil based on CT scanning images. Background Art

[0002] Many scholars use CT scanning, electron microscope scanning and other methods to directly observe the evolution characteristics of soil microstructure and cracks during the dry-wet cycle, and use CT number, gray value or three-dimensional imaging to characterize the crack development process. Indirect quantities such as soil pore size distribution characteristics and rock and soil acoustic wave velocity are also often used to study the influence of load and other effects on soil structural characteristics. For example, the mercury injection test can be used to determine the pore distribution of soil, which is simple to operate and has high accuracy.

[0003] CT scanning can obtain images of multiple sections of clay samples. The image directly records the CT number of each point of the sample, which is a relative value related to the density of the material. The CT number of air is generally defined as -1000 and the CT number of water is 0. Under certain test conditions, the CT number distribution of the sample obtained by CT scanning is stored in the form of pixel grayscale, that is, the image grayscale value and the CT number satisfy a one-to-one correspondence. For convenience, the grayscale of the sample CT image is often used for microscopic analysis, and its rules are consistent with the use of CT numbers.

[0004] Due to the limitation of the resolution of the CT workstation, the soil particle scale cannot be measured for clay. Each pixel of the CT image contains several soil particles and the corresponding pore water and gas. Therefore, it is necessary to study the method of determining the microstructural characteristics of clay based on CT scanning images, so as to statistically analyze the microstructural state and changes of the sample and lay the foundation for establishing the constitutive model. Summary of the invention

[0005] The purpose of the present invention is to provide a method for determining the microstructure characteristics of clay soil based on CT scanning images to solve the problems mentioned in the above background technology.

[0006] To achieve the above object, the present invention provides a method for determining the microstructure characteristic quantity of clay soil based on CT scanning images, comprising the following steps:

[0007] S1. Data preparation: Scan the clay sample using a CT workstation to obtain a CT scan image;

[0008] S2. Define the soil particles and corresponding pore water / gas corresponding to each pixel of the CT image as a microelement. The clay sample is a complex composed of several microelements to evaluate and study the microstructural characteristics of clay.

[0009] S3. Study the microstructure state and its changes of clay soil samples by statistical analysis of each microelement structure characteristics;

[0010] S4. Draw and analyze the cumulative distribution curve of the percentage of pixels in the CT scan image that are less than a certain grayscale, observe the changing trend, and verify the rule that the grayscale distribution of the CT scan image shifts to a larger grayscale value interval as a whole after the clay sample is loaded.

[0011] S5. By comparing the changes in the percentage of pixels under different test conditions, analyze the influence of different grayscale thresholds on the changes in the percentage of pixels, and select a suitable grayscale threshold;

[0012] S6. After determining the grayscale threshold, in order to quantitatively characterize the microstructural characteristics, the fabric state parameter Ω of the clay is defined.

[0013] Preferably, the grayscale value of the microelement in S2 reflects the distribution of soil particles and pore water, and each pixel is used as a microelement to form a pixel-based three-phase characteristic unit.

[0014] Preferably, the grayscale values ​​of the distribution curve in S4 are mainly concentrated in one interval, for example, the interval [80, 140].

[0015] Preferably, the tests in S5 include a dry-wet cyclic shear test, a conventional shear test and a conventional confined compression test.

[0016] Preferably, in S6, a grayscale of 137 is selected as a grayscale threshold for determining the microstructural characteristic quantity of clay soil.

[0017] Preferably, the fabric state parameter Ω in S6 is the percentage of pixels with grayscale values ​​greater than 137 in the CT image, and the formula is as follows:

[0018]

[0019] Therefore, the present invention adopts the above-mentioned method for determining the mesostructural characteristic quantities of clay based on CT scanning images, clarifies the mesostructural evolution law of clay based on CT scanning technology, and establishes a method for determining the mesostructural characteristic quantities of clay, thereby revealing the mesostructural evolution mechanism induced by stress and environmental changes. By quantitatively expressing the structural evolution process of clay, this method will contribute to the development of macro- and micro-constitutive models of clay.

