Basic Theory Analysis And Test Study On Long-short Piles In Loess Region With Big Thickness

With the national strategy “One Belt,One Road” being deeply implemented and the process of urbanization being accelerated,the construction projects and their scale in collapsible thick loess area in northwest are increased and updated consecutively.The construction sites are moved to high-terrace gradually,and the projects of pile foundation have been applied and developed tremendously.However,it has to be pointed out that the study of bearing characteristics of pile foundations is not mature since the design of pile foundation is still calculated according to half theory and half experience.There are obvious contradictions between existing codes and reality about the design of pile foundation.For this reason,not only deeply studying the problems about the effective length of single pile and the bearing characteristics of long-short pile is a necessity for the development of pile foundation’s theory,but also an urgent need in engineering field.Many study methods including theoretical analysis,indoor and outdoor experiments,and numerical simulations,were used in this paper.Firstly,the problems about effective length of single pile in collapsible thick loess area were analyzed and calculated.Secondly,the applicability of long-short piles in loess area was analyzed and the calculation method for the bearing capacity and deformation of long-short piles was established.Thirdly,field test for single pile’s bearing capacity and the indoor model test for long-short pile were carried out.Lastly,they were comprehensively analyzed and studied by numerical analysis.The main innovations and conclusions were as follows:1.Through analysis about the problems of effective pile length in collapsible thick loess area,considering stratified collapsibility phenomenon and multiple neutral points along the pile body,the sinusoid dis tribution for negative frictional resistance of pile foundation was proposed,and the mathematical expression was given.The calculation formula of effective single pile length considering pile weight in collapsible thick loess area is derived.Meanwhile,comparing it with the single pile field test and analyzing it,the rationality of the method has been verified.2.According to the in-situ test and stratified deposition characteristic of large thickness loess,the feasibility of long-short pile foundation in large thickness loess area was proposed and analyzed.The calculation method for neutral point and negative resistance for group piles was given by the theoretical approximate solution and the group pile effect coefficient,and the calculation method for the bearing capacity of long-short pile considering the influence of negative resistance was established based on it.Through shear displacement method,the calculation method for settlement and deformation of long-short pile foundation in large thickness loess area was given.The relative calculation method is programmed in MATLAB,which lays a foundation for the design of long-short pile foundation based on the deformation control.3.It can be found that the interlayer of foundation soil in loess tab leland distributes interactively,and the collapsibility is not continuous.The influence of soil layer around the pile on the transmission and distribution of inner force is obvious.Under the load of all levels,the q-s curve of the pile foundation develops slowly,which is the typical bearing characteristic of the friction pile.The axial force and side friction of the pile body are asynchronous.When designing pile foundation,combining with the experience,the bearing capacity and maximum allowable deformation can be controlled to resist compressive deformation and decrease the settlement of upper pile foundation by increasing the linear stiffness.From numerical simulation,it can be found that long-short pile foundation can make full use of the capability that long pile controls settlement and the bearing capacity of shallow layer of foundation soil,which saves the amount of long piles and costs of pile foundation.4.In this paper,an indoor model test device for long-short pile was developed.The bearing capacity and deformation characteristics was analyzed under conditions of single-pile,4 composite pile and 9 combined pile.It can be concluded that the load-settlement curve of the long pile has obvious inflection points,that is,the "piercing" damage happened.But this damage is not obvious on short piles.The bearing capacity of the long pile is 35.7% higher than that of the short pile.In the multi-pile combination test,the load of corner pile is larger than the load of side pile and center pile.Compared with the single pile test,The pile bearing capacity and side friction resistance of both long piles(corner pile,side pile)and short piles(central piles)are significantly improved.The load sharing ratio at top of corner piles is slightly higher than that of side piles.Under different horizontal thrusts,the F-S curves of pile foundation have obvious steeply descending sections,which are typical features of piercing failure.Short piles are damaged before long piles,and the long piles failed with deflection and deformation,and their horizontal bearing capacity is mainly controlled by the strength of the soil around piles or the horizontal displacement of the pile top.5.Using the finite element software of geotechnical engineering PLAXI S 3D V2016,establishing the calculation model of full-long piles,full-short piles and the combination of 5 long,4 short piles respectively,the influence of pile types and their distribution on bearing capacity and deformation are analyzed.The analysis shows that the corner piles have the least stress,the stress of side pile is in the middle,and the center piles have the largest stress.The vertical deformation of the rafts varies much with different pile type layouts;the pile space and pile length a lso have a great influence on the force and deformation of the pile body.Under the oblique load,the horizontal displacement accounts for a large proportion of total displacement,and the bearing capacity of the pile body is mainly controlled by the horiz ontal load.