Abstract: The search model of the slip surface and the calculation method of the safety factor are the two important contents to analyze the stability of the pile-anchor support structure of deep foundation pits. In this study, the most dangerous slip surface is searched by the tangential displacement gradient method, and the calculation model of the safety factor is improved based on the limit equilibrium theory. The sliding surface is searched in two steps: searching the direction with the maximum tangential displacement gradient as the tangent direction of the sliding surface; the next point of the sliding surface is determined according to the direction of the maximum tangential displacement gradient. In the calculation of the safety factor, the effect of tensile failure on the anti-sliding moment of the soil is considered through the stress ratio. Finally, the location of the slip surface and the tensile failure area are substituted into the improved safety factor formula to calculate the safety factor of the foundation pit under the corresponding working conditions. Through the analysis of numerical examples, it is found that the tangential displacement gradient has a maximum value in the range of 0° to 90°. The tensile failure zone is concentrated in the shallow soil layer and changes dynamically with the construction process.

Key words: sliding surface searching, safety factor, tangential displacement gradient, finite element, limit equilibrium method