Disintegration and strength weakening characteristics of red-bed soft rock in the Shengzhou-Xinchang area under dry-wet cycles

Abstract

Objective The red-bed soft rocks is easy to disintegrate and weaken its strength when it encounters water, which is easy to cause poor stability and economic loss or even casualties in slope construction. It is of great significance to reveal the variation characteristics of cohesion and internal friction angle of red-bed soft rocks under the action of dry and wet cycling for the design of slope treatment measures. Methods Investigating the three groups of red-bed soft rock formations within the Lower Cretaceous Chaochuan Formation situated in the Shengzhou-Xinchang region of Zhejiang Province, this study delves into the disintegration and strength attention tendencies inherent in these formations. This exploration unfolds through a comprehensive suite of method ologies, including dry-wet cycle tests, point load strength tests of disintegrated rock blocks and direct shear tests of disintegrated particles. Results The results reveal that the sample undergoes disintegrates, in the form of fragments, particle slag, and mud paste due to successive dry-wet cycles. The main disintegration process can be divided into four distinct stages: Initial disintegration, rapid disintegration, fine reconfiguration, and eventual stabilization. The point load strength of the sample decreases with the increase as the number of dry-wet cycles increases, while the disintegration resistance index Idn shows a positive exponential relationship with the point load strength Is(50). Notably, the point load strength Is(50) of the sample experiences rapid weakening between 80% to 100% and 50% to 80% of the disintegration resistance index Idn, demonstrating gradual attenuation characteristics. The peak shear strength of the sample ranges between 1.219 MPa and 0.567 MPa, which mostly occurs before and after the shear displacement of 3 mm. Under the consistent axial pressure, the peak shear strength of the same group decreases with an increasing number of cycles. Additionally, the internal friction angle of disintegrated particles of the sample ranges from 22.28° to 33.03°, while the cohesion is between 0.46 MPa and 0.74 MPa. Both the friction angle and cohesion of the sample show a negative exponential relationship with the number of dry-wet cycles. Conclusion The test results indicate that siliceous cementation exhibits better disintegration resistance compared to argillaceous cementation, and the rock with high clay minerals more easily disintegrates. Additionally, the water absorption and expansion capacity of "white mineral" albite significantly lag behind that of clay minerals, with a lesser variance in its content compared to clay minerals.

Keywords

• dry-wet cycle
• red-bed soft rock
• disintegration resistance
• point load strength
• shear strength
• shengzhou-xinchang area