Abstract: The side friction will decrease due to unloading rebound deformation after excavation, though it occurs only within a certain depth range near the bottom of pits. Unloading action can be considered as a uniformly distributed upward pressure on the bottom of pit when the side friction of piles after excavation is calculated. The additional stress of soils can be expressed by Mindlin solution when the load is under the ground. Excavation calculation depth Hc is defined as the depth at which change ratio of side friction is 5% between after and before excavation, and all the influential factors on Hc such as excavation depth, excavation wide, internal friction angle and Poisson’s ratio are considered. In addition, the calculation formula of Hc is derived. Although the result of vertical additional stress from Boussinesq solution is bigger than that from Mindlin solution, but the difference becomes smaller with the depth. The side frictions after excavation for 216 cases are calculated. It is shown that the maximum ratio ? of results from Boussinesq solution to that from Mindlin solution is 1.124, and the minimum value is 1.001. Therefore the side friction of pile after excavation can be calculated by Boussinesq solution for safety.

Key words: unloading of excavation, uniform upward pressure, Mindlin solution, Boussinesq solution, excavation calculation depth, side friction of pile