Under load, the subgrade will deform. As the load increases, the deformation of the subgrade gradually increases. In the initial stage, the stress in the subgrade soil is in an elastic equilibrium state and has a safe bearing capacity. When the load increases to the point where the shear stress at a certain point or at various points within a small area on a plane in a certain direction reaches the shear strength of the soil, shear failure occurs at that point or at various points within the small area, placing them in a limit equilibrium state, and the stress in the soil will be redistributed. This small-scale shear failure zone is called the plastic zone. The limit equilibrium state of a small area of the subgrade can mostly be restored to an elastic equilibrium state, and the subgrade can still tend toward stability and still has a safe bearing capacity. However, at this time the subgrade deformation is slightly larger, and it must be verified that the calculated deformation value does not exceed the allowable value. When the load continues to increase and a relatively large plastic zone appears in the subgrade, it will indicate that the subgrade bearing capacity is insufficient and stability is lost. At this point, the subgrade has reached its ultimate bearing capacity.