Excavation beneath the original basement can effectively utilize the underground space

which is one of the popular ways to improve the parking problem in the dense urban areas. Excavation beneath the original basement will change the behavior of original pile. If the excavation depth exceeds a certain value

it may lead to buckling instability of the original piles due to the reduction of ground confining pressure around piles. In this paper

a theoretical analysis is performed to estimate the bucking critical load and the effective length of a single pile. At first

the scheme of floor-addition of basement is briefly introduced with the case of Zhejiang Hotel Extension Project. Secondly

the total potential energy of the pile-soil system under the condition of excavation beneath the original basement is set up based on the Winkler elastic beam theory. Finally

the expressions of critical load and effective length of single pile are deduced by using the minimum potential energy principle. Based on the proposed theory

the influence factors of critical load

including half-wave number n and excavation depth

are analyzed. It is shown that the buckling critical load of pile shaft converges with an increase in half-wave number n; by increasing the excavation depth

the buckling critical load decreases rapidly. The proposed theory may provide guideline to estimate the supported pile behavior of excavation beneath original basement under existing buildings.