Optimization Design of Key Mechanical Properties of Cement-Based Thin Spray-On Liners for Monorail Transport Systems in High-Water-Seepage Areas of Coal Mines

Thin spray-on liners (TSLs) play a crucial role in the coal mine roadway support. However, the deterioration of their mechanical properties in high-water-seepage environments remains a critical challenge. This study optimizes the key mechanical properties of cement-based TSLs by adjusting the polymer-cement ratio (4-10%) of ethylene-vinyl acetate copolymer (EVA). Experiments employed full water immersion curing to simulate high-water-seepage conditions. The flexural strength, compressive strength, viscosity, and bond strength of EVA emulsion- and powder-modified TSLs under different polymer-cement ratios were systematically tested. Spraying experiments were conducted to evaluate their wall-adhesion performance. Results indicate that the flexural strength and bond strength significantly increased with higher polymer-cement ratios. EVA powder specimens with a 10% polymer-cement ratio exhibited a 51.1% improvement in flexural strength compared with the 4% ratio, achieving a bond strength of 1.82 MPa. Conversely, the compressive strength decreased with increasing polymer-cement ratios, with 10% EVA emulsion specimens showing the lowest compressive strength (15.3 MPa). Spraying experiments demonstrated that the 10% EVA powder formulation exhibited no sagging on wet rock surfaces, indicating optimal overall performance. This study provides theoretical and experimental foundations for designing TSLs in high-water-seepage areas, offering significant implications for enhancing the safety and cost-effectiveness of underground roadway support.