Principle And Theoretical Research On Self-anchored Loading Test For Pile Bearing Capacity

Pile foundation plays an important role in engineering construction,and accurate detection of the bearing capacity of pile foundation is the prerequisite to ensure the function of pile foundation.Therefore,the research group proposed a new pile test method,the self-anchored test pile method.The self-anchored test pile method is to divide the whole pile into the upper top-uplift pile and the lower top-down pile at the equilibrium point.The upper top-uplift pile can be regarded as an anchor pile.A through hole is added to the upper pile body and the force transmission rod is placed inside.And its main steel bars are anchored to the upper reaction beam.During the test,the jack is used to apply the vertical load,and the top of the upper pile is subjected to the uplift force by the anchoring reaction force,and the top of the lower pile is subjected to the pressure through the force transmission rod.At this time,the upper pile’s uplift bearing capacity and the lower pile’s compressive bearing capacity can be measured simultaneously.Finally,the vertical compressive bearing capacity of the entire pile can be obtained through the scientific conversion method.As a new pile test method,the self-anchored test pile method has the following advantages:(1)easy to operate,there is no need for heavy weight as counterweight;(2)simple devices,loading devices are all placed on the ground,which can be repaired and replaced at any time and recycled and reused;(3)low test cost;(4)high precision,suitable for large-tonnage pile foundations.At present,the corresponding research on the self-anchored test pile method is still blank.Therefore,this paper has carried out research on the bearing characteristics of the self-anchored test pile.The main research contents of this article are:(1)The indoor model test and field test of self-anchored test piles were carried out,and the detailed process of indoor model test and field test was fully explained,including test preparation,test installation,test loading and other steps.The indoor model test is carried out in a model box,and the soil is homogeneous silt,which includes five working conditions: the top-uplift pile A1,the traditional top-down pile B1,the self-anchored test pile’s upper section A2,lower section B2(loaded separately)and the self-anchored test pile C.The soil of the field test is stratified soil,including four working conditions: the top-down pile P1(without soil at the pile tip),the top-uplift pile P2,the traditional top-down pile P3 and the self-anchored test pile P4.(2)It is assumed that the pile side soil conforms to the double-broken hardening load-transfer function model,a complete set of analytical solution formulas for the top-uplift pile’s load-displacement curve under axial load is presented.Then,by fitting the measured data to get the parameters such as shear stiffness coefficient and elastic limit displacement,the load-displacement curve of top-uplift pile in the elastic stage completely,in the plastic stage partly and in the plastic stage completely can be obtained by the above analytical solution formulas.Finally,the accuracy of the analytical solution formulas is verified by the top-uplift piles’ measured data of the indoor model test,the field test and two engineering cases.The results show that the load and displacement obtained by the analytical solutions are relatively close to the actual measured data,which can be used to predict the behavior of pile foundations with different pile diameters and pile lengths on the same site.It can also be compared with the measured data to analyze the mechanical properties of the pile side soil,which has high engineering practicability.(3)Combining the existing analytical solution formulas for the load-displacement curve of the traditional up-down pile,a complete set of analytical solution formulas for the load-displacement curve of the self-anchored test pile is formed.It is assumed that the pile side soil and pile tip soil of the upper and lower sections of the self-anchored test pile conform to the double-broken load transfer model,then assume that the pile body is a linear elastic rod with constant cross-section;the soil around the pile is homogeneous soil;when the pile tip soil enters the plastic stage,the pile side soil has completely entered the plastic stage.Therefore,the analytical solution formulas for the load-displacement curves of the upper and lower sections of the self-anchored test piles are given.Finally,the accuracy of the analytical solution formulas is verified by the measured data of the indoor model test and the field test.It can be seen from the results that the upper and lower sections of the self-anchored test piles’ load-displacement curves obtained the analytical solution formula is closer to the measured curves,which can well reflect the load characteristics of the self-anchored test pile and meet the actual needs of the project.(4)Two methods are proposed to convert the test result of the self-anchored test pile into the equivalent load-displacement curve of traditional up-down pile.The first one is the simple conversion method,which converts two variables-load and displacement respectively equivalently.The second one is the analytical conversion method,which uses the analytical solution formula to convert the load-displacement curve of the self-anchored test pile into that of the traditional up-down pile.Finally,the verification results of indoor model test and field test show that both conversion methods are reasonable and practical.