Stress Distribution and Evolution Characteristics of Hard–Soft Interbedded Floor Strata Subjected to Coal Pillar Loading

Abstract

To reveal the stress transfer mechanism of overlying coal pillar loads in hard–soft composite floor strata during close-distance coal seam mining, this study comprehensively employed theoretical analysis, similar material simulation, and numerical simulation to systematically investigate the floor stress distribution characteristics under different pillar widths and rock combinations. This study focuses on the instantaneous elastic response of hard–soft composite floor strata under static coal pillar loading, providing a theoretical foundation for pillar design and roadway layout in multi-seam mining. The limitations and future research directions are also discussed. First, based on the elastic layered half-space theory, mechanical models for stress transfer in the floor under narrow coal pillars (unimodal load) and wide coal pillars (bimodal load) were established. Analytical expressions of stress at any point in the floor were derived, and the influence laws of key parameters, including Poisson’s ratio, interlayer spacing ratio, and shear modulus ratio, were clarified. Second, two typical physical models, namely “hard–soft–hard” and “soft–hard–soft”, were constructed. Experimental results revealed that the weak interlayer exhibits a significant “barrier effect” in the hard–soft–hard combination, causing the stress contours to contract in a “bulb-like” shape; whereas the hard rock layer plays a “bearing effect” in the soft–hard–soft combination, leading to stress contours diffusing in a “gourd-like” shape. Furthermore, numerical simulation revealed the controlling mechanisms of rock combination and thickness ratio: the hard rock layer dominates stress concentration, with the peak stress zone evolving from an “inverted water droplet” shape to a “platform” shape as the thickness increases; the soft rock layer governs stress diffusion and buffering. The depth of the plastic zone significantly decreases with increasing hard rock thickness ratio, achieving a reduction of 44.4%.