Study On Characteristics Of Piles In Collpsible Loess Sites Under Water Immersion Condition

There has a large area of collapsible loess distributing in China,and the Chinese loess is famous in the world as broad cover and great thickness.In allusion to the piles in collapsible loess,negative friction will appear when the loess immersed by water and the danger caused by negative friction has already been attended by engineering field early. With the expanding of construction scale in China,the application of pile foundation in great thickness collapsible loess becomes more and more.But the existing experiences and theories about negative friction of pile in collapsible loess can't meet the requirements of current practical engineering,it's urgent need to study the bearing behavior of the pile foundation in immersing collapsible loess.China has carried out field water immersion test studies on piles in eight loess sites of five provinces from 1970s to now,but there has an absence of systematic analysis to these test results.We also carded out a field water immersion test study which the test scale was biggest in similar tests for a practical engineering.This article summarized and studied the bearing behaviour and its relative problems of piles in immersing collapsible loess systematically based on this test results and other test results gotten by predecessors from the point view of field tests.The relative problems of bearing behaviour in immersing collapsible loess may include the following aspects:collapse characteristic of loess,pile additional settlement caused by water immersion,pile side negative frictional resistance and neutral point.This article mainly studied on these three aspects.The-collapse deformation of loess mass had been measured in the test carded out by author,but the measured results were different from the collapse deformation characteristic measured in other Loess Water Immersion Test,embodied in:(1)The settlement occurred after the immersion is larger than the settlement occurred in the process of immersion;(2)The Settlement Profile in the process of immersion presents the shape of saddle.The article explained the phenomena using the knowledge of Osmotic Mechanical,and analysed the different collapse characteristics corresponding to different permeability stratum assemblages,and also pointed out that the deformation occurred after the immersion should still be the Collapse Deformation,but not the Consolidation Settlement suggested in the past.Through the monitoring to pile top settlement in the process of test,combining with other research results gotten by predecessors,this article also studied the pile settlement characteristic in different conditions,the composition of pile additional settlement caused by immersion,and the ultimate bearing capacity of single pile in different conditions.The reseach results shows that:(1)Immersion early or late do not affect the ultimate bearing capacity of single pile,but affect the pile settlement;(2)The vertical load on pile top is larger in the process of immersion,the pile additional settlement is bigger, but also the final settlement corresponding to ultimate load is bigger;(3)The additional settlement includes the concrete creep of pile body and the pile end displacement caused by shear creep of pile side soil in addition to the instant settlement caused by load on pile top and downward load.Pile side negative frictional resistance is the main problem of pile foundation in collapsible loess site.The article analysed regularity of occurrence and development of the neutral point depth and negative frictional,resistance,and made a detailed study of their influencing factors.The neutral point depth is relative with the pile settlement and the loess collapse lower limit depth under overburden weight,the neutral point depth is 2～8m deeper than the collapse lower limit depth.The influencing factor of negative frictional resistance has many,it is relative with the violent degree of collapse,neutral point depth,load on pile top,test method,pile type etc.The negative frictional resistance may be estimated by Effective Stress Method.The relationship curve of negative frictional resistance and pile-soil relative displacement may be expressed by hyperbolic model,but the ultimate pile-soil relative displacement is not a constant which is different from the routine understanding.The research results of this article have some theoretical significance and practical value.