Creep Characteristics Of Structured Soft Clay And Its DSC Model

Soft clay distributes widely in China, has poor engineering properties such as high sensitivity, low strength and bearing capacity, consolidation for a long time and creep characteristics is remarkable. The rapid development of coastal areas huge-scale constructions presents us newer and higher tasks on engineering researches of soft clay, and the structural characteristics, creep characteristics and mechanical models of soft clay are the prominent and urgent ones up till now. The creep characteristics is an important factor that decides the settlement and stability of soft clay foundation and superstructure after construction, and the special structural characteristics of soft clay causes the creep characteristics―the complication‖. Therefore research on the creep characteristics of the structural soft soil and constitutive model has important theoretical significance for the correct description of the engineering properties of the soft soil, and have important practical significance for analyzing and evaluating soft clay ground deformation and stability.This paper was based on the soft clay in Huangshi and Zhangzhou and Qingdao areas, according the approaches that combined the soil mechanics and soil material, researched on soft clay structural characteristics, creep mechanics behavior, physical chemical composition, microscopic structure characteristics and their relationships, investigated the creep mechanism of the structural soft clay from two aspects which were macroscopic mechanics and microscopic structure. Through introducing the disturbed state concept to establish soft clay creep model, and verified the reliability of model through the creep test data fitting, for the purpose of forecasting reliably the foundation deformation of the soft clay, for the purpose of forecasting reliably the foundation deformation of the soft soil.The primary results and conclusions ware gained as following:1) Analyzed systematically the influence of the structural characteristic on solidifying characteristics, consolidation coefficient, secondary consolidation coefficient, stress-strain relations, pore-water pressure characteristics, strength envelope,viscosity coefficient,nonlinear creep characteristics and creep curves. Results showed that structural soft clay displayed special engineering characteristics relatived to non- structural soil and structural weak soil. Such as the phenomenon that the secondary consolidation coefficient Ca achieved peak value with the pressure increasing was caused by the structural characteristics, so estimating Ca through Ca /Cc must consider the influence of the soft clay structure; structural soft soil's viscosity coefficient achieved peak value along with shear stress increasing, and the bigger peak value of consolidation pressure the bigger the shear stress.2)According the structural strength region between the compression curve of reconstituted soil and the compression curve of primary soil, proposed the concept of structural damage coefficient S, this parameter was gained simply which showed soil structural damage condition in the process of compression, could be used to distinguish quantitatively the structural strength of the different structural soils, by using this method obtained that the structure characteristics of QingDao soft clay was the strongest and the HuangShi soft clay was second and the ZhangZhou soft clay was weakest.3)Some meaningful conclusions about test data processing and result expression were obtained. Such as soft clay creep test datas that exhibited nonlinear characteristics should use Chen's method; the complexity should be taken as the important parameters measuring soft soil pore or particle shapes during the creep, which not only could reflect microstructure outline shape, but also measure the size between the average size of the gap and the actual size; radar chart was used for showing directionality frequency distribution, that simultaneously contrast many group of datas, and it was advantageous of the observation on distributed intensity by setting the data standard ring.4)Researched on soft clay microstructure change rule in the pro and post creeping through qualitative and quantitative comparison of electron microscope photographs of the primary soil and the soil sample after the creep test. Results showed that soft clay mainly by flocculated structure, skeleton structure, honeycomb structure, aggregate structure. In the Condition of creep the particles contact forms transited the primarily form by side-to-side, side-to-face to the form by face-to-face, and the state was gradually approaching to the characteristics of reconstituted soil; micro-poor size changed fellow theory of homogenize which maropore reduced many and the fine pore changed was small,the pore distribution turned to be homogenization gradually in the volume compression process; the number of elongated pores and particles reduced, the equiaxed shape pore and particle were almost non-existent, on the whole the pore turned to be oblateness; the pore and particle outline was trended to be smooth, and the pore complexity reduced more bigger than the particle complexity reduction scope; particle's directionality trended orderliness. Because in the process of soft clay creep the soft clay microstructure parameters influenced by the test method more, should analyze comprehensively the microscopic test datas by different methods, for approaching to the actual situation more realistically.5)By the creep mechanism the analysis showed that: the structural soft clay's creep process could be summarized to the process that internal structure changed to be self-adjustment restoration in order to adapt the external force change. In this process, the soft soil experienced three stages which were structure basic complete, structure massive breakages, and reconstituted soil similar.The root cause of the soft clay creep characteristics was the soft clay formed the flocculated structure, skeleton structure, honeycomb structure, aggregate structure, which particles contacted form primarily by side-to-side,side-to-face. The clay mineral in the particle surface had electric charges, adsorbing massive free oxide and cationic, then bonding particles through the cemented joint, so that the soft clay had certain structural strength. Generally soft clay was characteristics that high moisture content large void ratio, high clay content, simultaneously soft soil contained massive clay minerals such as illite, montmorillonite and kaolinite, some marine clay also contained organic matter, and the soluble salt content is higher. These factors enabled soft soil to have the big water absorption, and holes were filled with massive bound waters, causing the soft soil showed remarkable timeliness by the external force.6)On base of the conception of disturbed state, replacing the disturbance function by the structural damage coefficient processed, established the creep model of the structural soft clay.According to the visual performance of disturbance function during the compression, confirmed that this model was possible to describe creep process that the structural characteristics weakened gradually. According to creep test data fitting by contrasting Burgers models the results showed that model can describe deceleration creep stage and steady creep stage well, verifying the reliability of the model.