[0020] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a flow chart of a method for determining microstructure characteristic quantities of clay soil based on CT scanning images according to the present invention;

[0022] Figure 2 The present invention is the evolution mechanism of the microstructure state of clay based on CT scanning images;

[0023] Figure 3 It is a cumulative distribution curve of the percentage of pixels with a grayscale value less than a certain grayscale in a CT image of a certain clay dry-wet cycle shear test of the present invention;

[0024] Figure 4 The graph is a curve of the percentage change of the number of pixels greater than a certain gray level in a CT image of a certain clay dry-wet cycle shear test of the present invention, wherein (a) is a curve under the conditions of a normal pressure of 100 kPa and a shear stress level of 0.5, and (b) is a curve under the conditions of a normal pressure of 200 kPa and a shear stress level of 0.5;

[0025] Figure 5 It is a curve diagram showing the change in the percentage of pixels greater than a certain grayscale in a CT image of a saturated clay shear test at a normal pressure of 100 kPa according to the present invention;

[0026] Figure 6 This is a curve diagram of the change in the proportion of pixels greater than a certain grayscale in the CT image of the saturated clay confined compression test of the present invention. DETAILED DESCRIPTION

[0027] The following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] Example

[0029] like Figure 1 As shown, the present invention provides a method for determining the microstructure characteristic quantity of clay soil based on CT scanning images, comprising the following steps:

[0030] S1. Data preparation: Scan the clay sample using a CT workstation to obtain a CT scan image;

[0031] S2. Define the soil particles and corresponding pore water / gas corresponding to each pixel of the CT image as a microelement. The clay sample is a complex composed of several microelements to evaluate and study the microstructural characteristics of clay.

[0032] The gray value of the microelement reflects the distribution of soil particles and pore water. Each pixel is regarded as a microelement, forming a pixel-based three-phase characteristic unit.

[0033] S3, such as Figure 2 As shown in the figure, the mesostructure state and its changes of clay samples can be studied by statistical analysis of the structural characteristics of each microelement.

[0034] S4. Draw and analyze the cumulative distribution curve of the percentage of pixels in the CT scan image that are less than a certain grayscale, observe the changing trend, and verify the rule that the grayscale distribution of the CT scan image shifts to a larger grayscale value interval as a whole after the clay sample is loaded.

[0035] The soil sample deforms after being loaded, and its structural evolution and displacement occur simultaneously. It is difficult to describe the structural evolution law of the sample as a whole by analyzing only the grayscale of a single pixel (microelement) or the overall grayscale mean. For this reason, the cumulative distribution curve of the percentage of pixels with a grayscale less than a certain value in the CT scan image is analyzed. Figure 3 As shown, it is found that the grayscale values ​​of the image are mainly concentrated in an interval ( Figure 3 The grayscale distribution after loading shifts to the interval with larger grayscale values, which is more obvious in a certain grayscale interval. This means that the microelements of the sample have undergone a trend change in a statistical sense. Therefore, it can be inferred that the mesostructural evolution caused by load can be quantified based on the change in the proportion of pixels in a certain grayscale interval.

[0036] S5. By comparing the changes in the percentage of pixel numbers under different test conditions, the influence of different grayscale thresholds on the changes in the percentage of pixel numbers is analyzed, and the appropriate grayscale threshold is selected; the tests include dry-wet cyclic shear test, conventional shear test and conventional confined compression test.

[0037] like Figure 4 The percentage of pixels greater than different grayscale thresholds in the CT image of a certain clay soil dry-wet cycle shear test is given. It can be seen from the figure that with the increase of the number of dry-wet cycles, the percentage of pixels greater than different grayscale thresholds increases monotonically, and the rate of increase gradually decreases. This law is consistent with the law of dry-wet cycle deformation development observed macroscopically. From the perspective of macro-micro integration, the percentage of pixels greater than a certain grayscale threshold can be used as a quantitative characterization of the characteristics of the microstructure under dry-wet cycle conditions.

