Abstract: The post grouting technology is used to format the screws of a grouting screw pile, which can be constructed easily in soft clay area and has been applied to engineering project currently. According to the stress change of soils around the screws in the uplift process of the grouting screw pile, the Mohr circle stress analysis is conducted to analyze the development of slip line along the screw; and it can also give the relationship between the angle of slip line and the ratio of screw pitch to screw dimensions，which is expressed in the mathematical equation of incremental normal stress and shear stress of soil around the screws. The condition to judge the shape change of slip line between the screws from cylinder to truncated cone is obtained. Based on the shape of slip line, the calculation of uplift capacity of screw pile can be divided into three cases, the formula of which is obtained through limit equilibrium method. The application and rationality of the presented method is proved by comparison with finite element analysis and a field test.

Key words: screwed pile, uplift capacity, Mohr stress circle, slip face