Study On Triaxial Creep Characteristics And Models Of Jingyang Loess

With the western development drive, loess is widely used as the building materials, building environment and the carrier of foundation engineering constructions. So the rheological problems such as deformation, strength and stability over time become very prominent. Practice has proved that: the destruction and instability problems of many engineerings such as the failure and instability of natural slope and artificial slope, the slow lateral displacement of excavation airport surface, the slow vertical displacement of foundation and subgrade, the deformation and instability of earth-rock dam, the settlement and deformation of the surface during the process of tunnel construction and operation, etc are not formed immediately after excavation or the completion of the project in most cases which caused by the continuous adjustment, change and development of soil stress and deformation with the passage of time. Therefore, further comprehensive and in-depth study on the creep characteristics of loess is still needed,which will enrich and perfect the loess creep theory, provide some guidances for the design, construction and stability analysis of the structure of actual engineering and have an important theoretical and practical significance to guarantee the long-term safety and normal use of structures on loess plateau areas.Based on the summary and thinking of the predecessors’ s achievements on three axial creep characteristics of loess, by taking the undisturbed and remolded samples of Q3 from a loess slope of Jingyang in Shaanxi province as the study object, a series of laboratory consolidated drained triaxial creep tests are performed by triaxial creep meter of SR-6 in this paper. The tests focus on the water content, confining pressure, deviatoric stress level, stress path, the initial principal stress angle and their coupling effects on the creep characteristics and based on this the creep constitutive models suitable for the loess in this area are established. The main research results are as follows:(1)The remolded and undisturbed loess in this area both have obvious creep properties overall showed as nonlinear attenuation creep and linear attenuation creep respectively. The creep characteristics of loess are obviously influenced by the deviatoric stress,confining pressure and water content. When the samples are under the condition with high deviatoric stress level, small confining pressure and high moisture, the creep phenomenon is obvious and the creep strain is large.(2)The creep law of remolded loess under three different stress paths is consistent. While under a certain level of the deviatoric stress, creep deformation difference is obvious, and arranged in descending order as: excessive loading, grading loading, respective loading.(3)Because of the nonlinear cumulative after effect of grading load,the results of measured data processed by two kinds of methods(the Boltzmann superposition method and Chen’s method) are different. So Chen’s method should be used to process the data of nonlinear creep under graded loading as far as possible. Based on mathematical statistic method, two kinds of empirical creep models are established which can aptly describe nonlinear attenuation creep characteristics of remolded loess with a 90% degree of compaction in this area under the partial stress level between 20% and 65%. The two models both have their own advantages, so they should be considered comprehensively in practical applications achieving mutual verification purpose.(4)The analysis of the basic rules of viscoelasto-plastic characteristic of the undisturbed loess loess by exerting circular incremental load and unload by steps shows that: When loading, the creep strain of the undisturbed soil samples mainly consists of four parts: the instantaneous elastic deformation, the instantaneous plastic deformation, visco elastic and visco plastic deformation. After unloading, the soil samples generate attenuated viscoelastic rebound deformation which decreases with the decrease of unloading and the increase of confining pressure and moisture content. But because the bond strength of the loess in this area is weak and it exists uneven large pores, it shows a strong plastic deformation characteristic at low stress levels. The rebound deformation of the undisturbed loess in this area is not obvious, and the proportion of viscoelastic deformation in the creep strain is small. Loading and unloading history has a cumulative effect on soil creep deformation which will increase the creep deformation; Based on the analysis of the basic rules of viscoelasto-plastic characteristic of the undisturbed loess loess by exerting circular incremental load and unload by steps, a viscoelasto-plastic nine-element creep model is established which can be applied to characterize the triaxial linear attenuation creep properties of undisturbed loess in this area and has clear physical meaning as well as the model parameters can be obtained by using the software-Origin.(5)The creep law of undisturbed soil samples with different initial principal stress angles is consistent. When the axial deviatoric stress is small, the initial principal stress angle has a certain influence on the creep characteristics of soil mass. At any time, the axial creep deformation increases with the increase of initial principal stress angle. Because the structural property of loess in this area is not obvious, there is a little difference between the deformation values. The primary structure of soil samples are crushed with the increase of axial deviatoric stress, so the soil itself has no structural anisotropy which will cause that the difference of creep deformation of soil samples with different initial principal stress angles decreases and also there is not a certain regularity.