| The engineering behavior of an unsaturated soil is commonly viewed as being more complex and more difficult to understand than that of a saturated soil. However, unsaturated soil has become the focus of soil mechanics research because of new engineering problems and cognitive law of simplicity to complexity. Now, laboratory test based on the reconstituted specimen is the main research means for the unsaturated soil. However, test results of unsaturated soil depend on the uniformity of reconstituted specimen because expensive test equipment and time-consuming. In traditional concepts, specimens reconstituted under the same condition, is uniform, but these specimens may be different under the unsaturated condition due to difference of pore structure. In this paper, differences of equilibrium saturation degree are analyzed based on the double pores system. Soil-water characteristic curves of four specimens were measured based on pressure plate extractor, and test results show that saturation degrees of different specimens are different under the same matric suction, in other words, specimens are not identical. The mechanical behaviors of specimens are affected by difference of equilibrium saturation degree, and the result is confirmed by direct shear test. The specimen preparation condition is adjusted to improve uniformity of specimen. S WCC of new specimens are measured based on pressure plate extractor, and difference of equilibrium saturation degree is analyzed again, then relative factors during the drying test are analyzed. Results of drying test and unconfined compressive strength test are analyzed. Based on the relationship between curves of saturation degree versus equilibrium time and mechanical behavior of specimen, the method for distinguish the uniform unsaturated specimen is proposed. Uniform unsaturated specimens are obtained by the method, and then are used to study the cyclic behavior of unsaturated soil based on the improved soil static and dynamic universal triaxial and torsional shear apparatus. The cumulative deformation, pore water pressure and volume change under the cyclic loading are investigated. At last, a series of meaningful conclusions are obtained and listed as follow:(1) Based on the double pores system, this paper makes a theoretical discussion for difference of equilibrium saturation degree, and the difference of equilibrium saturation degree is confirmed by the SWCC test. Results show that SWCC affected by difference of specimen can be described by the mean value in the random stage and test data of basis specimen in the linear stage. The fitting result shows that Fredlund&Xing model is most suitable, and this is relevant to coefficient C(ψ). The study shows that fitting effect can be improved by the assumed data. The results of direct shear test confirm that mechanical behaviors of specimens are affected by difference of equilibrium saturation degree. Further study shows that mean value can reduce effect of equilibrium saturation degree. The result can be ensured and specimen amount, if the mean values are selected only under the maximum and minimum matric suction. But how to use the method should think about the fact.(2) The difference of specimen is improved by the adjusting specimen reconstitute condition, but it does not disappear. The range of particle size obtained by the packing form is identical with the part of violent change on the grain size distribution curve, and the relevant matric suction range is identical with SWCC. So the test can carry out without these matric suctions. There is the evaporation effect in the drying tests, and the water loss caused by evaporation is dominant with the increasing of time, so time effect induced by evaporation should be thought. The SWCC is obviously influenced by side protecting, especially to the specimen with big ratio of height to diameter, evaporation will become main factor of water loss without side protecting. So the side protecting based on the improved compact cylinderis is worth popularizing.(3) The pressure plate instrument is employed during the drying, and the unconfined compressive strength test is performed on the unsaturated specimen. Curves of shear force versus shear displacement and curves of saturation degree versus time are investigated. Results show that the specimens with similar curves of saturation degree versus time have nearly identical mechanical behavior. In particular, the uniform specimens should be chosen within the lower equilibrium saturation degree because steady test results are presented. Based on the experiment research, new triaxial specimen preparation equipment is designed to solve questions of inhomogeneous specimen, hard to control dry density, specimen being disturbed. The location and seal of split mold equipment are comprehensively considered in order to improve precision of specimen and sealing effect. The dry density of specimen is controlled by the two-level control. The disturbance of specimen is avoided during the specimen preparation because is protected by the split mold.(4) The results under the critical cyclic stress show that with the increasing matric suction, the cumulative strain will decrease and then is up to steady. When the matric suction is constant value, the cumulative strain will increase with the increasing confining pressure. Further study shows that the ratio of origin strain and cumulative strain is the constant value under the constant confining pressure, and origin strain and matric suction are linear relationship. Based on the above analysis, cumulative strain within the specific range is predicted by the origin strain of saturated specimen. (5) Results of100kPa matric suction and100kPa confining pressure are analyzed, the research shows the shear strength of soil will increase with increasing matric suction, but tensile stress will become main failure factor of unsaturated specimen under the cyclic loading, especially the wean place of unsaturated specimen is easy to be destroyed by the tensile stress. The cyclic stress is important factor to tensile failure. In this paper, the tensile failure happens at rc=1.0; meanwhile the tensile failure is also limited by matric suction, and it happens only at the (ua-uw)=100kPa. Above results indicate that the cumulative strain is not monotone variation with increasing matric suction. However, it is correlated to the comprehensive influence of cyclic stress and matric suction. So final state of specimen under the rc=1.0is from lower matric suction to higher matric suction, that is shear failure, tensile failure and steady state; in other words, with increasing matric suction, there is not threshold value as like critical cyclic stress ratio but inflection point. This matric suction is named as key matric suction, and the key matric suction is100kPa in the paper.(6) Pore water pressure and volume change are analyzed thinking about cyclic stress, matric suction and confining pressure. The research shows that pore water pressure of unsaturated soil under the cyclic stress is affected by the matric suction. Under the lower matric suction, unsaturated specimen will generate excess pore water pressure, and pore water pressure will increase with the increasing cyclic stress. Under the high matric suction, pore water pressure is limited and it is not clear rule even for high cyclic stress. The volume change of unsaturated specimen is observably affected by cyclic stress, but there is not rule at the critical cyclic stress. Note that the mutation that volume change is from volume contraction to volume inflation happen, when cyclic stress ratio is from0.8to0.9. The mutation is not affected by matric suction and confining pressure. The phenomenon further confirms that tensile stress has clearly affected to unsaturated soil. |
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