Multifractal characteristics on pore structure of grouted coal using nuclear magnetic resonance for anti-seepage evaluation

Grouting technology is crucial for controlling rock deformation and ensuring stability in underground engineering. In this study, the grouting simulation experiments were implemented using superfine cement (SC) and a water-soluble modified resin (MR) as slurries to evaluate the grouting anti-seepage effect of broken coal. The nuclear magnetic resonance (NMR) experiments were conducted to analyze the pore structures of the various grouted specimens. In accordance with the NMR results, the multifractal characteristics of the various pore types within the grouted specimens were identified using the geometric fractal method. The results showed that the fractal dimension Ds, based on the seepage pore, ranged between 2 and 3, accompanied by high R2 values, effectively reflecting the pore structure and distribution characteristics. Specifically, the Ds values of the MR grouting specimen are closer to 3, and the NMR porosity of the SC specimens is 2.12 times greater than that of the MR specimens, indicating the superior anti-seepage efficacy of the MR material. Moreover, the uniaxial compressive experiments demonstrate that the mechanical strength and strain capacity of the MR specimens exceed those of the SC specimens. This finding implies that the lower porosity and denser structure of the MR specimens contribute to enhanced homogeneity of the grouted coal, thereby bolstering the overall structural integrity. Therefore, the NMR-based seepage pore fractal dimension method can be regarded as a scientific and feasible approach for evaluating the anti-seepage effect of grouted coal in actual engineering.