Rheological mechanical properties and its constitutive relation of soft rock considering influence of clay mineral composition and content

Rheological mechanical properties of the soft rock are affected significantly by its main physical characteristics-clay mineral. In this study, taking the mudstone on the roof and floor in four typical mining regions as the research object, firstly, the clay mineral characteristic was analyzed by the X-ray diffraction test. Subsequently, rheological mechanical properties of mudstone samples under different confining pressures are studied through triaxial compression and creep tests. The results show that the clay mineral content of mudstone in different regions is different, which leads to significant differences in its rheological properties, and these differences have a good correlation with the content of montmorillonite and illite-montmorillonite mixed layer. Taking the montmorillonite content as an example, compared with the sample with 3.56% under the lower stress level, the initial creep deformation of the sample with 11.19% increased by 3.25 times, the viscosity coefficient and long-term strength decreased by 80.59% and 53.94%, respectively. Furthermore, based on the test results, the damage variation is constructed considering the montmorillonite content and stress level, and the M–S creep damage constitutive model of soft rock is established. Finally, the test results can be fitted with determination coefficients ranging from 0.9020 to 0.9741, which proves that the constitutive relation can reflect the influence of the clay mineral content in the samples preferably. This study has an important reference for revealing the long-term stability control mechanism of soft rock roadway rich in clay minerals.