Field Test On Characteristics Of Piles Negative Friction In Loess Collapsible Under Overburden Pressure

There are more and more high-rise buildings(structures)on the Loess Plateau recently,piles has the advantages of high bearing capacity,small settlement and uniform settlement,and good seismic performance,was widely used in collapsible loess site.However,the pile on the self weight collapsible loess site after immersion,will produces negative friction,forms a downdrag.As a result,the bearing capacity of pile is greatly reduced,even damaged of the foundation.The negative friction is a key problem encountered in pile engineering of this kind of site,reasonable determination of negative riction is the key to the design of pile in this engineering site.The study of negative friction resistance of piles begins in soft soil area,there are relatively more research results in soft soil area,the research is still insufficient and not enough in the collapsible loess area.The mechanism of collapsible settlement of loess is essentially different from consolidation settlement of soft soil,the formation and development of its negative friction is also quite different.The negative friction of piles on collapsible loess site is different from of soft soil,mature research results in soft soil cannot be used directly.Therefore,it is of great practical significance to study the generation and regularity of pile side negative friction in self-weight collapsible loess area.In this paper,combined with the needs of the project,the representative self weight collapsible loess site near Jiuzhoutai was selected,and six test piles were set up under three working conditions of natural friction end bearing pile,friction pile and post wet immersion saturation state,the vertical compression static load test of single pile was carried out combined with the internal force test,the field test research on negative friction resistance of pile side was carried out to master this kind of pile comprehensively,the first-hand test data of negative friction of this kind of collapsible loess site were comprehensively mastered.The comparative analysis shows that the bearing capacity of the pile foundation under the natural water content state and the saturated state is quite different,the ultimate load of the non-immersion test pile is 1.67 times of the immersion.At the same time,the difference of pile top settlement is significant.The total settlement of immersion pile under ultimate load is 1.80 times of non-immersion.On the basis of experimental study,the applicability of load transfer method to calculate the negative skin friction of piles in large thickness collapsible loess site is evaluated.At the same time,pointed out the shortcomings of the research on the negative friction characteristics of piles in large thickness collapsible loess site and the contents to be carried out in the future.