Failure Mode And Load Transfer Characteristics Of The Self-anchored Test Pile

Pile foundation is widely used in civil engineering because of the reducing the settlement of buildings（structures）,reducing uneven settlement,improving the bearing capacity and other advantages.Accurate detection of pile foundation bearing capacity is a necessary prerequisite to ensure the full play of pile foundation.This article uses a new type of pile testing method-self anchoring pile testing method to determine the bearing characteristics of pile foundations.This pile testing method compensates for the limitations of axial single pile static load testing method（also known as traditional pile testing method）and self-balancing pile testing method.It has the advantages of simple and safe loading device operation,saving testing costs,and occupying less space,and is suitable for detecting large and high bearing capacity pile foundations.This article investigates the failure modes of the upper tension and lower compression piles of the self-anchored test piles by studying the range of the rupture surfaces between pile and the soil in horizontal direction.The expressions of the ultimate bearing capacity of the upper tension and lower compression piles of the self-anchored test piles are obtained through basic assumptions and differential equations of the rupture surface.By analyzing the load transfer characteristics between the pile and the soil in the vertical direction,assuming that the entire self-anchored test pile conforms to the three-fold hardening load transfer function model.Based on the basic differential equation of load transfer,load transfer function expression,boundary conditions,and continuity conditions,the relationship between load and displacement at the top of the upper tension pile and the lower compression pile of the self-anchored test pile is derived.Finally,the expressions of the ultimate bearing capacity and Q-s curves of the upper tension and lower compression piles of the self-anchored test pile are converted into equivalent traditional compression piles through conversion methods to obtain the ultimate bearing capacity expressions and Q-s curves of the entire self-anchored test pile.The feasibility of the theoretical method and accuracy of the theoretical result in this paper are verified through indoor model tests and in-situ test results.The main research content and conclusions are as follows:（1）According to the indoor model test of the self-anchored test pile,the unfold range of the rupture surface between the upper tension pile and the soil can be determined in the horizontal direction.The measured range of the rupture surface of the upper tension of the self-anchored test pile at the ground level is about 0.092 meters away from the pile side.The measured failure mode of the upper tension pile is a composite shear failure mode,and the rupture surface between the pile and soil is a trumpet shape.The ultimate bearing capacity of the upper tension pile and the lower compression pile is both 1300 N.According to the calculus analysis that the upper tension pile is a composite shear rupture surface,the theoretical expression of the ultimate bearing capacity of the upper tension pile of the self-anchored test pile is derived.After calculating the soil properties of the indoor model test,the theoretical unfold range of the upper tensile pile rupture surface is0.09 m away from the theoretical value of the pile side at the horizontal grand,which differs from the measured value by-2.17%.The theoretical ultimate bearing capacity of the upper section tensile pile is 1287.34 N,which compared with the measured value,the difference is+0.97%,indicating a high degree of agreement.The results indicate that the expression for the ultimate bearing capacity of the upper tensile pile obtained from the failure mode can be used to calculate the ultimate bearing capacity of the upper section tensile pile of self-anchored test piles with different soil properties and pile sizes on the pile side.According to the FLAC3D numerical analysis software fitting of the self-anchored test pile test,the failure mode of the lower compression pile of the self-anchored test pile is a Meyerhof type failure mode,and the rupture surface between the pile and the soil is pear shaped.At the same time,based on the rupture surface morphology of the compression pile in the lower compression of the self-anchored test pile,its theoretical ultimate bearing capacity expression can be derived.The soil parameters from the indoor model test were incorporated into the theoretical expression of the ultimate bearing capacity of the lower compression pile of the self-anchored test pile.The theoretical ultimate bearing capacity obtained was 1201.65 N,which compared with the measured value,the difference is-7.57%,the results are within a reasonable range.For the determined soil properties,the pile dimensions,and the pile materials,the failure mode of the pile and soil can be uniquely determined in the horizontal direction.Ignoring the interaction between the upper tension pile and the lower compression pile,the failure mode of the whole self-anchored test pile is formed by the respective failure pattern formation of the upper tension pile and the lower compression pile,and the development of the rupture surface shape of any pile section is not affected by the other pile section.And based on the position of the equilibrium point,two failure modes of the self-anchored test pile can be constructed:the equilibrium point is located inside the rupture surface of the lower compression pile and the equilibrium point is located outside the rupture surface of the lower compression pile.（2）By analyzing the Q-s curve of in-situ testing,it is assumed that the load transfer characteristics between the vertical self-anchored test pile and the soil are consistent with the three-line hardening load transfer model.Based on the expression of the three-line hardening load transfer function and the basic differential equations of the upper tension pile and the lower compression pile,the differential expressions of the pile displacement at each stage are obtained.After integration,the displacement expressions at each stage are obtained.At the same time,the undetermined calculation parameters are obtained through boundary and continuous conditions and in-situ test measured values.Finally,the analytical expressions of the self-anchored test pile load and pile displacement are obtained.After verification by in-situ testing,the fitting mean square errors between the displacement results on the Q-s curve of the upper tension pile and the lower compression pile analytical theory of the self-anchored test pile and the corresponding displacement data points on each measured Q-s curve are 0.383 mm and 0.279 mm,respectively.The results show that the theoretical Q-s curve obtained by fitting the theoretical analytical expressions of the upper tension pile and the lower compression pile of the self-anchored test pile has a good fitting effect with the measured Q-s curve in the in-situ test.It can be used to predict the load transfer characteristics of the self-anchored test pile at the same test site and different aspect ratios,and has strong engineering practicality.（3）Comparing the measured ultimate bearing capacity of P2 uplift pile and P1 soil free compression pile can obtain the conversion coefficient from negative frictional resistance to positive frictional resistance in indoor model tests.According to the principle of converting the bearing capacity of pile foundations,and taking into account the influence of the self-weight of the upper tension pile,the expression of the ultimate bearing capacity of the upper tension of the self-anchored test pile obtained through the composite shear failure mode is subtracted from its self-weight of the pile,and then divided by the conversion coefficient from negative to positive frictional resistance.Finally,the expression of the ultimate bearing capacity of the lower compressive of the self-anchored test pile obtained through the Meyerhof type failure mode is added.It can obtain the expression for the ultimate bearing capacity of the whole self-anchored test pile.After verification by the indoor model test results,the theoretical ultimate bearing capacity of the self-anchored test pile is 2726.16 N,while the measured value of the traditional compressive pile ultimate bearing capacity is 2800 N,with a difference of-2.64%between the two.The results indicate that the expression for the ultimate bearing capacity of the self-anchored test pile obtained by combining the bearing capacity conversion principle with the pile failure mode has good accuracy.After dividing the calculation parametersλ₁,λ₂,₁k,k₂,_bW,W_b1 and in the analytical formulas of the upper tension pile and the lower compression pile of the self-anchored test pile by the correction coefficientβ,the Q-s curve of the self-anchored test pile equivalent to the traditional compression pile is obtained through analytical transformation.By comparing the measured test data of the self-anchored test pile A4 in the in-situ test with that of the traditional compressive pile A3,the mean square error of the fitting between the theoretical displacement results of the self-anchored test pile and the corresponding displacement data points of the traditional compressive pile A3 is 1.159 mm.The results show that the Q-s curve of the self-anchored test pile after analytical conversion is generally in good agreement,and has certain practicality and accuracy.