Finite element analysis on mechanism of effect of extra-deep excavation on vertical load transfer and settlement of a single pile

The effect of deep excavation and heave of soil beneath pit bottom on the pile supporting extra-deep foundation has not been considered.The approaches adopted to determine the bearing capacity of a single pile in extra-deep foundation still employ conventional pile load tests on ground surface first and then deduct the skin friction along the pile shaft in the range of excavation from the results obtained by the conventional tests,where the effect of deep excavation is not taken into account.Here three different methods of pile load tests are simulated through the axisymmetric finite element model of a pile in homogeneous soil adopting parameters of soil,pile material and soil-pile interface verified by a case history.Through the simulation of deep excavation,its effect on the pile is investigated and on this basis the mechanism of load transfer and the settlement of the pile loaded after deep excavation are studied.Compared with the conventional load tests and load tests with sleeve,the bearing capacity and vertical stiffness of the pile loaded after excavation are reduced.Under the same load the pile experiencing deep excavation has a larger settlement and a larger proportion of pile rigid penetration in settlement than that of piles in the other two tests.Deep excavation also leads to a large pile-soil relative displacement when the skin friction is totally mobilized.In addition,deep excavation can cause considerable tension force in the pile,which means that the reinforcing steel bars in the lower part of the pile should not be reduced at discretion.