Nonlinear softening mechanism of argillaceous slate under water-rock interaction

The argillaceous slate obviously shows softening characteristics under water-rock interaction. The relationship between the uniaxial compressive strength (UCS), elastic modulus, Poisson's ratio and water absorption time in the softening process of argillaceous slate is analyzed by conducting uniaxial compression tests. The laws of pore generation, expansion and breakthrough in the softening process under water-rock interaction are studied by using the nuclear magnetic resonance (NMR). The relationship between porosity and water absorption time in the softening process is analyzed. The evolution rule of microstructure in the softening process of argillaceous slate under water-rock interaction is analyzed by the scanning electron microscope (SEM). Based on the fractal theory, the change rule of the fractal dimension under different soaking time is studied. The fractal dimension value of pore microstructure, porosity, UCS and elastic modulus are selected as the changes of describing the interaction system of argillaceous slate and water solution by using the nonlinear dynamics theory. The applicability of the model is verified with experimental data. The results show that UCS and elastic modulus decrease with the increase of water absorption time, showing a negative linear correlation, but there is no obvious relationship between the Poisson's ratio and water absorption time. At the early stage of immersion, the water-rock interaction is strong, and the micropores in the slate expand and penetrate to form larger pores, and the porosity increases rapidly. With the increase of water absorption time, the pores in the argillaceous slate are connected with each other to form large pores, which leads to the complex network structure. The fractal dimension of argillaceous slate increases logarithmically and tends to be stable gradually. The results calculated by the nonlinear model are close to the experimental data, which shows that the softening process of argillaceous slate has the obvious nonlinear dynamic characteristics, and the softening law of argillaceous slate under water-rock interaction can be better characterized by the nonlinear dynamic model. The results can provide a reference for the theoretical study of soft rock-water interaction.