The maximum collapsing depth of collapsible loess ground is closely related to the neutral point position of pile foundation. In order to solve the problem of improper pile foundation design

the accurate determination of the collapsing depth is of key importance. Based on a systematic analysis of experimental results obtained from Ningxia

Guyuan series of pile foundation and Heping town in Lanzhou ground treatment test and other area immersion test

the relationship between the maximum collapsing depth and neutral point position of pile foundation is revealed. The research results can be divided into the following four points. 1) Because of the limited vertical penetration depth of the homogeneous loess in the field immersion test (generally 20-25 m)

the maximum collapsing depth is limited as well. The collapse coefficient in indoor test cannot be directly adopted to evaluate the collapsing depth. 2) The recommended values of the collapse coefficient or dead weight collapse coefficient varying with the region and the collapsing depth are given. Meanwhile

the depth modification coefficient and earth immersion probability coefficient are suggested to be introduced into calculating collapsing depth and their reference values are provided as well; the difference between the indoor and outdoor can thus be effectively decreased. 3) The maximum collapsing depth calculated in indoor test is some kind of over conservative

the adverse effects of negative skin friction are overly exaggerated in most actual projects

rendering the smaller design load-carrying capacity. 4) When the depth of the neutral point of the pile foundation is less than 20-25 m and maximum collapsing depth is smaller than 20 m

the neutral point should be determined by the evaluation depth. The estimation method of negative skin friction is provided and It could be more appropriate if the negative skin friction of foundation pile is set as 20-35 kPa.