Study Of Numerical Analysis On Bearing Capacity Of Existing Building Pile Foundation Under The Condition Of Deep Excavation

Development and utilization of underground space has aroused people’s great attention with the rapid development of urban construction.Especially with the increases of private cars, defects of use function are getting more and more obvious, of which the lacking of parking space in existing buildings. When the existing buildings construct additional underground space, we need to use the original or addition new pile foundation to support the upper buildings, in order to excavate the soil beneath the buildings. Compared with the original pile foundations, the naked pile’s characteristics will change after the soil excavation, especially the lateral friction resistance. Therefore, the analysis of the existing buildings bearing characteristics of pile foundation under excavation is necessary.This paper use finite element program ANSYS analyzed the variation of pile foundation characteristics before excavation and different soil excavation depth around the pile foundations, including:bearing capacity of single pile, normal force of pile, pile side friction and pile-soil relative slip amount analysis. According to the basis of research on pile foundation excavation, and considering superstructure, base and foundation combined effect for a three-tier framework, a ANSYS finite element simulation process was conducted, it reached a varying pattern which is about different excavation depth bearing characteristics under various position of pile foundation.We can see from the results of the analysis, for single pile foundation, with the increases of the elastic modulus of soil around pile and the length, the bearing capacity of single pile will increase. Excavation of soil around the pile foundation will cause a significant increase in the amount of the settlement, and greater depth the excavation is, a greater increase the settlement will have Pile of soil deep excavation would make excavation section normal force increases due to its own weight, without excavation section normal force transmitted to the soil around the pile while maintaining a decreasing trend through the pile lateral friction. With the pile resistance the side of the trunk excavated section disappeared, side resistance of the none excavated section on pile body correspondingly increases, to compensate the loss of lateral resistance of the section being excavated. Meanwhile, the relative slip settlement between the pile and the pile of soil will also increase accordingly.Considering superstructure, foundation and foundation interaction modeling after analysis, before and after the excavation of soil around the pile, all piles at different location showed the center settlement is lager than the surrounding parts, it is like a "pot". With the increase of the depth excavation, the unloading effect makes the pile settlement and differential settlement reduce whenever the piles are in the center or in the corner. Under different depth of excavation, excavation section normal force will increase, but no excavated segment remains a decreasing trend, and the trend of the center piles reducing is larger, corner piles and boundary piles is slow. With the increase of excavation depth, the relative slippage between the pile and soil gradually increases. After the excavation of soil around the pile, the skin friction was disappears on the excavated section, not only does it need to increase the skin friction of the excavation portion on the pile body side for bearing, but also increase the tip resistance. Pile settlement, normal force, the relative slippage between the pile and the pile of soil resistance characteristics were all shown like---center pile is the largest, the pile side take the second place, followed by the smallest corner pile.