Spring-Based Trapdoor Tests Evaluating Pile-Supported Load Transfer Platforms with Different Fill Materials

Load transfer platform (LTP) plays an important role in load transfer between piles and subsoil in pile-supported embankments over soft soils. This study conducted spring-based trapdoor tests to investigate the load transfer mechanisms of LTPs with unreinforced and geosynthetic-reinforced river sand and lightweight aggregate (LWA) subjected to differential movement. Spring-based trapdoor tests were used to simulate the differential movement below the LTP under fill placement and surface footing loading to investigate the mobilization and degradation of soil arching and tensioned membrane effects above the trapdoor (simulating the subsoil settlement). To evaluate the trapdoor rigidity effect, both rigid (one-segment) and flexible (three-segment) trapdoors were utilized. Test results revealed that LWA, when utilized as an LTP material, could effectively improve the mobilization of soil arching, hinder the degradation of soil arching, and reduce the trapdoor settlement. LWA was proven to be a good alternative LTP fill material in pile-supported embankments. Geosynthetic reinforcement was beneficial to minimize degradation of soil arching. In the rigid trapdoor tests, the tensioned membrane effect was progressively mobilized with the applied footing pressure. However, the flexible trapdoor tests showed rapid mobilization of the tensioned membrane effect, followed by a sustained mobilization, and then a rapid arching degradation after a certain applied pressure was reached and continued. Under surface footing loading, the embankment with the LWA as the LTP fill material showed an inverted tower-shape deformation pattern, whereas that with the river sand as the LTP material exhibited a rectangular deformation pattern.