Spring-Based Trapdoor Tests Investigating Soil Arching Stability in Embankment Fill under Localized Surface Loading

Pile-supported (PS) embankments have been used increasingly to support highways and railways on soft subsoils. In addition to the self-weight of the embankment, this embankment system is often subjected to surface localized loading, such as traffic loading. In this embankment system, soil arching is a key load transfer mechanism. Stability of soil arching under localized surface loading is important because traffic loading applied on the embankment surface can transfer onto and between pile heads and affect the degree of mobilization and degradation of soil arching. Conventional trapdoor systems have a rigid trapdoor, control its displacement manually/automatically, and cannot represent load-induced subsoil settlement below the embankment. This study utilized a trapdoor supported on low-stiffness or high-stiffness compression springs that moved under the load above the trapdoor (called a spring-based trapdoor) to evaluate the effects of continuous trapdoor displacement on the soil arching stability under static footing loading. To investigate the trapdoor rigidity effect, a trapdoor consisting of three segments (called a flexible trapdoor) was utilized in this study as well. The trapdoor test results showed that soil arching was mobilized during fill placement as the fill height and the trapdoor displacement increased. Subsequently, under static footing loading, the degree of soil arching increased at a low applied pressure; however, it degraded under higher footing loading that caused a larger trapdoor displacement. The high-stiffness trapdoor increased the degradation pressure required to eliminate soil arching even though it reduced the degree of soil arching under a low applied pressure compared with the low-stiffness trapdoor. The flexible trapdoor resulted in a uniform stress distribution on the trapdoor but reduced the total load transferred to the supports. The conventional trapdoor resulted in a lower soil arching ratio compared with the spring-based trapdoor during the soil arching mobilization, but had a higher soil arching degradation rate than the spring-based trapdoor due to continuous soil movement.