Abstract: This study uses the composite pile-sheet wall introduced into the Lanzhou hub project by the Zhongwei–Lanzhou passenger dedicated line as the research object, and employs a combined field monitoring and numerical simulation approach to systematically investigate the passive soil-arch effect in front of piles in the embedded section of the existing–new side-by-side pile-sheet wall. Field monitoring yields measurements of pile deformation, bending moment, and soil pressure in front of the new pile, and a three-dimensional finite element (FE) model is constructed for comparative verification. Based on the numerical simulation results, an in-depth analysis is conducted of the formation mechanism, evolution, geometric characteristics, and spatial configuration of the passive soil arch in front of the composite pile-sheet wall. The results show that the contact soil pressure in front of both the existing and the new piles follows a parabolic distribution, with larger values near the surface and smaller values at depth. Under embankment widening and train loading, the increase in contact soil pressure in front of the existing piles is smaller than that in front of the new piles. As the loads on the new pile increase, both piles experience greater displacements, creating a substantial displacement difference with the soil between them. This causes the principal-stress direction in the inter-pile soil to deviate and leads to the formation of a passive soil arch in front of both the existing and the new piles. The evolution of the passive soil arch can be divided into the following stages. Under the load from the existing embankment, a stable soil arch forms in front of the existing pile. The stress relief caused by excavating the new pile holes is limited in extent, and the soil arch in front of the existing piles remains intact. With the increase of new load, the stress of the soil in front of the new and the existing piles overlaps, resulting in stress redistribution. The original soil arch in front of the existing pile disappears, and a new passive soil arch forms progressively between the new and existing piles. Before and after the embankment widening, as the load increases, the thickness of the passive soil arch remains relatively constant, but the rise-span ratio gradually increases. Under the same working conditions, the soil arch thickness and the rise-span ratio decrease with increasing burial depth. The research results can provide reference for the design and construction of new composite pile-sheet wall.

Key words: embankment widening, existing-new, side-by-side composite pile-sheet wall, passive soil arching reconstruction, soil arching geometric shape