Abstract: In order to determine the cushion parameters in design, the relationship between reinforcements and cushion can be considered as that the ground is under the action of inverted piles. The upward penetration of piles into cushion is likely to lead the cushion to such failures as monolithic shear, partial shear failure and piercing failures. Among them the rational model that can adjust the pile-soil stress ratio is monolithic shear failure. Based on the model, an iterative formula of upper limit value in terms of the internal friction angle of a cushion has been derived. According to Terzaghi's theory of slipping spirals, it is proved that the cushion thickness shall be the minimum one to set if the logarithmic slipping spirals intersect tangentially the bottom board of foundation. As a result, the adjustability of pile-soil stress ratio will be decreased if the cushion thickness is less than the minimum one to set, and the cushion width to set should be given on stress diffusion basis so as to prevent the cushion from failure.