Three soil-arching evolution patterns in unreinforced piled embankments were observed in a series of two-dimensional (2D) model tests using a multitrapdoor test setup. These include the triangular expanding pattern, the tower-shaped evolution pattern, and the equal settlement pattern. The inclination of the slip surfaces and the height of the vertical slip surfaces that enclose the tower-shaped arches were found to be the critical parameters describing the arching evolution. Three analytical models were proposed to describe the evolution processes of the three arching-evolution patterns and to find the stress distributions of the corresponding processes. Load distribution equations were also derived from these models. Using the empirical relationships between the inclination of the slip surfaces and the tower height and settlement, the stress distribution ratio during the entire evolution process was calculated. The models matched the model tests well.

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A trapdoor system has frequently been used to study soil arching and its development in recent years. The load transfer in the fill of piled embankments is very similar to a trapdoor system with multiple trapdoors. There are multiple arching models described in different standards and guidelines for piled embankments that can be subdivided into three archingmodel families. To study the soil-arching type and its development, a series of model tests with sand fills were carried out in a two-dimensional (2D) multi-trapdoor test setup. The tests considered four factors—the fill height, trapdoor width, pile width, and grain size of the sand—with four values for each factor. Triangular slip surfaces were found at very small deformations using the particle image velocimetry (PIV) technique. These surfaces evolved in ways that could be related to the three types of stress-distribution ratio curves, with development patterns similar to the arching families of piled embankments: (1) the rigidmodel family, (2) the equal-settlement-plane-model family, and (3) the limit-equilibriummodel family. The limit-equilibrium-model family occurred in tests with narrow trapdoor widths. @en