Research Article: Disturbance function for soil disturbed state strength based on X-ray computed tomography triaxial test

Date Published: May 2, 2019

Publisher: Public Library of Science

Author(s): X. Li, S. J. Wang, B. H. He, Y. Frank Chen, Peitao Wang.


Soil is a porous, multiphase, and loose medium, which is prone to serious disturbance by the activity of biological livings, accompanied with cracks and macro pores. These phenomena greatly affect the strength of soil and the degradation of land. Two different quantifiable values from X-ray computed tomography (CT) images are usually used to define the disturbance function for soil disturbed state strength: mean density (MD value) and standard deviation (SD value). Two types of disturbance functions are investigated quantitatively by CT-triaxial testing using four samples with different macro-pore sizes (disturbed samples) and one sample without a macro-pore (undisturbed sample). It is found that the shear strength of three disturbed samples with the same macro-pore size is close to each other. As with the shear strength, the SD value and its defined disturbance function have a similar correlation with the cross-sectional area of macro-pore. However, the MD value and its defined disturbance function have no regularity with the macro-pore size. Therefore, the disturbance function expressed in terms of the SD value is deemed more reasonable and appropriate in analyzing the soil disturbed state strength using the quantitative CT morphometry. While, the disturbance function defined by the MD value has often been used to characterize a rock damage.

Partial Text

Soil is a porous, multiphase, and loose medium, which is prone to be seriously disturbed due to the growth of plant roots and the activity of biological livings, accompanied with cracks and macro pores being the two important structural characteristics of soil. The potential disturbance caused by the fracture and pore greatly affects the strength and stability of soil. However, the detailed geometry and variation of the disturbances cannot be observed in the traditional triaxial test because most of the disturbances occur inside the soil. X-ray computed tomography (CT) being a non-invasive and non-destructive technique was initially used in the medical field. CT images are constructed by the spatial distribution of the so called CT value, which is defined as follows:
where μi is the coefficient of absorption at the scanning point, μw is the coefficient of absorption for water. Thus, the CT value of air should be -1000 because the coefficient of absorption for air is zero. Likewise, the CT-value for water is 0. CT images are presented with a shaded gray for low CT-value and light gray for high CT-value through the black to white range. It is well known that this CT-value, equal to the MD, is linearly related to material density.

The quantifiable information, including MD value and SD value, was obtained using a software in conjunction with the CT equipment during different test stages. S3 Fig shows the macro-pore section under triaxial compression. It consists of five rows and four columns with each row corresponding to one sample. They represent Samples A, B, C, D, and E respectively from the top to bottom of the figure. Each column corresponds to one testing stage. The left to right lists the scanned samples by the CT equipment corresponding to the axial strains of 0%, 5%, 10%, and 15% respectively.

Artificial cylindrical macro-pores disturbance with different macro-pore sizes and positions were generated in four compacted soil samples. The shear strength, the evolution of the MD and SD values subjected to shear deformation, the correlations between the structural disturbance functions D1 and D2, and the magnitude of macro-pore disturbance, were investigated using CT-triaxial tests under the same confining pressure and matric suction. The size rather than the position of macro-pore disturbance primarily affects the shear strength of repacked soil. The macro-pore disturbance effect on soil strength depends on the size of macro-pore and the moisture content of soil. The coupling effect between the moisture content and the disturbance degree on the strength of soil is a far more complicated issue deserving for a further study.




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