Abstract: The micro steel tube mortar composite pile is widely used in foundation reinforcement and pile replacement because of its clear force mechanism, simple construction, short construction period and low cost. However, there is rare theoretical study on instability or failure of micro steel tube mortar composite piles with small diameter and large length in soil. Therefore, on the basement of the theory of elastic foundation beam, the differential equation is solved according to the force relationship between the pile and the foundation soil. The specific expression of the bearing capacity for instability of the micro steel tube mortar composite pile is given. By comparing with the actual engineering in situ test, it is found that the theoretical and experimental results are close. An expression calcuating the minimum critical length is given to guide the construction of the project. Through a program writen in Matlab, the instability load refering to different pile lengths is calculated and analyzed under different stiffness ratios between soil and pile, which reflects the influence degree of soil on the instability and restraint force of micro steel tube mortar composite piles.

Key words: stability, analytical solution, k method, micro steel pipe mortar composite pile