Abstract: Combined with the project of mobile communication comprehensive building, Gansu Dingxi branch, China mobile in Dingxi new city, a lasting pile immersion test was made in the site of collapsible loess with large thickness; while the changing characteristics of pile negative friction and settlement process of soil were analyzed for different depths and limits (the diameter and length of test pile were 800 mm and 26 m respectively with 2 m pit in depth). The test results show that under the special immersion condition mentioned above, there appear multiple peak values along the pile, and positive and negative frictions presented staggered distribution phenomenon; so that the value of average negative friction couldn’t be given by Technical code for building pile foundation[1]; then with multimodal distribution of soil settlement amount, its changing tendency directly affects its trend in positive and negative friction along the pile; also, in the neutral point location, the single reference value provided by Technical code for building pile foundation wasn’t given; but several neutral points appeared along the pile; and the actual collapsible condition of loess is in sections, but not is sudden collapsible in the whole section above a certain neutral point as the traditional view states.

Key words: pile immersion test, layered settlement, negative friction, multiple peak of negative friction, neutral point