Experimental study on mechanism of soil arching effect induced by vertical unloading in deep underground construction

As shallow underground resources are depleted, urban development is extending to greater depths, necessitating a clear understanding of soil arching at various burial depth conditions. Laboratory trapdoor tests equipped with embedded soil–pressure cells and digital image correlation captured the ground–reaction curve and soil deformation. The results reveal pronounced discrepancies between shallow and deep burial. In shallow conditions, soil arching undergoes a “failure-reconstruction” process: soil pressure plunges, then rebounds to stability. In deep strata, the arching forms rapidly and attains stability almost immediately after the minimum pressure is reached. Shallow tests generate several horizontal displacement bands rising to 4.8 B ( B , trapdoor width); deep tests yield one stable band, with its influence height reduced to about 3.0 B . Vertical displacement above the trapdoor evolves through “triangular-tower-parabolic” stages to 4.3 B in shallow tests, but follows a persistent parabolic profile limited to 2.7 B in deep tests. Additionally, shear bands under deep conditions form at smaller angles and are more vertically oriented. These findings expose the fundamental differences in deformation mechanisms between shallow and deep burial and provide quantitative criteria for depth zoning in urban underground‑space development.