Study on the Mechanical Properties and Microstructural Characterization of Sand-Clay Mixtures Using Nuclear Magnetic Resonance

ABSTRACT To quantitatively reveal the complex mechanical and microstructural characteristics of sand-clay mixtures, a series of studies were conducted on different sand-clay proportions (sand content of 0 %, 15 %, 30 %, 45 %, and 60 %) using nuclear magnetic resonance testing and nonconsolidated and undrained triaxial shear tests. This research characterized microstructural index including porosity, pore size distribution, pore throat distribution, and pore fractal dimension, as well as the cohesion and internal friction angle of the sand-clay mixtures. It established quantitative relationships between the characterized microstructural parameters and the corresponding macroscopic mechanical properties. Results showed that porosity decreased progressively with increasing sand content in the mixture. Based on the T2 spectrum, the pores in the mixture were categorized as micropores, mesopores, and macropores, with pore sizes ranging from 0.002 to 0.07 μm, 0.15 to 1.5 μm, and 1.5 to 30 μm, respectively. With increasing sand content, micropores decreased while mesopores and macropores increased proportionally, with the proportion of micropores being the highest, exceeding 60 %. With increasing sand content, the small fractal dimension of pores in the sand-clay mixture increased, whereas the large fractal dimension decreased, indicating an increasing nonuniformity of small pores and a reduction in the nonuniformity of large pores. The increase in sand content significantly reduced the axial deviator stress of the sand-clay mixture. Additionally, a consistent rise in the internal friction angle corresponding to higher sand content, whereas the cohesion significantly decreased. The clay effectively filled the voids between sand particles and acted as a bridging agent between sand particles, thereby significantly reducing the proportion of large pores and increasing the cohesion of the specimens. These findings provide a scientific basis for systematic characterization of multiscale properties in sand-clay mixtures, from microstructural features to macroscopic behavior.