The reason of the failure of foundation treatment in a certain slope and deep artificial fill area is investigated by analyzing the site investigation and the settlement observation. The useful conclusions can be drawn:(1) the settlement in collapsible loess zone can be divided into 4 stages:the growth period, the stable period, the acceleration period and the last stable period. The increase of the rate of the ground subsidence is caused by the collapse of loess soil. The settlement observation time is suggested to be longer to assure the settlement of the construction in the last stable period.(2)the collapse of loess soil not only cause the differential ground subsidence, but also change the mechanical characteristics of pile foundation. Only relying on the stiffness of pile foundation to control the displacements of structure is dangerous, but the deformation of soil near the pile foundation should be considered as well. (3)When the foundation of the construction is in a certain slope, the cementation of soil grains in a low height is weaker than those in a high height, which causes the low area deforming largely.