Research On Swelling-Shrinking Characteristic And Pile-Soil Interaction Of Expansive Soil Foundation

In many civil engineering constructions such as railway, road, hydraulic and architecture engineering, etc., expansive soils (EPS) problems may be met sometimes. The repeated action by the soils swelling and shrinking, respectively, induced by expansive soils absorption of water and loss of water may greatly damage the constructions built on its area. In order to reduce the upward construction's movements as much as possible, pile foundation is commonly used in those constructions on the EPS. For the complexity of EPS itself, the soil-pile interaction (SPI) in the EPS have relatively big difference from the interaction in common soils. Pile in EPS is subjected to uplift force induced by soils swelling, and thus upward pile and upper structures' movements occur. Given inappropriate design or construction, the pile may be broken if the uplift force exceeds the strength of the pile, which greatly hurt the safety of the constructions on the EPS. So, further research on the soil-pile interaction (ISP) in the EPS attaches important value in theory and practical meaning in engineering. Based on present domestic and international research on pile in EPS and comprehensive analysis, this dissertation has been accomplished with the following achievements by means of combining the research with experiments in-door, model tests, theoretical analysis and numerical simulation:(1)Systematically studies on EPS swelling and shrinking characteristics has been made based on lots of in-door swelling and shrinking experiments of the samples from Tan-shao highway in Hunan province and Nanning, Guangxi province, followed by which the regression equations to calculate their swelling force and percent swelling, which provide basis for predicting and estimating the uneven deformation and settlements of the EPS.(2)A new method for calculating the heaves of isotropic EPS foundation and layered EPS foundation is suggested based on the swelling and shrinking characteristics of the EPS. The suggested method not only can calculate the final heaves at varying depth in EPS foundation, but also can predict the heaves' changes over time. Comparison between the obtained values and the measured ones are made, and good reliability and practicability are proved.(3)Analytical calculation formula is established in the dissertation for load transfer of a loaded pile in EPS foundation that is subjected to either soil swelling or soil shrinking, which is based on the EPS swelling and shrinking behaviors. Because the time-related effect of EPS swelling and shrinking is introduced in the formula, so the method can predict the changing laws of pile axial force, deformation and pile skin friction over time. 3 measured cases are provided to prove the reliability of the suggested method that is simple, practical and with high accuracy.(4)A new constitute model of EPS-pile interface is suggested through lots of direct shear tests of Nanning expansive soil samples, which is an addition of the present models. Five parameters required in the models can just be obtained by direct shear tests.(5)In-door tests are carried out to simulate the ISP in EPS foundation, and to systematically study the pile behavior under vertically statistic loads and the influence of introduction of water on the pile in the foundation, which provide experimental and theoretical basis for further numerical simulation of the ISP in the EPS.(6)Based on fluid-solid coupling mechanism theory, the processes of the EPS swelling and shrinking with changes of water are considered as a dynamic coupling process. Then a seepage-deformation coupling model is established by introducing relative unsaturated EPS theory. Finally fluid-solid coupling FEM programs of unsaturated EPS with good opening function are made in FORTRAN environment.(7)An analyzing modular is added in the above fluid-solid coupling FEM programs for the unsaturated EPS, which is used to the fluid-solid coupling analysis of the ISP in EPS foundation. Goodman joint elements are adopted as the interface elements and the suggested constitute model of EPS-pile is introduced. After comparing the obtained results to the measured ones, it is proved that both the suggested method and the adopted model are available and correct.(8)On the basis of previous work, numerical simulation analysis of the ISP in the EPS are carried out, focusing on the systematical analysis of the influences of the pile length, pile diameter, pile cap, bell-pile and soil-pile interfaces' characteristics on the ISP in the EPS foundation.