Research On Load Transfer Character Of Pile In Expansive Soil Foundation

Pile foundations are effectively able to reduce the proportion of structural deformation induced by expansive soils (EPS) swelling or shrinking, which has been successfully reported by many engineering cases and research on those constructions within the zone of EPS. As for a pile in EPS, it is pulled by upward or downward shear force resulted from change of water content in EPS, and those unstable forces may cause the pile to move upward, and depart the pile from cap or even broken the pile, which contributes to overall structural damage. Statistic shows that the EPS problems are the cause of distortions to a lot of transportation infrastructure, civilian infrastructure and infrastructure of city, which is directly responsible for more than several billion yuans annually in China. So, it's very essential and urgent to carry out systematical research on EPS. Based on present domestic and international research on pile in EPS and comprehensive analysis, this dissertation contributes many efforts to work toward load transfer character of pile in EPS with the research projects supported by the National Natural Science Fund. By means of combining the research with experiments in-doors, model tests, theoretical analysis and numerical simulation, the research work has been accomplished with the following achievements:1) Based on one-dimension oedometer tests, time independent deformation law of EPS has been described. The results are as follows:①Indicate the difference between the loaded swelling ratio-time curves and free swelling ratio-time curves: the formers around undergoing two stages could well be described by hyperbolic relationship in single-logarithmic scale, while the laters around undergoing three stages prefer being described by logarithmic function in the same scale.②Loaded shrinking ratio-time curves are almost of linear with time in logarithmic scale.Further more, corresponding functions have been established to describe all test curves with good agreements. Then, deformation law of EPS has been analyzed in physics view based on the test data. The study establishes basis for carrying out the research on time independent interaction between pile and EPS.2) The effects of vertical loads, water contents and dry densities on mechanical character at the interface of EPS and concrete block are studied through many direct shear tests of Nan-ning expansive soil samples by the improved direct shear apparatus. The study indicates that ultimate shear stress is mostly sensitive to water contents while residual shear stress vertical loads. Sensitive analyses accounting for effect laws of different factors are made by using the test data and relative references, which contributes to better understanding of EPS-structure interaction, to the basis of copping with EPS problem and to the continuing research work on EPS-pile interaction.3) The load-transferring characteristics regarding behaviors of P-s curve skin, axial force, pile friction, pile-end resistance and together with load distribution of the both are studied and analyzed by model tests for a pile in expansive soil. Meanwhile, comparison of the load transfer law of a pile in EPS and common clay soils has been made which contributes to better understanding of the load-transferring law for a pile in EPS.4) Model tests for a pile are used to study the interaction between pile and EPS in terms of force and deformation, under continuous introduction of water. The study mainly focuses on the changing laws of pile axial force, pile skin friction,neutral point,and it suggests the pile length that is for design could be obtained by solution of force equilibrium: total uplifting force is equal to total withholding force. Meanwhile the study obtains time independent behavior of soil around the pile, all which contributes experimental and theoretical foundation to numerically stimulate the pile-EPS interaction.5) Elastic differential equations of load-transfer for loaded single pile, under either soil swelling or soil shrinking, are firstly established in the dissertation, based on the mechanism of pile-soil interaction and the shear-deformation method. The derivation of analytic solution to load-transfer for single pile in expansive soil could hereby be obtained by means of superposition principle under EPS swelling or shrinking. The results obtained by theoretical method are in good agreement with tested data.6) Relative computer programs are designed for simulation analysis of load-transferring law of pile in EPS based on the above research, obtaining the following conclusions and results:①With the increase of pile length, upward pile movements reduce while tensile forces increase. ②Small diameter piles that are dug into lower clays below active zone may effectively decrease upward pile movements.③Axial force and displacement of a pile in EPS will increase followed by the increase of the heave at soil surface.④The load that stops the upward pile movement is as 2.5 times as the maximum tensile force of the unloaded pile in EPS.⑤The upward pile movements and axial forces will increase by unstopped introduction of water, but the increments of the both become less and less with time elapsing and till the time of swelling stability for EPS, thus the upward pile movements and axial forces achieve their stability as well.