Abstract: Focusing on red bed soft rock from the Yuanma Basin, uniaxial compression tests, electron microscope scanning tests, and elastic P-wave velocity tests were conducted to study the changes over time in the mechanical properties, microstructure, and elastic P-wave velocity of the soft rock under different water saturation conditions. Digital image processing technology was used to quantify the characteristics of rock microstructure, and the varying characteristics of rock microstructure were quantitatively analyzed. Based on damage mechanics, a failure model for water-saturated red bed soft rock was established. The results show that the strength of red bed soft rock decreases by 14%, 27%, 39%, and 80% respectively after 1, 2, 3, and 5 days of water saturation. As water saturation time increases, the uniaxial compressive strength of red bed soft rock decreases monotonically, and its failure mode gradually transitions from brittle to ductile, with the angle between the rock’s fracture surface and the horizontal plane gradually diminishing. The number of secondary cracks surrounding the main shear fracture initially increases and then decreases. The proportion of defect area in red bed soft rock increases linearly with water saturation time, and the number of pores is negatively correlated with the average pore size, showing a trend of increasing initially and then decreasing. The P-wave velocity of the rock decreases as water saturation time increases, and the expdecl function is suitable for fitting the relationship between P-wave velocity and water saturation time of red bed soft rock. A damage model for water-saturated red bed soft rock was established, and the calculated values from the model were compared with the experimental values, verifying the accuracy of the model. It is concluded that this model can accurately predict the uniaxial compressive strength of rocks after different water saturation times through non-destructive testing and can better describe the damage characteristics of red bed soft rocks after saturation.

Key words: red layer, soft rock, strength, deteriorate, damage model