Abstract: The residual soil in the red bed constitutes a fundamental material basis for the development of large-scale density-related hazards in its shallow deposits. The physical properties and pore characteristics of this soil are essential for understanding the infiltration and movement of the water within red bed soil slopes to study its physical properties and its vertical porosity characteristics. This study investigates the undisturbed residual soil samples collected at various depths. The mineral composition and microstructure of the soil are examined using the X-ray diffraction (XRD) and the scanning electron microscopy (SEM). Based on these analyses, conventional physical property tests are performed, and the pore characteristics of the soil are subsequently evaluated using the mercury intrusion porosimetry. The findings of this study demonstrate that: (1) The mineral composition of the residual soil in the Chishui Red Beds primarily consists of quartz, clay, potassium feldspar, and hematite. With the increasing of sampling depth, the clay content increases, while the quartz content decreases. (2) As the sampling depth increases, the natural moisture content of the residual soil in the red bed rises, leading to a gradual increase in density. The particle size variation is minimal, and the soil exhibits good gradation. (3) The porosity of the soil gradually decreases with increasing depth. The characteristic curves exhibit similarity, and the trend of the curves clearly shows a distinct boundary point. Based on the comprehensive fractal results and previous research, a suitable boundary pore size value for classifying the pore types of the red bed residual soil is proposed: pores smaller than 0.05 μm are classified as the intragranular pores; those between 0.05 and 0.35 μm are classified as the micropores; pores ranging from 0.35 to 3.5 μm are identified as the small pores; those from 3.5 to 25 μm are classified as the medium pores; and pores larger than 25 μm are categorized as the large pores. 

Key words: Red Bed Residual Soils, Mineral Component, Mercury Injection Test, Physical Property, Boundary Pore Size