| Expansive soil is a kind of clay mineral mainly composed of montmorillonite,illite and kaolinite.The expansive soil has strong hydrophilic property,super consolidation property and fissure property,and the repeated deformation property which is easy to cause expansion and shrinkage when it encounters water is different from that of normal cohesive soil.The construction of highway,railway,bridge and tunnel in expansive soil area is easy to be seriously affected.Jianshanying Bridge is located in Changming Town,Guiding County,Guizhou Province,through which the newly built High-speed railway from Guiyang to Guangzhou passes.The on-site monitoring data shows that the piers of the bridge appear to rise abnormally in rainy season and fall abnormally in dry season,and the displacement of the piers seriously affects the safety of the high-speed rail and beam structure of the bridge.Some studies show that the expansion of expansive soil in this area leads to abnormal uplift and subsidence of the piers of Jianshanying Bridge.In view of this actual condition,this paper takes the expansive soil and pile foundation of the bridge as the research object,conducts indoor model test based on similarity principle to study the pile-soil action mechanism of the bridge,and provides reference for the design and construction of pile foundation in this area.The main contents of this paper are as follows:(1)The similar materials of expansive soil required for model test materials were studied and selected,and bentonite,sand and kaolin were established as the similar expansion materials to simulate the expansive soil of Jianshanying extra-large bridge.(2)Based on laboratory tests,a material ratio with expansion characteristics of Jianshanying Bridge was developed with free expansion rate and liquid-plastic limit as the similar criterion.The ratio was montmorillonite: sand: kaolin = 7:6:1.(3)The change curve of lifting and settling of the model pile body was analyzed,and the pile body rose gently to the maximum value after two discontinuous and rapid lifting processes.The time of rapid uplift of the longest pile is half of the time of maximum uplift of the pile,while the time of rapid uplift of other piles is only 1/5 ～ 1/3 of the time of maximum uplift.(4)Under the same pile top load,the maximum uplift of the pile decreases with the increase of the pile length,presenting a nonlinear logarithmic form between them.(5)Based on the horizontal stress in the soil,the distribution curve of pile lateral friction resistance and pile axial stress along the pile body of model no.4 is obtained by calculation,and the load transfer mechanism of pile-soil system under vertical load is analyzed.(6)At the initial stage of water immersion,the upper part of the soil expands,while the lower part does not expand or slightly expand because the water does not penetrate.Generally,the swelling amount of the expanded soil decreases linearly with the increase of the depth of the soil. |