[0038] Figure 5 and Figure 6 The changes in the percentage of pixels in the CT image of saturated clay greater than different grayscale thresholds in the conventional shear test and the conventional confined compression test are given respectively. As can be seen from the figure, the changes in the percentage of pixels in these two types of tests generally show the same change pattern as the dry-wet cycle shear test. Therefore, it can be inferred that for the case of dry-wet cycle and stress coupling, the mesostructure feature can be taken as the percentage of pixels in the CT image greater than a certain grayscale threshold.

[0039] Further observation Figure 4-Figure 6It can be seen that when grayscale 135 is used as the threshold, the percentage change of the number of pixels in each test is higher than that of other grayscale thresholds. However, the change of the confined compression test at the grayscale 135 threshold is higher than that of the shear test and the dry-wet cycle test. Considering that the deformation and failure of the soil slope are more closely related to the shear properties of the soil, according to the macro-micro integration idea, the changes of different grayscale thresholds under different dry-wet cycles and stress coupling are comprehensively considered, and the grayscale 137 is selected as the grayscale threshold for determining the micro-structure characteristic of clay soil in this embodiment.

[0040] S6. After determining the grayscale threshold, in order to quantitatively characterize the microstructural characteristics, the fabric state parameter Ω of the clay is defined.

[0041] The fabric state parameter Ω is the percentage of pixels with grayscale values ​​greater than 137 in the CT image, and the formula is as follows:

[0042]

[0043] Therefore, the present invention adopts the above-mentioned method for determining the microstructural characteristics of clay based on CT scan images, reveals the influence of factors such as stress and environmental changes on the grayscale distribution of CT scan images, and proposes a grayscale threshold for determining the microstructural characteristics of clay.

[0044] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for determining the microstructure characteristic quantity of clay soil based on CT scanning images, characterized in that: The following steps are involved: S1. Data preparation: Scan the clay sample using a CT workstation to obtain a CT scan image; S2. Define the soil particles and corresponding pore water / gas corresponding to each pixel of the CT image as a microelement, and the clay soil sample is a complex composed of several microelements; S3. Study the microstructure state and its changes of clay soil samples by statistical analysis of each microelement structure characteristics; S4. Draw and analyze the cumulative distribution curve of the percentage of pixels with a grayscale value less than a certain value in the CT scan image, observe the change trend, and verify the rule that the grayscale distribution of the CT scan image of the clay soil sample shifts to the interval with a large grayscale value as a whole after being loaded; S5. By comparing the changes in the percentage of pixels under different test conditions, analyze the influence of different grayscale thresholds on the changes in the percentage of pixels, and select a suitable grayscale threshold; S6. After determining the grayscale threshold, define the fabric state parameter Ω of the clay soil.

2. The method for determining the microstructure characteristic quantity of clay soil based on CT scanning images according to claim 1 is characterized in that: The grayscale value of the microelement in S2 is used to reflect the distribution of soil particles and pore water. Each pixel is used as a microelement to form a pixel-based three-phase characteristic unit.

3. The method for determining the microstructure characteristic quantity of clay soil based on CT scanning images according to claim 1 is characterized in that: The grayscale values ​​of the distribution curve in S4 are mainly concentrated in one interval.

4. The method for determining the microstructure characteristic quantity of clay soil based on CT scanning images according to claim 1 is characterized in that: The tests in S5 include dry-wet cyclic shear test, conventional shear test and conventional confined compression test.

5. The method for determining the microstructure characteristic quantity of clay soil based on CT scanning images according to claim 1 is characterized in that: In S6, a grayscale of 137 is selected as the grayscale threshold for determining the microstructural characteristics of clay soil.

6. The method for determining the microstructure characteristic quantity of clay soil based on CT scanning images according to claim 5 is characterized in that: The fabric state parameter Ω in S6 is the percentage of pixels with grayscale values ​​greater than 137 in the CT image, and the formula is as follows:

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