Abstract: A method for calculating the unloading additional stress in the soil at the bottom of deep foundation pits under the condition of layered excavation is proposed, based on the Mindlin solution. The nonlinear relationship between the soil resilient modulus and the unloading additional stress during foundation pit excavation is considered in the calculation of rebound deformation using the layered summation method. The calculation results show that affected by the depth of layered unloading, the unloading additional stress under the condition of layered excavation is less than that under the condition of one-time excavation within the most of the depth range, and the difference between them is obvious within three times the depth of foundation pit below the pit bottom. With the same foundation pit area, the unloading additional stress at the bottom of long strip foundation pit is less than that of square foundation pit. This method can be used to evaluate the influence of pit bottom reinforcement on the unloading additional stress of soil and rebound deformation combining with the equivalent layer method. With the increase of resilient modulus or thickness of reinforced layer, the unloading additional stress and the rebound deformation decrease, and the extent of deformation reduction decreases correspondingly. The method has been verified to be reasonable through the case study of West Square of Hangzhou East Railway Station. Based on the parameter analysis, the equivalent design method of resilient modulus and thickness of the reinforced layer is proposed, which has guiding significance for practical engineering.

Key words: foundation pit, unloading additional stress, rebound deformation, reinforcement, resilient modulus