New and old subgrade stability is a crucial problem for widening projects in soft soil foundations because uncoordinated deformations can cause serious engineering accidents, even disasters. In order to ensure the stability of a widening expressway project near a pond in the Qinbei section, a series of on-site investigations, lab tests, and numerical analyses was performed. The settlement and displacement of on-site monitoring were carried out during the construction period to verify the analyzed results. It was found that the uneven settlement caused heavy settlement and displacement of the embankment; thus, net cracks and longitudinal cracks occurred in the expressway. The new embankment was also affected by the weak foundation; therefore, the foundation should be reinforced before the construction of the widening part. Considering the economy and effectiveness of the widening project, the replacement method was recommended for the weak foundation reinforcement based on the numerical analysis and on-site motoring results. Water pumping had a significant effect on the settlement of the embankment. Water pumping caused seepage in the foundation and increased the effective stress of the soil, making the foundation more consolidated. The pond slope should be reinforced before the construction of the embankment because it was not confined by the surrounding soil. Therefore, the slope soil could not provide sufficient passive soil pressure and easily slide.