Experimental Investigation of the Evolution of Suffusion in Gap-Graded Arched Soils at Different Soil Arching Ratios

Soil arching is a common phenomenon existing in the earth structures such as pile-supported embankments and buried pipes. Suffusion would be triggered in these structures during flood events after rainstorms and pipe ruptures, which poses significant threats to these infrastructures. A novel trapdoor-suffusion apparatus was developed in this study to investigate the initiation and progressive development of suffusion in infrastructures involving soil arching. A series of laboratory tests were conducted using gap-graded glass beads with an initial fine particle content of 15% by mass. Initial soil arching was established via trapdoor movement and then was subjected to a horizontal seepage flow with an increasing hydraulic gradient to induce suffusion in the arched soil. The presence of soil arching increased the susceptibility of local instability of the soil specimen within the stress reduction areas at significantly low local hydraulic gradients, due to the reduced stress restriction on fine particles. Subsequently, global suffusion was triggered at a critical hydraulic gradient much lower than the predicted value by the modified zero effective stress principle, and this critical hydraulic gradient decreased rapidly with the reduction of initial soil arching ratio (SAR). This study obtained an empirical formula in the form of power functions to characterize the variation of critical hydraulic gradient with the initial soil arching ratio. As the suffusion continued, the degradation of soil arching intensified under the low initial SAR, and became the dominant factor increasing the surface displacement of the soil. This paper reveals the increased risk of suffusion to earth structures with the presence of soil arching, and provides valuable guidance for the design and protection of earth structures against suffusion-induced challenges.