<p>In filling engineering, weak interlayers are a critical factor contributing to the abrupt settlement of the filling surface. Understanding the deposition patterns of sand filling in confined spaces is crucial for mitigating the formation of weak interlayers and ensuring the safety of the filling project. This study, focusing on the filling of abandoned flooded shafts, utilizes colored sand and CMC solution as experimental materials. Transparent acrylic tubes were utilized to simulate confined spaces, and filling experiments were conducted to examine the impact of wall roughness and fluid viscosity on sand sedimentation behavior and deposition patterns. The experimental results were effectively visualized, providing a clear and intuitive representation of the findings. The results indicate that fluid viscosity determines the sedimentation mode of sand filling in confined spaces. As fluid viscosity increases, the fine-grained interlayers in the sedimentary sand evolve from being thin and numerous to thick and sparse and ultimately vanish. The wall roughness in confined spaces primarily affects the thickness of the fine sand particle interlayers. As the roughness of the confined space wall increases, the thickness of the fine sand interlayer diminishes, and its quantity and distribution range progressively diminish. This study revealed the formation mechanisms of various sedimentation patterns, as well as the relationship between roughness, fluid viscosity, and the deposition patterns of filling sand. During the filling process in confined spaces such as coal mine shafts and deep karst caves, to prevent the formation of weak interlayers and avoid sudden settlement of the filling material, the presence of medium-viscosity fluids (about 50&#xa0;mPa) within the filling space should be avoided, and the filling interval time should be minimized.</p>