A new method of replacing soft soil with lightweight foam concrete (lightweight foam concrete replacement method, LFCRM) is carried out to treat deep soft foundation under the low-filled embankment. Two-dimensional models with different thicknesses of replacement are established by the finite element platform (MIDAS/GTS). Moreover, the deformation and stress distribution law of treated foundation is analyzed by numerical simulation with different models. The results are as follows: (1) LFCRM can offset part of dead load; (2) the transverse distribution of settlement presents an “arc” shape; (3) the replacement made of lightweight foam concrete can spread stress well; (4) increase of replacement thickness will cause the necking of the additional stress “bubble”; (5) initial excess pore water pressure is related to the additional stress, and its dissipation path is deduced; (6) preliminary analysis indicates the feasibility of LFCRM for treating deep soft foundation with the low-filled embankment.

(1) Rock bridge cracking is a key factor controlling toppling failure. Previous studies have suggested a potential relationship between fundamental natural frequency and rock bridge length, but frequency‐only identification may introduce systematic error when the damping effect becomes non‐negligible during rock bridge deterioration. (2) In this study, an improved dynamic analysis method was developed to quantify rock bridge length by jointly considering the fundamental natural frequency and damping ratio. The method was validated through indoor tests in the Y and Z vibration directions and was further examined using a dangerous toppling rock case in the Sichuan Province, China. (3) The experimental results show that the average error of the improved method is 6.28% and 6.36% in the Y and Z directions, respectively, which is lower than that of the original frequency‐only method (9.49% and 9.73%). The correction is most evident in the unstable stage, where the damping effect becomes more influential as the effective rock bridge shortens. (4) The field case provides preliminary engineering support for the proposed indirect identification framework. The method should be interpreted as an engineering approximation for evaluating the effective bonded portion of the structural plane, rather than as a direct measurement of the full three‐dimensional bonded area.