Study On Secondary Consolidation Properties Of Expansive Soil Based On Consolidation Tests

Based on Hunan traffic science and technology project " research on the prediction of water damage influence for expansive soil subgrade under extreme climate" (project number 200944), scientific research fund project for introduced higher-level talents in Central South University Of Forestory and Technology(project number 104-0094), Hunan innovation project for undergraduate students "experimental study on the basic mechanical properties improvement of expansive soil with biological enzyme " (project number CX2015B308), this paper carries out long-term one-dimensional compression test research and nonlinear rheological model research for Yiyang expansive soil under the engineering background of subgrade expansive soil treatment of Loudi-Yiyang expressway.Based on literature searching and review in early stage, contents of four aspects have been studied in this thesis. Firstly, this thesis experimentally studies the change rule of secondary consolidation coefficient affected by 3 factors. Secondly, it explores the optimum cement content for cement modified expansive soil based on the uniform design experiment method. Then, with a background of weather conditions encountered by the expansive soil, it conducts the long-term one-dimensional compression test after drying and watering cycle, so as to study the influence of drying and watering cycle on the long-term compression properties of expansive soil. Finally, based on rheological model theory and genetic creep theory, this paper establishes the nonlinear rheological model of Yiyang expansive soil, and verifies its correctness by comparing with testing data. So this paper has obtained some research results and innovative conclusions both from the perspective of practical engineering and theory. In summary, the research results and innovative conclusions is this paper are as following.1% Research on the influence law of dry density, water content and consolidation pressure to the secondary consolidation coefficient. Based on the long-term one-dimensional compression test for the expansive soil which is taken from the fourth section of Loudi-Yiyang expressway, it is found that the secondary consolidation coefficient increases with the increase of water content; secondary consolidation coefficient reduces with the increase of dry density; with the increase of consolidation pressure, secondary consolidation coefficient increases rapidly at first, then gradually slows down, and tends to a constant value finally. By orthogonal test analysis, the influence level of above three factors on secondary consolidation coefficient is respectively the water content, consolidation pressure and dry density in descending order.2、Research on the influence of long-term compression properties to cement modified expansive soil. In order to obtain the influence law of cement content to the secondary consolidation coefficient and optimum cement content, this paper designs two experiments to explore the two questions, and comes to following conclusions:(1) With the increase of cement content, the secondary consolidation coefficient reduces firstly and then increases later, showing a parabolic shape. The optimum cement content is preliminary determined between 7% and 8.5%.(2)Further experiment has been conducted through uniform experiment design method, and the optimum cement content has been determined by multiple regression analysis. According to the CBR and total swell-shrink ratio purposed by subgrade specifications, the improved soil with which the cement content is 8.5% can fully meet the requirements. It can also be considered as a new method to determine the cement content.3、Experimental research on the influence of drying and watering cycle on the long-term compression properties of Yiyang expansive soil. In order to simulate the influence of nature environment on long-term compression of soil sample, this paper designs the drying and watering cycle experiment, and compares with the sample that does not suffer drying and watering cycle. It is found that the secondary consolidation coefficient increases slightly after the drying and watering cycle, the secondary consolidation coefficient of unimproved expansive soil shows a logarithmic increase with cycle times after drying and watering cycle, the improved expansive soil shows a linear increase. From a perspective of variation range, the unimproved expansive soil is larger than that of improved soil, which verifies the effect of cement improvement to stabilize the long-term compression properties of expansive soil.(1) Research on simple constitutive model purely based on the test result. Based on analyzing the influence of various factors for long-term compression properties of expansive soil, the relationship between compression and time, consolidation pressure and dry density. The consolidation process is divided into primary consolidation and secondary consolidation, the corresponding result is almost identical with the result of standard correction method. Moreover, its physical meaning and consolidation process division is more clear.(2) Based on the experiment results of long-term one-dimensional compression, the model theory and genetic creep theory are applied to divided the nonlinear rheology of Yiyang expansive soil into linear viscoelastic part, linear visco plastic part and nonlinear visco plastic part. The former two parts apply the model theory and the nonlinear part applies the genetic creep theory. The model takes comprehensive account of aging characteristics of soil rheology, and fits material parameters with nonlinear equation. The established model can fit well with the measured curve.