Geometric Contour of Slip Surfaces and Loosening Earth Pressure in Sand Under Soil-Arching Effect

The soil-arching effect, in essence, is a stress transfer phenomenon triggered by the loosening of the internal structure of the soil. The loosening process is accompanied by the formation and expansion of slip surfaces. At present, research on the geometric contour of slip surfaces, loosening-affected areas, and influence of the dynamic evolution of loosening zones on the loosening earth pressure remains sparse. Therefore, Trapdoor tests were conducted to investigate the contour of slip surfaces and their evolution in sand under the soil-arching effect. In this way, differences in the characterization of the geometric contour of slip surfaces and their evolution modes under different filling heights, downward movements and widths of trapdoors, and sand densities were obtained. By defining a core loosening zone in the area affected by slip surfaces, a calculation method for the loosening stress based on the geometric contour of the core loosening zone was proposed. In addition, the curve characteristics of the loosening stress and downward movement of the trapdoor were analyzed to reveal the changes in curve characteristics with the filling height, downward movement and width of the trapdoor, and sand density under maximum and minimum arch states. The research results show that: 1) the contour of slip surfaces evolves from a triangle, to a bullet shape, and finally to an ellipse with the downward movement of the trapdoor. As the filling height gets greater, and the trapdoor becomes narrower, the contour of primary and secondary slip surfaces is sharper. 2) with the downward movement of the trapdoor, the height of the primary and secondary slip surfaces increases, while that of inflection points of the level-Ⅲ slip surface decreases. In addition, as the trapdoor moves downward, the angles of the primary and secondary slip surfaces both increase first and then decrease, while the upper and lower half-angles of the level-Ⅲ slip surface both increase. 3) the contour of the core loosening zone changes from a triangle, to a trapezoid, and finally to a rectangle with increasing filling height. The height of the core loosening zone is 0.5–0.8 times the filling height, and the angle and area of the zone both decrease in a quasi-linear manner with increasing internal friction angle. 4) the calculation method for the loosening stress based on the area of the core zone is more applicable than Terzaghi’s method, and it is suggested that it be used to calculate the critical stress of test groups with a low and a high filling height respectively. The research results provide more accurate descriptions and judgment criteria for the displacement and failure modes of the loosening zone in sand. They also provide a reference for evaluating the stability of the loosening zone.