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.