Microstructure and water-swelling mechanism of red-bed mudstone in the Xining region, Northeastern Tibetan Plateau

This paper examines the effect of the microstructure and electrical conductivity (EC) on the swelling ratio and pressure in red-bed mudstone sampled from arid areas in the Xining region in the northeastern Tibetan Plateau. A series of laboratory tests, including swelling experiments, X-ray diffraction (XRD), and scanning electron microscope (SEM), was carried out for mechanical and microstructural analysis. The coupled influence of the EC and microstructural parameters on the expansion ratio and pressure was investigated, and the weight coefficients were discussed by the entropy weight method. The results revealed an increasing exponential trend in EC, and the maximum swelling speed occurred at an EC of approximately 10 μS/cm. In addition, a method for predicting the expansion potential is proposed based on the microstructure, and its reliability is verified by comparing with swelling experimental results. In addition, according to the image analysis results, the ranges of the change in the clay minerals content (CMC), the fractal dimension (FD), the average diameter (AD) of pores, and the plane porosity (PP) are 23.75%–53%, 1.08–1.17, 7.53–22.45 μm, and 0.62%–1.25%, respectively. Moreover, mudstone swelling is negatively correlated with the plane porosity, fractal dimension and average diameter and is linearly correlated with the clay mineral content. Furthermore, the weight values prove that the microstructural characteristics, including FD, AD, and PP, are the main factors influencing the expansion properties of red-bed mudstones in the Xining region. Based on the combination of macro and micro-analyses, a quantitative analysis of the swelling process of mudstones can provide a better reference for understanding the mechanism of expansion behavior.