Abstract: Based on the strength reduction theory, the instability mechanism of a deep and large foundation pit slope in Chongqing under surrounding muck overload is studied using numerical simulation and insitu monitoring. The conclusions are drawn as follows. (1) The potential destructive type of slope with surrounding muck load is shearing and slide, and the inner essence of slope instability is that the maximum shear ratio is formed and extended from toe to top of slope, and is connected in the banded form. (2) The support method of soil layer is "large slope and mesh spray", and the soilslope ratio is 1∶1.75. (3) The support method of rock layer is "low slope and bolt mesh spray", and rockslope ratio is 1∶0.75. According to numerical simulation, insitu monitoring and economic calculation, the stability of foundation pit slope can be improved and the requirements of safe, high efficiency and economy can be met.

Key words: strength reduction theory, numerical simulation, insitu monitoring, deep and large foundation pit, combined support technology