Experimental Study On Mechanical Characteristics Of Foundation Piles Under Water Immersion In Collapsible Loess Sites

Pile foundation is a kind of deep foundation that has the advantages of high bearing capacity and small settlement.It is widely used in the large thickness of loess region.Engineering practices and many tests show that the mechanical properties of foundation piles vary with the different engineering conditions.For the foundation piles in self-weight collapsible loess site,site immersion is an important factor affecting the mechanical properties of foundation piles.Different immersion conditions also lead to different stress of the foundation pile,and the negative friction resistance caused is also different.Different treatment of this problem in the design measures is the premise of optimization design.Therefore,it is necessary to conduct an in-depth study on the stress characteristics of foundation piles in this kind of special cases in order to provide more basis for further optimizing the design of foundation piles in such projects on the basis of meeting the requirements of safe use.In this paper,field test combined with scale model test were carried out to study the influence of immersion and collapse of self-weight collapsibility loess site on the stress of foundation pile.The level of site collapsing was related to the stress of foundation pile.The specific researches of negative friction of piles were done in two aspects including the collapsibility characteristics of site self and the activating level of collapsibility by infiltration.The results are as following.（1）The collapsibility of site plays a decisive role on the stress characteristics of pile in self-weight collapsibility loess site after immersion.For the site with greater collapsibility,the axial force in everywhere pile length is greater after the site is infiltrated and saturated,and the average value and maximum value of negative friction resistance are also greater.Meanwhile,the distribution of negative friction resistance on the pile body is wider and the position of neutral point is lower.（2）The distribution of axial force and negative friction of foundation pile under different level of collapse can be obtained by controlling the amount of immersion.The results show that the axial force and negative friction of foundation pile increase with the increase of collapse activation level of the site.The maximum axial force and negative friction resistance of foundation piles under saturated water immersion conditions are 3 times and 3.9 times of 50 % collapsibility.The position of neutral point is 40 % lower than that of 10 % collapsibility.At the same time,the influence of the site collapse level on the axial force and negative friction resistance of foundation pile is also restricted by site conditions.Higher site collapse potential will lead to the influence of collapse activation level on the stress of foundation pile more significant.（3）The stress of foundation pile under different collapse activation level is also affected by pile tip bearing layer and pile top load.The settlement of pile with top load is larger than that of pile without load.The relative displacement between pile and soil is smaller than that of pile without load.The friction resistance is also relatively smaller under the same collapse activation level.When the bearing layer at the end of the pile is good,the axial force and the reaction force of the pile are relatively large,and the negative friction resistance of the pile is also large.