| Expansive soil was classified as an unsaturated soil in engineering construction. The influence of swell-shrinking deformation of expansive soil foundation is relatively small for the general railway engineering, which can be neglected frequently. The swelling and shrinkage deformation of expansive soil may cause line harshness intensified for the ballastless track on high-speed railway subgrade, affecting the normal operation of high-speed railway, which may even lead to major accidents. Therefore, the swell-shrinking deformation of swelling soil foundation must be considered in the design of high-speed railway engineering.At present, many passenger dedicated lines are constructed or designed in China, which traverse through expensive soil regions regularly, such as Yungui high-speed railway. To construct the ballastless track of subgrade with high standards on expansive soil foundation, especially low embankment or cutting, there are some main problems should be solved, Such as:(1) Swell-shrinking macroscopic response of expansive soil foundation under low embankment have not been revealed.(2) An accurate computational theory of swell-shrinking deformation for expansive soil foundation under low embankment have not been determined, especially the correction coefficient of swell-shrinking deformation.(3) Replacement thickness of low embankment and cutting bed for high-speed railway in expansive soil region have not been united in our country, then, the foundation treatment standards under subgrade bed surface is also a part without agreement.In response to main problems above mentioned, combined with scientific research project "Deepen research to settlement controlling of ballastless track of low compression soil and swelling soil foundation" supported by Chinese ministry of railway, some research methods such as the detailed investigation of research date at home and abroad, laboratory soil test of undisturbed expansive soil, In-situ tests, field soaking test under plate loading, field wetting and drying test under low embankment, numerical simulation, theoretical analysis, et al. Which were adopted to study swell-shrinking macroscopic response characteristics, coupled seepage and deformation, computational theory of swell-shrinking deformation and the technology of foundation treatment of expansive soil foundation under low embankment.All of them have made a number of innovative achievements, including the following major areas:(1) To master the physical-mechanical parameters and swell-shrinking deformation calculation parameters of undisturbed expansive soils, a number of series tests are carried out. Based on it, the equations considering the variation of consolidation coefficient for consolidation under stage continuous loading was derived by the relationship among coefficient of consolidation, effective stress and void ratio. According to analytical solutions of the above equations, a formula for settlement calculation under stage continuous loading was obtained. Combined with stage continuous loading K0 oedometer test of undisturbed expansive soil, the reasonableness of theoretical calculation formula was verified.(2) To obtain basic geotechnical parameters of swelling soil, such as compression modulus, foundation bearing capacity, et al. In-situ tests of expansive soil foundation in Mile town of Yun-Gui high-speed railway were carried out, which advantages, disadvantages and applicability of various in-situ testing methods was discussed. Meanwhile, to acquire relative swelling pressure and absolute swelling pressure of foundation, field soaking test under multi-plate loading was performed in the region of field wetting and drying test under low embankment. Finally, acquisition principle of compression settlement calculation parameters of expansive soil foundation was established, then, the expansion force of foundation with field soaking test under multi-plate loading was also suggested.(3) Field wetting and drying experiment was performed for the first time, which was used to study swell-shrinking macroscopic response characteristics of expansive soil foundation under low embankment. The result show that the critical height of embankment is about 5m for medium-strong expansive soil foundation without occurring after soaked and saturated, the ultimate relative expansion of expansive soil foundation is 14.22-8.14mm、the relative expansion during rainy season (May 2014-September 2014) of expansive soil foundation is 3.75-2.23mm under the height of 0.9-2.7m subgrade. From the end of expansive soil foundation fully saturated to the beginning of next rainy season (September 2013-May 2014), the relative shrinkage is 7.48～6.33mm under the height of 0.9-2.7m subgrade. The relative expansion of subgrade surface is small than that of ground surface. attenuating along the subgrade body, and the attenuation proportion decreases with the height of subgrade; When the height of subgrade is 0.9-2.7m, the ultimate relative shrinkage of subgrade surface is 11.95-5.8mm which attenuating 16.0-28.8% coMPare to the ground surface, and the relative shrinkage during drying is 7.03-5.65mm which attenuating 14.3-25.0%.(4) Based on the mass conservation of fluid, Darcy law and elastic constitutive equation of expansive soil, two-dimensional equations of coupled seepage and deformation in expansive soil is obtained. Then, both the numerical simulation considering coupling and that without considering coupling were used to analysis parameter sensitivity and swelling deformation characteristics of expansive soil foundation under low embankment. The study results indicate the vertical and lateral (caused by seepage) deformation of foundation increases rainfall, however, the vertical and lateral deformation of foundation is significantly reduced while increasing the elastic modulus、the height of embankment and Poisson’s ratio;The swelling deformation and the pore water pressure obtained from the numerical simulation which did not consider coupling is less than that obtained from the numerical simulation considering coupling at the same time. In addition, the numerical calculated values are coMPared with test results in field. It is pointed out that the numerical simulation without considering the effects of coupling is difficult to describe the swelling deformation of expansive soil foundation under the artificial soaking condition. On the other hand, the results from the numerical simulation considering the iMPacts of coupling agree with the measured results.(5) Combined with field large wetting and drying test under low embankment, A coMParative analysis of the advantages and disadvantages of the direct simulation method and constant volume method were carried out with calculation accuracy of swell-shrinking deformation and difficult degree of parameter acquired. It suggested that the direct simulation method can be used to compute expansion in preference, while the design of ballastless track on low embankment of high-speed railway performed in expansive soil region. On the basis of considering the relative expansion of subgrade surface attenuating along the subgrade body, modified calculation methods for swell-shrinking deformation of expansive soil under low embankment with ballastless track were proposed. Meanwhile, the correction coefficient of swell-shrinking deformation of ground surface and subgrade surface was definite (Ψer and Ψerf is 0.7、Ψs and Ψsf is 0.6).Based on the direct simulation method and field large wetting and drying test, the relationship of expansion calculation between rigid foundation and flexible infrastructure was revealled. In addition, the analysis thought for using field soaking test under plate loads to predict the swelling deformation of soil foundation under low embankment. Finally, through field large wetting and drying test under low embankment and the study of calculation theory of swell-shrinking deformation, the principles of foundation treatment were proposed, based on it, foundation treatment measures of medium-strong expansive soil foundation under low embankment with ballastless track was demonstrated and proposed. |
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