The differential reduction in the transition zone between the heap segment and rigid structures such as Slab on Pile is one of the critical problems in the construction of toll roads on soft soil, because it can reduce comfort, safety and service life. In the Serang-Panimbang Toll Road Section III Project, the handling method used is a combination of Free Draining Granular Material (FDGM) to support oprit and bored pile piles as the foundation of the Slab on Pile abutment. This study aims to evaluate the performance of existing treatment methods by comparing the prediction of decline using the 1D analytical method (Terzaghi for the stack and the Bousinessq method and the Poulos–Davis method for the abutment) against the 2D numerical modeling using PLAXIS. The decline profile in the transition zone was then analyzed to assess the potential for differential decline in the applicable permit limits. The validation results showed a high compatibility between analytical and numerical methods, with a deviation of 6.09% and abutment of 4.93%. However, the pile above FDGM experienced a total decrease of 51.90 cm, while the abutment of the Slab on Pile only decreased by 6.10 cm, resulting in a differential decrease of 45.80 cm which far exceeded the permit limit. This indicates that the use of FDGM alone is not able to overcome the thick soft soil compressibility at the study site.
Ihsani et al. (Mon,) studied this question.