During pile-sinking using the static pressure method, under-driven pile foundationsfrequently occur because of difficulties in jacked pile-sinking. To investigate the bearing performance of under-driven piles, this study conducted several on-site jacked pile-sinking tests based on the East Coast Railway Project in Malaysia. A discrete element model for jacked pile groups in layered foundations was developed using the particle flow code software, comparing the bearing behaviors of under-driven and jacked piles with designed lengths and then considerably analyzing the influence of pile spacing. The results indicate the following: first, there is an optimal pile spacing of 19D for under-driven piles, whose ultimate bearing capacity can reach 93.3% of those with designed length. Second, from the microforce chain propagation viewpoint, sand layers contribute substantial shaft friction and pile tip resistance, whereas clay layers primarily function as the bearing strata with pile tip resistance. Third, displacement cloud diagrams reveal that deep pile foundations exhibit superior deformation resistance, whereas shallow foundations are prone to horizontal spreading and displacement, and the stress form of the pile groups is similar to that of open-ended pipe piles. Finally, below optimal spacing, the ultimate bearing capacity decreases with reduced pile spacing. These findings offer critical insights for under-driven pile construction under static pressure challenges.