Moistening Test Study And Model Of Unsaturated Soil

In order to investigate the properties of stress, deformation and strength of collapsible soil in the case of moistening, a test method is raised in which ice crumbles are blended into dry soil while sampling and thus the samples are moistened evenly under prescribed stresses by controlling the meltage of ice crumbles. By this method the problem of uneven water contents after moistened, which is encountered in tests by means of infusing soil samples from outside, is solved and it is possible to obtain prescribed moistened water contents in quite a short time. Confined compression tests and tri-axial tests were carried out involving remolded compacted clay under unsaturated moistening. Based on the initial Tsinghua elasto-plastic model of soil, water content instead of matrix suction is introduced into the hardening parameter of the model to develop a constitutive model on unsaturated soil as handy and feasible as possible. With the yield surface and all parameters of the existing Tsinghua elasto-plastic model of saturated soil unchanged, the Tsinghua elasto-plastic model of unsaturated soil is derived and established. Only a set of isotropic compression tests with different water contents were employed to determine all the parameters of the model. It is shown in the tests that the model is able to assess the stress - strain relationship on unsaturated soil of different water contents. A uniform framework of both saturated and unsaturated soil is presented in this constitutive model. A series of moistening tests under isotropic compression were utilized to calculate the moistened strain under any stresses by the normality law. It is observed in the tests that the application to the moistening calculation of Tsinghua elasto-plastic model of unsaturated soil is capable to reasonably predict the whole stress-strain relationship of soil moistened from the air-dry state to any other water content. The phenomenon that the initial segment of the reloaded stress-strain curve presents a sharp straight rise, which is easily found in moistening tests, is explained in the yield-advancing theory. A function describing the relation ofwater contents and the hardening parameter is directly presented in the model and the location of yield surface after moistened is estimated according to the difference of hardening parameters. Consequently it is possible to reasonably predict and control the range of the elastic segment so as to assure that the linetype of the stress - strain curve and the strength well agree with the test results both before and after moistened. On the basis of the aforementioned tests, an application of the Tsinghua elasto-plastic model of unsaturated soil to dewatering calculation is investigated; it is shown in the results that the Tsinghua elasto-plastic model of unsaturated soil has the potential to reasonably assess the relationship between the stress-strain and the strength of unsaturated soil.The test method of involving ice crumbles while sampling introduces a new idea to perform unsaturated moistening tests. Inheriting from the initial Tsinghua elasto-plastic model, both the yield surface and the hardening parameter of the model are directly determined on the data of tests; without any assumption, it shows strong feasibility for all kinds of soil; it enjoys distinct advantages in terms of solving the complicated problems concerning special kinds of unsaturated soil; the complicated conception of suction is successfully hedged against throughout the model while a simple and explicit expression concerning water contents is employed to describe the properties of unsaturated soil, which may provide a convenient way of engineering design and computation on unsaturated soil.