Mechanical Compatibility and Pressure-Sensitive Sealing Behavior of an Illite–Kaolinite Clay-Based Functional Interlayer

Accurately reproducing continuous stress transfer and cross-layer gas sealing remains a major challenge in physical simulations of multiseam coal mining. Rigid separators can prevent gas leakage but create artificial mechanical discontinuities, whereas brittle sealing materials may crack during stress redistribution. This study evaluated an illite–kaolinite clay (IKC)-based material as a pressure-sensitive functional interlayer for simultaneously transmitting load and sealing gas. Triaxial stress–seepage tests were performed on all three specimen types; loading–unloading tests were conducted on the coal–IKC composite specimen; and gas-pressure-dependent permeability tests were conducted separately on the reconstituted coal and IKC-based specimens. The coal–IKC composite exhibited a maximum differential stress of 7.67 MPa, comparable to that of the reconstituted coal specimen (7.51 MPa), indicating that the IKC-based interlayer did not weaken the load-bearing continuity of the system. Meanwhile, the 10 mm IKC-based interlayer reduced the elastic modulus by 42.2%, produced a more ductile deformation response, and decreased the initial permeability from 2.13 × 10 − 15 m 2 to 0.62 × 10 − 15 m 2 . During loading, permeability further decreased to below 0.20 × 10 − 15 m 2 . Loading–unloading tests identified a critical compaction threshold at a mean effective stress of approximately 5.0 MPa, beyond which permeability remained below 0.10 × 10 − 15 m 2 during unloading. These results reveal a pressure-sensitive sealing response in which stress-induced compaction of the IKC-based interlayer reduces connected pore space and improves interfacial sealing. The methodological advance of this study is a unified coal–IKC composite testing framework that quantifies strength retention, stiffness regulation, permeability reduction, and unloading sealing stability under coupled stress and methane seepage. The results indicate that the interlayer can preserve load-transfer compatibility while maintaining low permeability through stress-induced compaction and improved interface contact.